Thermal stability nitrides

Cubic Phase of Boron Nitride c-BN. The cubic phase of boron nitride (c-BN) is one of the hardest materials, second only to diamond and with similar crystal structure. It is the first example of a new material theoretically predicted and then synthesized in laboratory. From automated synthesis a microcrystalline phase of cubic boron nitride is recovered at ambient conditions in a metastable state, providing the basic material for a wide range of cutting and grinding applications. Synthetic polycrystalline diamonds and nitrides are principally used as abrasives but in spite of the greater hardness of diamond, its employment as a superabrasive is limited by a relatively low chemical and thermal stability. Cubic boron nitride, on the contrary, has only half the hardness of diamond but an extremely high thermal stability and inertness. 

Cubic BC2N. Hetero-diamond B C—N compounds have recently received a great interest because of their possible applications as mechanical and optical devices. The similar properties and structures of carbon and boron nitrides (graphite and hexagonal BN, diamond, and cubic BN) suggested the possible synthesis of dense compounds with all the three elements. Such new materials are expected to combine the best properties of diamond (hardness) and of c-BN (thermal stability and chemical inertness). Several low-density hexagonal phases of B,C, and N have been synthesized [534] while with respect to the high-density phases, different authors report contradictory data [535-538], but the final products are probably solid mixtures of c-BN and dispersed diamonds [539]. 

The carbothermal nitridation of oxides is a common method of preparation of nitrides. It is generally applicable to nitrides of high thermal stability like TiN, VN and CrN. Its main drawback is that it produces impure phases. 

The imide is a clear yellow sohd which is mnch more resistant to hydrolysis than the amide and which possesses greater thermal stability. At 250 °C it evolves ammonia and converts to the binary nitride (equation 33), which is a red-orange solid that decomposes violently on heating. 

To clarify the origin of thermal stability of transition metal nitrides, an empirical approach and the DV-Xa molecular orbital calculation for several transition metal nitride have been executed. Thermally stable crystal phases in MOj jM N (M =Nb, Zr,... 

Ti) solid solution and simple transition metal nitrides are classified using the radius ratio of nonmetal to metal atoms and the number of valence electrons. The relationship of the generalized number of valence electrons instead of the average number of valence electrons per atom to the thermal stability of transition metal nitride has been discussed. 

It has been found that the bond overlap population of metal-metal bond plays an important role on the thermal stability of transition metal nitride. 

In this paper the thermal stability of several metal nitrides is discussed firstly with the number of valence electrons based on the information acquired experimentally. In the second part, the thermal stability is investigated by the electron theory using results of the discrete variational (DV)-Xct molecular orbital calculations for some transition metal nitrides. 

Experimental and empirical approach for thermal stability of transition metal nitride... 

Overlap population and thermal stability of transition metal nitrides 3.2.1 Ti-M-N system. 

It has been found for examining empirically the thermal properties of the metal nitrides that the number of valence electrons is more advantageous than the average number of valence electrons per atom. DV-Xa molecular orbital calculations for several metal nitrides reveal that the thermal stability of transition metal nitride is intensely dominated by the bond overlap population of the metal-metal bond. 

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