A general introduction on luminescence especially photoluminescence (PL) and

thermoluminescence ITL) has been presented A discussion on kinetics of

luminescence and different decay mechanisms is included. The methods of

preparation, properties, and applications o f different types of luminescent

materials are incorporated. The physical processes of doped phosphors and

doped glasses are discussed in a more general way with special emphasis on

glass formation from melt and sol-gel processing. The advantages and merits of

glasses as hosts for various rare earth ions and luminescent species are detailed.

The spectroscopic and optical features of rare earth ions in phosphors and

glasses, which are instrumental in arriving at qualitative and quantitative results,

are described. A brief account of enera transfer processes of rare earth doped

glassy materials has been incorporated The properties of alkaline earth sulphide

(AES) phosphors, their importance and applications in modern technology are

also outlined.

Chapter 1 2

1.1. Luminescence

Luminescence is a science closely related to spectroscopy, which is the study of

the general laws of absorption and emission of radiation by matter [I]. The

existence of luminous organisms such as bacteria in the sea and in decaying

organic matter, glow worms and fireflies have mystified and thrilled man since

time immemorial. A systematic scientific study of the subject of luminescence is

of recent origin, from the middle of nineteenth century. In 1852 English Physicist

G.C.Stokes identified this phenomenon and formulated his law of luminescence

now known as Stoke's law, which states that the wavelength of the emitted light

is grater than that of the exciting radiation. German physicist E. Wiedemann

introduced the term 'luminescence' (weak glow) into the literature in 1888. The

phenomenon of certain kinds of substance emitting light on absorbing various

energies without heat generation is called luminescence. Luminescence is

obtained under variety of excitation sources [2]. The wavelength of emitted light

is characteristic of the luminescent substance and not of the incident radiation [3].

The various luminescence phenomena are given names based on the type of

radiation used to excite the emission (Table 1 .I).

Table 1.1. Luminescence phenomena and the methods of excitation

General introducrron -- 3

The last three phenomena are linked together for the time scale over which the

light emission takes place. Fluorescence is a luminescent process, which persists

only as long as the excitation is continued. The decay time of fluorescence is

independent of temperature; it is determined by the probability of transition from

the excited level to the ground state. Phosphorescence is luminescence observable

after the removal of exciting source. The decay time is dependent on temperature.

Substances emitting luminescence are called luminophors or phosphors.

Luminescence study of materials in the form of polycrystalline phosphors,

crystalline solids. noncrystalline solids and glasses are reported [3]. Most

phosphors are basically semiconductors. describable in terms of energy band

model with valence and conduction bands and with localized energy levels in the

forbidden regions between the bands. The localized centers are associated with

the impurities or imperfections in the host lattice. Impurities that provide levels,

which permit radiative transitions, are called activators. Generally these levels

close to valence band are when occupied by electrons can also act as traps for

valence band holes. Certain other impurities can provide levels close to

conduction bands, if empty may act as electron traps. Typical excitation

mechanisms of luminescence involve raising an electron from the filled band or

from a filled activator level to the conduction band, or from an activator ground

level to some higher activator levels. Those electrons that reach the conduction

bands can return to activator level or may be trapped. If trapped it can return to

conduction band by absorbing sufficient thermal or other foms of energy. The

return of an electron from conduction band to an empty activator level yields

luminescence. Radiationless transition may also occur to trapped or free holes in

which energy is transferred to lattice in the form of phonons [I]. The complex

consisting of activator impurity and surrounding disturbed host lattice where

these transitions take place is known as 'luminescence center'.

Radiations from transitions between excited and ground state of activator produce

fluorescence while the delayed return of electron from traps through the

conduction bands yields phosphorescence. At low temperatures phosphorescence

can be 'frozen in' such that thermal energy is not sufficient for the release of

Chapter I 4

trapped electrons. Thermoluminescence (TL) is the release of the ' frozen in'

phosphorescence by raising the temperature. It is also known as thermally

stimulated luminescence (TSL) or thermally stimulated relaxation.

1.2. Photoluminescence

Luminescence in solids is the phenomenon in which electronic states of solids are

excited by some energy from an external source and the excited energy is

released as light. When the energy comes from short wavelength light, usually

ultraviolet light the phenomenon is called photoluminescence (PL). PL in solids

is classified in view of the nature of the electronic transitions producing the

luminescence. In the case of PL a molecule absorbs light of wavelength h ~ ,

decays to lower energy excited electronic state and then emits light of wavelength

h2 as it radiatively decays to its ground electronic state. Generally the wavelength

of emission h2 is longer t w the excitation wavelength, but in resonance

emission hl=h2.. Luminescence bands can be either fluorescence or

phosphorescence, depending on the average lifetime of the excited state, which is

much longer for phosphorescence than fluorescence. The relative broadness of

the emission band is related to the relative difference in equilibrium distance

between the excited emitting state and the ground electronic state. PL of a

molecular species is different from emission from an atomic species. In the case

Figure 1.1. Partial energy level diagram of a photoluminecent molecule. SI & S2

are singlet states and TI the triplet states

General introduction 5

of atomic emission both the excitation and emission are at the resonance

wavelengths, in contrast excitation of a molecular species usually results in an

emission that has a longer wavelength than the excitation wavelength. PL can

occur in gas, liquid and solid phases. An energy level diagram as in Figure 1.1

can illustrate the radiative and non-radiative transitions that lead to the

observation of molecular photoluminescence.

The spin multiplicities of a given electronic state can either a singlet @aired

electrons) or a triplet (unpaired electrons). The ground electronic state is normally

a singlet state and is designated as SO. Excited electronic states are either singlet

(SI. S2) or triplet (TI) states. When the molecule absorbs light an electron is

promoted within 10-14 - 10.'' seconds from the ground electronic state to an

excited state that posses the same spin multiplicity as the ground state. This

excludes a triplet-excited state, as the final state of electronic absorption because

the selection rules for electronic transition dictates the spin state should be

maintained upon excitation. A plethora of radiative and non-radiative processes

usually occur following the absorption light en routc to the observation of

molecular luminescence.

1.2.1.Non-radiative relaxation processes

(a) Vibrational relaxation: -Excitation usually occurs to higher vibrational level

of the target-excited state. The excited molecules normally relax rapidly to the

lowest vibrational level of the excited electronic state. These non-radiative

processes are called vibrational relaxation. It occurs within 1 0 " ~ - 1 0 ~ ~ ~ s, a time

much shorter than the typical luminescence lifetime. So such processes occur

prior to luminescence.

(b) Internal conversion: - If the molecule is excited to a higher energy excited

singlet state than SI (like S2 in Figure 1.1), a rapid non-radiative relaxation

usually occurs to the lowest energy singlet excited state (SI). Relaxation

processes between electronic states of like spin multiplicity such as SI and S2 are

called internal conversion. It normally occurs on a time scale of s

Chapter 1 6

(c) Intersystem crossing: - Non- radiative relaxation processes can also occur

between excited states of different spin multiplicity. Such relaxation process is

known as intersystem crossing. The relaxation from SI to TI in Figure 1.1 is an

example of intersystem crossing

(d) Non-radiative de-excitation: - The above mentioned non-radiative processes

occur very rapidly and release small amount of energy .The rest of the energy is

dissipated either radiatively, by emission of photons, or non-radiatively by the

release of thermal energy The non-radiative decay of excitation energy which

leads to the decay of excited molecule to the ground electronic state is called non-

radiative de-excitation. The amount of energy released in the form of heat is very

small and cannot be measured experimentally. The evidence for non-radiative de-

excitation process is the quenching of luminescence. In solid-state luminescent

materials the crystal vibrations (phonons) provide the mechanism for non-

radiative de-excitation.

1.2.2.Radiative processes -Fluorescence and Phosphorescence

Fluorescence refers to the emission of light associated with a radiative transition

from an excited electronic state that has the same spin multiplicity as the ground

electronic state. The radiative transition SI+ So in Figure 1.1 represents

fluorescence. Since fluorescence transitions are spin allowed they occur very

rapidly and average lifetimes of the excited states responsible for are typically

less than 1w6 s. Electronic transitions between states of different spin multiplicity

are spin forbidden, however it becomes more probable when spin orbit coupling

increases. The net result of spin orbit coupling is the mixing of excited singlet

and triplet states. This mixing removes the spin forbidden nature of the transitions

between pure singlet and pure triplet states. Therefore if intersystem crossing

populates the triplet-excited state then luminescence might occur from the triplet

state to the ground state. Phosphorescence refers to the emission of light

associated with a radiative transition from an electronic state that has a different

spin multiplicity from that of ground electronic state. The radiative transition

TI -+ So in Figure 1.1 represents the phosphorescence. Since phosphorescence

General introduction -- 7

transitions are spin forbidden they occur slowly and so the average lifetime for

such emission typically range from 10" to several seconds. Phosphorescence is

also known as 'delayed fluorescence.'

1.2.3. Configuration co-ordinate model of Luminescence

Most of the luminescent solid materials exhibit broad bell-shaped absorption

bands and emission bands corresponding to smaller energies. For explaining the

luminescence spectra different models were suggested and configuration co-

ordinate model is one among them. It is illustrated in Figure 1.2.

The ordinate is the total energy of the system for ground and excited states of the

center including both ionic and electronic terms. In the diagram the energy is

shown to vary parabolically as some co-ordinate. The configuration co-ordinate

represents changes of nuclear co-ordinates of all the lattice ions constituting the

Configuration eo-ordinate * Figure 1.2 illustration of configuration co ordinate model

luminescence center. There is a value for the co-ordinate for which the energy is

minimum, but this value is different for ground and excited state because of

different interaction of luminescent center with its neighbours [I]. Absorption of

a quantum of light causes a transition from A to B. The lifetime of the excited

state is of the order of 1w8s in the case of allowed transitions and is much longer

than the lattice vibration period. So just after the absorption has terminated

relaxation toward the minimum energy point in the excited state takes place

Chapter I 8

accompanied by the emission of phonons and reaches the position 'C'. From 'C'

it comes to the ground state 'D' by emitting a quantum of luminescent light. Then

the system goes back to A by means of energy dissipation through lattice

vibrations. This model can also explain the case governed by Stocke's law.

When the system is in equilibrium position 'C', it is not at rest, but migrates over

a small region around "2' because of the thermal energy of the system. As a

result the emission transition is not just to point 'D' on the ground state but

covers a region around 'D'. In the vicinity of D the ground state curve shows a

rapid change of energy so that even a small range of configuration co-ordinate

leads to a large range of energies in the optical transition. This explains the broad

emission and absorption bands observed

1.2.4. Kinetics of Luminescence

One generally comes across two kinds of luminescence processes. They are

(a) kinetics of first order (monomolecular mechanism) (b) kinetics of second

order (bi -molecular mechanism)

(a) First order kinetics

The number of excited electrons N decreases according to a constant probability law

dN/N = -adt

which gives the solution

N = N,e-"

The luminescence intensity I a dNIdt

Then I = 1,e'"'

The major characteristic is lifetime, which is the average stay of ion in a given

excited state.

(b) Second order kinetics

In this case the probability for recombination is proportional to the number of

centers. Then

General introduction - 9

Therefore

N = N,, /(l + Noat) ......... I . 6

Which shows that N decreases hyperbolically with time.

Since I cc dNidt

I = I , /(I + at)' ......... I. 7

where a = (1,a)' ' ......... I. 8

Here the decay become more rapid as the excitation intensity is increased. The

above kinetics can be applied only if the optical transition is associated with

dipole radiation or the lifetime .r - second. In the case of phosphorescence

the kinetics involved in the process depends on the spatial relation between

luminescence centers and on the motion of conduction electrons. The after glow

is longer at lower temperatures and shorter at higher temperatures showing the

strong dependence of phosphorescence on temperature.

1.2.5.Different Decay mechanisms

The process of phosphorescence arises due to a number of complicated terms

such as transition between bands, activators with transfer of electrons and the

hole involving temporary capture by traps, by the presence of traps with different

depths, by possible repeated trapping etc. In order to explain the mechanism of

decay, different methods are suggested.

(a) Simple exponential decay

The luminescence intensity from N atoms will be given by

I a -dN/dt = uN ......... I . 9

i.e. the number of photons emitted or the number of atoms de-excited at time t is

equal to the rate constant u times the number of excited atoms existing at time t.

Integration of this equation yields

Chapter I 10

where the rate constant a is equal to the reciprocal of the life time .r of the excited

state

- E / k T ......... i.e. a = l / r = S e 1. I 1

where S is the frequency factor, s is the time taken by the system to decay to e-'lo

ie.0.36788 lo, where I0 is the luminescence emission intensity at t = 0 ( i.e. at the

instant of cessation of excitation). Hence

So a plot of logarithm intensity I versus time t is a straight line for an exponential

decay. There are no external constraints on the free atoms assumed in this

example so that r is about lo-' seconds. A decay curve is intensity versus time

plot during phosphorescence, and the rate of decay at time t is the slope of the

tangent of the decay curve at the same time.

(b) Hyperbolic decay

This type of decay takes place in a system where on irradiation shallow traps are

also produced along with deeper traps. The decay of phosphorescence occurs

generally in two or three groups depending on the nature and concentration of the

traps. In such cases the observed intensity will be due to the superposition of all

the exponentials col~esponding to different traps and represented generally as

I, = l,Cb ......... 1. 13

where b is the decay constant. Also in this case logarithm of intensity I versus

time t plots are nonlinear indicating the nonexponential form of decay. However

the plots In I versus in t show linearity and the decay is said to be a hyperbolic

one. Again

I = ~ , t - ~ = CI~,,, exp(-~,t) ......... I. 14 m

where 10, is the phosphorescence intensity due to electrons in traps of energy Em

and

General introduction -

is the probability of an

~ . , < \ .

:, 'C ; .%

-- , , , 11 , .. " '

:* . electron escaping from'the trap. H ~ c ~ . & & h a l y s e s .,.- of the

decay curve are usually done by adopting the peeling - off procedure, where each

stage of decay will be associated with an exponential law [4]. The trap depth

corresponding to each exponential is calculated from the slopes of straight lines

of the semi-log plot using the relation

E = kT ln(S I a) ......... I. 16

where a is the slope of each linear portion of the curve. The number of

discontinuities in the in I versus In t plots correspond to the number of straight

lines into which the semi log plots can be split. The value of b is indicative of the

decay rate and also provides information about relative population of trapping

states at various depths. The relative population of trapping levels N, at t=O can

be obtained by the extrapolation of the In I versus time plot using the relation

where ~,=l iP , is the life time of the electron trapped in the trap of depth En. So

the ratio of N,(t)t=o/N.(t)t=l for the peeled off components of the graph indicate

the idea about the distribution of trapping levels.

1.2.6.Techniques to study optical properties of condensed matter

(a) Absorption spectra:- To study optical behavior of impurity atoms in

condensed matter such as crystals, phosphors, and glasses very simple method is

used. To characterize the material one can measure its absorption spectrum in the

uv-visible region [ 5 ] . It helps to identify the active impurities and gives

information on their environment. Now a days the measurement can be easily

done by using computer-controlled spectrophotorneters. In crystalline hosts the

polarization of absorption spectra can be used to conclude the site symmetries

and crystal field parameters. Along with ground state absorption to investigate

the excited state absorption characteristics, modified techniques are available by

using two excitation sources, one powerful beam to excite the material and the

other to measure the absorption after the excitation pulse.

Chapter I 12

(b) Excitation and emission spectra:- In certain materials ground state

absorption is too weak to measure [1,6]. A method known as zero order

excitation spectroscopy will provide equivalent information. Emission at all

wavelengths is directed to a detector without dispersion. By varying the

wavelength of exciting beam and recording the strength of emission at all

wavelengths, a spectra equivalent to that of absorption of all luminescent centers

can be recorded.

By using modem spectrofluorimeters two different types, the emission and

excitation spectra can be recorded. With special accessories the phosphorescence

spectra also can be recorded. In excitation spectra the emission wavelength that

corresponds to the emission peak of the sample is fixed whereas excitation

monochromator is scanned. In emission spectra the excitation wavelength is fixed

and emission monochromator is scanned. The excitation is usually fixed for a

wavelength at which the sample has significant absorbance. The much higher

sensitivity of luminescence technique compared to absorption technique is an

obvious advantage for excitation spectra over absorption spectra. Also the

relative ease of acquiring excitation spectra of materials like solids represents

another advantage. Finally excitation spectra can provide valuable information

about excited state processes such as energy transfer that cannot be obtained by

absorption spectra.

Thermoluminescence (TL) means not temperature radiation but enhancement of

the light emission of materials already excited electronically by the application of

heat. TL can be distinguished clearly from incandescence emission from a

material on heating. In incandescence, which is classical in nature, radiation is

emitted when the material is very hot. This radiation is in the invisible-far

infrared but at higher temperature, shifts to the visible region [7]. The

fundamental principles which govern the production of TL are essentially is the

same as those which govern all luminescence processes and hence TL is one

member of a large family of luminescence. The phenomenon is observed with

General introduction 1.4

some minerals and, above all, with crystal phosphors after they have been excited

by light. The crystal phosphors or doped glasses, which respond to TL, contain

certain traps. The traps are imperfections in the crystal lattice where electrons are

captured after they have been ejected from a luminescent centre by excitation

energy. The luminescent properties of phosphor centres are strongly dependent

on the chemical nature of the host crystal, showing that the same activator ions,

in different host crystals yield remarkably different-coloured emissions and decay

times. Prolonging the emission time of phosphors up to days or even longer

(production of phosphorescence of the phosphors) is possible by inserting traps

into the host crystal. Trapped electrons cannot return directly to the centre. In

order to be released from the traps they must first obtain additional thermal

energy-in this case, thermal energy stimulates luminescence-after which they

recombine with a centre and undergo radiative transition [8].

V K Mathur et a1 studied high dose measurements using thennoluminescence of

CaS04:Dy and reported that the range of high dose measurements can be

increased by an order of magnitude by increasing the concentration of

dysprosium in CaS04:Dy. Recently many works were reported on CaSO., doped

with rare earth ions ce3+ and sm3+ [9-111. Earlier workers suggested that the glow

peak and hence the trap depth depends on the property of host materials. But

recent observations [12] on doped phosphors shows that the broad emission is

quenched in favor of emissions from the rare earth (RE) impurity sites and the

degree of quenching varies between the REs. The spectral measurements showed

that the host material has minimal effect on the glow peak temperatures, T,,.

Above room temperature, the glow peaks are specific to the added RE ions and

do not show common peaks.

1.3.1.Understanding of TL on the basis of energy band model.

The phenomenon of TL can be understood by the energy band diagram of an

insulator. in which the forbidden gap between the valence band and conduction

band is of the order of few eV. According to this model the trace of impurities

and defects responsible for the luminescence in solids introduced in the host

Chapter 1 14

lattice by heat treatment can be imagined to form discrete energy (donor1

acceptor) levels within the forbidden energy gap. Other impurities and lattice

defects provide unoccupied energy levels called traps that have the capability of

detaining the charge carriers before their recombination with the luminescent

centers. These traps are denoted as hole traps or electron traps depending on

whether the trapped carriers are holes or electrons [I]. A schematic representation

of a simplified energy band model is shown in Figure 1.3 (a, b, c, d, e and f) [13].

When the substance is exposed to ionizing radiations, some of the electrons from

the valence band are sufficiently excited to reach the conduction band. While

most of them fall back immediately to valence band accompanied or un

accompanied by light emission or internal heating, some of them get trapped in

the forbidden gap at the donor level D or accepter level A (Figure 1.3 a).

The temperature at which the light emission occurs depends on the depth at

which the traps are located since it decides the amount of thermal energy required

Irradiation (a) (b) (4

Figure 1.3. Band model diagrams for TL process in an insulating crystal (a) On Irradiation (b) - (9 Alternative process on heating. D -Donor level A- Acceptor level, Ee and E h thermal activation energies.

General infroduction 15

to accomplish the detrapping and the consequent recombination of the electrons

and the corresponding holes. In simple cases the liberated trap can move to its

still trapped counterpart and recombine to give TL (Figure 1.3 b & c). Both

acceptor and donor levels can simultaneously move to meet and recombine at an

entirely new location known as luminescence or recombination centre L and give

TL (Figure 1.3 d). A trapped charge always has a finite probability that it gets

retrapped and then gets recombined. But it also has a finite probability that it get

retrapped. (Figure 1.3 e). The recombination probability for the detrapped charge

may change with time as the heating progresses. This is called as the second

order process where the recombination probability is constant with time, are

called the first order process. An isolated case where the detrapped charge gets

recombined without having to be excited to the valence band or to the conduction

band is also possible (Figure 1.3 f) which is also a first order process.

1.3.2.Thermoluminescence- configuration coordinate diagram

When TL occurs in isolated luminescence centers the process can be described by

the configuration coordinate diagram [ I]. The curves in Figure 1.4 represent the

potential energy of the luminescence center as a function of the distance between

---+ Configurational Displacement , X

h 313

2 W 3 . - * r: 3 8

T Figure 1.4. Configuration co-ordinate diagram for an impurity atom in an insulator

Excita 'on hv Emission hv'

an impurity atom and the first nearest neighbour called the configuration

coordinate. The configuration coordinate diagram defines the potential energy Ug

when the system is in the ground state and Ue when it is in an excited state [I].

Chapter I 16

During the irradiation the center will be raised to its excited state Ue (transition

AA'), where the system will be in a higher vibrational state and so relaxes to the

stable configuration B, dissipating the excess energy to the lattice in the form of

heat. The centre will emit light through the transition BB' and will again be in the

ground state Ug. The centre still being in the higher vibrational energy level in

the ground state will come to its minimum energy position A with a further

nonradiative loss of energy. The excited centre may also get trapped at T through

CC', wherein after remaining for some time t it might escape to the excited state

via CC' emitting light via BB'. Rejumping of the centre from T via C'C to the

excited state depends upon the probability of escape p per unit time denoted by

p = 1 1 ~ =S exp(- EIkT) where S is the frequency factor and E is the energy

required for the release of the center from the trap to reach its excited state. The

emission is now delayed (phosphorescence) by an amount of time that depends

upon the time .r that the carrier spends in the trap. In the case of deep traps the

heat energy at room temperature may not be sufficient to raise the centre to its

excited state. Here heating of the luminescent material after the irradiation has

been removed will raise the trapped center via C'C giving rise to emission via

transition BB'. It is this emission during heating that is called TL, and a plot of

intensity vs temperature is called glow curve.

1.3.3.Mathematical treatment

The occurrence of TL during a thermal scan of a previously excited material is

probably one of the most direct evidences that we have for the existence of

electronic trapping levels in these materials. TL spectrum normally consists of a

number of overlapped peaks, which are rarely isolated. Randall and Wilkins [14]

proposed the first theory of TL for first order kinetics followed by Garlick and

Gibson for second order kinetics [15].

During the irradiation of a sample by X-rays, Gamma rays or UV rays electrons

are excited from the valence band to conduction band leaving holes in the valence

band. Both electrons and holes move in the respective bands until either each

finds a localized defect where they are trapped or might recombine directly with a

General introduction 17

charge carrier resulting the emission of light. The localized energy levels below

the conduction band where the electrons are trapped are called electron traps.

Similarly the energy levels just above the top of valence band is known as hole

trap as it can trap holes present in the valence band. From thermodynamic

considerations it can be shown that the mean time .r spent in trap by an electron or

a hole can be written as

r = S" exp (EkT) ......... I . 18

where S is known as the frequency factor, k is Boltzmann's constant and T is the

temperature. During heating a trapped electron will gain energy to escape from

the trap and might recombine with a hole trap resulting in the emission of light.

In the first order kinetics (b=l) model of Randall and Wilkins, the possibility of

re-trapping is neglected .In such a case the rate of recombination is equal to rate

of release of electron from the traps. So the TL intensity I (t) as a function can be

expressed as

I(t) = -dn/dt = nS exp(-EikT) ......... I. 19

where n is the density of trapped electrons .For a linear heating profile

T = T , , t pt ......... I. 20

where To is the initial temperature at time t and P is the rate of heating

From eqs. (2) and (3) one can write

I (T) = no (S) exp (-E/kT) exp [(-SIP) exp ( - E ~ T ' ) d~I .1 ......... I. 21 i o

The TL intensity I(T) is maximum at a temperature T = T, given by

PE/(~T,~) =S exp(E/kT,) ......... I. 22

From equation (5) it is clear that the peak temperature T, depends on E, S and P.

1.3.4.Factot-s affecting TL

The factors on which the TL emission intensity and glow peak temperature

depend on various factors. In general highly pure substances are poor TL

exhibitors. For TL to be shown, the existence of defects or imperfections in the

crystal lattice is essential. An impurity that causes TL is called an activator.

Chapter 1 18

Certain activators activate the TL in the host whereas some impurities quench or

kill the luminescence present in the phosphor, are known as poisons. The

activator that increases the luminescence already present in the TL material is

called a sensitiser. Generally an activator is a luminescence emission center, a

poison prevents the energy transfer to the emission center and sensitiser increases

the energy absorption to useful luminescence emission [16,17].

Any thermal treatment after irradiation essentially erases the TL signal. A

thermal treatment before exposure increases the number of defects in the

substance resulting into enhanced sensitivity. The number of defects retained by

the crystal also depends on the rate of cooling employed in the thermal treatment

[18,19]. In the preparation of TL materialsflux is used to incorporate the dopants

into the matrix and generally it does not introduce any new trapping sites. This

indicates that flux serves to alter the relative importance of different group of

traps and not their mean trap depth [20].

If very high dose ofirradiation is given to a sample it can enhance the TL [21].

In some cases high dose of radiation causes crystal damages like production of

voids, aggregates etc. which reduces the TL output [22]. Some phosphors after

irradiation with gamma rays are exposed to UV rays may show some bleaching

effect in the gamma induced TL [23]. Preparatory parameters also affect the TL

properties. The storability of the phosphor can be improved by varying the

preparatory parameters like duration of firing rate of cooling ambient

atmosphere etc [20].

Rao et a1 reported that the TL output decreases with decrease of grain size [24].

Size of the particle influences the excitation of a phosphor as well as the emission

output by scattering and self absorption characteristics [25]. The fading behavior

of TL with storage for different duration of time is an important parameter in TL

dosimetry. Mathur et a1 [26] observed peak shift with decay time in case of

CaS:Ce. This effect may be due to the broad distribution of trap depth, which is

responsible for the release of electrons from the traps as they have two kinds of

energies; one is the result of thermal vibration and the other due to the

General infroduction 19

interaction-taking place between electrons trapped at the same depth. It has been

found that application of high electric fields on a TL phosphor during heating

enhances the TL output and sometimes affects the nature of emission [27]. This

effect was explained as due to either field ionization of electron traps or

acceleration of electrons after thermal release from traps.

In addition to the thermal and optical quenching discussed, quenching of

luminescence is also achieved by concentration quenching. 'Nambi et a1 [I31

studied the effect of Dy and Tm doping on the TL output from CaS04:Dy and Tm

are found to be important activators of the luminescence in the system such that

the TL intensity increases as the dopant concentration increases. However at

larger concentrations (= 0.1%) the TL output decreases gradually as the dopant

levels are increased further. This effect is known as concentration quenching. It

is stated that this behavior is characteristic of isolated activator centers. When one

activator ion is located in a certain radius of another, the luminescence is

quenched [3]. A separate effect known as impurify quenching is found to occur

due to the action of 'killer' centres. Upon introducing certain elemental

impurities especially heavy metals like Cu, Fe, Ni, and Cr into a TL material the

intensity of luminescence emission is seen to reduce drastically [28].

1.3.5.TL Applications

The applications of TL are numerous. Besides the measurement of radiation

exposure in nuclear plants and in medical fields, it is used in archaeology for the

determination of the age of the ancient objects and for checking its genuineness.

Also it is used for dating and deciding the origin of geological formation and

possibly for ore prospecting and earth quake predictions. It can also be used in

forensic science and in a wide range of quality control and analysis [29]. The TL

investigations of certain metal oxides, ceramics, and various papers are reported

which have been found to be suitable for accidental dosimetry to be used with TL

technique [30]. The 'rL studies on different type of cements by Gartia et a1

suggested that the same could be used as a tool quality control and in forensic

science [3 I]. Few of the recent trends are discussed below.

Chapter I 20

(a) Radiation Dosimetry

A technique commonly applied in radiation dosimetry is the use of

thermoluminescent dosimeters (TLDs). This technique is based on the use of

crystalline materials in which ionizing radiation creates electron-hole pairs.

During the period of exposure to the radiation, a growing population of trapped

charges accumulates in the material. The trap depth is the minimum energy that is

required to free a charge from the trap. It is chosen to be large enough so that the

rate of detrapping is very low at room temperature. Thus, if the exposure is

carried out at ordinary temperatures, the trapped charge is more or less

permanently stored. After the exposure, the amount of trapped charge is

quantified by measuring the amount of light that is emitted while the temperature

of the crystal is raised. The applied thermal energy causes rapid release of the

charges. A liberated electron can then recombine with a remaining trapped hole,

emitting energy in the process. In TLD materials, this energy appears as a photon

in the visible part of the electromagnetic spectrum. The total intensity of emitted

light can be measured using a photo multiplier tube set up and is proportional to

the original population of trapped charges. This is in turn proportional to the

radiation dose accumulated over the exposure period. The readout process

effectively empties all the traps, and the charges thus are erased from the material

so that it can be recycled for repeated use. One of the commonly used TLD

materials is lithium fluoride, in which the traps are sufficiently deep to prevent

fading, or loss of the trapped charge over extended periods of time. The elemental

composition of lithium fluoride is of similar atomic number to that of tissue, so

that energy absorbed from gamma rays matches that of tissue over wide energy

ranges [3].

Dosimeters are the instrument that measures exposure to ionizing radiation over a

given period. UV and nuclear radiations incident on a TL sample produce mobile

electrons and holes, which are caught in their respective trap states within the

band gap of the sample. The populations of these occupied traps are proportional

to the incident radiation dose. As TL intensity is a measure of the occupied trap

density the sample can act as a TL dosimeter. There are three types of dosimeters

General introduction 21

worn by persons who work with or near sources of radiation. The film badge is

the most popular and inexpensive. In it, photographic or dental X-ray film,

wrapped in light-tight paper, is mounted in plastic. Badges are checked

periodically, and the degree of exposure of the film indicates the cumulative

amount of radiation to which the wearer has been exposed. Thennoluminescent

dosimeters are nonmetallic crystalline solids that trap electrons when exposed to

ionizing radiation and can be mounted and calibrated to give a reading of

radiation level. The ion-chamber dosimeter, like the thennoluminescent one, is

reusable, but it is self-reading for immediate determination of exposure [32-341.

Several of the TL materials e.g.CaS04:Dy, MgzSi04:Tb, CaFZ:Mn, LiZB407:Mn

and LiF:Mg,Ti have been found especially valuable in this field . Glasses are

also used as TLD materials 1351. Another area of widespread application of TLDs

has been the intercomparison of radiation sources, particularly radiation therapy

equipment on a national or international scale. Monitoring of radiation doses in

body cavities of patients undergoing radiation therapy is one of the earliest

applications of TLD.

(b) Age determination

This application of TL was first suggested by Daniels et al [36] who offered the

premise that the natural TL from rocks is directly related to the radioactivity from

uranium, thorium and potassium present within the material. This radioactivity

results in the accumulation of so called 'geological dose'. If the rate of irradiation

from the radioactive minerals is established, then the length of time over which

the rock has been irradiated can be determined from the relation

Geological age= absorbed dose1 dose rate.

In addition to age determination utilized in geology TL is more sensitive for

detecting traces of radioactivity than conventional methods. Thus the technique

has found widespread application in radioactive mineral prospecting.

(c) Defects in Solids.

Townsend and Kelly 1371 estimated that the TL technique is capable of detecting

defect levels in a specimen. When coupled with the ability to separate these

Chapter I 22

energies of these levels TL provides in principle a unique tool in the

determination of the defect energies. Although it is fruitless to use TL alone to

describe the defect structure of a solid, it is very useful technique when combined

with other measurements.

(d) Forensic Sciences

In this application comparison of evidentiary materials with similar materials of

known origin is made. The material can be glass, soil, insulation material etc that

are encountered in criminal cases. This is exclusionary evidence, in which when

the TL characteristics do not match it can be said certainty that the sample has not

come from its known source [38]. The application of TL methods to detect art

forgeries is also a part of forensic sciences. TL studies of dental enamel is also in

progress, which is of great importance in criminal cases in relation with death of

living things.

(e) Biology and biochemistry

Application of TL in the study of biological and biochemical systems is favored

in recent times. Here the measurements are to be done at very low temperatures

[39,40]. The attempt has been successful in the study of hydroxy and amino

benzoic acids, urea, proteins, plant leaves and bacteria. The inter and intra-

molecular transfer of radiation damage in nucleic acids, proteins and their

constituents could be correlated with their TL behavior.

1.4.Luminescent Materials

Materials and substances that are capable of emitting light, particularly in the

visible range are termed as 'luminescent materials'. There are innumerous

luminescent materials but not all are efficient enough to be put to practical use.

The more efficient materials are those, which are prepared in laboratories and

have a specific composition. These materials exhibit a degree of repeatability and

reliability in their performance, which qualifies them for practical application.

Crystalline phosphors and non-crystalline glasses doped with certain rare earth

ions and transition metals are examples for luminescent materials

General introduction 23

1.4 .l.Phosphors

Solid materials in powder form that give luminescence when suitably excited are

called phosphors. From 1950 onwards the extensive study of luminescence

characteristic of sulphide phosphors has been started. The first phosphor

synthesized was probably an impure barium sulphide prepared with very low

luminescence efficiency and with the serious shortcoming that it was rather

quickly decomposed in moist air, yielding hydrogen sulphide. A more stable

sulphide type phosphor was produced in 1866 by heating zinc oxide in a stream

of hydrogen sulfide. In 1887 it became known that these sulphides do not

luminescence in a chemically pure state but only when they contain small

quantities of a so-called activator metal. The sulphides of zinc and of cadmium

are the most important basic materials of sulphide type phosphors. An important

condition of getting highly efficient phosphors is that these sulphides must first

be prepared to the highest possible chemical purity before the necessary amount

of activator can be added precisely. Effect of impurities on the energy levels of

sulphide phosphors has become an active field of investigation.

Alkaline earth sulphide phosphors activated with specific metallic impurities and

rare earth ions are of considerable practical importance. Many investigators have

reported luminescence studies of alkaline earth sulphides activated with one or

more rare earth ions and transition metal ions 141- 491. Most of the rare earth ions

exhibit good fluorescence In many crystalline phosphors the luminescent

emission originate in impurity systems called activators. In sulphide phosphors,

however these properties seem to be associated more with lattice itself than

activators. The impurities can be introduced in two ways (i) They may be

impurity atoms occurring in relatively small concentration in the host material.

(ii) They may be stochiometric excess of one of the constituents of the host

material, which is called self-activation. The incorporation of an activator in

crystalline solid gives rise to certain localized energy levels in the forbidden

band. Depending upon the energy levels involved we can distinguish

characteristic and non-characteristic luminescence. For characteristic

luminescence the energy levels involved are those of the activator atoms or

modified perhaps by the host lattice. Here an activator atom absorbs the incident

quantum of energy by the transition of one of these electrons from one quantum

state to another. When the excited atom returns to the ground state, it loses a part

of energy due to lattice interaction and hence emits a photon of less energy. In

non-characteristic luminescence a charge transfer through the lattice takes place.

This also involves the energy levels of the host lattice modified due to activator

atoms [42,43].

The activator ions are surrounded by host-crystal ions and form luminescent

centres where the excitation-emission process of the phosphor takes place. These

centres must not be too close together within the host crystal. For high efficiency,

only a trace of the activator may be inserted into the host crystal, and its

distribution must be as regular as possible. In high concentration, activators act as

"poisons" or "killers" and thus inhibit luminescence. The term killer is used

especially for iron, cobalt, and nickel ions, whose presence, even in small

quantities, can inhibit the emission of light from phosphors.

The co-activator is an additional impurity, which is necessary for luminescence in

sulphide phosphors. But it does not have the pronounced effect on emission

spectrum that the activator has. Usually co-activators are identified as donors and

the activators as acceptors. The lack of positive charges created due to the

addition of monovalent or trivalent impurity ions gets compensated adding

suitable flux materials such as sodium thiosulphate. The addition of flux only

serves to alter relative importance of different groups of traps and not their mean

depth or additional trapping levels. The flux facilitates the solution and

distribution of the activators in the host crystal on firing. It probably acts to

provide a charge compensating co activator, although the atoms of the flux do not

always go into the lattice. If they do the flux may also furnish trapping centers [6] .

Only small quantities of fluxes are integrated into the phosphor, but this small

quantity is highly important for its luminescence efficiency. The fluxes act as

coactivators by facilitating the incorporation of activator ions. Thus, many

luminescent centres will be produced, and strong activation will result. In

General introduction 25

describing a luminescent phosphor, the following information is pertinent: crystal

class and chemical composition of the host crystal, activator (type and

percentage), coactivator (intensifier activator), temperature and time of

crystallization process, emission spectrum and persistence. Copper-activated zinc

and cadmium sulphides exhibit a rather long afterglow when their irradiation has

ceased, and this is favorable for application in radar screens and self-luminous

phosphors. Alkaline earth sulphides are very versatile phosphor materials. These

sulphides produce different characteristic emission of different activators and co-

activators and it is possible to prepare hundreds of different phosphors with

different properties. Since their band gaps are large, the excited states of dopants

are not densely distributed between valence and conduction bands. Also these

host crystals provide environment around the impurities [20].

The study of phosphor chemistry has yielded a detailed picture of the role of the

above-mentioned activators and fluxes. Philipp Anton Lenard,, was the first

(1890) to describe activator ions as being distributed in zinc sulphide and other

crystalline materials that serve as the host crystal. That luminescent properties of

a centre are strongly dependent on the symmetry of neighboring ion groups with

respect to the whole phosphor molecule is clearly proved by the spectral shifts of

certain phosphors activated with lanthanide ions, which emit in narrow spectral

regions.

1.4.2. Applications of Phosphors.

Phosphors are transducers. whose output is light, in response to various forms of

energy used as input. The present utilization of phosphors in a variety of devices

is the result of scientific investigations on solid-state luminescence that began a

century ago. Phosphors are widely used for lightning applications and in solid-

state insulator (SSI) laser material research [50]. The classification of phosphors

is based on the type of energy, to be converted. Although there is certain overlap

in the type of materials used in lightning. CRT displays, and electra luminescence

and in thermoluminescence, the mechanisms for light generation are quite

different. In all cases the light generated may not be in the visible, but it could be

Chapter 1 26

in the IR or in the UV region. In either military or security applications,

phosphors can be used for the detection of infrared or near-infrared, radiation by

taking the advantage of outstanding detection sensitivity of photomultipliers. It

should also be possible to resort to IR to visible conversion to improve the light

output from incandescent sources rich in infrared output. Photostimulated

luminescence (PSL) in Eu, Ce and Sm co-doped sulphides (CaS) has been studied

by Kravets in order to develop a novel erasable and rewritable optical memory

using the photoluminescence method.

Some inorganic phosphors known as thermo luminescence (TL) phosphors,

which satisfies the below mentioned features are used in solid-state dosimetry.

(a). A high concentration of electron (or hole) traps and a high efficiency in the

light emission associated with recombination. (b) Sufficient storage stability of

the electrons (or holes) to cause no fading even during slight change in

temperature. (c) Should have resistance against potentially disturbing

environmental factors including light, humidity, organic solvents etc.(d) Low

photon energy response and a linear response over a wide dose range.MgSi04

(Tb), A1203(Dy), CaF2(Dy), CaS04 (Dy), CaF2 (Mn), LiB407(Mn), LiF(Mg,Ti)

etc are some commercially available TLD phosphors [32].

1.4.3.Melt Glasses.

Glass was first made in the ancient world, but its earliest origins are obscure.

Egyptian glass beads are the earliest glass objects known, dating from about 2500

BC [6,51]. Glass is an inorganic solid material that is usually transparent or

translucent as well as hard, brittle, and impervious to the natural elements. Glass

has been made into practical and decorative objects since ancient times, and it is

still very important in applications as diversified as building construction, house

wares, and telecommunications.

It is made by cooling molten ingredients such as silica sand with sufficient

rapidity to prevent the formation of visible crystals. The varieties of glass differ

widely in chemical composition and in physical qualities. Most varieties,

however, have certain qualities in common. They pass through a viscous stage in

cooling from a state of fluidity; they develop effects of colour when the glass

General inrroducrion 27

mixtures are fused with certain metallic oxides. They are, when cold, poor

conductors both of electricity and of heat; most types are easily fractured by a

blow or shock. Commercial glasses may be divided into soda-lime-silica glasses

and special glasses. Soda-lime-silica glasses are made from three main materials,

sand (silicon dioxide. or SiOz), limestone (calcium carbonate, or CaC03), and

sodium carbonate O\ia2C03). Fused silica itself is an excellent glass, but, as the

melting point of sand (crystalline silica) is above 1700°C and as it is very

expensive to attain such high temperatures, its uses are restricted to those in

which its superior properties-chemical inertness and the ability to withstand

sudden changes of temperature.

Glasses of very different, and often much more expensive, compositions are

made when special physical and chemical properties are necessary. For example,

in optical glasses, a wide range of compositions is required to obtain the variety

of refractive index and dispersion needed if the lens designer is to produce

multicomponent lenses that are free from the various faults associated with a

single lens, such as chromatic aberration. High-purity, ultra transparent oxide

glasses have been developed for use in fibre-optic telecommunication systems, in

which messages are transmitted as light pulses over glass fibres.

When ordinary glass is subjected to a sudden change of temperature, stresses are

produced in it that render it liable to fracture; by reducing its coefficient of

thermal expansion, however, it is possible to make it much less susceptible to

thermal shock. The glass with the lowest expansion coefficient is fused silica.

Another well-known example is the borosilicate glass used for making domestic

cookware, which has an expansion coefficient only one-third that of the typical

soda-lime-silica glass. In order to effect this reduction, boric oxide and some of

the lime replace much of the sodium oxide added as a flux by alumina. Another

familiar special glass is the lead crystal glass used in the manufacture of superior

tableware; by using lead monoxide (PbO) as a flux, it is possible to obtain a glass

with a high refractive index and, consequently, the desired sparkle and brilliance.

The agents used to colour glass are generally metallic oxides. The same oxide

may produce different colours with different glass mixtures, and different oxides

Chapter I 28

of the same metal may produce different colours. The purple-blue of cobalt, the

chrome green or yellow of chromium, the dichroic canary colour of uranium, and

the violet of manganese are constant. Ferrous oxide produces an olive green or a

pale blue according to the glass with which it is mixed. Ferric oxide gives a

yellow colour but requires an oxidizing agent to prevent reduction to the ferrous

state. Lead gives a pale yellow colour. Silver oxide gives a permanent yellow

stain. Finely divided vegetable charcoal added to a soda-lime glass gives a yellow

colour. Selenites and selenates give a pale pink or pinkish yellow. Tellurium

appears to give a pale pink tint. Nickel with a potash-lead glass gives a violet

colour, and a brown colour with a soda-lime glass. Copper gives a peacock blue,

which becomes green if the proportion of the copper oxide is increased .An

important class of materials is the chalcogenide glasses, which are selenides,

containing thallium, arsenic, tellurium, and antimony in various proportions.

They behave as amorphous semiconductors. Their photoconductive properties are

also valuable. Certain metallic glasses have magnetic properties; their

characteristics of ease of manufacture, magnetic softness, and high electrical

resistivity make them useful in the magnetic cores of electrical power

transformers.

Many different useful and decorative articles have been made from glass over the

centuries. Intermediates are those materials which do not form glasses on their

own, but help in the formation of glass. These can be present in large proportions

in glass if there is one glass former present in the material. The cations of the

intermediates are capable of entering the glass network and occupying positions

of the glass former. Some of the cations which at as intermediates in glass

formation are Ti, Zn, Pb, Al, Th, Be, Z, Cd etc. Modifiers form glasses only when

mixed in suitable proportions with the glass formers. The modifier ions are

capable of building up continuous network and they affect the glass structure. By

the addition of modifiers, the continuous glass network is disrupted and there are

now two ligands of oxygen ions: one that bridge two tetrahedrons and those that

do not or are non-bridging. Some of the commonly used modifier cations are Li,

Na, K, Ba, Sr, Hg, Cs, In, Nd, Pr, Sm, Tm, Er and Eu.

General introduction 79

1.4.4.Spectroscopy of Rare Earths in Glass

The spectra of the rare earth ions are composed of a set of sharp levels, which can

be directly traced to their free-ion Russell-Sanders origin [52]. The majority of

commonly encountered transitions are intraconfigurational i.e., 4f-4f and hence

are weak because of parity selection rules. 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 are often denoted using Russell-Saunders notation 2 S + ' ~ ~ arise 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 electron

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 allowed. The absorption and emission cross-sections are

therefore small and the luminescence life times can be quite long (ms). The

optical properties of trivalent 4f ions in the crystals in the visible and near infrared

are well understood in terms of the weak crystal field approximation [53,54].

There are two important effects that the crystal fields have on the energy levels of

the rare earth ions. Firstly the previously (2J+1) degenerate LSJ levels are split

into a number of Stark levels. Due to the shielding of the 4f electrons by the 5s2,

5p6 electrons these Stark shifts are quite small only around a few hundred inverse

centimeters. Secondly the crystal fields break the inversion symmetry of the ions

environment and this now permits electric dipole transition to occur between

Stark levels in different L.SJ multiplets. The oscillator strengths of these

transitions are very small of the order of which reflects the weakness of the

interaction of the 4f electrons with the crystal field. The magnetic dipole

transitions, which are typically many orders of magnitude weaker than fully

allowed electric dipole transitions, may nevertheless take place. Broadening of

Chapter 1 30

the rare earth ion fluorescence and absorption bands in glass is necessarily more

complicated than in crystals, since glasses have by definition a random structure

with each rare earth ion seeing a different electric field and therefore having a

different set of Stark levels. For a first approximation it is considered that at room

temperature the broadening of the absorption and fluorescence bands of rare earth

doped glasses is homogeneous in nature.

1.4.5.Energy transfer in doped glasses

The weak ion phonon or lattice coupling and sharpness of transitions make 4f

ions very useful as probes of various excitations and interactions in the

condensed phases. The energy transfer between two inorganic ions can occur

either by multipolar interaction or exchange mechanism in dilute systems or by

multi-step migration in concentrated systems. Forster 15.51 and Dexter 1561

developed the theory for the multipolar resonant energy transfer. According to

their theory energy transfer can occur between a donor and an acceptor provided

the emission transition of the donor overlaps with the acceptor absorption

transition. For the exchange coupling Inokuti and Hirayama 1571 proposed a

theoretical treatment assuming that the sensitizer ion is surrounded by a set of

acceptor ions and during the transfer process the environment of excited acceptor

ion changes with time in a nonexponential manner. The theory of nonresonant

energy transfer where the energy mismatch between the energy levels of the

donor and the acceptor ions is compensated by the emission or absorption of

phonons was developed by Miyakawa and Dexter 1581. As the concentration of

the impurity ions is increased the average distance between the ions decreases

which in turn causes greater interaction between the ions. Consequently, the

excitation energy residing on donor ion can non-radiatively be transferred to

acceptor ion. In rare earth containing glasses energy transfer between like ions is

known as cross-relaxation and it causes concentration quenching. However

unlike ions in glasses may cause dopant quenching and sensitizing. In the absence

of concentration quenching an excited sensitizer may either fluoresce or transfer

energy to an activator. If the sensitizer transfers its energy before it can fluoresce

General introduction 3 1

there will be an overall decrease in the fluorescence of the sensitizer and it

corresponds to the nonradiative transfer process. On the other hand, if the ion

fluoresces before transfer takes place the activator will absorb only part of this

fluorescence corresponding to one of its absorption peaks leading to the process

of radiative transfer.

1.4.6.Sol Gel glasses

The sol-gel process is one such method, which emerged as an important

processing technique for manufacturing new materials in different forms during

the last two decades This is not a new method for preparation of materials. As

early as 1864, Thomas Graham had prepared Gels of Silica. The word 'sol'

implies a dispersion of colloidal particles in a liquid. Colloids are in turn

described as solid particles with dimensions in the range of 10 to 1000 AD, each

containing 10' to 10' atoms. When the viscosity of the sol increases sufficiently,

usually throughout the practical loss of its liquid phase andlor polymerization of

the solid particles, it becomes a porous solid body; it is now termed 'Gel'. For

approximately hundred years, the potentials of the sol-gel process were not well

appreciated. Around 1980, the sol-gel process was 'rediscovered'. A great deal of

scientific knowledge has been generated at this time and many new materials

were prepared.

1.4.7. Sol-Gel Process

The sol-gel process is based on the mixing of liquid reactants on molecular scale and

subsequent solidification of the solution in to a porous amorphous oxide gel [59,60].

This is then subjected to suitable environments to produce different forms of

materials such as thin films, glasses. fibers etc. A schematic diagram of the

process is shown in Figure 1.5. The most important reagent in sol-gel processes

is a metal alkoxide, M(OR),. where OR is an alkoxyl group. For example

Tetraethyl orthosilicate (TEOS) is an alkoxide precursor commonly used for

preparing silica sol-gel materials.

Chapter 1 32

Solution LrJ Fiber

Aging

Glass fiber r"? Coating w Dense Film c 5

Ceramics 13 fi Figure 1.5. Schematic diagram of sol-gel processing

The basic chemical reactions are as given below.

First step is hydrolysis of the alkoxide precursor,

Si(OR)4+H20+ HO-Si(OR)3 + R(0H)

R is an alkyl group and R(0H) is an alcohol. Depending on the amount of water

and catalysts present hydrolysis may go to completion,

Si(OR)4 + 4H20 + Si(OH)4 + 4R(OH)

or stop while it is only partially hydrolyzed, Si(OR).,..(OH). .

Two partially hydrolyzed molecules can link together in a condensation reaction

such as,

OH-Si-(OR)3 + OH-Si (OR)3+(OR)3 Si-O-Si(OR)3 +H20

General inrroducrion 33

This condensation reaction grows and more and more Si-0-Si bonds are formed.

As polymerization continues, the viscosity of the solution increases until a solid

gel is formed.

Molecular and chemicals variations of the solution depends on

1. Selection of starting compounds and host medium.

2. Waterlalkoxide ratio

3. Solution pH

4. Catalyst

5. Reaction temperature

1.4.8. Potential Advantages of the Sol-Gel Process

The advantages of the sol-gel process are in general high purity, homogeneity,

and low temperature processing. Alkoxides are available in relatively free of

impurities. Homogeneity is obtained because the mixing is accomplished in

solution on the atomic scale in relatively short times. Even in the production of

dense materials. the porous gel can be compacted at temperatures 113 to 112 lower

than the melt temperature. Some compositions that cannot be made by

conventional means because of phase separation or diversification can be

prepared by sol-gel process. One of the primary advantages of the sol-gel process

for the formation of films is that the material used is efficient. Any excess

material is recovered and can be used again, leaving little waste. The only

disadvantage at this time is the cost of raw materials. --

Chapter I 34

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Chapter I 36

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