Nickel dendrites, interface

A variety of surface reactions have been observed with other metals on a-Si H (Nemanich 1984). For example, a similar silicide is formed with platinum and nickel at 200 °C and with chromium at 400 °C. Aluminum and gold form intermixed phases at low temperature, but do not form silicides. Instead both metals promote low temperature crystallization of the a-Si H film. Dendritic crystallization occurs at 200 °C at a gold contact, giving a very non-uniform interface, and aluminum causes crystallization at 250 C. The resulting Schottky contact for gold is surprisingly ideal, but is very poor for aluminum. 

In catalysis of cyclohexane on platinized spirals of nichrome heated by electric current, the author and Kotelkov observed 327) considerable coke formation. A puzzling feature of this process is that with the accumulation of coke the reaction rate not only does not fall but at first even increases. This was explained by the author with the help of the dendrite hypothesis of coke formation coke grows in the form of dendrites the foundations of which lie on the interface between chromium (or chromia) and nickel (see above). Dendrites do not prevent the molecules of cyclohexane from getting to the surface of nickel, just like trees that do not prevent the rain from getting into the ground in the forest. The initial acceleration of the reaction with the accumulation of coke was explained by the corrosion of metal because of the formation of carbides as the first stage of coke formation and of the decomposition of these carbides. 

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