Preparation And Properties Of Monodispersed Spherical Al₂O₃ And YAG Powders

The requirements of powders for high performance ceramics are small particle size,high purity and good dispersion.Spherical powders are considered to be favorable for preparation of high performance ceramics.The synthesis of monodispersed spherical alumina powders and yttrium aluminum garnet(YAG)powders were investigated,and the applications of spherical YAG powders in transparent ceramics and fluorescent materials were also discussed.The results are as follows:Mono-dispersed spherical alumina powders were prepared by the homogeneous precipitation method.It was found that using aluminum nitrate or aluminum chloride as the source of aluminum ions,uniform spherical precursors can be produced when sulfate ions were added.Particle size and agglomeration state are directly related to the molar ratio of ammonium sulfate to aluminum ion concentration(R value).The smaller the R value,the smaller the particle size.The particle size of the precursors can be adjusted from 200 to 800 nm by changing the R value.Monodispersed spherical alumina particles can be prepared only when the aluminum ion concentration is low(<0.006mol/L).Increasing reactant concentration will lead to agglomeration and irregular precursors.Morphology of the precursor can be regulated by organic dispersants.The spherical precursor transformed from amorphous into pure alpha alumina phase after calcining at 1100?.Mono-dispersed spherical YAG,Nd:YAG and Yb:YAG powders can be prepared by homogeneous precipitation.Spherical precursors can be prepared from yttrium nitrate and aluminum nitrate system with some amount of ammonium sulfate,but the surface of the spheres is not smooth and there is some degree of agglomeration.The addition of ammonium sulfate increased calcinations temperature from 1000? to 1100? in order to get pure YAG phase.Monodispersed YAG precursors can be prepared from yttrium nitrate,together with the mixture of aluminum nitrate and aluminum ammonium sulfate.With the cation concentration of 0.008mol/L,urea concentration of 0.16mol/L and the molar ratio of aluminum nitrate to aluminum ammonium sulfate(K value)of 1:1,monodispersed spherical powders were obtained with narrow particle size distribution.The average particle size of the precursors is 800 nm,and that of the fired powder is about 550 nm.The morphology,particle size and dispersion state of the YAG precursors depend on cation concentration,the amount of urea,reaction temperature,and sulfate ion concentration.The YAG particle size decreases with increasing K value.K reasonable value is from 0.7 to 1.25.The precipitation mechanism was confirmed to be a differential precipitation process,i.e.,aluminum ions precipitated firstly into approximately monodispersed spheres,followed by yttrium ion precipitation onto the surface of the existing spheres.Doping with certain amount of Yb and Nd still resulted in spherical mono-dispersed powders.After calcination at 1100?,the precursor turned into pure spherical YAG particlesYAG,Nd:YAG and Yb:YAG transparent ceramics can be prepared from homogeneous precipitated spherical powders by vacuum sintering.Ball milling was not used for the dispersion of the powders in order to avoid the introduction of impurities.When using TEOS as the sintering aids,the optimum amount is 0.6-0.7wt%.It was found that highest transmittance of the YAG transparent ceramics was achieved when the stoichiometric ratio of Y to A1 was 3:5.006.Monodispersed spherical powders are advantages for making close-packed structure,so centrifugal molding can be used in the preparation of YAG transparent ceramics.It was found that 1 wt%ammonium polyacrylate or 1 wt%sodium hexametaphosphate respectively with pH value close to 9 leads to best stability of the slurry.Close-packed structure was obtained after natural sedimentation for 60 days.The YAG greenbody after centrifugal molding reach close-packed state in some degree,and YAG transparent ceramics can be prepared after vacuum sintering.Monodispersed spherical YAG:Ce3+ phosphors can be prepared by homogeneous precipitation.The excitation spectra have two strong absorption bands in the vicinity of the 344 nm and 454 nm,and the emission spectra show a broad peak near 530nm.The luminescence intensity changes with varying K value(aluminum nitrate to ammonium aluminum sulfate molar ratio).When K is smaller than 1,the luminescence intensity of the phosphors increases significantly with increasing calcination temperature.When K is larger than 1,the increase in emission intensity is not so significant.Therefore,coarse particles with good crystallinity leads to better luminescence properties.The luminescence intensity of the YAG:Ce3+ phosphor powders increases first and then decreases with the increase of Ce3+ concentration.The maximum intensity appeared at x=0.045,when Ce3+substitute 1.5 at%of Y3+.Further increase of the amount of Ce3+ will lead to concentration quenching.The emission peak shows red shift with the increase of the amount of Ce3+.Treatment in weak reducing atmosphere does not have significant effect on the photoluminescence intensity.