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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.
References
[1] Zarzyki J, Glasses and the vitreous state, Cambridge University
Press, Cambridge (1991).
[2] Varshneya A K, Fundamentals of Inorganic Glasses, Academic
Press Inc., Harcourt Brace & Co., New York (1987).
[3] Zacharisen W H, J. Am.Chem.Soc., 54(1932)3841.
[4] Hagg G, J.Chem. Phys., 3(1935)42.
[5] Smekal A, J. Soc. Glass Technol., 35(1951)411.
[6] Sun K H, J. Am. Ceram. Soc., 30(1947)277.
[7] Turnbull D, Contemp. Phy., 10(1969)473.
General Introduction 31
[8] Rice R W, Ceramic Fabrication Technology, Marcel Dekker, Inc.,
New York(2003).
[9] Saito S, Fine ceramics, Elsevier Applied Science Publishers,
England (1985).
[10] Kingery W D, Bowen H K, Uhlmann D R, Introduction to ceramics,
2nd edition , John Wiley & sons(1976).
[11] Patil K C, Bull. Mater. Sci., 16(1993) 533.
[12] Subbarao E C, Aca. proced. eng. Sci., 13 (1988) 156.
[13] Brinker C J, Scherer G W, Sol-Gel Science, Academic Press,
London (1990)
[14] Hench L L, West J K, Chem. Rev., 90(1990)33.
[15] Mukherjee S P, J. Non-Cryst. Solids, 63 (1984) 35.
[16] Guglielmi M, Carturan G, J. Non-Cryst. Solids, 100(1988)16.
[17] Schmidt H, J. Non-Cryst. Solids, 100(1988) 51.
[18] Brinker C J, Scherer G. W. Sol-Gel Science: the phys. and chem. of
sol-gel processing, Academic Press, New York, (1989).
[19] Hench L L, Orcel G, J. Non-Cryst. Solids, 82 (1986)1.
[20] Courty P, Marcilly C, Preparation of Catalysts; Delmon B, Jacobs P
A, Poncelet G, (Eds), Elsevier Science: Amsterdam, (1976).
[21] Ward D A and Edmond I K, Ind. Eng. Chem. Res., 34 (1995) 412.
[22] Rigden J S, Walters J K, Dirken P J et. al, J. Phys. Condens.
Matter, 9(1997) 4001.
[23] Wolfgang R, Nicola H and Ulrich S, J. Mater. Chem., 12
(2002)2594.
32 Chapter 1
[24] Zachariasen W H, J. Am. Chem. Soc. 54 (1932)3841.
[25] Schlutz P C, Smyth H.T, Ultra-Low Expansion Glasses and their
structure in the SiO2-TiO2 system, in: Douglas P W, Ellis B,
John(Eds), Wiley & Sons (1972).
[26] Evans D L, J. Amer.Ceram.Soc, 53 (1970) 418.
[27] Kawachi M, Yasu M, Edahiro T, Electronics Lett. 19 (1983) 583.
[28] Satoh S, Susa K, Matsuyama I, J. Non-cryst. Solids, 146 (1992)
120.
[29] Sakka S, Miyaji F, Fukumi K, J. Non-Cryst Solids, 107 (1989) 171.
[30] Rivallin M, Benmami M, Gaunand A, Kanaev A, Chem. Phys.Lett.
398 (2004) 157.
[31] Svadlak, Shanelova J, Malek J, et.al, Thermochim. Acta, 414
(2004) 137.
[32] Petkov V, Himmel B, et.al, J. Non-Cryst. Solids, 231 (1998) 17.
[33] Wang Y, Ma C, Sun X, Li H, J. Non-Cryst. Solids, 319 (2003)
109.
[34] Wang T, Wang H, Chao S, et.al., Thin Solid Films, 334 (1998)
103.
[35] Bastow T J, Moodie A F, Smith M E, Whitfield H J, J. Mater.
Chem. 3(1993)697.
[36] Holland M A, Pickup D M, Newport R J, et.al., J. Mater. Chem., 10
(2000) 2495.
[37] Yoldas B E, J. Non-Cryst. Solids, 38 (1980) 81.
[38] Haro-Poniatowski E, de la Cruz Heredia M, Arroyo-Murillo R, J.
Mater. Res. 9 (1994) 2102.
General Introduction 33
[39] Tang H, Prasad K, Schmid P.E, J. Apply. Phys. 75 (1994) 2042.
[40] Sankur H , Gunning W, J. Appl. Phys. 66 (1989) 4747.
[41] Richard E. Mistler, Eric R. Twiname, Tape Casting: Theory and
Practice, Wily publishing(2000).
[42] Zhang Jingxian, Jiang Dongliang, Lars Weisensel, Peter Greil, J.
Euro. Ceram. Soc. 24 (2004) 147.
[43] Vasantha Kumari K G, Sasidharan K, Sapna M , Raghu Natarajan,
Bull. Mater. Sci., 28 (2005) 103.
[44] Das K, Nagarajan V, Nandagoswami M L, Panda D, Dhar A, Ray
S K, Nanotechnology, 18 (2007) 095704.
[45] Thomas V, Jose G, Jose G, Unnikrishnan N V, J. Sol-gel. Sci. &
Tech. 33 (2005) 269.
[46] Jose G, Jose G, Thomas V, Joseph C, Ittyachen M A,
Unnikrishnan N V, J. Fluoresc, 14 (2004) 733.
[47] Weber M J, J. No n-Cryst. Solids, 123 (1990) 208.
[48] Snitzer E, Phys. Rev. Lett., 7 (1961) 444.
[49] Jonscher A. K., Dielectric relaxation in solids, Chelsea Dielectrics
Press, London(1983).
[50] Bottelberghs. P. Solid Electrolytes, Academic Press, New York
(1978).
[51] Macdonald. J R,Garber. J A, J. Electrochem. Soc., 124 (1977) 1022
[52] Joseph R. Lakowicz, Plasmonics, 1 (2006) 5.
[53] Audrey Moores, Frederic Goettmann, New J. Chem.,30(2006)1121
34 Chapter 1
[54] Natalia Strekal, Olga Kulakovich, Valiantsin Askirka, Iosif Sveklo,
Sergey Maskevich, Plasmonics 4(2009)1.
[55] Qingbo Zhang, Yen Nee Tan, Jianping Xie, Jim Yang Lee,
Plasmonics 4 (2009) 9.
[56] Kevin J. Major, Chandrima De, Sherine O. Obare, Plasmonics 4
(2009) 61.
[57] Stern E A, Ferrell R A, Phys. Rev. 120 (1960) 130.
[58] Palik E. D, Hand book of Optical constants of Solids, Academic
Press, New York (1985).
[59] Stefanie Ahl, Petra J. Cameron, Jing Liu, Wolfgang Knoll, Jonah
Erlebacher, Fang Yu, Plasmonics 3 (2008) 13.
[60] Klimov V V, Lambrecht A, Plasmonics 4 (2009) 31.
[61] Oleg A. Y, Igor M. D, Alexandr A. A, Mykhaylo Y L, Andriy V. Kotko,
[62] Anatoliy O P Phys. Rev. 79(2009)235438.
[63] Intravavia F, Henkel C, Lambrecht A, Phys. Rev. 76(2007)033820.