Unusual structural phase transition in nanocrystalline zirconia

Abstract

The present work is the first example demonstrating that a hydrous zirconia formed by precipitation can yield a nearly pure nanocrystalline monoclinic zirconia at a temperature as low as 320 °C. The X-ray diffraction pattern of the hydrous zirconia heated to 310 °C shows that diffraction peaks begin to emerge and reveals a just crystallized mixture of predominantly monoclinic zirconia (70%) with some tetragonal zirconia(30%). In other words, the hydrous zirconia formed in the present work yields the predominantly monoclinic structure coexisting with the tetragonal one as soon as crystallization starts at low temperature (310 °C). This is an important exception to the general principle that amorphous zirconia precursors first convert to the tetragonal structure of zirconia with increasing calcination temperature and then transform to the monoclinic one at a higher temperature (∼600 °C). At the crystallization temperature (310 °C), the monoclinic crystallite size is about 17 nm and the tetragonal one 28 nm. The monoclinic crystallite is much smaller than the tetragonal one with which it co-exists. This result is also not consistent with the traditional view that a critical particle size effect is responsible for the stability of the tetragonal and monoclinic structures. When the temperature (310 °C) is slightly raised to 320 °C, the XRD pattern shows a nearly pure monoclinic zirconia. The crystallite size of the monoclinic zirconia is around 15 nm, and it does not change appreciably as calcination temperature is increased from 320 to or above 400 °C. The unusual structural phase transition has been investigated by several complementary experimental tools: X-raydiffraction and surface analyses, and infrared and Raman spectroscopies.