Role of Water Vapor in Crystallite Growth and Tetragonal‐Monoclinic Phase Transformation of ZrO ₂

The role of water vapor in crystallite growth and the tetragonal‐to‐monoclinic phase transformation of ZrO 2 was studied using three specially prepared samples: an ultrafine powder of monoclinic ZrO 2 obtained by hydrolysis of ZrOCI 2 , an aggregated powder of tetragonal ZrO 2 obtained by thermal decomposition of Zr(OH) 4 under reduced pressure, and an ultrafine powder of tetragonal ZrO 2 obtained by thermal decomposition of zirconyl acetate dispersed in caramel. The samples were heat‐treated up to 1000°C in dry and wet atmospheres saturated with water vapor at 90°C. It was found that water vapor markedly accelerated crystallite growth for both monoclinic and tetragonal ZrO 2 and facilitated the tetragonal‐to‐monoclinic phase transformation. Water vapor increases surface diffusion and thus enhances crystallite growth and decreases surface energy, which leads to stabilization of the tetragonal phase.