Dielectric constant at high frequencies

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

Electrical properties of a commercially available glass-bonded mica with respect to temperature frequency and humidity are shown in Figs. 2.45 to 2.48. The low dissipation factor and relatively stable dielectric constant at high frequencies persist to fairly high temperatures. Engineering properties of glass-bonded mica are listed in Table 2.12. 

Herei A andi B are the cavity radii for solvation of the isolated donor and acceptor ions, where A - - B+ A+ -F B I ab is the A-B distance at contact and is the familiar (relative) dielectric constant s, except that we add the subscript 0 to emphasize that we mean the static limit. The limiting value of the dielectric constant at high frequencies is roo- This is the dielectric constant of the solvent when only its electrons can respond. Because roo is but a small fraction of s, the reorganization energy is of the order of the energy of solvation of a charge and as such 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). 

Electronic polarization of the environment. This effect is related to the square of the refractive index, n1 2 (dielectric constant at the frequency of light). Here the spectral shift occurs instantly (10 15 s), and its evolution with time is not observed by the kinetic spectroscopic methods. The protein molecule is a medium with a relatively high electronic polarization (n= 1.5 -s-1.6). 

Figure 1 indicates the dielectric behavior of practically all tissues. Two remarkable features are apparent exceedingly high dielectric constants at low frequencies and three clearly separated relaxation regions a, 3, y of the dielectric constant at low, medium, and very high frequencies. Each of these relaxation regions is in its simplest form characterized by equations of the Debye type... 

The electrical properties of polyelectrolyte complexes are more closely related to those of biologically produced solids. The extremely high relative dielectric constants at low frequencies and the dispersion properties of salt-containing polyelectrolyte complexes have not been reported for other synthetic polymers. Neutral polyelectrolyte complexes immersed in dilute salt solution undergo marked changes in alternating current capacitance and resistance upon small variations in the electrolyte concentration. In addition, their frequency-dependence is governed by the nature of the microions. As shown in... 

Ice is a (leaky) dielectric and may therefore contain polarization charges from three sources making characteristic contributions to the dielectric constant. At optical frequencies, the high-frequency dielectric constant is caused by the displacement of electron clouds and of atomic nuclei from their equilibrium positions. It is temperature-independent. 

Dielectric relaxation results are proven to be the most definitive to infer the distinctly different dynamic behavior of the hydration layer compared to bulk water. However, it is also important to understand the contributions that give rise to such an anomalous spectrum in the protein hydration layer, and in this context MD simulation has proven to be useful. The calculated frequency-dependent dielectric properties of an ubiquitin solution showed a significant dielectric increment for the static dielectric constant at low frequencies but a decrement at high frequencies [8]. When the overall dielectric response was decomposed into protein-protein, water-water, and water-protein cross-terms, the most important contribution was found to arise from the self-term of water. The simulations beautifully captured the bimodal shape of the dielectric response function, as often observed in experiments. 

Similar property distributions occur throughout the frequency spectrum. The classical example for dielectric liquids at high frequencies is the bulk relaxation of dipoles present in a pseudoviscous liquid. Such behavior was represented by Cole and Cole [1941] by a modification of the Debye expression for the complex dielectric constant and was the first distribution involving the important constant phase element, the CPE, defined in Section 2.1.2.3. In normalized form the complex dielectric constant for the Cole-Cole distribution may be written... 

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... 

The dielectric constant at angular frequency to is e (ai) = e (u>) — ie" u>). The limiting dielectric constants arc e, (sometimes written e ) and e, (sometimes written e,). In Debye s model the relaxing clement (which represents a polar molecule) is a sphere of radius a containing an electric dipole of dipole moment p = qd (see Fig. 4.36). The sphere is immersed in a liquid of viscosity q. Under an electric field E the torque on the dipole is pE sin 0. The rotation of the dipole under this torque is resisted by the Stokes frictional torque Xnt/cr tl. The dipole will follow the field for low U biit not for high. The relaxation time is... 

Corona resistance Among the factors contributing to the breakdown of insulating materials is dielectric heating at high frequencies and, in many instances, the effect of corona. Corona is usually described as the partial breakdown of insulation due to the concentration of electrical stress at sharp edges, or actual breakdown of insulation when placed in series with another insulation having a different dielectric constant. 

The two effects, electronic or molecular rearrangement, can be discriminated by measuring the dielectric constant at different frequencies using electromagnetic radiation. When the frequency is low, the responses of molecular as well as electronic motion are measured. At high frequency, however, only the response due to adaptation of electronic motion is found. 

The sLCP has excellent dielectric characteristics at high frequencies and low moisture absorption as shown in Table 2.1(C). The dielectric properties of the sLCP film were formd to be constant up to a frequency of 25 GHz. Trials of sLCP as a substrate material of flexible printed circuit boards for higher frequency applications have been performed. Interestingly, the sLCP itself has a relatively high thermal conductivity. Its low viscosity allows the incorporation of a larger quantity of fillers to even further improve thermal conductivity. Trials have been continued in order to make use of sLCP as a substrate for metal based copper clad laminates (MCCL). 

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