Niobium Nitride NbN

Niobium nitride (NbN) from the reaction of NbC with nitrogen or ammonia in excess hydrogen at 1000-1100 Ct lH l... 

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. 

The metaniobates of the alkali metals, MNb03, the orthoniobates MjNbOj and the pyroniobates, M+NbeOy, where M is an alkali metal, can be prepared by various alkali carbonate or hydroxide fusion processes. Niobium fonns a nitride, NbN, and a carbide, NbC. 

Nagai and co-workers prepared alumina-supported niobium nitride from the eVD of NbCls, using a mixture of NH3 and H2 as a nitridation gas at 823 K under reduced pressure (65). NbN thin film of submicrometer thickness on carbon plate was prepared via the RF-sputtering method (RF power of 2.5 W/cm ) with a Nb plate and N2 atmosphere under vacuum chamber (66). A NbN thin film retained the fine particles of 100 nm. 

Li3(BN2) have already demonstrated the decomposition of (BN2) ions into boron nitride. The remaining nitride can lead to the formation of a binary metal nitride or reduce the transition metal ion under the formation of N2. Both mechanisms have been obtained experimentally, depending on the stability of the metal nitride. For instance niobium pentachloride forms NbN, titanium trichloride forms TiN, and nickel dichloride forms Ni, plus BN and nitrogen, respectively, in reactions with Li3(BN)2 (at 300-600°C) [24]. 

Thermodynamic considerations are useful for determining conditions for carburization and nitridation. For example, in the nitridation of niobium the particular phase of the product (e.g. NbN, / -Nb2N, or y-Nb4N3) will depend on temperature and pressure. This holds true also for carbides. However, for carbides, competing reactions such as... 

The plethora of structures known in the niobium-nitrogen system appears to result from varying amounts of oxygen contamination [300, 301]. The phase diagram has been established [302, 303. 304, 305]. NbN, [S-Nb2N and y-Nb4N, are known from neutron diffraction experiments [306. 307], but the fully nitrided Nb,N5 has not been observed [ 308],... 

Similar reactions have been reported for vanadium, niobium and tantalum nitride at temperatures higher than 900°C [85-87]. The superconducting/cc NbN was also prepared as a thin film by reacting NbCls with NH3 2 at 900°C [88, 89], The main problem is the deposition of single phase films, e.g., TaCls and ammonia produce the tetragonal phase Ta3Ns at temperatures of 600-1 000 °C [85]. 

Vanadium (V) (Aldrich Chemicals, -325 mesh, 99.5% purity), niobium (Nb) (Strem Chemicals, -325 mesh, 99.8% purity), and tantalum (Ta) (Aldrich Chemical, -325 mesh, 99.9% purity) powders were used as the reactants in this study. Metal powders were pressed into cylindrical specimens with a diameter of 7 mm and a height of 12.5 mm. In order to prepare test samples with different porosities, the powder compacts were formed with a packing density between 45% and 60% relative to the theoretical maximum density (TMD). For the study of the effect of nitride dilution on the SHS reaction, VN and NbN were employed as the diluents to add into the V and Nb powder compacts, respectively. The diluent content ranged from 5% to 30% by weight of the total powder mixture. No TaN dilution was applied for the Ta powder compact, because of no anticipation of melting upon considerably high melting points of Ta (2996 °C) and TaN (3090 C). 

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