Understanding Dielectric Materials: Polarization Mechanisms and Applications
Learn about dielectric polarization, charge density, permittivity, and the electric field in dielectric materials. Explore different types of polarization mechanisms such as electronic, molecular, and ionic polarization, as well as how polarization charge densities are modeled. Discover the significance of dielectric materials in various applications.
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Dielectric Materials 1. Dielectric Polarization 2. Polarization Charge Density 3. Permittivity 4. Electric Field in Dielectric Materials 5. Dielectric Materials and Applications 1
1. Dielectric Polarization Dielectric polarization - Negative/positive charges oriented toward the positive/negative electrode - Charges do not move freely in the material but reoritented. Fig. Polarization in materials [www.pcbdirectory.com] 2
Polarization mechanisms in dielectric materials - Electron displacements - Ion displacements - Dipole orientation - Space (interfacial) charge displacements Fig. Polarization mechanisms [Knowles] 3
Electronic polarization: - Elecrons are shifted by external E-field force. - Infinite dipoles (+ charge, charge separated to a distance) are generated in the material. = p d q = p E (dipole moment) : polarizability 4
Molecular polarization: - Some materials have permanent dipole moments. They are called polar molecules. = p d (dipole moment) q = p E N (torque at the dipole center) = F p E ( ) = p E r F + N (torque at points away from the dipole center) = p E (energy stored in the dipole) U 5
Ionic polarization: - If the molecule contains ionic bonds, then the E-fields stretches the lengths of these bonds. - The effect of this change in length is to produce a net dipole moment in the unit cell. 6
2. Polarization Charge Density Polarization charge density - The effect of polarization is modelled using charge densities. - Polarization charges cannot move around. Bound charges = P polarization dipole moment per unit volume p R 1 = (single dipole) V 2 4 R 0 P R 1 = (distributed dipoles) V dV 2 4 R V 0 P S P 1 1 ( ) d R = + V dV 4 4 R S V 0 0 (polarized bound surface charge density) P n b P (polarized bound volume charge density) b 7
Electric flux density in dielectric materials = + b f = = + = + E P (Gauss's law) 0 b f f + = E P ( ) 0 f + D E P (electric flux density) 0 = D f 8
3. Permittivity Permittivity of dielectric materials + = + = + = D E P E E E E (1 ) 0 0 0 0 e e + (1 ) (permittivity) 0 e = + = 1 (dielectric constant or relative permittivity) r e 0 9
3. Electric Field in Dielectric Materials Example: a dielectric slab with uniform polarization = , on the top surface P b = s = , on the bottom surface P b = = P 0 since is uniform. P b P / , inside the slab 0 = E 0, outside the slabe = + = D E P 0 everywhere 0 10
Example: dielectric in a parallel-plate capacitor = E E 0 D D | | | | Electric flux density is increased in the dielectric. 0 11
Example: dielectric in an external field = D D (continuity of electric flux; Gauss's law) 0 0 = E E 0 | | E E | | 0 12
Example: a point charge above a dielectric interface ( , , ): cylindrical coordiantes s z = 1 1 E = D D E 1 2 2 2 n n z z = = = , E E E E E E tan1 tan2 2 1 2 1 x x y y = E + 1 q R q R + = 1 2 , 0 z q q 4 1 1 2 1 2 = + 2 1 q R = 2 q q , 0 z 4 1 2 2 1 13
4. Dielectric Materials and Applications Properties - Dielectric polarization: high permittivity, used in energy storage applications (capacitor, supercapacitors) - Small material loss: high-frequency electromagnetic materials (circuit boards, radomes, antennas, waveguides) - High resistivity: used as insulators - High dielectric strength: used as insulators 14
Fig. Dielectric material applications thrusts [PennState Material Research Institute] 15
Dielectric material types 1) Ferroelectric materials ( ): - Have spontaneous electric polarization which can be reversed by external E-field - Have very large dielectric constant -All ferroelectric materials are pyroelectric. - Have hysteresis effect -Applications: ferroelectric capacitors, tunable capacitors, ferroelectric RAM - Materials Oragnic polymers: PVDF ferroelectric polymers, flexible electronics Perovskite oxide ferroelectrics: PZT, BaTiO3, LiNbO3, sensors, actuators, SAW devices Hydrogen bonded radicals: KDP, TGS, Rochelle salt 17
- Ferroelectric hysteresis - Hysteresis power loss per unit volume per cycle = P PdE H V Fig. Ferroelectric hysteresis curve [Cadence] 19
2) Piezoelectric materials: , - Electric charge accumulates by applied mechanical pressure -Applications: High voltage and power sources Sensors Actuators Frequency standard - Materials: Crystalline materials: quartz Ceramics III-V and II-VI semiconductors Polymers Fig. Piezoelectricity [Electronic Design] 20
Pyroelectric materials: - When heated or cooled, they can generate a voltage by lectric polarization -Applications: heat sensors, power generation, nuclear fusion - Pyroelectric material types Pyroelectric infrared sensors: motion, food, flame sensors Fig. Pyroelectric effect [R. Kishore, Materials 11(8)] 21
Thermoelectric materials: - Electric field generated with temperature change across the material - Seebeck effect, Peltier effect, Thomson effect -Applications: power generation, refrigeration - Material types: bismuth telluride (Bi2Te3) Fig. Thermoelectric effect [IDTechEx] 22
Insulator: - Electron densiy much smaller than conductors: ne= 106/m3 - Current does not flow in insulators - Resistivity: very high - Dielectric strength or dielectric breakdown: Emax before breakdown 20 (vacuum), 30 (air), 100 (porcelain), 1000 (glass), 2000 (mica) kV/cm -Atomic bond: shared electron pairs, like to catch electrons, no free electrons Ionic bond: grid network with electric forces, no free electrons - Insulating materials: ceramic (porcelain), plastic - Power factor (= dissipation factor): accounts for power loss in dielectrics 23
Solid insulator - Uses: electrical devices, capacitors, cables - Types: glass, polymer (PVC,DEHP, PTFE, PFA), porcelain, mica Liquid insulator (= Liquid dielectric) - Uses: electric power distribution equipments: transformer - Types: mineral oil, silicone oil, synthetic esters, vegetable oils - Dielectric strength: 30 - 90 kV/cm Dielectic gas - Uses: transformers, circuit breakers, switchgear, radar waveguides - Types (dielectric strength relative to air): air, nitrogen (1.15), sulfur hexafluoride (SF6) (3), R-114 (3.2), R-12 (2.9) 24
Dielectric strength table Material Dielectric strength (MV/m) Air 3 Sulfur hexafluoride (SF6) 8.5 9.8 Alumina 13.4 Borosilicate glass 20 40 Silicone oil, mineral oil 10 15 Benzene 163 Polystyrene 19.7 Polyethylene 19 160 Neoprene rubber 15.7 26.7 Distilled water 65 70 High vacuum (200 Pa) (field emission limited) 20 40 (depends on electrode shape) Fused silica 470 670 Waxed paper 40 60 PTFE (Teflon, extruded ) 19.7 PEEK (Polyether ether ketone) 23 Mica 118 Diamond 2,000 PZT (lead zirconate titanate), a piezoelectric ceramic material 10 25 25
Insulators - Electron densiy much smaller than conductors: ne= 106/m3 - Ceramic (porcelain), plastic - Dielectric materials - Dielectric breakdown: 20 (vacuum), 30 (air), 100 (porcelain), 1000 (glass) 2000 (mica) kV/cm 26
