Investigation of the displacement reaction in mixed AIN + TiO₂ powders

Abstract

Micrometre-size AIN + TiO₂ powders in the molar ratio 2/1.5 were mixed, cold-pressed and heat-treated to 1400-1800 K in an AIN + N₂ = 1 atm buffer environment. Coarse AIN particles in fine TiO₂ powder, and TiO₂ single-crystal chips in fine AIN powder were also processed similarly. From X-ray diffraction and direct microstructural observations it is described and discussed that to a first approximation the overall reaction proceeds in three major steps: (i) reduction of TiO₂ to Ti₄O₇ or Ti₃O₅ with concurrent Al₂O₃ dissolution, (ii) formation of Ak _x Ti _y O₅ phases of either monoclinic or orthorhombic structure depending chiefly on the initial TiO₂ particle size, (iii) decomposition of such mixed oxide into Al₂O₃+ TiN. Simultaneously with these steps, the progressive oxidation of AIN to Al₂O₃ takes place at a rate which does not limit the overall reaction kinetics. Evidence is also provided that the reaction takes place in part via the transport of gaseous AI- and Ti-containing species, which calls for further investigation to be reported in the companion paper.