Study On The Mechanism Of Self-propagation High Temperature Synthesis Of AlB₂-Al₂O₃ Composite Powders

AlB2-Al2O3 composite powders was prepared from the raw materials of Al powders, B2O3 powders, H3BO3 powders and B powders by self-propagation hightemperature synthesis(SHS) technique. The microstructural evolution during SHS process was investigated by combustion front quenching method(CFQM). The feasibility of reactions in the Al- B2O3 system, Al-H3BO3 system, and Al-B system was analyzed by thermodynamics calculation. The phase composition of synthesized product is investigated by XRD; The microstructure characteristics of synthesized product and microstructural evolution in the quenched sample was observed with fieldemission scanning electron microscopy(FESEM) and analyzed with dispersive spectrometry(EDS). The thermal behaviors of the Al-B2O3 system under the thermal exposure were characterized using differential scanning calorimetry(DTA) and thermogravimetric(TG). The flowing important results were obtained.(1) The adiabatic temperature of the Al- B2O3 system was 2327 K. That is, a combustion wave can theoretically self-propagate in the compact. However, when the initial temperature up to 470 K or 1000 K in the Al- H3BO3 system and Al- B system, the combustion wave can theoretically self-propagate in the compact.(2) The microstructural evolution during SHS process of AlB2-Al2O3 composite powders could be described with a reaction-dissolution-precipitation mechanism. It could be obtained that the combustion reaction started from the melting of the B2O3 and Al particles, which was followed by the formation of B2O3-Al solution. The ignition temperature of this system was determined to be around 1073 K. B and Al2O3 were then precipitated from the solution. AlB2 could then be formed by the direct reaction between Al and B. Based on a reaction-dissolution-precipitation mechanism, a model corresponding to the dissolution-precipitation mechanism was drawn.(3) The effect of the different starting materials on the phase constituents of final product was very important during synthesis of AlB2-Al2O3 composite powders. In addition to AlB2 and Al2O3, there are the impurity phase, AlB12, in the Al-B2O3 system, which was same to the Al-B system. It is found that the presence of impurity phase was associtted with the decomposition of AlB2 at high temperature. Moreover, the presence of impurity, H18B4O33, in Al- H3BO3 system was related to the reaction between Al2O3(generated by thermite reaction) and B2O3(decomposted by H3BO3) at high temperature.