Chapter II EXPERIMENTAL TECHNIQUES 2.1 …shodhganga.inflibnet.ac.in/bitstream/10603/22396/9/09...Each batch was then melted in porcelain crucibles in an electric furnace at about

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Chapter II

EXPERIMENTAL TECHNIQUES

2.1 INTRODUCTION

Different experimental techniques are employed for obtaining the

structural and other information about the glasses [1-3]. In the present

investigation the alkalihalo borate glasses containing the second glass

former P2O5 were prepared and characterized by different experimental

techniques. A brief description of the various experimental methods

employed is given in this chapter.

2.2 SAMPLE PREPARATION BY MELT QUENCHING METHOD

The conventional melt quenching method has been employed for the

preparation of the glass samples in the present investigation. A brief

description of the melt quenching method is presented below. The melt

quenching is a process of cooling the melt at a sufficient rate to bypass

crystallization so that the disorder of the liquid is retained in the glassy

state. In the melt quenching technique, the materials initially in crystalline

or polycrystalline state are grounded to achieve fine mixing. The mixture is

then taken in a suitable crucible and is melted in an electrically heated

furnace. The melt is then quenched on to a steel plate maintained at room

temperature or higher temperatures depending on the material to be

prepared into glass. A steel block is used to dissipate the heat from the melt

at a rate suitable to bypass crystallization of the material.

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In the present work, different glass samples were prepared by using

reagent grade Boric acid (H3BO3), Phosphorous pentoxide (P2O5), Potassium

carbonate (K2CO3), Sodium carbonate (Na2CO3),Sodium chloride (NaCl),

Sodium bromide (NaBr), Potassium chloride (KCl),Potassium bromide (KBr),

Potassium iodide (KI),Copper oxide (CuO) and Vanadium pentoxide (V2O5).

Required starting materials (in mole %) for various compositions were

weighed to get 10 grams. Each batch was then melted in porcelain crucibles

in an electric furnace at about 1150 K for about 30 minutes. The melts were

then quenched by pouring them on to a stainless steel plate maintained at

373 K. The glasses were annealed for 24 hours at the same temperature to

relieve the mechanical stress.

The details of the composition of the glasses studied are given in

Chapters III and IV.

Different experimental techniques employed in the present

investigation are described below.

2.3 X-RAY DIFFRACTION

X-ray diffraction technique was used to confirm the glassy nature of

the samples. In the present study, fine powder of the glass samples is used

for X-ray diffraction investigations. Philips X pert PRO XRD (Pan Analytic)

model powder X-ray diffractometer with copper tube was used to record

the X-ray diffractrograms at room temperature.

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Typical X-ray diffractograms of different glassy systems thus prepared

are shown in Fig’s. 2.1 to 2.4. X-ray diffraction analysis of each glass

sample revealed the absence of crystallinity. The recorded diffractograms

were featureless and peak free revealing the amorphous nature of all the

glass samples prepared.

2.4 DIFFERENTIAL SCANNIG CALORIMETRY (DSC)

Differential Scanning Calorimetry (DSC) is one of the significant

experimental techniques to study the thermal properties of solids. The glass

transition temperatures can be determined from the DSC studies.

In the present study, DSC is used to determine the glass transition

temperature of the glass samples. The glass samples in the form of flakes

are crushed are hermitically sealed in aluminum pans using TA instruments

sample encapsulating press. The empty aluminum pans pierced by the

encapsulating press were used as reference material. The sample weighing

approximately about 15 mg was kept in the DSC cell and nitrogen at a flow

rate of 90cm3/mm is used as purge gas. The sample was heated at a

constant heating rate of 100 C/min. After the run was completed, the glass

transition temperatures (Tg) were calculated from the thermal analysis

software available in the PC attached to TA instrument.

In the DSC technique the sample is heated in a programmed way and

the heat (H) flow of the sample and the reference (chemically inert) material

are compared. In a DSC trace, the Y-axis represents dH/dt that is actually

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FIg 2.1 XRD of 1KCl-29K2O -35B2O3-34P2O5-1CuO glass

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FIg 2.2 XRD of 1NaBr-29Na2O -35B2O3-33P2O5-2V2O5 glass

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FIg 2.3 XRD of 1KBr-29K2O -35B2O3-33P2O5-2V2O5 glass

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Fig 2.4 XRD of 5NaCl-25Na2O-35B2O3-34P2O5-1CuO

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the differential power (in mW) and the X-axis represents the temperature (T).

The DSC determines the temperature and heat flow associated with material

transitions as a function of time and temperature. The thermal events are

recorded for a particular heating rate dT/dt in the form of endothermic or

exothermic peaks superimposed on the horizontal base line.

In the present investigation, the glass transition temperatures (Tg) of

all the glass systems were determined using TA instruments DSC 2010

thermal analysis PC based instrument.

A complete set of DSC 2010 system of TA instruments includes the

2010 instrument and a controller. Both temperatures and the heat flows

associated with transitions in materials can be rapidly measured by the

system. If a sample and the inert reference are heated at a known rate in a

controlled environment, the increase in the sample and reference

temperature will be about the same depending on the specific heat

differences, unless a heat-related change takes place in the sample. If this

change takes place, the sample either evolves or absorbs heat. In DSC, the

temperature difference between the sample and the reference form such a

heat change is directly related to the differential heat flow.

The 2010 DSC cell is used to measure differential heat flow. The

sample and reference materials are placed in pans that sit on raised

platforms on a constantan disc and heat is transferred through the disc up

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into the sample and the reference. The differential heat flow is monitored by

chromel-constantan thermocouple, placed beneath the disc.

A silver-heating block, capped with a vented silver lid, encloses the

constantan disc. Purge gas is pre-heated within the block before entering

the sample chamber through the purge gas inlet. Gas exits through the vent

hole in the silver lid. A bell jar placed over the cell and sealed with an o-ring,

protects the operator from evolved gases and permits cell evacuation.

Fig 2.5 and 2.6 show the DSC thermograms.

The glass transition temperatures (Tg) are given in Tables 2.1 and 2.2.

2.5 ELECTRON PARAMAGNETIC RESONANCE (EPR) SPECTROSCOPY

The EPR spectrometer operates at fixed microwave frequency and

scans the EPR spectrum by linear variation of the magnetic field. The

majority of EPR spectrometers operate in the X-band frequencies (~ 9 GHz).

The next widely used are Q-band (~35 GHz) EPR spectrometers. The

important features of EPR spectrometers are

1. A source of microwave radiation of constant frequency with

variable amplitude.

2. A means of applying microwave power to the paramagnetic sample.

3. An arrangement to measure the power absorbed from the

microwave field and

4. Homogeneous and variable magnetic field.

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Fig 2.5 DSC plot of 1KBr-29K2O-35B2O3-35P2O5 glass

Fig 2.6 DSC plot of 5NaCl-25Na2O-35B2O3-35P2O5 glass.

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Table 2.1 Glass transition temperature (Tg 0C) of xKBr-(30-x)K2O-35B2O3-35P2O5

Sl. No. Glass Composition Tg (0C)

1 KBKBPC1 1KBr-29K2O-35B2O3-35P2O5 412





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Table 2.2 Glass transition temperature (Tg0C) of xNaCl-(30-x)Na2O-35B2O3-35P2O5

Sl. No. Glass Composition Tg (0C)

1 NCNBPC1 1NaCl-29Na2O-35B2O3-35P2O5 415

2 NCNBPC2 2NaCl-28Na2O 35B2O3-35P2O5 425




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The EPR spectra of the glasses containing transition metal ions were

recorded on a JEOL EPR spectrometer that works in X-band frequency

range with 100 kHz field modulation. The principle and construction of the

EPR spectrometer were shown in Fig. 2.7 and 2.8. The main parts of the

EPR spectrometer are (1) microwave unit (2) cavity resonator (3)

electromagnet and excitation power supply (4) signal averager and (5) CRO

and recorder.

2.5.1 EPR Spectrometer

The EPR spectrometer consists of a magnetic field control unit, a 100

kHz modulation unit, oscilloscope and recorder. The microwave unit

consists of a Gunn diode oscillator, magic T, automatic frequency control

(AFC) unit and a Gunn diode power supply with the Hall element, the linear

field sweep unit, the decoder, modulation coil etc. Its magnetic control unit

controls the magnetic field over a range of 0-600 milli Tesla. The oscilloscope

screen provides a facility for mode check and observation of EPR signal. The

DYT type recorder records the EPR signal on a chart of width 250x360mm.

The instrument includes microwave unit, pre amplifier, AFC and

Gunn diode oscillator power supply. The Gunn oscillator produce

microwaves in the frequency range 8.8 to 9.6 GHz (X-band) at power

variable from 0.1 mW to 200mW. The oscillation frequency is mechanically

varied by varying the cavity resonator frequency. The AFC can be used to

maintain the stability of the frequency so that the oscillation of the Gunn

diode oscillator matches the frequency of the sample cavity resonator.

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Fig 2.7 Principle of X-band Electron Paramagnetic Resonance Spectrometer

Fig 2.8 The Construction of JEOL FE-1X EPR spectrometer.

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Microwaves from Gunn diode oscillator are allowed to pass through

the isolator. The reference line and the signal line are divided by means of

the directional coupler. The signal is attenuated to the required power and

finally enters the cavity resonator. When the cavity resonator coupling is

adjusted for critical coupling, there are no reflected waves from the cavity

resonator. When EPR is exited, microwaves from the cavity resonator are

reflected and enter the balance mixer that is made up from the magic T and

the crystal mount. The waves are detected and amplified by the pre-

amplifier.

The cavity resonator consists of an ultraviolet irradiation aperture, a

cooling constant temperature device, a 100 kHz modulation coil, bayonet

connectors for connecting the variable temperature attachments, a nitrogen

gas inlet port and so on. The sample tube held by a sample tube holder is

inserted into the sample insertion port located on the upper wall of the

cavity and secured by means of a clamp screw.

The electromagnet produces a magnetic field having a maximum field

sweep of ± 2500 G. The Hall element supplies ac voltage to the magnetic

field control unit. Excitation power supply supplies a highly stabilized

excitation current to the electromagnet. The homodyne crystal detector is

used for the detection of the signal. The sample is placed in the cavity

resonator and adjusted to be at the middle of the poles of the electromagnet.

Now the sample subjected to microwave magnetic field of constant

frequency, which is perpendicular H. The magnitude of H is changed by

varying the electromagnet excitation current and when the resonance

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condition is fulfilled, a part of microwave energy is absorbed into the sample

as a result, the cavity resonator Q value changes. This Q variation is

detected, amplified and recorded.

While the magnetic field is varied and when the frequency is kept

constant, an absorption signal is observed. Later on a differential curve is

derived from the absorption curve. Since the crystal noise output is inversely

proportional to the modulation frequency, the amplitude of high frequency

magnetic field modulation enables high sensitivity EPR measurements to be

carried out.

2.6 OPTICAL ABSORPTION SPECTROSCOPY

UV-VIS spectroscopy is an important experimental technique involving

the measurement of the absorption of the UV and visible light by a sample.

The UV and visible spectrum originates from the electronic

excitations. The absorption of UV and visible radiation by a molecule leads

to transitions among the electronic energy levels of the molecules.

In the present study, optical absorption spectra of the glass samples

doped with transition metal ions (Cu2+) were recorded at room temperature

using Shimadzu-UV 3100PC, UV-VIS-NIR spectrophotometer. The block

diagram of the spectrophotometer was shown in Fig. 2.9.

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Fig 2.9 Functional block diagram of UV-VIS spectrophotometer.

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The spectrophotometer is a computer controlled double beam, direct ratio

measuring system. It compares the data obtained from the sample with the

data from the reference. The spectrophotometer operates in the wavelength

region of 190 nm to 3200nm with a slit width variation of 0.1 to 30 nm and

a resolution of 0.1nm. The spectrophotometer (Fig. 2.9) consists of a light

source, a mono-chromater, a detector, amplifier and the recording devices

[4]. The source incorporates a tungsten halogen lamp for wavelengths

greater than 375 nm and a deuterium discharge lamp for values below that

and a solenoid operated mirror, which automatically deflects light from

either one as the machine scans through the wavelengths. The detector is a

photo multiplier tube (R-928 detectors for UV and visible, 190-800nm) and

the ratio of reference to sample beam intensities (I0 / I) is fed to a pen

recorder. The relative intensities of the two beams are given in the form of

optical density (O.D).

O.D = log10(I0/I) …… (1)

The optical absorption spectra were recorded for the glass samples in

the wavelength region 200 to 1200 nm. The peak positions of the spectrum

were obtained by using peak-pick facility provided in the spectrophotometer.

The uncertainty in the measurement was about ±1 nm.

2.7 DC IONIC CONDUCTIVITY

The dc electrical conductivity is defined as the steady state current

which flows in the sample, subjected to unit electric (d.c) field. Conductivity

is obtained from the conductance value using the relation

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σ = ( ) 1cmohmAt.G −− ……(2)

where ‘t’ is the thickness, ‘A’ the area of cross section of the sample and ‘G’

is the conductance which is the ratio of steady state current to applied

voltage.

The dc electrical conductivity measurements on the glass samples

were performed using a laboratory built apparatus.

The ionic conductivity apparatus is shown in Fig. 2.10. It consists of

two electrodes A and B. The lower electrode (A) and the upper electrode (B)

are circular in shape and of diameter 2 cm2 surface area. The lower circular

electrode is supported on a single centrally placed ‘vitreosil’ cylinder of 2 cm

length. This vitreosil is a good insulator even at high temperatures. The

lower end of the vitreosil cylinder is fixed to a steel frame and the other end

is inserted in a socket, which is the end point of a steel rod ‘R’ of about 8cm

in length. This rod can be moved up and down through a groove made in the

steel frame. At the end of the steel rod ‘R’, a spring is provided so that a

pressurized contact is given to the sample ‘C’. This spring is just outside the

furnace and retains its spring action even though it acquires certain

temperature. Two flexible silver wires are brazed into silver electrode and are

brought out of the furnace. The two leads are fixed to amphinole connectors

J1 and J2. Fine bored alumina tubes shield the two wires, which prevents

them from coming in contact with the steel frame ’F’. S is a thin copper foil,

which is earthed and acts as a shield. The furnace is of resistance type and

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Fig 2.10 Ionic Conductivity Apparatus

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consists of alumina muffle of 30 cm long and 10 cm diameter. The

heating element is a super kanthal wire wound on the muffle. The number

of windings on the muffle is more at the place where the sample is placed.

A temperature controller controls the temperature of the furnace. The

outer surface of the furnace is covered with a metallic sheet, which in turn

connected to earth in order to avoid pick up currents. The stainless steel

frame ‘F’ hangs into the muffle over a steel rod support. A d. c voltage is

applied to the sample through a battery placed in a separate box that is

thermally insulated. The temperature of the sample is measured by

connecting the output of the chromel-alumel thermocouple arranged very

close to the sample to a digital voltmeter.

The sample used for the conductivity measurement was polished and

then painted with silver paste for good electrical contacts. The current was

measured using Digital Keithley Electrometer model 614. The polarizing

effects during the conductivity measurement were minimized by applying

smaller voltage for a short interval of time and short-circuiting the sample

leads after every reading. The error in the measurement is about +3%.

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2.8 REFERENCES

1. S.R.Elliott, “Physics of Amorphous materials”, 2nd ed., Longman Sci. &

Technical, New York (1990).

2. S.Chandra, “Super Ionic Solids: Principles and Applications”, North -

Halland Pub.Co., (1981).

3. Colin N.Bandwell and Elaine M.McCash “Fundamentals of molecular

spectroscopy” 4th ed., Tata Mc Graw Hill Pub., New Delhi (1999).

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Chapter II EXPERIMENTAL TECHNIQUES 2.1 …shodhganga.inflibnet.ac.in/bitstream/10603/22396/9/09...Each batch was then melted in porcelain crucibles in an electric furnace at about