Synthesis Mechanism And Property Of Al₄SiC₄ By Natural Raw Material

Al₄SiC₄ has some excellent properties, such as melting point about 2037℃, high strength, stability of chemistry and thermal expansion coefficient, excellent hydration resistance and oxidation resistance, etc. Al₄SiC₄ material is a kind of potential materials applying to high-temperature structural materials or high-quality refractories. So far, the synthesized Al₄SiC₄ materials were mainly used as starting materials. Although the single phase Al₄SiC₄ material could be synthesized, the synthesis cost of Al₄SiC₄ material is very high. Therefore, those methods have not been industrialized.We took two natural raw materials (flint clay and white clay powders) in Al₂O₃-SiO₂ system, aluminum powder and active carbon powder as the starting materials. To synthesize Al₄SiC₄ materials, we adopted three projects as following, (1) natural raw materials and C, (2) the secondly synthesis, (3) the impact of adding Al on natural raw materials and C. The samples were died under 100MPa pressure, then dried out, and synthesized at high temperature in Ar atmosphere.The feasibility on the synthesis of Al₄SiC₄ was analyzed under Ar atmosphere through thermodynamic calculation. The phase compositions of the sintered samples were determined by X-ray diffractometry (XRD). The microstructures of the synthesized samples were examined by scanning electron microscope (SEM), and the elemental and quantitative compositions of the specimens were determined by the energy dispersive X-ray spectroscopy (EDX). The oxidation process of the synthesized samples was researched by the differential thermal analysis (DTA) and the thermal gravity analysis (TG). Erosion-resisting property was studied by the crucible method.By analyzing the experimental results, we got some conclusions as follows.(1) Using natural raw materials in Al₂O₃-SiO₂ system and active carbon powders as the starting materials, we did not found Al₄SiC₄ but the middle product Al₄O₄C and Al₂OC in the sintered samples. In the course of the Secondly synthesis, when adding the metal Al powder, we got a little Al₄SiC₄. So adding the metal Al was very important to synthesized Al₄SiC₄.(2) In the course of the impact of adding the metal Al on synthesizing Al₄SiC₄ by natural raw materials and C, the quantity of adding the metal Al had great impact on reaction mechanism and end product. When adding insufficient the metal Al, it was difficult to synthesizing Al₄SiC₄. But when adding excessive the metal Al, Al₄SiC₄ was synthesized and we got the optimum reactant ratio. The reaction mechanism was that the metal Al melted at about 660℃, firstly. The mullite in the samples reacted with the active carbon, and produced Al₂O₃ and SiC; meanwhile, the melted Al reacted with the active carbon, and produced Al₄C₃. With the increasing of the firing temperature, the produced Al₄C₃ reacted with SiO₂ in the samples, and produced Al₂O₃ and SiC. The produced Al₄C₃ also reacted with the produced Al₂O₃, and produced Al₄O₄C. Subsequently, the produced Al₄O₄C reacted with the active carbon, and produced Al₂OC. Finally, the produced Al₂OC and the produced SiC reacted with the active carbon, and produced Al₄SiC₄ at higher temperature.The shape of the synthesized Al₄SiC₄ in the synthesized multi-phase materials was flake or block. The grain size was about more than 10 microns to tens of microns. The grain size of the produced Al₄O₄C was a little smaller and existed in the surrounding and gap of Al₄SiC₄ grain.About the multi-phase materials, we found that it had good oxidation-resisting property and erosion-resisting property.