Defects refractory metal compounds

As with corresponding stoichiometric carbides (Chapter 2), cubic nonstoichiometric phases MC, (x < 1) were historically the first, and remain the most studied defect refractory compounds. The main studies carried out in this direction are listed at the end of this chapter. Most calculations have been carried out for the iv and v group d-metal carbides, and we shall consider these to discuss both the approaches used and the main ideas of the electron state rearrangements caused by lattice defects. 

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. 

Aluminum oxides and related compounds have long been technologically important as abrasives (corundum) and in refractories and ceramics in the a-crystalline modification. In the y modification, a more open, defect structure, aluminum oxide becomes activated alumina and is useful in chromatography and in catalysis. A third modification occurs on the surface of the metal on exposure to air and serves as the well-known protective oxide. A more recent technological achievement is the production of remarkably uniform cylindrical fibers of AI2O3. These fibers can be incorporated in a variety of fabrics, papers, ropes, and so on, which gain the advantage of stability to very... 

Hard-coating materials range from ultra-hard materials such as diamond-like carbon through the refractory compounds to alloys. However, the transition-metal carbides and nitrides have achieved by far the highest level of commercial success. Perhaps, the most important property of this group of carbides and nitrides is their defect structure. Ideal stoichiometry is generally not found in these phases deviations from the stoichiometry are found to be far more common. The transition-metal carbides and nitrides are typically metallic in their electrical, magnetic and optical properties. 

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