Supercritical combustion synthesis of titanium nitride

To increase the interstitial mass loading of nitrogen for enhanced product yield in the self-propagating, high-temperature synthesis (SHS) of metallic nitrides, the synthesis was conducted in cryogenic nitrogen at supercritical states just above the critical point. This state regime has the unique characteristic that the density increases significantly with pressure. The viability of this concept has been experimentally substantiated for loosely packed titanium powders. Results show that the bubbling that characterized the synthesis process in liquid nitrogen was avoided and that liquidlike initial densities were maintained. Yields of titanium conversion to titanium nitride of almost 75% were achieved. This level of conversion is comparable to the highest of previously reported values obtained for titanium in gaseous nitrogen at ultrahigh pressures. Analysis of the present supercritical and literature data on high-pressure gas synthesis shows that the yield does not correlate well with pressure in accordance with the previous concept that filtrational transport is the controlling mechanism, especially for the after-burn condition. The data, however, were satisfactorily correlated against fluid density. Mass diffusion is the apparent relevant process for after-burn in low-nitrogen-density environments, and nitrogen loading in the interstices of the powdered titanium controls the yield at high-initial-nitrogen densities.