High-frequency dielectric constant

The terms polarizability constant and dielectric constant can be utilized interchangeably in the qualitative discussion of the magnitude of the dielectric constant. The k values obtained utilizing dc and low-frequency measurements are a summation of electronic E, atomic A, dipole P0, and interfacial /, polarizations. Only the contribution by electronic polarizations is evident at high frequencies. The variation of dielectric constant with frequency for a material having interfacial, dipole, atomic, and electronic polarization contributions is shown in Figure 6.1. 

In compound crystals, the ujn values considered are wlo, the frequency of the longitudinal optical phonons on the high-energy (h-e) side, and wto, the frequency of the transverse optical phonons, on the low-energy side. The dielectric constant at frequencies above c lo is denoted as while that below wto is denoted as s (the index s represents static, despite the fact that s shows a small dispersion between the value just below ujto and the one at radiofrequencies1). It can be seen from expressions (3.14) and (3.15) that above ujo, the ionic contribution decreases such that qo is smaller than s. Typical values are given in Table 3.1. 

It is evident from this discussion that the power loss and heat dissipation in a dielectric will be aided by a high dielectric constant, high dissipation factor, and high frequency Therefore, for satisfactory performance electrical insulating materials should have a low dielectric constant and a low dissipation factor but a high dielectric strength (Table 3.6) and a high insulation resistance. 

Figure 10.111 Dielectric constant vs. frequency and temperature for Chevron Phillips Chemical Ryton R-4—40% glass fiber-filled, high-strength PPS resin [13].

Figure 7 shows the variation of dielectric constant with frequency at room temperature for SPE and NCPE films. It is observed that dielectric constant decrease with increasing field frequency. High value of dielectric constant at low frequency can be explained by the presence of space charge effects due to acciunulation of enhanced charge carrier density near the electrode. As frequency increases, the periodic reversal of the electric field occurs so fast that there is restriction of excess ion diffusion in the direction of applied field and hence dielectric constant decreases. Thus conductivity... 

High Dielectric Constant (Microwave Frequencies) Polymer Composites... 

Muscovite has a high dielectric strength that averages between 3,000 and 6,000 volts/mil at 60 Hz for specimens 1 to 3 mils thick. It has a relatively stable dielectric constant over a wide frequency range, as is shown in Fig. 2.43. Muscovite also has a dissipation factor (tan S) that decreases with frequency. This makes ruby mica an especially useful dielectric for high-frequency apphcations. 

Here e , is the high frequ y limit of s, So is the static dielectric constant (low frequency limit of s ). So - Soo = A is the dielectric increment, fR is the relaxation frequency, a is the Cole-Cole distribution parameter, and P is the asymmetry parameter. The relaxation frequency is related to the relaxation time by fa = (27It) A simple exponential decay of P (oc,P = 0) is characterised by a single relaxation time (Debye-process [1]), P = 0 and 1 < a < 0 describe a Cole-Cole-relaxation [2] with a symmetrical distribution function of t whereas the Havriliak-Negami equation (EQN (4)) is used for an asymmetric distribution of x [3]. The symmetry can be readily seen by plotting s versus s" as the so-called Cole-Cole plot [4-6]. 

Tetralluoroethylene polymer has the lowest coefficient of friction of any solid. It has remarkable chemical resistance and a very low brittleness temperature ( — 100°C). Its dielectric constant and loss factor are low and stable across a broad temperature and frequency range. Its impact strength is high. 

High Frequency Dielectric Strength. Dielectric strength at high frequency is important in microwave power uses such as radar (see Microwave technology). Because SF has zero dipole moment, its dielectric strength is substantially constant as frequency increases. At 1.2 MHz, SF has... 

In air, PTFE has a damage threshold of 200—700 Gy (2 x 10 — 7 x 10 rad) and retains 50% of initial tensile strength after a dose of 10" Gy (1 Mrad), 40% of initial tensile strength after a dose of 10 Gy (10 lad), and ultimate elongation of 100% or more for doses up to 2—5 kGy (2 X 10 — 5 X 10 rad). During irradiation, resistivity decreases, whereas the dielectric constant and the dissipation factor increase. After irradiation, these properties tend to return to their preexposure values. Dielectric properties at high frequency are less sensitive to radiation than are properties at low frequency. Radiation has veryHtde effect on dielectric strength (86). 

For most commercial voltages and frequencies used in power distribution, the capacitance effects are negligible. At relatively high voltages the current due to capacitance may reach sufficient value to affect the circuit, and insulation for such an appHcation is designed for a moderately low dielectric constant. 

Electrical Properties. AH polyolefins have low dielectric constants and can be used as insulators in particular, PMP has the lowest dielectric constant among all synthetic resins. As a result, PMP has excellent dielectric properties and alow dielectric loss factor, surpassing those of other polyolefin resins and polytetrafluoroethylene (Teflon). These properties remain nearly constant over a wide temperature range. The dielectric characteristics of poly(vinylcyclohexane) are especially attractive its dielectric loss remains constant between —180 and 160°C, which makes it a prospective high frequency dielectric material of high thermal stabiUty. 

Material Dielectric constant at high frequency Density, kg/m Knoop hardness, kg/mm Thermal conductivity, W/(m-K) Melting point, °C... 

Acryhc resins have excellent moisture resistance, dielectric properties, and reworkabiUty, but poor abrasion resistance. Their dielectric constant, which decreases with increasing frequency, makes them attractive candidates for high frequency appHcations. 

Electrical Properties. Polysulfones offer excellent electrical insulative capabiUties and other electrical properties as can be seen from the data in Table 7. The resins exhibit low dielectric constants and dissipation factors even in the GH2 (microwave) frequency range. This performance is retained over a wide temperature range and has permitted appHcations such as printed wiring board substrates, electronic connectors, lighting sockets, business machine components, and automotive fuse housings, to name a few. The desirable electrical properties along with the inherent flame retardancy of polysulfones make these polymers prime candidates in many high temperature electrical and electronic appHcations. 

The attenuation of ultrasound (acoustic spectroscopy) or high frequency electrical current (dielectric spectroscopy) as it passes through a suspension is different for weU-dispersed individual particles than for floes of those particles because the floes adsorb energy by breakup and reformation as pressure or electrical waves josde them. The degree of attenuation varies with frequency in a manner related to floe breakup and reformation rate constants, which depend on the strength of the interparticle attraction, size, and density (inertia) of the particles, and viscosity of the Hquid. 

Because of very high dielectric constants k > 20, 000), lead-based relaxor ferroelectrics, Pb(B, B2)02, where B is typically a low valence cation and B2 is a high valence cation, have been iavestigated for multilayer capacitor appHcations. Relaxor ferroelectrics are dielectric materials that display frequency dependent dielectric constant versus temperature behavior near the Curie transition. Dielectric properties result from the compositional disorder ia the B and B2 cation distribution and the associated dipolar and ferroelectric polarization mechanisms. Close control of the processiag conditions is requited for property optimization. Capacitor compositions are often based on lead magnesium niobate (PMN), Pb(Mg2 3Nb2 3)02, and lead ziac niobate (PZN), Pb(Zn 3Nb2 3)03. 

Polymers with outstandingly high resistivity, low dielectric constant and negligible power factor, all substantially unaffected by temperature, frequency and humidity over the usual range of service conditions. 

At low frequencies when power losses are low these values are also low but they increase when such frequencies are reached that the dipoles cannot keep in phase. After passing through a peak at some characteristic frequency they fall in value as the frequency further increases. This is because at such high frequencies there is no time for substantial dipole movement and so the power losses are reduced. Because of the dependence of the dipole movement on the internal viscosity, the power factor like the dielectric constant, is strongly dependent on temperature. 

Poly(methyl methacrylate) is a good electrical insulator for low-frequency work, but is inferior to such polymers as polyethylene and polystyrene, particularly at high frequencies. The influence of temperature and frequency on the dielectric constant is shown in Figure 15.9. 

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