Dielectric materials frequency effects

Eq.(l 13) clearly indicates that at high frequency the dielectric constant decays with For a dielectric material, the effective conductivity CTeff is usually expressed as ... 

A number of areas in which plastics are used in electrical and electronic design have been covered there are many more. Examples include fiber optics, computer hardware and software, radomes for radar transmitters, sound transmitters, and appliances. Reviewed were the basic use and behavior for plastics as an insulator or as a dielectric material and applying design parameters. The effect of field intensity, frequency, environmental effects, temperature, and time were reviewed as part of the design process. Several special applications for plastics based on intrinsic properties of plastics materials were also reviewed. 

Impedance spectroscopy is discussed in depth in the monograph edited by J.Ross Macdonald [17]. It has its origins in the classical work of K.S. Cole and R.H. Cole, published more than 60 years ago, concerned with methods of plotting the response of a dielectric material to applied voltages as a function of frequency. The method assists in identifying observed relaxation effects with processes at the atomic and microstructural levels. For a system having a single well-defined... 

Capacitively coupled electrodes are frequently employed in electrical measurements to study such effects as interfacial polarization, dielectric polarization, and high-frequency effects [38]. Capacitive electrodes are also sometimes used with semi-insulators to generate a field inside the specimen that is well-defined, and have been applied to the study of the initiation of PbN [39,40]. However, caution must be exercised in such an enterprise. A material with conductivity (ohm m)" is generally viewed as a good insulator. Yet it has an... 

Many dielectric materials are conductive. This complicates the TDS study of conductive samples, and the effect of low-frequency conductivity needs to be corrected for. Usually, for a low-conductivity system the value of the d.c. conductivity can be evaluated as described in Sec. III.A. 

In all real systems, some deviation from ideal behavior can he observed. If a potential is applied to a macroscopic system, the total current is the sum of a large number of microscopic current filaments, which originate and end at the electrodes. If the electrode surfaces are rough or one or more of the dielectric materials in the system are inhomogeneous, then all these microscopic current filaments would be different. In a response to a small-amplitude excitation signal, this would lead to frequency-dependent effects that can often be modeled with simple distributed circuit elements. One of these elements, which have found widespread use in the modeling of impedance spectra, is the so-called constant phase element (CPE). A CPE is defined as... 

Effective medium theories characterize the frequency-dependent transport in systems with large-scale inhomogeneities such as metal particles dispersed in an insulating matrix [118,119]. An IMT in the effective medium model represents a percolation problem where a finite a c as T 0 is not achieved until metallic grains in contact span the sample. To understand the frequency dependence of the macroscopic material, an effective medium is built up from a composite of volume fraction /of metallic grains and volume fraction 1 — / of insulator grains. The effective dielectric function semaCw) and conductivity function (Tema(w) are solved self-consistently. 

Dielectric High-frequency voltage applied to film or sheet causes material to melt at bonding surfaces. Material cools rapidly to effect a bond. Most widely used with vinyls. Fast seal with minimum heat applied. Only for film and sheet. Requires rf generator, dies, and press. Operation can range from hand-fed to semiautomatic with speeds depending on thickness and type of product being handled. Units of 3 to 25 kW are most common. 

When the dielectric material is subjected to an alternating field, the orientation of dipoles, and hence the polarization, will tend to reverse every time the polarity of the field changes (Nelson et al., 1991). At low frequencies the polarization follows the alterations of the field without any significant lag and the permittivity is independent of frequency and has the same value as in static field. When the frequency is increased the dipoles will no longer be able to rotate sufficiently rapidly so that their oscillations will begin to lag behind those of the field (Galasso, 1969). The above effect leads to a fall in dielectric constant of the material with frequency as in (Figure 16.6) (Vrejoiu et al., 2002). 

These devices create a resonant impedance to reduce reflections at multiple frequencies corresponding to multiple quarter-wavelengths of the dielectric material. Multilayers, or Jaumann absorbers, are generally more frequency broadband absorbers and may even be graded multidielectric or multimagnetic materials and thus increase the effectiveness at angles off of normal. Knott has shown that the thickness of the absorber can be reduced and the bandwidth considerably expanded if the sheets are allowed to have capacitive reactance in addition to pure resistance (Knott and Lunden, 1995). 

---