Preparation Of Ultrafine Mullite Powders

Ultrafine mullite powders were prepared by the calcination of the mullite precursor powders obtained by a co-precipitation method and followed by n-butanol azeotropic distillation process using aluminium nitrate , water glass and ammonium carbonate as raw materials. The effects of precipitating process conditions on the precipitation form of products and the properties of mullite powders obtained were studied and the optimum process conditions for the manufacture of mullite precursor powders were determined. The changes of the thermochemical behaviours, phase structure, particle size, morphology, BET specific surface area and apparent density of mullite precursor powders in the process of calcination were investigated, and TG-DTA, XTD, TEM, IR spectra , nitrogen, adsorption and some other experiment methods have been employed to characterize the products. In addition, the sintering behavior of the mullite precursor powders was also studied after cold isostatic pressing of the powders at 180 MPa. The research results show:(l)Ultrafme mullite powders, with particle size in the range 60-90 nm can be obtained by calcinating at 1300 for 1 h the mullite precursor powder, which was in the shape of short fiber, with 5-10 nm in width and 50-100 nm in length , and can be prepared by n-butanol azeotropic distillation of the wet aluminum silicate gels (or precipitates) synthesized under the conditions of reaction temperature 38-40, A1(NO3)3 concentration 0.3mol/L, (NH4)2CO3 concentration 1.5mol/L. MLtNOs addition 0.36mol/L and PEG (200,1540,6000) addition 0.35%.(2)The sequence of phase transformation of the mullite precursor powders during calcinating was found to be Y -AlO(OH)+ vague SiO2 amorphous Al2O3 + amorphous SiO2 Al-Si Spinel + amorphous SiO2 mullite .(3) Calcining the mullite precursor powders below 600 can decrease the apparent density ,but has only slight influence on the specific surface area, while calcining above 600 will reduce the specific surface area and increase the apparent density .The specific surface area and apparent density of mullite powders were 30m2/g and 1.5g/ml respectively at the temperature of 1300.(4)Mullite precursor powders calcined at 1000 can be reactively sintered to 98.6% relative density at 1550 for 4 h.