Nitrides formation

There is a reaction between beryllium and nitrogen that starts at about 750°C and is appreciable at 850°C, beryllium nitride being formed". The reaction with oxygen is less sluggish and at 900°C in oxygen oxidation proceeds at about twice the rate of nitride formation. Thus when beryllium is heated in air, beryllium nitride forms only a small proportion of the total scale —about 0-75% after 1 h at 1 000°C. 

Nickel, K., Riedel, R., and Petzow, G., Thermodynamic and Experimental Study of High-Purity Aluminum Nitride Formation from Aluminum Chloride by Chemical Vapor Deposition, /. Amer. Ceram. Soc., 72(10) 1804-1810 (1989)... 

In some cases it is necessary to add a reactive gas to either assist the loss of ligands and/or reach better decomposition conditions, or to introduce a heteroelement into the deposit such as for nitride formation for instance. 

In February 1909, the results of the experiments on nitride formation had led to the outline of a patent application which covered the preparation of metal nitrides in the presence of auxiliary substances. Following a hypothetical concept of the action of these additions, they were defined as flux promoters." This draft of an application ended with the following sentence Finally, it is also advantageous to add a flux promoter to metals or alloys which serve as catalysts for the ammonia synthesis. This statement was made in view of the early catalytic experiments in which we had observed the synthesis of traces of ammonia in the presence of catalysts similar to those which acted favorably for the nitride formations. 

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. 

In finely divided form, hafnium is pyrophoric, igniting in air spontaneously. However, bulk metal reacts slowly in oxygen or air above 400°C. The rate of oxidation increases with temperature. The product is hafnium dioxide, Hf02. It combines with nitrogen, carbon, boron, sulfur and silicon at very high temperatures to form hafnium nitride HfN, hafnium boride HfB, hafnium sulfide HfSi2, respectively. Nitride formation occurs at 900°C. 

Reactive sputter deposition is a useful technique not only for coating but also for obtaining metastable phases, especially nitrides. Nitrogen gas is activated in a plasma to enhance nitride formation and heating at high temperature is not required. It is possible to obtain metastable nitrides by sputter deposition in which species in the plasma are quenched on substrates. Compounds with different crystal structures normally do not form solid solutions, but by co-sputter deposition metastable solid solutions may form. 

Nitrides. Nitride formation on the surface of molybdenum has been revealed by high-temperature etching with ammonia, and the nitrides were characterized by X-ray diffraction spectrometry.378 The diffusion layer formed on molybdenum consists of Mo2N at >950°C and MoN at <950°C. The interaction of ammonia with a tungsten surface can generate dense adlayers such as W2N3H.379... 

The very active metals are capable of combining directly with nitrogen to form nitrides. In the air the oxide is formed so much more readily, that nitride formation is likely to escape notice but if the metal is presented in powdered form in a thick mass the oxygen is all combined in the surface layer and only nitrogen penetrates to the interior where pure nitride is formed. 

Thermodynamic calculations (Table 2) indicate tl nitride formation from the elements is favorable, a indeed, many nitrides can be so prepared. For examp TiN, VN, and CrN can be made at 1 bar with me... 

The Van Arkel process can also be used to make metal nitrides. Because of the tendency of NH3 to decompose at the temperatures needed for nitride formation (1300-1900 K), the gaseous reactant is a mixture of H2 and N2. The nitride forms as a layer around the tungsten wire. 

Heated in vacuo or in nitrogen it begins to decompose at 500° C., and at 600° C. nitrogen is rapidly evolved. This reduction takes place even under a pressure of 18 atmospheres of nitrogen. For this reason there is no nitride formation when iron is heated at these temperatures in nitrogen.9... 

McCarty K. F. et al., On the low-temperature threshold for cubic boron nitride formation in energetic film deposition. Diamond Relat. Mater., 5 (1996) pp. 1519-1526. 

The only reactions of molecular nitrogen at ambient temperature are the formation of lithium nitride Li3N, reactions with certain transition metal complexes, and nitrogen fixation with nitrogenase in the bacteria of the root nodules of legumes and in blue algae (Sections 14.1.1 and 14.1.2). Above 500 °C nitrogen reacts with some elements, especially with metals (nitride formation). 

Mn, Co, Ni, Tc, Rh and their alloys (no nitride formation under synthesis conditions)... 

Skibska, M., Szulc, A., Mukasyan, A. S., and Rogachev, A. S., Microstructural peculiarities of silicon nitride formation under high nitrogen pressures. Part I Hje influence of initial Si particle size distribution on SiaN4 SHS morphology. Int. J. SHS, 2,39 (1993a). 

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