2 Chapter 2

Chapter 2

Experimental Methods

63

2.1 Introduction:

This chapter deals with the detailed description of the methods used in the

preparation of lead antimony borate glasses doped with rare earth ions and also various

experimental techniques employed in the characterization of glasses. The apparatus

used and detailed procedures followed in studying X-ray diffraction, FTIR spectra,

optical absorption and photoluminescence are described.

2.2 Glass preparation

For present study the following compositions of the glasses are chosen.

1. 30 PbO-25 Sb2O3-(45-x) B2O3 –x Dy2O3 (x = 0.2 to 1mol% in steps of 0.2)

2. 30 PbO-25 Sb2O3-(45-x) B2O3 –x Er2O3 (x = 0.2 to 1mol% in steps of 0.2)

3. 30 PbO-25 Sb2O3-(45-x) B2O3 –x Nd2O3 (x = 0.2 to 1mol% in steps of 0.2)

The various methods of preparation of glasses were reviewed by Zarzycki [1]

and are given as

a. Melt quenching method g. Thermal evaporation

b. Chemical vapour deposition h. Glow-discharge decomposition

c. Electrolytic deposition i. Gel-desiccation

d. Glow discharge deposition j. Chemical Reaction

e. Sputtering k. Shear amorphisation

f. Irradiation l. Shock wave transformation

For the preparation of samples of present system, we have chosen melt

quenching method [2-4].

Melt quenching method:

The analytical grade reagents of PbO, Sb2O3, H3BO3, Dy2O3, Er2O3 and Dy2O3

were purchased from Sigma Aldrich with 99.99% purity. Fig. 2.1 shows schematic

diagram of furnace to prepare the glasses by melt quenching method. The compounds of

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required composition were thoroughly mixed in agate mortar. The mixture is taken in

platinum crucible and melted in an electric furnace about 1 hour. The melt was

quenched by pouring it on a thick brass plate and pressed by another brass plate. The

glass samples were annealed in another furnace at about 250 °C for four hours to

remove thermal stains. The samples prepared were ground and optical polished to the

dimensions of 1cm ×1cm × 0.2cm.

Fig. 2.1 Photograph of electric furnace.

2.3 Physical parameters

(a)Density

The density (d) of the samples was determined by using standard Archimedes principle

using O-xylene as the buoyant liquid at room temperature by means of the following

formula

21

1

ww

wd

−=

where w1 is the weight of glass sample in air and w2 is the weight of the glass sample in

buoyant liquid at room temperature.

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(b) Molar volume

Molar volume can be calculated by dividing average molecular weight with

density.

Molar volume, Vm =���������� �����

�������

(c) Oxygen packing density

Similarly oxygen packing density can be calculate by using the formula

Oxygen packing density, O (g atom/L) =�������������������������������

(d) The dopant ion concentration (Ni)

The concentration (C) of the rare earth ion in the glass was calculated in the

units of mol/lit by using the equation

1000××=MW

d

y

xN i

where x is mass of the rare earth salt, y is the mass of total composition, d is the density

of glass and MW is molecular weight of rare earth salt

The concentration in mol/lit can be converted in to ions/cm3 by multiplying the

above value with a factor of NA/1000 where NA is Avogadro number.

(e)Inter-ionic distance

Inter-ionic distance ri (Å) =

3/11

iN

(f) Polaron radius

Polaron radius, rp (Å) =

3/1

62

1

iN

π

66

(g) The field strength

The field strength (Fi) of dopant ion in the glass network is described

through the oxidation number (Z) and the polaron radius (rp) of the transition

metal ions by:

Field strength, Fi (cm–2) = 2pr

Z

(h) Refractive index

The refractive index (nd) of the glasses was measured by Abbe refractometer

with monobromo naphthalene as contact liquid between the glass and refractometer

prism at room temperature with a wavelength λ=589.3 nm.

Working principle

The main part of Abbe’s refractometer is prism lens. It consists of two Abbe’s

prism of flint glasses of high refractive index, 1.75 cemented into mounting hollow

cases. The cases act as jackets and water from the thermostat can be circulated around

the prism for controlling temperature. The upper face of the lower prism is mounted so

that it will act as a diffusion screen giving rays in all directions and confines a thin

sample of the liquid. The lower face of the upper prism, known as the refractive prism is

highly polished. The two prism cases are lingered together, when the prisms are closed

and touched by a locknut with a space of 0.1 to 0.14 mm between the faces of the prism.

A small quantity of the sample under experiment is introduced in this narrow space.

The case of upper prism is rigidly attached to the index arm, which carries a

scale at its upper end. The scale is graduated in terms of refractive index from 1.3 to 1.7

up to 3rd decimal.

67

Fig. 2.2. The schematic diagram of Abbe refractometer

A telescope is rigidly fixed into the sector which is attached to upright of base in

such a way that the axis or the sector coincides with the rotation of the prism lens. The

prism lens can be rotated by means of the arm which can be moved by a knob. The line

of demonstration corresponds to the critical angle is seen through the telescope and the

index arm is rotated so that the border line passes exactly through the refractive index,

thereby intersecting the cross wires. The refractive index is read through the eye piece

at the upper end. The schematic diagram of Abbe refractometer was shown in the Fig.

2.2.

2.4 Characterization of the samples

2.4.1 X-ray diffraction

Working principle of X-ray diffractometer is based on diffraction of X-rays

which have the same order of wavelength with the lattice parameters in the crystal. Cu

Kα1 radiation is generally used for diffraction. This is produced from the cupper target

in X-ray generator. Wavelength of this radiation is 1.54056 Å. There is some X-ray

68

optics which is located in front of the X-ray tube and it can be changed according to the

application. This optics consists of some slits and monochromators. There is a counter

detector in the diffracted beam side. This detector is generally scintillation counter.

Additionally, several attenuators (Al, Cu and Ni) can be located in detector arm when it

is needed. Sample is located on sample holder on goniometer. Diffracted X-ray beam is

detected by scintillation detector. This is a kind of photomultiplier. The Schematic

diagram of X-ray diffraction was presented in Fig. 2.3.

Fig. 2.3 (a) Schematic diagram of X-ray diffraction (b) Diffraction pattern of amorphous material and (c) Diffraction pattern of crystalline material.

The X-ray diffraction of the samples was recorded on Rigaku Diffractometer mini flex

with CuKɑ radiation. The specifications of the instrument are given in the Table 2.1.

Glass or amorphous materials do not have the long range atomic order i.e. the atoms in

the samples are arranged randomly. Hence the diffraction pattern of these samples do

not contain sharp peaks as observed in crystalline samples. All the glass samples were

subjected to X-ray diffraction. The resultant X-ray diffractograms are observed to be

unsymmetrical without containing any sharp peaks confirmed the glassy nature of the

samples. The Basic components, working principle and X-ray diffractometer diagram

are presented in Fig. 2.4.

Fig. 2.4. X-ray diffraction mechanism (a) Basic components of X-ray diffractometer (b) Rigaku diffractometer mini flex (c)

Schematic diagram and working principle of X-ray diffractometer (d) X- ray difractograms of Er3+ doped lead antimony

borate glasses

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Table 2.1 : Specifications of X-ray diffractometer

Software Instrument control Control & Measurement

Data analysis PDXL

Generator Maximum power 600 W

Tube voltage 40 kV

X-ray Tube Cu, Co, Fe, or Cr

Optics Divergence slit Fixed or variable

Filter Kβ foil filter

Goniometer Type Vertical

Radius 150 mm

Scanning range -3 to 145° (2θ)

Scanning speed 0.01 to 100°/min (2θ）

Detector Scintillation counter NaI scintillator

D/teX Ultra (Optional) High speed silicon strip detector

Power Supply Main body 100 to 240 VAC 1φ ±10%

50/60 Hz ±1% 1.0 kVA

50/60Hz ±1% 1.1kVA

2.4.2 Differential scanning calorimetry

The glass transition temperatures of these glasses were determined by differential

scanning calorimetry traces that were recorded using universal V23C TA differential

scanning calorimeter with a programmed heating rate of 20 oC per minute in the

temperature range 30 oC -700 oC to an accuracy of ±0.1 oC.

Differential scanning calorimetry (DSC) monitors heat effects associated with

phase transitions and chemical reactions as a function of temperature. In a DSC the

difference in heat flow to the sample and a reference at the same temperature, is

recorded as a function of temperature. The reference is an inert material such as alumina

or just an empty aluminum pan. The temperature of both the sample and reference are

71

increased at a constant rate. Since the DSC is at constant pressure, heat flow is

equivalent to enthalpy changes:

���� !" =�$�

Where %&%' is the heat flow measured in mcal sec-1. The heat flow difference

between the sample and the reference is:

ferenceSample dT

dH

dT

dH

dT

dH

Re

−

=

and can be either positive or negative. In an endothermic process heat is absorbed and

therefore, heat flow to the sample is higher than that to the reference. Hence

∆dT

dH is

positive. Other endothermic processes include helix-coil transitions in DNA, protein

denaturation, dehydrations, reduction reactions, and some decomposition reactions. In

an exothermic process, such as crystallization, some cross-linking processes, oxidation

reactions, and some decomposition reactions, the opposite is true and

∆dT

dHis

negative.

The calorimeter consists of a sample holder and a reference holder as shown in

Fig1. Both are constructed of platinum to allow high temperature operation. Under each

holder, there is a resistance heater and a temperature sensor. Currents are applied to the

two heaters to increase the temperature at the selected rate. The difference in the power

to the two holders, necessary to maintain the holders at the same temperature, is used to

calculate

∆dT

dH. A schematic diagram of a DSC is shown in Figure 2. A flow of

nitrogen gas is maintained over the samples to create a reproducible and dry

atmosphere. The nitrogen atmosphere also eliminates air oxidation of the samples at

high temperatures. The sample is sealed into a small aluminum pan. The reference is

usually an empty pan and cover. The pans hold up to about 10 mg of material.

Fig. 2.5 (a) Schematic diagram of differential scanning calorimeter. (b) Graph between time Vs. heat flux. (c) Graph between temperature Vs. heat flux.

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During heating of a sample, for example, from room temperature to its

decomposition temperature, peaks with positive and negative

∆dT

dH may be recorded;

each peak corresponds to a heat effect associated with a specific process, such as

crystallization or melting

The following information is obtainable from a DSC curve.

a) The first and most direct information is the temperature at which a certain process

occurs, for example, the melting point of a polymer. The temperature at which a

reaction, such as decomposition, may start is another important parameter. The peak

temperature is associated with the temperature at which maximum reaction rate occurs.

A special case in which the temperature of a phase transformation is of great

importance in polymers and glasses is the glass transition temperature, Tg. This is the

temperature at which amorphous (non-crystalline) materials are converted from a

brittle, glasslike form to a rubbery flexible form. This is not a true phase transition but

one that involves a change in the local degrees of freedom. Above the glass transition

temperature certain segmental motions of the polymer are comparatively unhindered by

the interaction with neighbouring chains. Below the glass transition temperature, such

motions are hindered greatly, and the relaxation times associated with such hindered

motions are usually long compared to the duration of the experiment.

The operative definition of glass transition temperature is that at this

temperature, or within a few degrees, the specific heat, the coefficient of thermal

expansion, the free volume, and the dielectric constant (in the case of a polar polymer)

all change rapidly.

Since the mechanical behavior of polymers changes markedly at the glass

transition temperature, it is an important characteristic of every polymer. In the DSC

experiment, Tg is manifested by a drastic change in the base line, indicating a change in

the heat capacity of the glass. No enthalpy is associated with such transition (for which

74

reason it is also called a second order transition); therefore, the effect in a DSC curve is

slight and is observable only if the instrument is sensitive enough.

b) The second direct information obtainable from DSC curves is the enthalpy associated

with certain processes.

2.5 Spectroscopic properties

Under spectroscopic properties, FTIR spectra, optical absorption spectra and

luminescence spectra of lead antimony borate glasses doped with different

concentrations of Dy2O3, Er2O3 and Nd2O3 are studied.

2.5.1 FTIR spectra

The FTIR transmission spectra of these glasses were recorded using a Perkin Elmer

Spectrometer in the wavenumber range 400 cm-1-4400 cm-1 by KBr pellet method.

Working principle of FTIR spectrometer

Fig. 2.6 (a) shows a continuous wave spectrometer. A source generates light

across the spectrum of interest. A monochromator (in IR this can be either a salt prism

or a grating with finely spaced etched lines) separates the source radiation into its

different wavelengths. A slit selects the collection of wavelengths that shine through the

sample at any given time. In double beam operation, a beam splitter separates the

incident beam in two; half goes to the sample, and half to a reference. The sample

absorbs light according to its chemical properties. A detector collects the radiation that

passes through the sample and in double-beam operation, compares its energy to that

going through the reference.

The detector puts out an electrical signal which is normally sent directly to an

analog recorder. A link between the monochromator and the recorder allows you to

record energy as a function of frequency or wavelength, depending on how the recorder

is calibrated.

75

Although very accurate instruments can be designed on these principles, there

are several important limitations. First, the monochromator/ slit limit the amount of

signal one can get at a particular resolution. To improve resolution, you must narrow

the slit and decrease sensitivity. Second, there is no easy way to run multiple scans to

build up signal-to-noise ratios. Finally, the instrument must be repetitively calibrated,

because the analog connection between the monochromator position and the recording

device is subject to misalignment and wear.

Fig. 2.6 : Schematic diagram of (a) Continuous wave spectrometer. (b) FTIR spectrometer.

Fig. 2.6 (b) shows FTIR spectrometer. We still have a source, a sample and a

detector, but everything else is different. Now, we send all the source energy through

an interferometer and onto the sample. In every scan, all source radiation gets to the

sample. The interferometer is a fundamentally different piece of equipment than a

monochromator. The light passes through a beam splitter, which sends the light in two

directions at right angles. One beam goes to a stationary mirror then back to the beam

splitter. The other goes to a moving mirror. The motion of the mirror makes the total

path length variable versus that taken by the stationary-mirror beam. When the two

meet up again at the beam splitter, they recombine, but the difference in path lengths

creates constructive and destructive interference: an interferogram:

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Fig. 2.7 Block diagram of an FTIR spectrometer.

The recombined beam passes through the sample. The sample absorbs all the

different wavelengths characteristic of its spectrum, and this subtracts specific

77

wavelengths from the interferogram. The detector now reports variation in energy

versus time for all wavelengths simultaneously. A laser beam is superimposed to

provide a reference for the instrument operation.

Energy versus time is an odd way to record a spectrum, until you recognize the

relationship between time and frequency. A mathematical function called a Fourier

transform allows us to convert an intensity (vs) time spectrum into an intensity (vs)

frequency spectrum. A common FTIR spectrometer consists of a source, interferometer,

sample compartment, detector, amplifier, A/D convertor and a computer. The source

generates radiation which passes the sample through the interferometer and reaches the

detector. Then the signal is amplified and converted to digital signal by the amplifier

and analog-to-digital converter respectively. Eventually, the signal is transferred to a

computer in which Fourier transform is carried out. Fig. 2.7 is a block diagram of an

FTIR spectrometer.

2.5.2 Spectrophotometry

The optical absorption spectra of all the glasses were recorded at room

temperature in the spectral wavelength region ranging from 300nm-2100nm with a

spectral resolution of 1nm was recorded by using JASCO Model V670 UV-VIS-NIR

spectrophotometer.

A spectrophotometer is an instrument that measures the fraction of the incident

light transmitted through a solution. In other words, it is used to measure the amount of

light that passes through a sample material and by comparison to the initial intensity of

light reaching the sample, they indirectly measure the amount of light absorbed by that

sample.

Spectrophotometers are designed to transmit light of narrow wavelength ranges

(Ultra violet, Visible and near infrared of electromagnetic spectrum). A given

compound will not absorb all wavelengths equally-that is why things are different

colors (some compounds absorb only wavelengths outside of the visible light spectrum,

and that is why there are colorless solutions like water). Because different compounds

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absorb light at different wavelengths, a spectrophotometer can be used to distinguish

compounds by analyzing the pattern of wavelengths absorbed by a given sample.

Additionally, the amount of light absorbed is directly proportional to the concentration

of absorbing compounds in that sample, so a spectrophotometer can also be used to

determine concentrations of compounds in solution. Finally, because particles in

suspension will scatter light (thus preventing it from reaching the light detector),

spectrophotometers may also be used to estimate the number of cells in suspension.

In spectrophotometry, a compound in solution is kept in sample holder called a

cuvette and places it in the spectrophotometer. Light of a particular wavelength passes

through the solution inside the cuvette and the amount of light transmitted (passed

through the solution-transmittance) or absorbed (absorbance) by the solution is

measured by a light meter. While a spectrophotometer can display measurements as

either transmittance or absorbance; we are usually interested in the absorbance of a

given sample.

79

Because other compounds in a solution (or the solvent itself) may absorb the

same wavelengths as the compound being analyzed, we compare the absorbance of our

test solution to a reference blank. Ideally, the reference blank should contain everything

found in the sample solution except the substance you are trying to analyze or measure.

The amount of light transmitted through a solution is referred as transmittance (T). The

transmittance is defined as the ratio of the light energy transmitted through the sample

(I) to the energy transmitted through the reference blank (I0).

0I

IT = 2.1

This number is multiplied by 100 to determine the transmittance percent (%T),

the percentage of light transmitted by the substance relative to the reference blank.

Transmittance percentage is 100%0

×=I

IT

100%0

×=I

IT 2.2

A certain portion of the light will be absorbed by the compound in the test

cuvette therefore its %T will be lower than that of the blank (by definition, 100%).

For most applications however, we measure absorbance (A, also referred to as optical

density or OD, where λ is the wavelength used for the measurements), the amount of

light absorbed by a solution. Absorbance is related logarithmically to transmittance by

the equation.

A = -log T

2.3

The basic components of spectrophotometer are given bellow

Fig. 2.8 (a) Schematic diagram of spectrophotometer. (b) Photograph of JASCO V670 UV-Vis-NIR spectrophotometer. (c)

Schematic diagram of filter.

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Table 2.2. Performance specifications of JASCO Model V670 UV-VIS-NIR

spectrophotometer.

Item Specification

Optical System Single monochromator; dual-grating, dual detector

UV/VIS region: 1200 lines/mm plane grating

NIR region: 300 lines/mm plane grating

Light Source Deuterium lamp: 190nm to 350nm

Halogen lamp: 330nm to 2700nm (3200nm, option)

Detector Photomultiplier tube (PMT) Peltier cooled PbS detector

Detector/ Grating Exchange

Grating and detector exchange is user selected in the range of 750nm to 900nm.

Wavelength Range 190nm to 2700nm

Wavelength Accuracy ±0.3nm (using a spectral bandwidth of 0.5nm;

Scanning Speed 10nm-4000nm/min

(a) Source

Fig. 2.8 shows the schematic diagram of spectrophotometer. The sources of the

lights are deuterium ark lamp for UV region (wavelength range 190nm-420nm),

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halogen lamp for visible and NIR region (wavelength range 350nm-2500 nm).

Occasionally xenon lamp is also used for UV-Visible region (wavelength range 190nm

800 nm). The light from the source will be reflected by the mirror M1 and collimated in

to fine beam by the collimating lens and passed in to monocromator.

(b) Monocromator

The light will be made into fine pencil of beam by passing through entrance slit

and made to fall on mirror M2. The reflected light is made to fall on a rotating grating.

The grating will split the light in to spectrum and this spectrum of light is made to pass

through exit slit via mirrors M2 and M3.The wavelength of light reaching the exit cell

changes with respect to the rotation of grating. This light will be splitted into two parts

by the beam splitter in which one beam passes through sample cell and the other will be

passed through reference cell.

(c) Filter

The light from the monocromator is made to pass through filter which converts

wide band input radiation to narrow band radiation. The filter wheel is driven by a

stepping motor to be in synchronization with the monocromator. Depending upon the

wavelength from the monocromator, the appropriate filter will be automatically placed

in the path of the beam.

(e) Beam splitter

The light from the filter will be splitted into two rays by the beam splitter. One

ray will passes through the sample cell and the other ray will passes through the

reference cell which will have identical environment except the sample. Both the rays

will be ensured to travel the same path before entering the photo multiplier.

(f) Photo multiplier

The photo multiplier will contain the photo detectors which will detect the

intensity variation from both the reference and the electrical system will gives the

output of absorption of light with respect to wavelength.

83

2.6 Excitation and photoluminescence spectra

The emission, excitation and decay measurements of Re3+ doped glasses were

recorded on a Jobin-Yvon Fluorolog-3 spectrofluorimeter using xenon arc lamp (450 W)

as an excited source. The photograph of the Jobin-Yvon fluorolog-3 spectrofluorimeter

and its optical layout are shown in Fig. 2.9. and Fig. 2.10 respectively.

The beam of the light emitted by xenon arc lamp was focused on the sample by a

built in excitation monochromator. Emitted light from the sample passes through built in

emission monochromator and the intensity is recorded by a built in photomultiplier tube

(PMT). The slits of the monochromator can be adjusted depending upon the type of

experiment being carried out. A filter is placed in front of the detector to prevent lamp

light from entering. Otherwise second order light would screw spectra. Finally,

spectrofluorimeter is connected to a computer with its own software (Spectra) which

gives the intensity output by integrating the value recorded by the PMT over a period of

time. The data is exported and analyzed by origin software.

Fig. 2.11. Experimental set up of JOBIN- YVON Fluorolog- 3 Spectrofluorimeter.

84

Fig. 2.12. Optical layout of JOBIN- YVON Fluorolog- 3 Spectrofluorimeter.

85

References [1] J.Zaezycki, Glasseseous states and the vitreous state, Cambridge University Press,

Cambridge (1991).

[2] A. Paul, Chapman & Hall, London (1982).

[3] S. R Elliot, Physics of amorphous materials, Longman, London 1990.

[4] J. F. Shackl Ford, Introduction to materials science for engineers, Macmillan, New York, 1985.