Spherical alumina is widely used in daily life due to its excellent thermal conductivity and regular morphology.However,do you know how it was prepared?There are currently various methods for preparing spherical alumina,among which,in addition to high-temperature melting,there are also three common preparation methods.
　　1.Homogeneous precipitation method
　　The precipitation process in a homogeneous solution is the formation of crystal nuclei,followed by aggregation and growth,and finally precipitation from the solution.It is usually a non equilibrium state,but if the concentration of the precipitant in the homogeneous solution can be reduced,or even slowly generated,a large number of small crystal nuclei will be uniformly generated,and the resulting small precipitate particles will be uniformly dispersed throughout the entire solution,And it will maintain an equilibrium state for a considerable period of time,and this method of obtaining precipitation is called homogeneous precipitation method.With Al2(SO4)3,Al(NO3)3 and urea as raw materials,the spherical precursor of Aluminium hydroxide was prepared by using the Hydroxide ion produced by the slow hydrolysis of urea as the precipitant in an oil bath at 98℃.The size of precursor particles can be adjusted by adjusting the ratio of SO42-to NO3-.The precursor can still maintain spherical shape after calcination.The Aluminium hydroxide sol was prepared by Al2(SO4)3 at high temperature,and the colloidal particles obtained also have very good spherical morphology.Using Al2(SO4)3 and urea as raw materials,spherical and hollow spherical alumina powders are prepared under different conditions.For homogeneous precipitation method,if the size of the precipitation particles obtained is within the range of colloidal particles,this method is also called sol gel method.In addition to the conditions for the existence of SO42-,it is usually difficult to form alumina powder with high sphericity through the gel of sol particles.Therefore,people thought of using emulsification technology,which formed the sol lotion gel method.
　　2.Sol lotion gel method
　　This method is developed on the basis of the sol gel method.In order to obtain spherical powder particles,people use the interfacial tension between the oil phase and the water phase to produce small spherical droplets,so that the formation of sol particles and gel are limited to small droplets,and finally obtain spherical precipitation particles.Spherical aluminum oxide powder is prepared by hydrolysis of aluminum alkoxide through the sol gel process,in which 50%of the octanol dissolved in aluminum alkoxide,40%of the acetonitrile solvent,9%and 1%of the octanol and butanol dispersed in water respectively,and hydroxypropyl cellulose is used as the dispersant to obtain a spherical shape with excellent sphericityγ－Aluminum oxide powder.
　　3.Dropping ball method
　　The ball dropping method is to drop the alumina sol into the oil layer(usually paraffin,mineral oil,etc.),form spherical sol particles by the effect of surface tension,then gel the sol particles in the ammonia solution,and finally dry and calcine the gel particles to form spherical alumina.This method is a further improvement on the process of the sol lotion gel method.The lotion technology is applied to the aging stage of the sol,and the oil phase is kept still,eliminating the separation of the powder and the oily reagent.