Crystallographic Study of Product Phases of Carbothermic Reduction and Nitridation of Hafnium Dioxide.

Details of the carbothermic reduction/nitridation to synthesize hafnium nitride (HfN) and hafnium carbide (HfC) are scarce in the literature. Therefore, this current study was carried out to evaluate two pathways for synthesizing these two refractory materials: direct nitridation and carbothermic reduction/nitridation. Two mixtures of hafnium dioxide and carbon with C/HfO 2 molar ratios of 2.15 and 3.1 were nitridized directly using flowing nitrogen gas at elevated temperatures (1300-1700 °C). The 3.1 C/HfO 2 molar ratio mixture was also carbothermically reduced under flowing argon gas to synthesize HfC, which was converted into HfN by introducing a nitridation step under both N 2(g) and N 2(g) -10% H 2(g) . X-ray diffraction results showed the formation of HfN at 1300 and 1400 °C and HfC 1- y N y at ≥1400 °C under direct nitridation of samples using a C/HfO 2 molar ratio of 2.15. These phase analysis data together with lower lattice strain and greater crystallite sizes of HfC 1- y N y that formed at higher temperatures suggested that the HfC 1- y N y phase is preferred over HfN at those temperatures. Carbothermic reduction of 3.1 C/HfO 2 molar ratio samples under an inert atmosphere produced single-phased HfC with no significant levels of dissolved oxygen. Carbothermic reduction nitridation made two phases of different carbon levels (HfC 1- y N y and HfC 1- y ' N y ' , where y' < y ), while direct nitridation produced a single HfC 1- y N y phase under both N 2 and N 2 -10% H 2 cover gas environments.