Study On The Preparation Of Well Dispersed α-Al₂O₃ Powder By Chemical Precipitation Method

Aluminum powders have attracted much attention because of their excellent mechanical and chemical performance,these powders have been widely used in many fields such as optical ceramics,refractory materials and circuit substrates.Since the Al₂O₃ transparent ceramics has been prepared by Dr.Coble,and the high-quality yttrium aluminum garnet transparent ceramics prepared successfully,alumina powder has become one of the most important ceramic raw materials and will surely have bright prospects for development.Synthesized ultrafine powders by the liquid-phase precipitation method show considerable improvements in sintering activity and chemical purity.Therefore,the liquid-phase precipitation method has been widely studied to synthesize Al₂O₃ powders.Alumina powder is an amphoteric hydroxide,which leads to agglomeration during the preparation process.The phenomenon of agglomeration is a difficulty in the preparation of high-performanceα-Al₂O₃ powder,which restricts its development and application.In order to synthesize sphericalα-Al₂O₃ powder with good dispersity,fine particle size,and high sintering activity,the spray precipitation method is used.The factors of titration method,feeding method and calcined temperature on the performance of the powder is studied.At the same time,the ball milling process was improved and step by step milling was used to increase the sphericity and dispersibility of the powder.The research work is as follows:（1）Al₂O₃ powders is synthesized by the conventional precipitation method.The solution is mixed in the form of droplets The reaction solutions generated precursor precipitates during vigorous stirring of a four-bladed turbine impeller.The influence of the parameters,such as the pH,feeding methods,the concentration of the Al³⁺solution on the precursor and calcinedα-Al₂O₃ powders were studied.The results show that during the preparation of alumina powder by normal titration precipitation method,the titration endpoint pH has a significant effect on the morphology and phase formation temperature of the precursor powder and the calcined powder.The higher the titration endpoint pH is,the higher the temperature at which the prepared precursor is converted into the pure phaseα-Al₂O₃.At low calcination temperatures,the calcined powder contains other transition phases at the high titration endpoint pH.In contrast,during the preparation of alumina powders by reverse titration,the dispersibility and particle size uniformity of the precursors and calcined powders are much improved.The grain size of the calcinedα-Al₂O₃ powders is increased when the Al³⁺concentration was increased.When the molar ratio of SO₄^2-ion to Al³⁺as dispersant is 15%([SO₄^2-]/[Al³⁺]=15 mol.%),the prepared powder has better dispersibility.The particle size distribution is more uniform than the normal titration method,and the average particle size is 196.7 nm.（2）High sintering activityα-Al₂O₃ powder is the key to the preparation of high optical transparent ceramics.However,due to the large droplet size produced by the titration method,the concentration of the local reaction solution is too high,which is not conducive to the homogeneity of the raw solution,resulting in a large particle size and serious particle agglomeration.The stirring process provided better understanding with the assistance of the computational fluid dynamics software ANSYS CFX.The flow pattern of a 4-PBT（4-pitched-blade turbine）impeller is axial flow with axial symmetry.Then,a spray precipitation device was designed in this study and the device could atomize the aluminium nitrate solution into many small droplets with a small nozzle.The effects of Al³⁺concentrations,calcination temperature and holding time on the morphology,particle size distribution and phase evolution of aluminium powders were systematically investigated.Through test the powders,the parameters of the spray precipitation method are optimized.The results show that the Al³⁺concentration was 0.1 mol/L,the NH₄HCO₃ concentration was 1.5 mol/L,and 15 mol.%of ammonium sulfate(（NH₄）₂SO₄,[SO₄^2-]/[Al³⁺]=15 mol.%)was used as the dispersant and added into the NH₄HCO₃ solution during the vigorous stirring for 30min,the injection rate is 3 ml/min,the calcination temperature is 1150 ^oC,holding time is 2 h.The average particle size ofα-Al₂O₃ powders with higher sintering activity was approximately 62.5 nm,and the specific surface area was above 22.4m²·g^-1.At last,in order to obtainα-Al₂O₃ powder with well dispersibility,spherical shape and the narrow particle size distribution.A step-by-step horizontal ball-milling process was systematically optimized.The results show thatα-Al₂O₃ powders with improved dispersibility were achieved by a step-by-step ball-milling method for co-precipitation precursor.There were no impurity phases observed from all calcined powders.The work shows that step-by-step horizontal ball-milling achieves a well-dispersibleα-Al₂O₃ powder with quasi-spherical morphology and narrow size distribution.