Electrical properties of hard materials

Some materials are as hard as ceramics but contain conduction electrons i.e., "metallic ceramics". The ones described are monocarbides of Group IV and V transition metals: TiC, ZrC, HfC, and VC, NbC, TaC, and one of Group VI: WC. The IV and V materials can be useful because of certain properties - high melting points, high hardness, low diffusion rates of metal and carbon, little change in resistivity at high temperatures, low electromigration, corrosion resistance; the applications can be to microelectronic devices and to high-temperature switches and resistors. One reason for the saturation of this electrical resistivity at high temperatures is a large residual resistivity due to scattering of conduction electrons by random carbon vacancies (point defects) in these non-stoichiometric compounds, e.g., TiCx, where x represents the carbon-to-metal ratio, and 1-x represents the carbon vacancy concentration, which can range from 3% to 50%. A second reason is a parallel resistor effect due to interband scattering of these electrons in the complex electron band structure. But specific compositions of these non-stoichiometric compounds can shift to periodic structures and eliminate the residual resistivity, thus increasing the temperature dependence at low temperatures but not at high temperatures because of interband scattering. WC, which is stoichiometric, has a near zero residual resistivity and a strong temperature dependence. (C) 1999 Elsevier Science Ltd. All rights reserved.