Carbothermal synthesis of titanium nitride

Abstract

The conversion of TiO₂ (both anatase and rutile) to TiN by carbothermal reduction in nitrogen was found to proceed by progressive reduction through a homologous series of Magneli phase oxides Ti_nO_2n−1, where n has values of 4–10. The next phase to be observed by X-ray diffraction is Ti₃O₅, whose formation from the most stable Magneli phase (Ti₄O₇) is not predicted by the phase diagram, nor by simple thermodynamic calculations. The conversion of triclinic Ti₄O₇ to monoclinic Ti₃O₅ appears to be the slow step in the reaction sequence. Formation of TiN then proceeds directly, apparently without the intervention of Ti₂O₃ or TiO, as usually assumed. A possible cubic oxynitride intermediate was shown by surface analysis to contain no more than 5% oxygen. A small but significant amount of CO is evolved during the formation of Ti₄O₇, with a larger CO evolution occurring during the nitridation of Ti₃O₅. Although no direct mass spectroscopic evidence was found for the formation of C₃O₂, as suggested by previous workers, the present observed weight losses appear to be consistent with this concept. Both the observed reaction sequence and the pattern of CO evolution is consistent with thermodynamic calculations made using a computer program which takes into account the initial nitrogen concentration, and iterates over small temperature intervals.