LUMINESCENCE STUDIES OF RARE EARTH DOPED

STRONTIUM ALUMINATE PHOSPHORS

REG. NO: 10118002

EXAM CODE: 63009402

SUBMITTED TO PARTIAL FULFILMENT FOR THE REQUIREMENTS

OF THE AWARD OF DEGREE OF MASTER OF SCIENCE IN PHYSICS

AUGUST 2012.

LUMINESCENCE STUDIES OF RARE EARTH DOPED

STRONTIUM ALUMINATE PHOSPHORS

Project report submitted to the University of Kerala in partial fulfilment for the

requirements towards the award of Master of Science degree in physics

by

ARUN T R

Reg:No :10118002

Under the supervision of

JAYASUDHA S.

DEPARTMENT OF PHYSICS

MAHATMA GANDHI COLLEGE

THIRUVANANTHAPURAM

AUGUST 2012

CERTIFICATE

This is to certify that the dissertation work entitled “ Luminescence Studies of

Rare Earth doped Strontium Aluminate Phosphors” is an authentic record of

work carried out by ARUN T R under my supervision and guidance towards the

partial fulfilment of the requirements for the award of Master of Science

Degree in Physics of the University of Kerala during the Academic Year 2011-

12.

Dr. V. Anup Kumar Jayasudha.S

Head, Dept. of physics Asst.Professor

Mahatma Gandhi College Mahatma Gandhi College

Thiruvananthapuram Thiruvananthapuram

ACKNOWLEDGEMENT

I wish my deep sense of gratitude and indebtness to Ms. Jayasudha S, Asst.

Professor, Dept. of Physics, Mahatma GandhiCollege for her valuable guidance

and encouragement throughout the course of the project.

I am extremely grateful to Mrs. Resmi.G.Nair, Research Fellow, Dept. of

Physics, Mahatma Gandhi College, Thiruvananthapuram for her valuable and

sincere help and valuable suggestions for the progress of this work.

I express my deep gratitude to Dr. V.P MahadevanPillai and Research

scholars of the Department of Optoelectronics, University of Kerala Karyavattom

for their assistance for measurement and study of Photoluminescence spectra of the

prepared phosphors.

I also express my deep gratitude to Dr. T.S. Elias, Professor Regional Cancer

Centre, Trivandrum, for assisting me in irradiating the sample.

I wish to acknowledge my grateful thanks to the teaching and non-teaching staff of

the Dept. of Physics, Mahatma Gandhi College, Thiruvananthapuram.

PREFACE

Phosphors owe their practical importance to their properties of absorbing

incident energy and converting it into visible radiation. This phenomenon, known

as luminescenceas, is driven by electronic processes in the material due to the

presence of trapping levels created by the presence of impurity atoms or lattice

defects. The study of absorption and emission characteristics of a phosphor enables

the understanding of electronic energy levels and, in turn, the design of phosphors

of practical relevance. Current work deals with the study of Thermoluminescence

(TL) and Photoluminescence (PL) properties of Strontium aluminates, doped with

Europium and Dysprosium dopants in varying concentrations, through

Combustion route. This work contains 5 chapters.

The first chapter contains the general introduction to the phenomenon of

Thermoluminescence and photoluminescence and their theoretical background.

Second chapter includes the important applications of Thermoluminescence

and Photoluminescence. A detailed review of different methods of phosphor

preparation is included in the third chapter.

The sample preparation using combustion method and the characterization

studies using the X-ray diffraction, TL and PL are described in the fourth chapter.

The chapter also describes about the TL Reader, the instrument used for the

measurement of TL response.

Finally, the project concludes with a comparison of the performances of the

developed phosphors with respect to nature of the dopants, concentration of the

dopants and dosage of irradiation.

CONTENTS

1. INTRODUCTION

1.1 THERMOLUMINESCENCE

1.1.1 TL PHOSPHORS

1.1.2 TL DOSIMETRY

1.1.3 TL DOSIMETRY PHOSPHORS

1.1.4 LUMINESCENCE OF INORGANIC CRYSTALS

1.1.5 LUMINESCENCE CENTRE

1.2 PHOTOLUMINESCENCE

1.2.1 INTRODUCTION

1.2.2 PL SETUP

1.2.3 PL MEASUREMENT PRINCIPLES

2. APPLICATIONS OF TL AND PL

2.1 APPLICATIONS OF TL

2.1.1 GEOLOGY

2.1.2 ARCHEOLOGY

2.1.3 MATEREOLOGY

2.1.4 FORENSIC SCIENCE

2.1.5 BIOLOGY AND BIOCHEMISTRY

2.1.6 RADIATION DOSIMETRY

2.1.7 OTHER APPLICATIONS

2.2 APPLICATIONS OF PL

2.2.1 SOME INTERESTING APPLICATIONS

3. METHODS OF PREPARATION OF PHOSPHORS

3.1 INTRODUCTION

3.2 CO-PRECIPITATION METHOD

3.3 RECRYSTALLISATION (SLOW EVAPORATION)

3.4 SOLID STATE SYNTHESIS

3.5 MELT TECHNIQUE

3.6 GROWTH OF CRYSTALS BY GEL METHOD

3.7 CHEMICAL REACTION METHOD

3.8 ACID EVAPORATION METHOD

3.9 COMBUSTION METHOD

4. PREPARATION AND CHARACTERISATION OFSrAl2O4:Eu/Dy

PHOSPHORS

4.1.1 INTRODUCTION

4.1.2 STRUCTURE OF SrAl2O4

4.1.3 COMBUSTION METHOD

4.1.4 CHARACTERISATION

4.1.4.1 X-RAY DIFFRACTION ANALYSIS

4.1.4.2 TL STUDIES

4.1.4.3 PL STUDIES

5. CONCLUSION

6. REFERENCES

CHAPTER 1

INTRODUCTION

1.1 LUMINESCENCE

The term “luminescence” (the literal translation from Latin is “weak glow”)

wasintroduced into the literature by Wiedemann (1888). There are several types of

luminescence depending on the cause or duration, as described below.

The presence of vacant lattice sites and other casual impurities or activator

atoms in the crystal leads to the appearance of localized energy levels in the band

gap. Some of them are deep, i.e. they are located at a considerable distance from

the top of valence band or from the bottom of the conduction band. Such levels are

often metastable and play the role of traps for charge carriers. For the electron at

the metastable level to be able to recombine with a hole, it should first be

transferred to the conduction band. This process can be stimulated by an electric

field, by infrared light, or by simply warming the crystal. The luminescence, in the

form of light, of a pre-excited crystal phosphor stimulated by heating is termed

thermoluminescence (TL). It is the release of previously absorbed radiation energy

and is quite different from incandescence light emission from a substance heated at

high temperatures. A large number of dielectric materials, including minerals,

rocks, inorganic (amorphous, singlecrystals and polycrystalline) semiconductors

and insulators, glasses and ceramics exhibit TL. Depending on the duration of the

emission, one can distinguish between two general classes of luminescence-

fluorescence and phosphorescence--the line of demarcation being about 10-8 sec,

which is the lifetime of the excited atoms in the gaseous state.

Phosphorescence and thermoluminescence are due to one and the same

process, the onlydifference being the fixed and the rising temperature, respectively,

of the emitting materialduring the time the emission is observed.

In phosphorescence, the presence of vacant lattice sites or other impurities,

latticedefects, and/or irregularities in the host lattice, provide unoccupied states

(traps) and delay the luminescence by detaining (trapping) the charge carriers

(electrons/holes) before their radiative recombination with the luminescent centers.

When we use X-rays as a source of excitation, the phenomenon is called X-ray

induced luminescence and when we use high-energy electrons (cathode rays) as the

means of excitation; the phenomenon is called cathodoluminescence (CL). On the

other hand, electroluminescence (EL) is a direct, non-thermal emission of light due

to recombination of minority and majority carriers across the bandgap of crystals

resulting from the application of an electric field to a material. In EL, the source of

energy is the electric field and the conversion of electric energy to light is unlike

that in cathodoluminescence, where there is a multistage process with acceleration

of electrons in a vacuum and the generation of secondary electrons. High-field EL

consists of excitation of luminescence centers by majority charge carriers

accelerated under the action of strong electric fields of the order of 1 to 2MeV/cm.

The field may be either ac or dc. Emission of energy in the form of visible light

during chemical or biochemical processes is called chemiluminescence or

bioluminescence. There are numerous organisms, both terrestrial and aquatic, that

emit light (Hastings 1983). Chemiluminescence appears during oxidation reactions

or free-radical recombination.

THERMOLUMINESCENCE

Thermoluminescence is the emission of light from an insulator or

semiconductor when it is heated. This is not to be confused with the light

spontaneously emitted from a substance when it is heated to incandescence.

Thermoluminescence is the thermally stimulated emission of light following the

previous absorption of energy from radiation. There are three essentional

ingredients necessary for the production of thermoluminescence. Firstly, the

material must be an insulator or semiconductor. Metals do not exhibit luminescent

properties. Secondly, the material must have previously absorbed energy during

exposure to radiation. Thirdly, the luminescence emission is triggered by heating

the material. In addition, there is one important property of thermoluminescence,

which cannot be inferred from this statement as it stands at present. It is a

particular characteristic of thermoluminescencethat once heated to excite the light

emission; the material cannot be made to emit thermoluminescence again by

simply cooling the specimen and reheating. In order to re-exhibit the luminescence

the material has to be re-exposed to radiation, where upon raising the temperature

will once again produce light emission. Thermoluminescence is a type of delayed

phosphorescence, where the photon energy is released when a crystalline material

is heated after subjecting it to ionizing radiations.

Thermoluminescence (TL) has been an active field of research during the

last few decades on account of its wide application potential. Its most striking

application has been in its use in radiation dosimetry. Ionizing radiation

dosimeters, which rely on the thermoluminescence properties of materials, have

helped in the solutions of many dosimetric problems due to their long time storage

capacities, independence of dose with radiation intensities, ease with which

measurements are done and light weight.

Experimental observations of luminescence can be grouped into three

categories. The substance can be heated at a linear rate and the light output may be

recorded as a function of temperature. Graph showing such a relation is called

glow curve. Usually emission over entire spectral range is collected. One can, of

course select a particular wavelength with the help of monochromator. Such glow

curves are referred to as monochromatic glow curves. Normally a number of

trapping sites exist having different activation energies. This results in appearance

of number of glow peaks. Each glow peak represents one type of trap.

In other type of observations, the substance is maintained at some elevated

temperature and decrease in emission intensity with time recorded. Graph between

intensity and time recorded under each condition is known as ‘Isothermal decay

curve’. Spectral distribution of TL can also give valuable information about the

process involved. For making such observations, the substances is rapidly heated to

the desired temperature and maintained at the temperature. The light emitted is

passed through the monochromator and thus various wavelengths are separated.

The decomposed spectrum is then recorded. Graph between intensity and

wavelength is known as “thermoluminescence emission spectrum”.

The peak location in thermoluminescence glow curve may be influenced by

factors such as heating rate, extent of initial excitation (radiation dose) etc. Hence

thermoluminescence characteristics of may be specified by TL glow curve at a

given dose and heating rate.

1.1.1 THERMOLUMINESCENT PHOSPHORS

It had been known since the work of Wiedmann and Schmidt in Lithium

Fluoride that the presence of impurities within a crystal enhances the

thermoluminescence response. The exact nature of impurity responsible for the

enhanced luminescence was not known at that time. Such impurities in the crystal

lattice are known as dopants and the process of addition of impurities in the lattice

is known as doping. The research on the use of alkali halide materials for TL

applications was initiated in the early 1920’s, with a view to study the structure and

defects of crystals and thermoluminescence associated with it. The first theoretical

foundation of the phenomenon of thermoluminescence was developed by Randall

and Wilkins. Slowly thermoluminescence gained importance in the measurement

of radiation doses, which came to be known as Thermo Luminescent Dosimetry

(TLD).

1.1.2 THERMOLUMINESCENCE DOSIMETRY

Thermoluminescencedosimetry (TLD) is a versatile technique used for

radiation dose measurements by making use of the phenomenon of

thermoluminescence. The basic principle used in this technique is that the amount

of light energy obtained on heating a material, which has been previously exposed

to ionizing radiations, will depend on the radiation dose received by the material.

The amount of light emitted from the phosphor can be correlated with the exposure

to radiation it has been subjected to, so that an unknown exposure can be

estimated.

TLDs are widely used in personal as well as environment radiation

monitoring. One of the most important applications of TL dosimetry has been in

the field of medical physics. TLD has become an integral part of medical physics

departments in all major hospitals especially cancer hospitals. Thermo luminescent

dosimeters have become popular and are being substituted for film badges because

of their high sensitivity, miniature size, tissue equivalence, high stability to

environmental conditions, low TL fading, re-usability, linear dose response and

sufficient precision and accuracy.

1.1.3 THERMOLUMINESCENCE DOSIMETRY PHOSPHORS

The phenomenon of thermoluminescence has been known for a long time.

Though the use of thermoluminescence for radiation measurements has been stated

to be made as in 1895, the work on the topic took momentum with the report of

Daniels et al based on the extensive work on the feasibility of using

thermoluminescence in dosimetry and other related applications. Thus if the light

sum of a phosphor can be co-related with the exposure it has received, then an

exposure can be estimated from the thermoluminescence intensity corresponding to

the exposure to be estimated. Efforts of Daniels saw the developments of a LiF

based phosphor. It was made commercially available by Harshaw. This was the

dawn of the new research fields, dosimetry of ionizing radiations using

thermoluminescence: abbreviated as TLD. Continuous efforts are going on since

then to obtain phosphors with improved characteristics.

Though a large number of organic solids exhibit thermoluminescence, only a

small number of them possess all the characteristics necessary for the use in

dosimetry; and in fact not a single solid possess all the characteristics of a good

TLD phosphor. The characteristics of a good TLD phosphor are,

i. High sensitivity.

ii. Emission wavelength falling in the range of commonly available detectors.

iii. No fading up on the post irradiation storage of the sample under normal

conditions of temperature, humidity, light etc.

iv. Simple and reproducible glow curve structure, which will not change with

exposure over a wide range.

v. Good correlation between exposure and thermoluminescence intensity,

complete absence of pyroluminescence, spurious thermoluminescence,

thermoluminescence excited by room light etc.

vi. Easy method of preparation, which will lead to batch homogeneity.

vii. Practically infinite shelf life.

viii. Reusability after the read out.

ix. Insensitivity to exposure conditions such as humidity, temperature,

atmosphere etc.

x. Properties such as toughness and non toxicity which will facilitate easy

handling.

xi. Tissue equivalence, which leads to energy independence.

xii. Selective response to various types of radiations.

Not a single material has been found out which possess all these

characteristics. Compromising on some factor or the other, several materials have

been considered as thermoluminescencedosimetry phosphors (TLDs).

1.1.4 LUMINESCENCE OF INORGANIC CRYSTALS

The luminescence of an organic compound like anthracene is an inherent

molecular property, characteristic also of the material in the vapour or solution

phases. By contrast the luminescence of inorganic crystals is a crystalline property,

and it is not normally exhibited in other phases.

The majority of efficient inorganic luminescent materials are impurity-

activated, which means that their luminescence is due to the presence of small

concentrations of specific impurities. Typical systems are the alkali halides

activated by heavy metals such as thallium (e.g. NaI:Tl, CsI:Tl) or oxides doped

with rare earth ions (LSO:Ce, YAP:Ce). Apart from crystals, oxide or fluoride

inorganic glasses may also be activated by similar impurities and luminescent. In

some crystals the activator is not an added impurity, but a stoechiometric excess of

one of the constituents of the solid (e.g. BGO, BaF2). The excess ions occupy

interstitial positions in the crystal lattice and function as luminescent centres. Such

crystals are called self-activated. A few pure crystals, notably diamond, are also

luminescent. In this case it appears that the luminescence centres are associated

with defects in the crystal lattice, and that atoms or ions situated near these defects

act as activators. Thus the general pattern for luminescence in an inorganic solid is

a crystal or glass containing emission centres, which may be either interstitial or

substitutional impurities, excess atoms or ions, or atoms or ions associated with

defects.

1.1.5 THE ENERGY BAND MODEL

1.1.5.1 PERFECT CRYSTALS

A suitable model for the discussion of inorganic crystals is provided by the

collective electron or band theory from Bloch (1928). The electronic energy states

of an isolated atom or molecule consist of a series of discrete levels defined by

Schrödinger’s equation. In an inorganic crystal lattice the outer electronic energy

levels are perturbed by mutual interactions between the atoms or ions, and they are

broadened into a series of continuous allowed energy bands, separated by

forbidden energy regions. The inner energy levels are practically undisturbed and

retain their normal character. A schematic figure of the energy bands is shown in

(a). For an insulator or a low temperature semi-conductor the lower energy bands

are completely filled while the higher bands are empty. The highest filled band, the

valence band, is separated from the lowest empty band known as the conduction

band, by an energy gap Eg

of a few electron volts. Electrons from the valence band

may be raised into the conduction band by the absorption of a photon leaving

positive holes in the valence band. Photoconduction can then take place by the

independent motion of the electrons in the conduction and the holes in the valence

band.

Figure-1.1

(a) Energy bands in ideal insulating crystal and (b) energy bands in impurity-activated crystal

phosphor showing excitation, luminescence, quenching and trapping processes.

Alternatively the excited electron may remain bound to the positive hole.

This system, which constitutes an exciton, carries no charge and is free to migrate

through the crystal lattice. The exciton band corresponds to a band of energies

below the conduction band. In principle the two bands can be distinguished by the

absence of any photoconduction associated with exciton migration. By analogy to

single molecules, promotion of an electron into the exciton band constitutes

excitation, while similar promotion into the conduction band constitutes ionization.

Electrons in the conduction band and holes in the valence band may subsequently

recombine to form excitons.

In semi-conductors the energy gap Eg

between the valence and the

conduction band is sufficiently small for some electrons to be able to acquire the

excitation energy thermally. In insulating crystals the energy gap is large enough

that the concentration of free carriers is negligible at normal temperatures, in the

absence of high electric fields of excitation by ultraviolet or ionizing radiation.

1.1.5.2 IMPERFECT CRYSTALS

The simple model is only true for insulators having a perfect crystal lattice. In

practice, variations in the energy bands due to defects and impurities in the crystal

lattice occur, producing local electronic energy levels in the normally forbidden

region between conduction and valence bands. If these levels are unoccupied,

electrons (or excitons) moving in the conduction band may enter these centres.

There are mainly three types of centres:

a) Luminescence centres, in which the transition to the ground state is

accompanied by photon emission.

b) Quenching centres, in which radiationless dissipation of excitation energy

can take place.

c) Traps, which have metastable levels from which the electrons may

subsequently return to the conduction band by acquiring thermal energy, or

fall to the valence band by radiationless transition.

The same centres may contain luminescence, quenching and/or trapping levels

since their relative population is determined by the Boltzmann statistical

distribution. The luminescence centres and traps arise from impurities like

interstitial ions and/or defects. They introduce local discrete energy levels

corresponding to the ground and excited state of a centre. The excitation of a centre

requires the capture of an electron from the conduction band and a hole from the

valance band either by capturing an exciton or by electron-hole recombination at a

centre. The traps arise from other lattice variations and provide additional levels

for the electrons below the conduction band (or holes above the valence band). The

energy level system for an impurity-activated crystal phosphor is shown in Figure

1.1.

1.1.6 Luminescence Centre

The theoretical model of configuration co-ordinate from Hippel (1936) and

Seitz (1938, 1939) is often used to discuss the conditions for luminescence

emission or thermal quenching of a centre. It is a rather general model and it is

suitable for all luminescent materials (including organic scintillators). In Figure the

potential energies of the ground and excited electronic state of the luminescent

centre are plotted against the x-configuration coordinate of the centre. The curves

aAa’ and bBb’ represent the vibrational amplitude of the centre in the ground and

excited state, respectively. The minima of the curves (point A and B) indicate the

stable energy positions of the two states. At room temperature, the thermal

vibrations lead to displacements from the minimum potential energy position

according to energies of kT.

If a centre absorbs a photon with the energy hν (or is excited by the capture

of an exciton) a transition from the ground to the excited electronic state takes

place. The transition occurs along a vertical line AC on the diagram, since by the

Franck-Condon principle, electronic transitions involved in absorption or emission

occur in a short time compared with that of atomic or ionic movements. Directly

after this transition the system moves to the point of minimum potential energy

(from C to B) and gives the excessive vibrational energy thermally to its

neighbours. The time spent in B depends on the probability of the optical transition

which gives rise to the luminescence emission hν’. After this transition the centre

in the ground state returns from D to A by thermally dissipating the rest vibrational

energy shows the origin of the absorption and emission spectra corresponding to

the AC and BD transition, respectively.

Figure-1.2

Potential energy diagram of luminescent centre. aAa’ represents the ground state and bBb’ the

excited state. AC is the absorption transition, BD the luminescence emission and FF1 the region of

internal quenching.

a) The emission spectrum is at lower energies (longer wavelengths) than the

absorption spectrum (Stoke’s law).

b) The overlap of the absorption and emission spectrum depends on the relative

positions of the potential energy curves of the ground and excited states.

c) Vibrational structure in the absorption and emission spectrum arise from

transitions to the vibrational sub-levels of the excited and ground states.

d) In inorganic scintillators, the emission spectrum tends to be broader than the

absorption spectrum of the luminescence centres.

Figure-1.3

Absorption and luminescence emission transition showing the origin of the overlap of absorption

and emission spectra.

1.1.7 Physical Mechanism of Scintillation

The scintillation process can be divided in three stages: a) the primary

interaction of radiation with matter, relaxation and thermalization of the resulting

electrons and holes to electron-hole pair energies roughly equal to the bandgap

energy Eg, b) further relaxation, formation of excitonic states, and energy transport

to the luminescence centres, and c) luminescence. It is well-known that crystals

having a high light output under photoexcitation can give a rather low light

emission under excitation by charged particles, x-rays or γ-rays . This is primarily

a consequence of energy losses in stages a) and b). The primary interaction of an

x-ray or γ-ray will result in the production of one or more energetic electrons,

depending on the energy and the interaction mechanism (photoelectric effect,

Compton effect, electron-positron pair formation). X-rays produced by

recombination of electrons with holes in core levels (from the photoelectric effect)

will also produce electrons by the mechanism mentioned above. Heavy charged

particles and low-energy electrons transfer their energy primarily by ionization,

thus producing electrons and holes. High-energy electrons transfer their energy by

production of bremsstrahlung, which in turn gives pair formation.

Various models have been used for the description of these complicated

processes of energy dissipation: simple phenomenological , “crazy carpentry” ,

plasmon, and polaron . These models consider only a single mechanism and do not

take into account the whole variety of processes taking place. A detailed

description of these models would go beyond the scope of this thesis and can be

found in literature.

1.2 PHOTOLUMINESCENCE

1.2.1 INTRODUCTION TO PHOTOLUMINESCENCE

In this experiment, the energy levels in a semiconductor quantum well

structure are investigated using the technique of photoluminescence (PL). A laser

is used to photoexcite electrons in a GaAs semiconductor and when they

spontaneously de-excite they emit luminescence. The luminescence is analyzed

with a spectrometer and the peaks in the spectra represent a direct measure of the

energy levels in the semiconductor.

GaAs is a popular and useful semiconductor material. The importance of

electronic devices using GaAs is second only to devices using the more ubiquitous

semiconductor, Si. Since GaAs has a higher electron mobility than Si, it is used for

higher-speed electronics. Cell phones use GaAs power amplifiers for generating

the output signal at 1.8-1.9 GHz, and 2.5 GHz frequencies. In addition, GaAs has a

so-called “direct band-gap,” unlike Si which has an indirect band-gap. This results

in a strong interaction with light which makes it useful for generating light in

LEDs (light emitting diodes) and laser diodes. Alloy of GaAs, especially with Al

and In, are used exclusively for sources in optical communication.

All solids, including semiconductors, have so-called “energy gaps” for the

conducting electrons. In order to understand the concept of a gap in energy, first

consider that some of the electrons in a solid are not firmly attached to the atoms,

as they are for single atoms, but can hop from one atom to another. These loosely

attached electrons are bound in the solid by differing amounts and thus have many

different energies. Electrons having energies above a certain value are referred to

as conduction electrons, while electrons having energies below a certain value are

referred to as valance electrons. This is shown in the diagram where they are

labeled as conduction and valance bands. The word band is used because the

electrons have a multiplicity of energies in either band. Furthermore, there is an

energy gap between the conduction and valance electron states. Under normal

conditions electrons are forbidden to have energies between the valance and

conduction bands.

If a light particle (photon) has an energy greater than the band gap energy,

then it can be absorbed and thereby raise an electron from the valance band up to

the conduction band across the forbidden energy gap. (See figure 1.4). In this

process of photoexcitation, the electron generally has excess energy which it loses

before coming to rest at the lowest energy in the conduction band. At this point the

electron eventually falls back down to the valance band. As it falls down, the

energy it loses is converted back into a luminescent photon which is emitted from

the material. Thus the energy of the emitted photon is a direct measure of the band

gap energy, Eg. The process of photon excitation followed by photon emission is

called photoluminescence.

Figure-1.4 Mechanism of photoluminescence

Photoluminescence in solids is the phenomenon in which electronic states

of solids are excited by light of particular energy and the excitation energy is

released as light. Stokes (1852) formulated the first law in the history of

luminescence (Stoke’s rule), which states that the wavelength of emitted light

generally is equal to or longer than that of the exciting light (i.e., of equal or less

energy). This difference in wavelength is caused by a transformation of the

exciting light, to a greater or lesser extent, to non-radiating vibration energy of

atoms or ions. In rare instances--e.g. when intense irradiation by laser beams is

used or when sufficient thermal energy contributes to the electron excitation

process—the emitted light can be of shorter wavelength than the exciting light

(anti-Stokes radiation). A general introduction on the photoluminescence

spectroscopic technique can be found in the classic work of Bebb and Williams

(Bebb 1972), in the book by Pankove (Pankove 1975), and in the review article by

P. J. Dean (Dean 1982). In photoluminescence spectroscopy, photons with energy

greater than the bandgap of the material studied are directed onto the surface of the

semiconductor material, the incident monochromatic photon beam is partially

reflected, absorbed, and transmitted by the material being probed. The absorbed

photons create electron-hole pairs in the semiconductor. The electrons are excited

to the conduction band, or to the energy states within the gap. In addition, electrons

can lose part of their energy and transfer from the conduction band to energy levels

within the gap. Photons produced as a result of the various recombinations of

electrons and holes are emitted from the sample surface and it is the resulting

photon emission spectrum that is studied in photoluminescence (PL). The photon

energies reflect the variety of energy states that are present in the semiconductor.

Different energy states are produced by different defects, and by the many different

ways impurities are incorporated into the lattice. As a consequence, a PL emission

spectrum provides information concerning the point defect nature of a material by

determining not only the presence, but also the type of vacancies, interstitials, and

impurities in the lattice. A direct conduction band-to-valance band recombination

is rarely observed in PL spectra. Even if direct band-to-band recombinations occur,

the crystal will strongly reabsorb the photons emitted. Therefore, in PL spectra,

recombination processes are observed with emission energies less than Eg. These

processes include excitonic recombinations and indirect transitions, which involve

the trapping of electrons (or holes) by impurities. Recombination processes in

semiconductors are described with great care in the recent book by Landsberg

(Landsberg, 1991).

1.2.2 PHOTOLUMINESCENCE SETUP

Figure 1.5 Photoluminescence setup

1.2.3 PHOTOLUMINESCENCE MEASUREMENT PRINCIPLES

Photoluminescence is the luminescence of a material after excitation by

high energy photons. Photoluminescence properties of a material are characterized

by both absorption (excitation) of the material by a primary excitation source and

emission of light by the material. A typical experimental arrangement for

determining excitation spectra is shown in Figure 1. 6. In this example the

excitation source is the output of a monochromator which, like a prism, resolves

the excitation light source into its component wavelengths.

Figure 1.6 Schematic diagram for the measurement of excitation spectra.

The excitation wavelength of interest illuminates the sample. Then intensity of

the luminescence emission is measured by a photomultiplier tube. The optical cut-

off filter placed between the sample and the photomultiplier tube is selected so that

it will pass the luminescence emission but will absorb the reflected excitation

radiation. The output of the photomultiplier tube is amplified and then fed into the

y axis of an x-y recorder. The value of the excitation wavelength selected is plotted

on the x-axis. Thus, one obtains an x-y plot which shows the intensity of the

luminescence emission as a function of the wavelength of the excitation radiation.

The spectrum is obtained using a monochromator equipped with an appropriate

light detector. In the case of an excitation spectrum, the relationship is obtained by

observing changes in the emitted light intensity at a set wavelength while varying

the excitation energy.

The excitation source consists of the light source and a

monochromator, which selects a specific wavelength range from the incoming

light. A filter can do a similar job. The light emitted from the sample is analyzed

by a monochromator equipped with a light detector. The light detector transforms

the photons into electrical signals. A laser is an excellent monochromatic light

source and has a radiative power at a given frequency several orders of magnitude

greater than that of other light sources. They can either operate in continuous or

pulsed mode. Common gas lasers used for the study of luminescence are the He-

Ne, Ar+ ion, Kr+ ion and He-Cd. The He-Cd laser uses a mixture of the He gas

and Cd metal vapour, and has emission peaks in the ultraviolet and visible region.

When it is operated in the continuous wave (CW) mode, the 325 nm peak is most

prominent, with output powers of 100 mW. This laser is very useful as an

ultraviolet excitation source for measuring photoluminescence spectra.

The luminescence properties of a phosphor can be characterized

by its emission spectrum (wavelength), brightness and decay time. The emission

spectrum is obtained by plotting the intensity against the wavelength of the emitted

light from a sample excited by an appropriate excitation source of constant energy.

The experimental arrangement for the determination of an emission spectrum is

shown schematically in Figure 1.7s. A single excitation wavelength is selected.

The optical cut-off filter serves the same purpose as previously described. The

emission of the sample is analyzed by means of a monochromator.

Figure 1.7 Schematic diagram of a typical experimental arrangement for recording the

emission spectrum of a phosphor.

CHAPTER 2

APPLICATIONS OF TL AND PL

2.1 APPLICATIONS OF THERMOLUMINESCENCE

The most important applications of TL are in radiation dosimetry. Apart

from this, TL phenomenon finds numerous applications in the field of Geology,

Archaeology, Meteorology, Biology and in biochemistry and in Forensic science.

2.1.1 GEOLOGY

Thermoluminescence has been applied in recent years to solve a variety of

geological problems. Radioactive mineral prospecting is one of the important TL

applications in geology. Field experiments were done in Kerala to identify the

region from where the radioactive minerals (mostly monazite) originate. Many of

the rock forming minerals give characteristic TL.

2.1.2 ARCHAEOLOGY

The possibility of using TL in archaeological dating was first suggested by

Daniel et al. According to them the natural TL from rocks is directly related to the

radioactivity from uranium, thorium and potassium, which are present within the

material. This radioactivity results in the accumulation of ‘geological dose’ in the

material. This radioactivity results in the accumulation of ‘geological dose’ in the

material. If the rate of irradiation from the radioactiveminerals is established then

the length of time over which the rock has been irradiated can be determined from

the relation,

Geological age = Absorbed dose / Dose rate (annual dose)

TL is more sensitive for detecting traces of radioactivity than conventional

methods and hence it is widely used in radioactive mineral prospecting.

2.1.3 METEOROLOGY

Rock forming minerals give characteristic TSL (glow peak positions as well

as spectral emission). Hence by examining the TL of igneous rocks, their minerals

contents can be guessed.

2.1.4 FORENSIC SCIENCES

The major work in a forensic science laboratory is based on a comparison of

evidentiary materials with similar materials whose origin is known and sometimes

certain clue materials are required to be identified also. The examination of such

clue materials is carried out by evaluating the physic-chemical properties, which

are characteristics of the substances under examination.

In scientific methods of criminal investigation, TL characteristics is one of

the most efficient methods for differentiating between samples of several types of

contact traces, such as soil, paint chips, glass etc; commonly encountered in

criminal cases. This sometimes helps to lead to conclusive evidences whether the

samples has come from its known sources. TL studies of dental enamel are also

useful in criminal investigation.

2.1.5 BIOLOGY & BIO-CHEMISTRY

In biochemical examinations also TL studies are useful. Radiation damage in

tissues, nucleic acid, proteins etc could be measured using their TL characteristics.

In these applications the measurements are to made at very low temperatures.

Tatake has studied some applications of thermoluminescence technique on the post

irradiation phenomena in bio molecules. Special attention was given to the

observations made on nucleic acids, proteins and their constituents.

2.1.6 RADIATION DOSIMETRY

Radiation is a ubiquitous and universal phenomenon. People are exposed to

natural as well as man made sources of radiation. Radiation dosimetry deals with

the monitoring techniques of the radiation dose received by individuals working in

various radioactive installations.

As early as 1895, the physical process for the thermal release of the stored

radiation induced luminescence was used for the detection of ionizing radiation by

Wiedmann and Schmit. They irradiated a large number of minerals and in organic

compounds with cathode rays and found that natural fluorites show a very intense

luminescence when they are heated in darkness and there is no decay of the stored

luminescence even after storage for a few weeks. All types of radiations such as

gamma rays, alpha rays, x-rays and light rays can excite materials to widely

different extents.

2.1.7 OTHER APPLICATIONS

TL of materials like glass, soil, etc that are commonly encountered at the

scene of crime can be used as additional evidence. TL signals using few milligrams

of the sample can be compared with those similar samples collected from the

suspected places.

In ceramic industries an effective quantity control can be done using TL

testing. The thermoluminescence output of a ceramic sample (after exposure to

radiation) is directly proportional to its field spar content. This method can be

regularly used in industry for maintaining the quality of products such as glass,

ceramic, semi conductor etc. Recently, it has been shown that the variation in TL

glow curves depends on the structure of fabrics. However not much attention has

been given to this aspect in the industrial sciences.

2.2 APPLICATION OF PHOTOLUMINESCENCE

Photoluminescence has wide application in the field of photoluminescent

paints on art –canvas, photoluminescent ink on clothing , military, emergency etc.

Some of the important application of photoluminescence are listed given below.

2.2.1 Some interesting applications of photoluminescence

Photoluminescence is a process where a material aborbs photon energy

(light) at one wavelength; stores it by exciting an electron to a higher energy state;

photoluminescence (light emission) is observed when the excited electron returns

to the lower energy state. The typical process of excitation and light emission takes

10s of seconds. By arrangement of molecules via doping with additives, one can

extend the photoluminescent periods to more useful time scales of 10 minute to 20

hours. The rocks below show photoluminecence as they are photographed in the

dark.

Figure 2.1Rocks exhibiting photoluminescence

Over the past decade, we have become familiar with photoluminescent or

glow in the dark products. We have seen novely items such as children pajamas,

toys at fair which glow and attract attention. We have also seen nonpowered

photoluminescent exit signs and arrows which direct individuals out of an office or

hotel in power failure situations. Further, use of photoluminescent ink on clothing,

cars, plates and posters is becoming more common. The use of photoluminescent

paints on art--canvas and glass are also gaining popularity. You may have

also seen woven products which have been sewn or embroidered

with photoluminescent thread. Besides these areas, photoluminescent products are

finding applications in security and tamper-evident markers.

Application includes:

Safety road worker vests, traffic signs, rail crossing markers.

Dials, buttons and switches in Autos and Aircraft.

Rescue

Fire and Ambulance handles and latches

Emergency

Emergency signage, Low-level lighting of escape routes

Life rafts and vests, Trunk releases, Fire Extinguisher pins and Hangers

Sports

Diving markers, camping gear, Hunting vests, Boating markers and Bicycle

parts.

Convenience

Cell phone buttons, Light switch covers, Doorbell buttons, Appliance

Dials/Buttons.

Dials buttons and switches in ships, tanks, trucks and planes. Gun sites.

Electrical breaker switches, Commode seats, House numbers, TV remote

buttons.

These newer phosphors are expensive and loadings must be higher than

normal colorants, so colouring costs for “Glow in the Dark” plastics are high.

Figure 2.2

A PHOTOLUMINESCENCE FOOTBALL

Figure 2.3

The football pictured above shows one of the application of

photoluminescence. Remember all the times when it became too dark to see the

football and the game was tied. Now, you can continue playing for atleast 30

minutes longer.

CHAPTER 3

METHODS OF PREPARATION

3.1 INTRODUCTION

Thermoluminescence phosphors find wide range of applications in the field

of personal dosimetry, environmental monitoring, medical dosimetry, dating and

geology. The first step of phosphor preparation is the development of crystals of

suitable materials undergoing the change gradually uniformly and continuously

looses their random character and achieves crystalline solid character so that

crystals are grown in the course of phase transition. This is done in the presence of

activator ions so that the ions get incorporate in the crystal lattice as dopant in

certain proportion. At present, various methods of phosphor preparation such as

co-precipitation, acid evaporation (recrystallisation), solid state synthesis, melt

technique are being used. A brief description of these methods is given below.

3.2 Co-precipitation Method

This method is used when the reactants are soluble in the medium of

precipitation, generally water. Definite concentration of reactants together with the

activator is mixed together under suitable pH and temperature conditions and the

precipitate formed is separated, washed, dried and annealed at suitable

temperature. For example LiF was precipitated from aqueous solution of LiCl with

NH4F. Dopants Mg and Ti were incorporated in the form of MgCl2 and TiL4. The

precipitate LiF:Mg, Ti was separated, repeatedly washed with water, then heated

in an oven up to certain temperature, and then quenched. The materials so formed

were powdered and then sieved to get a phosphor having a definite grain size.

3.3 Recrystallisation (Slow Evaporation)

The process involves the slow evaporation of saturated solution of the material

to be crystallized in which appropriate amounts of dopants are added. Alkali halide

crystals are grown by this method. For substances, which do not dissolve in water,

another technique known as acid evaporation is commonly used.

3.4 Solid state synthesis

The process involves a change from one solid phase to another accompanied by

a change in crystal structure of the reactants involved. The solid state synthesis is

the most commonly used method used for the preparation of very fine

polycrystalline powders. In this method, the desired phosphor is synthesized by

direct mixing of the constituent oxides with dopants and then firing at high

temperatures. This method involves a series of mixing, grinding and heating

cycles.

3.5 Melt Technique

The most successful method for the preparation of large single crystals consists

of growing the crystals from mell. As compared to vapour or solution method,

large growth rate are possible here because the solid is in constant touch with its

own molecules and there is no presence of solvent, contamination is not a problem.

Hence, crystals of high purity can be obtained from non-reactive system. The

method is applicable only if the materials melt congruently and without irreversible

decomposition. This method is preferred to grow alkali halide crystals, as their

melting points are not very high (below 1000’c). Both amorphous and crystalline

phosphors are synthesized using this technique.

3.6 Growth of crystals by Gel Method

Using the gel method, different types of crystals- ionic, organic, metallic and

even biological crystals such as Cholesterol have been grown at ambient

temperature. The gel method has also been used to grow mixed crystals.

3.7 Chemical Reaction Method

The chemical reaction method is suitable for crystals which are mostly

insoluble or sparingly soluble in the water and which decompose below their

melting temperature. In this method two suitable reactants are allowed to diffuse

through a gel where they react and form an insoluble or sparingly soluble

crystalline product. The chemical reaction taking place can be represented as

AX + BY AY + BX

Where A & B are cations and X & Y are anions. Annealing of grown crystals is

the second stage of phosphor preparation. Annealing involves heating the crystal

up to a certain temperature, maintaining that temperature for a period of time and

then cooling it. The parameters such as rate of heating / cooling, duration of

annealing and annealing environment influences luminescence efficiency of the

phosphor.

3.8 Acid Evaporation Method

In this technique, the material to be developed as a phosphor is dissolved in

a suitable acid along with the required amount of dopants and the acid is slowly

evaporated under a constant temperature gradient. Yamashita was the first scientist

who developed a method for the preparation of CaSO4 phosphor using this

method. The preparation by open evaporation of sulphuric acid has some draw

backs like corrosion and air pollution, contamination by external impurities and

loss of sulphuric acid. The assembly of phosphor preparation is shown in the figure

3.1

Figure 3.1 Setup for Phosphor preparation by Acid Evaporation Method

The assembly consists of one liter two necked flask mounted on an isomantle,

which provides a temperature gradient. A thermocouple is inserted through one

neck to monitor the temperature. The other neck of the flask is connected to a

condenser unit so that the boiling sulphuric acid can be cooled and collected in a

receiving vessel kept at the end of the condenser. A water suction pump is used to

speed up the collection of sulphuric acid and extract fumes which have not been

condensed. An empty container is kept between the receiver and the suction pump

to avoid any explosive reaction between sulphuric acid and water in case of back

suction of water. At present, this method is widely used for the preparation of

sulphate based phosphor.

3.9 Combustion method

Almost all known advanced materials (both oxide and nonoxide) in various

forms (nanosize, films, whiskers) have been made by a combustion process.The

combustion process to prepare the precursor powders, however, is very facile and

only takes a few minutes, which has been extensively applied to the preparation of

various oxide materials. In this method the grade metal nitrate is treated with

aluminium nitrate, oxides of earth material and urea.Then weighed quantities of

each nitrate and urea were mixed together and crushed into mortar for 1 hour to

form a thick paste. The resulting paste is transferred to crucible and introduced into

a vertical cylindrical muffle furnace maintained at 600oC. Initially the mixture

boils and undergoes dehydration followed by decomposition with the evolution of

large amount of gases (oxides of carbon, nitrogen and ammonia). The process

being highly exothermic continues and the spontaneous ignition occurs. The

solution underwent smoldering combustion with enormous swelling, producing

white foamy and voluminous ash.The flame temperature, as high as 1400 - 1600

ºC, converts the vapor phase oxides into mixed aluminates. The flame persists for

~30 seconds. The crucible is then taken out of the furnace and the foamy product

can easily be milled to obtain the precursor powder.

CHAPTER 4

PREPARATION AND CHARACTERISATION OF

SrAl2O4: Eu/Dy PHOSPHORS

4.1 Introduction

The solid-state reaction process has been used intensively for phosphor

synthesis, but this process often results in poor homogeneity and requires high

calcinating temperature. Moreover, the grain size of phosphor powders prepared

through solid-state reaction method is in several tens of micrometers. Phosphors of

small particles must be obtained by grinding the larger phosphor particles. Those

processes easily introduce additional defects and greatly reduce luminescence

efficiency. With the development of scientific technologies on materials, several

chemical synthesis techniques, such as co precipitation, sol–gel, microwave,

pechini and combustionsynthesis methods have been applied to prepare rare earth

ions activated alkaline earth aluminate phosphors. All of these methods were

conducted in liquid phases so that each component can be accurately controlled

and uniformly mixed. The combustion process to prepare the precursor powders,

however, is very facile and only takes a few minutes, which has been extensively

applied to the preparation of various oxide materials. In this paper combustion

process was chosen to prepare rare earth doped Strontium aluminates. The

luminescent properties also studied. Compared to solid-state method, the

combustion process is safe, instantaneous and energy saving.

4.2 THE CRYSTAL STRUCTURE OF SrAl2O4

SrAl2O4 adopt a stuffed tridymite-type structure consisting of corner

sharing AlO4 tetrahedral which connect together to form six-membered rings. Each

oxygen ion is shared by two aluminium ions so that each tetrahedron has one net

negative charge. The charge balance is achieved by the large divalent cation Sr2+

,

which occupies interstitial site within the tetrahedral frame-work. SrAl2O4 exists in

two different phases, namely monoclinic (M) i.e. P21 (a = 8.447 Å , b = 8.816 Å, c

= 5.163 Å, = 93.42o) and hexagonal P6322, (a = 5.140 Å c = 8.462 Å (H) i.e.

P6322. It undergoes a phase transition from a low-temperature monoclinic

distorted structure to hexagonal tridymite structure at 650o C.

The ideal undistorted structure of SrAl2O4 is desribed by cell parameters

close to those of high tridymite. The monoclinic SrAl2O4, being stable at

temperatures below 950 K is a distorted form of a hexagonal SrAl2O4. The

distortion involves a reduction in the symmetry of the trigonally distorted rings.

The monoclinic SrAl2O4 has two strontium sites. The distances between the

strontium ion and its neighbouring oxygen ions are different for the two strontium

sites. In one site, the oxygen atoms are at a larger distance from the strontium ion

than the other. The structure has channels in the a- and c-directions where Sr2+

ions

are located. This can be revealed by the parallel projections of the polyhedral

forms for the directions-c and -a shown in Figure 4.1.

Figure 4.1

Schematic views of the monoclinic phase of SrAl2O4 along the a- and c-directions.

Phosphorescent phosphors have great potential in several applications for

devices and luminous paints and have been widely studied. A phosphorescent

material such as the ZnS matrix has been well known as a longlasting phosphor but

did not show sufficient brightness and long phosphorescent behavior. The chemical

instability and fast luminance change of such a sulfide material has been proposed

as the problems in practical applications. Eu2+

doped alkaline earth aluminates,

MAl2O4:Eu2+

(M: Ca, Sr, Ba) phosphors with a strong photoluminescence in the

blue-green visible range have been studied by many researchers. These materials

have important industrial applications with a long persistence luminescence.

4.3 COMBUSTION METHOD

The synthesis of solids possessing desired structures, composition and

properties continues to be a challenge to chemists, material scientists and

engineers. Formation of solids by the ceramic method is controlled by the diffusion

of atoms and ionic species through reactants and products and thus requires

repeated grinding, pelletizing and calcination of reactants (oxides or carbonates)

for longer durations (than soft chemical routes) at high temperatures. Attempts

have recently been made to eliminate the diffusion control problems of solid

synthesis by using various innovative synthetic strategies. One such approach is

‘combustion synthesis’ also known as‘self-propagating high-temperature

synthesis’ (SHS) and fire or furnaceless synthesis. The process makes use of highly

exothermic redox chemical reactions between metals and nonmetals, the

metathetical (exchange) reaction between reactive compounds or reactions

involves redox compounds/mixtures. The term ‘combustion’ covers flaming (gas-

phase), smouldering (heterogeneous) as well as explosive reactions. The

combustion method has been successfully used in the preparation of a large

number of technologically useful oxide (refractory oxides, magnetic, dielectric,

semiconducting, insulators, catalysts, sensors, phosphors etc.) and nonoxide

(carbides, borides, silicides, nitrides etc.) materials. To date more than 500

materials have been synthesized by this process, many of which are commercially

manufactured in Russia.

In recent years, there has been tremendous interest in the combustion

synthesis of materials because it is simple, fast, energetically economic and yields

high purity products compared to the conventional routes used to prepare these

materials. As it is a high-temperature process, only thermodynamically stable

phases can be prepared. At the same time, rapid heating and cooling rates provide

the potential for the production of metastable materials with new and unique

properties. A quarterly journal, ‘International Journal of Self-Propagating High-

Temperature Synthesis’ devoted to SHS has been published by Allerton Press Inc;

New York, since 1992 with Alexander G Merzhanov as the ‘General Editor’.

Three International Symposia on SHS have been held, in 1991 (Alma-Ata,

Kazakhstan), 1993 (Honolulu, USA) and 1995 (Wuhan, China) and the fourth one

is being planned in Spain during October 6-10, 1997. A two part review on

combustion synthesis of advanced materials by Moore and Feng gives an account

of the historical perspectives of SHS; parameters that control the SHS process; and

materials that have been prepared and their applications. Various types of SHS

reaction and proposed models of reaction are discussed, the thermodynamics and

kinetics of SHS reactions are also presented.

Important parameters that control combustion synthesis such as the

particle size and shape of the reactants, ignition techniques, stoichiometric ratio,

processing of reactant particles (green density, i.e. the density of the pellet before

sintering) and the adiabatic temperature (Tad) which is a measure of the

exothermicity of the reaction, have been discussed in detail . For this reason no

attempt is made here to elaborate on these points. In this article, the recent trends in

the SHS, thermite, solid-state metathesis (SSM) and flame syntheses, used in the

preparation of inorganic materials will be discussed. The latter part of the article is

devoted to the combustion synthesis of oxide materials using redox compounds

and mixtures.

4.3.1 Preparation of SrAl2O4: Eu

SrAl2O4: Eu phosphor particles were prepared by solution combustion

synthesis method followed by heating the precursor combustion ash at 900° C. In a

cylindrical quartz container, stoichiometric composition of aluminium nitrate

(Al(NO3)3), strontium nitrate (Sr(NO3)2), and europium nitrate (Eu(NO3)3) were

dissolved in a minimum amount of distilled water together with urea as fuel. The

precursor solution was introduced into a muffle furnace maintained at 500 °C.

Initially, the solution boiled and underwent dehydration, followed by

decomposition with the evolution of large amounts of gases. Then, spontaneous

ignition occurred and underwent smouldering combustion with enormous swelling,

producing white foamy and voluminous SrAl2O4:Eu. The whole process is over

within less than 3 min. The voluminous and foamy combustion ash can be easily

milled to obtain the precursor powder of SrAl2O4:Eu. The well-milled precursor

powder is subsequently annealed at 900 °C for 2 h in a programmable furnace,

producing SrAl2O4:Eu phosphor.Samples containing dopant Eu in different

concentration of 0.1, 0.2, 0.3, 0.4 and 0.5 mol% were prepared.

4.3.2 Preparation Of SrAl2O4: Dy

SrAl2O4: Dy phosphor particles were prepared by solution combustion

synthesis method followed by heating the precursor combustion ash at 900° C. In a

cylindrical quartz container, stoichiometric composition of aluminum nitrate

(Al(NO3)3), strontium nitrate (Sr(NO3)2), and Dysprosium nitrate (Dy(NO3)3) were

dissolved in a minimum amount of distilled water together with urea as fuel. The

precursor solution was introduced into a muffle furnace maintained at 500 °C.

Initially, the solution boiled and underwent dehydration, followed by

decomposition with the evolution of large amounts of gases. Then, spontaneous

ignition occurred and underwent smouldering combustion with enormous swelling,

producing white foamy and voluminousSrAl2O4: Dy. The whole process is over

within less than 3 min. The voluminous and foamy combustion ash can be easily

milled to obtain the precursor powder ofSrAl2O4: Dy. The well-milled precursor

powder is subsequently annealed at 900 °C for 2 h in a programmable furnace,

producing SrAl2O4: Dy phosphor. Samples containing dopant Dy in different

concentration of 0.1, 0.2, 0.3, 0.4 and 0.5 mol% were prepared.

EXPERIMENTAL SET UP FOR COMBUSTION SYNTHESIS

Figure 4.2 preparation setup for combustion method

4.4 CHARACTERISATION TECHNIQUES

4.4.1 X-RAY DIFFRACTION

In the present work X-ray diffraction technique is used to study the phase

formation of host material. The basic principle of X-ray diffraction is that when a

monochromatic beam of X-ray falls on a crystal, there will be only limited number

of angles at which the diffraction of the beam can occur. The diffraction of the

beam is governed by Bragg’s law given by 2dSinθ = nλ. Diffraction data depends

on the lattice parameters, which are unique for one particular material and can be

used for proper identification purposes.

Figure 4.3

The spectrum matches with JCPDS file no 31-1336 for SrAl2O4.

4.4.2 Thermoluminescence Studies

Thermoluminescence studies are carried out using a thermoluminescence

Glow Curve Reader (TL Reader).

CaCOM

Operations: Smooth 0.150 | Background 0.000,1.000 | Import

File: SAIFXR120430A-02(CaCOM).raw - Step: 0.020 ° - Step time: 31.2 s - WL1: 1.5406 - kA2 Ratio: 0.5 - Generator kV: 40 kV - Generator mA: 35 mA - Type: 2Th/

Lin

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0

1000

2000

3000

2-Theta - Scale

3 10 20 30 40 50 60 70 80

4.4.2.1 The TL Reader

It consists of an arrangement for heating the sample, light detector, signal

processor and a recorder. The block diagram of a TL reader is shown in the figure

4.4.

Figure 4.4 The block diagram of PC based TL reader

A cylindrical shell containing the Photo Multiplier Tube (PMT) is fitted on

to a rectangular base drawer block containing a heater arrangement and

thermocouple (Cr-Al). Heater current is provided through a step-down power

transformer. Kanthal strip (72%Fe, 23% Al and 2% Cr or Nichrome) is used as a

heating element. The strip has a circular depression to place the sample. Cr-Al

thermocouple spot-welded to the bottom of the heater strip is used to sense the

temperature. Any temperature from room temperature up to 500’c can be

maintained using the temperature controller setup. Regulated power supply (0v-

1500v), 1mA is used to bias the various dynodes of the PMT, DC amplifier, AD-

DA card etc, are incorporated in the system.

4.4.2.2 Radiation Units

The quantity of ionizing radiation is most often expressed in terms of (a) its

ability to produce ionization in air or the ‘exposure’ and (b) the amount of

radiation energy imparted in a medium, or the ‘absorbed’ dose. While both of the

exposure and the absorbed dose are some physical quantities of radiation, another

unit of radiation, the ‘rem’ was also used in radiobiological and radiation

protection work.

4.4.2.3 Radiation Exposure

The unit of exposure is the Roentgen (R). It is defined as the quantity of

gamma or X-rays which, when interacting with one kilogram of air, liberate

energetic electrons that produce 0.000258 coulombs of charge by ionization when

the electrons are completely stopped.

For many years Roentgen was used as a unit of radiation quantity and as a

unit of absorbed energy (dose) as well. Later on Roentgen was assigned for the unit

of exposure and another unit ‘rad’ has been designated as the unit of absorbed

dose. The current definition of the unit of exposure is as follows.

Exposure is the quotient Δq by Δm, where Δq is the sum of all the electrical

charges on all the ions of one sign produced in air when all the electrons liberated

by photons in a volume element of all air whose mass in Δm are completely

stopped in air. As a special unit of exposure, one ‘Roentgen’ or one ‘rad’ is defined

as one electrostatic unit of charge (1esu=3.33×10^-10 coulomb) produced in the

mass of 1cc of air at standard temperature and pressure or 0.001293 gm of air.

1 Roentgen = 2.58×10-4

coulomb/kg

4.4.2.4 Absorbed Dose

The absorbed dose is defined as the energy imparted by radiation per mass of

absorbing material; the material here includes all types of exposed material. The

absorbed dose is a quantity that refers to how much energy is deposited in a

material by irradiation. The term “rad” is derived from the expression “radiation

absorbed dose”.

D=ΔE/Δm

where ΔE is the energy imparted by ionizing radiation to the matter in a

volume and Δm is the mass of matter in that volume element.

1rad=100 erg/g

It should be emphasized that the term exposure only applies to photon beam

(X-ray or gamma radiation in an air medium). If the radiation is not a photon beam

or if the medium is not air, the roentgen can no longer be used.

In June 1971, the International Commission on Radiation Units and

Measurements (ICRU) adopted a new unit, the gray (Gy), as the SI unit of

absorbed dose.

1 gray (Gy) = 1joule/kg=100 rad

4.4.2.5 Dose Quantities

Ionizing radiation cannot be directly detected by human senses, but they can

be detected and measured by variety of means, such as Photographic films,

Thermo luminescent materials, Geiger tubes and Scintillation counters.

The biological effect of radiation depends on the type of radiation interacting

with the tissues. For example 1Gy to tissue from alpha radiation is more harmful

than 1Gy from beta radiation. To put all ionizing radiations on an equal footing

with regard to potential for causing harm, another quantity known as dose

equivalent is used in practice. It is the absorbed dose multiplied by the factor that

takes account of the way a particular radiation distributes energy in tissue causing

harm. For gamma ray, X-ray and beta particles, the factor is set at 1 so that both

absorbed dose and dose equivalent are numerically equal.

4.4.2.6 Source of Irradiation

Radon make X-ray unit working with 10mA, 30 KV supply was used for

irradiating the sample. 10mg phosphor was used for TL measurement each time.

4.4.2.7 The Glow Curve

The glow curve obtained for SrAl2O4: Dy irradiated with cobalt 60 γ and read

using the TL Reader is shown in the figure 4.5.

Figure 4.5 Glow Curve of SrAl2O4:Dy sample recorded with a heating rate of 20°C/s.

Figure 4.6 Glow Curve of SrAl2O4:Eu sample recorded with a heating rate of 20°C/s.

Figure 4.7 Glow Curve of undoped SrAl2O4 sample recorded with a heating rate of 20°C/s.

4.4.3 PHOTOLUMINESCENCE STUDY

The PL study of the developed phosphors was done using a

Photoluminescence (PL) studies were carried out to study the type of trapping

centres presentin the crystal lattice. Hitachi (F-4000) Fluorescence

Spectrophotometer was used for PL studies.

4.4.3.1 THE PL STUDIES

The PL reader consists of the source of primary excitation (laser/light). The

excitation source can be the UV light, electron beam or laser. The spectrum is

obtained using a monochromator equipped with an appropriate light detector. The

block diagram of a PL Reader is shown in figure 4.8.

Figure 4.8 The block diagram of PL Reader.

PL Emision spectrum for SrAl2O4:Eu0.2% andSrAl2O4:Eu0.3% Phosphors

Figure 4.9 Excitation spectrum of SrAl2O4:Eu ( 0.2% and0.3% ) ( λem = 394nm )

PL Emision spectrum for SrAl2O4:Dy0.2% andSrAl2O4:Dy0.3% Phosphors

Figure 4.10 Emission spectrum of SrAl2O4:Dy ( 0.2% and 0.3% ) ( λem = 350nm )

Results and Discussion

The XRD pattern of prepared samples are in good agreement with JCPDS data

file number 31-1336 forSrAl2O4. SrAl2O4doped with Eu and Dy are prepared using

combustion synthesis. Samples were irradiated using cobalt 60 γ source for 5Gy.it

was found thatSrAl2O4:Dy is more sensitive thanSrAl2O4:Eu. Undoped sample

gave a very low response. Samples prepared using combustion synthesis were also

subjected to PL analysis. The excitation wavelength for Dy was 350 nm and Eu

was 394 nm.SrAl2O4:Dy gave very intense PL curve than SrAl2O4:Eu.

CHAPTER 5

CONCLUSION

In the present work TL and PL analysis works were carried out on Eu and

Dy doped SrAl2O4 phosphors. It is observed that SrAl2O4:Dy is more sensitive to

TL and PL than SrAl2O4:Eu, prepared by combustion method. The prepared

phosphor is sensitive to low dose gamma radiations, radiations of dose below

10Gy. Hence the phosphor may find applications in personal and environmental

radiation dosimetry.

REFERENCE

1. Ruixi Zhang, Gaoyi Han, Liwei Zhang, Binsheng Yang, Materials

Chemistry and Physics 113 (2009) 255–259.

2. Changliang Zhao, Donghua Chen, Yuhong Yuan, Ming Wu, Materials

Science and Engineering B 133 (2006) 200–204.

3. Ramdas B. Khomane, AmitAgrawal, Bhaskar D. Kulkarni, Materials Letters

61 (2007) 4540–4544

4. TianyouPenga, HuanpingYanga, XuliPua, Bin Hua, ZuchengJianga,

ChunhuaYanb Materials Letters 58 (2004) 352– 356.

5. K.Madhukumar, HK Varma, ManojKommath, TS Elias, V. Padmanabhan

and CMK Nair Bull.Mater.sci.,Vol30,No5,pp527-534October2007.

6. U. madhusoodanan, Mt Jose, A Tomita, AR Lakshmanan J. Of Lumin. 87-

8992000) 1300-1302.

7. MT Jose. AR Lakshmanan, Optical Materials 24(2004)651-659.

8. Chen Yongjie, Qiu Guanming, gengXiujuan, yang Ying, Shi shuang, Wang

Hui, Zhang Zhonqiu J. of rare earths, Vol.25,Jun2007, p.113.

9. Nambi K S V 1977 Thermoluminescence: Its understandingand applications

(Sao Paulo, Brazil: IEA Informacao).

10. J. A. DeLuca, Journal of Chemical Education, 57 (1980) 541.

11. C. L. Kane, E. J. Mele, Phys. Rev. Lett. 90, 107401 (2003).