Chemical Dissolution-assisted Ultrafine Grinding For Preparation Of Quasi-spherical Colloids Of Zinc Oxide And Alumina

Zinc oxide（ZnO） and alumina（Al₂O₃） are both important inorganic materials of modern industry.ZnO is a multifunctional material that has unique catalytic,thermoelectric,piezoelectric and photoelectric characteristics.It is widely applied in many fields,such as electronics,biomedical,coating,packing,ceramics and catalysts.Al₂O₃has some superior properties like chemical stability,corrosion resistance,high rigidity,great insulativity,high melting point,high tenacity,etc..As a ceramic engineering material,Al₂O₃is widely used as grinding,polishing,electronic and refractory materials.The modern inorganic oxide powders are required to have ultrafine particle size（submicron-or nano-sized）,narrow size distribution and regular particle morphology（i.e.,spherical or quasi-spherical shape）.This is since ultrafine spherical or quasi-spherical particles have excellent properties of great fluidity,high dispersibility and high stacking density,consequently enhancing the performance of subsequent applications.The methods of preparing ultrafine spherical oxide powders mainly include physical methods（i.e.mechanical grinding,plasma,flame melting）and chemical methods（i.e.chemical vapor condensation,sputtering,precipitation,emulsion,sol-gel,hydrothermal synthesis and spray methods）.Due to the limitation of these methods,it is thus necessary to explore an economic and facile method to prepare submicron-sized spherical oxide particles.The purpose of this dissertation is to investigate a method for the preparation of submicron-sized quasi-spherical ZnO and Al₂O₃particles by wet ultrafine grinding with chemical dissolution assistance bases on a coupling mechanism of chemical dissolution of particles and shear/compressive stresses on particles in stirred media milling.In this dissertation,submicron-sized quasi-spherical ZnO particles were prepared via a vertical high-energy density stirred media mill in different acid or alkali solutions（i.e.,hydrochloric acid,acetic acid,oxalic acid,citric acid,ammonia/ammonium chloride and sodium hydroxide solution）.The effects of solution type,concentration,solid content and grinding time on the particle size/size distribution and sphericity were investigated.The particle size/size distribution were characterized by laser particle size analysis,and the uniformity coefficient of size distribution was calculated by the Rosin-Rammler-Bennett（RRB）model.The morphology of the ground particles was analyzed by a scanning electron microscopy（SEM）,and the particle sphericity was evaluated by a software named Image-Pro Plus.The results show that submicron-sized quasi-spherical ZnO particles with the median size of 370 nm,uniformity coefficient of 2.28 and sphericity of 0.91 can be prepared under optimum condition（i.e.,0.010 mol/L of acetic acid solution,solid content of 20 wt.%and grinding time of 30 min）.Besides,the chemical dissolution mechanism of ZnO in ultrafine grinding process was analyzed by inductively coupled plasma emission spectroscopy（ICP-OES）,X-ray diffraction（XRD）and p H value analysis.It is indicated that the chemical dissolution on ZnO particle ground in acetic acid solution is accelerated due to the mechanochemical effect,thus improving the particle size and sphericity.Also,the breakage mechanism for ultrafine grinding of ZnO particles with and without chemical dissolution assistance was discussed via the selection and breakage functions of population balance modeling.The breakage mechanism of ZnO particles in stirred media mill is mainly due to shearing,and the addition of acetic acid solution can improve the particle size/size distribution.Furthermore,the results of the photocatalytic performance of ZnO particles show that ZnO particles ground in acetic acid solution have an enhanced photocatalytic performance because of the finer particle size and quasi-spherical morphology.In addition,submicron-sized quasi-spherical Al₂O₃particles were also prepared via a vertical high-energy density stirred media mill in different acid or salt solutions（i.e.,hydrochloric acid,sulfuric acid,acetic acid,citric acid,ammonium chloride,sodium fluoride and ammonium fluoride）.The effects of solution type,concentration,solid content and grinding time on the particle size/size distribution and sphericity were investigated.The results show that submicron-sized quasi-spherical Al₂O₃particles with the median size of 333nm,uniformity coefficient of 2.48 and sphericity of 0.88 can be obtained under optimum condition（i.e.,0.010 mol/L of ammonium chloride solution,solid content of 20 wt.%and grinding time of 60 min）.Also,the friction coefficient of Al₂O₃particles before and after grinding in different solutions was analyzed by a friction coefficient analyzer.It is shown that the friction coefficient of Al₂O₃particles ground in ammonium chloride solution is smaller than that of the particles ground in water,indicating that the particles ground in ammonium chloride solution become more regular with the increased sphericity.Besides,the chemical dissolution mechanism of Al₂O₃particles in wet ultrafine grinding was discussed.It is indicated that the addition of the ammonium chloride solution can promote the chemical dissolution due to the mechanochemical effect.In addition,the grinding mechanism of Al₂O₃particles with or without chemical dissolution was also simulated via the selection and breakage functions.The results show that the chemical dissolution effect in ammonium chloride solution in ultrafine grinding can reduce the particle size and improve the size distribution.Finally,the dissertation gave the main conclusions and future work according to the results obtained and the corresponding analysis.