Heating dielectric

Dielectric losses are usually dissipated as heat inside the dielectric material, and in the surroundings (e.g., air). The dissipated power (i.e., power input for the material), expressed in watts (W), is given by the following equation, where/is the frequency in Hertz (Hz), with 2nf= co  

The dissipation of heat strongly depends on the applied parameters such as the frequency,/, of the electric field strength and its amplitude, E, as well as intrinsic properties of the material such as the loss factor and thermophysical properties of the material, geometrical factors such as the shape and dimensions of the material, and finally the type and nature of the surroundings. On the other hand, this behavior is used extensively to heat industrial materials in the plastics industry, in woodworking, and for drying many insulating materials. 

Dielectric heating occurs when a dielectric material is placed in an electric field that alternates at high frequency. A dielectric material is an electric insulator and it has a low conductivity (high resistivity). Materials with a resistivity higher than 10 ohm-cm are generally considered to be dielectric most polymers fall into this cate- 

Dissipation factor, power factor, loss angle, etc. are important terms in dielectric heating. They are defined as follows  

For most polymers, the loss angle is quite small, thus sin8 tanS in other words the power factor and dissipation factor are almost equal. 

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Beside continuous horizontal kilns, numerous other methods for dry pyrolysis of urea have been described, eg, use of stirred batch or continuous reactors, ribbon mixers, ball mills, etc (109), heated metal surfaces such as moving belts, screws, rotating dmms, etc (110), molten tin or its alloys (111), dielectric heating (112), and fluidized beds (with performed urea cyanurate) (113). AH of these modifications yield impure CA. 

Use of specific forms of radiant energy, infrared, ultraviolet, dielectric heating, etc., can allow specific separations to be made. The separation of clear and colored grains of glass and the separation of different metals are possible apphcations of the thermoadhesive method being considered in the field of solid-waste processing. 

Abstract Current microwave-assisted protocols for reaction on solid-phase and soluble supports are critically reviewed. The compatibility of commercially available polymer supports with the relatively harsh conditions of microwave heating and the possibilities for reaction monitoring are discussed. Instrmnentation available for microwave-assisted solid-phase chemistry is presented. This review also summarizes the recent applications of controlled microwave heating to sohd-phase and SPOT-chemistry, as well as to synthesis on soluble polymers, fluorous phases and functional ionic liquid supports. The presented examples indicate that the combination of microwave dielectric heating with solid- or soluble-polymer supported chemistry techniques provides significant enhancements both at the level of reaction rate and ease of purification compared to conventional procedures. 

In recent years, parallel to the emergence of SPOS, microwave-mediated organic synthesis has come to hght and has developed into a popular field [24-31]. The main advantage of microwave dielectric heating compared to other conventional methods, such as hot plate, oil bath or isomantle, is the tremendous rate enhancement generally observed under microwave irradiation conditions. Various theories have been proposed to explain the source of the rapidity of microwave chemistry [32,33]. However, the gener-... 

Investigation of the microwave-assisted attachment of Fmoc-protected amino acids onto 2-chlorotrityl chloride resin indicated higher loadings and increased rates compared to standard room temperature procedures [146]. In this comparative study standard procedures yielded 0.37 mmol/g loading after 1 hour, whereas at 110 °C using microwave dielectric heating, a similar result (0.38 mmol/g) was obtained after only 15 min (Fig. 7). 

Microwave curing of polymeric materials requires the presence of dipolar materials for effective modification to occur through dielectric heating [45]. This is not an essence in the case of EB modification of polymers which requires the presence of only labile reactive site, e.g., hydrogen in the polymeric stmcture. 

Microwave (dielectric) heating in solution may distinguish three situations, which determine heating characteristics ... 

In addition to the above mentioned thermal/kinetic effects, microwave effects that are caused by the unique nature of the microwave dielectric heating mechanisms (see Section 2.2) must also be considered. These effects should be termed specific... 

Fig. 2.10 Selective dielectric heating of water/chloroform mixtures.

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