Nitridant, effect formation

The contact resistance between cBN and electrodes of conventional materials is usually lO -lO n atroom temperature (184). Although the contact resistance decreases at high temperatures, it is still quite large. In order to form an ohmic contact and reduce the contact resistance, a few materials have been examined. Trials of materials such as Cu (208), Ag (4,184), Au (210), A1 (210), Cr-Ni (177), and Mo and Pt (200) have been reported in the literature and patents. Ohmic electrodes with relatively low contact resistance have been made using Ti-Au and Al-Au on Be-doped p-type crystals (210). The contact resistance of Ti-Au was 10 -10 Q at room temperature. Annealing procedures are considered to be effective. Formation of Ti or A1 nitride probably occurred. 

Another interesting lithium-based system is Li3N/Li2NH [53]. Lithium nitride can be hydrogenated to lithium imide and lithium hydride (5.4 wt% H2). The latter reaction can be used for reversible storage at 250°C. The formation of ammonia can be completely avoided by the addition of 1% TiCl3 to the system, which has the positive additional effect to improve the kinetics [54]. Very fast kinetics has been reported for a partially oxidized lithium nitride [55]. 

The preliminary plastic deformation considerably effects on the phase formation, structure, microhardness and thickness of nitrided layers in -Fe and Fe-Ni alloys. The high microhardness of the diffusion layers results from the formation of the s- and y- nitrides. Iron doping with Ni leads to changing of the s-, f-phases composition. The existence of narrow intervals of deformations of 3-8 % and 20-30 %, in which the considerable (about 2 times) rise of microhardness of surface nitrided layers due to accelerated formation of s- and f-phases, was found. 

The main emphasis was laid, in this initial work, on Haber s catalysts, e.g., osmium and uranium compounds, as well as on a series of iron catalysts. Some other metals and their compounds which we tested are, as we know today, less accessibble to an activation by added substances than iron. Therefore, they showed no improvement or only small positive effects if used in the form of multicomponent catalysts. Finally, the substances which we added to the metal catalysts in this early stage of our work were mostly of the same type as those which had proved to favor the nitride formation, e.g., the flux promoting chlorides, sulfates, and fluorides of the alkali and alkaline earth metals. Again, we know today that just these compounds do not promote, but rather impair the activity of ammonia catalysts. 

Munir and Holt examined theoretically the effect of nitrogen pressure and sample porosity in the simultaneous formation of nitrides and solid solutions. Using thermodynamic arguments, they derived minimum values of nitrogen pressures required for the nitridation of various transition metals however, several experimental studies demonstrated that nitridation could take place at much lower pressures than those predicted by these authors. 

There are a limited number of techniques used for the synthesis of ternary and quaternary nitrides. In fact, the vast majority of these nitrides have been synthesized using only two methods (1) the reaction of a metal nitride with a metal or another metal nitride and (2) the reaction of two metal powders with nitrogen gas or ammonia. The first method has been the most common approach (Table 8.1, Reaction 1). Since these reactions usually require high temperatures (1073-2073 K), the inductive effect is Used to effect product formation. Li3N is a favored starting material due to its stability and relatively low melting temperature (mp = 1086 K), which... 

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