Self-controlled growth in highly stableα-Al2O3 nanoparticles in mesoporous structure

Highly stable α-Al2O3 nanoparticles are obtained in a mesoporous structure (10–25% porosity) by reconstructive decomposition of a mesoporous AlO(OH) x H2O, x ∼ 0.7, powder followed by annealing at 1475–1900 K. At x ∼ 0.7, self-controlled AlO(OH) x H2O → Al2O3 molecular decomposition occurs in a controlled desorption of H2O through pores at 330–650 K. It was achieved by a novel hydrolysis method of Al metal with nascent surfaces in water at 295 K. Average α-Al2O3 crystallite size hardly grows to 30–50 nm (several hundred nanometers otherwise in a bulk sample) at 1475–1900 K. They are arranged through pores in a specific fashion as observed in TEM micrographs. A network structure forms of surface atoms in high-energy crystallites in a high configurational entropy (governs improved stability and superplasticity or other properties) by minimizing the Gibbs free energy. A metastable α-Al2O3 phase exists in processing at low temperature such as 1475 K. It has a modified X-ray diffraction or IR spectrum of equilibrium phase after annealing (reorders interstitial vacancies) at 1525 K or higher temperatures. It is proposed that a mobile Al3+ hole in a site neighboring an AlO6−δ architect defect creates a center to a plane slipping or twin structure formation. Mesopores provide sites for crack initiation and also for crack arrest in confined cracks (support a high failure strain). (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) An Editorial Note to this paper has been published in phys. stat. sol. (a) 201, No. 3 (2004): http://dx.doi.org/10.1002/pssa.200406784