Dielectric Properties of insulators.pptx

Dielectric materials are insulating materials that can store electrical energy when exposed to an electric field. Unlike conductors, dielectrics do not allow the free flow of current. Instead, they become polarized, meaning that electric charges within the material are slightly displaced, creating an internal electric field that can store potential energy. Dielectric materials are essential in many electrical and electronic applications, especially in capacitors, insulators, and other devices where energy storage, insulation, or controlled electric fields are required.

Key Concepts in Dielectric Materials
Dielectric Properties

Dielectric Constant (Permittivity): This is a measure of a material's ability to store electrical energy in an electric field. The dielectric constant (often represented as ε) compares a material's ability to store charge relative to a vacuum. Higher dielectric constants mean better energy storage capabilities.
Dielectric Strength: This is the maximum electric field a material can withstand without breaking down (i.e., without becoming conductive). Dielectric strength is crucial for insulators, as it determines the material’s ability to block high voltages.
Polarization in Dielectrics

When a dielectric material is placed in an electric field, its internal charges are slightly displaced. This displacement creates "polarization," where positive and negative charges are separated within the material.
There are different types of polarization, such as electronic polarization (displacement of electron clouds in atoms), ionic polarization (displacement of ions in a lattice), and orientation polarization (alignment of dipoles in polar molecules).
Types of Dielectric Materials

Ceramic Dielectrics: These are often used in capacitors and high-frequency applications because of their high dielectric constant and stability.
Polymeric Dielectrics: Plastics and polymers like polyethylene and Teflon are flexible, lightweight, and have good dielectric properties, making them suitable for insulation in cables and wires.
Glass and Mica: These materials have excellent dielectric strength and are used in high-voltage applications as insulators.
Gaseous Dielectrics: Gases like air, nitrogen, and sulfur hexafluoride (SF₆) are used as insulators in high-voltage applications due to their dielectric properties.
Dielectric Loss

Dielectric loss refers to the energy dissipated as heat in a dielectric material when it is exposed to an alternating electric field. This loss is caused by the friction of moving charges within the material as it polarizes and depolarizes. Minimizing dielectric loss is important in applications like high-frequency circuits to improve efficiency.
Applications of Dielectric Materials

Capacitors: Dielectrics are used between capacitor plates to store energy. Materials with high dielectric constants are chosen to maximize the amount of energy stored.

1. Electrical Engineering Materials Dielectric Properties of insulators

2. Dielectric Materials Dielectric materials are materials which do not conduct electricity. They are insulators having very low electrical conductivity . So, what is the difference between dielectric material and insulating material? The difference is that insulators block the flow of current but the dielectrics accumulate electrical energy.

3. Dielectric properties of insulation The dielectric properties of insulation include breakdown voltage or dielectric strength, dielectric parameters like permittivity, conductivity, loss angle and power factor . The other properties include electrical, thermal, mechanical and chemical parameters. We can discuss the main properties in detail below.

4. Dielectric Strength or Breakdown Voltage The dielectric material has only some electrons in normal operating condition. When the electric strength is increased beyond a particular value, it results in breakdown. That is, the insulating properties are damaged and it finally becomes a conductor .

5. Cont’d… The electrical field strength at the time of breakdown is called breakdown voltage or dielectric strength . It can be expressed as a minimum electrical stress that will result in breakdown of the material under some condition It can be reduced by ageing, high temperature and moisture. It is given as Dielectric strength or Breakdown voltage = Where: V = Breakdown Potential. t =Thickness of the dielectric material.

6. Relative permittivity It is also called specific inductive capacity or dielectric constant. This gives us the information about the capacitance of the capacitor when the dielectric is used. It is denoted as ε r The capacitance of the capacitor is related with separation of plates or we can say the thickness of dielectrics, cross sectional area of the plates and the character of dielectric material used

7. We can see that if we substitute air with any dielectric medium, the capacitance (capacitor) will get improved. The dielectric constant and dielectric strength of some dielectric materials are given below. Examples of Dielectric Materials include; Solid dielectrics: Ceramics, Plastics, Mica, Glass, Paper etc Liquid Dielectrics: Distilled water, Silicone fluids, Mineral oil hydrocarbons Gas Dielectrics: Dry air, Vacuum, Nitrogen, Helium

8. An alternating voltage applied to a dielectric material produces an oscillating electric field inside the material. This alternating field interacts with the dipoles and causes them to rotate and align themselves with the field. As time passes, the electric field reverses its direction, and the dipoles must rotate again to remain aligned with the correct polarity. As they rotate, energy is lost through the generation of heat (friction) as well as the acceleration and deceleration of the rotational motion of the dipoles. Interaction of Electromagnetic Field with Matter Figure showing the rotation of a polar diatomic molecule under ac field excitation.

9. Dissipation Factor, Loss Angle and Power Factor When a dielectric material is given an AC supply, no power utilization takes place. It is perfectly achieved only by vacuum and purified gases. Here, we can see that the charging current will lead the voltage applied by 90 o which is shown in figure below. This implies there is no loss in power in insulators.

10. The current now leads the voltage by a very small angle, φ , less than 90°, where the difference δ (Greek letter delta) is termed as the dielectric loss angle , as seen in the figure below. The fraction of the maximum energy lost in each cycle, divided by 2  is termed as the ‘loss factor’ and its value is given by tan δ (‘tan delta ’). From the phasor diagram I R

11. Dielectric loss (because I R = I cos  ] (because ) Therefore, (using ) where V = applied voltage in volts, A = area of the electrode f = supply frequency in Hz, d = thickness of dielectric medium  = is the absolute permittivity of the medium Where: Xc = Capacitive reactance ( )   cosφ = sinδ ; in most cases, δ is small, so we take sinδ = tanδ So, tanδ is known as power factor of dielectrics

12. Power loss per unit volume (using vol = Ad ) Power loss per unit volume in terms of applied electric field E, (using E = V/d )

13. Table 1 shows some typical values of dielectric constant and loss factor. The AC values are measured at 1MHz. Table 1: Some typical values of dielectric parameters material  r tan δ air 1.0006 polycarbonate 2.3 0.0012 FR-4 4.4 0.035 alumina 8.8 0.00033 barium titanate 1200+ 0.01

14. Dielectric of a material under influence of applied field The dielectric material under the influence of a charge undergoes motion of charges inside it. This leads to the storage of electrical Energy and Capacitance Where: Q = Charge, proportional to area, A ) and V is the potential difference between the plates (V)  

15. Electric field intensity applied, , d = separation in meters  

16. Polarization of dielectric materials Polarization is defined as the dipole moment per unit volume. The polarization is induced by electric field and therefore it is a function of electric field.

17. Polarizability is a relative tendency of a charge distribution ( E.g Electron cloude of an atom or a molecule) to be distorted from its normal shape by an external electric field. p is the dipole moment, = the polarizability  

18. Contribution to Polarizability + + + Where: = Polarizability of an atom = Electronic polarizability = Ionic polarizability = Dipolar polarizability = Space charge polarizability  

19. Electronic polarizability,     The applied field deforms the symmetry of the structure of atoms, causing a small displacement of the center of the distribution of negative charge of electrons relative to the center of positive charge in the nucleus. Thus, the atoms acquire electric dipole moment, and finally each atom is characterized by an electronic polarizability. The value of electronic polarizability increases with the number of electrons, since the outer shells’ electrons in multielectronic atoms are less bonded to the nuclei, having a higher contribution to  α . This happens at higher frequencies of about Hz ie within UV range of EM spectrum  

20. Ionic or atomic polarizability ( α i ) Ionic polarizability occurs in ionic compounds when an electric field causes the displacement of positive and negative ions relative to each other. This shift induces dipoles within the ionic lattice. The degree of polarizability depends on the size and charge of the ions The considerably higher mass of ions or atoms compared to electrons leads to a significantly slower polarization process relatively to electronic polarization. However, both processes are regarded as “fast,” characterized by low relaxation times. This is known as displacement of irons This lies at a frequency between and Hz  

21. Dipolar polarizability,   Dipolar polarizability refers to the reorientation of permanent dipoles (molecules with permanent positive and negative poles, such as water) under the influence of an external electric field. This reorientation aligns the dipoles with the field, contributing to the overall polarizability. It is more pronounced in polar molecules than in non-polar ones. This lies at a frequency between and Hz Occurs in polar molecules eg. Water and carbon-dioxide  

22. Space charge polarizability,   This happens as a result of long range migration of charge This lies at a very low frequency

23. Frequency dependence of dielectric constant Where: = Real part of the dielectric constant = imaginary part of the dielectric constant also known as dielectric loss   Because of the AC frequency dependence of the dielectric constant, ε r is in general a complex quantity which can be expressed in the form

24. Ferroelectric materials Ferroelectric materials are dielectric materials in which polarization remains permanent, even after removing the applied electric field. Moreover, the direction of the dipole moment can be switched by applying electric field Application of ferroelectric materials Multi-layer capacitors Ferroelectric random access memory

25. Dielectric relaxation This refers to the momentary delay in the dielectric constant of a material. This is caused by the delay in molecular polarization with respect to a changing electric field in a dielectric medium. This relaxation is often described in terms of permittivity as a function of frequency. Where: = the permittivity at the high frequency limit ∆ ε = - and = the static low frequency permittivity τ = relaxation time of the medium ε ( ω ) = dielectric constant as a function of frequency  

26. When the relaxation time is much slower than the frequency of the applied electric field, no polarization occurs When the relaxation time and the frequency of the applied field are similar, a phase lag occurs and energy is absorbed. This is called dielectric loss, it is normally quantified by the relationship