Preparation Of Ultrafine Alumin A Powders By Pyrolysis Of Ammonium Aluminium Carbonate Hydroxide

Ultrafine alumina powders were prepared by pyrolysis of the aluminium precursor powders obtained by precipitation method using aluminium nitrate and ammonium carbonate as raw materials. The effects of precipitating process conditions on the precipitation form of products and the properties of alumina powders obtained were studied and the optimum preparation conditions for manufacture of ammonium aluminium carbonate hydroxide were determined. The changes of the thermochemical behaviours, phase structure, particle size, morphology, specific surface area of precursor in the process of calcinations were studied, and TG/DTA XRD TEM BET and some other experiment methods had been employed to characterized the products. In addition,the sintering ability of a~phase alumina powders after cold isostatic pressing of the precursors at 200MPa was researched and the sintering mechanism of alumina powder was also discussed.Experiment results show:(1) Ultrafine alumina powders ,with particle sizes in the range of 40-50nm and specific surface area >30 m2/g ,can be obtained by calcinating the aluminium precursors at 1150 for 2h ,which were synthesized under the conditions of aluminium nitrate solution dripped into (NH4)2CO3 solution at dropwise velocity < 0.7L/hr , reaction temperature 37+2,the molar ratio of ammonium carbonate to aluminium nitrate 3, (NH4)2CO3 concentration 2.0mol/L, Al(NO3)3 concentration 0.30mol/L ,dosage of PEG 0.6%.(2) The pyrolytic reaction of the precursor was found to be two stages. The first was D2, ie , didimensional phase-interface reaction with the reaction order 2 and the other was Fl , ie, simple first order reaction with the activation energy 115 + 5KJ/mol.(3) The phase change sequence of precursor was found to be:Ammonium aluminium carbonate hydroxide amorphous Al2O3-y-Al2O3-a-Al2O3 .(4) The specific surface area of the precursor was decreased gradually during the calcinating process. The specific surface area of Y -Al2O3 and a -Al2O3 obtained at 800 and 1150 were 182. 8m2/g and 32. 5m2/ g, respectively.(5) Samples for sintering were isostatically compacted at a pressure of 200MPa.The sintered body had a dense microstructure with a relative density of 99.7% at 1400 for 4h.The alumina sintering was found to be governed by two types of mechanisms, the viscous or plastic flow mechanism and the surface diffusion mechanism at the first stage and the second stage of the sintering , respectively.