Synthesis and structural transformation of zirconia aerogels

Zirconia (ZrO[sub 2]) aerogels were prepared by the sol-gel method using zirconium n-propoxide in n-propanol followed by supercritical drying with carbon dioxide. This synthesis, without the use of dopants, formed a high surface area material and stabilized the tetragonal phase at low temperatures. By optimizing the water and nitric acid amounts, the authors formed a zirconia aerogel with a surface area of ca. 130 m[sup 2]/g after calcination 773 K for 2 h. The effect of heat treatment on the physical characteristics of the aerogel was determined by nitrogen adsorption, X-ray diffraction, Raman spectroscopy, electron microscopy, and differential thermal analysis. The untreated, highly porous, amorphous aerogel decreased in surface area and pore volume upon heating. By variation of the heat treatment, the zirconia aerogel existed in either a completely amorphous, tetragonal, or monoclinic form at room temperature. In situ X-ray diffraction measurements were used to examine the tetragonal-to-monoclinic phase transformation, which was controlled by embryo formation and growth and was not a simple function of crystallite size. Finally, the time required for gel formation controlled the type of oxide network formed, which in turn dictated the physical characteristics of the aerogel and the number of defects for embryo formation. 53 refs., 22 figs., 2 tabs.