Preparation And Properties Of Yttrium Aluminium Garnet Powders And Transparent Ceramics

Yttrium aluminium garnet?YAG?,having the cubic garnet structure,is best known as an important laser-based and phosphor-based material due to its excellent optical,mechanical and thermal properties,and high transparency in the infrared and visible wave band.Yb:YAG transparent ceramic has great potential to replace the Yb:YAG single crystal and glass laser materials.YAG:Ce has been widely used in white LED in recent years,which has excellent yellow fluorescent emission and can be combined with the blue light from the InGaN.For the preparation of either high optical performance Yb:YAG ceramics or YAG:Ce fluorescent materials,the powder properties directly affect the quality of the final products.The present study was intended to develop novel techniques for preparation of high performance YAG powders,aiming at a precise control of particle size,dispersion and sintering activity.Based on the synthesized powders,the preparation of Yb:YAG transparent ceramics and fluorescence properties of YAG:Ce phosphors were investigated.The main results are as follows:?1?A stearate melting method was first introduced to synthesize YAG powder.Compared with other wet chemical methods,the stearate melting method has various advantages.First,the melting points of yttrium stearate and aluminum tristearate are low and similar.It can guarantee homogeneous mixing of cations at atomic level through direct stirring of the mixed melts.Second,absorption of water is not observed on the surface of yttrium stearate and aluminum tristearate,avoiding the particle aggregation due to surface tension and capillarity force between particles.Furthermore,yttrium stearate and aluminum tristearate can be directly weighed without considering the absorbed water and crystalline water,guaranteeing an accurate control of cationic stoichiometry.Well dispersed YAG nanopowders with narrow particle size distribution were produced by the stearate melting method.Pure YAG phase can be obtained at a temperature as low as 750?,without YAM and YAP transition phases.Transparent ceramics can be obtained after powder compaction and vacuum sintering.The YAG ceramic exhibits uniform grain size distribution and nearly pore-free micro structure.In addition,YbrYAG nanopowder was produced through stearate melting method,and the powder was sintered into transparent ceramic by vacuum sintering.The microstructure is similar to that of the YAG ceramic.?2?[(Y_1-xLu_x)_1-yCe_y]₃Al₅O₁₂?x=0-0.5,y=0.005-0.03?garnet powder was prepared by the stearate melting method.The results show that phase-pure garnet can be obtained at a relatively low temperature of 800 ?.The YAG:Ce,?Y,Lu?AG:Ce phosphor powders are well dispersed and uniform in size.The quenching concentration of Ce³⁺ and quenching mechanism were studied through changing the Ce³⁺ concentration.PL band of the phosphor exhibits red shifts with increasing Ce³⁺ content.While the strongest excitation and emission bands are blue shift with increasing Lu³⁺ doping.The fluorescent lifetime of the YAG:Ce powder gradually reduces with increasing Ce³⁺ content or calcination temperature.The fluorescent lifetime of the?Y,Lu?AG:Ce powder has a weak downtrend with increasing Lu concentration.The(Y_0.985Ce_0.15)AG phophors calcined at 1300 ? has the best powder properties considering practical application.?3?Monodispersed spherical Y₂O₃ powder was synthesized by a homogeneous precipitation method.Using this spherical Y₂O₃ powder,together with the commercial ultrafine Al₂O₃ powder and Yb₂O₃ powder,(Y_1-xYb_x)AG?x=0,0.02,0.06,0.10 and 0.15?transparent ceramics were obtained by the solid-state reaction method.Compared with the traditional solid-state method,the present method is advantageous in mixing homogeneity and reduction of formation temperature of YAG phase.The(Y_1-xYb_x)AG ceramic samples have similar linear shrinkage rate.Transparent ceramics,can be obtained by vacuum sintering at 1700 ?,and the maximum transmittance exceeded 80%.Due to the small radius of Yb³⁺ ion,it is easy to enter into the Y³⁺ position of the YAG crystal lattice,and no segregation of Yb³⁺at the grain boundary was observed.Monodispersed spherical Al₂O₃ was produced,and was used with spherical Y₂O₃ powder to fabricate(Y_1-xYb_x)AG ceramic samples.Transparent ceramics can be obtained by vacuum sintering at 1700 ?,and the maximum transmittance is close to 80%.?4?A homogeneous co-precipitation method was proposed to synthesize YAG powders using urea as the precipitant.The method has the advantages of both the homogeneous precipitation method and the co-precipitation method.The co-precipitation of Y³⁺ and Al³⁺was achieved,and pure YAG phase can be obtained at a comparatively low temperature of 900?.The addition of ammonium sulfate plays a positive role in the dispersion of the precursor and calcined powders,inhibiting fast growth of the crystallite grains.Transparent YAG ceramic can be obtained through vacuum sintering at 1700?.In addition,(Y_1-xCe_x)AG?x=0.005-0.03?powders were prepared by the homogeneous co-precipitation method,and the powders are well dispersed and homogeneous in particle size distribution.The fluorescence properties of the powder,including fluorescence lifetime and fluorescence mechanism,were studied.(Y_0.995Ce_0.005)AG transparent ceramic can be obtained through vacuum sintering at 1700?.