Formation and crystal-chemical studies of metastable and stable mullite-type aluminum borates

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Whiskers of mullite-type aluminum borates, especially Al18B4O33 (A9B2) and Al4B2O9 (A2B) are used as reinforcement in aluminum alloys due to their highly refractory, elastic and mechanical properties. The main objective of this proposal is the investigation of the formation and the crystal-chemical characterization of mullite-like phases in the binary system Al2O3 – B2O3 with an emphasis on Al4B2O9 and Al18B4O33 (Al5BO9) compounds. In this context, the question which factors (chemical composition, reaction route, temperature) essentially control the formation of mullite-type compounds at ambient pressure and hydrous or anhydrous environments is of special interest. Another central point of the study is the understanding of aluminum borate formation from their metastable precursor phases obtained by sol-gel syntheses and whether these phases have also mullite-type structures. A main focus will be given to the understanding of the crystallization mechanisms of the aluminum borates with or without formation of intermediate metastable phases. The investigations should lead to a revised phase diagram of the binary system, considering possible solid solutions. Based on various synthesis routes (nitrate decomposition, solid-state reaction, hydrothermal synthesis) the powders will be characterized by HT X-ray and neutron diffraction methods, NMR, HT-IR and HT-Raman spectroscopy, and thermal analyses. The chemical compositions will be determined by prompt gamma activation and ICP-OES analyses.