Upload
others
View
5
Download
0
Embed Size (px)
Citation preview
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
64
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.
65
(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
70
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.
73
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:
76
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
78
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.
81
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),
82
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.