Dielectric Materials: Properties and Applications

Dielectric Materials: Properties and Applications Content 1. Dielectrics : Properties 2. Fundamental definitions and Properties of electric dipole 3. Various polarization mechanisms involved in dielectric : 3.1 Electronic polarization, 3.2 Ionic polarization, 3.3 Orientation polarization, 3.4 Space charge polarization; 3.5 Total polarization 4. Active and Passive Dielectrics 5. Frequency and Temperature on Polarization of Dielectrics : 5.1 Frequency Dependence, 5.2 Temperature Dependence, 6. Internal field or Local field : 6.1 Definition, 6.2 Derivation, 6.3 Clausius Mosoti Equation 7. Dielectrics and Loss Tangent; 7.1 Loss in purified gas; 7.2 Loss in commercial dielectric ; 7.3 Power loss 8. Dielectric Breakdown: 8.1 Types of dielectric breakdown; 8.2 Remedies for breakdown mechanisms 9. General Applications 10. Applications of dielectric materials : 10.1 Dielectrics in capacitors, 10.2 Insulating materials in transformers 11. Ferro-electrics : 11.1 Properties, 11.2 Applications 1

Understanding Dielectric: Solids which have an energy gap of 3eV or more are termed as insulators. In these materials, it is almost not possible to excite the electrons from the valence band to conduction band by an applied field. Generally dielectrics are also called as insulators, thereby poor conductors of electricity. However they allow movement of some electrons at abnormally high temperatures, causing a small flow of current. Dielectrics are non-metallic materials of high specific resistance ρ, negative temperature coefficient of resistance (-α), large insulation resistance. Insulation resistance will be affected by moisture, temperature, applied electric field and age of dielectrics. Understanding Dielectric: Dielectric materials are electrically non-conducting materials such as glass, ebonite, mica, rubber, wood and paper. All dielectric materials are insulating materials. The difference between a dielectric and an insulator lies in their applications. If the main function of non-conducting material is to provide electrical insulation, then they are called as insulator. On the other hand, if the main function of non-conducting material is to store electrical charges then they are called as dielectrics. 2

Properties Generally, the dielectrics are non-metallic materials of high resistivity. They have a very large energy gap (more than 3eV). All the electrons in the dielectrics are tightly bound to their parent nucleus. As there are no free electrons to carry the current, the electrical conductivity of dielectrics is very low. They have negative temperature coefficient of resistance and high insulation resistance. Fundamental Definitions And Properties Electric Dipole A system consisting of two equal and opposite charges n(+q, -q) separated by a distance (d) is called an electric dipole. DIPOLE MOMENT (P) The product of the magnitude of the charge (q) and distance between two charges (d) is called as dipole moment. Dipole moment P = qd (coulomb-metre) 3

Fundamental Definitions And Properties Electric Dipole PERMITTIVITY (ε) The permittivity represents the dielectric property of a medium. It indicates easily polarizable nature of material. Its unit is farad/metre DIELECTRIC CONSTANT (ε r ) A dielectric characteristic of a material is determined by its dielectric constant. It is a measure of polarisation of the dielectrics. Definition It is the ratio between absolute permittivity of the medium (ε) and permittivity of free space (εo). Dielectric constant = Absolute permittivity (ε) / Permittivity of free space (ε o ) ε r = ε / ε o Fundamental Definitions And Properties Electric Dipole POLARIZATION Definition The process of producing electric dipoles inside the dielectric by the application of an external electrical field is called polarization in dielectrics. POLARISABILITY (α) It is found that the average dipole moment field (E). μ = α E Where (α) is the polarisability. α = μ / E Polarisability is defined as the ratio of average dipole moment to the electrical field applied. Its unit is farad m 2. 4

Active and Passive Dielectrics The dielectric materials can be classified into active and passive dielectric materials. i. Active dielectrics When a dielectric material is kept in an external electric field, if it actively accepts the electricity, then it is known as active dielectric material. Thus, active dielectrics are the dielectrics, which can easily adapt themselves to store the electrical energy in it. ii. Passive dielectrics Passive dielectrics are the dielectrics, which restrict the flow of electrical energy in them. So, these dielectrics act as insulators. Examples: All insulating materials such as glass, mica, rubber etc., 6

Basically, there are four mechanisms of polarization: Electronic or Atomic Polarization This involves the separation of the centre of the electron cloud around an atom with respect to the centre of its nucleus under the application of electric field (see (a) in figure below). Ionic Polarization This happens in solids with ionic bonding which automatically have dipoles but which get cancelled due to symmetry of the crystals. Here, external field leads to small displacement of ions from their equilibrium positions and hence inducing a net dipole moment (see (b)). Dipolar or Orientation Polarization This is primarily due to orientation of molecular dipoles in the direction of applied field which would otherwise be randomly distributed due to thermal randomization (see (c and d)) and finally Interface or Space Charge Polarization This involves limited movement of charges resulting in alignment of charge dipoles under applied field. This usually happens at the grain boundaries or any other interface such as electrode-material interface (see (e and f)) 8

Dielectrics and Loss Tangent 13

Ferroelectric A material that shows spontaneous and reversible dielectric polarization. 22

Piezoelectric A material that develops voltage upon the application of a stress and develops strain when an electric field is applied. 25

2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning is a trademark used herein under license. The (a) direct and (b) converse piezoelectric effect. In the direct piezoelectric effect, applied stress causes a voltage to appear. In the converse effect (b), an applied voltage leads to development of strain. Direct piezoelectric effect Reverse (converse) piezoelectric effect 26

E E 28

Direct piezoelectric effect P = d P = d d o d: Piezoelectric coupling coefficient (piezoelectric charge coefficient) Table: The piezoelectric constant d (longitudinal) for selected materials Material Piezoelectric constant d (C/N = m/v) Quartz 2.3 x 10-12 BaTiO 3 100 x 10-12 PbZrTiO 6 250 x 10-12 PbNb 2 O 6 80 x 10-12 29

PZT: PbZrO 3 -PbTiO 3 solid solution or lead zirconotitanate 30

Table: Properties of commercial PZT ceramics Property PZT-5H (soft) PZT4 (hard) Permittivity ( at 1 khz) 3400 1300 Dielectric loss (tan at 1 khz) 0.02 0.004 Curie temperature (T c, C) 193 328 Piezoelectric coefficients (10-12 m/v) d 33 593 289 d 31-274 -123 d 15 741 496 Piezoelectric coupling factors k 33 0.752 0.70 k 31-0.388-0.334 k 15 0.675 0.71 Table: Measured longitudinal piezoelectric coupling coefficient d, measured relative dielectric constant, calculated piezoelectric voltage coefficient g and calculated voltage change resulting from a stress change of 1 kpa for a specimen thickness of 1 cm in the direction of polarization. Material d (10-13 m/v) * g (10-4 m 2 /C) Cement paste (plain) Cement paste with steel fibers and PVA Cement paste with carbon fibers Voltage change (mv) 0.659 0.031 35 2.2 2.2 208 16 2700 8.7 8.7 3.62 0.40 49 8.5 8.5 PZT 136 1024 15 15 * Averaged over the first half of the first stress cycle At 10 khz 31

Piezopolymer Moonie Cymbal Composites with piezoelectric/ferroelectric material sandwiched by metal faceplates fo enhancing the piezoelectric coupling coefficient 32

Pyroelectric The ability of a material to spontaneously polarize and produce a voltage due to changes in temperature. p dp dt d o, dt p = pyroelectirc coefficient P = polarization Material p(10-6 C/m 2.K) BaTiO 3 20 PZT 380 PVDF 27 Cement paste 0.002 Px V = ( -1) o Voltage sensitivity dv P dx x dp = d ( -1) d ( 1) d o Compliance o Piezoelectric coupling coefficient d 33

Piezoelectric composite When any material undergoes polarization (due to an applied electric field), its ions and electronic clouds are displaced, causing the development of a mechanical strain in the material. polarization. This phenomenon is known as the electrostriction. 34

Examples of ceramic capacitors. (a)single-layer ceramic capacitor (disk capacitors). (b) Multilayer ceramic capacitor (stacked ceramic layers). Types of Dielectric Materials Dielectric materials can be divided into following groups: Solid Dielectrics - are of following types: Mica is inorganic material and is crystalline in nature. Glass is inorganic material made by fusion of different oxides. Rubber is a organic polymer, which can be natural or artificial. Ceramic is non-metallic organic compound such as silicates 35

Types of Dielectric Materials Liquid Dielectric includes following: Mineral Insulating Oils obtained from crude petroleum & have high oxidation resistance. Synthetic Insulating oil are very much resistant to oxidation & fire hazards. Miscellaneous Insulating oils Vaseline, vegetable oils, silicon oils belongs to this. Gaseous Dielectric includes Air Nitrogen Sulphur hexafluoride Inert gases 36