Transition metal nitrides phases

For quenching experiments, which were necessary to freeze-in the equilibrium states between the phase bands, the samples were reheated in a vacuum furnace under 500 mbar N2 to the original annealing temperature and dropped onto a copper plate kept below 470 K (200°C). With this treatment the Group 5 transition metal nitrides underwent a slight change in surface composition because it was impossible to adjust the appropriate nitrogen pressures. This, however, did not influence the compositions at the interface boundaries inside the diffusion couples. 

The synthesis of ternary transition metal nitrides through the reaction of two binary nitrides (Table 8.1, Reaction 1) or by nitriding two metals (Table 8.1, Reaction 2) has been relatively unsuccessful. Consequently, until recently, there was little or no information concerning the synthesis and structures of these phases. Lately, several transition metal ternary nitrides have been made through other synthetic routes, such as metathesis reactions and ammonolysis. For example, the ternary nitride CuTaN217... 

Laser pyrolysis has been shown to produce a wide variety of crystalline transition metal nitride and carbide nanoparticles with diameters as small as 2 nm. The nanopowders are in many cases monophasic and single crystalline. By varying the reaction conditions it is possible to control the particle diameter, and in some cases, the crystalline phase produced. Improvements in the synthesis are needed to control surface oxidation. 

The stmeture of transition metal carbides are closely related to those of the transition metal nitrides. However, transition metal carbides feature generally simpler stmeture elements as compared to the nitrides. In carbides, the metal atoms are arranged in such a way that they form close-packed arrangements of metal layers with a hexagonal (h) or cubic (c) stacking sequence or with a mixtme of these (see Nitrides Transition Metal Solid-state Chemistry). The carbon atoms in these phases occupy the octahedral interstitial sites. A crystallochemical rule claims that the phases of pure h type can have a maximum carbon content of [C]/[T] = 1/2 and the c type phases a maximum carbon content of [C]/[T] = 1 hence in stractures with layer sequences comprising h and c stractme elements the maximum nonmetal content follows suit. 

Carbon does not stabilize the hep low-temperature modifications of these metals (in contrast to nitrogen) and the solnbility of C in the a-phases is much smaller as compared to the group 4 transition metal nitride systems. 

Because of the evident structmal similarities between transition metal carbides and transition metal nitrides the carbon atoms in group 4 and 5 carbides can be replaced completely by nitrogen without changing the structme of the binary phases. So far only one distinct ternary phase Cr2 (C,N)2 has been reported. Intersolubihty between the binary nitrides and carbides in the group 6 carbonitride systems Cr-C-N and Mo-C-N is not complete because of the differences in the crystal structmes of the carbide and nitride phases. 

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,... 

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... 

The most important transition metal nitride, TiN, has complete miscibility with ZrN, HfN, VN, and NbN at high temperatures but miscibility gaps occur upon lowering the temperature [36]. The two-phase mixtures which form upon demixing have increased hardness and are thus interesting for commercial applications. 

Electro-catalyst supports play a vital role in ascertaining the performance, durability, and cost of PEMFC and DMFC systems. A myriad of nano-structured materials including carbon nanostructures, metal oxides, conducting polymers, transition metals nitrides and carbides, and many hybrid conjugates, have been exhaustively researched to improve the existing support and also to develop novel PEMFC/DMFC catalyst support. One of the main challenges in the immediate future is to develop new catalyst supports that improve the durability of the catalyst layer and, in a best-case scenario, also impact the electronic properties of the active phase to leapfrog to improve catalyst kinetics. 

---