| The preparation of spherical silica powder, subsphaeroidal Al2O3-coated SiO2composite powder and its sintering have been studied in this paper.1) Silica powder was prepared by a sol-gel spray drying process usingNa2SiO39H2O and sulfuric acid as raw materials. The chemical agents andtechnical parameters used are: the concentration of Na2SiO3solution is8%, theconcentration of sulfuric acid solution is1.0mol/L, stirring rate is500r min-1andsurfactant is2%PEG6000. The results of XRD, SEM and particle size analysesshowed that silica prepared by sol-gel spray drying process is in amorphous state,and smooth spherical shape with an average grain diameter of6.3μm;2) Silica prepared in1) was further coated with Al2O3by a heterogeneousnucleation process using Al（NO3）39H2O as raw material and acetic acid-sodiumacetate buffer solution as precipitating agent, to form Al2O3-coated SiO2compositepowder. Theoretical analysis and experimental study showed that: when the pH ofbuffered solution is controlled at about5and a concentration of0.1mol/L Al3+isused, the surface of silica can be coated with a layer of graininess or schistoseamorphous Al（OH）3. The coated Al（OH）3was transformed into amorphous Al2O3incompacted grains completely after heat treated at800℃and5hours. The sampleswere still composed of the dispersed composite particles and no clear sinteringphenomenon was observed.3) The Al2O3-coated SiO2composite powder was calcined at differenttemperatures to study its structures. The results showed that the crystallisationtransformation occured at1100℃with clear observation of mullite phase.Crystalline SiO2peaks were found at1200℃for5h calcinations. When thetemperature reached1400℃, the mullite transformation process was basicallycompleted and mullite-coated crystalline silica composite powder was formed.However, the spherical degree and dispersity of the composite powder deterioratedwith the increase calcining temperature. |
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