View
238
Download
3
Category
Preview:
Citation preview
73
CHAPTER - 3
EXPERIMENTAL TECHNIQUES AND INSTRUMENTS
This chapter gives a brief description about the basic experimental techniques/
instruments used / employed in the present investigation. Details of experimental
procedure are separately discussed in the relevant chapters. The basic techniques used
are: X-ray diffraction for ascertaining the amorphous nature of synthesized samples,
Differential Scanning calorimeter for determining the glass transition temperature,
Vickers Hardness Tester for determining the mechanical properties, Impedance
analyzer for ac / dc measurements, MAS NMR Spectrometer for understanding the
structure and dynamics of nucleus, FTIR Spectrometer for obtaining structural
information of glasses, Raman Spectrometer for studying the rotational, vibrational and
other low frequency modes in a system, ESR Spectrometer for studying the
paramagnetic centers of the sample, Photoluminescence Spectrometer and UV
absorption Spectrometer for studying the nature of glasses and hence find its potential
applications.
3.1 Test Samples
The general formulae and compositions of the samples used for the synthesis are listed
below:
Sample Chemicals used Composition
Na2O- B2O3–V2O5
Sodium carbonate (Na2CO3)
Orthoboric acid (H3BO3),
Vanadium pentoxide (V2O5)
[(100 – x)0.5( Na2O – B2O3 )
– x V2O5] (where x = 10, 15,
20, 25, 30, 35, 40, 45, 50 mol
%)
74
NaPO3–ZnO –MnO2
Sodium Hydrogen Phosphate
(NaHPO4)
Zinc oxide (ZnO)
Manganese oxide (MnO2)
[(80– x) NaPO3 – 20 ZnO –
x MnO2] (where x = 1, 3, 5,
7.5, 10 mol %)
NaPO3 –ZnO –Nd2O3
Sodium Hydrogen Phosphate
(NaHPO4)
Zinc oxide (ZnO)
Neodymium oxide (Nd2O3)
[(80 – x) NaPO3 – 20 ZnO –
x Nd2O3] (where x = 0, 0.1,
0.3, 0.5, 0.7 mol%)
3.2 X-Ray Diffraction
Powder X-ray diffraction (XRD) method is used to measure the diffraction of
X-rays from the plane of atoms within the material to investigate and quantify the nature
of materials. Diffraction occurs when X-rays having wavelength of the order of a few
angstroms interact with a structure whose repeat distance is about the same as the
wavelength of X-rays. Depending upon the atomic arrangement, the reinforcement
between the reflected rays occurs based on the selective Bragg’s law condition 2d sin θ
= n λ, where ‘d’ is the inter-planar spacing of the specimen, ‘’ the glancing angle, ‘n’
the order of the reflection and ‘λ’ the wavelength of the X-rays used.
XRD pattern is a non-destructive testing method for the identification of
crystalline and amorphous phases. The main components of the system are the
monochromatic X-ray source, finely powdered sample which is rotated against the
center (goniometer) and data collector such as film, strip chart or detector systems.
Fig.3.1 represents the schematic representation of X-ray diffractometer.
75
Fig. 3.1. Schematic representation of X-ray diffractometer
The X-rays after undergoing diffraction from the sample gets concentrated on
the detection slit, which are detected by scintillation detectors and are converted into
electrical signals. The pulse height analyzer picks up these signals after eliminating its
noise components. Spectra is recorded with the help of a chart recorder which runs
synchronous with the goniometer. The plot of angular positions and intensities of the
resultant diffraction peaks produces a pattern which is characteristic of the sample. The
glassy or amorphous materials do not have a long-range atomic order, therefore, a
diffraction pattern containing sharp peaks are not observed as in the case of crystalline
materials. The nature of the glasses, whether it is amorphous or crystalline, have been
determined with the help of X-ray diffraction (XRD) using Rigaku Model DMAX-TC
(SSCU, IISc, Bangalore). The photograph of the instrument used for the analysis is
shown in Fig.3.2.
76
Fig. 3.2. A photograph of X-ray diffractometer (XRD) used in the experimental
analysis.
3.3 Modulated Differential Scanning Calorimeter (DSC)
Calorimetry is an efficient technique, normally used for the study of the
thermodynamic properties of materials like phase transformation, specific heat, glass
transition temperature, heat capacity etc. A schematic diagram of DSC is shown in
Fig.3.3.
Fig. 3.3. Schematic diagram of Differential Scanning Calorimeter (DSC).
77
It can measure enthalpy changes in samples because the changes in their
physical and chemical properties are dependent on time or temperature. In this
technique, the difference between the temperature and heat flow of a sample and a
reference material are studied by subjecting them to temperature variation in a
controlled atmosphere. There is a constant temperature difference ‘ΔT’ between the
sample and reference material, since they have different heat capacities. This difference
in temperature (ΔT), between sample temperature (Ts ) and reference temperature (Tr)
is detected during heating/cooling and plotted against sample temperature ‘Ts’ and is
known as baseline. Glass transition (Tg) is manifested by a drastic change in the base
line, indicating a change in the heat capacity of the sample under consideration. If the
sample undergoes a physical change or a chemical reaction, its temperature will change
while the temperature of the reference material remains the same. That is because
physical changes in a material such as phase changes and chemical reactions usually
involve changes in enthalpy, the heat content of the material. In the sample, endotherms
or exotherms are obtained during phase transformation depending on the energy
absorbed or released. DSC has been widely used to find the glass transformation
temperature (Tg), changes in heat capacity, etc.
Glass transition temperature (Tg) and heat capacity (Cp) of the investigated
glasses were recorded using a Differential Scanning Calorimeter using METTLER-
TOLEDO DSC-1 (SID, IISc, Bangalore) at a heating rate of 2 degree per minute. Glass
transition temperature (Tg) were determined using the intersection of the extended
linear region in the thermograms. The photograph of the instrument used for the
analysis is shown in Fig. 3.4. High temperature furnace together with a sample carrier
suitable for Cp measurements and blank aluminium crucibles were used as reference
samples. All the recordings were carried out in nitrogen atmosphere to prevent samples
78
from oxidation. The glass transition temperature and other glass forming ability
parameters are evaluated to an accuracy of ± 1o C.
Fig 3.4. A photograph of Differential Scanning Calorimeter (DSC) used in
the experimental analysis.
3.4 Fourier Transform Infrared (FTIR) Spectrometer
Infrared (IR) spectroscopy is one of the most powerful analysis techniques
which offers the intensity measurements for the quantitative analysis. This technique
is based on the fact that the specimen under study shows marked selective absorption
in the infrared region. After absorption of the IR radiation, the molecules of the
specimen vibrate and give rise to close-packed absorption bands, called an IR
absorption spectrum which may extend over a wide wavelength range. If the natural
frequency of vibration of some part of the molecule is the same as the frequency of the
incident radiation then a molecule will absorb IR radiation [Griffiths 1983]. When
exposed to infrared radiation there occurs a change in the dipole moment of the
molecules due to selective absorption of radiation of specific wavelengths by the
molecules in the test sample. Thus, the vibrational energy levels of these molecules
shift from ground to excited state. The energy gap of the vibration is used to determine
79
the frequency of the absorption peak and the number of absorption peaks can be
determined by the number of vibrational freedom of the molecule. The intensity of
absorption peaks can be correlated to the change of dipole moment and the feasibility
of the energy level transition. Various bands in the IR spectrum will correspond to the
characteristic functional groups present in a chemical substance. Therefore, one can
readily obtain adequate information on the structure of the molecule by analyzing the
infrared spectrum. The IR spectrum of a chemical specimen is a finger print for its
identification. The absorption radiation of most organic compounds and inorganic ions
is found to be in the most common region of infrared absorption spectroscopy as its
radiation is found to be in the range of 4000 ~ 400 cm-1. One of the simplest types of
interaction of an external electromagnetic field with solids is the absorption of infrared
light. The frequency of IR radiation lies in the range 30 cm-1 ≤ 0 ≤ 2000 cm-1 and thus
coincides with typical frequencies of atomic vibrations. In glasses, the atomic vibrations
in the IR domain are quite similar to those in crystalline materials.
FTIR Spectrometer consists of a source, interferometer, sample holder, detector,
amplifier, convertor and an analyzer. The radiation generated from the source asses
through the interferometer to the sample and reaches the detector. Then the amplifier
amplifies the signal using an amplifier. This amplified signal is converted to digital
signal by an analog-to-digital converter. Finally, the Fourier transform is carried out by
transferring the signal to a computer [Perkins 1986]. The basic components of an FTIR
spectrometer are shown schematically in Fig.3.5.
80
Fig. 3.5. Schematic illustration of an FTIR system [Perkins 1986]
In the present investigation, FTIR Spectrometer (Thermo-Nicolet 6700 Range: 400 to
4000 cm-1 at SID, IISc, Bangalore) was used to carry out measurements in the range of
400-4000 cm-1 using a KBr beam splitter and a deuterated L-alanine doped triglycine
sulfate (DLaTGS) detector with a KBr window. The photograph of the instrument used
for the analysis is shown in Fig.3.6.
Fig. 3.6. A photograph of FTIR Spectrometer used in the experimental analysis.
81
3.5 Raman Spectrometer
Raman Spectrometer usually consists of source, sample holder, monochromator
and a detector. The sample is illuminated with a monochromatic light and the scattered
light is observed at right angles to the incident radiation. Light from the helium-neon
laser beam enters the sample compartment horizontally. Then the scattered light from
the sample cell is focused on the entrance slit of the monochromator. If depolarization
measurements are to be made, the Raman emission is first allowed to pass through an
analyzer prism before entering a monochromator which is a double –pass grating. A
double-pass grating acts as a filter which keeps stray light from the unshifted laser
wavelength to a minimum. The spectrum is further focused onto a Photomultiplier tube.
This detector is connected to an amplifier and a recorder which directly provides the
Raman spectrum. A block diagram of Raman spectrometer [Chatwal Anand 1979] is
shown in Fig.3.7.
Fig. 3.7. Block Diagram of Raman Spectrometer
82
In the present investigation, Lab RAM HR (UV) system (HORIBA Jobin Yvon
CCNSE-CHL-OPTCL-LRHR-E01 at CNSE, IISc, Bangalore) was used which is fully
automated with fast 2D and 3D imaging capability with a wide range of detectors and
visualization options. This system allows the collection of large area Raman images,
using the X, Y and Z mapping features in few seconds / minutes with the help of 325
nm and 514 nm LASER using CCD detector. The measurements were carried out in
the range of 200-1500 cm-1 making use of an excitation wavelength of 514.5 nm from
a 1mW laser at room temperature. The photograph of the instrument used for the
analysis is shown in Fig. 3.8.
Fig. 3.8. A photograph of Raman Spectrometer used in the experimental
analysis.
83
3.6 Nuclear Magnetic Resonance (NMR) Spectrometer
NMR is a phenomenon found in magnetic system that possess both magnetic
moments and angular momentum. The magnetic energy levels are created by keeping
the nuclei in a magnetic field. Without the magnetic field the spin states of nuclei are
degenerate and energy level transition is not possible. But as soon as the magnetic field
is applied, the separate levels and radio frequency radiation can cause transition
between these energy levels. The excitation of nuclear spin states occurs due to the
absorption in the low-energy radio-frequency part of the spectrum. Certain nuclei like
1H, 13C, 19F and 31P are tuned to NMR spectrometers. This high-resolution spectroscopy
can distinguish and count the atoms in different locations of the molecules of certain
nuclei. NMR is a powerful tool for investigating nuclear structure and is used for
understanding mainly the structure and dynamics of molecules in the matter. Fig.3.9
represents the block diagram of computer based NMR spectrometer [Constantin Job et
al 1994].
Fig. 3.9. Overall block diagram of computer based NMR Spectrometer
84
The Receiver, Preamplifier and Amplifier comprises the basic spectrometer. RF
amplifier, Receiver and Preamplifier are enclosed in an external aluminum enclosure
to minimize the electrical noise and interference with outside sources and to reduce
temperature effects. Virtually all the components of a standard NMR spectrometer
console are enclosed within the personal computer and are directly connected to the
microcomputer.
3.7 MAS NMR Spectrometer
MAS NMR spectra of finely powdered glass samples were recorded on
BRUKER DSX-300 NMR Spectrometer (NMR Lab, IISc, Bangalore) operating at
96.28 MHz. Sample spinning speeds were generally 7 kHz / minute. The chemical shift
values were referenced against saturated trimethyl boron and neat VOCl3 liquid by
using saturated NaVO3 aqueous solution (-578 ppm) for the 11B and 51V spectra,
respectively.
The measurements were made at room temperature (298K). The magnetic field
is of the range 7.04 Tesla. This spectrometer can be extensively used for Magic Angle
spinning studies and has multinuclear facilities and covers all the nuclei resonating
between 44 - 121 MHz. For Magic Angle Spinning, it has a capacity to function at a
temperature range of 300 -363 K. The achievable spinning speed is 10 - 12 KHz. The
spinning angle can vary from 0 - 90 degrees for angle dependent studies in spinning
experiments. NMR of boron and vanadium samples were studied with this instrumental
set up. The photograph of the instrument used for the analysis is shown in Fig. 3.10.
85
Fig. 3.10. A BRUKER DSX-300 NMR Spectrometer used in the
experimental analysis.
3.8 Electron Paramagnetic Resonance (EPR) Spectrometer
EPR is another important and powerful technique to study paramagnetic
centers (transition metal ions having free radicals, partially filled inner electron shells
etc). It involves magnetic dipole interactions between Zeeman levels produced by the
removal of spin degeneracy by an applied magnetic field. These transitions are induced
when an appropriate electromagnetic field at right angles is applied to the direction of
observation [Ayscough 1967]. The EPR spectrum like NMR, results from transition
from one spin state to the other state of an electron. Each spin state has an energy level,
the transition in EPR is induced by the radiation of microwave frequency rather than by
86
radio frequency. The main parts of an EPR spectrometer consists of source, sample
cavity, microwave bridge, magnets, detectors, oscilloscope and pen recorder. Fig.3.12
shows the block diagram of EPR spectrometer. The radiation source is usually a
Klystron, which is a stable high power microwave source. The sample is stationed in a
resonant cavity which passes microwaves through an iris. The weak signals from the
sample are amplified with the help of cavity located in the middle of an electromagnet.
Around 100 mg powder of the sample is placed in a sample cavity and is subjected to
microwave magnetic field of constant frequency which is perpendicular to magnetic
field ‘H’. H is varied by the electromagnetic excitation current and when the resonance
condition is fulfilled, a part of the microwave energy will be absorbed by the sample.
When H is varied, the detector output at each point on the absorption signal forms a
sinusoidal wave which is amplified by a selective amplifier. In practice, the microwave
bridge control contains most of the external components, such as the source and
detector. Additionally, other components like attenuator, field modulator and amplifier
are also included to enhance the performance of the instrument. The EPR spectra is
recorded either by first differential curve or second differential curve of the absorbed
signal.
In the present work, Bruker EMX spectrometer operating at X-band (9.025
GHz) microwave frequency was used. EPR of magnesium and vanadium samples were
studied using the spectrometer shown in Fig.3.11 [www.pharmatutor.org].
87
Fig. 3.11. Block diagram of ESR spectrometer showing essential components.
3.9 Ultraviolet - Visible (UV) absorption Spectrometer
The Ultraviolet and Visible spectroscopy is a valid and precise analytical
laboratory experimental technique that allows the analysis of a substance. Specifically,
the absorption, transmission and emission of ultraviolet and visible light by matter is
measured by the ultraviolet and visible spectroscopy. The wide ranging electromagnetic
radiation spectrum consists of only a small portion of the Ultraviolet and visible light.
In the practical sense, spectroscopy measures the absorption, emission or scattering of
electromagnetic radiations by atoms or molecules. In the present study we are
calculating the band gap and the related optical parameters of glass samples.
Instruments for measuring the absorption of UV or visible radiation comprises of
88
source, monochromatic, sample holders (reference cell and sample cell), detector,
signal processor and chart recorder. Fig. 3.12 [www. chemguide. co. uk] shows the
schematic representation of components and working of UV absorption Spectrometer.
Fig. 3.12. Block diagram of UV absorption Spectrometer showing essential
components.
When a light source consisting of the entire visible spectrum plus the near ultra-
violet is made to fall on a rotating diffraction grating, it allows light from the whole
spectrum (a tiny part of the range at a time) into the rest of the instrument. The light
coming from the diffraction grating and slit will strike the rotating disc and gets
reflected alternately through the reference cell. The sample cell contains a solution of
the substance and the solvent is chosen so that it doesn't absorb any notable amount of
light in the wavelength range (200 - 800 nm). Chart recorders usually plot absorbance
against wavelength.
In the present work Perkin - Elmer (Lambda-35) spectrometer [CNSE, IISc,
Bangalore], which is a double beam spectrophotometer with variable band width from
0.5 - 4 nm was used. It provides sensitive measurements with accessories such as fiber-
89
optics probes and integrating spheres for solid samples for online and macro sample
testing with advanced color software. Its wavelength ranges from 190-1100 nm and has
a variable bandwidth for higher resolution. It possess 50 mm integrating sphere for
reflectance measurements and thus measures absorbance of liquid, thin films and
powder samples. Fig. 3.13 shows the experimental setup used for the optical absorption
measurements in the present investigation.
Fig. 3.13. Perkin- Elmer (Lambda-35) spectrometer
3.10 Photoluminescence (PL) Spectrometer
Photoluminescence (PL) occurs under optical excitation with the emission of
light from the sample under consideration. When light of sufficient energy is incident
on the sample, photons are absorbed and electronic excitations takes place. Finally, if
this relaxation is radiative, the electrons return to the ground state and the emitted light
will be photoluminescence signal. From the intensity of the PL signal, the measure of
the relative rates of radiative and non-radiative recombinations can be obtained.
Because PL depends much on the nature of the optical excitation, the excitation energy
usually selects the initial photo-excited state and governs the depth of the penetration
90
of incident light. Rare-earth–doped glasses can be usefully investigated by using PL
techniques. The experimental set up of PL is as shown in Fig. 3.14.
Fig. 3.14. Diagram of PL experiment set-up
Photoluminescence occurs when a laser source adjusted to a wavelength close
to the band gap energy of the sample is lead onto the sample. Light will be emitted from
the sample at wavelengths based on the composition of the sample. The sample is
aligned such that the reflected laser beam and the PL emission transmit in different
directions. The emitted light is targeted into a fiber optic cable and then to a
spectrometer. Filter placed in front of the fiber input will remove the stray incident
laser light. Wavelengths are diffracted in different directions with the help of
diffraction grating inside the spectrometer towards an array of photo-detectors, which
measures the intensity of each component of wavelength. The computer finally displays
a PL spectrum by interpreting the digital information. The relative intensities of light
of different wavelengths entering the detector is an indicator of the spectrum [Gfroerer
2000].
91
In the present investigation, Lab RAM HR (UV) system (HORIBA Jobin Yvon
CCNSE-CHL-OPTCL-LRHR-E01 at CNSE, IISc, Bangalore) was also used for PL
measurements. The measurements were carried out in the range of 200-1500 nm using
an excitation laser source at 325 and 514.5 nm (depending upon the sample). The
photograph of the instrument used for the analysis is shown in Fig. 3.8.
3.11 Electrical conductivity measurements
Electrical properties of glasses are some of the most important aspects to be
studied for their applications in electrical and electronic industries. A substance is said
to be electrically conducting when free electrons or ions within make the flow of current
possible. This property is characterized by the parameter called electrical conductivity,
which is reciprocal of resistivity. Electrical conductivity measurements [Veeranna
Gowda et al 2013] were carried out as a function of frequency and temperature by
employing a Hewlett- Packard (HP 4192A) impedance gain phase analyzer of
frequency range 10Hz -10MHz and the temperature range of 303 K - 343 K. A home
built cell assembly (having two terminal capacitor configuration and spring loaded
silver electrodes) was used for the measurements. The bulk electrical resistivity is
determined by the complex impedance analysis of the frequency- dependent
capacitance and conductance data. A schematic diagram of the fabricated conductivity
cell is shown in Fig. 3.15.
92
Fig. 3.15. Schematic representation of the cell used for conductivity
measurements [Veeranna Gowda et al 2013].
The sample temperature was measured using a Pt–Rh thermocouple positioned very
close to the sample. The temperature was controlled using a Heatcon (Bangalore, India)
temperature controller. Annealed glass pieces were coated with silver paint on both
sides so as to serve as electrodes for measurements. The capacitance (C) and
conductance (G) of all the samples were measured from the impedance analyser. These
were used to evaluate real and imaginary parts of the complex impedance using
standard relations (Macdonald 1983; Sundeep Kumar and Rao 2004).
The conductivity is calculated using the values of bulk resistance (Rb), radius
(r) and thickness (t) of the glass pieces using the equation . The d.c
conductance’s were determined from the semicircular complex impedance plots. Thus,
the conductivity ‘σ’, activation energy ‘Ea’, dielectric permittivity ‘ ε ' and electric
modulus M* can be calculated from the real and imaginary parts of the impedance data.
Recommended