Dielectric excitation frequency

The relaxation methods employed are Dynamic Mechanical Thermal Analysis (DMTA) and Dielectric Thermal Analysis (DETA). Generally in both cases a single excitation frequency is used and the temperature is varied,... 

I he notation 0e indicates that this is the dielectric function at frequencies low i ompared with electronic excitation frequencies. We have also replaced co0 with l (, the frequency of the transverse optical mode in an ionic crystal microscopic theory shows that only this type of traveling wave will be readily excited bv a photon. Note that co2 in (9.20) corresponds to 01 e2/me0 for the lattice vibrations (ionic oscillators) rather than for the electrons. The mass of an electron is some thousands of times less than that of an ion thus, the plasma liequency for lattice vibrations is correspondingly reduced compared with that lor electrons. 

The relaxation methods employed are Dynamic Mechanical Thermal Analysis (DMTA) and Dielectric Thermal Analysis (DETA). Generally in both cases a single excitation frequency is used and the temperature is varied, typically over a range between — 100 °C and +200 °C. Changes in molecular motion, and hence 7, are detected by both techniques, but in the case of DETA the process has to involve movement of dipoles or fully developed electrical charges on the polymer in order to be detected. Thus the two techniques can be used to complement each other, since transitions can be detected on DMTA and assigned as due to dipoles according to whether or not they also occur with DETA. 

In static operations, the lifetime is mainly limited by the humidity, which penetrates through the external insulation layer and leads to a leakage current increase. A larger leakage current can lead to an electrical breakdown. Due to the dielectric and mechanical losses, the piezoelectric actuator warms up under continuous excitation. Losses are mainly non-linear and depend on the excitation frequency, the voltage amplitude and the humidity. To avoid a depoling effect of the ceramic, the temperature in the actuator should be monitored to ensure that it stays well below the ceramics Curie temperature. So a typical range of temperatures is —40°C to 80 °C. 

From Eq. (12) it is apparent that when efcjif = -2so, the value of g becomes infinite, which would maximize G. This situation will occur at a given value of (Ol thus the system may be tuned into resonance by changing the excitation frequency, (o. However, the dielectric functions are complex variables so that a zero in the denominator of g giving an infinite enhancement is, in fact, not possible since Si should be expressed as e, = + is2- Assuming... 

It should be remembered that the dielectric functions are dependent on the exciting frequency, wl. It is clear that the DSP resonance occurs when there is a zero in the denominator of Eq. (30), i.e., when... 

Various data sources (44) on plasma parameters can be used to calculate conditions for plasma excitation and resulting properties for microwave coupling. Interactions ia a d-c magnetic field are more compHcated and offer a rich array of means for microwave power transfer (45). The Hterature offers many data sources for dielectric or magnetic permittivities or permeabiHty of materials (30,31,46). Because these properties vary considerably with frequency and temperature, available experimental data are iasufficient to satisfy all proposed appHcations. In these cases, available theories can be appHed or the dielectric parameters can be determined experimentally (47). 

The spectra from strong oscillators have special features which are different from those from metallic and dielectric substrates. Different structures in tanf and A are observed on a metallic substrate, dependent on the thickness of the film (Fig. 4.65). For very thin films up to approximately 100 nm the Berreman effect is found near the position of n = k and n < 1 with a shift to higher wavenumbers in relation to the oscillator frequency. This effect decreases with increasing thickness (d > approx. 100 nm) and is replaced by excitation of a surface wave at the boundary of the dielectric film and metal. The oscillator frequency (TO mode) can now also be observed. On metallic substrates for thin films (d < approx. 2 pm) only the 2-component of the electric field is relevant. With thin films on a dielectric substrate the oscillator frequency and the Berreman effect are always observed simultaneously, because in these circumstances all three components of the electric field are possible (Fig. 4.66). 

The dynamic viscoelasticity of particulate gels of silicone gel and lightly doped poly-p-phenylene (PPP) particles has been studied under ac excitation [55]. The influence of the dielectric constant of the PPP particles has been investigated in detail. It is well known that the dielectric constant varies with the frequency of the applied field, the content of doping, or the measured temperature. In Fig. 11 is displayed the relationship between an increase in shear modulus induced by ac excitation of 0.4kV/mm and the dielectric constant of PPP particles, which was varied by changing the frequency of the applied field. AG increases with s2 and then reaches a constant value. Although the composite gel of PPP particles has dc conductivity, the viscoelastic behavior of the gel in an electric field is qualitatively explained by the model in Sect. 4.2.1, in which the effect of dc conductivity is neglected. 

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