Transition tantalum nitrides

The early transition metal nitrides of titanium, zirconium, hafnium, tantalum, and tungsten as well as titanium and tantalum carbide are effective diffusion... 

This chapter is a review of the properties and general characteristics of the interstitial nitrides formed by the metals of Group IV (titanium, zirconium, and hafiiium) and Group V (vanadiiun, niobium, and tantalum). As mentioned in Ch. 10, these six nitrides are the only refiactory transition-metal nitrides. Th have similar properties and characteristics and, of the six, titanium nitride has the greatest importance from an application standpoint. 

Transition metal carbides, such as tungsten carbide and its alloys, tantalum carbide, titanium carbide, and molybdenum carbide (Cowling et al, 1970,1971 Voorhies et al., 1972 Scholl et ah, 1992,1994 Borup et al., 2007), have been studied as catalysts for electrochemical reactions. However, it has been found that these transition metal carbides are unstable under high potentials and in acid solution, and this limits their application as PEM fuel cell catalysts (Borup et al., 2007). Transition metal nitrides have been studied as electrochemical catalysts in PEM fuel cell environments, and Zhong et al. (2006) showed that molybdenum nitride supported on carbon powder resulted in a cell performance of about 0.3 V at 0.2 A cm, and the catalyst was stable for 60 h of cell operation. However, the long-term performance durability is still questionable. 

These are usually reactions of anhydrous transition and B metal halides with dry alkali metal salts such as the sulphides, nitrides, phosphides, arsenides etc. to give exchange of anions. They tend to be very exothermic with higher valence halides and are frequently initiated by mild warming or grinding. Metathesis is described as a controlled explosion. Mixtures considered in the specific reference above include lithium nitride with tantalum pentachloride, titanium tetrachloride and vanadium tetrachloride, also barium nitride with manganese II iodide, the last reaction photographically illustrated. 

In general, the nitrides of the early transition metals are either highly conducting (e.g., TiN) or superconducting, e.g., NbN. However, the nitrides of zirconium, hafnium, and tantalum, with the metals in their highest oxidation state, are transparent insulators. 

The transition metal carbides and nitrides have often been called interstitial compounds [70] however, this is somewhat misleading. The small boron, carbon, or nitrogen atoms certainly occupy octahedral or trigonal prismatic voids of the metal sublattice, but the arrangement of the metal atoms themselves is different from that of the element. In the monocarbides the transition metal atoms show cubic close packing. However, titanium, zirconium, and hafnium are packed hexagonally and vanadium, niobium, and tantalum are body centered cubic [1]. Thus, these monocarbides are inorganic compounds with their individual crystal structures and they should not be considered as an interstitial compound of a transition metal host lattice. 

The metals of the nine early-transition elements, i.e., titanium, zirconium, and hafiiium of Group IV, vanadium, niobium, and tantalum of Group V, and chromium, molybdenum, and tungsten of Group VI, fit the criteria of size and site availability, and form interstitial nitrides.1 1... 

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