Defects carbides

Estimates of the metallic vacancy formation energy ( ) in the systems under discussion show that > EP-. This gives a qualitative explanation for the known difficulties in the synthesis of metal-defect carbides and nitrides and confirms the conclusions on this question drawn from cluster calculations. 

E Storms, B Calkin, A Yencha. The vaporization behavior of the defect carbides. Part 1 The Nb-C system. High Temp Sci 1 430, 1969. 

Carbides Cemented Carbide Producers Association 712 Lakewood Center North Cleveland, Ohio 44107 Standards Developed bj Cemented Carbide Producers Association ie, standard shapes, sizes, grades, and designations and defect classification. 

Refractory Compounds. Refractory compounds resemble oxides, carbides, nitrides, borides, and sulfides in that they have a very high melting point. In some cases, they form extensive defect stmctures, ie, they exist over a wide stoichiometric range. For example, in TiC, the C Ti ratio can vary from 0.5 to I.O, which demonstrates a wide range of vacant carbon lattice sites. 

Once initiated, zirconium and carbon powders react exothermically in a vacuum or inert atmosphere to form zirconium carbide. With the greater availabiHty of relatively pure metal powders, this technique is coming into common use for the production of several refractory carbides. Zirconium carbide is not a fixed stoichiometric compound, but a defect compound with a single-phase composition ranging from ZrCQ to ZrCQ at 2400°C. 

The chains of hollow carbon may be initially chains consisting of Ni (or carbide) particles covered with graphitic carbon. The chains lying on the hot surface of the cathode are heated, and Ni atoms evaporate through defects of the outer graphitic carbon because the vapor pressure of Ni is much higher than carbon. Thus, the carbon left forms hollow graphitic layers. 

The hardness of boron carbide (carbon hexaboride) is not well defined because it is made as sintered compacts which have variable densities, compositions, and defect densities. It is very hard (up to 4400kg/mm2), and of relatively low density, so it has been used extensively as body-armor (McColm,... 

Jefferson s studies of the pyroxenoids has added greatly to our application of the way in which, through the intermediary of planar - or planar and Kinke - faults one structure is converted into another (45). And Audier, Jones and Bowen (46) have revealed how unit cell strips of Fe C may be accommodated as extended defects in the Fe C structure. Both these carbidic phases can be readily identified by HREM at the interface of iron catalysts used for the disproportionation of CO (to yield C j+CC ). 

The treatment assumes that the point defects do not interact with each other. This is not a very good assumption because point defect interactions are important, and it is possible to take such interactions into account in more general formulas. For example, high-purity silicon carbide, SiC, appears to have important populations of carbon and silicon vacancies, and Vsj, which are equivalent to Schottky defects, together with a large population of divacancy pairs. 

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