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CHAPTER-4
CHARACTERIZATION TECHNIQUES
Characterization Techniques
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CHAPTER –IV
4.0 CHARACTERIZATION TECHNIQUES
The characterization of Barium Titanate nanoparticles synthesized using
optimized wet chemical process is required to be explored for its different
properties using different analytical techniques. The structural characterization of
the samples was carried out using the X-ray Diffraction (XRD). The particle size
of the dielectric materials has a strong impact on various dielectric properties.
Thus, detailed studies were carried out on particle size of the materials. The
agglomerated particle sizes were characterized and confirmed using the
Scanning Electron Microscopy (SEM). The purity of samples and the absence of
the byproducts were checked and confirmed by Elemental dispersive X-ray
Spectrum (EDXs). The presence of the functional bonds was confirmed by
Fourier Transform Infrared techniques (FT-IR). The detailed description of the
analysis of the above mentioned techniques is given below.
4.1 Instrumental Characterization Techniques
Three analytical techniques namely X-ray diffraction (XRD), Energy Dispersive X-
ray spectroscopy (EDXs) and Scanning Electron Microscopy (SEM) has been
utilized for the structural characterization of the ceramics. SEM in association
with energy dispersive X-ray analysis is used for microstructural investigations as
well as micro-area quantitative analyses while XRD is used for phase
identification.
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4.1.1 X-ray diffraction (XRD)
Considering the crystalline nature of the materials, all solids can be broadly
classified into amorphous and crystalline states. In amorphous state the atoms or
molecules are arranged in a random manner similar to the liquid or gaseous
materials whereas in the crystalline state the arrangement of atoms and
molecules are regular in all directions throughout the solid.
About 95% of all solid materials are described as crystalline. In 1919, A. W. Hull
wrote a paper entitled “A new method of chemical analysis.” Where he pointed
out that every crystalline substance gives a pattern and the same substance
always gives the same pattern, whereas in a mixture of substances each
produce its pattern independently of the other [4.1].
The X-ray powder diffraction method is most suited for characterization and
identification of polycrystalline materials and its different phases. It is a rapid
analytical technique primarily used for identification of phases in the crystalline
materials and can provide information about the unit cell dimensions. When X-
rays interact with a crystalline substance it gets diffracted and a diffraction
pattern is obtained. The X-ray diffraction pattern of a pure substance is therefore
like a fingerprint of the substance.
Today more than 50,000 inorganic and around 25,000 organic single
components, crystalline phase diffraction pattern have been collected and stored
on magnetic or optical media as standards.
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Working Principle of X-Ray Diffraction
An X-ray beam incident on a pair of parallel planes P1
and P2
Figure 4.1 Bragg’s Law
The two parallel incident rays 1 and 2 make an angle (θ) with these planes. A
reflected beam of maximum intensity will result if the waves represented by 1'
and 2' are in phase. The difference in path length between 1 to 1' and 2 to 2'
must be an integral multiple of wavelengths (λ). We can express this relationship
mathematically in the form of Bragg's Law. This law relates the wavelength of
electromagnetic radiation to the diffraction angle and the lattice spacing in a
crystalline sample.
and so on
separated by an interplanar spacing d as shown in Figure 4.1.
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The Bragg’s law is express as:
2d Sin θ = nλ ….. (4.1)
Where θ is the angle of incidence
d is the inter planer spacing
n is the order of reflection and
λ is the wave length of the incident beam used.
Rewriting Bragg’s law we get
Sin θ = nλ/2d ….. (4.2)
Therefore the possible 2θ values where we can have reflection are determined
by the unit cell dimensions. However, the intensities of the reflections are
determined by the distribution of the electrons in the unit cell. The highest
electron density is found around atoms. Therefore, the intensities depend on
what kind of atoms we have and where in the unit cell they are located. Planes
going through areas with high electron density will reflect strongly and planes
with low electron density will give weak intensities. The interaction of incident
rays with the sample produce constructive interference (and a diffracted ray)
when Bragg’s conditions are satisfied. These diffracted X-rays are then detected,
processed and counted by scanning the sample through a range of 2θ angles. All
possible direction of the lattice should be attained due to the random orientation
of the materials because each material has a set of d-spacing, which is achieved
by comparison of d-spacing with standard data [4.2-4.3].
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X-Ray diffraction was performed on a Bruker AXS D8 shown in Figure 4.2.
Advance X-Ray diffractometer uses Ni filtered Cu-Kα radiation. Normal XRD
scans with step resolution of 0.02° with time step of 0.5 sec was used. To ensure
stability of the measurements with respect to change in resolution in angular co-
ordinates and time, measurements were repeated with angular step size (in 2θ)
of 0.05° with time step of 2s. The Cu-Kα2 diffraction signal was removed by a
standard stripping procedure to obtain the correct lattice parameters and grain
size.
Figure 4.2 Powder XRD
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4.1.2 Scanning Electron Microscopy (SEM)
The scanning electron microscope (SEM) is a type of electron microscope that
images the sample surface by scanning it with a high energy beam of electrons
in a raster scan pattern. The electrons interact with the atoms that make up the
sample producing signal that contain information about the sample. Electronic
devices are used to detect and amplify the signals and display them as an image
on a cathode ray tube in which the scanning is synchronized with that of the
microscope. The image is displayed on a computer monitor.
The type of signals made by SEM can include secondary electrons, back
scattered electrons, characteristic X-rays and light (cathodoluminescence).
These signals come from the beam of electrons striking the surface of the
specimen and interacting with the sample at or near its surface. In its primary
detection mode secondary electron imaging, the SEM can produce very high
resolution images of a sample surface, revealing details about 1 to 5nm in size.
SEM micrographs are useful for understanding the surface structure of a sample.
This great depth of field and the wide range of magnifications (commonly about
25 times to 250,000 times) are available in the most common imaging mode for
specimen in the SEM. Characteristic X-rays are the second most common
imaging mode for the SEM. X-rays are emitted when the electron beam removes
an inner shell electron from the sample causing a higher energy electron to fill
the shell and give off energy. The characteristic X-rays are used to identify the
elemental composition of the sample.
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Back scattered electrons (BSE) that come from the sample may also be used to
form an image. BSE images are often used in analytical SEM along with the
spectra made from the characteristic X-rays due to the elemental composition of
the sample [4.4].
4.1.2.1 Scanning Process
In a typical SEM, electrons are thermionically emitted from a tungsten filament
cathode and are accelerated towards an anode. Tungsten is normally used in
themionic electron guns because it has the highest melting point and lowest
vapour pressure of all metals, thereby allowing it to be heated for electron
emission. However, lanthanum hexaboride (LaB6) cathodes are also used.
Alternatively electrons can be emitted using a field emission gun (FEG). The
electron beam, which typically has an energy ranging from a few hundred eV to
100 keV, is focused by one or two condenser lenses into a beam with a very fine
focal spot sized 0.4nm to 5nm. The beam posses through pair of scanning coils
or pairs of deflector plates in the electron optical column, typically in the objective
lens, which deflect the beam horizontally and vertically so that it scans in a raster
fashion over a rectangular area of the sample surface. When the primary
electrons beam interacts with the sample, the electron loses energy by repeated
scattering and absorption within a teardrop-shaped volume. The size of the
interaction volume depends on the electron landing energy, the atomic number of
the specimen and its density.
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4.1.2.2 Secondary Electron Imaging
This mode provides high-resolution imaging of fine surface morphology. Inelastic
electron scattering caused by the interaction between the samples electron and
the incident electrons results in the emission of low-energy electrons from near
the samples surface. The topography of surface features influences the number
of electrons that reach the secondary electron detector from any point on the
scanned surface. This local variation in electron intensity creates the image
contrast that reveals the surface morphology.
4.1.2.3 Backscatter Electron Imaging
This mode provides image contrast as a function of elemental composition as
well as surface topography. Backscattered electrons are produced by the elastic
interactions between the sample and the incident electron beam. These high-
energy electrons can escape from much deeper than secondary electrons, so
surface topography is not as accurately resolved as for secondary electron
imaging. The production efficiency for backscattered electrons is proportional to
the sample materials mean atomic number which results in image contrast as a
function of composition i.e. higher atomic number material appears brighter than
low atomic number material in a backscattered electron image. The optimum
resolution for backscattered electron imaging is about 5.5 nm.
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4.1.2.4 Resolution of the SEM
The special resolution of the SEM depends on the size of the electron spot which
in turn depends on both the wavelength of the electrons and the magnetic
electron-optical system which produces the scanning beam. The resolution is
also limited by the size of the introduction volume or the extent to which the
material interacts with the electron beam. The SEM has advantages to image a
comparatively large area of the specimen, the ability to image bulk material and
the variety of analytical modes available for measuring the composition and
nature of the specimen.
The morphology and the agglomerated particle size of the samples were
analyzed using Scanning Electron Microscope. SEM analysis was carried out on
cryogenically broken samples using field emission SEM on JEOL JSM-6380LV,
Japan at accelerating voltage of 20 KV. The samples were coated by Platinum
Sputter Coater vacuum coater (JEOL, JFC 1600, Auto fine Coater) to minimize
electrostatic charging. Figure 4.3 represents the SEM instrument.
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Figure 4.3 Scanning Electron Microscope
4.1.3 Energy dispersive X-ray spectroscopy (EDS)
EDS provides quantification with reasonable accuracy (≥ 5 at. %) by invoking
ZAF correction. It is called so because it accounts for the influence of atomic
number (Z), X-ray absorption (A) and secondary fluorescence (F) effects. The Z
effect comprises two factors namely (a) the proportion of incident electrons
backscattered out of the sample and therefore not available for X-ray excitation,
and (b) influence of the electron stopping power. The absorption correction takes
into consideration the attenuation of X-rays by the specimen on their way out to
the detector while the fluorescence correction takes into account the
fluorescence of one element by the X-ray from another element.
It is also equipped with a Si(Li) detector for automated quantitative EDS
measurements. An ultrathin X-ray window enables the detection of elements
down to carbon. The scattering chamber is evacuated to 10−5 bar by a diffusion
pump. The observed microstructure is captured at appropriate magnification as
video images and stored into a computer.
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4.1.3.1Theory of Description
The incident electron beam can create a vacancy (hole) in the inner-shells of an
atom, which leaves the atom as an ion in the exited state. It can relax to its
ground state via two competitive processes. One of it involves transitions of
outer-shell electrons leading to the emanation of characteristic X-rays as shown
pictorially in Figure 4.4. The Auger process is the other competitive process. The
detection of the characteristics X-rays while performing the microstructural
investigations by SEM is extremely useful for microarea analysis providing
elemental identification and quantification. The detection of the characteristic X-
rays can be performed either by a wavelength dispersive spectrometer (WDS) or
by an energy dispersive spectrometer (EDS).
Figure 4.4 Schematic representation of emission of characteristic X-rays
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Table 4.1 summarizes the merits of the two detection systems. Again due to
scattering and large interaction volume and in view of larger penetration depth of
X-rays the, the spatial resolution is much higher than the probe size, and can
even exceed 1μm. The detection sensitivity of EDS is about 1 at. % while it is
better for WDS due to improved signal to noise ratio.
Table 4.1 Comparison between wavelength dispersive (WDS) and energy
dispersive (EDS) spectrometers
Operating characteristics
WDS Crystal diffraction
EDS Silicon, Energy dispersive
Geometrical collection efficiency
Variable , <0.2 % Variable, 30 %
2 % ≈100 % for 2 – 16 keV
Overall quantum efficiency
Detectors Z ≥ 4
Detectors Z ≥ 10 (Be window) Detectors Z ≥ 4 (windowless or thin window)
Resolution
Crystal dependant (5 eV)
Energy dependent (140 eV at 5.9 keV)
Instantaneous acceptance range
≈ The spectrometer resolution
The entire useful energy range
Maximum count rate
50,000 cps on an X-ray line
Resolution-dependent; < 2000 cps over full spectrum for best resolution
Minimum useful probe size
≈ 2000 Å
≈ 50 Å
Typical data collection time Tens of minutes
Minutes
Spectral artifacts
Rate
Major ones include: escape peaks, Pulse pileup, electron beam scattering and window absorption effects
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Energy Dispersive X-Ray Spectroscopy was used for the elemental analysis of
the materials. Energy Dispersive X-Ray Spectroscopy JEOL JSM-6380 LV INCA
X-Sight, Model 7583 manufactured by Oxford instruments was used to detect the
elements present in the inorganic fillers thus confirm the purity of samples. It was
also used to determine the distribution of inorganic filler in the polymer matrix
using Mapping technique. Energy Dispersive X-Ray Spectroscopy analysis with
an accelerating voltage of 20 kV and a energy resolution of 85 eV, was employed
to identify the elements on and under the nanocomposite surface. The samples
were kept at 10-15 mm from the detector at a takeoff angle of 35°. Special care
was taken to attach the sample to the sample holder on the SEM machine in
such a way that it actually stand outside the sample holder. This prevents false
element identification by making sure that the sample holder is relatively far away
from the electron beam. SEMs are equipped with a cathode and magnetic lenses
to create and focus a beam of electrons, and since the 1960s they have been
equipped with elemental analysis capabilities. A detector is used to convert X-ray
energy into voltage signals; this information is sent to a pulse processor, which
measures the signals and passes them on o analyzer for data display and
analysis. EDAX-SEM elemental mapping analysis was carried out on
cryogenically broken samples and samples were coated platinum Sputter Coater
(JEOL, JFC 1600, Auto fine Coater) to minimize electrostatic charging before
analysis.
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4.2 Functional Group Analysis by Fourier Transform Infrared
Spectroscopy (FT-IR)
Fourier Transform Infrared Spectroscopy (FTIR) identifies the functional groups
and chemical bonds in the synthesized materials by producing an infrared
absorption spectrum. When infrared radiations passed through the samples
some of radiations are absorbed and some are transmitted. The resulting
spectrum represents the finger print of the sample with absorption peaks, which
correspond to the frequencies of vibrations between the bonds of atoms making
up the material. As each material is unique combination of chemical bonds with
different atoms making up the unique material so no two compounds have the
similar type of vibration and thereby, does not produce the exactly same infrared
spectrum. Therefore the FTIR can give the confirmation of the synthesized
material, positive identification of different kind of the samples. Thus the
technique provides qualitative information about the samples.
4.2.1 Basic Principle
The FT-IR technique is based on the physical principles of the total energy
posses by the molecule. The total energy composed of translation, rotational,
vibrational and electronic energies. Molecular bonds vibrate at various
frequencies depending on the element and type of the bonds that it holds. For
any given chemical bond there are several specific frequencies at which it can
vibrate. According to quantum mechanics, these frequencies correspond to the
ground state (lowest frequency) and the several excited states (higher
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frequencies). One way to cause the frequency of molecular vibration to increase
is to excite the bond by having it absorb light energy. For any given transition
between two states the light energy (determined by wavelength) must be exactly
equal to difference in the energy between the two states [usually ground state
(E0) and the first excited state (E1)].
Difference in Energy states = Energy of Light Absorbed
E1 - E0
Different alignments of the molecules results in changes in the intensity of a
number of infrared modes. Because of each interatomic bond may vibrate in
different modes such as stretching and bending. Stretching includes the
symmetric stretching and asymmetric stretching. Similarly bending has four
different sub modes as rocking, scissoring, twisting and wagging. Each mode has
different absorption frequencies. The change in dipole moment is required for the
visible bands. Thus individual chemical bond may absorb at more than one IR
frequency. Symmetric stretching vibration modes are not observed due to
change in dipole moment (µ = 0). Thus symmetrical vibration does not cause
absorption of IR radiation. Asymmetric stretching absorptions usually produce the
strong peaks than bending. However, the weaker bending absorptions can be
= h c / λ ….. (4.3)
Where, h = Planck’s constant
c = speed of light
λ = Wavelength of light
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useful in differentiating similar type of bonds (for example aromatic substitution).
The most important factors that determine whether a chemical bond will absorb
or not are the bond order and type of atoms joined by the bond. Conjugation and
nearby atoms (polarity) shift the frequency to lesser degree. Therefore, the same
or similar functional group in different molecules will typically absorb within the
same and specific frequency range.
FT-IR spectra of samples were scanned on Perkin Elmer Spectrum BX Fourier
Transform Infrared (FTIR) system (Huenenberg Switzerland) in the range of 400-
4000 cm-1. This technique has been used to identify and confirm the presence of
required chemical bonds in the synthesized material. The scanning rate was 0.4
sec.cm. The infrared double beam spectrometer instrument plots intensities (as
percent transmission) versus wave number (wavelength).
To confirm the presence of functional groups the IR of each test specimen was
taken, and studied. The scans were obtained from Perkin Elmer Spectrum BX
Fourier transform infrared (FTIR) system (Huenenberg Switzerland). The
samples for IR analysis were prepared as potassium bromide pellets. The
photograph of FTIR spectrophotometer instrument is shown in Figure 4.5.
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Figure 4.5 Photograph of Perkin Elmer Spectrum BX Fourier Transform
Infrared (FTIR) system
4.3 Electrical Properties
The electric measurements for ceramic material were carried out by preparing
the pellet from powder followed by sintering. Thus the sintered pellet is then used
for the electric measurements such as capacitance, dielectric constant, volume
resistance and the dissipation loss. These electric measurements can be easily
carried out using the LCR meter.
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4.3.1 LCR Meter
This is the most widely used instrument in dielectric testing on a small scale. It
provides an “all-in-one” approach to capacitance measurement. The instrument
can measure capacitance and parasitic resistance, and the dielectric constant
can easily be calculated if the physical dimensions of the parallel plate capacitor
are known.
Working Operation
By applying a small sinusoidal signal of frequency f and amplitude A to the
capacitor, the LCR meter can measure the voltage (V) across the capacitor and
the displacement current (I) through the capacitor and find the complex
impedance (Z) from Ohm’s Law.
….. (4.4)
….. (4.5)
If the capacitance under test is small the reactance X is large, and more likely to
be affected by a parallel parasitic resistance. If the capacitance under test is
large, the reactance is small and more likely to be affected by a series
resistance. The capacitances seen in measurement were quite small, and the
parallel resistance model was more fitting, so it will be discussed over the series
resistance model.
To make the math easier, the admittance (Y) is found instead of the impedance.
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….. (4.6)
In these equations, G is the conductance, measured in Siemens, and B is the
susceptance, also measured in Siemens. All this math is done inside the LCR
meter, and the capacitance (C) is displayed along with the parasitic parallel
resistance (Rp). Also useful are the quality factor (Q) and the dissipation factor
(D). These provide a metric for the ratio of parasitic resistance and capacitance
[4.5]
….. (4.7)
If the capacitor were ideal, no parasitic resistance would exist and the quality
factor would be infinite. From this a high quality factor, or low dissipation factor, is
desirable. Once the capacitance is found, the dielectric constant can be easily
calculated by back solving the parallel plate capacitor equation.
….. (4.8)
By adding a DC offset voltage, the internal ferroelectric domains can be
polarized, thereby studying the effect of polarization on the dielectric constant.
The pictorial representation of the LCR Bridge and the fixture are shown below in
Figure 4.6.
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Figure 4.6 LCR bridge Instrument
The study of the electrical and other related properties of dielectric materials in
relation to their chemical composition and structure play a vital role in
understanding the physical mechanism of phase transition as well as its practical
utility for device applications. Under the electrical properties of ferroelectric or
nonlinear dielectric materials, we are mainly concerned with the study of
dielectric properties such as dielectric constant, dielectric loss, ac conductivity
and dc conductivity. A brief theoretical account of all these properties is
mentioned here as under.
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4.3.2 Dielectric Studies
4.3.2.1. Basic Concept of Dielectric Constant
Consider a parallel plate capacitor of area ‘a’ each and separated by a distance
‘d’ apart from each other. Let Q be the charge (in coulombs) due to potential
difference of V volts between the plates. The electric flux lines will travel straight
across one plate to other and the electric field strength E between the plates will
be V/d volts/m.
Now dielectric constant is the ratio of flux density to the electric field,
ε = D/E = (Q/a) = Qd (V/d) Va
but Q/V = C, where C is capacitance in farads, a is area of electrode
Therefore, ε = Cd/a Farad/m ..… (4.9)
For the case of vacuum between the plates, the value from (equation 4.9) will
give the value of dielectric constant for free space usually denoted by ε0 and its
value is ε0 = 8.85 × 10
-12 Farads/m.
But the relative permittivity εr is defined as
ε = εr ε0 Farad/m
& εr = ε/ ε0
Where ε is the absolute permittivity and it always includes a negative power of 10
where as relative permittivity is generally a number greater than unity and has
been defined as a property of the medium.
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If a parallel plate condenser has a capacitance C0 in air and the space between
the plates is filled by a medium of permittivity εr then the capacitance becomes
C = εr C0
or εr= C/C0
When an alternating current voltage V is applied across the condenser an
alternating current I will flow and provided the dielectric is a ‘perfect dielectric’.
I = j ω εrC0 V ….. (4.10)
In general, however, an in-phase component of current will appear corresponding
to a resistive current between the condenser plates. Such current is entirely due
to the dielectric medium and is a property of it. We therefore characterize it by a
component of permittivity by defining relative permittivity as
εr = ε’ – j ε’’ ….. (4.11)
The current in condenser then becomes
I = j ω (ε’– j ε’’) Co
V
I = j ω Co
V ε’ + ω Co
V ε’’ .… (4.12)
The magnitude of ε’ is defined by the magnitude of the in-phase or loss
component of the current.
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4.3.2.2 Dielectric Loss
Let us plot a phasor diagram of current and voltage in a capacitor energized by
an alternating voltage. If the power were not dissipated at all in the dielectric of
the capacitor (an ideal dielectric) the phasor current I through the capacitor would
be ahead of the phasor of voltage V precisely by 90o
and the current would be
pure reactive (Figure 4.7)
But in actual practice the phase angle Φ is slightly less than 90o
and the total
current I through the capacitor can be resolved in to two components. The active
current Ia and the reactive current Ir. Thus, the phase angle describes a capacitor
from the view point of losses in a dielectric (the power losses in the capacitor
plates and leads are neglected). Since the phase angle is very close to 90o
in a
capacitor with a high quality dielectric, the angle δ is a more descriptive
parameter which when added to the angle Φ brings the angle Φ to 90o
Figure 4.7 Active and reactive components of current
.
δ = 90 – Φ … (4.13)
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The angle δ is called the dielectric loss angle and the tangent of this angle is
equal to the ratio between the active and reactive currents.
tan δ = Ia / Ir ….. (4.14)
tan δ can also be defined in terms of power and it is defined as the ratio of the
active power (power loss) P to the reactive power Pq
tan δ = P/Pq ….. (4.15)
The dielectric loss angle is an important parameter for the material of a dielectric
and the quality factor of an insulation portion is determined as
Q = 1/tan δ = cot δ ….. (4.16)
For good quality dielectrics, tan δ may be of the order of 1/10-3 or 1/10-4
The value of dielectric constant depends on frequency of the applied voltage. As
the time required for electronic or ionic polarization to set-in is very small as
compared with the time of the voltage sign change i.e. with the half period of
or even
more.
4.3.3 Factors Affecting the Dielectric Constant of a Material
The dielectric constant of a dielectric material depends on the various factors
such as the frequency of the applied voltage, temperature, pressure, humidity
etc. Brief descriptions of these factors are given below:
4.3.3.1 Effect of Frequency
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alternative voltage even for the highest frequencies. But the situation is different
in the case of dipole polarization where the frequency of the alternating voltage
increases the value of dielectric constant of a polar dielectric at first remain
invariable but beginning with a certain critical frequency (fo
In the case of polar dielectrics the molecule cannot orient themselves at low
temperature region. When the temperature increases, the orientation of dipoles
facilitated and permittivity increases. As the temperature grows the chaotic
thermal oscillation of molecules are intensified and the degree of orderliness of
their orientation is diminished. Due to this reason the dielectric constant versus
), when polarization
fails to settle itself completely during one-half period, dielectric constant begins to
drop approaching at very high frequencies, the values typical of non-polar
dielectrics.
4.3.3.2 Effect of Temperature
Temperature does not affect the process of electronic polarization in nonpolar
dielectrics and the electronic polarizability of molecules does not depend on
temperature. However, due to thermal expansion of matter, the ratio of the
number of molecules to the effective length λ of the dielectric decreases when
temperature increases and for this reason dielectric constant decreases with the
increase of temperature.
In the case of ionic solids the ionic mechanism of polarization increases with the
increase of temperature and hence dielectric constant increases. This is the case
in most alkalihalide crystals, inorganic glasses and ceramics.
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temperature curve passes through a maximum and then decreases again. The
temperature at which permittivity attains its maximum value is known as critical
temperature/curie temperature or transition temperature (Tc).
4.3.3.3 Effect of Humidity
In hygroscopic dielectrics with dielectric constant smaller than dielectric constant
of water, the permittivity increases with the increase of moisture content. But
presence of humidity also deteriorates the important parameters of dielectric
materials; such as decrease in resistivity, increase in the loss angle and
reduction in dielectric strength etc.
4.3.3.4 Effect of Voltage
In linear dielectrics the dielectric constant is almost independent of applied
voltage. But in case of ferroelectrics (non-linear dielectrics) the permittivity
depends strongly on the strength of applied voltage.
4.3.4. Factors Affecting Dielectric Loss of the Material
The value of dielectric loss (tan δ) for a given specimen of material is not strictly
constant but depends on various external factors. Brief accounts of the effect of
these factors are given below:
4.3.4.1 Effect of Frequency
Dielectric loss decreases with increasing frequency and it occurs due to different
orientations in the ceramic materials.
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4.3.4.2 Effect of Temperature
The value of dielectric loss increases with increasing temperature. With the
increase in dielectric loss, resistivity (ρ) decreases at high temperature. This
change in dielectric loss is brought about by an increase both in the conduction
of residual current and the conduction of absorption current.
4.3.4.3 Effect of Humidity
The value of dielectric loss increases in hygroscopic dielectrics when humidity
increases. A high humidity impairs the properties of electrical insulation.
4.3.4.4 Effect of Voltage
The dielectric loss increases in proportion to the square of the voltage applied to
the insulation. However, the dependence of tan δ on voltage is almost invariable
with same values of voltage, but the curves of tan δ vs. voltage (V) abruptly rises
when voltage grows above a definite limit Vion. The curve is known as ionization
curve (Figure 4.8) and a point on the curve where ionization starts to increase is
called the ionization point (point A in Figure 4.8) and confirms to the beginning of
ionization in the insulation of air or other gases inside the insulation [4.6-4.7].
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Figure 4.8 Ionization Curve
4.3.5 Volume Resistivity
Volume resistivity measures the ability of the material to resist moving charge
through its volume. The samples were prepared and five measurements of
thickness on each specimen were taken, and their average values were
recorded. High Resistance Meter of Hewlett-Packard (Model-4329 A) make was
used for the measurements. One sample was inserted at a time into the sample
holder (cell) with the surface in contact with the graded electrode and charged for
1 min at 500 V, DC. The volume resistivity measurements were carried out at
27± 2 ˚C and were measured in Ohm-cm. Figure 4.9 shows the High resistance
meter.
Volume Resistivity (Ω-cm) = 19.6 Rv / t
Where, Rv
t = Thickness (cm)
= indicated volume resistance (Ω)
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Figure 4.9 High Resistance Meter
4.3.6 Dielectric Strength
The dielectric strength of an insulating material is defined as the maximum
voltage required to produce a dielectric breakdown. The brass electrodes
consisting of opposing cylindrical metal rods of 1mm in diameter with edges
rounded to a radius of 0.8 mm were taken in a wooden set- up and polished prior
to testing. The electrode faces were kept parallel and held exactly opposite to
one another. The sample was kept between the two electrodes and applied
voltage method was used to measure the voltage at 27± 2 ̊C. According to this
method, the voltage raised from zero at a uniform rate at 500 V/sec (as per Test
Method ASTM D 149) such that break down occurs on an average between 10-
20 sec. Five measurements of breakdown voltage were taken and the electric
strength was calculated in kV/mm. The set up for the measurement is shown in
Figure 4.10. The different sources supply for measurement on different kind of
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samples. The dielectric strength of the materials were calculated using the below
mentioned equation.
Electric strength (kV/mm) = Breakdown Voltage (kV)/ Average thickness
(mm)
Figure 4.10 Measurement Set-Up for Dielectric strength
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REFERENCE
4.1 Hull, A.W. J. Am. Chem. Soc., 1919, 41, 1168–75.
4.2 Cullity, B.D. Elements of X-ray Diffraction; Addison-Wesley Publishing
Company: USA, 1978
4.3 Woolfson, M.M. An Introduction to X-ray Crystallography; Cambridge
University Press Vikas Publishing House Pvt. Ltd: New Delhi, 1970.
4.4 Wing, L.N. Ph. D. Thesis City University of Hong Kong; Hong Kong, 2008.
4.5 "Agilent 4248A precision LCR meter operation manual," Agilent
Technologies, Japan, 2001.
4.6 Tareev, B. Physics of Dielectric Materials; Mir Publication: Moscow, 1979.
4.7 Raghvan, V. Materials Science and Engineering; Prentice Hall of India
Pvt. Ltd.: New Delhi, 2000.