ZIRCONIA AEROGELS - EFFECT OF ACID-TO-ALKOXIDE RATIO, ALCOHOLIC SOLVENT AND SUPERCRITICAL DRYING METHOD ON STRUCTURAL-PROPERTIES

Zirconia aerogels have been prepared by the sol-gel method using tetra -n-butoxy zirconium(IV) and acid catalysis. The nitric acid-to-alkoxid e ratio and the alcoholic solvent, as important sol-gel parameters, we re varied. After aging, the solvent entrapped in the sol-gel products was removed applying two different methods: supercritical drying at hi gh-temperature and low-temperature extraction with supercritical CO2. Finally, for gel stabilization and removing of organic residues the ae rogels were calcined at 573 and 773 K, respectively. The effects of th e varied sol-gel parameters, different supercritical drying methods, a nd calcination temperature on the structural and textural properties w ere investigated. The aerogels were characterized by means of N-2 phys isorption, X-ray diffraction, thermal analysis, scanning and transmiss ion electron microscopy. Depending on the kind of alcoholic solvent, n itric acid-to-alkoxide ratio and drying method, the specific surface a reas of the mesoporous aerogels varied from 55 to 205 m(2) g(-1) after calcination in air at 773 K. The aerogels supercritically dried at hi gh-temperature possessed larger pores (17-65 nm) and BET surface areas (143-205 m(2) g(-1)) compared to the aerogels dried by low temperatur e extraction (< 20 nm and 55-112 m(2) g(-1), respectively). The width of the pore size distribution of all high-temperature supercritically dried gels became smaller with increasing amount of acid. All aerogels dried by low temperature extraction with supercritical CO2 were X-ray amorphous. Upon calcination in air at 773 K, the amorphous aerogels c rystallized to tetragonal zirconia. The high-temperature supercritical ly dried aerogels contained predominantly tetragonal zirconia. The fra ction of monoclinic ZrO2 shares increased with increasing nitric acid- to-alkoxide ratio, the use of a long-chained branched alcoholic solven t, such as t-butanol, and increasing calcination temperature. (C) 1998 Elsevier Science B.V.