Hydrothermal oxidation of niobium metal

Abstract

Reactions of niobium metal powder with high-temperature high-pressure water were studied in an open system under conditions of 300° - 800°C, 10 - 100MPa and 3-72h. Niobium pentaoxide (Nb2O5), niobium hydride (NbHx), and NbO2 were formed as products. Niobium changed into orthorhombic Nb2O5 and niobium hydride at 400°C under 100MPa for 3h. Only orthorhombic, or/and monoclinic Nb2O5 were obtained above 700°C under 100MPa for 3h. The formation of oxides and hydrides caused cracking on the surface above 400°C (100MPa, 3h) resulting in disintegration of grains above 500°C (100MPa, 3h). Partial crystal growth of Nb2O5 was observed above 700°C (100MPa, 3h). Temperature and reaction duration affected the hydrothermal oxidation of Nb metal, but pressure did not.