Study On The Industrialized Preparation And Application Of Mullite Whiskers

Mullite (3Al₂O₃·2SiO₂) is the only stable compound of Al₂O₃-SiO₂ with many excellent performances such as high creep resistance, excellent thermal shock resistance and corrosion resistance, high melting point, high shear modulus, low dielectric constant and dielectric loss. Mullite whiskers have not only excellent performances of mullite but also good mechanical properties as a integrity single crystal with the high aspect ratio. And it is a superior reinforcement of ceramic matrix composites and metal matrix composites materials.This paper studied the processing conditions of the preparation of mullite whiskers by Sol-Gel process. Industrial Si-sol and Al-sol were used as raw materials, and aluminum fluoride was used as whisker growth-promoting agent. The effects of the alumina and silicon oxide molar ratio, Sol-Gel process parameters, the amount of whisker growth-promoting agent, sintering processing were studied. Mullite whiskers with the diameter of 2 to 5μm and the aspect ratio of 20-30 were prepared.When alumina and silicon oxide molar ratio is 6:5, mixed powder fineness maintain between 60 to 200 mesh, the quality ratio of aluminum fluoride and dry silica-alumina gel is 4:10, the sintering temperature is 1450℃and the soaking time is five hours, mullite whiskers with good shape and high aspect ratio could be obtained.The performances of mullite whiskers were investigated by means of SEM,XRD,EPMA. The main phase of the whisker was mullite. The shape of the mullite whisker was cylindrical structure and the cross-section was polygon. Mullite whiskers with the diameter of 2 to 5μm and the aspect ratio of 20-30 were prepared. The elements of the sample were Al, Si, O, and the ratio of Al: Si: O was 6:2:13 which was similar to the ratio of mullite Al, Si, O elements.Mullite whiskers could be prepared with industrial raw materials under the industrialized production conditions. By studying the technical parameters in the industrial production of mullite whiskers, we could concluded that the mixing of citric acid and hydrochloric acid with the ratio of 1:3 was used to adjust the pH value of the solution, the tap water is more suitable for diluting the solution in industrial production, the powder should be filled into alumina crucibles sparsely when sintering, breaking compacted samples of the product by 2h milling is a better approach.Mullite whiskers can be very good in enhancing the mechanical strength of aluminum titanate ceramics, inhibiting decomposition, retaining its low coefficient of thermal expansion. Mullite whiskers, with good wetting and relatively low prices, could be used as the reinforcement of aluminum matrix composites. When the content of mullite whisker is 7%, the bending strength of the composites with the highest value of 86.2 MPa was obtained and could be increased 100%.Aluminum matrix composites reinforced mullite whiskers have the point of low-cost and high-performances. The tensile strength of aluminum BH135 could be increased 24% and the modulus of elasticity could be increased 34.0% by adding 5% of mullite crystals.