Effect Of Different Starting Materials On Synthesis Of Al₈B₄C₇and Al₂OC By Carbothermal Reduction

In Al-B-C ternary systems, because of its low density （2.69g/cm3）, high temperaturestability （²103K）, low coefficient of thermal expansion, excellent semiconductor performanceand excellent oxidation resistances and excellent hydration resistances, Al₈B₄C₇is used as theraw material of the non-oxide and structural ceramic materials in refractories and can also beused as excellent antioxidants in carbon-containing refractories. Using oxide raw materialsreplace elemental powders as starting materials reduces the synthesis costs of Al₈B₄C₇, which isbeneficial to industrialization production. Carbothermal reduction method is considered to be apromising synthetic method because its simple process. Al₂OC was formed as intermediateproducts in the process of synthesizing Al₈B₄C₇. Al₂OC not only has good corrosion resistance,high melting point and high temperature stability but also has good thermal shock resistance,high thermal conductivity and low thermal expansion coefficient, so it is a high performancerefractory and also a fine antioxidant in carbon-containing refractories. But there are fewreports about the synthesis of Al₂OC. It is necessary to research and analysis the synthesis andperformance of Al₂OC. Using oxide materials to synthesize Al₈B₄C₇and Al₂OC bycarbothermal reduction method, the reaction mechanism and the influence factors wereanalyzed.This work researched the synthesis of Al₈B₄C₇by heating a mixture of Al/Al₂O₃, B₂O₃andactivated carbon powders through a carbothermic reduction process. The reaction mechanismand the effects of different synthesis temperatures and raw materials ratios on synthesis ofAl₈B₄C₇were investigated by means of X-ray diffraction （XRD） and scanning electronmicroscopy （SEM）. A12OC ceramic powder was successfully synthesized via a carbothermalreduction method using Al₂O₃, B₂O₃and activated carbon powders as raw materials. The effectsof synthesis temperature and the amount of B₂O₃addition on the phase transformation andmicro-morphology of A12OC were investigated. Oxidation properties of Al₈B₄C₇and Al₂OCpowders were investigated.Using Al/Al₂O₃, B₂O₃and activated carbon powder as raw materials to synthesize Al₈B₄C₇by carbothermal reduction, at1800℃, a large amount of Al₈B₄C₇was generated. And0.5molarexcess of B₂O₃addition promoted the formation of Al₈B₄C₇.But caused hydratable Al₄C₃assecond phase. Al2O, AlO₂, Al2O₂and B₂O₂gases were generated from carbothermal reductionof Al₂O₃and B₂O₃, which participated in the reaction for synthesizing Al₈B₄C₇, Al₈B₄C₇particles had a hexagonal plate-like morphology, synthesis mechanism mainly is gas-solidreaction. Using Al₂O₃, B₂O₃and activated carbon powder as raw materials to synthesize Al₂OCby carbothermal reduction, at1700℃, a large amount of hexagon plate-like A12OC with thickness of5-10μm and size of about50μm was generated. The amount of B₂O₃additionwhich kept molar ratio of Al₂O₃/C/B₂O₃was1:6:0.3¹:6:0.7promoted the formation of A12OC,Synthesis mechanism of Al₂OC mainly was gas-solid reaction.Al₈B₄C₇powder resisted oxidation up to700℃in air and the product was Al18B4O33at1500℃. Al₂OC powder resisted oxidation up to720℃in air and the product was Al₂O₃at1500℃. Al₈B₄C₇and Al₂OC were beneficial to improve the oxidation resistance ofmagnesia-carbon bricks and the effect of Al₈B₄C₇was remarkable.