Ultrasound, Cavitation and Cleaning

Benjamin Paul Wilson B.Sc. Hon’s (Wales)

A thesis submitted in fulfilment of the requirements of the degree of Master of Philosophy in the University of Wales

Department of Materials Engineering

University of Wales, Swansea September 1997

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Disclaimer Declaration: This work has not previously been accepted in substance for any degree and is not being currently submitted in candidature for any degree. Signed: _____________

Date:____________

Statement: This thesis is the result of my own investigations, except where otherwise stated. Other sources are acknowledged by footnotes giving explicit references. Signed: _____________

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Aim of the Thesis This thesis describes work initially aimed at elucidating the interaction of

power ultrasound and electrolytic current in the removal of oxide scale from metal

(steel) surfaces. The descaling of metals is currently carried out using an acid

‘pickling’ process and electrolytic descaling in neutral salt would be an

‘environmentally friendly’ alternative. As such the work has three parts:

I. A systematic study of ultrasound propagation and ultrasonically induced

cavitation in aqueous solution proximal to an ultrasound transducer. This

work was undertaken by using the sonogenerated chemiluminescence (SCL)

of luminol to produce images of ultrasonically generated cavitation fields.

The background to ultrasound and cavitation in aqueous systems is

introduced in sections 1.1 and 1.4. the phenomenon of luminol SCL is

introduced in section 3.4

II. A systematic study of the influence of ultrasound intensity and electrolytic

current density on the rate of oxide scale removal from a steel surface. The

theory of electrochemical rate processes is introduced in section 1.5. The

nature and causation of oxide heat scales is introduced briefly in section 1.3.

III. A study of the influence of ultrasound on the rate of cathodic hydrogen

evolution and cathodic oxygen reduction at a titanium “sonotrode” (i.e. an

electrode that is also an ultrasonic transducer.) This last piece of work was

undertaken in order to determine how, and to what extent, ultrasound might

affect the rate of interfacial electron transfer processes occurring at a metal

solution interface.

The original intention was to produce a body of knowledge, which would be

of use in the design of equipment for the rapid, environmentally friendly, descaling

of metal surfaces. However, in the course of the above work information came to

light, which may be of more general significance. For this reason an effort was

made to investigate the propagation of ultrasound in cavitation fields and the

operation of a "sonotrode” at as fundamental level as time allowed.

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Acknowledgements

I would like to thank Dr. Neil McMurray for his supervision and for being a

source of inspiration, encouragement and astonishment (in varying degrees)

throughout the duration of this project. Thanks must also go to Duncan MacDonald

for allowing me the opportunity to participate in this research and all at Maysonic

Ultrasonics Ltd. for providing technical assistance throughout the year.

Personal thanks to Sue, Fiona, Jafar, Ahmed, Siva and Dave for all the help,

drama, comedy, and for all those hours you’ve had to endure the less than pleasant

hullabaloo that has been emanating from my little “box of tricks” in the corner of

the lab. Thanks once again to Justin for putting up with lack of computer literacy

your assistance is always much appreciated! Finally a big “diolch yn fawr” for my

good friend Sarah, for all the hours of fun and entertainment that we have both

experienced during the course of our respective write ups.

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Ultrasound, Cavitation and Cleaning

Aim of the Thesis....................................................................................... iii

Acknowledgements ................................................................................... iv

Chapter 1 Introduction...............................................................................2

1.1 Ultrasound and Ultrasonic Mechanisms .................................................................................... 2 1.1.1 Background ......................................................................................................................... 2 1.1.2 History of Ultrasound.......................................................................................................... 2

1.2 Applications of Ultrasound ........................................................................................................ 5 1.2.1 High Frequency (or Diagnostic) Ultrasound ....................................................................... 5 1.2.2 Power or (Low Frequency) Ultrasound............................................................................... 5 1.2.3 Ultrasonic Welding ............................................................................................................. 5 1.2.4 Biological Uses of Ultrasound ............................................................................................ 6 1.2.5 Ultrasound in Medical Applications ................................................................................... 6 1.2.6 Engineering and Ultrasound, ............................................................................................... 7 1.2.7 Dentistry.............................................................................................................................. 8 1.2.8 Ultrasonic Cleaning............................................................................................................. 8

1.3 Heat Scale Formation and Composition..................................................................................... 9 1.3.1 Introduction......................................................................................................................... 9 1.3.2 The Classical Three Layer Scale Formation ....................................................................... 9

1.4 Ultrasonic Effects on Aqueous Media ..................................................................................... 11 1.4.1 Introduction....................................................................................................................... 11 1.4.2 Acoustic Cavitation and Streaming................................................................................... 13 1.4.3 Factors Affecting Cavitation ............................................................................................. 16

1.5 Electrochemistry ...................................................................................................................... 20 1.5.1 Electrochemical Reactions ................................................................................................ 20 1.5.2 The Electrical Double Layer Hypothesis .......................................................................... 27

1.6 Ultrasound in Electrochemistry................................................................................................ 29 1.6.1 Introduction....................................................................................................................... 29 1.6.2 Mass Transport in Electrochemistry ................................................................................. 31 1.6.3 Mass Transport Boundary Layer (Nernst Diffusion Layer) .............................................. 31

Chapter 2 Experimental Set-up and Protocol ........................................34

2.1 Materials .................................................................................................................................. 35 2.1.1 Chemicals.......................................................................................................................... 35

2.2 Methods ................................................................................................................................... 36 2.2.1 Ultrasound Probe............................................................................................................... 36 2.2.2 Photomultiplier Tube ........................................................................................................ 36 2.2.3 Low Light Camera ............................................................................................................ 36 2.2.4 Galvanostat ....................................................................................................................... 36 2.2.5 Function Generator ........................................................................................................... 38 2.2.6 Potentiostat........................................................................................................................ 38

2.3 Calibration of the Ultrasound Probe ........................................................................................ 38 2.3.1 Calorimetry ....................................................................................................................... 38 2.3.2 Equipment Utilised in the Calibration of the Ultrasound Probe........................................ 39 2.3.3 Method .............................................................................................................................. 39 2.3.4 Results............................................................................................................................... 39

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2.4 Luminol Sonochemilumunescence Experiments ..................................................................... 42 2.4.1 Experimental Set-up.......................................................................................................... 42

2.5 Wire Cleaning Experiments ..................................................................................................... 43 2.5.1 Wire Samples .................................................................................................................... 43 2.5.2 Sample Preparation ........................................................................................................... 43 2.5.3 Electrolyte Preparation...................................................................................................... 47 2.5.4 Electrolytic Current........................................................................................................... 47 2.5.5 Current Density ................................................................................................................. 47

2.6 Sonotrode Experiments ............................................................................................................ 48

Chapter 3 The Visualisation of Ultrasonically Induced Cavitation with

Luminol ..................................................................................................50

3.1.1 Introduction....................................................................................................................... 51 3.2 Materials .................................................................................................................................. 51

3.2.1 Chemicals.......................................................................................................................... 51 3.2.2 Experimental Equipment................................................................................................... 51

3.3 Experimental Details................................................................................................................ 52 3.3.1 Kinetic Measurements....................................................................................................... 52 3.3.2 Image Capture and Analysis ............................................................................................. 53

3.4 Results and Discussion............................................................................................................. 53 3.4.1 Kinetic Investigations ....................................................................................................... 53 3.4.2 Mechanism: sonochemical generation of OH• and O2

•- .................................................... 58 3.4.3 SCL Image Analysis ......................................................................................................... 64 3.4.4 Acoustic Attenuation in Cavitating Water. ....................................................................... 73 3.4.5 Conclusions....................................................................................................................... 75

Chapter 4 Determination of the Effect of Ultrasound Intensity and

Proximity on Wire Cleaning Kinetics ......................................................76

4.1 Introduction.............................................................................................................................. 77 4.2 Experimental Details................................................................................................................ 77

4.2.1 Samples ............................................................................................................................. 77 4.2.2 Method .............................................................................................................................. 77 4.2.3 Ultrasonic Configuration................................................................................................... 78 4.2.4 Measurement of Surface Cleaning. ................................................................................... 78

4.3 Results and Discussion............................................................................................................. 81 4.3.1 Influence of Ultrasound Power and Transducer Surface Distance. ................................... 81 4.3.2 Ultrasound Shadowing. ..................................................................................................... 84

Chapter 5 Hydrogen Evolution at the Titanium Sonotrode...................89

5.1 Introduction.............................................................................................................................. 90 5.2 Experimental Details................................................................................................................ 91

5.2.1 Methodology ..................................................................................................................... 91 5.3 Results and Discussion............................................................................................................. 92

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Appendices ...............................................................................................98

Paper: ‘Hydrogen evolution and oxygen reduction at a titanium

sonotrode’ Chem. Commun. (1998) ........................................................99

References ..............................................................................................100

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Chapter 1 Introduction

2

1.1 Ultrasound and Ultrasonic Mechanisms

1.1.1 Background

Sound comprises of a series of waves transmitted through a medium - solid,

liquid or gas – which possesses elastic properties allowing the periodic

displacement of molecules from the mean position. Sound waves are ‘longitudinal’

waves and comprise of a series of alternating compressions and rarefractions, along

the axis of propagation, as shown in Figure 1.1. In effect, the movement/vibration

of the sound source is communicated to a layer of molecules of the medium, which

in turn transmits the motion to an adjoining layer before returning to an

approximately unperturbed position. This is contrast to electromagnetic waves (e.g.

light, radio waves, x-rays)1 which are transverse waves oscillating at right angles to

the direction of propagation. The pitch of sound depends solely on its frequency i.e.

the higher the pitch the higher the frequency. For humans, the range at which sound

waves are audible is around 16 to 17,000 Hz (vibratory waves/cycles per second).

Ultrasound is a term that is used to describe sound waves that possess a frequency

in excess of that discernible by the human ear (>17kHz). The upper level of the

ultrasound is fairly indistinct but is usually taken to be in the region of 5 MHz for

gases and 500 MHz for liquids and solids.

1.1.2 History of Ultrasound

The beginnings of modern day ultrasound technology can be traced back as

far as the 1800’s with the discovery of ultrasonic generation and detection

techniques. In 1847 Joule 2 revealed the phenomenon of Magnestriction which

involves the modification of a magnetic material’s dimensions whilst exposed to a

magnetic field. The Curie Brothers followed this, in 1880 3,4 with their observation

of the Piezoelectric Effect and it’s inverse. Piezoelectricity involves the production

of electrical charge on certain crystalline surfaces when they are put under tension

or pressure and is a method of ultrasound detection. The inverse of this effect uses

alternated potential, which, when applied to the crystal, will be converted from

electrical energy to mechanical (sound) energy – akin to a loudspeaker. If the

alternating potential is of a high enough frequency then ultrasound is generated.

Galton 5 discovered ultrasound’s first practical use in 1883. His specially

designed whistle ‘transducer’ had a resonance cavity which was adjustable and

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allowed sound of a known frequency to be generated. The so-called ‘Silent Dog

Whistle’ was originally used to investigate the audible range for humans and

animals. As its name suggests it is still in use today for certain applications.

Commercial exploitation of ultrasound didn’t occur until 1917,when

Langevin developed an ultrasonic echo sounding technique for water depth

estimation. The impetus for the invention came out of a competition to detect

icebergs in the open sea and thus prevent another Titanic disaster. This early ‘Echo-

sounder’ method utilised a simple pulse of ultrasound from the ship’s keel to the sea

bottom that reflected it back to a detector. From Equation 1 for seawater:

waterin sound of velocity time21 Travelled Distance ××=

Equation 1

The depth could be gauged and any foreign object between the bottom of the sea

and the ship appeared as an echo in advance of that from the bottom. This system

was the forerunner of what is known as SONAR (SOund, Navigation And Ranging)

today.5,6

Sir John Thornycroft and Sidney Barnaby were the first to observe the

phenomenon of cavitation 7 in 1895. Problems with HMS Daring (a newly built

destroyer) led to discovery that the propeller blades were incorrectly aligned

causing the water structure to be torn apart by the mechanical action and the

induction of cavitation bubbles. The periodic rarefractions produced by intense

‘Power’ ultrasound fields in water can also produce cavitation 5. Since the 1940’s,

an increased understanding of ultrasound and its associated cavitation phenomena

has led to significant developments in the application of power ultrasound to

chemical processes – Sonochemistry. Modifications to chemical reactions by

ultrasound are caused by the occurrence of cavitation.

4

Rarefaction

Axis ofpropagation

Compression

Wavelength

Figure 1.1: An illustration of the periodic compressions and rarefractions present in a sound wave along the axis of propagation.

Frequencies

(Hz)0 101 102 103 104 105 106 107

Bat navigation signals(70 kHz)

Dolphin whistle(120 kHz)

Grasshopper(7 kHz)

Bumble bee(150 Hz)

Range of Human Hearing (16 Hz – 17 kHz) High Frequency Ultrasound (1MHz – 10 MHz)

Low Frequency Ultrasound (20 kHz – 100 kHz)

Figure 1.2: A diagram of frequency range.

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1.2 Applications of Ultrasound

Practically, ultrasound can be divided into two main areas, High Frequency

or Diagnostic ultrasound and Low Frequency or Power ultrasound.

1.2.1 High Frequency (or Diagnostic) Ultrasound

This utilises ultrasound within the 2-10 MHz range and the effects of the

medium on the wave (attenuation). Typical uses of high frequency ultrasound

include non-destructive materials testing, SONAR and medical scanning of

foetuses.

1.2.2 Power or (Low Frequency) Ultrasound

The second area is known, most commonly, as Power Ultrasound and

involves the region between 20 and 100 kHz in frequency. These are high-energy

waves and are used for a variety of applications that include the welding of plastics,

cleaning and modifications to chemical reactivity. (See Figure 1.2)

1.2.3 Ultrasonic Welding

Ultrasonic welding is one of the major uses of power ultrasound in industry

today. It has the advantages of not involving long heating and cooling cycles in

comparison to more traditional methods. Welding of plastics in this manner also

provides a weld with a high joint strength that is equivalent to between 90-98% of

the normal material strength.8 Application of such technology to is not just limited

to plastics, it has also been used weld snap on lids to paint tins and weld aluminium,

which is extremely difficult by more conventional means due to it’s hard oxide

layer. Welding metal 5 ultrasonically, by producing lateral vibratory movement,

leads to the oxide layer being broke up and adsorbed into the metal surround the

weld. As with the plastic, this forms a high strength weld by preventing the

formation of brittle inter-metallic compounds. Such flexibility also allows for the

precision welding of delicate components e.g. musical instruments instead of using

the more common hard soldering method.

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1.2.4 Biological Uses of Ultrasound

The main use of ultrasound within the field of biology is for the disruption

of cell walls to release their contents (particularly genetic material) for further

examination. Outer cell wall collapse is achieved using a 20 kHz probe to produce

cavitation. The cavitation acts in ‘machine gun’ fashion, with bubbles driven at very

high speed from the tip of the probe into the cell wall, eventually leading to

penetration and subsequent disruption. This method reduces the cells to their

components with limited denaturation of the cellular material, e.g. macromolecular

protein and nucleic acids, if appropriate measures to limit the bulk heating effects

caused by cavitational collapse are used, i.e. keeping the sample cool during

sonication.

1.2.5 Ultrasound in Medical Applications

(i) Therapeutic applications

Power ultrasound found its first medical use as an alternative to massage in

the 1930’s. The mechanical movement of the tissues by the waves of ultrasound

was found to mimic the rubbing movements of a masseur and as a consequence

improve circulation and muscle physiology. Today, the ultrasound can be applied

direct to the skin of the afflicted area, caused by e.g. sporting injuries, strains etc.,

by the use of flat, earthed, quartz crystals and is a common treatment in the

physiotherapist’s armoury.

Another, more recent, use of power ultrasound has been the removal of

kidney stones. By using the mechanical effect of the ultrasound the stone can be

annihilated and excreted via the patient’s urine, without the need for invasive

techniques. Ultrasonic baths have also been used for sterilisation of medical

instruments e.g. scalpels, forceps etc. (see section 1.2.8)

(ii) Diagnostic Uses

This utilises high frequency (diagnostic) ultrasound to provide a non-

invasive technique for human body scanning. It finds particular in foetal imaging9

and continuous/non-hazardous visualisation of surgical instruments during in-vitro

procedures. The basis for what are termed ‘Percussion Techniques’ stem from an

18th century method of tapping on the skin of a patient and listening to the resonant

note produced from either the either air or fluid areas of the body. Diagnosis is then

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based on the pitch and quality of the resonant note. Modern day ‘percussion’

involves high frequency pulses of acoustic ultrasound (3-10 MHz) of short duration.

Backscattering of the pulse from the boundaries in the tissues leads to a series of

echoes, which can be detected at the skin.

1.2.6 Engineering and Ultrasound

(i) Ultrasonic Machining (USM)

With the increasing use of hard, brittle materials e.g. carbides, stainless

steel, glass, ceramics, an alternative to the more usual methods of machining had to

be developed.10 Ultrasonic machining usually involves a 20 kHz cutting tool in an

abrasive slurry (containing alumina, silicon or boron carbide) and is shaped so as to

produce the required profile in the workpiece. An alternative is to use ultrasound to

augment traditional machining methods, which gives an increased efficiency.

(ii) Abrasive Jet Machining (AJM)

Abrasive Jet Machining is similar to conventional sandblasting with the

exception that is more controllable and uses a finer abrasive. Such advantages mean

it can be used to clean, deburr and cut a variety of hard and brittle materials – mica,

germanium, glass, ceramics 5.

(iii) Drilling and Cutting

Use of ultrasound by the aerospace industry includes the use of

ultrasonicated tungsten carbide cutting blades for chiselling complex carbon fibre

shapes prior to sealing with epoxy resin. This allows continuous production of a

quality which would be unattainable by more established methods. Ultrasound is

also used to drill carbide turbine blades in aero-engines due to their fragile nature.

(iv) Metal Tube Drawing

Application of ultrasound to the cold drawing of metal tubing has led to

significant improvements. Ultrasonication of the die with 20 kHz vibrations leads to

a reduction in draw pressure and draw times, and gives a better finish to the final

product. Similar results have been found for the ultrasonic drawing of wire.

8

1.2.7 Dentistry

As with engineering applications (see section 1.2.6) ultrasound is used in the

dentist’s surgery for a whole host of tasks like cleaning, descaling, polishing,

drilling and root canal treatments. Cleaning involves an instrument resonating at 25

kHz that ultrasonicates a spray of sodium carbonate, water and air. Projection of

this spray on to a tooth surface gives a cleaning/polishing action, which is much

gentler for tooth enamel than the previous use of pumice stone grinding. The use of

a diamond-tipped drill attachment and alumina slurry allows for the drilling of tooth

enamel. The same instrument, with the addition of a file, can be used for root canal

treatment that is self-cleaning due to effect of the ultrasonic vibrations transmitted

through the irrigation fluid.

1.2.8 Ultrasonic Cleaning

Since the 1950’s power ultrasound has been utilised for surface cleaning,

particularly in industry, in areas as diverse as the electronics, optics and medicines.

The usefulness of ultrasound for cleaning depends greatly on the nature of the

material to be cleaned. Sound absorbing materials like rubber suffer from mediocre

cleaning quality, whereas the method is highly effective for sound-refracting items

made of glass, metal and plastic. Known in some quarters as ‘Brushless

Scrubbing’,11 ultrasonic cleaning achieves its effects via cavitation bubbles which

slowly erode the insoluble surface contaminants. A majority of the bubbles formed

by cavitation are transient in nature, but a significant number can remain in the

solution on a semi-permanent basis and oscillate for multiple acoustic cycles.12

Hence, hard surface contaminant removal can be achieved by the combination of

the transient cavitations, which crack the contaminant layers, and the stable bubbles

which can lift surface contamination by forming in the cracks between the coating

and the surface. Another phenomenon, which occurs during ultrasonication of a

liquid medium is ‘microstreaming’, whereby liquid is rapidly convected away from

the ultrasound transducer surface and through the bulk medium. This convection

further enhances the process of cleaning by accelerating the dissolution of

contaminants and constantly supplying a fresh solution to the surface of the item to

be cleaned.

For over forty years the advantages of ultrasonic cleaning have been

exploited in a whole host of areas. The technique allows significant time saving and

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more importantly allows unusually formed articles with indents and holes to be

cleaned with similar efficiencies. As recently as 1980, Geckle13 stated that

“…ultrasonic cleaning proves faster than any conventional cleaning method for the

removal of soil and contaminants.” C.T. Walker and R. Walker14 provided further

evidence for this when they demonstrated that, ultrasonication of metal items in an

alkali solution led to a reduction in cleaning times by up to a factor of 1500 when

compared to that achieved in a stirred solution.

1.3 Heat Scale Formation and Composition

1.3.1 Introduction

While the oxygenation of steel in air is kinetically slow at room temperature,

under conditions of intense heat, the increase in rate is such that oxidation products

rapidly accumulate on the metal. When, after a relative short period of time, this

surface contamination has grown into a hard and thick oxide layer, it is referred to

as ‘heat scale’. While consisting predominately of an iron oxide matrix, a heat scale

may contain some of the alloying elements in the steel as silicone and chromium,

which also have a great affinity for oxygen.15 Consequently, determining the scale

composition of highly alloyed steels can become an extremely complex problem.

The following section will limit itself to a general outline of the heat scale

formation of low carbon steels, which, in essence, mechanistically follow the high

temperature oxidation of pure iron.

1.3.2 The Classical Three Layer Scale Formation

The oxygen partial pressure (PO2) and temperature of the surrounding

atmosphere both strongly influence the type of oxide formed on exposed iron

surfaces. For example, a classical three layer scale, such as the one shown in Figure

1.3 is formed when oxygen when the oxygen partial pressure of the furnace gas is

high (such as for air) and the heating temperature is above 570°C.16 Under such

conditions, a compact adherent scale is formed consisting of three distinct layers,

each representing a different oxide phase. Their sequential appearance in the scale

is dictated by the relative availability of oxygen. At the oxide/gas boundary where

oxygen is abundant, a layer of hematite (Fe2O3) develops, as it is

thermodynamically the most stable oxide of iron at high PO2. In the middle region

10

Steel

FeO

Fe3O4

Fe2O3

Figure 1.3: Classical three layer scale (not to scale) consisting of würsite (FeO), magnetite (Fe3O4) and hematite (Fe2O3).

11

of the scale where the oxygen availability is moderate, the formation of magnetite

(Fe3O4) dominates. Finally, at the metal/oxide interface, a layer of würsite since it is

the most stable iron oxide at low PO2. This innermost layer is normally represented

as the stoichiometric compound FeO but in reality, is a grossly non-stoichiometric

phase. This is due to its high iron deficiency and, in consequence, is more

accurately described as Fe1-yO, where y is a measure of the concentration of the

vacancies,17 typically 0.95.18 Diffraction studies have shown that the complex

clusters of crystals, which often exist in würsite19,20 comprise of vacant octahedral

sites (‘normal’ iron vacancies) as well as tetrahedral sites occupied by iron ions (i.e.

interstitial iron ions).19 the chemical formulae of magnetite and hematite, Fe3O4 and

Fe2O3 respectively, represent their ideal composition but both tend to be non-

stoichiometric. However, such deviations are significantly smaller (particularly for

hematite) than is the case for FeO. Würsite is known to exhibit the rock salt crystal

structure, magnetite exhibits a mixed Fe2+ and Fe3+ spinel structure, and hematite is

known to have a corundum structure.21 The source of such heat scales on the

surface of wires comes from the various stages of manufacture that are used to

produce the final product, which include wire drawing, annealing, patenting,

hardening and tempering.

1.4 Ultrasonic Effects on Aqueous Media

1.4.1 Introduction

In general, most cleaning applications involve the use of power ultrasound

(within the region of 20-100 kHz, see section 1.2.2) within an aqueous medium.

The sound energy transmitted into the solution at these lower frequencies is

significantly greater than is the case for high frequency ultrasound – giving rise to

the rapid formation of cavitation bubbles, which are responsible for the ‘scrubbing’

action of the ultrasound.22

Application of the longitudinal ultrasonic waves (see section 1.1.1) to a

solution leads to the creation of physical compressions and rarefractions along the

axis of propagation. (See Figure 1.1) The speed of this propagation has been

reported to be as much as 1505ms-1 in degassed water with an equilibrium

temperature of 31°C23. The passage of sound waves through the liquid causes the

propagation of periodic pressure and velocity variations as the water molecules

oscillate around equilibrium positions. These variations are clearly illustrated by

12

Figure 1.4: An illustration of the phase differences between sound wave pressure and velocity. (Reproduced from Ref.24).

Figure 1.5: Illustration of the standing wave pattern set up due to interference between incident and reflected sound waves. (Reproduced from Ref.25)

VELOCITY

PRESSURE

13

(Figure 1.4) and show the relative velocity and pressure changes in successive

liquid layers. Both velocity and pressure variations occur along the axis of wave

propagation, but they are out of phase by 90° with the pressure lagging behind

velocity. When these propagating (travelling) sound waves reach an interface –

liquid/air or liquid/solid – they are reflected. This reflection leads to the formation

of a ‘Standing Wave Pattern’ caused by the interference between the incident and

reflected waves (see Figure 1.5). Areas where pressure excersion is at a minimum

are termed ‘Pressure Nodes’ and can be observed at three key areas:

(I) Transducer/liquid interface

(II) The half wavelength point (1/2λ)

(III) The liquid/air boundary.

Conversely areas of maximum pressure (‘Pressure Antinodes’) occur at the

1/4λ and 3/4λ points. For travelling waves velocity nodes are out of phase by 90°

(1/4λ) with pressure nodes in a standing wave pattern.

1.4.2 Acoustic Cavitation and Streaming

“Cavitation occurs when bubbles – cavities – filled with gas or vapour

develop within the body of a liquid.”26 A good example of this phenomenon is the

effervescence of supersaturated gas containing liquids. As previously mentioned (in

section 1.1.2), cavitation was first alluded to by Sir John Thornycroft during

investigations into HMS Daring’s propeller corrosion. This was followed by

Langevin’s 27 observation of acoustic cavitation in liquids due to ultrasound. By

1936, the first observations of the presence of cavitation in degassed liquids (at

room temperature and pressure) were reported by Söllner.28 The late 1930’s saw the

interest in cavitation increase as numerous researchers including Harvey 16, Boyce 16 and Kornfeld/Suvorov 29 began to investigate various facets of the phenomenon

and increase understanding of the mechanisms of its inception.

(i) Acoustic cavitation

The formation of cavities in response to an alternating acoustic pressure is

known as ‘Acoustic Cavitation’. This can be generalised to include all observable

14

activity i.e. bubble formation, motion and lifetime caused by the acoustic field.

Henceforth ‘cavitation’ will be taken to mean acoustic cavitation.

Applying ultrasound of a high intensity to aqueous media leads to the

formation of alternating cycles of compressions and rarefractions (see section

1.4.1). Passage of these sonic-waves through the liquid establishes a sinusoidal

wave of varying acoustic pressure (PA):

fTπ2 sin P P Aa =

Equation 2

Where PA is oscillating acoustic pressure amplitude, ƒ Frequency, T time.

This Applied Acoustic Pressure is superimposed on the pre-existing, ambient

Hydrostatic Pressure (PH).

Pressure reaches a maximum during the positive half cycle when the

molecules in the solution reach a maximum compression. Therefore maximum

liquid pressure (PL) is defined as:

aHL P P P +=

Equation 3

Conversely the negative half cycle occurs when the molecules reach their

most spaced, or rarefied state, leading to the minimum liquid pressure:

aHL P- P P =

Equation 4

During the negative half-cycle the liquid pressure (PL) can actually become

negative i.e. when Pa > PH. Should –PL exceed the intermolecular binding forces

within the fluid the molecules are torn apart and cavitation is induced.

An ‘ideal’ cavity is one that is purely a void and collapses almost

instantaneously during the following compression cycle. This collapse releases huge

15

quantities of energy that can generate temperatures as high as 5000°K, pressures in

the region of 1000 atmospheres 30, weak light emission or sonoluminesence 31

arising from the collapse of cavities often accompanies sonication of water at 25°C.

Such violent implosions are known as ‘Transient Cavitations’32 and are generally

formed by lower frequency ultrasound with intensities in excess of 10Wcm-2.19

The existence of transient cavitation bubbles is short-lived (usually less than

one cycle) and typically involves expansion to a radius that is two times greater than

the initial radius of the nucleus, before ending in the violent collapse. This

implosion is considered to be so extreme as there is so little time from initiation to

collapse for gas or vapour to diffuse into the bubble and cushion the impact of the

imploding liquid surfaces.

Transient cavitation is not the only type of cavitation that can be observed.

‘Stable Cavitations’ last for many acoustic cycles and oscillate in a non-linear

fashion around an equilibrium size. It is thought that such micro-bubbles owe their

stable nature to the presence of gas and/or vapour within the bubble preventing the

implosion. Thus the lifetime of cavitation bubbles is dependent on whether there is

sufficient time for gas and vapour diffusion into the cavity during the rarefraction

period. If there is, this allows for the formation of stable cavitation, if there isn’t the

bubble is mainly a void (or highly rarefied solvent vapour) tending to favour a more

transient nature to the cavitation.

(ii) Acoustic Streaming

In addition to the production of cavitation, application of ultrasound to

liquid media creates a continuous displacement of particles around their equilibrium

positions in what is termed ‘Acoustic Streaming’.

This displacement is also sinusoidal in nature (like the variation in acoustic

pressure) and is given by:

Tsin2 YY A fπ=

Equation 5

Where Y is particle displacement,

YA particle displacement amplitude,

ƒ Frequency,

16

T time.

Production of a continuous movement of the particles within the medium

produces the effect of displacing loose contaminants from a contaminated surface

and constantly supplying fresh solution to the surface during ultrasonic cleaning.

(See section 1.2.8)

1.4.3 Factors Affecting Cavitation

A number of experimental parameters can influence the extent and type of

cavitation formed within a liquid medium by the application of ultrasound. This is

of prime importance as the production of cavities is key to the ability of solution to

provide ultrasonic cleaning via the scrubbing action.

(i) Effect of Temperature

Increasing temperature causes a reduction in the intensity of ultrasound

required for the formation of cavitation within an ultrasonicated medium. This is

probably due to the lowering of the surface tension and/or viscosity of the liquid,

either of which would lead to a reduction in the cohesive forces within the fluid and

the energy required to tear it apart. However, near a liquid’s boiling point, the high

temperature causes a significant increase of vapour pressure. So, whilst high

temperature favours the nucleation of cavities the presence of liquid vapour within

the bubble leads to a cushioning of the implosion during the compression cycle.

Hence stable cavitation is favoured and the impact of ultrasound is reduced, as the

shock wave released on cavity implosion becomes less intense.

The effect of temperature on cavitation in tap water is clearly illustrated in

Figure 1.6. Increasing temperature to 55°C leads to an analogous increase in the

intensity of cavitation. Further increases in temperature towards water’s boiling

point sees a reduction in cavitation intensity observed due to the increase in vapour

pressure and it’s associated cushioning effect. Also noteworthy is the hysteresis that

occurs when the water is heated to boiling and then cooled back down to the 50°C -

60°C optimum. The upper curve in Figure 1.6 shows that the intensity of cavitation

is increased when compared to the initial optimum, probably due to the degassing of

solution caused by the boiling of solution.

17

Figure 1.6: The effect of temperature on cavitation in tap water and its hysteresis effect (Reproduced from Ref. 33)

Figure 1.7: Effect of ultrasonic frequency on the cavitation threshold (Reproduced from Ref.34)

18

(ii) Dissolved Gas and Particulate Matter A majority of liquids are heterogeneous in nature and contain a certain

amount of dissolved gas or gas bubbles. These gas bubbles can have a positive

effect on the frequency of cavitation when contained in an ultrasonicated liquid as

they can act as nuclei for the growth of cavitation bubbles. (See Figure 1.7)

Unfortunately if there is a large amount of dissolved gas present it can also have a

detrimental effect on the cleaning effects of cavitation as the cushioning of shock

wave intensity provided by such relatively large quantities of gas far outweighs the

increase in cavitation frequency.

An analogy often used to explain the cushioning effect of dissolved gas is

that of the fizzy drink bottle. When the bottle is sealed it is done so under pressure,

so as to maintain the drink’s fizziness by super saturating it with gas. On opening

the bottle’s pressure is suddenly reduced, leading to an immediate release of

dissolved gas that floats to the surface and diffuses into the surrounding

atmosphere. A similar thing happens as the pressure is quickly reduced during the

rarefraction period of an ultrasound cycle. Here the reduction in pressure causes

dissolved gas to be drawn towards and into a cavitation bubble. In the succeeding

compression cycles this gas softens, and can even prevent, cavity collapse.

Prevention of collapse inevitably leads to an increase in gas attracted into the

bubble on subsequent rarefractions causing it to grow, until it floats to the surface

and discharges into the atmosphere.

Particulate matter can also lower the cavitation threshold by acting as

sources of trapped vapour gas nuclei.

(iii) Ultrasound Intensity (Irradiation Power)

Increasing the intensity intensifies the vibration amplitude. As a

consequence, collapse pressure rises causing faster and more violent transient

cavitation implosion. However there is a limit to the intensity of irradiation that can

be applied to a system due to practical and engineering considerations.

At sufficiently high acoustic intensities a ‘Decoupling Phenomenon’ occurs

leading to a loss of power being transferred into the medium from the source. This

decoupling is due to the source of ultrasound being unable to remain in contact with

the liquid medium for the complete cycle.

19

Another consequence of high power ultrasound on a liquid medium is the

increase in the number of cavitations per unit volume. A large concentration of

cavities can cause a significant amount of conglomeration leading to the formation

of larger, more stable bubbles 26. Such bubble clusters also have the effect of

dampening the sound energy as it passes through the solution causing a decrease in

ultrasound impact and it’s application to surface cleaning for example.

From the point of view of transducer design, an increase in irradiation

intensity requires a greater dimensional change in the transducer material. Such

changes can increase the strain on these components causing them to break and

hence reduce the equipment’s lifetime. Compromise is therefore necessary to give

both optimum performance and longevity.

(iv) Ultrasound Frequency

To produce gas or vapour filled voids by completely rupturing a liquid

requires a finite time. High frequency sound waves can suffer problems as the time

needed to create a bubble is longer than the time available during the rarefraction

cycle. For example at 20 kHz the rarefraction cycle lasts 25µs (=½ƒ) with

maximum negative pressure at 12.5µs, whereas at 20 MHz the rarefraction cycle is

0.025µs.35 Consequently as frequency is increased, cavitation bubble formation

becomes more and more difficult. This problem can be overcome by increasing

ultrasound intensity, which increases acoustic pressure amplitudes and in turn gives

a more powerful rarefraction cycle than can overcome the liquid’s intermolecular

forces. Figure 1.7 demonstrates this quite clearly, illustrating the variation of

threshold intensity with ultrasonic frequency. As can be seen on the plot, there is a

significant rise in the ultrasound intensity required to produce cavitation above

~100 kHz. Operating transducers at this and higher frequencies with sufficient

intensities is extremely difficult. With this in mind, the typical range used for a

power ultrasound application e.g. cleaning, is normally between 20-50 kHz

(anything lower maybe audible which can produce discomfort in the ear of the

user.)

20

1.5 Electrochemistry

1.5.1 Electrochemical Reactions

Electrochemical reactions are concerned with the reduction and oxidation

(so called 'REDOX') processes that occur heterogeneously at a surface which is

electrically conducting i.e. an electrode. Reduction processes or 'Cathodic

Reactions' are so called because they occur at the cathode and have the general

formula:

Red Ox ne n- →+ +

Equation 6

Where Red and Ox are the reduced and oxidised forms of the ‘Redox Couple’. Such

cathodic reduction reactions include metal deposition from an ionic solution 36

(s)-

)( n M ne M →++

aq

Equation 7

And the cathodic evolution of hydrogen gas:

(g) 2-

)( H 2e 2H →++aq

Equation 8

The opposites of such reactions are the oxidation processes or 'Anodic

Reactions’ that occur at the anode. The general form for these anodic oxidation

reactions is:

-n ne Ox Red +→ +

Equation 9

Commonly encountered anodic reactions include the solvation of metal ions:

21

-(aq)

n (s) ne MM +→ +

Equation 10

E.g. -(aq)

2 (s) 2e Fe Fe ++ +

Equation 11

And anodic evolution of oxygen from water:

- 22 4e O 4H O2H ++→ +

Equation 12

It is worthy of note that both the anodic and cathodic processes can occur at

the same electrode surface simultaneously. Each redox couple has its own, unique

electrode potential (Eeq) where there is no net oxidation or reduction occurring i.e.

the couple is in equilibrium (N.B. Cathodic and anodic rates are non-zero here, a

finite exchange velocity is associated with the process of equilibrium.) When such

conditions are reached the value of Eeq can be determined by the Nernst Equation37:

[ ][ ]RedOxlnE E eq

nFRTφ=

Equation 13

Where Eǿ is the standard electrode potential of a couple

R is the universal gas constant

F is the Faraday constant

N is the number of electrons exchanged

T is the temperature

[Ox] concentration of the oxidised form of the couple

[Red] concentration of the reduced form of the couple

22

Application of an external current produces an ‘overpotential’, i.e. causes

the electrode potential to be displaced from Eeq resulting in the net oxidation or

reduction of the redox couple. When this occurs electrical currents are related to

observed chemical rates by Faraday’s law, i.e.:

rate) (anodic nF iA =

Equation 14

(Anodic currents are positive by convention)

rate) (cathodic nF- i C =

Equation 15

(Cathodic currents are negative by convention)

And

CA i i i +=

Equation 16

Where iA is the anodic current component

iC is the cathodic current component

i the external circuit current

The rates (and therefore current contributions) of electrochemical reactions

depend, exponentially on the potential of the electrode and are expressed in the

following form:

For an anodic reaction38

)E-(E )nF-(1 exp i i eqoA

=

RTα

Equation 17

23

Figure 1.8: Anodic and cathodic Tafel Plots (Reproduced from Ref.39)

Pote

ntia

l

Log Current

Tafel Region(Potential Controlled)

Diffusion Limited(Diffusion Controlled)

Figure 1.9: Illustration of the Tafel plot diffusion limited current plateau

24

A cathodic reaction

) E- (E RT

nF- exp i- i eqoC

= α

Equation 18

where io is the exchange current of the redox equilibrium

(A measurement of the exchange velocity)

∝ is the 'Asymmetry Factor' of the redox reaction

When the applied overpotential | E – Eeq| is large (>75mV), then reverse

currents become insignificant i.e. either iA or iC predominates within i leading to

straight line plot of E verses Log i. These plots are known as 'Tafel Plots' and can be

observed experimentally. (Figure 1.9) The current increases exponentially with

potential until it eventually becomes limited by diffusion processes, leading to a

plateau on the Tafel plot (Figure 1.8.)

Another factor that can have a bearing on the appearance of the Tafel plot

for anodic metal dissolution is metal surface passivation. To simplify the

understanding of the metal redox system, the most likely anodic reactions for a

given metal are plotted verses pH on what is termed a 'Pourbaix Diagram'. (See

Figure 1.10.)

Such a diagram for a metal indicates the following zones:

(i) Corrosion (Active)

This is where anodic metal dissolution occurs, E > Eǿ M/Mn+ leading to the

production of water soluble species (ions) and corrosion of the metal (where Eǿ is

the standard electrode potential for the metal.)

(ii) Immunity

Here E < Eǿ M/Mn+ and anodic metal dissolution is impossible,

thermodynamically.

25

Figure 1.10: Pourbaix diagram showing the equilibrium potential-pH for the FE-H2O system, illustrating the areas of immunity, corrosion and passivation. m = the equilibrium of H2O/O2 at Po2 = 1, I = H2O/H2 equilibrium at pH 2. (Reproduced

from Ref.38)

26

Log i

E

EF

Epp

Ec

Transpassive Region

Passive Region

Active or Tafel Region

Figure 1.11: Diagram to illustrate the active-passive regions of a Tafel plot

27

(iii) Passivation

E > Eǿ M/Mn+ and although dissolution is thermodynamically possible the

reaction leads to water insoluble products (usually and an oxide or hydroxide

species.) Deposition of such a non-conductive substance prevents further

dissolution by blocking the electrode surface resulting in a kinetic inhibition.

When the metal surface becomes passive the Tafel plot displays a rapid

reduction in current. (See Figure 1.11) The current increases with electrode

potential up to the 'Passivation Potential' (Epp). Once the electrode attains the

'Breakdown Potential' (EC) the passive film decays and current again begins to

increase. Such an electrode is termed 'Transpassive' 40

Reduction of the electrode potential sees a similar current-potential

relationship with the exception that the passive-active transition is observed at the

'Flade Potential' (Ef). The difference between the Flade Potential and the

Passivation Potential (Ef < Epp) is probably due to potential drops that occur as the

pre-passive film forms at the active regions upper end and local pH changes

associated with anodic current.40

The shape of a Tafel plot for a metal and it's associated values of EC, Epp and

Ef depends on the sort of electrolyte used. Some electrolyte anions may cause the

complexation of the oxidised metal species produced by the anodic reaction, leading

to them becoming soluble and hence hindering the formation of a passive film at the

metal surface. Likewise these anions can breakdown a pre-formed passive film.

This property is termed 'Aggressiveness' i.e. the ability of an anion to complex and

solubilise anodic reaction products. Hence an aggressive anion is one that readily

forms soluble complexes and reduces a metal passivity. In contrast a non-aggressive

anion is one that either doesn't form a complex with the products of the anodic

reaction or reacts to form insoluble complexes, causing the metal surface to remain

or become passive. Halide ions are an example of a highly aggressive anion,

sulphate to is also termed aggressive though is quite mild when compared to

chloride.

1.5.2 The Electrical Double Layer Hypothesis

The interface between an electrode and electrolyte is analogous to that of a

circuit, (Figure 1.12) with a 'Charge Transfer' resistance (associated with the

28

RctCdl

Figure 1.12: The electrode/electrolyte interface is analogous to an electrical circuit in which Rct

is a ‘charge transfer’ resistance and Cdl is the double layer capacitance.

Current

Potential

Time

Figure 1.13: Depiction of the electrode potential–current lag caused by double layer capacitance charging

29

electrochemical reaction) Rct and a double layer capacitance Cdl.. Values of Rct are

potential dependent due to the exponential relationship of the electrochemical

reaction to the electrode potential (see section 1.5.1.) Cdl values are also known to

vary with potential.41 However this does not prevent a simple equivalent circuit

(shown in Figure 1.12) from being used to explain, qualitatively, pulsed or transient

electrochemical current phenomenon.

Application of pulsed or stepwise current to an electrode causes a lag

between electrode potential and the transient current due to double layer

capacitance charging 88 – illustrated by Figure 1.13.

Also impedance (Z) of the double layer varies with f :

dlfCπ21Z =

Equation 19

Whereas values of Rct are frequency independent.

Hence low frequencies of AC current produce electrochemical reactions as

they tend to pass 'Faradically' through Rct, alternatively high frequency AC

produces no electrochemical reaction as it passes through Cdl 'Non-Faradically'.

1.6 Ultrasound in Electrochemistry

1.6.1 Introduction

Sonoelectrochemistry 5, 42 involves the coupling of electrochemical systems

and power ultrasound together to produce new processes caused by the influence of

ultrasound on reaction kinetics. This phenomenon can be compared to other

synergistic approaches in which two independent sources of “activation” energy are

joined together. Such techniques usually result in new methodology for the study of

each activation source separately and the detection of new reactions caused by so-

called ‘Dual Activation’43. Ultrasound may merely initiate processes by activation

but also by influences mass transport.

Direct exposure of homogenous and heterogeneous chemical reactions to

intense ultrasound has a profound effect on reactivity 11,44. Ultrasound has also been

30

usefully applied to organic synthesises that involve electrochemistry,45,46

electroanalytical problems, degradation/mineralisation of toxic materials and

wastewater treatment 47. The application of ultrasound, particularly in electroplating

has led to significant improvements in terms of higher density/quality metal surface

film build-up. Similarly, ultrasound has been shown to produce better quality

conducting polymer films when these are generated by electrodepostition. 48

Recent studies involving ultrasound in conjunction with analytical

electrochemical techniques found that the application of ultrasound to be most

useful under certain conditions. Experiments made by Bard 49 utilised High Speed

Coulometry with ultrasound induced mass transport and Dewald and Peterson 50

found that using pulsed ultrasound led to a new form of Hydrodynamic Modulation

Voltametry. Compton 51 has been particularly active in this field having been

involved in a number of studies including determining the effect of ultrasound on

current/voltage characteristics 52 and developing a form of Anodic Stripping

Voltametry with an added ultrasonic component 53. Brett et al 54 have also shown

the usefulness of in-situ ultrasonicated anodic stripping voltametry and the cleaning

effect provided by ultrasound during voltametric nucleic acid detection.

Studying the effects of ultrasound on an electrochemical system is not

without its difficulties. Ultrasound is known to influence a large number of physical

and chemical processes 55 and its application to an electrode surface must be

carefully controlled. In an ideal situation the sound intensity should be known and

be able to consistently applied.

The effect of ultrasound in electrochemistry can be divided into two areas:

(i) Homogeneous Effects

Such effects can be induced by cavitation, which is coupled to the pressure

changes within the solution (see section 1.4.2.) The collapse of cavitation bubbles,

and the associated high transient pressures and temperatures 56, 57 may cause

homolytic cleavage of chemical bonds in compounds present within, or close to the

bubble. Such cleavage gives rise to the formation of free radicals e.g. hydroxyl

radicals 58, halogen radicals 59 and others 60 depending on the solution composition.

Confirmation of such radical formation has been obtained from radical

trapping/spectroscopic detection experiments.61

31

If a solvent medium contains dissolved macromolecules, ultrasound can

mechanically rupture bonds within these macromolecules by the inducing strong

shear forces in solution. Such effects have resulted in the application of ultrasound

to limit molecular weight, and hence control material properties during

polymerisation processes.

(ii) Heterogeneous Effects

The most dramatic effects caused by ultrasound on heterogeneous processes

are the significant change in mass transport at phase boundaries and solid surface

erosion.62 Erosion is caused by asymmetric cavitation near the solid/solution

interface 63 and is more prominent on materials such as lead and copper materials,

which have relatively low hardness.

Mass transport plays a substantial role in electrochemical as well general

heterogeneous reactions. A great number of heterogeneous/electrochemical

processes are controlled by mass transport and the variation in transported material

to and from interfaces can cause a change in the reaction pathway involved. 64

Electrochemical mass transport depends on the mass transport boundary layer

thickness, δ, (see below) and control of this thickness can be used to vary the nature

of the chemical process.

1.6.2 Mass Transport in Electrochemistry

Transfer of charge at an electrode surface is accompanied by ion transport

within the electrolyte. The transport of uncharged species is dependent on

convection and diffusion. However, charged species are influenced by the effect of

electric fields and for such species charge migration also contributions to mass

transport. Electrochemical processes often owe their limitation to the rate at which

reactant is transported to the surface of the electrode. This limited mass transport

rate can be increased in a variety of ways including an increase in solution

temperature, reactant concentration or fluid agitation.65

1.6.3 Mass Transport Boundary Layer (Nernst Diffusion Layer)

The complex nature of the mass transport boundary layer means that

simplifications are required. One of these simplifications is to assume that there is

32

Con

cent

ratio

n

Distance from electrode0

Csurface

Cbulk

δ

Extrapolation of initialconcentration

Actual concentration

Figure 1.14: Nernst diffusion layer model. The solid line represents the actual concentration profile and the dashed line from Csurface is the extrapolation of the

initial slope

33

the presence of a laminar sub-layer close to the electrode surface that possesses a

linear concentration gradient. This is equivalent to the assumption that near the

electrode mass transport occurs exclusively by diffusion across a constant diffusion

barrier. The concentration gradient of the electroactive species the boundary layer

(known as the ‘Nernst Diffusion Layer’ see figure sad) determines the flux density

and hence current at the electrode surface. The thickness of the Nernst layer (δ) can

be used as a measure of resistance to mass transport. For a given electrode δ is

hydrodynamically determined i.e. the greater the fluid agitation, the thinner the

boundary layer and easier the resultant mass transport.

Nernst diffusion layer treatment of a typical electrochemical system gives an

equation for mass transport of:

δ)c - (c nFD

i surfacebulklim =

Equation 20

Where ilim is the limiting current

n is the number of transferred electrons

F is the Faraday Constant

D is the diffusion co-efficient

C is the concentration

δ is the diffusion layer thickness

The limiting current may be increased by the use of ultrasound because this

thins the diffusion layer thickness by increasing the agitation within the fluid.

Hence there is an enhancement of mass transport due to the influence of ultrasound.

This increase in mass transport has been observed in a number of experiments

carried out by both Compton et al 66 and Walton and co-workers.67

34

Chapter 2 Experimental Set-up and Protocol

2.1 Materials

2.1.1 Chemicals

All the chemicals utilised in this work, listed below, were of

ANLAR purity (unless otherwise stated) and all solutions made up using doubly

distilled water.

Supplier Chemicals Used

Fisher Chemicals UK Sodium Hydroxide (NaOH)

Sodium sulphate (Na2SO4)

Sodium chloride (NaCl)

Hydrochloric acid (HCl)

Sulphuric acid (H2SO4)

Di-sodium Hydrogen orthophosphate

(Na2HPO3)

Aldrich Hydrogen peroxide (H2O2)

3-Aminophtalhydrazine

(Luminol)

Diaminoethanetetra-acetic acid EDTA

(CH2N[CH2COOH]2)2

BOC, UK Argon (Ar)

Albright and Wilson, UK Ltd Phosphoric acid (H3PO4)

36

2.2 Methods

2.2.1 Ultrasound Probe

The ultrasound source in all experiments was a Branson Sonifier 250

variable power 20 kHz ultrasound generator with piezo electric transducer horn.

The transducer horn was used in combination with a variety of 10mm diameter

cylindrical titanium tips including a standard flat tip and a tip ended in a 450 wedge

section, both supplied by Branson Ultrasonics Ltd, UK.

2.2.2 Photomultiplier Tube

All non-spatially-resolved light intensity measurements were performed

using an Electron Tubes Ltd, model QL30F photomultiplier working into a type A1

transconductance amplifier. Light was focussed onto the pmt photocathode using a

3cm-diameter compound glass lens with a 10cm focal length.

2.2.3 Low Light Camera

Luminescence imaging and spatially resolved light intensity measurements

were carried out using a Merlin Low Light camera with fibre optic interface (LTC

216F40E) and type (CCU 2025) control unit supplied by Custom Cameras. The

camera was fitted with a Ziess 55-mm f 2 lens. All images were captured with

integration over either 16 or 64 frames (at a sample rate 25 frames per second) to

improve the signal to noise ratio and produce clearer images. Greyscale images

were digitised in the form of a 320 x 240 resolution matrix of 8-bit pixels using a

Xciplite software.

2.2.4 Galvanostat

The galvanostat apparatus comprised of a Wenking Instrument LB75

Potentiostat operated in galvanostatic mode by passing current through a standard

1Ω resistor as shown Figure 2.6. This apparatus was used for all experiments, which

involved electrolytic scale loosening.

a b

Figure 2.1: A illustration showing the configurations of the ultrasound transducer horn tips a) the standard flat and b) the 45° angled wedge.

38

2.2.5 Function Generator

A Thandar TH501 model, 5MHz Function Generator, RS Components

Limited was used in conjunction with the galvanostat (described above) to produce

current pulse waveforms.

2.2.6 Potentiostat

The potentiostat used for all potentiostatic and potentiodynamic experiments

was a Solartron Instruments electrochemical measurement unit (Model SI 1280)

under computer control, utilising Omega Pro DC software.

2.3 Calibration of the Ultrasound Probe

2.3.1 Calorimetry

Before any of the experimental work could be carried out a calibration of the

ultrasonic probe was undertaken to determine the ultrasound output power

associated with each of the defined generator intensity settings. This was achieved

by the use of calorimetry68. A rise in temperature produced by the addition of any

given amount of energy (in this case ultrasound) to a body is determined by its heat

capacity. Thus when the body in question is a volume of water the input of energy

from the ultrasound probe can be calculated from:

tT c m (W)Power

∆∆×=

Equation 21

Where m is the mass of water (in grams)

c is the specific heat capacity (water = 4.2 JK-1)

∆T is the change in temperature (°C)

∆t is the time taken for the observed temperature

rise (in seconds)

39

2.3.2 Equipment Utilised in the Calibration of the Ultrasound Probe

Figure 2.2 illustrates schematically the experimental set-up used for the

calibration of the ultrasound probe’s power output.

2.3.3 Method

A 100ml of distilled water was measured out and placed in the polystyrene

cup within the insulated beaker. The solution was then allowed to equilibrate for 5

to 10 minutes until a constant temperature reading was achieved.

After equilibration, the water was then sonicated at generator intensity one,

with readings every 10 seconds until an appreciable temperature rise was observed

(usually a few degrees Celsius). This was then repeated for the individual generator

intensities, the results of, which can be observed in Table 2-1 and graphs Figure 2.3

2.3.4 Results

Intensity Energy Input - m x

c x ∆T (Joules)

Time - ∆S

(seconds)

Power Output

(Watts)

1 5 x (4.2 x 100) =

2100

404 5.2

2 8 x (4.2 x 100) =

3360

228.78 14.68

3 10 x (4.2 x 100) =

4200

148.7 28.24

4 8 x (4.2 x 100) =

3360

78.78 42.65

5 20 x (4.2 x 100) =

8400

148.95 56.39

6 10 x (4.2 x 100) =

4200

59.35 70.8

Table 2-1: Power Outputs for the respective ultrasound generator settings

B

A

C

ED

Figure 2.2: Diagram of the apparatus used for calibrating the ultrasound probe power output. A = 20 kHz ultrasound generator, B = ultrasound

probe, C = insulated beaker, D = polystyrene reaction vessel, E = electronic thermometer

20

25

30

35

40

45

50

0 50 100 150 200 250 300 350 400 450 500

Time (seconds)

Tem

pera

ture

Figure 2.3: Calibration curves for ultrasound generator power output levels: 1, 2, 3, 4, 5, 6.

2.4 Luminol Sonochemilumunescence Experiments

2.4.1 Experimental Set-up

The apparatus was set-up as illustrated below. The ultrasound was

introduced into the buffered solution of EDTA, luminol and peroxide (e) via the

ultrasonic transducer horn (a) at varying power levels. The temperature of the

solution was maintained at 50°C by the constant circulation of thermostated water

(f) around the cell (e). For the capture of non-spatially resolved SCL data the

apparatus was used in conjunction with (d) the photomultiplier tube, focussed on

the area directly beneath the tip of the ultrasound transducer horn with a 10cm focal

length, compound glass lens. For the capture spatially resolved data a low light

(CCD) camera was employed.

a

e

b

d

g

c

f

f

Figure 2.4: Schematic diagram of the experimental set-up (a) ultrasound transducer; (b) thermostated cell; (c) optically flat window; (d) photon multiplier tube/low light camera; (e) buffered solution of EDTA, luminol and peroxide; (f)

thermostatic fluid; (g) lightproof box.

43

2.5 Wire Cleaning Experiments

2.5.1 Wire Samples

Wire supplied by Garphyttan Wire Limited was used throughout all the

experiments. This wire consisted of steel with small amounts of carbon (0.50%),

silicon (1.40%), chromium (0.70%) and manganese (0.70%). As a result of the

manufacturing process there was an oxide layer of approximately 15 µm

(determined by microscopic analysis) covering the surface of the metal.

2.5.2 Sample Preparation

Test samples for electrolytic/ultrasonic cleaning consisted of 15cm lengths of

wire complete with a watertight tip of epoxy resin. A coating of insulating tape was

utilised to provide a predetermined surface area for electrolysis (See Figure 2.5).

Steel wire

Exposed area

Waterproofepoxy resin tip

Insulating tape

Figure 2.5: An illustration of the wire sample preparation.

44

B

A

C

To workingelectrode

Frompotentiostat

output

To potentiostat reference inputs

Figure 2.6: Set-up of the 1Ω resistor to provide galvanic output from the

potentiostat. A = ± 1V square waveform, B = 1 Ω resistor, C = ± 1A square

waveform.

45

Figure 2.7 illustrates the equipment used for the ultrasonic used for the

ultrasonic probe distance/cleaning experiments. In light of earlier research 69,70 all

cleaning experiments employed 2 minutes of square wave current with a 95%

anodic duty cycle, frequency 1 Hz and 1A of current provided by the

galvanostat/potentiostat (C) via the function generator (B) to soften the surface

scale to ensure that all observed descaling was entirely due to ultrasound intensity

alone. After the electrolytic pre-treatment the wire was then subjected to 10 second

bursts of ultrasound (the minimum time required by the generator to produce a

stable ultrasound field) for a maximum of 120 seconds in total. After application of

ultrasound the surface was examined and the percentage of the remaining scale

visually assessed. The experiments were repeated for a number of probe to wire

distances and at a number of ultrasound output powers.

C

E

B

F

D

A

G

Figure 2.7: Diagrammatic illustration of the experimental set-up for the probe distance/cleaning tests. (A = 20 kHz generator, B = function generator, C = potentiostat/galvanostat, D = ultrasound probe, E = wire sample, F = graphite counter electrode, G = inert plastic support

47

2.5.3 Electrolyte Preparation

All experiments utilised a 10% w/w solution of sodium sulphate at pH 7. A

standard volume of solution (1400ml) contained in a beaker - thermostated at 50°C

by a stainless steel coil connected to a Grants Model L14 waterbath - was allowed

to reach a thermal equilibrium before any pH adjustments were made. Adjustments

were carried out by the addition of small quantities of sulphuric acid or sodium

hydroxide. Measurement of the pH was made using a Jenway (Model 3071) pH

Meter that was calibrated prior to each use with buffers of pH 4 and pH 10 at 20°C.

The cleaning solution was then subjected to 45 minutes sonication in an ultrasound

bath to ensure that total degassing of the liquid and optimum ultrasound efficiency

for the descaling process.

2.5.4 Electrolytic Current

Pulsed anodic current is generated by connection of the galvanostat to a

function generator (see Figure 2.6) which allowed the anodic duty cycle, current

waveform and frequency to be set.

Anodic duty cycle is defined as the percentage time during which the sample

is anodically polarised during the total current time period:

100 period totalperiod anodic Cycle Duty Anodic ×=

Equation 22

2.5.5 Current Density

Current density on the wire samples was kept constant at 1Acm-2 by keeping

the current passing between the working and the counter electrode at 1A and the

exposed area on the prepared wire sample constant at 1cm2, this can be calculated

from:

area exposedcurrent density Current =

Equation 23

48

2.6 Sonotrode Experiments

Experiments were carried out to determine the current voltage

characteristics (Polarisation Curves) of hydrogen evolution and oxygen reduction at

the tip of the titanium sonoprobe, with and without ultrasonic stimulation. The

experimental set-up is illustrated in Figure 2.8 and measurements were recorded

with a Solartron Instruments electrochemical measurement unit (Model SI 1280)

under computer control, utilising Omega Pro DC software. The reference electrode

used for these experiments was a saturated calomel electrode (SCE) which has a

stable potential of 0.268 Volts when compared to the standard hydrogen electrode

(SHE). For experiments that involved deaerated conditions the electrolyte solution

was purged with pre-purified argon (supplied by BOC, UK) for a period of thirty

minutes to ensure total removal of the air present.

Cathodic polarisation curves were obtained by using a Linear Sweep

Voltametry (LSV) technique 71. LSV involves the potential of the electrode of

interest (working electrode) being varied relative to a fixed reference (reference

electrode potential) at a constant rate. The flow of current between the working

electrode and the electrode solution is recorded as a function of the working

electrodes potential.

For all experiments to determine the cathodic polarisation characteristics of

the tip of the titanium sonoprobe, the LSV potential sweep commenced at –2.5 V

and was swept at a rate of 0.014 Vs-1 to 0 V verses SCE. To ensure that all

voltammograms would be reproducible, the potential of the sonoprobe tip was

cycled from –2.5 V to 0 V at a rate of 0.014 Vs-1 without ultrasonic stimulation, and

before the collection of data, until a stable pattern was observed (typically < three

cycles.)

Electrolyte

Perspex

Reference electrode

Magnetic stirrer

Ar inlet fordegassing

Graphite counter electrode

INSULATEDSONIC HORN

Ti tip

Transducer

Electrochemical measuring unit

To sonic horncontrol unit

Computer

Figure 2.8: Schematic Diagram of sonotrode and electrochemical cell arrangement (Reproduced from Ref. 72

Chapter 3 The Visualisation of Ultrasonically Induced Cavitation with Luminol

51

3.1.1 Introduction

Alkaline solutions of luminol have been known for quite sometime to emit

light when exposed to a source of power ultrasound. The observed light intensity is

significantly more (orders of magnitude) intense than the visible sonoluminescence

caused by the direct sonication of aerated water 63,73,74,75,76 and is thought to be due

to an oxidative chemiluminescent process involving OH• generated by ultrasound.

This method of sonochemically inducing the chemiluminescence of luminol has

been used in a number of studies to investigate the cavitation mechanics 63,76. The

spatial distribution of cavitation in solution has also been determined by using this

set-up in conjunction with either a scanning fibre optic probe 75 or by

photographically capturing images 74,75. As for the kinetics and mechanisms

involved in the sonochemiluminescence of luminol not a great deal is known.

Previous studies of both luminol chemiluminescence 77,78 and electrogenerated

chemiluminescence 79,80 in aqueous solution have demonstrated that pH and

hydrogen peroxide concentration have a dynamic effect on the emitted light

intensity.

Investigation of sonoluminescence involving luminol with additional H2O2

is affected by divalent transition metal cations 81,82,83,84 that catalyse a non-

sonochemical/background chemiluminescence. Solution contamination by transition

metals is introduced through reagent impurities or via leachate from equipment,

which is nigh on impossible to avoid totally. The solution to the problem is to add a

chelating agent, like EDTA, which is able to complex and thus prevent the catalytic

actions of the divalent transition metal cations 81,84.

3.2 Materials

3.2.1 Chemicals

All chemicals used in this work (detailed below) were of ANLAR grade and

all solutions created with doubly distilled water.

3.2.2 Experimental Equipment

The experimental set-up used is illustrated in Figure 2.4. Image analysis and

iso-luminescence contour plot generation was performed using “Surfer”

cartography software obtained from Golden Software Ltd. In all cases where

digitised images were subject to quantitative analysis care was taken that the image

52

bitmap contained no areas of saturation i.e. that all the 8 bit pixel values fell

between zero and 255. Care was also taken that the camera’s depth of field was

adequate to keep the luminescent feature in sharp focus. The object distance was

typically 40cm.

3.3 Experimental Details

3.3.1 Kinetic Measurements

(i) Effect of pH and H2O2 concentration

Experiments to determine the effect of H2O2 concentration on the intensity

of luminol sonoluminescence over a range of solution pH (pH 7-13) at an

ultrasound power of 60W.

A luminol stock solution containing 10-3M of luminol, aqueous phosphate

buffer Na2HPO4 (0.1M) and 5 x 10-6M EDTA was prepared using doubly distilled

water. A 50ml aliquot of this stock solution was taken, heated to 50°C and the pH

adjusted by addition of either 0.3M aqueous NaOH or 0.1M aqueous H3PO4.

Variation in concentration of H2O2 was achieved by additions of known volumes of

0.02M aqueous H2O2 prepared by volumetric dilution of a 30% stock. All H2O2

additions came from prepared solutions that were less than 12 hours old to ensure

no decay of the peroxide concentration.

The aliquot was then placed in the reaction cell, allowed to thermally

equilibrate for 5 minutes before sonication by the ultrasound probe. Results were

plotted as relative intensities verses hydrogen peroxide concentration. (See Figure

3.1)

(ii) Effect of Ultrasound on Sonochemical Luminescent Intensity.

50ml of the luminol stock solution (10-3M luminol, 0.1M Na2HPO4, 5 x 10-6

EDTA) was adjusted to pH 12 with 0.3M aqueous sodium hydroxide solution. This

was then placed in the glass reaction cell and allowed to equilibrate to 50°C before

addition of 10-4M H2O2. The solution was then sonicated at each of the calibrated

ultrasound generator intensities (see section 2.3) and the results plotted as a graph of

power input (W) verses the relative intensity of sonoluminescence. (Figure 3.2)

53

(iii) Effect of EDTA

A solution containing 10-3M luminol, 0.1M Na2HPO4 and 10-4M H2O2 was

prepared. A 50ml aliquot of this solution was adjusted to pH 12 by using 0.3M

aqueous sodium hydroxide, placed in the glass reaction vessel and allowed to

equilibrate to 50°C. Before sonication, a silent or “background” reading was taken

from the photomultiplier tube with another reading recorded during sonication.

The experiment was then repeated with the addition of 5 x 10-7M EDTA and

the results recorded by the photomultiplier tube. EDTA was then added in a

stepwise fashion up to a maximum of 5 x 10-5M.Results were recorded prior to

(background/silent) and during sonication. These results were plotted as relative

‘background’ (unsonicated) or sonicated intensities against concentration of EDTA

and are illustrated in Figure 3.3.

3.3.2 Image Capture and Analysis

The apparatus was used in conjunction with a solution comprising of

luminol (10-3M), H2O2 (10-4M), Na2HPO4 (0.1M) and EDTA (0.02M) adjusted to

pH 12 as previously described, with sodium hydroxide. Experiments were initially

carried out on the flat, 10mm diameter probe tip. Sonication of the luminol solution

was carried out at the calibrated ultrasound generator intensities, two, four and six

(see 2.3) with an individual image being captured for each separate intensity by

CCD video camera. These images were stored on PC via the video capture unit as

greyscale images, digitised in the form of a 320 x 240 resolution matrix of 8-bit

pixels using a Xciplite software prior to further analysis using “Surfer” cartography

software. The experiments were then repeated with a wedge shaped tip (also 10 mm

in diameter).

3.4 Results and Discussion

3.4.1 Kinetic Investigations

The Influence of H2O2 concentration ([H2O2]) on spatially unresolved SCL

intensity was studied over a range of solution pH at constant temperature and using

constant ultrasound power. Figure 3.1 shows the relative intensity of SCL emission

(ISCL) as a function of [H2O2] obtained from a solution containing luminol and

54

EDTA at pH values between pH 7 and pH 13. It may be seen from Figure 3.1 that

any given value of [H2O2] ISCL increases monotonically with solution pH up to pH

12. Above pH 12 the trend becomes reversed and ISCL is decreased at pH 13. It may

also be seen from Figure 3.1 that at pH ≤ 10 increasing [H2O2] has no significant

effect on ISCL. At pH > 10 however, ISCL increases monotonically with [H2O2] for

[H2O2] < 10-4M. The effect is most pronounced at pH 12 where the value of ISCL

approximately doubles as [H2O2] increases from 10-6M to [H2O2] = 10-4M. At

[H2O2] > 10-4M the trend is reversed and ISCL decreases with increasing H2O2

concentration.

The dependence of ISCL on ultrasound power was investigated at pH 12 using

a solution containing luminol, EDTA and 10-4M H2O2. Figure 3.2 shows the

substantially linear relationship (linear correlation coefficient = 0.9998) relationship

observed between ISCL and ultrasonic output power under these conditions. The

observed dependencies of ISCL mechanisms shall be discussed in the subsequent

sections.

The effect of EDTA in suppressing the background (silent)

chemiluminescence of aqueous luminol / H2O2 is illustrated in Figure 3.3. Figure

3.3 shows the relative intensity of light emitted by a solution containing luminol and

H2O2 at pH 12 as a function of EDTA concentration, under both sonicated and

silent conditions. It may be seem from Figure 3.3 that the addition of EDTA has

little effect on the ISCL, which decreases by < 5% as the EDTA concentration

increases from zero to 5 x 10-5M. However it may also be seen from Figure 3.3 that

the intensity of chemiluminescence under silent conditions decreases monotonically

with increasing EDTA concentration and is reduced by approximately 95% at an

EDTA concentration of 5 x 10-5M.

Aqueous solutions of divalent transition metal cations such as Cu2+, Fe2+,

Co2+ and Mn2+ are well known to catalyse the chemiluminescence of luminol in the

presence of dissolved O2 and/or H2O2.81,82,83 In the presence of H2O2, significant

increases in emitted light intensity may be produced by even trace concentrations of

transition metal cation.83 The exact mechanism of this transition metal catalysis is

not known but the production of HO• through a Fenton type reaction has been

suggested as an important step81 i.e.

10-6 10-5 10-4 10-3

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

I SCL,

arbi

trary

uni

ts

[H2O2] , M

Figure 3.1: The effect of H2O2 concentration on spatially unresolved ISCL at different values of solution pH. pH 7, pH 8, pH 9, pH 10, pH 11, pH 12, pH 13 (10-3M luminol, 10-4M EDTA, Temperature 50ºC ultrasound power 70W).

0 20 40 60 80 1000.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

light

inte

nsity

, arb

itrar

y un

its

Ultrasound power, Watts

Figure 3.2: Relationship between ultrasound power output power and spatially unresolved ISCL (10-3M luminol, 10-4M H2O2, 10-4M EDTA, Temperature 50ºC, pH 12).

0 1 2 3 4 5 6

0.0

0.2

0.4

0.6

0.8

1.0

1.2

light

inte

nsity

, arb

itrar

y un

its

[EDTA], M x 10-5

Figure 3.3: The effect of EDTA concentration on spatially unresolved intensity of luminol chemiluminescence under: sonicated and silent conditions (10-3M luminol, 10-4M H2O2, pH 12, Temperature 50ºC).

58

HO HO MO H M -322

2 •++ ++→+

Reaction 1 It is also known that the complexation of divalent transitional metal cations by

chelating agents such as EDTA greatly diminishes their ability to catalyse luminol

chemiluminescence.81,83 On the basis of the above it is proposed that the levels of

background light emission observed in the absence of added EDTA result from the

luminol chemiluminescence catalysed by traces of transition metal cation present in

the reagents and/or water used in the experiments. It is further proposed that the

observed suppression of background chemiluminescence by EDTA result from the

complexation and deactivation of the trace metal cations. The finding that ISCL is not

significantly reduced at EDTA concentrations up to 5 x 10-5 M implies that EDTA

acts specifically to inhibit the background chemiluminescence. This suggests that

the EDTA cannot be acting as a reductive “quencher” for HO• or O2• - radicals (vidi

infra) and so tends to confirm the hypothesis that is the properties of EDTA as a

chelating agent that are important here.

3.4.2 Mechanism: sonochemical generation of OH•••• and O2•••• -

The propagation of ultrasound waves in aqueous solution leads to cyclic

pressure variations, which cause the nucleation, growth and collapse of microscopic

cavitation bubbles filled with gas and/or vapour 85,86 (see section 1.4.2.)

Furthermore, it has been shown that the extremely high local temperatures and

pressures may be generated during the collapse or implosion of such bubbles.85,86

Consequently, it is generally accepted that it is within the cavitation bubble, or the

layer of solution immediately contacting the cavitation bubble, that the

sonochemical effects of molecular activation and dissociation take place.85,86,87, 88

In air saturated water the principal sonochemical dissociation processes

involve the homolytic cleavage of H2O and dissolved O2.89,90,91

•• +→ HO H OH 2

Reaction 2

O O O2 +→

Reaction 3

59

Such cleavage products may then recombine or participate in further reactions:

22O H HO HO →+ ••

Reaction 4 •• +→+ HO HO O H O 2

Reaction 5 •• →+ 22 HO O H

Reaction 6

22222 O O H HO HO +→+ ••

Reaction 7

Both hydroxyl radicals (HO•) and hydrogen atoms (H•) have been detected in ESR

spin trapping experiments when water containing permanent gases in solution is

subjected to ultrasound.61,89,92 The production of hydroperoxyl radical (HO2•) has

been presumed from the involvement of this species in specific sonochemical

reactions.93, 94, 95 Furthermore, the sonochemical generation of H2O2 is well

documented, and reactions 4-7 are reported to be significant routes of H2O2

formation91,92,96

It should be noted that HO2• is itself a weak acid with a pKa = 4.8 97, which

causes it to immediately dissociate at pH’s that are neutral or alkali in nature to

form the superoxide radical (O2• -).

-

22 O H HO •+• +↔

Reaction 8

H2O2 is also a weak acid with a pKa = 10-11.65, and dissociates to give the

hydroperoxyl anion (HO2-). The HO• radical has a redox potential of 2.8V 98 and is

readily capable of oxidising both H2O2 and HO2-. Hence when significant

60

concentrations of H2O2 are present in the sonicated solution the following reactions

become of critical importance

+•• +→+ O H O O H OH 3

-2

k22

8

Reaction 9

O H O HO OH 2-

2k

29- +→+ ••

Reaction 10

k8 and k9 are second order rate constants and have been determined to have

the values of k8 = 3.7 x 107 M-1s-1 and k9 = 6.7 x 109 M-1s-1 respectively.99 Thus

leads the rate of O2• - production through reactions 9 and 10 is predicted to increase

with pH as a result of H2O2 dissociation.

There have been a number of mechanistic pathways proposed that contribute

to the oxidative chemiluminescence of luminol and the inter-relationship of such

pathways is far from straight forward. 77,99,100 The principal mechanism that is

proposed for sonoluminescence for the experimental conditions utilised is

illustrated by Scheme 1 and is similar to that proposed for gamma-ray radiolysis

induced luminol chemiluminescence.77 Luminol is known to be a weak dibasic acid

with first and second pKa values of 6.3 101 and approximately 13.80 It may therefore

be understood that over the experimental pH range the predominant luminol species

will be the luminol monoanion (I). Step (i) in Scheme 1 shows the oxidation of the

luminol monoanion (I) to produce the diazaquinone radical anion (II). Step (ii) is

the reaction of the diazaquinone radical anion (II) with the superoxide radical, O2• -

to form the hydroperoxide addition product (III). (III) is a weak acid, pKa = 10.4,99

and it is only the monoanion form of (III) which decomposes through step (iii) to

give the excited state of the aminophthalate monoanion (IV).77,99 The neutral form

of (III) decomposes via a dark reaction (iv) to give the starting material (I) and O2.

Step (iii) is thought to proceed via a concerted mechanism involving an unstable

endoperoxide intermediate99 and the aminophthalate product (IV) relaxes to the

ground state with emission of light at 430nm.

NH2

N

N

O

NH2

N

N

O

O O

NH2

N

N

O

OOHONH2

O

O

O

OH

*

OH

+ N2

-HpKa10.4

O2

(ii)

(iii)

(iv)-O2 + H+

NH2

N

N

(I) (III)

(IV)

O

(II)OOH

HO

(i)

Scheme 1: Reaction pathways of luminol sonogenerated chemiluminescence (SCL)

62

If we consider SCL occurring in the absence of added H2O2, conditions

approximated by the lowest H2O2 concentration data in Figure 3.1, O2• - will be

produced directly through the dissociation of HO2• produced by reaction 6. Some

H2O2 will be generated internally through reactions 4 and 7 and this may be

oxidised to O2• - through reactions 9 and 10. In addition O2

• - is produced through the

slow reaction of (II) with O2 through reaction 11 below. Thus sufficient O2• - will be

present for light to be emitted through Scheme 1. The monotonic increase in ISCL

with pH seen in Figure 3.1 at pH ≤ 12 may be explained by the progressive

dissociation of the hydroperoxide (III).99 The decrease in ISCL at pH > 12 is

consistent with the known decrease in quantum yield of aminophthalate at high

pH.99 The finding that increasing H2O2 concentration only increases ISCL at pH ≥ 10

may be explained if we assume the rate of reaction 10 is fast enough to significantly

increase the steady state concentration of O2• - whereas the rate of reaction 9 is not.77

This assumption would seem reasonable given the relative values of k9 and k10.

Regarding the ISCL maxima observed in the pH 12 and pH 13 data shown in

Figure 3.1, it may be understood that reaction 10 and step (i) of Scheme 1 are in

direct competition for sonochemically generated HO• . Furthermore, it has been

shown that this competition leads to maximum light emission occurring when the

steady state concentrations of (II) and O2• - are equal.77 Thus the observed reduction

in ISCL values at H2O2 concentrations > 10-4M may be ascribed to the depletion of

HO• through reaction 10 suppressing reaction step (i) and leading to a condition

where steady state concentration of (II) < concentration of O2• -. The second order

rate constant for step (i) of Scheme 1 has been reported as 8.7 x 109 M-1s-1,102 i.e.

similar to the value of k10 and close to the diffusion limit. This would suggest that at

pH ≥ 12 maximum light emission should occur when the concentration of luminol

and hydrogen peroxide are approximately equal, provided (II) and O2• - are

consumed at a similar rate. The finding that ISCL is maximal at luminol:H2O2

concentration ratio of ~10:1 suggests that (II) is actually consumed more rapidly

than O2• -. This would seem probable given the additional reactions in which (II) is

known to be involved (vidi infra).

63

In addition to involvement in step (ii) of Scheme 1 (kii ≅ 10-7M-1 s-1)77 (II)

has been shown to react slowly (forward rate constant kf (O2 + II) ~ 550 M-1s-1)

with molecular oxygen through the formal equilibrium

nediazaquino O O -22 ++↔+ • III

Reaction 11

to give a neutral diazaquinone product.100 (II) is also known to undergo rapid self-

recombination and quantitative dismutation (k12 = 5 x 108 M-1s-1)99,103 to give (I)

and the same neutral diazaquinone.

-

2 HO nediazaquino I O H II II ++→++

Reaction 12 The neutral diazaquinone product my either be destroyed through hydrolysis (k

(OH- + diazaquinone) ≅ 108 M-1s-1)99 or react with HO2- to produce the

hydroperoxide adduct (III) (k (HO2- + diazaquinone) ≅ 108 M-1s-1)99. The small

value of kf for reaction 11 implies that the formal equilibrium is never actually

attained. In comparison with (II) O2• - is relatively stable with respect to dismutation

to O2 and H2O2 over the range of experimental pH used here.104 Furthermore, the

steady state concentration of O2• - may be augmented through reaction 11 and the

reaction of neutral diazaquinone with HO2-.

One consequence of the competition between reaction 10 and step (i) of

Scheme 1 is that ISCL is primarily determined by the ratio of luminol and H2O2

concentrations. The absolute concentrations of luminol and H2O2 are immaterial

provided they are greater than the steady state concentration of HO• .77 Under

conditions of alkaline pH and constant H2O2: luminol concentration ratio it has been

shown that the integrated intensity of light emission is linearly dependent on γ-ray

pulse radiolytic dose, and hence HO• yield.77 Thus the linear dependence of ISCL on

ultrasound output power shown in Figure 3.2 is consistent with rate of

sonochemical HO• generation being directly proportional to the ultrasound power

entering the solution. It should be noted that the non-linear, and even inverse

relationships, have been reported between ISCL and ultrasound at high transducer

64

power levels.73,74 However, such relationships have not been observed under the

stated experimental conditions.

3.4.3 SCL Image Analysis

Figure 3.4 and Figure 3.5 show images of SCL in the region of a plane and

wedge ended 1cm-diameter cylindrical titanium sonoprobe tips, respectively. These

images were each obtained by the integration of 64 video frames, captured at 25-

frame sec-1. For both figures (3.4 and 3.5) a primary lobe or “plume” of

luminescence can be seen extending into solution such that the luminescent plume

is co-axial ultrasound transducer horn. In addition, the wedge shaped sonoprobe tip

displays secondary lobes of luminescence that are centred on the base angles of the

wedge section joins the main body of the tip. The localisation of SCL activity in

Figure 3.4 is similar to that reported previously in the case of a plane ended 20kHz-

transducer horn.76 Furthermore, the SCL images obtained using the planar

sonoprobe tip were uninfluenced by the volume of the sonicated solution, or the

dimensions of the sonication cell, provided that these were such that resonance and

a standing wave pattern did not arise.

It is difficult to extract quantitative information from the images shown in

Figure 3.4 and 3.5 ‘by eye’. However the digital nature of the images permit a full

analysis. Figure 3.6 and Figure 3.7 show the iso-luminance contour plots of

spatially resolved ISCL data calculated from the pixel values associated with Figure

3.4 and 3.5 respectively. Iso-luminance contour lines were constructed using the

‘inverse distance squared’ method of weighted interpolation and are spaced at

intervals corresponding to 10% of maximum light intensity. It may be seen from

Figure 3.6 that in the case of the plane tip, maximum ISCL values are located at, or

very near, the transducer-solution interface and decay rapidly with distance from

that interface. However, Figure 3.6 shows that, in the case of the wedge tip, an area

of maximum ISCL values occur approximately 0.25mm from the tip vertex and that

this area is elongated coaxially with the principal axis of the transducer horn.

Images similar to, of SCL emission at the plane ended sonoprobe tip were

obtained over a range of transducer power levels. These images were analysed with

the intention of determining the extent to which spatial distribution of SCL activity

was influenced by ultrasound intensity. In all cases, ISCL was found to decay

exponentially with perpendicular distance (d) from the transducer surface.

65

Figure 3.4: Image of luminol SCL activity, proximal to the standard, flat; 10mm diameter titanium sonoprobe tip. (10-3M luminol, 10–4M H2O2, 0.02M EDTA,

Temperature 50ºC power output - 30W).

66

Figure 3.5: Image of luminol SCL activity, proximal to the wedge shaped, 10mm diameter, titanium sonoprobe tip. (10-3M luminol, 10–4M H2O2, 0.02M EDTA,

Temperature 50ºC power output - 30W).

67

A-A’

B-B’

Figure 3.6: Iso-luminance contour plot of SCL activity proximal to the plane ended, 10mm diameter, titanium sonoprobe tip. (Contours spaced at 10 percent light

intensity intervals).

68

Figure 3.7: Iso-luminance contour plot of sono-chemiluminescent activity for the wedge-ended 10mm diameter, titanium sonoprobe tip. (Contours spaced at 10

percent light intensity intervals).

69

However the half-length of this exponential decay was found to decrease

significantly with increasing transducer output power. Figure 3.8 shows a semi-

logarithmic plot of normalised ISCL values a line coaxial with the principal axis of

the ultrasound horn, as indicated by the line A-A’ in Figure 3.6. The data in Figure

3.8 correspond to at least three half-lengths of ISCL-d decay. The solid lines shown

in Figure 3.8 were constructed by least squares linear regression and exhibit

gradients of 4.3 ± 0.1 cm-1 and 10.3 ± 0.4cm-1 for transducer output powers of 5W

and 70 W respectively. Both data sets exhibit a linear correlation coefficient > 0.98.

By contrast the radial distribution of ISCL in the luminescent plume

generated by the plane ended sonoprobe tip was substantially independent of

ultrasound output power. Figure 3.8 shows the distribution of normalised ISCL

(image pixel values) along a line normal to principal axis ox the ultrasound

transducer horn, as indicated by the line B-B’ in Figure 3.6 for ultrasound powers

5W and 70W. It may be seen from Figure 3.9 that neither the shape nor the absolute

widths of the radial ISCL distribution are changed significantly by the change in

ultrasound output power. Furthermore, the diameter of the sonoluminescent plume

did not change rapidly with perpendicular distance, d, from the transducer surface,

i.e. the plume was not strongly divergent. In no case did the width-at-half-maximum

value of the radially resolved ISCL distribution increase by more than 10% in going

from d = 2.5mm to d = 5mm.

In the previous section it has been proposed that ISCL is proportional to the

rate of sonochemical HO• generation. However, the extent to which Figure 3.4 to

Figure 3.7 can be interpreted, quantitatively, as maps of cavitational intensity is

unclear. One unknown quantity is the extent to which light scattering from

cavitation bubbles 105 contributes to the SCL images. It has been previously stated,

in the case of luminol SCL stimulated by a titanium tipped 20kHz-ultrasound horn,

that “Luminescence is located on the surface of the of the titanium horn”.76 For this

statement to be literally true the appearance of a the luminescent plumes observed

in Figure 3.4 and Figure 3.6 could be explained by the surface of the titanium tip

acting like a plane mirror and reflecting a beam of light out into solution. Scattering

of light would then cause the beam to be visible. That this is not the case is made

evident by Figure 3.5 and Figure 3.7, where the direction of the main plume of

luminescence remains co-axial with the long axis of the transducer horn (possibly

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7-3

-2

-1

0

Ln (n

orm

alis

ed I SC

L)

distance from surface, cm

Figure 3.8: Normalised ISCL as a function of perpendicular distance d from the plane ended sonoprobe tip i.e. along the line A-A’ shown in Figure 3.6 at various ultrasound power values. (10-3M luminol, 10–4M H2O2, Temperature 50ºC, and ultrasound power: 5W, 70W.)

-0.5 0.0 0.5-0.2

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

norm

alis

ed I SC

L

distance from centre axis, cm

Figure 3.9: Normalised radial ISCL distribution proximal to the plane ended sonoprobe tip i.e. along the line B-B’ shown in Figure 3.6 at various ultrasound power values. (10-3M luminol, 10–4M H2O2, Temperature 50ºC, and ultrasound power: 5W, 70W.)

73

due to the constructive interference of waves propagating from the tip surfaces)

even though the plane surfaces of the wedge ended sonoprobe tip lie at

approximately 45º to that axis. Thus, we can conclude that, whilst there is probably

a contribution from scattering, the light intensity distribution shown in Figure 3.4 to

Figure 3.7 derive principally from the spatial distribution of SCL activity.

The next difficulty in interpreting images shown in Figure 3.4 (and its ilk) is

the fact that they representations, in two dimensions, of a three dimensional

phenomenon. The light contributing to a single point on the image will therefore

derive from a volume of luminescent solution. If the object distance is sufficiently

great that the light rays entering the camera lens are effectively parallel, and the

absorption and scattering of light in solution is negligible, the observed light

intensity will be linearly dependant on the optical path length (L) through the

luminescent volume along the axis of observation. Given that the spatially resolved

ISCL data in Figure 3.4 and Figure 3.5 derive from an approximately cylindrical

plume of SCL activity the radially resolved ISCL profiles shown in Figure 3.9 are

expected to derive substantially from radial variation of L. Conversely, As the

diameter of the luminescent plume changes very little over the first few millimetres

from the transducer surface, i.e. L is approximately independent of d, the axially

resolved ISCL data shown in Figure 3.8 are expected to correspond closely to the

axial distribution of sonochemical activity. Thus, the exponential decay in ISCL

shown in Figure 3.8 reflects an exponential fall off in the rate of sonochemical HO•

generation with perpendicular distance, d, from the transducer surface.

3.4.4 Acoustic Attenuation in Cavitating Water.

Given the preceding arguments, the axially resolved ISCL – d data shown in

Figure 3.8 may be used to characterise the propagation of ultrasound travelling

waves in the volume of cavitating solution proximal to the transducer surface. In

order to facilitate the necessary analysis it is assumed that the axially resolved ISCL

values are proportional to the local acoustic intensity (I, Wcm-2). The assumption

that, microscopically, ISCL ∝ I is not unreasonable given the linear macroscopic

relationship between spatially unresolved ISCL values and transducer output power

shown in Figure 3.2. Obviously, this relationship will only apply, macroscopically

or microscopically, above the cavitation threshold.

74

When a plane acoustic wave propagates through a homogenous medium the

intensity of the wave decreases with the distance from the radiation source due to

the absorption of acoustic energy and its conversion into heat. Absorption results

from: viscous effects, thermal conduction and chemical relaxation processes

occurring within the medium.106,107 The acoustic intensity, I, at some distance, d,

from a source of intensity I0 is given by:

d)(-2 exp I I 0 α=

Equation 24

Where α is the acoustic absorption coefficient.106,107 For acoustic intensities below

the cavitation threshold below the cavitation threshold the value of α depends

predictably on the mechanical and thermodynamic properties of the medium and

increases with the square of the acoustic frequency, f,106,107 For water the quantity

α/f2 has a measured value of 21 x 10-17 cm-1 s2 over a wide range of frequencies,108

implying α = 8.6 x 10-8 cm-1 at 20 kHz. However, bubbles, such as those produced

through cavitation, are known to be effective absorbers and scatterers of acoustic

energy109,110,111,112,113. Sound absorption occurs through the damping bubble

oscillations by: viscous, re-radiative and thermal conduction mechanisms,109 and the

absorption cross-section of a bubble near its resonant frequency may be 1000 times

its geometrical cross section 110. For these reasons the value of α is predicted to

increase significantly in the presence of cavitation104,113,114 and will depend on the

number concentration and size distribution of cavitation bubbles. However, under

these conditions the ultrasound wave will be subject to multiple scattering from

cavitation bubbles and any experimental value of α would be more properly

regarded as an “attenuation” coefficient containing both absorption and scattering

contributions.

Equation 24 is immediately consistent with the exponential form of the

axially resolved ISCL – d data shown in Figure 3.8, given the assumption that I ∝

ISCL as argued above. This being the case, the gradients of the lines shown in Figure

3.8 correspond to the α values of 4.3 ± 0.1 cm-1 and 10.3 ± 0.4 cm-1 at I0 values of

6.4 Wcm-2 and 89 Wcm-2 respectively. The finding that, in the presence of acoustic

cavitation, α values in water may increase by > 8 orders of magnitude at the

75

cavitation producing frequency implies that, when present, cavitation is the

predominant mechanism of acoustic energy absorption. It also helps to explain the

“shielding” effect,113 significantly reducing acoustic intensities elsewhere in a

sonicated aqueous solution. The observed increase in α with I0 suggests that either

the number concentration of cavitation bubbles increases with acoustic intensity, or

there is an increase in their individual adsorption cross-sections, or both. This

finding also tends to support the notion that an enhancement of acoustic absorption

through cavitation may contribute to the phenomenon of ‘decoupling’34 whereby the

efficiency of energy transfer from the ultrasound transducer a liquid medium

decreases progressively with increasing I0 at high I0 values.

3.4.5 Conclusions

The sonogenerated chemiluminescence (SCL) of aqueous luminol is

strongly influenced by pH and by the concentration of H2O2. In the presence of 10-4

M H2O2 the intensity of SCL is linearly proportional to ultrasound transducer output

power. EDTA (10-4M) reduces the background (silent) chemiluminescence of

luminol/ H2O2 solutions by >95% whilst minimally affecting the intensity of SCL.

These findings are consistent with SCL light emissions following the decomposition

of a hydroperoxide adduct formed through the reaction of luminol mono-anion with

sonogenerated HO• and O2• -. Spatially resolved light intensity information derived

from digitally captured SCL video images may be analysed to provide quantitative

data on the spatial distribution of sonochemical activity in solution, provided

variations in optical pathlength are taken into account. SCL intensity (ISCL) decays

exponentially with perpendicular distance (d) from a planar ultrasound transducer-

solution interface and that the decay half-length decreases with increasing

transducer output power. Acoustic attenuation coefficients (α) in cavitating solution

may be estimated non-invasively using ISCL-d data by assuming a linear

macroscopic relationship between ISCL and transducer input power. The α values

thus obtained increase with transducer power and may be >108 times greater than α

values for homogenous water

76

Chapter 4 Determination of the Effect of Ultrasound Intensity and Proximity on Wire Cleaning Kinetics

77

4.1 Introduction

Previous studies115,116 have shown that oxide heat scale may be removed from

the surface of steel wires by a combination of electrolysis and ultrasonication in

neutral (or near neutral) electrolyte. It is thought that electrolysis serves to disrupt

the bonds, which hold the scale to the metal surface. Ultrasound then acts to break

up and remove the loosened scale.115,116 No synergistic interaction has been found

between the electrolytic current and ultrasound. Best results are obtained when

these two methodologies are applied separately, with electrolysis preceding

ultrasonication.

The work to be described was aimed at investigating the influence of

ultrasound transducer power and transducer-wire distance in determining the rate of

removal of an electrolytically loosened heat scale. A further aim was to relate the

rate of scale removed to the intensity of cavitation proximal to the scaled surface. It

is generally assumed that it is cavitation and the impingement of microjets

generated by the collapsing cavitation bubbles, which are responsible for the surface

cleaning effects of ultrasound. (See section 1.2.8). However, the difficulties in

quantifying cavitational activity in a spacial resolved manner make this hypothesis

hard to test. Here the relationships demonstrated in the preceding chapter between

ultrasound transducer power, distance and cavitational driven sonochemical activity

are correlated with scale removal rates.

4.2 Experimental Details

4.2.1 Samples

All the work was carried out on a wire used in piston ring manufacture

(described in section 2.5.1). The wire, consisting of a high carbon, silicon

manganese steel, was covered with a heat scale layer of approximately 15 µm.

Preparation of the steel wire samples followed the protocol outlined in section 2.5.2

to obtain a current density of 1 Acm-2 during all electrolytic treatments.

4.2.2 Method

The experimental set-up for the kinetics of wire descaling experiments is

shown in Figure 2.7. All experiments featured a constant, 95% anodic duty cycle of

1 Hz frequency with square wave characteristics. All electrolytic baths consisted of

78

a 10% w/w aqueous solution of sodium chloride adjusted to pH 7 and thermostated

at 50 °C by a stainless steel coil connected to a waterbath. Ultrasound was applied

at a variety of intensities and wire to probe distances as outlined below.

4.2.3 Ultrasonic Configuration

Variation of Intensity and Probe Distance Tip to Wire Distance

The distance of the wire samples from the tip of the ultrasound probe was

varied between 5-25mm in 5mm steps by the use of an adjustable inert plastic

support stand. After the initial electrolytic pre-treatment the wire was then exposed

to a 10-second burst of ultrasound from the probe (this was the shortest time that

produced a stable ultrasound field). After visually inspecting the surface of the wire

to determine the progress of the cleaning, the cycle of ultrasound/inspection was

repeated until either the sample was 100% scale free or a consistent percentage of

scale remained.

The experiments were repeated for the various probe to wire distances at

calibrated ultrasound intensities 1, 2, 4 and 6. (See section 2.3.4)

4.2.4 Measurement of Surface Cleaning.

The prepared wire sample and graphite counter electrode were immersed in

the 10% sodium sulphate solution and connected to the galvanostat in preparation

for electrolysis (See Figure 2.7.) For all the wire samples used the wire was

prepared in such a way that the surface current density was equivalent to 1Acm-2

(see Figure 2.4.) The subsequent cleaning was achieved by an initial two minutes

electrolytic treatment of the wire followed by 10-second bursts of ultrasound as

described in previous section. Experiments were repeated three times in total for

each of the different conditions employed from which an average cleaning rate was

determined.

Cleaning progress was monitored by withdrawal of the wire sample from the

sodium sulphate solution after each 10-second exposure to the ultrasound. An

assessment of the remaining heat scale soiling was determined by the viewing of the

upper surface (hemi-cylinder see Figure 4.4) of the wire through a millimetre grid.

Estimated results are presented as percentage of remaining soil verses total

sonication times.

79

(a) 5.2W

0

20

40

60

80

100

0 20 40 60 80 100 120 140

Cleaning Time (seconds)

Perc

enta

ge s

cale

rem

aini

ng

(b) 14.68W

0

20

40

60

80

100

0 20 40 60 80 100 120 140

Cleaning time (seconds)

Perc

enta

ge s

cale

rem

aini

ng

80

(c) 42.65W

0

20

40

60

80

100

0 20 40 60 80 100 120 140

Cleaning time (seconds)

Perc

enta

ge s

cale

rem

aini

ng

(d) 70.8W

0

20

40

60

80

100

0 20 40 60 80 100 120 140

Cleaning Time (seconds)

Perc

enta

ge s

cale

rem

aini

ng

Figure 4.1: The variation in the percentage of scale removed with distance from the ultrasound probe tip at various power outputs of (a) 5.2W, (b) 14.68W, (c) 42.65W,

(d) 70.8W ( -5mm, -10mm, -15mm, -20mm, -25mm.)

81

NB These DO NOT reflect total experimentation time, ONLY the

accumulative of the times that the wire was exposed to ultrasound

4.3 Results and Discussion

4.3.1 Influence of Ultrasound Power and Transducer Surface Distance.

Figure 4.1 shows the fraction of the exposed metal surface which remains

covered with scale as a function of time for various wire-probe distances at probe

ultrasound powers of a) 5.2W b) 14.6W c) 45W and d) 70.8W. It maybe seen from

Figure 4.1 that for each probe output power the rate of scale removal increases

markedly with decreasing probe-wire distances. It may also be seen from Figure 4.1

that at any given probe-wire distance the rate of scale removal increases with

increasing probe output power. It should be noted that the minimum experimental

periods of sonication was 10 seconds and at higher probe output powers and lower

probe-wire distances the surface is completely de-scaled after 10 seconds of

sonication. This means descaling times indicated in Figure 4.1 under these

conditions must be regarded as minimum estimates of the true descaling time

Figure 4.2 shows the fraction of the exposed wire surface, which has been

de-scaled after 120 seconds of ultrasonication as a function of transducer output

power at various transducer-wire distances. The data in Figure 4.2 is replotted from

Figure 4.1 and represents a crude estimation of scale removal rates over the 120

seconds experimentation period. Figure 4.2 serves to illustrate in a more condensed

fashion how the rate of scale removal increases with increasing transducer output

power and increases with decreasing transducer-wire distance. Again with the

caveat that the fastest rates of scale removal are underestimated due to the

experimental procedure used. A more refined estimate of scale removal rate was

made by constructing tangents to the scale area verses time plots shown in Figure

4.1 at time zero. It may be appreciated that the gradient of such a tangent represents

an estimate of the initial rate of scale removal (Ri).

Figure 4.3 shows a semi-logarithmic plot of Ri as a function of transducer-

wire distance for various values of transducer output power. It may be seen from

Figure 4.3 that for Ri values < 10 the plots of lnRi verses transducer wire distance

are approximately linear. The slopes of the linear portions of the lnRi-distance plots

were all approximately equivalent at 3.5cm-1 ± 0.1 cm-1, which correlates quite

readily with the calculated α value of 4.3 ± 0.1 cm-1 at lower ultrasound intensities.

0 10 20 30 40 50 60 70

0.0

0.2

0.4

0.6

0.8

1.0

Frac

tiona

l Are

a C

over

age

(Ac/

A)

Power (Watts)

Figure 4.2: Graph illustrating the changes in the area of the wire surface covered with scale as a fraction of the total surface area (Ac/A) with applied ultrasound density for distances of 5mm; 10mm; 15mm; 20mm; + 25mm; from the surface of the sonoprobe.

0.0 0.5 1.0 1.5 2.0 2.5-6

-4

-2

0

2

lnR

i

Distance from probe tip(cm)

Figure 4.3: Graph to show the changes in initial cleaning rate (ln Ri) with distance from the sonoprobe tip for various ultrasound power densities: = 5.2W. = 14.68W; = 42.65W;= 70.8W.

84

Figure 4.3 shows clearly that the maximum measured value of Ri is 10 and all the

experimental plots converge to this limit as Ri increases. Once again the limiting

value of Ri almost certainly arises from the minimum experimental period of 10

seconds and the error that this introduces when de-scaling is rapid.

The finding that (non-limiting) Ri values vary with exponentially with

transducer-wire distance is directly consistent with the exponential relationship

between cavitationally driven sonochemical activity and distance from the

transducer surface shown in the previous chapter. Furthermore, the slope of the ln

Ri-distance plots are of a similar magnitude to the cavitation field acoustic

coefficients (α) values of 4.3 ± 0.1 cm-1 and 10.3 ± 0.4 cm-1 determined by the

analysis of luminol SCL image data. These findings tend to support the hypothesis

that it is cavitation, and its related properties, which are responsible for the

ultrasonic removal of electrolytically loosened oxide scale from the wire surface. It

further implies an approximately linear dependence of de-scaling rate on ultrasound

intensity.

4.3.2 Ultrasound Shadowing.

It remains to explain the observation that only the half of the wire surface

proximal to the ultrasound transducer becomes significantly de-scaled over the 120

seconds experimental period (see Figure 4.4). It may be appreciated from the

preceding section that the observed exponential decrease in scale removal rate with

transducer-surface distance might be considered as a contributing factor. That is to

say the distal side of the wire is more distant than the ultrasound transducer and is

therefore more slowly de-scaled. However, the experimental ln RI-distance scores

are insufficient to explain the marked dissimilarity in observed descaling rates

An alternative explanation is that the wire-solution interface scatters

ultrasonic energy such that ultrasound intensity is significantly attenuated in

solution lying behind the wire. That is to say that the wire casts an ultrasound

shadow thus reducing the intensity of the cavitational activity near the distal portion

of the wire surface. To test this hypothesis a luminol SCL imaging experiment was

carried out, as described in the preceding chapter. In this experiment a prepared

wire sample (See section 2.5.2) was placed 5mm from the transducer tip and

sonicated. The image was captured and analysed as before.

85

A)

UltrasoundProbe

Wire Oxide

UltrasoundProbe

Wire Hemicylinder ofcleaned wire

B)

Figure 4.4: The hemi-cylindrical cleaning of the oxide scale of wire: A) diagrammatic illustration and B) the appearance of the wire surface after combined

electrolytic-ultrasonic de-scaling.

86

Figure 4.5: Image of luminol SCL activity proximal to the plane ended, 10mm diameter, titanium sonoprobe tip during wire de-scaling. (10-3M luminol, 10–4M

H2O2, 0.02M EDTA, Temperature 50ºC power output – 30W.)

87

Figure 4.6: False colour iso-luminance contour plot of SCL activity proximal to the plane ended, 10mm diameter, titanium sonoprobe tip during wire cleaning.

88

Both Figure 4.5 and Figure 4.6 clearly show a reduction in luminol SCL in

the region of solution immediately behind the wire. The SCL intensity at points A

and B in Figure 4.6 was 26 and 12 pixel values respectively. Assuming the linear

relationship between ultrasound intensity and SCL light intensity argued in the

preceding chapter these values suggest that the ultrasound intensity immediately

behind the wire is less than half the ultrasound intensity found immediately in front

of the wire. This difference offers a possible explanation for the observed difference

in the cleaning rate between the two sides of the wire’s surface.

89

Chapter 5 Hydrogen Evolution at the Titanium Sonotrode.

90

5.1 Introduction

The cleaning of metal surfaces electrolytically involves, by definition, some

type of electrolytic cell. This usually consists of two electrodes, (I) the working

electrode, which for metal cleaning purposes is usually the item to be cleaned and

(II) the counter electrode, which is traditionally an inert substance that allows the

passage of current (e.g. graphite). Both of these electrodes are immersed in an ionic

conductor like an aqueous salt solution, which completes the ‘electronic circuitry’

and allows the passage of electric current between the electrodes.

Effects caused by Power Ultrasound (15-40 kHz) cavitation on electrode

processes is known to fall into five distinct modes:

(i) Mass transport enhancement (see section 1.6.2) caused by increased

turbulence and microstreaming 67,85,117,118 ,

(ii) Continuous electrode surface activation 119,

(iii) Radical, ion or other high energy intermediate formation 117,

(iv) Product desorption 120 and

(v) The increase in heterogeneous electron transfer within the chemical

processes 121.

Traditionally ultrasonic baths and probes have been utilised to indirectly

stimulate electrodes 122. More recent research carried out by Compton, Ecklund et al 123 as well as Reisse, Francois and co-workers 124 has, however, indicated that a far

greater rate enhancement can be achieved by using the tip of an ultrasound

transducer horn as an electrode. Direct stimulation of these so-called ‘Sonotrodes’

causes the observed increase in observed reaction rate.

For this study experiments were carried out to investigate the beneficial

effects that the direct application of ultrasound would have, if any on hydrogen

evolution (Reaction One) and oxygen reduction (Reaction Two) in aqueous

electrolyte. This was achieved by substituting the working electrode for a

‘sonotrode’ – in this case the tip of a titanium ultrasound horn.

91

(g) 2-

(aq) H 2e 2H →++ (1)

(aq) --

2(aq)2 4HO 4e O2H O →++ (2)

Both of these reactions play an important part in both cathodic acid pickling,

AC based electrolytic de-scaling and metallic corrosion in aqueous systems. (The

presence of cavitation phenomena causing increased corrosion rates being of

notable importance 125.) Each of the reactions outlined above involve complex,

multistep mechanisms that incorporate the presence of chemisorbed intermediates 126,127 and whose reaction could be affected by any over the previously stated modes

of action (i) – (v). The present chapter aims to determine to what extent intense

ultrasound cavitation influences such processes.

5.2 Experimental Details

5.2.1 Methodology

(i) Aerated

The experimental set-up employed is illustrated in Figure 2.8. The

electrolytic cell bath consisted of a 10% w/w solution of sodium sulphate (Na2SO4)

adjusted to pH 7 with 0.1M NaOH. The power level used throughout all

experiments was 26Wcm-2, which was determined by calorimetry 68 (see section

2.3.1). Temperature control was provided by a Grants Y14 waterbath, which

circulated thermostated water through a stainless steel cooling coil 123 and

maintained electrolyte temperature constant (to within +/- 1ºC). A Solatron 1280

potentiostat controlled by computer was used to carry out all voltametric

measurements at a temperature of 30ºC. The collection of data for the linear sweep

voltammograms involved a sweeping rate of 0.014Vs-1 of the sonotrode potential

between –2.5V to 0V verses SCE (Standard Calomel Electrode). To ensure that the

resultant voltammogram was reproducible the potential of the sonotrode was cycled

from –2.5 to 0V at a rate of 0.014V-1 without sonication, prior to data collection,

until a stable pattern was observed (typically < three cycles).

92

(ii) Deaerated

For experiments that involved deaerated conditions, the electrolyte was

purged for 30 minutes with pre-purified argon (supplied by BOC) prior to

measurements being carried out.

5.3 Results and Discussion

The results of the investigations are displayed as Tafel plots. Figure 5.1

shows the voltametric responses under deaerated conditions and Figure 5.2 the

effect of aerated electrolyte. Evolution of hydrogen was clearly observable at

potentials <-1.6V. Both Figure 5.1 and Figure 5.2 show that the distinctive

appearance of the Tafel plot is more or less unchanged for sonicated and silent

conditions, the only difference being that the values of current density increase in

the presence of ultrasound, for all potentials.

The reproducibility, reversibility and magnitude of the observed sono-

electrochemical effect are emphasised by Figure 5.3 and Figure 5.4. For deaerated

and aerated conditions respectively these figures (5.3 and 5.4) demonstrate how the

sonotrode’s current responses are time dependent/transient when short pulses of

sonication are imposed on it at constant potential. The result of such bursts of

ultrasound in deaerated electrolyte at a potential of –1.3V can be found in Figure

5.3 whereas Figure 5.4 displays the response of the sonotrode in aerated electrolyte

at a constant potential of –1.25V. For both conditions, the changes in current

density caused by application of ultrasound are seen to be instantaneous and

reversible. Such results mean that bulk heating effects that can be caused by

ultrasound are not responsible for producing the rapid increases in current density.

Titanium sonotrodes rapidly form a TiO2/TiO3 layer (which acts like an n-Type

semiconductor) when exposed to air. This layer has been found to limit

electrochemical reactions –especially anodic- at titanium sonotrodes 123 but the

results in Figure 5.1 to Figure 5.4 show little evidence of this. The figures

demonstrate that the titanium sonotrode is more than acceptable cathode for

reaction (1) and (2) at the stated potentials and that sonication leads to a substantial

enhancement of reaction rate.

-6 -5 -4 -3 -2 -1 0-3

-2

-1

0

Pote

ntia

l (V)

Figure 5.1 Tafel plot of titanium sonotrode voltametric response under silent (---) and sonicated (___) conditions in deaerated neutral 10% wt Na2SO4 solution

Log I (A cm-2)

-6 -5 -4 -3 -2 -1 0-3

-2

-1

0

Pote

ntia

l (V

)

Figure 5.2: Tafel plot of titanium sonotrode voltametric response under silent (---) and sonicated (___) conditions in aerated neutral 10% wt Na2SO4 solution

Log I (A cm-2)

150 200 250

-2.5

-2.0

-1.5

-1.0

-0.5

0.0

Time (s)

Figure 5.3: Transient (time dependent) current response of the titanium sonotrode in neutral 0.7M aqueous sodium sulphate at 30ºC (Deaerated, 5 second ultrasound pulses.

Cur

rent

/ m

A cm

-2)

50 150

-4.0

-3.0

-2.0

-1.0

0.0

Time (s)

Cur

rent

/ m

A cm

-2

100

Figure 5.4: Transient (time dependent) current response of the titanium sonotrode in neutral 0.7M aqueous sodium sulphate at 30ºC (Aerated, 1 second ultrasound pulses

97

For the reaction under deaerated conditions (Figure 5.1) it can be assumed

that reaction (1) is entirely responsible for the observed cathodic currents. The Tafel

plot under silent conditions is more or less linear between –1.25 and –1.6V with a

Tafel slope of 190mV decade-1. This corresponds to reaction (1) being activation

controlled. When the Tafel plot enters the –1.7V region it curves away suggesting

that reaction becomes current limited. On the application of ultrasound the Tafel

plot becomes anodically shifted by a value in the region of 500mV and there is also

an observed change in the linear region’s Tafel slope, which increases by about

80mV decade-1 to 270mV decade-1. A change in Tafel slope indicates that reaction

(1) has undergone a change in mechanism or rate determining step caused by the

ultrasound. The enhancement of current density at constant potential by sonication

has the greatest magnitude within the linear, activation controlled region of the

Tafel plot and show an increase approximate to a factor of five. A less dramatic

effect is seen in the current limited portion of the plot where there is a factor of two

increase observed. As the greatest effect of ultrasound is seen in the activation-

controlled region of reaction (1) it would be sensible to conclude that ultrasound

interacts through modes (ii) - (v) with the electrochemical process.

In the case of aerated conditions, the silent Tafel plot (Figure 5.2) possesses

an unmistakable current plateau that spans the –0.9 to –1.4V region. This is caused

by the O2 in reaction (2) on the sonotrode surface becoming mass transport limited 128. Therefore it can be assumed that at potentials cathodic of –1.4V, reaction (2)

predominates whereas at potentials anodic of –1.4V a combination of reaction (1)

and (2) leads to the cathodic currents seen. The increases in current density at

constant potential caused by sonication are most profound (greater that one order of

magnitude) in the region anodic of –1.4V. The presence of the diffusion limited

current plateau also becomes obscured in the sonicated Tafel plot. Between –0.9

and -1.4V the current density must be due to mode (i) and suggests that the Nernst

diffusion layer (see section 1.6.3) has been reduced tenfold by the sonication. At

potentials anodic of –0.9V where reaction (2) is not limited by mass transport,

modes (ii) – (v) may play a role but mode (i) is still likely to make significant

additions to the current increase caused by the sonication.

98

Appendices

99

Paper: ‘Hydrogen evolution and oxygen reduction at a titanium sonotrode’ Chem. Commun. (1998)

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101

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