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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


110 | P a g e

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


111 | P a g e

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.

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