Dielectric Loss Mechanisms

The moisture content of a plastic affects such conditions as electrical insulation resistance, dielectric losses, mechanical properties, dimensions, and appearances. The effect on the properties due to moisture content depends largely on the type of exposure (by immersion in water or by exposure to high humidity), the shape of the product, and the inherent behavior properties of the plastic material. The ultimate proof for tolerance of moisture in a product has to be a product test under extreme conditions of usage in which critical dimensions and needed properties are verified. Plastics with very low water-moisture absorption rates tend to have better dimensional stability. 

Other relevant fields in physics are dielectric loss, mechanical loss modulus... 

Cure rate of an actual adhesive film can also be determined by several useful analytical methods. With these methods, fundamental properties of the adhesive, such as dielectric loss, mechanical damping, or exotherm, are measured as a function of time and temperature as the adhesive cures. Several of these test methods are described in Chap. 20. 

Ingram, M. D. (1980) Conduction and Dielectric Loss Mechanisms in beta-Alumina and Glass A Discussion Based on the Paired Interstitialcy Model, J. Am. Ceram. Soc., 63, 248-263. 

A yield stress equation was also derived on the basis of the dielectric loss mechanism, as described in the preceding chapter. Under the assumption that only interfacial polarization would contribute to the ER effect and the ER particle would form the bet structure under an electric field, a yield stress equation could be expressed in Eq. (69) or Eq. (70) in Chapter 8, which obviously indicates that the yield stress of an ER fluid would increase with the square of the applied electric field, the particle volume fraction and the dielectric constant of the liquid medium. Those predictions agree very well with previous experimental results [75-77]. 

FIGURE 8.39 Dielectric loss mechanisms as a function of frequency. 

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. 

Relaxations of a-PVDF have been investigated by various methods including dielectric, dynamic mechanical, nmr, dilatometric, and piezoelectric and reviewed (3). Significant relaxation ranges are seen in the loss-modulus curve of the dynamic mechanical spectmm for a-PVDF at about 100°C (a ), 50°C (a ), —38° C (P), and —70° C (y). PVDF relaxation temperatures are rather complex because the behavior of PVDF varies with thermal or mechanical history and with the testing methodology (131). 

Grade G-10, glass fabric with epoxy resin binder, has extremely high mechanical strength (flexural, impact, and bonding) at room temperature and good dielectric loss and electric strength properties under both dry and humid conditions. 

Principles in Processing Materials. In most practical apphcations of microwave power, the material to be processed is adequately specified in terms of its dielectric permittivity and conductivity. The permittivity is generally taken as complex to reflect loss mechanisms of the dielectric polarization process the conductivity may be specified separately to designate free carriers. Eor simplicity, it is common to lump ah. loss or absorption processes under one constitutive parameter (20) which can be alternatively labeled a conductivity, <7, or an imaginary part of the complex dielectric constant, S, as expressed in the foUowing equations for complex permittivity ... 

The interaction of microwaves with ferrites (qv) has many complicating features. Low field loss mechanism (41), nonlinear effects, and losses at high power levels (41,43) as well as dielectric losses are among these. 

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. 

The observed dielectric constant M and the dielectric loss factor k = k tan S are defined by the charge displacement characteristics of the ceramic ie, the movement of charged species within the material in response to the appHed electric field. Discussion of polarization mechanisms is available (1). 

Dielectric loss The dielectric loss factor represents energy that is lost to the insulator as a result of its being subjected to alternating current (AC) fields. The effect is caused by the rotation of dipoles in the plastic structure and by the displacement effects in the plastic chain caused by the electrical fields. The frictional effects cause energy absorption and the effect is analogous to the mechanical hysteresis effects except that the motion of the material is field induced instead of mechanically induced. 

Fig. 2.5.3 Typical NMR resonant tank circuit, showing coil loss mechanisms. This LC circuit is then placed in series with two matching capacitors (Cmatch). The resistance of the circuit is represented by Rco], the inductive losses by Rm and the dielectric losses by Ci, Cd and Rd.

The statistical mechanics of such impurity systems has been treated by Lidiard61-53 and his method has been widely employed in the interpretation of experimental data, e.g. ionic conductivity,6 51 dielectric loss,8 thermoelectric power,16-36 diffusion,31... 

High performance/ cost ratios High mechanical performances Low dielectric loss... 

The electric properties of polymers are also related to their mechanical behavior. The dielectric constant and dielectric loss factor are analogous to the elastic compliance and mechanical loss factor. Electric resistivity is analogous to viscosity. Polar polymers, such as ionomers, possess permanent dipole moments. These polar materials are capable of storing... 

---