Solids autowave processes

VI. Theoretical Treatment of Autowave Processes in Solid-State Cryochemical Conversions (The Simplest Model)... 

VI. THEORETICAL TREATMENT OF AUTOWAVE PROCESSES IN SOLID-STATE CRYOCHEMICAL CONVERSIONS (THE SIMPLEST MODEL)... 

The realization of the slower wave involved certain difficulties. Not in every experiment could it be initiated by the slow local heating. Frequently the faster wave was initiated as well. There were cases when the well-developed slower wave transformed into the faster one in the course of its propagatioon, so that the first thermocouple in its path registered a front profile similar to that of the solid curve in Figure 11, and the second, a profile similar to the dashed curve. The higher sensitivity of the slower propagation mode to various disturbances has already been noted in the theoretical treatment of the autowave process. 

Comparison of the experiments carried out in glass and metallic cuvettes did not show any appreciable effect of the substrate material on the characteristics of the autowave processes in solid-state conversions. The critical doses required for realization of the self-sustained regimes of conversion in films turned out to be noticeably greater than in massive samples. 

Therefore, neither the appreciable plastic deformation (both in the case of uniform compression and of local fracture) of the solid reaction systems studied nor their static state of high stress is a factor conditioning the critical phenomena and autowave processes observed during the chemical conversion in the systems. In other words, this series of experiments has provided another telling argument for the decisive role of brittle fracture in the mechanism of the phenomena considered. 

I. M. Barkalov, V. V. Barelko, V. I. Goldanskii, D. P. Kiryukhin. and A. M. Zanin, Threshold Phenomena and Autowave Processes in Low-Temperature Solid-State Chemical Reactions , preprint, Inst. Chem. Physics AN SSSR, Chernogolovka (1983) (in Russian). 

The data on the autowave phenomena considered in this chapter may turn out to be significant for the general chemistry of solids outside the low-temperature region. Probably, it is autodispersion of the solid matrix of reactants (the process that brings a chemical reaction from a volume to a highly active surface of fracture) that accounts for the extremely high velocities of conversion not infrequently observed experimentally in systems of diverse chemical nature. They may be comparable with or even be far above the... 

The extensively studied (especially during the recent years) transitions of solids from the metastable amorphous state to the polycrystalline state (see ref. 58 and the references therein) are of autowave character and resemble very much the above regimes of solid-state cryochemical reactions. The action of autodispersion, which facilitates phase transition by allowing it to proceed on the surface of a fracture instead of in the glass volume, cannot be excluded in the case of those processes either. Actually, the two classes of processes are similar in their physical nature both are connected with rearrangement of the solid matrix and are of exothermic character, differing only in the extent of the thermal effect. It should be added that fracturing and autodispersion of the sample are very typical of the autowave destruction of amorphous states and can be seen even by the unaided eye. 

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