Experimental studies on self-propagating combustion synthesis of niobium nitride

An experimental investigation of self-propagating high-temperature synthesis (SHS) of niobium nitride (NbN) was conducted with niobium compacts in gaseous nitrogen. A detailed characterization of the flame propagation mode was performed in this study. Effects of sample density, nitrogen pressure, and diluent content on the degree of conversion, flame-front velocity, and combustion temperature were studied. It was found that the self-sustained combustion of the niobium/nitrogen SHS process was featured by the spinning combustion wave traveling on the sample surface. Prior to the self-sustained combustion stage, the igniter heat input contributed the longitudinal propagation of a planar front. The flame-front velocity increased with nitrogen pressure, but decreased with sample density. Measured temperature profiles and SEM examinations showed that the sample remained in solid phase, implying that the sample retained its porosity for the filtration of nitrogen gas in the afterburning stage. For the undiluted samples, the conversion percentages ranged between 56 and 70% were independent of the nitrogen pressure, due to the afterburning nitridation. The addition of diluent optimally yielded an increase of about 13% in the degree of conversion. The XRD analysis indicated that the dominant nitride phase synthesized in this study was δ-NbN. For the undiluted samples, a small amount of β-Nb2N was found in the combustion product.