Dielectric Materials types & properties, adavantages, lossess

1. Unit II
Dielectric Materials

2. What is a Capacitor?
A capacitor is a two-terminal electrical device that possesses the ability to
store energy in the form of an electric charge. It consists of two electrical
conductors that are separated by a distance. The space between the
conductors may be filled by vacuum or with an insulating material known
as a dielectric. The ability of the capacitor to store charges is known as
capacitance.

3. As V= 0 Volt. or if no voltage is applied across capacitor, net charge on
conducting plates are zero. It means that capacitor plates has equal
amount of positive as well as negative charge as shown in the figure
V= 0 Volt

4. If finite voltage is applied across the capacitor, free electrons from one
plate entered in positive terminal of the battery & plate remains with only
positive charges. Also electrons from negative terminal of battery
combines with positive charges on another plate remains only negative
charges. At the end one plate consist only positive charges & another
plate consist only negative charges which is called charged capacitor. Due
to this charges voltage developed between two plates equal to supply
voltage

5. Due to the voltage across the plate there is presence of electric field between
the plates.
E = V/d
The ability of capacitor to stored the electric charge is called as capacitance
C = Q / V
Its unit is Farad.
If you want to store more charge on the capacitor we need to increase the
voltage across it but it has certain voltage limitation.
https://youtu.be/R2QQ0yiqH_U

6. Factors affecting the value of capacitor
C = 𝜺.A / d
Where, A = Area of the plates
d = distance between two plates
𝜺 = permittivity of the dielectric material

7. How permittivity affects the value of capacitance
If no voltage is applied(V = 0) across the capacitor polar molecules are aligned
randomly as shown figure.

8. As finite voltage applied across the capacitor then capacitor holds some charge
across it & polar molecules aligned according to charges. So alignment of polar
molecules are opposite to the electric field which decrease the electric field
between the plates. Which results decrease in voltage.
E = V/d
Which result value of capacitor increases
C = Q / V

10. Dielectric Losses

13. Dielectric Properties of Polymeric Material
Dielectric Constant:
● The dielectric constant is a measure of the influence of a particular
dielectric on the capacitance of a condenser.
● It measures how well a material separates the plates in a capacitor
and is defined as the ratio of the capacitance of a set of electrodes
with the dielectric material between them to the capacitance of the
same electrodes with a vacuum between them.
● The dielectric constant for a vacuum is 1 and for all other materials it
is greater than 1.

15. Power Factor
The power factor is a measure of the energy absorbed by the material as
the alternating current constantly changes direction and the dipoles try to
align themselves with the field.
As the dipoles try to align themselves with the external field they will
always be slightly out of phase and will ‘lag’ behind the field.
The amount of lagging is measured by the phase angle (q) and the power
factor is defined as cos q.
The power factor can be thought of as a measure of the internal friction
created by the alternating current and will define how much a material
heats up when placed in an alternating field.

16. Dielectric Strength
The dielectric strength is the DC voltage between two electrodes at which
dielectric breakdown occurs and is an indicator of how good an insulator
the material is. The voltage is increased until the material breaks down,
there is an arc across the electrodes and substantial current flows.
Most plastics have good dielectric strengths (in the order of 100 to 300
kV/cm)

17. Surface Resistivity
The surface resistivity is a measure of the resistance of the material to a
surface flow of current. It is the ratio of the applied direct voltage and the
resulting current along the surface of the material per unit width. Surface
resistivity is measured in Ω.

18. Tracking and Arc Resistance
These are measures of how long a material can resist forming a
continuous conduction path under a high voltage/low current arc

19. Breakdown in Gases-Townsend discharge

20. The Townsend discharge or Townsend avalanche is a gas ionisation
process where free electrons are accelerated by an electric field, collide
with gas molecules, and consequently free additional electrons. Those
electrons are in turn accelerated and free additional electrons. The result
is an avalanche multiplication that permits electrical conduction through
the gas. The discharge requires a source of free electrons and a
significant electric field; without both, the phenomenon does not occur.

37. Intrinsic or Ionic Breakdown
Intrinsic breakdown depends upon the presence of free electrons which are
capable of migration through the lattice of the dielectric. Usually, a small
number of conduction electrons are present in solid dielectrics, along with
some structural imperfections and small amounts of impurities. The
impurity atoms, or molecules or both act as traps for the conduction
electrons up to certain ranges of electric fields and temperatures. When
these ranges are exceeded, additional electrons in addition to trapped
electrons are released, and these electrons participate in the conduction
process. Based on this principle, two types of intrinsic breakdown
mechanisms have been proposed.

38. Electronic Breakdown Intrinsic breakdown occurs in time of the order
of 10-8 s and therefore is assumed to be electronic in nature. The initial
density of conduction (free) electrons is also assumed to be large, and
electron-electron collisions occur. When an electric field is applied,
electrons gain energy from the electric field and cross the forbidden
energy gap from the valence band to the conduction band. When this
process is repeated, more and more electrons become available in the
conduction band, eventually leading to breakdown.

43. Requirements / Characteristics of a Good Insulating Material
1. Large insulating resistance.
2. High dielectric strength.
3. Uniform viscosity—it gives uniform electrical and thermal properties.
4. Least thermal expansion.
5. When exposed to arcing should be non-ignitable.
6. Should be resistance to oils or liquids, gas fumes, acids and alkalis.
7. Should have no deteriorating effect on the material, in contact with it.

44. 10. High mechanical strength.
11. High thermal conductivity.
12. Low permittivity.
13. High thermal strength.
14. Should be homogeneous to avoid local stress concentration.
15. Should be resistant to thermal and chemical deterioration.

45. Dielectric Materials - Mica
● chemically inert
● dielectric
● elastic
● flexible
● hydrophilic
● insulating
● lightweight
Properties Applications
● electrical insulators
● thermal insulation
● gauge “glass”
● windows in stove and kerosene heaters
● dielectrics in capacitors
● insulation in motors and generator
armatures
● field coil insulation

46. Ferroelectricity
Ferroelectricity is a characteristic of certain materials that have a
spontaneous electric polarization that can be reversed by the application
of an external electric field.

47. If we take a ferroelectric material, and an electric field is given to it. We
get a nonlinear polarization. It also exhibits nonzero spontaneous
polarization without a peripheral field.
We can also see that by inverting the direction of the applied electrical
field, the direction of polarization can be inverted or changed.Thus, we
can say that the polarization will depend on the present and the previous
condition of the electric field. The hysteresis loop is obtained as in figure

48. Ferroelectric materials:
● BaTiO3
● PbTiO3
● Lead Zirconate Titanate (PZT)
● Triglycine Sulphate
● PVDF

49. Application of Ferroelectric Materials
● Thermistors
● Oscillators
● Non-volatile memory
● Filters
● Capacitors
● Light deflectors
● Transchargers
● Electro-optic materials
● Modulators
● Piezoelectrics
● Display etc

50. Piezoelectricity
Piezoelectric Effect is the ability of certain materials to generate an
electric charge in response to applied mechanical stress. The word
Piezoelectric is derived from the Greek piezein, which means to squeeze
or press, and piezo, which is Greek for “push”.

51. One of the unique characteristics of the piezoelectric effect is that it is
reversible, meaning that materials exhibiting the direct piezoelectric effect
(the generation of electricity when stress is applied) also exhibit the
converse piezoelectric effect (the generation of stress when an electric
field is applied).

52. Piezoelectric Material
1. Barium Titanate.
2. Lead zirconate titanate (PZT).
3. Rochelle salt.

53. Piezoelectric Applications
1. In microphones, the sound pressure is converted into an electric
signal and this signal is ultimately amplified to produce a louder
sound.
2. Automobile seat belts lock in response to a rapid deceleration is
also done using a piezoelectric material.
3. It is also used in medical diagnostics.
4. It is used in electric lighter used in kitchens.
5. They are used for studying high-speed shock waves and blast
waves.
6. Used infertility treatment.
7. Used in Inkjet printers