CHAPTER-1

INTRODUCTION

1.1 General Concepts of Glasses

The presence of glasses in our surroundings is so common that we rarely notice of

their existence. Ancient Egyptians considered glasses as precious materials as evidenced

by the glass beads found in the tombs of ancient Pharaohs. Humans have been producing

glasses by melting of raw materials for thousands of years. The first crude manmade

glasses were used to produce beads or to shape into tools requiring sharp edges.

Eventually, methods for production of controlled shapes were developed.

The advent of the age of technology created many new opportunities for the

application of glasses. Recently, the development of glass optical fibers has

revolutionized the telecommunication industry, with fibers replacing copper wires and

radically expanding our ability to transmit flow free data throughout the world. Unlike

many other materials, glasses are also esthetically pleasing to an extent which far exceeds

their mundane applications as drinking vessels and ashtrays, windows and bottles and

many other everyday uses.

What is a Glass? The glasses used by mankind throughout of our history have

been based on silica. Is silica an essential component of a glass? Since we can form an

almost limitless number of inorganic glasses, which do not contain silica, the answer is

obviously no, silica is not an essential component. Glasses are traditionally formed by

cooling from a melt; we can form glasses by vapour deposition, by sol-gel processing of

solutions and by neutron and heavy ion irradiation or by pressure induced amorphization

of crystals. Traditionally glasses are inorganic and non-metallic. Metallic glasses are

becoming more common with the passage of time. Obviously the chemical nature of the

material cannot be used to define a glass. Every glass found to date, shares two common

characteristics. First no glass has a long-range order, periodic arrangement of constituent

2

atoms or ions. Even more importantly, every glass exhibits the time dependent behavior

known as glass-transformation behavior. This behavior occurs over a temperature range

known as glass transformation region. A glass can thus be defined as ‘an amorphous solid

completely lacking in long range, periodic atomic structure and exhibiting a region of

glass transformation behavior. Any material inorganic, organic or metallic formed by any

technique, which exhibits glass transformation behavior, is a glass. Another definition of

glass given by C.A. Angell (1995) is:

“A glass is an amorphous solid which is capable of passing continuously into

viscous liquid state, usually but not necessarily accompanied by an abrupt increase in

heat capacity.”

This definition puts metallic glass materials to the grey world of amorphous solids

because although formed from a liquid they crystallize before ever achieving the

supercooled liquid state. On the other hand, the definition admits many substances

produced initially by routes, which never involve a liquid state [Angell (1995)].

We traditionally discuss glass transformation behavior on the basis of either

enthalpy or volume versus temperature diagrams as shown in Fig. 1.1. Since enthalpy and

volume behave in a similar fashion the choice of the parameter is arbitrary. In either case,

we can envision a small volume of a liquid at a temperature well above the melting

temperature of that substance. As we cool the liquid the atomic structure of the melt will

gradually change and will be a characteristic of the exact temperature at which the melt is

held, cooling to any temperature below the melting temperature of the crystal would

normally result in the conversion of the material to the crystalline state with the formation

of a long range periodic atomic arrangement. If this happens, enthalpy will decrease

abruptly to the value appropriate for the crystal. Continued cooling of the crystal will

result in a further decrease in enthalpy due to the heat capacity of the crystal. If the liquid

can be cooled below the melting temperature of the crystal without crystallization a

supercooled liquid is obtained.

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The structure of the liquid continues to rearrange as the temperature decreases but

there occurs no abrupt decrease in enthalpy. As the liquid is cooled further the viscosity

increases.

Fig. 1.1: Schematic illustration of the change in volume/enthalpy with temperature as a

supercooled liquid is cooled through the glass transition temperature (Tg).

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This increase in viscosity eventually becomes so great that the atoms can no

longer completely rearrange to equilibrium liquid structure during the time allowed by

the experiment. The structure begins to lag that exist if sufficient time was allowed to

reach equilibrium. The enthalpy begins to deviate from the equilibrium line following a

curve of gradually decreasing slope, until it eventually becomes determined by the heat

capacity of the frozen liquid i.e. viscosity becomes so great that the structure of the liquid

becomes fixed and is no longer temperature dependent. The temperature region lying

between the limits where the enthalpy is that of the equilibrium liquids and that of frozen

solid is known as glass transformation or transition region. The frozen liquid is now a

glass.

Since the temperature where the enthalpy departs from the equilibrium curve is

controlled by the viscosity of the liquid i.e. by kinetic factors, use of a slower cooling rate

will allow the enthalpy to follow the equilibrium curve to a lower temperature. The glass

transformation region will shift to the lower temperature and the formation of completely

frozen liquid or glass will not occur until a lower temperature. The glass obtained will

have lower enthalpy than that obtained using a faster cooling rate. As indicated above, the

glass transformation occurs over a range of temperature and cannot be characterized by

any single temperature. This temperature, which is termed either, the glass transformation

(Tg) or the glass transition temperature is rather vaguely defined by changes in either

thermal analysis curves or thermal expansion curves. Tg has traditionally been defined as

the temperature at which viscosity becomes ~1012 Poise. The most commonly used

definition of Tg, which is called Cponset definition is that it corresponds to the temperature

at which molecular liquids have viscosity ~1010 Poise. Another commonly used definition

is the “Cpmidpoint

” determined during heating where the viscosity is 109 Poise. All these

temperatures depend on the manner in which the system is prepared [Angell (1996);

Winderlich (1949)].

Strong /fragile glasses: The concept of liquid fragility was introduced by Angell and

Goldstein (1986) [Angell (1995); Angell et al. (2000); Angell et al. (2000)] building on

earlier work by Uhlmann (1977) and Uhlmann (1980). Liquid fragility is a measure of

departure from Arrhenius law viscosity temperature behaviour. A fragility plot shown in

5

Fig.1.2 is produced when the viscosity temperature relations for different liquids are

scaled against the calorimetric glass transitions (Tg). SiO2 is typically used to define the

‘‘strong’’ Arrhennian limit. More ‘‘fragile’’ liquids show increasing degrees of curvature

in their viscosity when scaled to Tg.

Fig. 1.2: Behavior of strong and fragile glasses versus temperature [Debenedetti

and Stillinger (2001)].

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SiO2 is typically used to define the ‘‘strong’’ Arrhennian limit. More ‘‘fragile’’

liquids show increasing degree of curvature in their viscosity when scaled to Tg. Fragile

liquids therefore show non-linear increase in viscosity in the supercooled liquid regime.

The relationship between the thermodynamic properties of a liquid and the viscosity is

considered to be a reflection of the contribution of configurational entropy. This is the

basis of the Adam–Gibbs model of viscosity and is seen in the jump in heat capacity

(ΔCP) at the glass transition temperature [Debenedetti (1996)]. A large change in heat

capacity occurs in a fragile liquid and indicates a strong temperature-dependence of the

liquid structure. The entropy differences between the liquid species in the two-state

models should, therefore, correspond to differences in the rheological properties of the

liquids. Liquids dominated by the high-density species will be more fragile. Since the

higher density species will be stable at greater pressures, therefore higher-pressure liquids

will be more fragile and will have increased configurational entropy. However, the exact

structural changes that occur are unclear and has led Angell and others [Angell and

Moynihan (2000); Moynihan and Angell (2000)] to develop versions of the two state

model that are not based on specific liquid species but on the degree of excitation of the

liquid structure (bond-breaking).

1.2 Structural Theories of Glass Formation

The question ‘Why do certain materials readily form glasses on cooling a melt? is

one of great practical and technological importance. In many cases this question may be

reformulated as ‘why do certain chemical compositions of materials have a greater glass-

forming ability (GFA) than others? This remains one of the great unsolved mysteries of

glass science and although empirical theories have been developed which are reasonably

successful in accounting for the glass-forming tendencies in certain specific cases, there

is no general rule which may be used universally to predict GFA of a given material. The

question of the ease of glass forming on cooling a melt is intimately related to the

problem of how glasses form. The first attempt to explain the structure of covalent

glasses around 1930, were based on the very natural hypothesis, the amorphous materials

consist of very large number of elemental microcrystals, randomly arranged into a very

7

fine polycrystalline structure which appears as amorphous. All attempts to fit the

experimental data with such microcrystalline models failed and the early idea of

continuous random network (CRN) of Zacharisen (1932) has become the only viable

alternative.

It was proposed by Zacharisen that “atomic arrangement in glass is characterized

by an extended three dimensional network which lacks symmetry and periodicity”. In

Fig. 1.3 we show the historical schematic diagram of Zacharisen, which represents the

CRN model of an analogue of amorphous silica in two dimensions. Zacharisen noted that

the silicate crystals, which readily form glasses instead of recrystallizing after melting

and cooling, have a network as opposed to close-packed structures. These networks

consist of tetraherda connected at all four corners, just as in the corresponding crystals

but the networks are not periodic and symmetrical as in crystals. These networks extend

in all three dimensions; such that the average behavior in all directions is the same i.e. the

properties of the glasses are isotropic. Zacharisen contends that the ability to form such

networks thus provides the ultimate condition for glass formation. After establishing that

the formation of a vitreous network is increasing for glass formation, he considered the

structural arrangement, which could produce such a network and gave the following

rules:

(a) The co-ordination number of the cation must be small.

(b) An oxygen ion may not be linked to more than two cations.

(c) The oxygen polyhedrons may share only corners, not edges or faces.

(d) At least three corners of every oxygen polyhedron must be shared by other

polyhedrons.

These conditions are fulfilled by the oxides of the type R2O3, RO2 and R2O5 which

is conformed through the occurrence in vitreous form of for example B2O3, As2O3, SiO2,

GeO2 and P2O5. The first model based on the Zacharisen’s idea was built much later. One

of those was for SiO2. This model was quite successful in explaining the x-ray diffraction

data. The radial distribution function was computed for this model and was in good

agreement with the diffraction data of Bell and Dean (1966, 1972); Mozzi and Warren

(1969).

8

Fig. 1.3: Two-dimensional schematic of the structure of vitreous SiO2. Si atoms are

represented by smaller circles.

A number of other theories of glass formation are based on the nature of bonds in

the material. Smekal (1951), for example, proposed that glasses are only formed from

melts, which contain bonds that are intermediate in character between, purely covalent

and purely ionic. Since purely ionic bonds lack any directional characteristics, highly

ionic materials do not form network structures. On the other hand, highly covalent bonds

tend to face sharply defined bond angles, preventing the formation of a non-periodic

network. Glass forming substances thus fall into the categories of either inorganic

compounds which contain bonds, which are partially ionic and partially covalent, or

either inorganic or organic compounds within the chains and Vander Waals bonds

between the chains. Stanworth (1950) attempted to quantify the mixed bond concept by

the use of the partial ionic character model. He classified oxides into three groups on the

basis of electronegativity of the cation. Cations which form bonds with oxygen with a

fractional ionic character near 50%, should act as network formers (group I) and produce

good glasses. Cations with slightly lower electronegativity (group II) which form slightly

9

more ionic bonds with oxygen, cannot form glasses by themselves, but can partially

replace cations from the first group, they are known as intermediates. Cations which have

very low electronegativity (group III) and therefore form highly ionic bonds with oxygen

never act as network formers. Since these ions only serve to modify the network structure

created by the network forming oxides, they are termed as modifiers.

Bond strength has also been used as a criterion for predicting the ease of glass

formation. Sun (1947) argued that strong bonds prevent reorganization of the melt

structure into the crystalline structure during cooling and thus promote glass formation.

In this particular case the bond strength was defined as the energy required to dissociate

an oxide into its constituent atoms in the gaseous state. Use of this criterion yields results

similar to those of Stanworth (1950) with groups of network former, intermediate and

modifier cations. Finally Rawson (1967) suggested that Sun ignored the importance of

temperature in his model. He suggests that high melting temperature mean that

considerable energy is available for bond disruption, while low melting temperatures

mean that significantly less energy is available. It follows that a material with large single

bond strength and a low melting temperature will be a much better glass former than one

with similar single bond strength but a much higher melting temperature.

1.3 Structure of Glasses

By structure we refer to a precise description of the substance in terms of atomic

positions, bond lengths and bond angles. In case of a crystal the arrangement of the atoms

or ions is periodic in three directions of space. The detailed description of such a structure

is complete once the dimensions and the content of the unit cell are specified. The

position of all atoms is then determined by translation of this cell along the three

directions of space. Crystals are said to possess both a short and long range order and the

crystallographic methods, which have been developed, are based on the properties of

point groups and translational groups which characterize a given structure. The case of

disordered materials such as glasses and liquids is more complex. Only short-range and

medium range order is present and the unit cell cannot be defined.

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1.3.1 Structure of borate glasses:

Borates and borosilicates are very important ceramic, metallurgical, and in

particular, glass-forming materials. A number of physical and chemical properties make

borates one of the outstanding components in the commercial glass industry: borates are

among the best glass-forming substances; boron is highly soluble in silicate melts, and it

lowers the solidus and liquidus temperatures of silicate systems; boron reduces the

viscosity and thermal expansivity of borosilicate melts; and borosilicates have high

chemical durability. Borosilicate glass is nowadays used as the host matrix for the

immobilization of high-level radioactive wastes because of the relatively high chemical

durability and its amenability to be processed at significantly lower temperatures in

relation to that of other competing materials.

Structures of borate and borosilicate glasses have been extensively studied by 11B

MAS NMR and Raman spectroscopy. These investigations have focused on changes in

the structure of the borate and silicate glass networks, as a function of metal oxide

content. 11B, 29Si and 17O MAS NMR, have led to widely accepted models for both the

borate and borosilicate systems [Jellison et al. (1978), Feller et al. (1982), Roderick et al.

(2001) Dell et al. (1983) and Chen et al. (2004)]. Pure B2O3 is an excellent glass former

and it has been well investigated. The most widely accepted model for the structure of

vitreous B2O3 is random network of corner linked BO3 triangles as suggested by

Zacharisen (1932). Fig.1.4 shows the two dimensional representation of random network

model of glassy B2O3. The triangular BO3 structural units are deduced from the boron-

oxygen configuration in crystalline borates. Although boron occurs in both trigonal and

tetrahedral coordination in crystalline compounds, it is believed to occur only in

triangular state in vitreous boric oxide at ambient pressures. Crystalline boron trioxide

was first obtained by Kracek and structural investigations of this material were carried

out by Berger by X-ray diffraction. These studies concluded that oxygen atoms in B2O3 –

I polymorph formed two different distorted tetrahedral about the boron atoms with B-O

distance ranging from 1.31 to 2.14 Å [Kracek et al. (1938) and Berger (1952)]. The

11

structure was criticized by Wells (1962) and is inconsistent with NMR studies made by

Svanson et al. (1962) and Kline et al. (1968), indicating same triangular co-ordination in

B2O3 –I polymorph as found in B2O3 glass. Later on Strong and Kaplow (1968) presented

the structure as ribbons of interconnected BO3 triangles. Fig. 1.5 shows structure of

crystalline form of B2O3-I proposed by Gurr et al. (1970). The unit cell of B2O3 contains

two structurally distinct boron atoms with B-O bond lengths between 1.336 and 1.404 Å

and a distribution of O-B-O angles range from 12.3 to 133.4° with a mean value of

130.7°.

Fig. 1.4: Random network model of B2O3 glass.

The structure of vitreous boric oxide is also believed to contain a large

concentration of units consisting of three boron-oxygen triangles joined to form boroxol

ring structure as shown in Fig. 1.6 .The presence of boroxol groups was first suggested

by Goubeau to explain extremely sharp line in the Raman spectrum at 808 cm-1 [Goubeau

(1953)]. Evidences for existence of boroxol rings in vitreous B2O3 is summarized by

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Krogh-Moe that glass has a network of BO3 triangles with a comparatively high fraction

of boroxol rings similar to that is illustrated in two dimensions in Fig. 1.7 [Krogh-Moe

(1969)].

Fig. 1.5: Crystalline form of B2O3. Small solid circles represent boron atoms and large

hollow circles represent oxygen atoms in a single ribbon.

Fig. 1.6: Boroxol ring structure

13

The structural model proposed by Mozzi and Warren (1970) explains that not all the BO3

triangles in vitreous B2O3 form part of boroxol group, a fraction of 0.6±0.2 of boron

atoms are present in boroxol rings. Johnson et al. (1982) proposed that vitreous B2O3

should be considered as two structural systems in which boroxol groups are joined by

BO3 group so that B-O-B angle is variable and twisting out of plane of the boroxol group

can occur (Fig. 1.8).

Fig. 1.7: Glassy B2O3 structure containing boroxol rings.

Fig. 1.8: Boroxol ring structure in boric oxide glass.

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The introduction of oxygen atoms from a modifier oxide to boric oxide glass can create

non-bridging oxygen (NBO) i.e. oxygens linked to only one network cations like B3+,

Si4+ etc whereas bridging oxygens (BO) are those which link two network cations.

or can convert boron from a three co-ordination state to four co-ordination state i.e. B3 to

B4 as shown below:

In the BO3 group the oxygens are fully bridging and hence one negative charge

from each oxygen satisfies the three positive charges on the boron atom. After conversion

from B3 to B4 all the oxygen remain bridging. When alkali metal oxides are added to

boric oxide many of the properties show anomalous behaviour, for example a minimum

in thermal expansion coefficient and maximum in Tg occurs at higher alkali oxide

concentrations. Since such behavior are not observed in alkali silicate glasses which has

been the subject of numerous earlier studies, this behavior was considered to be

anomalous for borate glasses and hence this phenomenon is termed as borate anomaly.

Krogh-Moe proposed a model which explains the structure of borate glasses by the

formation of various atomic groups with the alkali or other metal oxides. The borate

glasses contain well-defined and stable polyborate grouping which also occur in borate

crystals as shown in Fig. 1.9.

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Fig. 1.9: Structural groups present in alkali borate glasses (a) boroxol (b) pentaborate

(c) triborate (d) diborate (e) metaborate (f) pyroborate (g) orthoborate (h) loose

N4.

The relative concentrations of these borate groups are a strong function of the

glass composition [Meera et al. (1990), Meera and Ramakrishna (1993)].

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1.3.2 Structure and properties of lead glasses:

Binary lead borate glasses have been extensively studied during the last four

decades by a variety of techniques such as density measurements [Shaw and Uhlmann

(1969); Osaka et al. (1974); Shelby (1982); Doweidar and Oraby (1997) and George et

al. (1999) and Khanna (2000)], Raman vibrational spectroscopy [Meera et al. (1990);

Meera and Ramakrishna (1993) and Witke et al. (1994)], x-ray diffraction [Takashi et al.

(2000)] and 11B NMR spectroscopy [Bray et al. (1963); Leventhal and Bray (1965); Kim

et al. (1976) and Mao and Bray (1992)].

Mol%

Fig. 1.10: Phase Diagram of PbO-B2O3.

The PbO-B2O3 system has a very wide glass formation range of 20-80 mol%

PbO. These glasses exhibit high densities, and high transmittance in the UV-visible

region of the electromagnetic spectrum. These specific properties are favorable for many

applications; in fact, these glasses and crystals have been widely used in electronic and

optical technologies and are excellent materials for non-linear optical and magneto-optic

17

devices [Pan et al. (1995); Terashima et al. (1997) and Dimitrov and Komatsu (1999)].

The phase diagram of PbO-B2O3 system is shown in Fig. 1.10. The two well known

crystalline phases of this system are PbB4O7 (lead tetraborate) and Pb6B10O21, the former is a

promising material in non-linear optics [Bartwal et al. (2001)] and has all boron atoms in

tetrahedral co-ordination.

Structural investigations of lead borate compounds by 11B MAS NMR

spectroscopy, Raman spectroscopy, x-ray diffraction and molecular dynamics simulations

show that at low PbO concentrations, Pb2+ cations act as network modifiers. To maintain

charge balance throughout the material, diboron trioxide undergoes a change in

polymerization from three-coordinated ([3] B) bridging species to a four-coordinated ([4]B)

bridging species with a negative charge delocalized over the tetrahedral unit. It is also

well known that the addition of metal oxides like alkali, alkaline earth or heavy metal

oxides in binary, ternary and quaternary borate network based glasses results in similar

transformations [Ollier et al. (2004); Yiannopoulos et al. (2001); Hayashi et al. (2002);

Yamashita et al. (1999) and Prasad et al.(2006)]. For higher PbO concentrations,

increasing number of lead atoms act as glass formers and the coordination of Pb with

oxygen neighbors decreases from 8 to 3, as revealed by recent 11B and 207Pb MAS NMR

studies on lead borate glasses [Shaw et al. (2006)] The dependence of the boron

coordination number in borate glasses and melts as a function of glass composition and

temperature [Chryssikos et al. (1990); Stebbins et al. (1995); Sen et al. (1998); Yano et

al. (2003) and Majerus et al. (2003)] can be highly informative. Unlike silicates, borate

glasses consist of boron-oxygen coordination numbers of three and four, and various

mechanical, thermal, optical and electrical properties of borate glasses depend critically

on the fraction of tetrahedral boron units dispersed throughout the glass network. While

pure diboron trioxide glass has only triangularly coordinated boron atoms at ambient

pressures, recent experiments show almost all (>95%) of these boron atoms become

tetrahedrally coordinated at high pressures [Lee et al. (2005); Lee et al. (2007); Chason et

al. (1988) and Du et al. (2004)].

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Among experimental techniques for structural investigations of glasses, 11B NMR

spectroscopy, pioneered by Bray, is probably the best and most direct method for

determining the fraction of tetrahedral boron units, N4, in borate glasses and crystals

[Bray et al. (1963); Leventhal and Bray (1965); Kim et al. (1976) and Mao and Bray

(1992)]. This technique has now been greatly improved by modern, state of the art,

higher magnetic field NMR instrumentation and faster MAS probes, which enable

complete resolution of the [3]B and [4]B sites, thus allowing determination of N4 with

greater accuracy and precision [Prasad et al.(2006); Shaw et al. (2006); Du et al. (2004);

Kroeker et al. (2001) and Holland et al. (2004)]. 11B NMR experiments carried out by

Bray and coworkers on lead borates showed that the fraction of [4]B atoms in lead borate

glasses increases with the PbO content up to 50 mol%, while further increase in PbO

content lowers its value [Takashi et al. (2000); Bray et al. (1963); Leventhal and Bray

(1965); Kim et al. (1976) and Mao and Bray (1992)].

Glasses with high heavy metal contents are promising materials for use in the

field of nonlinear optics because of their high linear refractive index that is mainly

attributed to highly polarisable heavy metal cations. There has been an increase in interest

regarding sintered glasses using amorphous powder as a raw starting material and their

application to complex-shaped filters, composites and glass–ceramics [Gong et al.

(2000)]. In particular, significant developments in various electronic industries such as

flat panel displays, low temperature co-fired ceramics, and the packaging industry, need a

variety of glass types, which can be easily densified at low temperatures. PbO-containing

glass systems have become popular as commercial low temperature sinterable glass due

to their high structural stability, low glass transition temperature and good thermal and

electrical characteristics. One of the advantages of PbO glasses is that they do not easily

crystallize even when they contain 70 % of PbO. This is because the PbO glass systems

form PbO4 structures easily since Pb plays the role of an intermediate due to its own ionic

field strength [Hwang et al. (2002), Kim et al. (1999), Pascual et al. (2002) and Hwang et

al. (2002)]. However, recent environmental regulations have restricted the wide use of

PbO systems, so the development of Pb-free or Pb-saving materials, which can replace

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PbO, has been undertaken. Bi2O3, BaO and ZnO have been employed as candidate

materials that can replace PbO.

The third order non linear optical susceptibility for PbO-B2O3 has been measured

by Dimitrov et al. (1993). These values for lead borate glasses are about 11 times larger

than that for pure silica. The structure of lead borate and lead silicate glasses has been

extensively studied; lead borosilicate glasses have received limited attention. These early

studies showed that the role of PbO changes from primarily a modifier to a glass former

with increasing mol% of PbO [Bray et al. (1982)]. Zahra and Zahra (1993) studied the

PbO-B2O3 glasses containing lead in the range of 20 to 67 mol% of lead oxide. They

observed maxima in transition temperature (Tg) at 27 mol% of PbO and with the increase

in lead oxide concentration; the appearance of NBO’s decreases the stability of the

network and leads to decreasing transition temperature. El-Damrawi (2005) studied

density, molar volume and DC electrical conductivity of lead borosilicate glasses. They

interpret the structure of lead borate and lead silicate glasses to throw more light on the

correlation between the change in the network parameters and the electrical properties of

lead borosilicate glasses with low silica content. Khanna (2000) has investigated the

effects of melt annealing on the mechanical and optical properties of lead borate glasses

and concluded that the glass density, longitudinal modulus and transparency to visible

light show a large dependence on melt annealing time. But Khanna’s results are not

reliable as glass preparation was performed in porcelain crucibles which react unkindly

with borate melts [Khanna (2000)].

Meera et al. (1990) studied lead borate glass system by Raman spectroscopy

covering a wide range of lead oxide concentration varying between 22 and 85 mol%. The

conversion of three fold to four fold coordinated boron takes place on adding PbO, for

high lead content it is observed that there is back conversion of four coordinated borons

to three coordinated borons. Pan et al. (1995) found that both Raman cross-section and

non-linear index increases with increasing lead oxide content in PbO-B2O3 glasses.

Doweidar and Oraby (1997) have analyzed the densities of PbO-B2O3 in terms of volume

of the various B-O structural units. They presented a model to obtain the volume as a

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function of composition with the use of NMR and Raman spectroscopy studies of lead

borate glasses. Shaw et al. (2006) examined lead borate glasses to find co-relations with

their photo elastic response. 11B and 207 Pb NMR showed the familiar dependence of N4

on composition and distinct change in Pb-O co-ordination number from high to low with

increase in PbO content which indicated the role of latter as network former respectively.

Takashi et al. (2000) has been investigated the structure of PbO-B2O3 by using XRD, 11B

NMR technique. They first observed the well-separated peaks due to Pb-O and Pb-Pn

pairs in the radial distribution function and peak deconvolution by using pair function

method and proposed the structural models of lead borate glasses.

As mentioned earlier borosilicate glasses are technologically very important

materials and in the nuclear industry they find use as the host matrix for the

immobilization of high-level radioactive wastes because of the relatively high chemical

durability and its ability of being processed at significantly lower temperatures in relation

to that of other competing materials. Gohar et al. (1990) investigated alkali and alkaline

earth borosilicate glasses and discussed the formation of non–bridging oxygen and BO4-

tetrahedra at low silica content assumed that the shift of the UV edge to lower or longer

wavelengths may be due to the phase separation process in these glasses.

Klyuev and Pevzner (2003) studied the properties and structure of several borate

and aluminoborate glasses containing lithium, sodium and barium oxides. Their studies

revealed that the addition of Al2O3 can increase the structural thermal expansion

coefficient (STEC) values of glasses. The dependence of properties of the glasses on their

composition can be explained satisfactorily by coordination changes of boron with

oxygens and by the appearance, disappearance of a layered structure due to boroxol rings.

The effect of Al2O3 substitution for lead oxide has been studied in terms of density,

hardness, transition temperature (Tg), crystallization temperature (Tc), chemical durability

and structure of lead zinc aluminum phosphate glasses. Increase of hardness, Tg and Tc

indicate that the higher concentration of Al2O3 in the glasses, the stronger and more cross

linked glass network of the glasses may be formed [Ding et al. (2002)]. Klyuev and

Pevzner (2000) investigated BaO-Al2O3-B2O3 system over a wide composition range and

21

found that the introduction of Al2O3 into barium borate leads to the formation of AlO4

tetraherda and decreases the concentration of BO4 tetrahedra. Glass structures in the PbO-

Al2O3-SiO2 system, corresponding to stable chemical compounds in the respective phase

equilibrium diagram were investigated by Mylyanych et al. (1999). These authors found

that the introduction of Al2O3 into lead silicate glasses increases the mechanical strength

of the glasses and lowers their crystallization ability and that there exist micro-in

heterogeneities silicate glasses and melts. Multiple-quantum magic-angle spinning

(MQMAS) 17O NMR spectroscopy has been applied to several borate, borosilicate and

boroaluminate glasses by Wang and Stebbins (1999) who found multitypes of B–O–B

resonances in B2O3 and borate and borosilicate compositions. Bunker et al. (1991) have

studied the local structure of alkali earth boroaluminate glasses and crystals by 11B and 27Al MAS NMR spectroscopy. They concluded that most boroaluminate glasses contain

B3, B4, Al4, Al5 and Al6 structural units.

The addition of SiO2 into PbO-B2O3 is interesting as it raises the question of how

the borate network is modified by another network former, and in particular, what is the

impact on N4 by silicate tetrahedra. Alumina doping in borate and borosilicate glasses is

known to enhance glass formation ability and simultaneously cause significant reductions

in N4 [Araujo and Schreurs (1982) and Nassar and Adawi (1982)]. Lead

boroaluminosilicate glasses are of commercial importance as they find applications as

low melting point solder glass, porcelain glazes and enamels. And yet, there are few

reports in the literature on the mechanical, optical and thermal properties of borosilicate

and boroaluminosilicate glasses.

1.3.3 Structure and properties of bismuth glasses

Bismuth borate glasses have attracted enormous interest in recent years due to

their several outstanding properties like wide glass formation range of 20–80 mol%

Bi2O3, high density and refractive indices, and large coefficients for second and third

harmonic generation. The high atomic weight of both bismuth and lead oxides

contributes to a remarkable increase of the refractive index of the glasses. The phase

22

diagram of Bi2O3-B2O3 system was first studied by Levin and McDaniel (1962) is and is

presented in Fig.1.11.

Fig. 1.11: Phase diagram of Bi2O3-B2O3 system [Levin and McDaniel (1962)].

Five stable crystalline phases: Bi24B12O39 (boron sillenite), Bi4B2O9, Bi3B5O12,

BiB3O6 (bismuth triborate), Bi2B8O15 (bismuth octaborate) and one metastable phase:

BiBO3 (bismuth orthoborate), and its two polymorphs (BiBO3-I and BiBO3-II) are known

to exist. Out of these crystalline bismuth borate phases, Bi2B8O15 has all borons in

tetrahedral coordination with oxygen. Bi3B5O12 contains upto 40 % of boron atoms in

tetrahedral co-ordination [Kityk and Majchrowski (2004)], while Bi4B2O9 phase has only

isolated trigonal BO3 structural units [Levin and Daniel (1962]. Recently Egorysheva et

al. (2005) prepared single crystals of all bismuth borate phases and characterized them by

mid-infrared absorption spectroscopy. The IR peak positions like the X-ray diffraction

23

peaks are characteristic and are very useful for phase identification [Egorysheva et al.

(2005)].

The structure and properties of bismuth borate glasses has been investigated by

several authors. George et al. (1999) extended the glass formation range of Bi2O3-B2O3

system to 88 mol% Bi2O3 by using roller quenching method and reported very high

density values ~9 g cm-3 at high Bi2O3 concentration. These authors found a maximum in

glass transition temperature, Tg, at Bi2O3 concentration of 23 mol%.

A very interesting glass matrix effect on the UV-visible absorption and

fluorescence properties of bismuth borate glasses has been reported recently [Murata and

Mouri (2007)]. It has been found that Bi ion containing borate glasses show an optical

absorption band around 440 nm, which is absent in silicate glasses. It is further reported

that there is a large influence of melting conditions like highest melting temperature, on

the optical properties of oxide glasses containing Bi2O3 [Sanz et al. (2006)].

Transmission electron microscopy (TEM) studies by these workers concluded that at high

bismuth oxide concentration, Bi3+ ions reduce to Bi2+. The oxidation state of Bi ions can

critically influence the optical absorption and fluorescence properties of bismuth borate

glasses.

Kim et al. (2007) investigated Pb free Bi2O3 –B2O3-SiO2 glasses as a function of

Bi2O3 content and evaluated glass transition temperatures, the optimum sintering

temperatures and the crystallization temperatures of the glasses. They found that both Tg

and Tc decreased as the Bi2O3 content increased. Many studies have been made on

borosilicate glasses using MAS NMR and Raman spectroscopy. These have focused on

changes in the structure of the borate and silicate glass networks, as a function of alkali

content. Structural studies, using 11B, 29Si and 17O (MAS NMR), have led to widely

accepted models for both the borate and borosilicate systems [Jellison et al. (1978); Feller

et al. (1982); Roderick et al. (2001); Dell et al. (1983) and Chen et al. (2004)].

Combining bismuth oxide with boric oxide thus allows to tune the optical

properties in a wide range depending on the composition. Both, refractive indices and

ultraviolet absorption edge, show an expressed dependence on composition [Opera et al.

24

(2004)]. The decrease in the values of the glass transition temperature (Tg), from DTA

studies and optical band gap from optical transmittance analysis, and increase in UV and

IR cut-off wavelength from transmittance analysis indicate that the glass network

becomes less tightly packed and degree of disorder increases with increase of

concentration of Bi2O3 in bismuth borosilicate glass system [Gao et. al (2009)]. Yildirim

and Dupree (2004) use 17O 3Q MAS NMR technique to study Na-aluminosilicate glasses.

They observed main peak and small peak attributed to Si-O-Al site and Al-O-Al site

respectively on the basis of their quadrupole coupling constants. Terashima et al. (1997)

have measured the third order nonlinear optical properties of PbO-Bi2O3 –B2O3 glasses

by the third harmonic generation method and investigated the structure using Raman and 11B NMR spectroscopy. Sen et al. (1998) have used high resolution NMR spectroscopy

to quantitatively determine temperature dependent structural changes in Na-borate, Na-

borosilicate and Li-boroaluminate liquids. They found transformation of BO3 units from

boroxol rings to non-ring configuration in the borate and BO4 units into asymmetric BO3

units in the borosilicate liquid with increasing temperature.

Fujimoto and Nakatsuka (2006) discovered a new infrared fluorescence from

bismuth doped silica glass. These authors used 27Al NMR for characterization of their

glasses. They found that aluminium ions effectively help construct the Bi luminescent

center because; due to the presence of aluminium in these glasses the luminescent

intensity is drastically increased. The role of Bi2O3 in phosphate glasses has been

investigated by Rani et al. (2008). They observed increase in density and shift of cut off

wavelength (λcut-off) towards red with the increase in Bi2O3 content which is due to the

increase in non bridging oxygens. The structural investigations also showed the rapid

depolymerization of phosphate chains with increase in Bi2O3 content and formation of P–

O–Bi bonds. Saritha et al. (2008) studied ZnO-Bi2O3- B2O3 glasses and measured their

optical and structural characteristics. They found an expected decrease in glass transition

temperature (Tg), shift of the absorption edge or cutoff wavelength to longer wavelength

and increase in density with the increase in Bi2O3 concentration. Si et al. (2000) have

studied the non-linear optical properties of Bi2O3-B2O3 –SiO2 glasses with high Bi2O3

content. The large third-order nonlinear optical susceptibility of bismuth glasses which

25

originates from pure electronic polarization has been found to be beneficial for

photoinduced (SHG) second harmonic generation [Si et al. (2000)]. Similarly Lin et al.

(2007) have reported ultrafast third order non-linear optical properties of Bi2O3-B2O3-

SiO2 doped with GeO2 glasses (known as BI glass). Er3+ doped bismuth borosilicate

glasses are also considered to be promising materials for 1.5 m optical amplifier

[Tanabe et al. (2000), Dai et al. (2006)]. Although bismuth borosilicate glasses are

promising materials for variety of optical applications, detailed studies on their structure

and properties have not been carried till date.

1.3.4 Structure and properties of silicate glasses:

Most glasses used in industry contain numerous components out of which silica (SiO2)

forms the main ingredient. SiO2 glass by virtue of its composition and technical

applications has a prominent position among the single component glass and is called

vitreous silica. In the vast majority of silicates, the Si atom shows tetrahedral

coordination, with 4 oxygen atoms surrounding a central Si atom. The most common

example is seen in the quartz crystalline form of SiO2. In each of the most

thermodynamically stable crystalline forms of silica, on average, all 4 of the vertices (or

oxygen atoms) of the SiO4 tetrahedra are shared with others, yielding the net chemical

formula: SiO2, Crystalline silicon dioxide has a number of distinct crystalline forms

(polymorphs) in addition to amorphous forms. The structure of covalent amorphous

material, such as a-Si and a-SiO2 can be best described as continuous random network:

each atom in the amorphous solid has the same number of covalent bonds as in their

crystalline phase; the amorphous nature of the structure is reflected by the random

network made by the covalent bonds. An illustrative picture of an amorphous silicon

dioxide structure is shown in Fig. 1.12.

Usually, the glass formation process is treated taking into account bound strength

considerations. The stronger the bonds are in the melt, the more sluggish the

rearrangement process will be. Silicate glasses are well described by the continuous-

random-network model of Zachariasen (1932). Although borate glasses are interpreted

using the same concepts as silicate glasses the borates exhibit many peculiarities that

26

need additional discussion [Weyl and Marboe (1964) and Vogel (1994)]. Silicates have

complex structure, which changes from random network of pure silica to a gradually

broken-down structure containing chains and/or rings, upon addition of metal oxides to

the to the system [Zhang and Jahanshahi (1998)].

Fig.1.12. The continuous random network structure of amorphous silicon dioxide, notice

that each Si atom (gold sphere) has 4 bonds and each oxygen atom (red sphere)

has 2 bonds.

In silicate glasses, silicon occurs in tetrahedral coordination with oxygen and its

coordination and oxidation state remains fixed unlike borates, where boron atoms can

have triangular and tetrahedral co-ordination with oxygens. Qn nomenclature is used to

indicate the number of bridging oxygens (BO) per SiO4 tetrahedron, where n is the

number of bridging oxygen (BO) and (4-n) is the number of non-bridging oxygen (NBO)

in the silicate network. The number of BO’s in SiO4 tetrahedron can vary from 0 to 4

[Parkinson et al. (2007)]. From the variation of Qn, one can determine the polymerization

state of silicate network and understand the structural modifications that occur in silicate

27

network with the addition of metal oxides like PbO [Parkinson et al. (2007) and Fayon et

al. (1998)]. 29Si MAS-NMR studies on lead silicate glasses determined that the number

of NBO’s per SiO4 tetrahedron increases steadily with PbO content, as a result, the glass

transition temperature decreases continuously upon adding PbO and parallels with the

depolymerization of the silicate network [Fayon et al. (1998) and Zahra et al. (1993)].

Suzuya et al. (1997) have reported finding intermediate-range order in lead metasilicate

(50PbO-50SiO2) glass by x-ray and neutron scattering studies. Watanabe and Tsuchiya

discovered two-photon absorption and non-linear refraction in commercial lead silicate

glass at 532 nm [Watanabe and Tsuchiya (1997)].

The importance of borosilicate glasses that contain both B2O3 and SiO2 is

demonstrated through their large variety due to their outstanding properties like chemical

resistance and high softening temperatures.. Chemically and mechanically resistant

materials constitute one of their most important large-scale industrial applications.

Moreover, new and forefront technologies induce the development of materials for which

borosilicate glasses are also candidates. In particular, their use as sealing components

constitutes one important domain in the current materials technology. Conventional glass

compositions have been traditionally used as sealants in television tubes or bulb lamps,

but the increasing development of microelectronics and new demands, like the worldwide

research programs on fuel cells, necessitates new glasses. Recent works have concerned

this type of special sealing glasses; in particular, borosilicate glasses have been developed

as sealants for Molten carbonate fuel cells (MCFC) [Pasucal et al. (2002) and Pasucal et

al. (2002)]. The structure of borosilicate glasses has been widely studied [Xiao (1981),

Zhong and Bray (1989) and Dell et al. (1983)]. Martens and Mu¨ller-Warmuth (2000)

have also demonstrated that both borate and silicate networks and modifier cations are

statistically mixed, with no evidence for distinct compositional regimes.

1.4 Scope of the Thesis:

Borates have been subject of interest for many decades, motivated by their

extraordinary optical properties. In the literature variety of alkali, alkaline, heavy metal

oxide and rare earth ion containing borate and borosilicate glasses have been prepared

28

and characterized by techniques like UV-visible, IR, Raman spectroscopy, NMR, X-ray,

neutron diffraction and x-ray photoelectron spectroscopic techniques. Because boron may

adopt not only triangular but also tetrahedral coordination with oxygen, borate glasses

unlike the silicates have many anomalous properties. The structure of borate glasses is

quite different from that of silicates and boron oxide is widely used to make borosilicate

glasses which have excellent thermal properties. In this thesis we made structure-property

correlations in lead and bismuth borosilicate, aluminoborate and aluminosilicate glasses

by density measurements, DSC, UV-visible optical absorption high field 11B and 27Al

MAS-NMR spectroscopy studies. A direct and accurate measurement of boron co-

ordination number in borates and borosilicates can be carried out by 11B MAS NMR

spectroscopy.

The objective of this thesis is to measure the boron co-ordination number in lead

and bismuth borate, borosilicate, aluminoborate and boroaluminosilicate glasses, and the

concentration of tetra, penta and hexa coordinated aluminum-oxygen units in glasses

containing alumina, and correlate this structural information with glass density, optical

and thermal properties.

29

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