Synthesis of monolithic aerogel-like alumina via the accumulation of mesoporous hollow microspheres

Monolithic aerogel-like alumina was synthesized via the accumulation of mesoporous hollow microspheres through an epoxide-driven sol-gel method. The addition of propylene oxide to the solution controls the gelation process, while the aging time and H2O/ethanol molar ratio in the initial component determine the morphology of the gel. The bulk density of the as-prepared sample was 0.133 g/cm(3), and the BET surface area was 505.6 m(2)/g, which were very close to the values for the common alumina aerogel synthesized through the supercritical drying method. The microstructural evolution of the aerogel-like material as a function of aging time was carefully studied by SEM and TEM. The transition from solid particles to urchin-like mesoporous hollow microspheres can be explained as a self-templating process according to the Ostwald ripening mechanism. In addition, the samples were heat treated to 800 degrees C, 1000 degrees C and 1200 degrees C for 2 h. With an increase of the heat treatment temperature, the crystalline phases of the aerogel-like alumina varied from gamma-phase alumina to theta-phase alumina. The microstructures as well as the physical properties, such as the thermal conductivity and elastic modulus, of samples heat-treated at different temperatures were also measured and revealed that the samples exhibited excellent stability against high temperature. (C) 2014 Elsevier Inc. All rights reserved.