Transition metal nitrides, properties

Transition metal nitrides and carbides can be described, generally speaking, as insertion compounds of nitrogen or carbon in the metal network.1 In fact, strong metal-nonmetal interactions exist which induce structural modifications.1,2 These compounds form a class of materials with unique physical3,4 and catalytic1,5,6 properties. The term platinoids has been used to illustrate their potential in reactions traditionally catalysed by noble metals. 

This chapter summarizes recent experimental findings on transition metal carbides and nitrides and shows, in section 25.2, that measurable surface shifts do exist both in the metal and nonmetal levels. A few application examples are presented in section 25.3. These experimentally determined surface shifts may stimulate new theoretical efforts to explain their existence and lead to a better understanding of the surface electronic properties. To date, no ab initio calculations of surface shifts for transition metal nitrides and carbides, including final state relaxation effects, have... 

Transition-metal nitrides have many properties that make them suitable for industrial applications, including high wear resistance, high decomposition temperature, and high microhardness. 

The parallelism in properties between fee transition metal carbides and nitrides is also valid for room-temperature electrical resistivity, which decreases with increasing carbon content and increases with increasing temperature. Generally the transition metal carbides have higher electrical resistivities than the transition metal nitrides. Figure 7 gives the electrical resistivities of Groups 4 and 5 transition metal carbides as a function of carbon content, as compiled by Storms, compare also Ref 32. For the electrical resistivity of 5-HfC, values... 

Table 1 gives a briefsurveyofthe most important properties of the well-characterized binary transition metal nitrides. It should, however, be emphasized that practically all properties... 

Interestingly, the transition metal carbides and nitrides do not show the expected decrease in heat conductivities with temperature but instead an increase. Figure 10 illustrates this property for the group 4 nitrides and shows a comparison with group 4 carbides and carbonitrides. The substitution of nitrogen by carbon in group 4 transition metal nitrides significantly reduces the heat conductivities. 

Table 1 Bulk properties of important transition metal nitrides... 

The characteristics and properties of some combustion-synthesized transition metal nitrides are presented in Table III. 

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. 

The thermodynamic properties of hexagonal AIN and cubic transition metal nitrides and carbides are known, but for the calculation of the ranges of metastability of the cubic and hexagonal phases in the AlN-MeN or AlN-MeC systems, the energy differences for the transitions from the stable to the metastable structures, e.g. MeNj i, —> MeN and AlNi, —> AlN b primary importance. In addition, the... 

Because of the importance of the cubic to hexagonal transition in determining the properties of coatings for particular applications, a series of calculations have been carried out for a number of AlN-MeN systems (MeN is a transition metal nitride). 

These materials are generally black and hard since thin films are easy to lay down [273], some are used as tool coatings [ 274J. These desirable mechanical properties have made transition metal nitrides the subject of several reviews [275. 276. 277J and books [278. 279],... 

Transition element carbides and nitrides are applied as cutting tools because of their extreme hardness and wear resistance. In some cases nitrides and carbides (e.g., of titanium) form solid solutions over the entire compositional range other transition metal nitrides and carbides exhibit fairly different structures and are not completely soluble. Carbon contents within the range of few percentage points usually do not influence the mechanical properties of transition metal nitrides, and vice versa. Hence, completely carbon-free nitrides or nitrogen-free carbides are not required, especially for the titanium compounds. 

TABLE 5.2 Physical and Chemical Properties of Common Transition Metal Nitrides and Carbides... 

Despite the fact that this section deals with transition-metal nitrides, we should also mention that rare-earth nitrides emerge as superior luminescent materials because of their very special properties see also Section 3.3. 

A mixture of metallic, covalent and ionic components prevails in the bonding of transition metal carbides, nitrides, and carbonitrides. The metallic character is shown by the high electrical conductivities of these compounds. The bonding mechanism has been described extensively by a variety of approaches for calculating the density of states (DOS) and hence the electron density in f.c.c. transition metal carbides, nitrides, and oxides [11]. In the DOS of these compounds there is a minimum at a valence electron concentration (VEC) of 8, which corresponds to the stoichiometric composition of the group ivb carbides TiC, ZrC, and HfC. Transition metal carbides have a lower DOS at the Fermi level than the corresponding transition metal nitrides, hence the electrical properties such as electrical and thermal conductivity and the superconducting transition temperature, T, are lower than those of the nitrides. 

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