Thermal path effect

After a brief overview of the thermal conversion data (thermal dependence of dielectric properties, electric field focusing effects, hydrodynamics) thermodynamic aspects and athermal effects of electric fields will be examined. Next, the effects of thermal path and hot spots induced by microwave heating will be analyzed in term of kinetic effects. 

The phase or optical path length variation — (d x /) is a function of radial position in the ZnSe and CVD diamond windows and is shown in Fig. 20. The variation for CVD diamond over the effective beam width is over a factor of 250 smaller. Three factors combine to yield this large difference the smaller temperature rise in CVD diamond due to the higher thermal conductivity (AT a factor of more than 7 smaller in CVD diamond), the smaller value of dn/df for diamond (a factor of 5.8), and the thickness of the window, which for CVD diamond is only l/6th of that in ZnSe. For an example similar to that shown in Fig. 18 the thermal lensing effect in the ZnSe window was found to be equivalent to a lens of 3-5 m focal length, whereas for the CVD diamond window the effect was negligible [27,34]. 

While the results of the thermal motion effect are somewhat exaggerated in the top two ellipsoids in Figure 2.4, this thermal motion leads to errors, often in the range of 0.005-0.010 A, with the experimental bond length always being found to be too short. When one reduces the temperature, the thermal ellipsoids become smaller, as shown with the lower set of ellipsoids. Even so, the two atoms are still following curved paths, and the bond length obtained will be too short, but the effect will obviously be much reduced at lower temperatures. 

The construction principles of such chip calorimeters are similar to those of conventional calorimeters The heater corresponds to the furnace, and the center of the membrane corresponds to the calorimeter system, including the sample container. The thin membrane serves as the thermal path between the heater and the sample with very low thermal resistance and very low effective heat capacity. The thermopile measures the temperature difference between the sample site and the chip frame (surroundings). Because of the much larger lateral dimension of the membrane of at least two orders of magnitude, the heat exchange between the sample and the frame can be neglected. The chip calorimeter can therefore be considered a quasi-adiabatic calorimeter when vacuum is applied. 

Phonon transport is the main conduction mechanism below 300°C. Compositional effects are significant because the mean free phonon path is limited by the random glass stmcture. Estimates of the mean free phonon path in vitreous siUca, made using elastic wave velocity, heat capacity, and thermal conductivity data, generate a value of 520 pm, which is on the order of the dimensions of the SiO tetrahedron (151). Radiative conduction mechanisms can be significant at higher temperatures. 

Two examples of path-dependent micromechanical effects are models of Swegle and Grady [13] for thermal trapping in shear bands and Follansbee and Kocks [14] for path-dependent evolution of the mechanical threshold stress in copper. 

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