CHAPTER 1

General Introduction

Abstract

A brief introduction of glasses and ceramics with their

scientific and technical applications is given in this chapter. An

outline of the sol-gel processing and tape casting synthesis,

respectively of TiO2 based glassy matrices and ceramic systems is

presented. Sol-gel process is a low cost method known for its low

temperature preparation and since the precursors are in the form of

solution a high homogeneity and purity could be promised for the

end product. Tape casting process is a low cost method used for

making large area of thin ceramic sheets of controlled thickness and

high quality. Such ceramic sheets find application in the field of

actuators and fuel cells. The basic ideas of rare earth ion

spectroscopy and dielectric spectroscopy are given. The

phenomenon of surface plasmon resonance (SPR) and its

associated features are briefly touched upon.

General Introduction 3

1.1 Glass

Glass is one of the most ancient materials known to mankind.

The word glass is derived from a late-Latin term glaesum used to

refer to a lustrous and transparent material. The main importance of

glass materials is due to its wide variety of applications such as

lenses, mirrors, optical switches and fiber optics to filters and

substrates for active materials etc. Glass is essentially a

noncrystalline solid obtained by freezing super cooled liquids.

However, there are many different methods by which we can

produce amorphous solids, which we cannot classify as glass. On

defining the glassy state, two different approaches are taken by

glass scientists. One definition is based on the concept of disorder.

By disorder we mean mainly that the spatial arrangement of atoms,

ions and molecules do not exhibit three dimensional periodicity

(translational symmetry) and the long range order of the crystalline

state is destroyed. We can define the glassy state as a state with

short range order and long range disorder. The short range order is

of great significance to the optical and spectroscopic properties as

all the electronic movements take place in the short range region

within or among the atoms or ions. The second definition is based

on the internal stability of the materials. Classic glasses are

characterized not only by the absence of crystallinity but above all by

their ability to pass progressively and reversibly to a more and more

fluid state as the temperature is increased. In the course of this

change, there is modification of properties at a point called the glass

transition. Gradual softening with increasing temperature is

4 Chapter 1

moreover, extremely important in technical applications and

constitutes one of the fundamental properties of a glass material.

Based on these conditions Zarzyki [1] adopted the definition “A glass

is a noncrystalline solid exhibiting the phenomenon of glass

transition.” He named the corresponding physical state as vitreous

state originating from the Latin word vitrum. Varshneya [2] adopted

the definition of glass as a ‘solid with liquid like structure’, ‘a

noncrystalline solid’ or simply ‘an amorphous solid’. Zachariasen [3]

postulated that, as in crystals, the atoms in glass must form

extended three dimensional networks. However, Hagg [4] pointed

out that an infinite three dimensional network may not be a

necessary condition for glass formation. He concluded: It seems as

if a melt contains atomic groups which are kept together with strong

forces and if these groups are so large and irregular that their direct

addition to the crystal lattice is difficult. Such a melt will show a

tendency to super cooling and glass formation. According to Smekal

[5] mixed chemical bonding in a material is necessary for glass

formation. Sun [6] showed that bond strengths in glass forming

oxides are particularly high. Turnbull [7] pointed out that bond type,

cooling rate, density of nuclei and various material properties like

crystal-liquid surface tension and entropy of fusion are significant

factors which affect the tendency of different liquids to form glasses.

The sol-gel process of making a glass avoids the normally high

temperature employed for fusion of glass.

General Introduction 5

1.2 Ceramics

The word ‘ceramic’ derives from the Greek word ‘Keramos’

which means burnt-stuff or pottery. The term covers inorganic non-

metallic materials which are formed by the action of heat [8].

Traditional ceramics are older and usually based on clay and silicon.

They have been used for over 25,000 years and include high volume

items such as porcelain, brick, earthenware, refractory, cement,

glass etc. Ceramics for today’s engineering applications can be

considered to be non-traditional and have been developed within the

last 100 years. The new and emerging families of ceramics are

referred to as advanced, new or fine, and utilize highly refined

materials and new forming techniques. Advanced ceramics are

characterized by their high chemical purity, careful and precisely

controlled processing, reproduction of properties and good stability

of useful properties. The advanced or fine ceramics, when used as

an engineering material, possess several properties which can be

viewed to metal-based systems [9].

Ceramics are also defined as the art and science of making

and using solid articles which are in large part composed of

inorganic non-metallic materials. This definition includes not only

materials such as pottery, porcelain, refractories, structural clay

products, abrasives, porcelain enamels, cements, and glass but also

non-metallic magnetic materials, ferroelectrics, manufactured single

crystals and glass-ceramics. Another definition is the art and science

of making and using solid articles formed by the action of heat on

earthy raw materials, an extension of the Greek word ‘keramos’ ,

6 Chapter 1

and is much broader than a common definition such as “ pottery” or

“earthenware”

Traditionally, the word ceramic is associated with clay-based

products such as bricks, tiles, pottery, table ware, sanitary ware and

glass. Naturally occurring minerals like sand, quartz, bauxite,

feldspar etc. are used in the manufacture of these materials.

Advanced ceramics differ from conventional ceramics in their high-

mechanical strength, fracture toughness, wear resistance, refractory,

dielectric, magnetic and optical properties. Advanced ceramics or

fine ceramics are high value-added inorganic materials produced

from high purity synthetic powders to control microstructure and

properties.

From the beginning of civilization, ceramics provided objects

of utility and beauty. Shelter for mankind is based on adobe, brick,

tile, cement and window glass. Cooking and storing of food has

always been done in ceramic ware ranging from clay pots to modern

glass-ceramics. The abundant and widespread availability of basic

ceramic raw materials clay, sand and other minerals in nature

enabled ceramics to meet basic human needs over the millennia.

Growth of ceramics has been an important qualitative change in

recent decades, based on deeper scientific understanding of the

composition, structure, processing and property correlations of

ceramics. Advanced ceramics have applications in electronics,

space, and energy to name a few.

Tape casting, chemical vapour deposition, in situ oxidation,

injection moulding, followed by fast, often low temperature, sintering

General Introduction 7

are the novel fabrication techniques used for advanced ceramics as

compared to conventional ceramics. Alumina is by far the most

important base material for advanced ceramics and accounts for

over 80 % of the raw materials employed in engineering ceramics.

The other materials of important are BaTiO3, TiO2, lead zirconate

titanate (PZT), lead lanthanum zirconate titanate (PLZT), ZrO2, SiC,

Si3N4 and SiAlON.

All the conventional ceramic processes such as pressing, slip

casting, extrusion, drying, firing etc., are available for the synthesis

of advanced ceramics. Some of the new fabrication technologies

developed to meet these specific requirements are (i) isostatic and

hot isostatic pressing to achieve higher end point density and

freedom from defects such as laminations, density gradients etc., (ii)

tape casting to produce thin sheets of large area from a slurry of

alumina, titanates etc. spread by a blade on a moving plastic sheet.

The surface of such substrates has a high degree of evenness.

These sheets are often stacked in the green condition, with metal

electrode paste applied over the entire area or as thin lines, and

then the stack is fired to produce multilayer capacitors. Alumina

sheets with a network of resistor and conductor paste, in the form of

thin lines, are stacked and fired to produce multilayer ceramic chips.

(iii) injection moulding for small intricate shape with projections and

perforations. (iv) chemical vapour deposition of preforms from which

optical fibers are drawn and (v) in situ oxidation. A component is

cast from molten aluminium alloy and oxidation is allowed to take

place at the moving liquid/solid interface to result in an alumina

8 Chapter 1

component of the desired shape. The multitude of steps normally

employed in the fabrication of alumina components are replaced by

a single casting-oxidation step, which is completed at a much lower

temperature and shorter time than those employed for sintering of

alumina ceramics.

Since the advanced ceramics are often used in engineering

applications, often as components in large assembly, their

dimensional tolerance, integrity and reproducibility are considerably

more critical than conventional ceramics. The properties of

advanced ceramics are both quantitatively and qualitatively different

from those of conventional ceramics. Following the successive

replacement of vacuum tubes by transistors, integrated circuits and

large scale integrated circuits, miniaturization of other circuit

elements such as ceramic capacitors become necessary. These are

achieved by gradual increase in the dielectric constant of capacitor

materials. TiO2 and BaTiO3 are new class of dielectric materials and

the improvements in the last three decades are due to the

modification of these basic materials. These innovations include fine

grain size to inhibit domain reorientations and stacking of thin layers

in multilayer capacitors [9-12].

1.3 Synthesis Methods for Glass and Ceramics

1.3.1. Glass Synthesis (Sol-Gel Method)

The term sol-gel [13, 14] was first coined in the late 1800s.

Sol-gel processing is a low temperature method used for the

fabrication of inorganic or composite organic-inorganic materials

General Introduction 9

of high homogeneity and purity. This route can be used to

produce very sophisticated nanomaterials and to tailor the

materials to very specific applications. The sol-gel process starts

either from a chemical solution or colloidal particles to produce

an integrated network. This is then subjected to suitable

environments to produce different forms of materials such as thin

films, glasses, fibers, ceramics etc. A schematic diagram of the

process is shown in Figure 1.1.

The word 'sol' implies a dispersion of colloidal particles in

a liquid. When the viscosity of the sol increases sufficiently,

usually throughout the practical loss of its liquid phase and/or

polymerization of the solid particles, it becomes a porous solid

body; it is now termed 'Gel'. Gelation can occur after a sol is

cast into a mould, and if the smallest dimension is greater than a

few millimeters, the object is called a monolith [15]. Typical

precursors are metal alkoxides and metal chlorides, which

undergo hydrolysis and polycondensation reactions to form a

colloid dispersed in a solvent. The sol evolves then towards the

formation of an inorganic continuous network containing a liquid

phase (gel).

10 Chapter 1

Formation of a metal oxide involves connecting the metal

centers with oxo (M-O-M) or hydroxo (M-OH-M) bridges, thereby

generating metal-oxo or metal-hydroxo polymers in solution. The

drying process serves to remove the liquid phase from the gel thus

forming a porous material, and then a thermal treatment may be

performed in order to favour further polycondensation and enhance

mechanical properties.

There are a number of different types of precursor materials

that can be used [16]. All should be soluble in organic solvents and

easily converted to the relevant oxide preferably by hydrolysis but

alternatively by chemical reaction or thermal or oxidative

decomposition [17]. The precursors for the preparation of a colloid

consist of a metal or metalloid element surrounded by various

ligands. Metal alkoxides are members of the family of metal organic

Figure 1.1 Schematic diagram of the sol-gel processing

General Introduction 11

compounds, which have an organic ligand attached to a metal or

metalloid atom M(OR)x, where OR is an alkoxyl group. Metal

alkoxides are popular precursors because they undergo hydrolysis

depending on the amount of water and catalyst present.

1.3.1.1 Sol-gel Processing Steps

The processing involved in making sol-gel derived monoliths

samples comprises of seven steps: (Figure 1.2)

Step 1: Mixing. A liquid alkoxide precursor, such as Si(OR), where R

is most commonly CH3, C2H5, or C3H7, is hydrolyzed by mixing with

water. Since the mixing takes place at a molecular level, the

homogenity of the sol-gel process is achieved at the very first step.

Step 2: Casting. The sol is a low-viscous liquid, it can be cast into a

mould. The mould must be carefully selected to avoid adhesion of the

gel.

Step 3: Gelation. With time the colloidal particles and condensed

silica species link together to become a three-dimensional network.

The physical characteristics of the gel network depend greatly upon

the size of particles and extent of cross-linking prior to gelation. At

gelation, the viscosity increases sharply, and a solid object results in

the shape of the mould. With appropriate control of the time-

dependent change of viscosity of the sol, fibers can be pulled or

spun as gelation occurs.

Step 4: Aging. Aging of a gel, also called syneresis [18], involves

maintaining the cast object for a period of time, hours to days.

During aging, polycondensation continues along with localized

12 Chapter 1

solution and reprecipitation of the gel network, which increases the

thickness of interparticle necks and decreases the porosity. The

strength of the gel thereby increases with aging. An aged gel must

develop sufficient strength to resist cracking during drying.

Step 5: Drying. During drying the liquid is removed from the

interconnected pore network. Large capillary stresses can develop

during drying when the pores are small (<20 nm). These stresses

will cause the gels to crack catastrophically unless the drying

process is done in a controlled manner.

Step 6: Dehydration or Chemical Stabilization. The removal of

surface silanol (Si-OH) bonds from the pore network which results in

a chemically stable ultra porous solid.

General Introduction 13

Step 7: Densification. Heating the porous gel at high temperatures

causes densification to occur. The pores are eliminated, and the

density ultimately becomes equivalent to fused quartz or fused silica.

The densification temperature depends considerably on the

dimensions of the pore network, the connectivity of the pores and

surface area etc. [18, 19].

1.3.1.2 Multicomponent Glasses

The basic process of sol-gel glasses for single component

was briefly discussed. Now the versatility of this process can be

extended to the production of binary and ternary oxide composite

glasses also. This method can ensure high degree of

homogeneity and good compatibility of two materials.

For the multicomponent glasses it has to be remembered that

the problem of phase separation will emerge besides crystallization.

Some of the largely explored multi-component glasses are TiO2-

SiO2, GeO2-SiO2, alkali containing glasses etc. The sol-gel glasses

and thin films in the TiO2-SiO2 systems are attractive from many

points of view including their interesting optical properties being

used for integrated optics applications. Technological and

industrial applications are significant due to their high thermal and

chemical stability, low thermal expansion coefficients, porosity

tailoring, tunable refractive index in a wide range, intrinsic

antireflecting properties, superscratch resistance, enhanced

photocatalytic activity, superhyrophilicity, superhyrophobicity etc.

[20-23]

14 Chapter 1

It is known that TiO2 does not form glass by melt quenching.

Therefore, Zachariasen [24] classified TiO2 into intermediate oxide

or network-modifying oxide. On the other hand, it is known that

many binary and ternary titanates form glasses without any help of

a network-forming oxide. So far, the binary glasses containing TiO2,

such as Cs2O-TiO2, Rb2O- TiO2, K2O-TiO2, BaO-TiO2, Na2O-TiO2

etc, were prepared by melt quenching. But the intake of Ti ions in a

glass is restricted to much less than 11% due to the phase

separation of Ti ions and its tendency of segregation to form TiO2

crystals. Also very high temperatures around 1700oC are needed to

produce such glasses. Some of the results and observations for the

atomic and micro structure for TiO2-SiO2 are summarized as

follows: upto 10 wt% addition of TiO2, Ti4+ ions substitute for Si4+

ions in glass and take four-fold coordination, lowering the thermal

expansion coefficient, that is, TiO2 makes solid solution in SiO2. The

TiO2-SiO2 glasses have very low thermal expansion coefficients

values in the range from 25 to 700oC and so they are used as

lenses for astronomical telescopes. Clear glasses containing upto

16.5 wt% TiO2 can be obtained but more than 16.5 wt% turns the

glass opaque due to phase separation and precipitation of crystals.

Heat treatment of glasses containing 12-17 wt% TiO2 causes phase

separation and anatase formation, considerably raising the thermal

expansion coefficient. Also sol-gel prepared TiO2-SiO2 amorphous

coatings are attracting attention as planar waveguide in the field of

photonics [25-27].

General Introduction 15

Satoh et.al [28] successfully made TiO2-SiO2 bulk glass from

silicon and titanium alkoxides. Anderson et.al. prepared TiO2-SiO2

glasses containing 8, 18, 41 mol% TiO2. He also reported that Ti4+

ions are 4 fold coordinated for the sample containing 8 mol% TiO2

and sample containing 18 mol% has 4 fold and 6 fold coordination.

Sakka et.al. [29] reported that the binary glasses consisting of TiO2

and alkali oxide were prepared by quenching the melts with a twin-

roller. Sol-gel process of titanium dioxide was investigated by

Rivallin et al. [30] and nanocrystallization of anatase in gel was

reported by Svadlak et al. [31]. Trial of amorphisation of TiO2 was

recently reported by Petkov et.al. and Wang et.al. [32, 33].

The titanium dioxide nanocrystals have been really explored

for its photocatalytic property along with the presence of some

amount of silica. The anatase phase of titanium dioxide really

exhibits the photocatalytic activity which finds applications in many

promising and interesting fields like stain decomposition, anti-

fogging usage, water quality modification, water treatment, water

cleanliness, deodorization, air treatment, antibacterial usage

(coating the walls and floors of clean and sterile rooms, hospitals

etc). The wide band gap of titanium dioxide is exploited for its

dielectric, photochemical and catalytic properties [34].

1.3.1.3 Sol-Gel Preparation of Multicomponent Glasses

An inherent problem for the preparation of mixed oxides by

sol-gel processing of a mixture of different metal alkoxides is that

phase separation may occur because of different hydrolysis and

condensation rates (Figure 1.3).

16 Chapter 1

For example, addition of water to a solution of Si(OR)4 and

Ti(OR)4 results in the precipitation of titania. Common solutions

to this problem which allows the preparation of homogeneous

(well mixed on the nanometer scale) silica-titania gels involve

pre-hydrolyzing Si(OR)4 as the slower reacting component or

lowering the reactivity of the faster reacting precursor, Ti(OR)4,

by chemical modification, eg. by replacing part of the alkoxide

groups by chelating or bridging ligands, such as carboxylates or

P-diketonates, acetic acid, valeric acid, acetylacetate or

acetoacetate. A third possibility is to use single-source

precursors. In these precursors, the titanium and silicon atoms

are linked by oxo bridges. The first solution can produce glasses

but only with few percentage of TiO2 (<5-6%). Second solution by

chemical modification or addition of such stabilizing agents or

complexing ligands can bring out further problems, for example

acetyleacetate has a strong absorption in the visible region.

Hence producing homogenous SiO2-TiO2 gels without any

stabilizing complex agents is a real challenge due to the fast

reactivity of titanium alkoxide [35-37].

General Introduction 17

Another challenge faced by the conventional sol-gel process of

SiO2- TiO2 production is the crystallisation of TiO2 at the early stages

of heat treatment. TiO2 is known to crystallize as rutile (tetragonal

phase), anatase (tetragonal) and brookite (orthorhombic). Pure TiO2

crystallize in the anatase phase at 350oC and in the rutile phase at

temperatures of 600oC and above. In the sol-gel process, heat

treatment of the gels is required at least upto 500oC to burn off the

carbonaceous compounds and also to eliminate the water and

hydroxyl groups associated with the matrix. Thus there should be

sufficient amount of SiO2-TiO2 linkage in its gel state and should be

homogenously distributed throughout the matrix which will prevent

TiO2 from segregating to form crystallites [38-40].

1.3.2 Ceramic Synthesis (Tape Casting)

Since the inception of the modern ceramics era, there have

been several advances in ceramic processing technology. One of

the newest and most prolific of these advances has been the

development and implementation of tape casting as a

manufacturing process for the production of thin sheets of ceramic

materials. Tape casting is also known as doctor blading and knife

coating, and under these names the process is well known in many

industries, including paper, plastic, and paint manufacturing. The

"doctor” is a scraping blade for the removal of excess substance

from a moving surface being coated. The technique has long been

used in the paint industry to test the covering power of paint

formulations. Films of paint with few millimetres thick are uniformly

18 Chapter 1

coated on a standard black-and-white background, and the degree

to which the background is hidden is measured optically.

The chief advantage of the tape casting process is that it is

the best way to form large-area, thin, flat ceramic or metallic parts.

These are virtually impossible to press and most difficult if not

impossible to extrude. The difficulties are compounded in dry

pressing, when the plate is to be pierced with numerous holes

because of the increased problem of uniform dye fill. Punching

holes and slots of various sizes and shapes into unfired tape is

relatively easy and essential to the multilayered ceramic packages

being designed and manufactured today. The thin ceramic sheets

are essentially two-dimensional structures, since they are large in

the x and y directions and very thin in the z dimension. In today's

technology, very thin is defined in microns, with tapes as thin as 5

microns being reported by equipment manufacturers [41].

Tape casting is an established method for manufacturing

wide and flat sheets of ceramic materials. Most applications of tape

casting technology refer to the electronic industry, while some

authors have also used this technique for obtaining thin ceramic

sheets and multilayered structures for different applications such as

solid oxide fuel cells or laminated composites. A tape casting slurry

is a complex system in which each component has a substantial

effect on the slurry properties. To obtain a reliable processing of high

quality products, a homogeneous dispersion of ceramic particles is

highly required. Stable suspensions are achieved by polymer steric

stabilization or electrostatic repulsion or both according to the type of

General Introduction 19

dispersant and solvent used. The powder dispersion is not only

dependent on the dispersant but also on the type of solvent used.

The major function of solvent is to acts as a dispersing vehicle and

to ensure the dissolution of the organic components. Mixture of

solvents may be useful to achieve a good compromise between

dielectric constant and surface tension (for dispersion) and low

boiling point, and an adequate viscosity (for handling and drying).

Azeotropic solvent mixtures were reported to have the advantage of

improving the organic solubility, and preventing preferential

volatilization and polymeric surface skin formation. However, very

few works have appeared in the literature dealing with the effect of

solvent on the tape casting process. Although PVB is well known as

binder other than dispersant due to its high molecular weight. Good

dispersibility is also observed with it as dispersant [42]

The doctor blade tape casting process is a low cost process

for the manufacture of large areas of thin ceramic sheets of

controlled thickness and high quality. Tape casting of different

materials requires different slurry formulations, which include

solvents, dispersants, binders, plasticizers and homogenizers to

produce high quality products. To optimize the formulation, a better

understanding of the interaction between ceramic powders, organic

additives and solvent as well as a better understanding of the

influence of their interaction on the processing step are desired.

Both non-aqueous and aqueous based routes can be attempted for

tape casting of ceramics. Organic-solvent based tape casting

systems are widely used mainly because one can obtain improved

20 Chapter 1

quality of tape and because of easy and fast evaporation of solvent.

The most important characteristics of a tape casting slurry are (i) a

well dispersed homogenous stable system, (ii) minimum viscosity,

(iii) shear thinning behaviour and (iv) high solid loading. The degree

of dispersion, deagglomeration and dispersion of the powder in the

solvent have profound effect on the microstructure of the green

body. Breaking of agglomerates could be achieved by mechanical

agitation of the powder. Dispersion of fine ceramic powder in a liquid

medium is usually achieved by the addition of optimum amount of

dispersant. For maximum effectiveness the dispersant must have

access to each particle surfaces. The repulsive interaction can be

provided by two different general mechanisms or a combination

thereof, one is electrostatic repulsion as a result of development of

an electrical double layer around each particle upon dispersing a

powder into a polar liquid. This produces a repulsive force, which

decreases with increasing separation between particles. The second

one is polymeric stabilization in which the stability is conferred by

long chain polymers adsorbed onto the particle surface. The

dispersion and stability of the suspension are achieved when the

repulsive forces are high enough to dominate over the attractive

London-van der Waals forces.

Sedimentation technique, a well accepted method to

establish the degree of particle dispersion and packing, gives a

visual representation of deflocculation and dispersion. The efficiency

of dispersion is evaluated by slower settling rate and higher packing

density. During the dispersion stage, as far as the rheological

General Introduction 21

characterization is concerned, the system that gives the minimum in

viscosity and near-Newtonian flow behavior is considered as the

best dispersed condition. An important characteristic of tape casting

slurry is the rheological characteristic, i.e. flow behavior. Optimized

final tape casting slurry composition exhibits pseudoplastic

rheological characteristics [43].

1.4 Spectroscopy of Rare Earths in Glass

Sol-gel matrices are found to be a good host for rare earth

ions. Rare earths such as Er3+, Dy3+, Eu3+, Tb3+ and Sm3+ ions are

also incorporated in sol-gel matrices. The absorption and

luminescence properties of rare earth ions doped sol-gel matrices

have found different applications in the field of lasers,

telecommunications and also in the production of a wide variety of

optical components. The unsaturated 4f electronic structure of rare

earth elements make them have special properties in luminescence,

magnetism and electronics, which could be used to develop many

new materials such as phosphors, magnetic and magnetiostrictive

materials, hydrogen storage materials and catalysts. The 4f electrons

are largely shielded from the surrounding crystal field and are not

involved in chemical bonding. But some interactions with the crystal

field take place, because depending on its symmetry, higher

electronic states of opposite parity can be mixed with the 4f electronic

states. Ultimately any transition occurring within the lanthanides 4f

orbital is going to be only weakly coupled to crystal lattice vibrations,

and the resulting spectra will appear free-ion-like. [44-46].

22 Chapter 1

Optical spectroscopy has been used as an important tool to

study the nature of glasses. Optimization of new or improved optical

quality glasses doped with rare earth ions have been characterized

by absorption and emission transition probabilities which are

influenced by the ligand field of the surrounding rare earth ions.

When 4f ions are introduced into glasses they replace the network

cation, which forms the glass, or they act as network modifiers. The

precise manner in which the dopants enter in the glass structure

depends on the relative sizes, valency and bonding of the

constituents involved [47]. Rare earth activated lasers are reported

by Snitzer [48] as early in 1961.

The spectra of the rare earth ions are composed of a set of

sharp levels. The special optical properties of trivalent rare earth ions

result from the fact that the electrons of their partially filled 4f shell

are shielded from the surrounding completely filled 5s and 5p shells.

The energy levels of the 4f shell arise from spin-spin and spin-orbit

interactions. These levels are often denoted using Russel–Saunders

notation 2S+1LJ in which S is the total spin quantum number, L the

total orbital angular momentum and J the total angular momentum.

The most important feature of energy levels of rare earth ions is that

all the levels of a particular ion have the same electronic

configuration and consequently all of them have same parity. Since

the electric dipole matrix elements between the two states of same

parity are identically zero, electric dipole transition between any two

levels of the ions is totally forbidden. However in a solid, the slight

mixing with odd parity wave functions makes the transition slightly

General Introduction 23

allowed. The absorption and emission cross-sections are therefore

small and the luminescence lifetimes (ms) can be quite long.

1.4.1 Europium Ion

Europium is a member of the lanthanide series of elements,

characterized by partially filled 4f-electron shell. The element

europium is characterized by having two stable valencies Eu(II) and

Eu(III).Transitions within the 4f6 shell (S=3,L=3 and J=0)of Eu3+ are

responsible for the observed spectra. Eu2O3 is the commonly used

source of Eu3+ ions in the production of phosphors and laser

materials. Figure 1.4 gives the absorption and luminescence

channels for the Eu3+ ion.

0

5

10

15

20

7F

0

7F

6

5D

0

5D

1

5D

2

En

erg

y (

x 1

03 c

m-1)

Figure 1.4 Luminescence channels of Eu3+

ions

24 Chapter 1

1.5 Dielectric Spectroscopy

Dielectric spectroscopy is a very versatile electrochemical tool to

characterize intrinsic electrical properties of any material and its

interface. Glasses are generally considered as moderate dielectric

constant materials. The dielectric constant of glass material can be

varied by introducing various rare earth ions, transition metal ions,

semiconductor nanoparticles etc. By studying the dielectric

parameters of glasses such as dielectric constant, dielectric loss and

conductivity over a wide range of frequencies, we get information

about the insulating character, conducting behavior and structural

aspects of materials. The presence of nanoparticles are known for

the enhancement in permittivity for glassy matrices. The basis of

dielectric spectroscopy is the analysis of the impedance (resistance

of alternating current) of the observed system subject to the applied

frequency and exciting signal. This analysis provides quantitative

information about the conductance, the dielectric constant, the static

properties of the interfaces of a system, and its dynamic change due

to adsorption or charge-transfer phenomena. Dielectric spectroscopy

measures the dielectric properties of a medium as a function of

frequency and temperature. Dielectric spectroscopy is sensitive to

dipolar species as well as localized charges in a material. It

determines their strength, their kinetics and their interactions [49].

Thus, dielectric spectroscopy is a powerful tool for the electrical

characterization of non-conducting or semiconducting materials in

relation to their structure and also for electronic or sensor devices.

General Introduction 25

There are different types of electrical stimuli which are used in

dielectric spectroscopy. The most common and standard one is to

measure impedance directly in the frequency domain by applying a

single frequency voltage to the interface and measuring the phase

shift and amplitude, or real and imaginary parts, of the resulting

current at that frequency. These frequency dependent measurements

have been recognized as an important tool for the electrical

characterization of materials [50-51]. Commercial instruments are

available which measure the impedance as a function of frequency

automatically in the frequency range (1 Hz to 1 MHz) and which are

easily interfaced to laboratory microcomputers. The advantages of

this approach are the availability of these instruments and the ease of

their use, as well as the fact that the experimentalist can control the

frequency range.

1.6 Surface Plasmon Resonance Studies

The surface plasmon Resonance (SPR) is a phenomenon

due to the presence of metallic nanoparticles, in solution or in the

solid phase. The interaction of metallic nanoparticles with incident

light, results in the collective resonance of the conduction electrons

of the nanoparticle. Metallic nanoparticles exhibiting surface plasmon

resonance (SPR) is one of the interesting subjects and has the

potential applications in the field of chemical, biological sensing and in

optical devices. Controlling the size, shape and structure of metallic

nanoparticles is technologically important because of the optical,

electrical and catalytic properties. The two-dimensional nature of

plasmonic structures make them compatible with modern lithographic

26 Chapter 1

methods used for preparation of integrated circuits. These metallic

nanostructures are already known to display unusual and

unexpected optical properties. The plasmonic fields generated in

such structures provide opportunities for new experimental

capabilities such as sub wavelength optical imaging [52-54].

Plasmonics of nanoparticles is an integrated bottom-up

technique to fabricate the advanced devices and materials using the

surface plasmon resonance (SPR) of nanostructured metals. The

character and performance of the fabricated devices and materials

are largely depend on composition, dimension (size and shape), and

arrangement of the nanoparticles [55]. Nanoparticles (NPs) of Ag, Cu

and Au exhibit plasmon absorption bands in the visible wavelength

region [56]. The absorption spectrum is sensitive to various factors

such as particle size and shape, the electron density on particle, the

dielectric properties of the surrounding medium, aggregation state

and interparticle interaction. The underlying theory behind SPR is

defined by the Mie solutions to Maxwell’s equations by Stern and

Ferrell [57], which showed that an interface plasmon mode can exist

at the boundary between two metals. For the case of a bimetallic

particle, the two different metals have different electron densities and

therefore, two different plasmon frequencies [58]. To account for

bimetallic structures, the dielectric function used is a weighted linear

combination of dielectric constants of the constituent metals.

Theoretical simulations were compared to experimentally produced

Au/Ag nanoparticles and the data revealed relatively high correlation

[56]. This correlation is consistent when the particle sizes are above 5

General Introduction 27

nm, for which Mie theory holds. Cottancin et al. have developed a

theory that combines the classical Mie theory with quantum effects

that play an enhanced role at this size regime [57, 58]. Probably the

most interesting property of metallic nanoparticles is the ability to tune

their plasmon resonances across a wide range [59].

The SPR of Ag NPs with high symmetry, like spheres or

ellipsoids, can be calculated with good accuracy by analytical

expressions developed in the frame of the Mie theory. However,

when particle’s symmetry is lowered, the exact solution of the

electromagnetic problem is not possible, and numerical approaches

are necessary. One of the most frequently used numerical methods

for the calculation of the SPR in metal nanoparticles is the discrete

dipole approximation (DDA). In DDA, each object is approximated

with a cubic array of dipoles each having a definite polarizability. The

result of DDA calculations is considered reliable if the nanoparticles

are described with an adequate number of dipoles and with the

adequate size-corrected dielectric function. To describe the van der

Waals interactions between a nanoparticle and a surface, usually a

dipole approximation is used, which is valid only for large enough

distances between the particle and the surface. The van der Waals

energy between realistic metallic surfaces was shown to be

dominated by surface plasmon oscillations for small distances [60].

Oleg et al discussed the strong size and morphology dependence of

the fluorescence spectra of silver nanoparticles embedded in silica

[61]. When the size of the particle is decreased the fluorescence

intensity increases indicating a strong influence of the surface of the

28 Chapter 1

nanoparticle. Forster developed a theory relating the process of

excitation energy transfer (EET) from a donor to an acceptor. The

distance dependence of the rate of resonance energy transfer

between metallic particles is currently a subject of great interest

because of its potential use in material science and biomedical

applications [62]. The existence of interaction between two spherical

particles separated by a distance is explained by the Vander Waals

and Casimir forces [63].

The research work reported in the thesis is focused on to the

research problem concerning the structural, optical, dielectric and

plasmonic characterizations of TiO2 based glasses. The research

problem also includes the rheological characterization of TiO2 based

ceramic composites. The detailed research work has been

described in the following chapters. The main objectives of the work

presented in the thesis are (i) Synthesis of SiO2-TiO2 glassy

matrices via novel ultra-low hydrolysis sol-gel processing. (ii) The

structural and optical characterization of silver nanocrystallites/

Eu3+: SiO2-TiO2 matrices synthesized through sol-gel route

(iii) Fluorescence enhancement studies in silver nanocrystallites

/Eu3+: SiO2-TiO2 matrices and its explanation by invoking various

structural and optical phenomena (iv) Synthesis of Ag/Eu3+ and

ZnO/Eu3+ doped titanosilicate glass samples through sol-gel route

(v) Dielectric and AC conductivity studies of Ag/Eu3+ and ZnO/Eu3+

doped titanosilicate glasses (vi) Experimental observation of SPR

and emission spectra of silver nanoparticles in SiO2-TiO2 matrix

General Introduction 29

(vii) Numerical calculation of SPR of silver nanoparticles,

quantitative estimation of excitation energy transfer (EET), van der

Waals (vdW) energy and Casimir energy with spherical morphology

on the basis of discrete dipole approximation (DDA) method

(viii) Dispersion behaviour of TiO2 in different solvent systems in

combination with two different dispersants and optimization for the

dispersion of TiO2 (ix) Application of double doctor blade tape

casting process to produce TiO2 based ceramic tapes with optimum

properties.

The novelty of the research work lies in the fact that the

presence of TiO2 in other glass matrices enhances the glass forming

ability, chemical durability, mechanical and insulating strength of the

glasses. We report the enhancement of the fluorescence and

excitation spectra of the europium ions in the presence of silver

nanoparticles in SiO2-TiO2 system prepared by the sol-gel method.

Our experimental results show that the energy transfer from the

silver nanoparticles to Eu3+ ions is mainly responsible for

fluorescence enhancement. It is confirmed that the asymmetry ratio,

surface plasmon resonance surface roughness and more also

favour the enhancement. The conductivity variation with the Ag /

ZnO content in the Eu3+ doped SiO2-TiO2 system has been

explained by correlating the presence of ionic contribution to the

electrical conductivity process. The investigations of surface

plasmon resonance (SPR) of Ag nanoparticles: SiO2-TiO2 matrix

have shown promising properties for the realization of the matrix as

plasmonic material. Based on the optimized TiO2 tape casting slurry

30 Chapter 1

composition (xylene–ethanol solvent system and MFO dispersant)

novel tapes free from visible defects were obtained using the double

doctor blade technique.

The motives for carrying out several experiments to extract

the different structural, optical and dielectric properties are as

follows. We have used the experimental techniques EDX, XRD,

FTIR, AFM, SEM and TEM for elucidating the structural properties

of the synthesized materials. In order to extract the optical features

we have used UV-Vis spectrophotometer and spectrophotofluorimeter

and the porosity measurements were made using Tristar 3000

analyser. The dielectric and conductivity measurements were done by

using impedance spectroscopy. We have used optical microscope

image to ensure the defect free tapes as end products.

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