LUMINESCENCE STUDIES OF RARE EARTH DOPED STRONTIUM ALUMINATE PHOSPHORS
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Transcript of LUMINESCENCE STUDIES OF RARE EARTH DOPED STRONTIUM ALUMINATE PHOSPHORS
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
(C
ou
nts
)
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.
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