Journal of Sol-Gel Science and Technology 14, 233–247 (1999) c  1999 Kluwer Academic Publishers. Manufactured in The Netherlands. The Evolution of Microstructure in Nonhydrolytic Alumina Xerogels Y. DE HAZAN, G.E.SHTER AND Y. COHEN Chemical Engineering Department, Technion, Haifa 32000, Israel C. ROTTMAN AND D. AVNIR Institute of Chemistry, The Hebrew University of Jerusalem, Jerusalem 91904, Israel G.S. GRADER Chemical Engineering Department, Technion, Haifa 32000, Israel Received April 30, 1998; Accepted August 17, 1998 Abstract. The effect of drying, aging and thermal treatment of alumina xerogels prepared by the nonhydrolytic route was investigated using SAXS, BET and HR-SEM techniques. The microstructure of the fresh xerogels pre- pared under different procedures varied drastically, ranging from aerogel-like mass fractals to narrow pore size distribution materials. By variation of the drying conditions the N 2 -BET surface area was varied from an immeasur- able low level up to 600 m 2 /g. The initial microstructure has a significant influence on the xerogel behaviour during the post-drying heating stage. The ability to produce aerogel-like mass fractal materials from the nonhydrolytic systems is discussed. Finally, a brief theoretical treatment of the drying process of mass fractals is presented as well. Keywords: nonhydrolytic alumina xerogels, SAXS, BET, mass fractal, surface fractal 1. Introduction Porous ceramics have wide applications in ceramic cat- alysts and catalyst supports, filters and membranes, adsorbents, ceramic foams, entrapment matrices, and precursors for ceramic metal composites [1, 2]. For all these applications, the elucidation of the microstructure evolution during heat treatment and the determination of the thermal stability are crucial for the selection of appropriate materials, working conditions and process optimization. The structure of gels and aerogels has been inves- tigated and characterized extensively by using com- plementary techniques such as small angle scattering analysis coupled with gas adsorption/desorption, po- rosimetry and electron microscopy [1, 2]. However, less attention was given to a full description of the ma- terial microstructure evolution during heat treatment [3–7] much of which was restricted to the microstruc- ture evolution of silica aerogels. Nonhydrolytic sol-gel chemistry provides a general route for the preparation of single and multicomponent metal oxides [8–16]. The nonhydrolytic xerogels allow one to obtain unique chemical compositions; provide materials with high crystallization temperatures; pro- vide high homogeneity of multicomponent materials; and allow simplicity of preparation, which are unatta- inable or difficult to reach by hydrolytic processes. Recently, nonhydrolytic alumina gels were shown to posses unique microstructure and give rise to xerogels with a wide ranging surface area extending from seve- ral hundreds m 2 /g to closed pores materials [17, 18].