Properties dielectric factors

Sihcone fluids have good dielectric properties, loss factor, specific resistance, and dielectric strength at normal operating conditions, and the properties vary only slightly with temperature (59,328,350). The properties in combination with relatively low flammabiUty have led to the use of siUcones in transformers and other large electrical appHcations (351). The dielectric constant of a 1000-cSt oil is 2.8 at 30°C and 2.6 at 100°C. The loss factor is low,... 

Electrical properties — dielectric constant (e), representing polarization dissipation factor (tan 8), representing relaxation phenomena dielectric strength (EB), representing breakdown phenomena and resistivity (pv), an inverse of conductivity — are compared with other polymers in Table 5.14.74 The low dielectric loss and high electrical resistivity coupled with low water absorption and retention of these properties in harsh environments are major advantages of fluorosilicone elastomers over other polymeric materials.74... 

The Physicochemical Properties of Solvents and Their Relevance to Electrochemistry. The solvent properties of electrochemical importance include the following protic character (acid-base properties), anodic and cathodic voltage limits (related to redox properties and protic character), mutual solubility of the solute and solvent, and physicochemical properties of the solvent (dielectric constant and polarity, donor or solvating properties, liquid range, viscosity, and spectroscopic properties). Practical factors also enter into the choice and include the availability and cost of the solvent, ease of purification, toxicity, and general ease of handling. 

Apart from the type of phospholipids the formation of phospholipid structures such as bilayers, micelles or inverted micelles are directly dependent on the degree of hydration, the hydrophobic forces on the tatty acyl chains, and the electrostatic forces that are present on the polar head group region of the bilayer. The properties of the aqueous medium (pH, ionic strength, dielectric properties) are factors that influence the type of phospholipid structures. 

An application of continuum solvation calculations that has not been extensively studied is the effect of temperature. A straightforward way to determine the solvation free energy at different temperatures is to use the known temperature dependence of the solvent properties (dielectric constant, ionization potential, refractive index, and density of the solvent) and do an ab initio solvation calculation at each temperature. Elcock and McCammon (1997) studied the solvation of amino acids in water from 5 to 100°C and found that the scale factor a should increase with temperature to describe correctly the temperature dependence of the solvation free energy. Tawa and Pratt (1995) examined the equilibrium ionization of liquid water and drew similar conclusions. An alternative way to study temperature effect is through the enthalpy of solvation. The temperature dependence of is related to the partial molar excess enthalpy at infinite dilution,... 

Polysulfone is UL listed for continuous service at 320F, although it will withstand higher temperatures intermittently. It offers a good combination of electrical properties dielectric strength and volume resistivities are high, while dielectric constant and dissipation factor are low. 

Figure 5.25 shows the AC properties (dielectric constant/ dielectric loss factor) at 23°C as a function of the frequency. The differences found in the volume resistivity of these systems are also reflected in the AC properties. The strong decrease of the dielectric constant as a function of the frequency for the system cured with EPIKURE 160/161 indicates a certain amount of capacitive coupling between the conducting carbon black particles. The dielectric loss factor/frequency relation of this system is nearly linear with a slope of about -l, pointing at a pure resistive behaviour (see equation 5.18). 

Electrical Properties Dielectric Strength (V/Mil) Dielectric Constant ( 1 MC dry) Dissipation Factor ( 1 MC dry)... 

Intrinsic properties dielectric constant and dissipation factor. Their changes describe the modifications in the electronic density distribution in various macromolecules (Figs. 1-9). 

FIG. 33 Dependencies of electrical properties dielectric constant (a) and dissipation factor (b) on irradiation aging measured at various frequencies (accelerated electrons dose 10 MGy dose rate 3 kGy/h). (Adapted from Ref. 67.) Column 1 1 kHz, nonirradiated materials column 2 1 kHz, irradiated materials column 3 1 MHz, nonirradiated materials column 4 1 MHz, irradiated materials. 

The dipole moments of the hydrogen halides decrease with increasing atomic number of the hydrogen, the largest difference occurring between HF and HCl, and association of molecules is not an important factor in the properties of FICl, HBr and HI. This change in dipole moment is reflected in the diminishing permittivity (dielectric constant) values from HF to HI. 

The time-temperature superpositioning principle was applied f to the maximum in dielectric loss factors measured on poly(vinyl acetate). Data collected at different temperatures were shifted to match at Tg = 28 C. The shift factors for the frequency (in hertz) at the maximum were found to obey the WLF equation in the following form log co + 6.9 = [ 19.6(T -28)]/[42 (T - 28)]. Estimate the fractional free volume at Tg and a. for the free volume from these data. Recalling from Chap. 3 that the loss factor for the mechanical properties occurs at cor = 1, estimate the relaxation time for poly(vinyl acetate) at 40 and 28.5 C. 

Electrical Properties. The low polarizabiHty of perfluorinated Hquids makes them exceUent insulators. Theh dielectric strengths are about 40 kV (ASTM D877) dissipation factors are about 0.0001 at 1 MH2 dielectric constants are about 1.8 volume resistivities are about 1 x 10 ohm-cm (ASTM D257) (17). 

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). 

Electrical Properties. CeUular polymers have two important electrical appHcations (22). One takes advantage of the combination of inherent toughness and moisture resistance of polymers along with the decreased dielectric constant and dissipation factor of the foamed state to use ceUular polymers as electrical-wire insulation (97). The other combines the low dissipation factor and the rigidity of plastic foams in the constmction of radar domes. Polyurethane foams have been used as high voltage electrical insulation (213). 

Electrical Insulation. The substitution of a gas for part of a soHd polymer usuaUy results in large changes in the electrical properties of the resulting material. The dielectric constant, dissipation factor, and dielectric strength are aU generaUy lowered in amounts roughly proportional to the amount of gas in the foam. 

Electrical. Glasses are used in the electrical and electronic industries as insulators, lamp envelopes, cathode ray tubes, and encapsulators and protectors for microcircuit components, etc. Besides their abiUty to seal to metals and other glasses and to hold a vacuum and resist chemical attack, their electrical properties can be tailored to meet a wide range of needs. Generally, a glass has a high electrical resistivity, a high resistance to dielectric breakdown, and a low power factor and dielectric loss. 

The most important electrical properties of insulation are dielectric strength, insulation resistance, dielectric constant, and power factor. Corona resistance, although not stricdy an electrical property, is usually considered also (10). 

Power factor, like the dielectric constant, is a property that represents a power loss that takes place when a wire insulation becomes the dielectric of a condenser because of a surrounding sheath or other conducting medium. 

Fig. 1. Properties of foods near 2.45 GHz as a function of temperature, where A represents distilled water B, cooked carrots C, mashed potatoes D, cooked ham E, raw beef F, cooked beef and G, com oil (a) dielectric constants and (b) load factors, e = etan6 (32).

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. 

---