Aluminum borate in the combustion synthesis of alumina/boride composite

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Aluminum borate in the combustion synthesis of alumina/boride composite Yonghe Liu,a) Sheng Yin, Zhimeng Guo, and Lai Hoyi P/M Group, School of Materials Engineering, University of Science and Technology Beijing, Beijing 100083, People’s Republic of China (Received 17 July 1997; accepted 14 January 1998)

The formation mechanism of aluminum borate in the combustion synthesis of Al2 O3yB4 C composite with Al, B2 O3 , and C as starting materials is proposed. Based on the formation mechanism, several approaches taken to eliminate them are discussed. The unconverted B2 O3 is the major cause of the formation of 9Al2 O3 ? 2B2 O3 when the reaction proceeds in the SHS mode. The amount of 9Al2 O3 ? 2B2 O3 formed is very sensitive to the excess B2 O3 to the stoichiometry 4Al 1 2B2 O3 1 C and influenced by the size of B2 O3 powder. The high dispersion of the reactants is helpful in prompting the thermite reaction to consume B2 O3 and hence inhibiting the aluminum borate formation.

Boride dispersed alumina composites have several potential applications such as neutron absorber, wear resistance, and cutting tools materials. One of the limitations of the development of these materials is the cost of the procurement and the fabrication of the composite.1 Combustion synthesis or self-propagation high-temperature synthesis (SHS) is a promising technology in the preparation of the ceramic composite because of its low cost and efficiency.2,3 Many boride dispersed alumina composites have been prepared by SHS based on thermite reactions4–6 ; however, the actual application of SHS in preparing composite is still quite limited because of the problems in the control of the microstructure and hence the properties. In the thermite type SHS, some by-products are frequently found in the reacted samples.4–7 The elimination of the by-products to obtain the desired composite in SHS is important to the control of the microstructure and properties of the materials. Among the by-products in the SHS of Al2 O3yB4 C composite, aluminum borates are frequently detected.5–7 Two kinds of aluminum borates having the chemical compositions 9Al2 O3 ? 2B2 O3 and 2Al2 O3 ? B2 O3 are shown in the phase diagram proposed by Gielise.8 9Al2 O3 ? 2B2 O3 had been prepared by various methods in which Al2 O3 and B2 O3 are employed as the initial reactants.9,10 Ray10 found the formation of 2Al2 O3 ? B2 O3 at 1073–1273 K and the formation of 9Al2 O3 ? 2B2 O3 above 1373 K when Al2 O3 and B2 O3 powders were calcined. However, Al and B2 O3 are used in the initial reactants in the thermite SHS reactions. Logan6 found the formation of 9Al2 O3 ? 2B2 O3 when Al and B2 O3 powders were heated to a temperature just above the a)

On leave from School of Materials Engineering, Shandong University of Technology, Jinan 250014, People’s Republic of China. J. Mater. Res., Vol. 13, No. 7, Jul 1998

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melting point of aluminum (933 K). Wang and Munir5 found 2Al2 O3 ? B2 O3 at even more than

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