The Preparation Of Ultrafine-La₂O₃by Solid State Reaction With Mechanical Ball Milling

As a typical rare earth oxide, La₂O₃has some excellent physical and chemical properties,such as high chemical stability, high thermal stability, so it has been widely applied in thepurification of automobile exhaust gas, oil cracking, optical glass, thermoelectric materialsand ceramic glazes and so on.By consulting literature, this paper expounds the preparation of the ultrafine La2O3. Thepreparation methods have been compared, including the precipitation method, sol-gel method,hydrothermal method, polyethylene glycol method and micro emulsion method. The mainapplications of ultra-fine La₂O₃have been described. To overcome the limitation and theoperation difficulty existed in the above processes, combined with mechanical milling, thesolid phase reaction method has been used to prepare ultrafine powder of La₂O₃with regularshape and uniform particle size. The TG-DSC, XRD and SEM were adopted to detect thethermal decomposition temperature of the precursor, phase constituent, and the morphologyof the product. The effects of milling process parameters （the ratio of ball to material, millingtime） and roasting conditions （roasting time and roasting temperature） on the sample phaseconstituent and SEM morphology have been investigated. And the solid phase reactionmechanism of the mechanical ball milling has been discussed.Mechanical ball milling can provide strong shearing force, which can increase thechances of the particle collision, crush and disperse particles, improve the nucleation rate,obtain small particles. The ratio of ball to material, milling time and the dispersantpolyethylene glycol, will affect the sizes and morphology of the precursor particles. Based onthe compare experiments under different ratio of ball to material and different milling timewith0.5%polyethylene glycol, the precursor of lanthanum oxalate has been prepared by theoptimal ratio of ball to material5:1and milling time6hours, which has an average particlesize of about400nm and better dispersion.When the roasting temperature is between in600℃and700℃, intermediate productLa₂O₂CO₃is obtained. At750℃, intermediate productLa₂O₂CO₃andLa₂O₃is obtained.When the heating temperature is more than800℃, relatively pure powder can be obtained.With the increasing of heating temperature and time, the crack between the particles areincreased, diffusion become easy, powder shape tends to be spherical, but the sizes graduallygrow up. Heated at800℃for2h,La₂O₃powder is in a long strip, the average particle size isabout120nm; for3h,, the powder is similar to a spherical, the average particle size isincreased to about150nm, but the crack between particles is not sufficient. Heated at900℃ for2h, the particle is almost completely cracked, the shape is spherical approximately, withclear contour, uniform size, and about180nm of the average size.Using lanthanum chloride with seven water and oxalic acid as raw materials, withmechanical ball milling, under appropriate ball milling and baking process conditions, bysolid state reaction method, ultrafine powders are prepared. The optimized process conditionsare: with0.5%dispersing agent polyethylene glycol, the ratio of ball to material of5:1, ballmilling time of6hours, heated at900℃for2h,the ultrafine and uniformLa₂O₃powdersare prepared, and the average particle size is about180nm.