Low-temperature Synthesis of Chromium Nitrides by Metathesis Reactions between Chromium Oxides and Boron Nitride

Cr2N and CrN were synthesized using chromium oxides and boron nitride (BN) as starting materials. Cr2N powder with particle sizes of 0.1–5 μm was obtained together with Na3BO3 (melting point (m. p.): 675°C) by heating a mixture of Cr2O3, BN, and Na in an Ar atmosphere at 500–1100°C. CrN powder with particle size of 1 μm and NaBO2 (m. p.: 968°C) were obtained as the main phases by heating NaCrO2 and BN in an Ar atmosphere at 900–1000°C, while Cr2N crystal grains with sizes of 10–100 μm and NaBO2 were obtained at 1100°C. The formation reactions of the chromium nitrides are metathesis reactions in which the O in chromium oxides and N in BN are exchanged, resulting in the formation of sodium borates (Na3BO3 and NaBO2). The melts of these sodium borates were found to accelerate the formation reactions of chromium nitrides, and the NaBO2 melt, in particular, contributes significantly to the grain growth of Cr2N.