Study On The Mechanism Of Self-propagation High Temperature Synthesis Of Calcium Aluminate Powders

In this paper, calcium aluminate powders were prepared from the raw materials of CaCO₃ powdes, Al powders, Al₂O₃ powdes and MO₂ by self-propagation high temperature synthesis（SHS） technique. The feasibility of reactions in the MO₂-AlCaCO₃-Al₂O₃ system, and the system was analyzed by thermodynamics calculation.The microstructural evolution during SHS process was investigated by combustion front quenching method（CFQM） to study self-propagating high temperature synthesis mechanism of the calcium aluminate. The information of the microstructure change and phase transformation in the reaction process was mintained in a quenched sample. The phase composition of the SHS product were investigated by X-ray diffraction（XRD）; The thermal behaviors of the MO₂-Al-CaCO₃ system under the thermal exposure were characterized using differential scanning calorimetry（DSC） and thermogravimetric（TG）; And the process of microstructure transformation was observed with fieldemission scanning electron microscopy（FESEM） in the quenched sample. The ingredients change of the micro-area was analysised with the Energy Disperse Spectroscopy（EDS）. The flowing results were obtained.The analysis showed that reaction process of SHS of calcium aluminate powders could be described with a solid-phase diffusion mechanism. It could be obtained that the combustion reaction started from the decomposition of the MO₂ and the melting of the Al particles. Oxygen from the decomposition of calcium peroxide and Al reacted to produce Al₂O₃. CaCO₃ decomposed to produce CaO. As the SHS reaction proceeded, intermediate phases C12A7 were produced preferentially by the solid phase reaction between CaO particles and Al₂O₃ particles by diffusion of calcium ions. CA2 and CA could then be formed and ransformed from the C12A7. With increasing of the temperature, CA cound be transformed from the CA2. Finally, the CA2 and CA powders were obtained. And the formation of CA2 had precedenced over that of CA.Moreover, the effect of the ratio of starting materials on the phase constituents of final product of CS was very important. Results showed that, the combustionsynthesized product consists of CA2, CA and small amounts of C12A7 and Al phases in MO₂-Al-CaCO₃-Al₂O₃ system. The combustion-synthesized product is no presence of Al in MO₂-Al-Al₂O₃ system. With the increase of the content of Al₂O₃ in MO₂-AlAl₂O₃ system, the combustion-synthesized product consists of CA2, CA and small amounts of C12A7. In addition, the formation of only CA and CA2 phases without C12A7 in final product was feasible when Al₂O₃/（Al₂O₃ + CaO）（mole） is 0.55, that the starting mixture of MO₂, Al₂O₃ and Al was in a mass ratio of 37.50: 53.13: 9.38.