Tailoring of Boehmite-Derived Aluminosilicate Aerogel Structure and Properties: Influence of Ti Addition
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Tailoring of Boehmite-Derived Aluminosilicate Aerogel Structure and Properties: Influence of Ti Addition Frances I. Hurwitz,1 Haiquan Guo,2 Erik J. Sheets,3 Derek R. Miller,4 and Katy N. Newlin5 1 Structures and Materials Division, NASA Glenn Research Center, 21000 Brookpark Rd., Cleveland, OH 44135, U.S.A. 2 Ohio Aerospace Institute, Cleveland, OH, U.S.A 3 Purdue University, W. Lafayette, IN, U. S. A. 4 Michigan State University, E. Lansing, MI, U.S.A. 5 University of Louisville, Louisville, KY, U.S.A.
ABSTRACT Aluminosilicate aerogels offer potential for extremely low thermal conductivities at temperatures greater than 900˚C, beyond where silica aerogels reach their upper temperature limits. Aerogels have been synthesized at various Al:Si ratios, including mullite compositions, using Boehmite (AlOOH) as the Al source, and tetraethoxy orthosilicate as the Si precursor. The Boehmite-derived aerogels are found to form by a self-assembly process of AlOOH crystallites, with Si-O groups on the surface of an alumina skeleton. Morphology, surface area and pore size varies with the crystallite size of the starting Boehmite powder, as well as with synthesis parameters. Ternary systems, including Al-Si-Ti aerogels incorporating a soluble Ti precursor, are possible with careful control of pH. The addition of Ti influences sol viscosity, gelation time pore structure and pore size distribution, as well as phase formation on heat treatment. INTRODUCTION Aluminosilicate aerogels are of interest as constituents of thermal insulation systems with potential for use at temperatures higher than those attainable with silica aerogels, which densify and sinter at temperatures above 800˚C.1 It is anticipated that the effectiveness of aluminosilicate aerogels as thermal insulators will be influenced by their composition (Al:Si ratio), backbone structure, morphology, pore size distribution, physical and skeletal densities. The present study focuses on the synthesis of aluminosilicates from Boehmite precursors as the Al source, and tetraethylorthosilicate (TEOS) as the Si source. Titanium is introduced as a third metal in a ternary Al-Si-Ti system using titanium isopropoxide. The objectives of the work are to establish a fundamental understanding of the relationship among constituent ratios and synthesis parameters on aerogel structure and thermal properties, including densification and sintering, and to evaluate the effect of Ti addition on these properties.
EXPERIMENT Synthesis Aluminosilicate aerogels were prepared by a method similar to that of Aravind2 at Al:Si ratios of 8Al:1Si and 3Al:1Si using a number of different Boehmite powders which varied in crystallite size. An Al2TiO5 formulation was synthesized as a model compound. Boehmite (AlOOH) powders designated as P2 (4.9 nm crystallite size), and T25 (6.9 nm crystallite size), were obtained from Sasol North America. The powders were dispersed in nitric acid solution or water, and sonicated using a Misonix 4000 ultrasonic processor with a 419A tip for 2 minutes at 60 percent power.
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