Oxidation dielectric losses, effects

For the sample PPX -f Cu the calculated fractal dimension Df is equal to 2.609 [70]. It should be noted that the above-mentioned size distribution of metal nanoparticles leads to the mutual charging of such particles in the percolation cluster. This effect is discussed in the following section in coimection with catalysis by nanoparticles. As stated in reference 70, the specific low-temperature peak of dielectric losses in the synthesized composite samples PPX -t-Cu is probably due to the interaction of electromagnetic field with mutually charged Cu nanoparticles immobilized in the PPX matrix. The minor appearance of this peak in PPX -i- Zn can be explained by oxidation of Zn nanoparticles. 

The data show that there is a significant oxidation effect on both the dielectric constant and the loss tangent of residual carbon content in the composites. However, for the Nextel 312 (BN) system, heat treatment at temperatures above 450°C is definitely not recommended, because of degradation of the boron nitride interface at those temperatures. 

Stable mixtures of hydrophilic and hydrophobic silicas with water at different ratio between the concentrations of these oxides and a constant total amount of oxides were also studied using electrophysical methods (Mironyuk et al. 1999). The conductivity (a) of the oxide blend at different frequencies of the applied electromagnetic field increases relative to that for hydrophilic silica. However, the a value at the constant electrical current decreases as the concentration of hydrophobic silica increases. An increase in the frequency from 0.1 to 10 kHz gives the exponential growth of the ratio between dielectric loss (e") and dielectric permittivity (e ) due to a strong decrease in the e value. These effects are caused by additional ordering in the system to reduce the contact area between hydrophobic silica particles and water that influences formation of micelles with a high polarizability. 

A review of the photo-degradation of polystyrene includes a mechanism proposed to explain the breakdown of that polymer. A novel technique for following the extent of photo-oxidation of bulk polymer has been developed by Weir who measured the dielectric loss of films during irradiation in vacuum and in oxygen. Under these conditions the increases in dielectric constant were attributed to products of degradation, and measurement of the loss-peak due to carbonyl compounds provided a sensitive indicator of extent of reaction. - A study of the role of stabilizers showed that for specific types the stabilization mechanism involved the screening effect as well as the ability to quench excited states of the polymer. A study of photo-oxidation (A>300nm) of films of styrene-type copolymers produced a mechanism in which it was proposed that initiation of the... 

Dielectric measurements have been made by Buckingham and Belling [96] on ethylene/propylene copolymers. The data show an increase in the loss in range of 10 -10 Hz due to some more polarity of PP. Unlike low dielectric losses in PE, the copolymer spectrum exhibits a a-relaxation peak at 10 Hz at room temperature. The effect of oxidation on the microwave behaviour of PE can be seen in the data of Ref. [95]. A loss peak at 10 Hz (room temperature) increased with time of oxidation. 

It is important when choosing the additives to consider what effect they will have on the properties of the sintered body or whether they can be totally removed by oxidation at high temperatures. For instance, when making a piece of zirconium tin titanate, a dielectric resonator material whose densification is promoted by the addition of zinc oxide, it is sensible to consider adding a lubricant such as zinc stearate but not one such as calcium stearate which could have adverse effects on the dielectric losses of the sintered body. Also it would be acceptable to use an ammonium based deflocculant, e.g. Dispex A40, but not a sodium based one, e.g. Dispex N40. In the first instance it will be possible to remove all of the dispersant at a suitably high temperature but in the second sodium ions will not be removed by burning off. The sodium ions would probably increase the losses of the dielectric resonators. 

The effect of moisture on the p relaxations in amorphous phenylene polymers is well documented (93-95) for many of the systems cited previously. For example, Allen and co-workers (93) determined that both the dielectric and mechanical P relaxations in polysulfone, polycarbonate, polyphenylene oxide, and polyether sulfone were dependent on the water content of the samples. In addition, the amount of water absorbed depended on the polarity of the molecule. The effect of moisture on the dielectric p process in these polymers appears to be greater than for the mechanical loss process, as illustrated in Figures 41 and 42 (93). 

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