In situ synthesis of porous ceramics with a framework structure of aluminium borate whisker


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Abstract

New porous ceramics with a framework structure of aluminium borate (9Al2O3 · 2B2O3) whiskers, in which the whiskers are distributed uniformly and randomly, can be synthesized in situ by firing of a green powder compact of a mixture of aluminium hydroxide, boric acid and an additive of nickel oxide above 1100°C. During firing, the whiskers of aluminium borate grow in situ in the compact, and are bonded together by sintering. The porous aluminium borate consists solely of whiskers, has a porosity of 85%–50%, which corresponds to a volume fraction of whiskers of 15%–50%, and a flexural strength of 2.2–56.1 MPa. Because the whiskers are strongly bonded to other whisker(s), the problem of whiskers scattering, that can be an inhalation hazard, is solved. The aluminium alloy matrix composite using this porous aluminium borate as reinforcement was fabricated by the squeeze-casting method. The tensile strength of the composite material with a whisker volume fraction of 20% can be improved by up to about 90% compared with the unreinforced matrix alloy at 350°C.

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