Material transition metal borides

Most borides are chemically inert in bulk form, which has led to industrial applications as engineering materials, principally at high temperature. The transition metal borides display a considerable resistance to oxidation in air. A few examples of applications are given here. Titanium and zirconium diborides, alone or in admixture with chromium diboride, can endure temperatures of 1500 to 1700 K without extensive attack. In this case, a surface layer of the parent oxides is formed at a relatively low temperature, which prevents further oxidation up to temperatures where the volatility of boron oxide becomes appreciable. In other cases the oxidation is retarded by the formation of some other type of protective layer, for instance, a chromium borate. This behavior is favorable and in contrast to that of the refractory carbides and nitrides, which form gaseous products (carbon oxides and nitrogen) in air at high temperatures. Boron carbide is less resistant to oxidation than the metallic borides. 

The transition metal borides also show characteristics of covalent and metallic materials. The bonding in the borides is also complicated by the fact that there are interactions between the B atoms to form chains, layers, or three-dimensional networks. In the carbides and nitrides there are no C-C or N-N interactions. Despite these complexities we can still use some of the same approaches that we use for simple oxides (Chapter 6) to predict the crystal... 

Transition metal borides are mainly explored for their mechanical properties. Since they exhibit metallic transport properties such as high electric and thermal conductivity with a negative temperature coefficient they are also of interest as electrode materials, for heating elements and sensors. 

The transition metals form carbides, nitrides, and borides with nonstoi-chiometric ratios. Some of these materials are interstitial compounds, which means there are large gaps in between the metal atoms that can be filled by other small elements such as hydrogen, nitrogen, and carbon. When interstitial compounds form, the additional atoms are locked into place in the metal framework, making these materials stronger than the metal by itself. This is important in a number of industrial applications such impregnating a small amount of carbon into an iron matrix in order to make steel. 

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