Dielectric loss effects

Ordinarily the value of k evaluated by means of (S) includes a background conductivity k, which is due to transport of firee-cfaarge carriers like ions and/or electrons. Since Xg corresponds to a finite value of K at oj -> 0 we then have apparently c oo in the limit of low frequencies. In most studies of dielectric-loss effects such complication is avoided by discussing only the dynamic conductivity k — xg. 

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. 

Since the harmonic disorders occur at higher frequencies than the fundamental (/, > /). they cause higher dielectric losses due to a higher skin effect. 

If the wire is to be used to carry much higher frequency currents, the design problem in geometry and plastic selection becomes more complicated. The dielectric constant and dielectric loss values for the plastics become important in the design. At a frequency of one megahertz the effect of the dielectric on the power transmission behavior of the wire is substantial and, even at frequencies of 10 to 100 kilohertz, the insulation on the wire must be considered in the design as a major electrical element in the circuit. More on the subject of insulation will be following this section. 

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. 

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. 

Dielectric losses arise from the direct capacitive coupling of the coil and the sample. Areas of high dielectric loss are associated with the presence of axial electric fields, which exist half way along the length of the solenoid, for example. Dielectric losses can be modeled by the circuit given in Figure 2.5.3. The other major noise source arises from the coil itself, in the form of an equivalent series resistance, Rcoii. Exact calculations of noise in solenoidal coils at high frequencies and small diameters are complex, and involve considerations of the proximity and skin depth effects [23],... 

The objective of this first part of the book is to explain in a chemically intelligible fashion the physical origin of microwave-matter interactions. After consideration of the history of microwaves, and their position in the electromagnetic spectrum, we will examine the notions of polarization and dielectric loss. The orienting effects of the electric field, and the physical origin of dielectric loss will be analyzed, as will transfers between rotational states and vibrational states within condensed phases. A brief overview of thermodynamic and athermal effects will also be given. 

This macroscopic theory justifies the complex nature of the dielectric permittivity for media with dielectric loss. The real part of the dielectric permittivity expresses the orienting effect of the electric field with the component of polarization which fol-... 

Fig. 1.7 Effect of frequency and temperature on the complex dielectric permittivity (s is the real part and the imaginary part or the dielectric loss) [12].

If the molecular effects of the electric field are irrelevant to microwave heating of solutions, this assumption could be envisaged in the use of operating conditions very far from current conditions. On one hand, it will be necessary to use an electric field of higher amplitude, or to reduce the temperature according to the Langevin function. This last solution is obviously antinomic with conventional chemical kinetics, and the first solution is, currently, technologically impossible. It will, on the other hand, be necessary to avoid reaction media with dielectric loss. The molecular effects of the microwave electric field could, paradoxically, be observed for a medium which is not heated by the action of microwave irradiation. 

For liquid products (solvents), only polar molecules selectively absorb microwaves, because nonpolar molecules are inert to microwave dielectric loss. In this context of efficient microwave absorption it has also been shown that boiling points can be higher when solvents are subjected to microwave irradiation rather than conventional heating. This effect, called the superheating effect [13, 14] has been attributed to retardation of nucleation during microwave heating (Tab. 3.1). 

In order to quantify diffiisional effects on curing reactions, kinetic models are proposed in the literature [7,54,88,95,99,127-133]. Special techniques, such as dielectric permittivity, dielectric loss factor, ionic conductivity, and dipole relaxation time, are employed because spectroscopic techniques (e.g., FT i.r. or n.m.r.) are ineffective because of the insolubility of the reaction mixture at high conversions. A simple model, Equation 2.23, is presented by Chem and Poehlein [3], where a diffiisional factor,//, is introduced in the phenomenological equation, Equation 2.1. 

The dielectric constant is generally different in the crystalline and amorphous phases. In this case, Xc in Eq. (95) does not mean the volume fraction of the crystalline phase itself. Furthermore, the amorphous phase exhibits dielectric relaxation in the region where the mechanical relaxation occurs. Considering this effect, Xc should be taken as a complex quantity with a negative imaginary part. However, the inequality (100) is valid in so far as the dielectric loss tangent is smaller than the mechanical one. The inequality (101) holds more generally because the effect of dielectric relaxation enhances the positive value of d"jd. ... 

Traditionally, charge transfer mechanisms have been studied by such methods as conductivity, the Hall effect, and thermoelectric effect. Details of these applications may be found in Experimental Methods of Physics, Vol. 6, Pt. b (12), the article on ionic conductivity by Lidiard (70), and in many of the original papers quoted. More recently, techniques such as electron spin resonance (13), dielectric loss and pulsed photoconductivity methods (5—8) have been used to study semiconduction in organic materials. 

For many applications low-temperature flexibility of the plasticized composition is also important. Plasticizers of low viscosity and low viscosity-temperature gradient are usually effective at low temperature. There is also a close relationship betv/een rate of oil extraction and low-temperature flexibility plasticizers effective at low temperature are usually rather readily extracted from the resin. Plasticizers containing linear alkyl chains are generally more effective at low temperature than those containing rings. Low-temperature performance is evaluated by measuremen t of stiffness in flexure or torsion or by measurement of second-order transition point, brittle point or peak dielectric loss factor. 

Baghurst, D.J., Mingos, D.M.P. and Watson, M.J., Application of microwave dielectric loss heating effects for the rapid and convenient synthesis of orga no metallic compounds, /. Organomet. Chem., 1989, 368, C43. 

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