Abstract
When niobium is heated in static air, layers of oxides and nitrides form on the surface, depending on the temperature. At 1150–1500°C, a layer of solid Nb2O5 forms with linear growth kinetics. At 1500–1900°C, a layer of Nb2N forms under a two-phase layer of liquid oxide+solid NbO2. The two-phase oxide layer has linear growth kinetics, while the Nb2N layer has parabolic kinetics. Above 1900°C, successive layers of Nb2N, NbN, and liquid oxide form, with both nitrides exhibiting parabolic growth kinetics. The activation energies of layer growth are as follows: Q=105 kJ/mol for Nb2O5, 120 kJ/mol for the two-phase oxide layer, and 220 kJ/mol for Nb2N (Q for liquid oxide and NbN layer formation was not determined). Above 1500°C, the nitride layers are somewhat protective, in that they slow the oxide formation rate and prevent ignition.
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Hennessey, T.P., Morral, J.E. Oxidation and nitridation of niobium in air above 1150°C. Oxid Met 38, 163–187 (1992). https://doi.org/10.1007/BF00665050
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DOI: https://doi.org/10.1007/BF00665050