Dielectric stiffness constant

Commonly used materials for cable insulation are poly(vinyl chloride) (PVC) compounds, polyamides, polyethylenes, polypropylenes, polyurethanes, and fluoropolymers. PVC compounds possess high dielectric and mechanical strength, flexibiUty, and resistance to flame, water, and abrasion. Polyethylene and polypropylene are used for high speed appHcations that require a low dielectric constant and low loss tangent. At low temperatures, these materials are stiff but bendable without breaking. They are also resistant to moisture, chemical attack, heat, and abrasion. Table 14 gives the mechanical and electrical properties of materials used for cable insulation. 

Composite piezoelectric transducers made from poled Pb-Ti-Zr (PZT) ceramics and epoxy polymers form an interesting family of materials which highlight the advantages of composite structures in improving coupled properties in soilds for transduction applications A number of different connection patterns have been fabricated with the piezoelectric ceramic in the form of spheres, fibers, layered, or three-dimensional skeletons Adding a polymer phase lowers the density, the dielectric constant, and the mechanical stiffness of the composite, thereby altering electric field and concentrating mechanical stresses on the piezoelectric ceramic phase. 

For most polymers, the relationship between the glass transition temperature 7g and T), is 7g = T), -E C where C is a constant that in many dielectric studies is about 50 K [9, 73, 74]. On the basis of the usual uncertainty in C and the above Tq = 285 K, we estimate that Tg for dry annealed chitin is 62 10 °C. This low value for the 7g of a stiff polymer such as a-chitin is slightly surprising, but it is consistent with the low Ig s found in polypeptides (-70 to -50°C) [75-77]. The common denominator between polysaccharides and polypeptides is extensive hydrogen bonding significant thermal dismption of H-bonding and the onset of main chain molecular motions are probably closely related. 

PROPERTIES OF SPECIAL INTEREST Intractable pwlymer with no melting behavior below temperatures of significant degradation. It polymerizes directly into the crystalline state, thus showing high tensile stiffness, low dielectric constant, and dimensional stability at high temperatures. 

The polarity and strong inter-chain attraction gives a higher hardness and stiffness than polyethylene. Thus PVC has a higher dielectric constant and power factor than polyethylene, although at temperatures below the glass transition temperature the power factor is still comparatively low (0.01-0.05 at 60 Hz) because of the immobility of the dipole. PVC is... 

Tables 4.4-3-4.4-21 are arranged according to piezoelectric classes in order of decreasing symmetry (see Table 4.4-2), and alphabetically within each class. They contain a number of columns placed on two pages, even and odd. The following properties are presented for each dielectric material density q, Mohs hardness, thermal conductivity k, static dielectric constant Sij, dissipation factor tanS at various temperatures and frequencies, elastic stiffness Cmn, elastic compliance s n (for isotropic and cubic materials only), piezoelectric strain tensor di , elastooptic tensor electrooptic coefficients r k (the lat-...

General Static dielectric constant Dissipation factor Oastic stiffness tensor Elastic compliance tensor Elastooptic tensor... 

Specifically, the reinforcement of polymers with glass fibers substantially improves mechanical properties such as strength and stiffness improves dimensional stability, mold shrinkage, and chemical resistance and improves the dc electrical properties of dielectric strength and arc resistance as well as ac dielectric constant and dissipation factor. Both increases and decreases are noted in volume resistivities. Reinforcement also reduces percent elongation and thermal expansion properties. 

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