Short range order , carbides

Bil] Billingham, J., Bell, P.S., Lewis, M.H., Vacancy Short-Range Order in Substoichiometric Transition Metal Carbides and Nitrides with NaCl Structure. 1. Electron Diffraction Studies of Short-Range Ordered Compounds , Acta Crystallogr, A, 28, 602-606 (1972) (Crys. Structure, Experimental, 21)... 

The material has a lot of stmctural polytypes. The silicon carbide atoms are in state of sp3-hybridization and form a bond of a tetrahedron. In the crystal lattice of silicon carbide the short-range order is always the same but the long-range can differ that is why there are many polytypes of this material. The stmetural difference causes difference in physical and chemical properties (e.g., thermal resistance, electrical and optical characteristics). It makes one or another polytype being more preferred for different application. 

The papers presented in the conference span the spectrum of activity in the science of alloys. The theoretical presentations ranged in content from fundamental studies of electronic structure, to first-principles calculations of phase diagrams, to the effects of charge transfer, to the temperature dependence of short-range order parameters. They encompassed the study of mechanical properties, the properties of dislocations, of phase evolution, and computer simulations. Experimental studies were presented based on a variety of state of the art experimental techniques, from TEM to synchrotron diffraction. The phenomena studied varied from the precipitation of nitrides in steel, to the wetting of interfaces between two different crystal structures, to the ordering of vacancies in carbides. And the materials whose properties were measured ranged from Transition metals, to the Lanthanides, to the Actinide series of compounds and alloys. 

Hofer et al. [29] used a magnetic method to measure the isothermal kinetics of the decompositions of cobalt and nickel carbides. The reaction of CojC was zero-order (0.20 < a< 0.75) with Z , = 226 kJ mol in the range 573 to 623 K, and became deceleratory thereafter. The behaviour of NijC differed in that there was a short induction period, but there was again a period of constant rate (0.3 < ar < 0.9) with , = 295 kJ mol between 593 and 628 K and the final period was deceleratory. There was no evidence for the intervention of a lower carbide. The mechanisms of these reactions were not discussed. 

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