Synthesis of high temperature stable Alumina:Silica aerogels in monolithic form

Kurzfassung

Inorganic aerogels are well established among high-performance insulating materials. The thermal conductivities are regularly indicated with values below 0.020 W m-1 K-1. Brittle nature and shrinking behavior of these materials at temperatures of 500°C and above, limit the practical applicability [1]. A promising way to increase the durability in temperature ranges above 500°C, is to include Al2O3 at different molar ratios into the molecular structure. One of the most thermally stable materials in this class is mullite, reported to be of acceptable shrinkage and low crack propagation property at high temperatures of up to 1200°C [2, 3, 4]. Thus a route to synthesize hybrid alumina/silica aerogels was developed. In literature describes synthesis routes of those aerogel materials using toxic and carcinogenic reagents like propylene oxide (PO). Thus the biggest challenge was the substitution or minimization of those. In this context synthesis routes to obtain advanced hybrid aerogel of crack-free monolithic samples based on a mullite composition under minimal use of PO are developed. Systematics studies with different ratios of Al:Si, Al:Si:solvent; and Al:PO were performed. The hybrid aerogels were characterized for shrinkage, density and thermal conductivity after a temperature treatment for 24 h at 600°C, 800°C and 1000°C. The aerogel microstructure was examined by scanning electron microscopy (SEM). The investigation for the detection of the mullite phase has been carried out via X-ray diffraction (XRD) and a simultaneous thermal analyzer (STA). Calculated reaction rates of PO were confirmed by Karl-Fisher titration. The most promising composition in terms of thermal stability was used for bench scale experiments. A half shell prototype of 200 mm in height and of a nominal diameter of 125 mm was produced proofing the scalability of the route. High pressure supercritical drying method using CO2 was performed in a in a 60 L extractor unit with 500 mm inner diameter in order to receive the advanced aerogels. Within the presentation all results on synthesis of gel bodies, supercritical drying and aerogel properties combined with relevant examples of application will be reported. References: [1] Wong, J.C.H.; Kaymak, H.; Brunner, S.; Koebel, M.M., Mechanical properties of monolithic silica aerogels made from polyethoxydisiloxanes. Microporous and Mesoporous Materials 2014, 23-29. [2] Hurwitz, F.I.; Gallagher, M.; Olin, T.C.; et al., Optimization of Alumina and Aluminosilicate Aerogel Structure for High-Temperature Performance. International Journal of Applied Glass Science 2014, 276-286. [3] Chen, H.; Sui, X.Y.; Zhou, C.L., Preparation and characterization of monolithic Al2O3-SiO2 aerogel. Journal of the Ceramic Society of Japan 2016, 442-447. [4] Wu, X.D.; Shao, G.F.; Shen, X.D.; et al., Novel Al2O3-SiO2 composite aerogels with high specific surface area at elevated temperatures with different alumina/silica molar ratios prepared by a non-alkoxide sol-gel method. RSC Advances 2016, 5611-5620.