Study On Process Factors For Preparation Of High Dispersion Spherical α-Al₂O₃ Powders By Microwave-hydrothermal Method

Sphericalα-Al₂O₃ powders have characteristic of high insulation,corrosion resistance,high temperature resistance and wear resistance.Due to its special axisymmetric structure,they have the advantages of smaller specific surface area,larger bulk density,better fluidity and higher thermal conductivity compared to other morphology Al₂O₃ powders.Therefore,they can be widely used in the field of high-performance functional and structural ceramics,and are still a research hotspot at home and abroad.At present,preparation of sphericalα-Al₂O₃ powders generally have problems of expensive raw materials and equipment,complicated process flow,unadjustable particle size and easy agglomeration of powders,which limits their production and application.In this paper,conventional raw materials,hydrothermal method,and microwave technology are used to prepare high-dispersion sphericalα-Al₂O₃ powders with good performance at low cost.Main research contents and conclusions of full text are as follows:（1）Firstly,using Al₂（SO₄）₃·18H₂O as raw material and CO（NH₂）₂ as precipitant,precursors were synthesized by hydrothermal method,and sphericalα-Al₂O₃ powders were prepared after calcination in muffle furnace.Using DSC-TG,FT-IR,XRD,SEM,BET,laser particle size analysis and other characterization methods,influence of process factors such as raw material ratio,hydrothermal temperature,filling degree and calcination temperature on properties of thermal reaction process,chemical composition,phase composition,micro-morphology,specific surface area and particle size of precursors and calcined products was studied.The results show that under action of hydrothermal high temperature and high pressure,OH^-obtained by the hydrolysis of urea combines with Al³⁺,then uniformly nucleates and grows isotropically,and finally forms hydrated Al₂O₃ precursors containing SO₄^2-with spherical morphology.Among them,SO₄^2-promotes formation of precursors.Hydrogen heating at 120℃for2 h can obtain precursor particles with good sphericity,narrow particle size distribution range,particle size of 2μm,and specific surface area of 7.0161 m²/g.If hydrothermal temperature is too high,probability of collision between particles will increase,which will lead to serious agglomeration of precursors;when it is too low,urea will not undergo hydrolysis reaction and precursors cannot be obtained.When precursors are calcined,they mainly undergo processes such as dehydration,desulfurization,and crystal form transformation.Transformation process is from amorphous toγphase,and then toαphase.Calcination at 1100℃for 2 h can obtain sphericalα-Al₂O₃ powders.Particle size of sphericalα-Al₂O₃ powders can be adjusted between 1～5μm through change of raw material ratio and filling factor,and corresponding specific surface area is in the range of 6～15 m²/g.（2）Secondly,precursors were synthesized by above hydrothermal method and further calcined by microwave to obtain sphericalα-Al₂O₃ powders（Microwave frequency:2.45 GHz）.Effects of microwave calcination temperature,heating rate and holding time on powders’phase composition and micro-morphology were investigated.The research shows that,compared with traditional muffle furnace calcination,heating rate of microwave calcination can be increased to 20℃/min,which is three times heating rate of muffle furnace,and holding time is also shortened by 1/4,which only takes 90 minutes.Precursors were calcined by microwave at 1100℃to obtainα-Al₂O₃powders with particle size of 1μm and good sphericity and uniformity.Dispersion of powders were also improved compared with muffle furnace calcination.This is because microwave calcination uses coupling between precursors and microwave to achieve rapid and uniform heating of whole,eliminating effect of temperature gradient on performance of powders.Therefore,microwave heating efficiency is higher than traditional muffle furnace,and powder performance is also greatly improved.（3）Finally,using Al₂（SO₄）₃·18H₂O and CO（NH₂）₂ as raw materials,microwave-hydrothermal method was used to synthesize precursors,and microwave-calcined to prepare sphericalα-Al₂O₃ powders.Effects of hydrothermal temperature,reaction time,pressure and surfactant addition on phase composition and micro-morphology of precursors and calcined products were studied.The results show that use of microwave hydrothermal can promote hydrolysis reaction of urea.At 100℃hydrothermal for 30min,spherical amorphous hydrated Al₂O₃ precursors containing SO₄^2-are formed,which is 20℃lower than traditional hydrothermal process.However,when hydrothermal temperature is too high,precursors with a cluster-like boehmite structure are formed.Reaction time is only1/4 of traditional hydrothermal method.If reaction time is too long,boehmite will continue to be generated on spherical surface,destroying spherical structure of precursors and reducing dispersion.In microwave-hydrothermal process,pressure in reactors should be between 0.2～0.4 MPa.A moderate increase in pressure will increase number of uniform nucleation at the same time,thereby making products more uniform and fine.Obtained spherical precursors have particle size of about 1μm,with good dispersion and uniformity.If pressure is too small,growth rate of crystal nucleus will be different,and particle size distribution will become larger.If it is too large,collision probability between particles will increase,then serious agglomeration will occur and affect the spherical morphology.Adding an appropriate amount of PEG as a surfactant can play a steric hindrance role,which significantly improves dispersion of powders.Too much dispersant will lead to entanglement of polymer chains,and hinder isotropic growth of crystal nuclei,thereby destroying spherical morphology of precursors.