Study Of The Processing And Mechanisms Of Mullite Reinforced Al₂O₃-SIC Composites By Microwave Synthesis

Microwave sintering as an energy-efficient and environmental heating method has received widely attention around the world. This method were widely used in the preparation of various materials because of its unique heating behavior. In this work,mullite reinforced Al2O3-Si C composites were prepared by hybrid microwave sintering. This thesis explored the influence of sintering process on the growth of mullite whiskers and revealed the interaction mechanisms between microwave and materials.50Al2O3-50 Si C(vol.%) composite was designed in this study. ?-Al2O3 particles were prepared by microwave pyrolysis of aluminum hydroxide precursor at 1100 o C. A sol-gel method was carried out to coat 10 vol.%Si O2 on Si C particles. Then the coated composite particles were mixed with ?-Al2O3 particles. The sintering process of Al2O3-Si C composites was study by changing the sintering temperature, holding time and Si C particle size. The heating rate was controlled by changing input power during the microwave sintering process of Al2O3-Si C composites. XRD, SEM, and EDS techniques were carried out to characterize phases and microstructure of Al2O3-Si C composites. Density, thermal shock resistance, and fracture toughness were also measured.The results showed that Al2O3-Si C composites reinforced by mullite whiskers were successfully prepared by microwave sintering. The optimized sintering temperature was 1500 o C for 30 min, and the optimized Si C particle size of50Al2O3-50 Si C composites was ~5μm, corresponding to the highest density, the best thermal shock resistance and fracture toughness. Samples retained 40% of initial flexural strength after 5 cycles, and retained almost 30% of its initial flexural strength after 10 and 15 thermal shock cycles. The fracture toughness value is 2.12±0.21Mpa·m1/2.Microwave absorbed by Si C particles that caused a local hot spot effect during microwave sintering, which rose the local temperature instantaneously. Heat spread from inside to outside which facilitated the reaction between Al2O3 and Si O2 forming mullite. With the increasing of input power, the strengthen local hot spot effect promoted the anisotropic growth of mullite crystal forming mullite whiskers between particles. Conventional sintering depended mainly on thermal radiation and thermalconduction. Heat spread from outside to inside resulted in high energy consumption and low heating rate in the process of conventional sintering, which limited the anisotropic growth of mullite crystal.In the process of microwave sintering, when the undersized Si C particles(~500nm) distributed in Al2O3-Si C composites, the ability of generating heat of Si C particles by absorbing microwave was lower than the ability of the heat diffusion of the surface of Si C particles. That limited the heat concentration on the surface of particles and the formation of mullite. When the oversized Si C particles(~150μm)distributed in Al2O3-Si C composites, the ability of generating heat of Si C particles by absorbing microwave was more than the ability of the heat diffusion of the surface of Si C particles, which limited the anisotropic growth of mullite. There is a critical size~5μm, when the Si C particles distributed in it, the ability of generating heat of Si C particles by absorbing microwave was equal to the ability of the heat diffusion of the surface of Si C particles, which promoted the anisotropic growth of mullite crystal forming mullite whiskers.