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Dielectrics_1
- 1. Dielectrics Prof. V. Krishnakumar Professor and Head Department of Physics Periyar University Salem – 636 011
- 2. IInnttrroodduuccttiioonn • Dielectric materials: high electrical resistivities, but an efficient supporter of electrostatic fields. • Can store energy/charge. • Able to support an electrostatic field while dissipating minimal energy in the form of heat. • The lower the dielectric loss (proportion of energy lost as heat), the more effective is a dielectric material. • Another consideration is the dielectric constant, the extent to which a substance concentrates the electrostatic lines of flux.
- 3. Capacitance • Two electrodes separated by a gap define a capacitor. • When a bias is applied across the capacitor plates, one charges positively, the other negatively. • The amount of charge that the capacitor can store (Q) is proportional to the bias (V) times how good the capacitor is, the ‘capacitance’ (C). • The capacitance is related to the area of the plates (A), their separation (d), and the Dielectric Constant (εεo) of the dielectric between the plates • Dielectric constant of vacuum; εo = 8.85x10-12 F/m=55.2 Me/(V*m) C = ee oA d - Q A V m * 2 ee * = = = - V e m m V d e o *
- 4. Why does charge built up? There is generally not a built-in electric field between the plates of an unbiased capacitor. When an electric field is applied, any charged carriers or species within the material will respond. For a conductor or semiconductor, e- will flow to the + plate, and possibly also holes will flow to the - plate. Current is carried=no charge buildup. For an insulator, there aren’t a significant number of free carriers. There are highly ionic species, however, but they aren’t very mobile at low temperatures. No appreciable current is carried=charge buildup.
- 5. Polarization in Insulators Positively charged species in insulators shift/rotate/align toward the negative electrode and negatively charged species shift/rotate/align towards the positive electrode; creating dipoles. The dipole moment density is termed the Polarization (P) and has the units of C/m2. Electron Cloud Electron Cloud + + - E Electronic polarization, occurs in all insulators - - - + - + - + + + + E Ionic polarization occurs in all ionic solids: NaCl, MgO… - - - + + - + + E Molecular polarization, occurs in all insulating molecules; oils, polymers, H2O… Electric Dipole Moment p = q × x Polarization q A P = p º V
- 6. Dielectric Effects Metal plates Dielectric C = e A d What makes e different from e0? POLARIZATION e e e = + = 1 0 e c r r In electrostatics, the CONSTITUITIVE RELATION is D E E P e e = = = + Polarization P E 0 0 ce Susceptibility
- 7. Dielectric Effects POLARIZATION arises from charge shifts in the material— there is a macroscopic separation of positive charge (e.g., the ions) and negative charge (e.g., the BONDING ELECTRONS). Induced DIPOLE MOMENT POLARIZATION is then di = q × x0 There are many sources of dipoles. Amount of charge shift P = Ndipolesdi
- 8. Definitions •Permittivity is a physical quantity that describes how an electric field affects and is affected by a dielectric medium and is determined by the ability of a material to polarize in response to an applied electric field, and thereby to cancel, partially, the field inside the material. Permittivity relates therefore to a material's ability to transmit (or "permit") an electric field…The permittivity of a material is usually given relative to that of vacuum, as a relative permittivity, (also called dielectric constant in some cases)….- Wikipedia Dk Df 'r e " r e
- 9. Permittivity and Permeability Definitions Permittivity (Dielectric Constant) k = e = = - r r r e e je ' " e 0 •interaction of a material in the presence of an external electric field.
- 10. Permittivity and Permeability Definitions Permittivity (Dielectric Constant) k = e = = - r r r e e je ' " e 0 •interaction of a material in the presence of an external electric field. Dk
- 11. Permittivity and Permeability Definitions k = e = = - ' " r r r e e je ' " e 0 •interaction of a material in the presence of an external electric field. m = m = - 0 mr jmr m interaction of a material in the presence of an external magnetic field. Permittivity (Dielectric Constant) Permeability Dk
- 12. Permittivity and Permeability Definitions k = e = = - ' " r r r e e je ' " e 0 •interaction of a material in the presence of an external electric field. m = m = - 0 mr jmr m interaction of a material in the presence of an external magnetic field. Permittivity (Dielectric Constant) Permeability Dk
- 13. STORAGE Electric Magnetic Fields Fields Permittivity Permeability ' " Electromagnetic Field Interaction MUT mr = mr - jmr ' " e r = e r - je r STORAGE
- 14. STORAGE Electric Magnetic Fields Fields LOSS Permittivity Permeability ' " Electromagnetic Field Interaction MUT mr = mr - jmr ' " e r = e r - je r STORAGE LOSS
- 15. Loss Tangent d = e " re ' tan r e r 'r e ''r e Energy Lost perCycle Energy Stored perCycle tand = D = 1 = Q D Dissipation Factor Q Quality Factor Df
- 16. Relaxation Constant t "t = Time required for 1/e of an aligned system to return to equilibrium or random state, in seconds. = 1 = 1 wc pfc t 2 100 1 1 10 Water at 20o C 10 100 f, GHz most energy is lost at 1/t 'r e " r e e w e e e = + - ¥ j wt s+ ¥ 1 Debye equation : ( )
- 17. Dielectric Effects e s =e static e ¥ =e optical e 0 ln(w) wLO wP 30-50 meV 10-15 eV visible infrared Major source of POLARIZATION is distortion of the bonding electrons around atoms. This leads to the normal semiconductor dielectric constant. In POLAR materials, like GaAs and SiC, the different charge on the A and B atoms can be polarized as well, leading to a difference between the optical and the static dielectric constants. In Appendix C, the two values for GaAs are reversed!
- 20. Definition: A photonic crystal is a periodic arrangement of a dielectric material that exhibits strong interaction with light
- 23. Piezoelectric Effect In materials with NO REFLECTION SYMMETRY (like GaAs or many molecular species) the applied electric field produces a DISTORTION OF THE LATTICE (size change) and vice versa. FORCE ELECTRIC FIELD A common piezoelectric is Poly-Vinylidene Flouride, which is used in a variety of stereo headsets. The most common is crystalline quartz used as frequency control crystals—pressure applied to the quartz has a resonance which can be used in a feedback loop to create a highly-stable oscillator—the quartz crystal oscillator.
- 26. Phase sensitive multimeter interfaced with impendence analyzer for dielectric measurements
Notes de l'éditeur
- There are lots of different terms in use today to describe electromagnetic properties of materials. And lots of confusion on what they all mean and how they relate to each other. So, in the next few slides, I’ll give a definitions of the terms I will be using and what they mean.
- Permittivity (e), also called dielectric constant, and sometimes designated by the greek letter Kappa, describes the interaction of a material with an external electric field. Kappa is equivalent to epsilon sub r, and is equal to the absolute permittivity, epsilon, relative to the permittivity of free space, epsilon sub zero. And it is a complex number: epsilon sub r prime minus jay epsilon sub r double prime. Which is quite mouthful! So I will drop the sub R. When I say epsilon or permittivity, you can assume I mean the permittivity relative to free space.
- To make matters more confusing, the real part of permittivity can also be called dielectric constant or Dk So in an effort to minimize the ambiguity, I will say “the real part of permittivity”.
- Permeability or mu describes the interaction of a material with an external magnetic field. And it is also a complex number.
- Both permittivity and permeability are complex but not constant (another reason why the term dielectric constant is ambiguous). Many materials exhibit considerable change over frequency and temperature. Some materials such as iron (ferrites), cobalt, nickel and their alloys have appreciable magnetic properties for which it is valuable to measure permeability; however, many materials are non-magnetic. All materials, on the other hand, have dielectric properties.
- When electric and magnetic fields pass through a material, each can interact with that material in two ways: First: Storage: Energy may be exchanged between the field and the material, in a bi-directional (lossless) manner This energy storage is represented by the real part of permittivity or permeability.
- Second Loss: Energy may be permanently lost from the field, and absorbed in the material (usually as heat). This energy loss is represented by the imaginary part of permittivity and permeability.
- Another term we will talk about, and you will often see on data sheets is loss tangent. This is also called tan delta. When complex permittivity is drawn as a simple vector diagram, the real and imaginary components are 90o out of phase. The vector sum forms an angle delta with the real axis. If we remember our trigonometry, we recall that the tangent of an angle is equal to the side opposite to the angle divided by the side adjacent to the angle. So, in this case, is the imaginary part of permittivity, divided by the real part of permittivity. This is why the term tan delta came about. Loss tangent is also equivalent to the dissipation factor and one over the quality factor. It is a measure of the energy lost relative to the energy stored. The term Df is also commonly used for Dissipation factor. It is no wonder people get confused, all these terms mean the same thing, Loss Tangent, tan delta, Dissipation factor, and Df.
- Also of interest to many applications involving liquid and polar materials, for example microwave heating, Specific Absorption Rate, chemical processing, etc.. is the relaxation constant or time, For dipolar dielectrics (such as water), describes the time required for dipoles to become oriented in an electric field. (Or the time needed for thermal agitation to disorient the dipoles after the electric field is removed.) At low frequencies, the dipole rotation can follow the field easily; ’ will be high and ” will be low. As the frequency increases, the loss factor, ” increases as the dipoles rotate faster and faster. The loss factor ” peaks at the frequency 1/. Here, the dipoles are rotating as fast as they can, and energy is transferred into the material and lost to the field at the fastest possible rate. As the frequency increases further, the dipoles can not follow the rapidly changing field and both ’ and ” fall off. is one of the terms needed in the Debye equation that is often used to model the theoretical permittivity of polar liquids. This model works very well for water. The other terms are the predicted value of s, the DC or static value of permittivity, and at infinity.