Dielectric constant, at high

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. 

For comparison, one of the most common potyimides used as a dielectric in electronics applications is PMDA-ODA (9), made Ity reaction of pyromellitic dianhydride (PMDA) and 4,4 -diaminodiphenylether (ODA). For PMDA-ODA, the dielectric constant at 0% RH is 3.10 and at 58% RH the dielectric constant increases to 3.71. The smaller change in dielectric constant as a function of relative humidity for 5a, 5b, and 5c compared to PMDA-ODA may offer a performance advantage in microelectronic applications where low dielectric constant at high relative humidities is required. 

A. D. Buckingham, /. Chem. Phys., 25, 428 (1956). Theory of the Dielectric Constant at High Field Strengths. 

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). 

Solvent. The solvent properties of water and steam are a consequence of the dielectric constant. At 25°C, the dielectric constant of water is 78.4, which enables ready dissolution of salts. As the temperature increases, the dielectric constant decreases. At the critical point, the dielectric constant is only 2, which is similar to the dielectric constants of many organic compounds at 25°C. The solubiUty of many salts declines at high temperatures. As a consequence, steam is a poor solvent for salts. However, at the critical point and above, water is a good solvent for organic molecules. 

Historically, materials based on doped barium titanate were used to achieve dielectric constants as high as 2,000 to 10,000. The high dielectric constants result from ionic polarization and the stress enhancement of k associated with the fine-grain size of the material. The specific dielectric properties are obtained through compositional modifications, ie, the inclusion of various additives at different doping levels. For example, additions of strontium titanate to barium titanate shift the Curie point, the temperature at which the ferroelectric to paraelectric phase transition occurs and the maximum dielectric constant is typically observed, to lower temperature as shown in Figure 1 (2). 

The effectiveness of nitromethane can be attributed to its high dielectric constant, at least in part, which tends to promote reactions which involve electron-rich intermediates. It may also result from the low solubility of the indole products in nitromethane since the indoles precipitate out of the reaction mixture in many cases. ... 

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). 

The sodium chloride (NaCl) crystal shows very high absorption and reflectivity in the infrared region, known as the Restrahlen region. The real and imaginary relative dielectric constants at 6000 nm are, respectively, ei = 16.8 and S2 = 91.4. At this wavelength, estimate (a) the refractive index and the extinction coefficient, and (b) the optical density and the reflectivity at normal incidence for a 1 mm thick NaCl sample, (c) If the previous sample is illuminated (at normal incidence) by a beam of intensity Iq at 6000 nm, estimate the intensity of this... 

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