Niobium nitrides

Gallagher and Sinclair (93) studied the oxidation of niobium nitride, NbN0.9, in a flowing atmosphere of 0.1% 02 in Ar or air, using TG and other techniques. There was a pronounced tendency for thermal runaway near 600°C because of the highly exothermic nature of the reaction 

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A process has been developed to recover niobium from ferroniobium (30). The need for this process came about when Brazil would only export niobium in the form of ferroniobium. The process starts with a hydriding step, so as to be able to cmsh the alloy. Screening precedes a nitriding step, followed by an acid leach of the iron nitrides. This leaves the niobium nitride for further processing to the pure niobium metal. 

Niobium nitride (also known as columbium nitride) has three phases Nb2N, Nb4N3, and NbN. The mononitride, NbN, is the most common phase and the only one reviewed here. NbN is an excellent superconductor. It is produced by CVD mostly on an experimental basis. Its characteristics and properties are summarized in Table 10.5. 

Niobium metal absorbs nitrogen, similar to hydrogen, forming interstitial solid solution. The absorption occurs at 300°C and the solubility of nitrogen in the metal is directly proportional to the square root of the partial pressure of nitrogen. The reaction is exothermic and the composition of such interstitial solid solution varies with the temperature. When the metal is heated with nitrogen at temperatures between 700 to 1,100°C, the product is niobium nitride, Nb2N or (NbNo.s) [12033-43-1]. When heated with ammonia at these temperatures, niobium forms this nitride. Another niobium nitride exists, NbN [24621-21-4], with a face-centered cubic crystalline structure. 

Niobium, discovered in 1801 and formerly called colum-bium, was a special favorite. A soft, ductile, gray-blue metal, it is present in several minerals, forms a number of compounds and complexes, and is used to strengthen welded joints and certain steels. In 1941, scientists reported superconductivity in niobium nitride with a Tc around 16° K, fairly high compared to earlier superconductors. Progress slowed after that until 1953, when John Hulm, of the University of Chicago, put together a material... 

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