Combusion Synthesis Process And Crystallization Behavior Of Al₂O₃/Y₂O₃ Metastable State Powder

Several compounds in Al₂O₃/Y₂O₃ binary system have attracted the attention of researchers for their great application value in laser field,among which Yttrium Aluminium Garnet(YAG,Y₃Al₅O₁₂)has been the most widely studied and applied.YAG’s excellent physical and chemical properties and cubic crystal structure make it an ideal optical functional material.In this paper,Al₂O₃/Y₂O₃ metastable amorphous powder was innovatively prepared by combustion synthesis with water atomization method,and the amorphous crystallization behavior of the powder was investigated,and then the feasibility of combustion synthesis with water atomization method combined with amorphous crystallization to prepare YAG（phosphor）powder was evaluated.In this paper,the relationships among the target product composition,the ratio of raw materials and the adiabatic temperature in the combustion system were firstly calculated,and the combustion synthesis system was designed according to the calcu-lation results.Several groups of metastable powder with different components were prepared by the combustion synthesis water atomization method.The phase and microstructure of the prepared metastable powder were characterized.The main crystal phase of each group of powders is amorphous,andγ-Al₂O₃ was formed by liquid phase Al₂O₃ fast cooling exists in some powders.The metastable powder has high sphericity,good dispersibility.The particle size of the powder is distributed within several to dozens of microns and the composition is uniform without chemical segregation.During the heating process,metastable powder will transition to thermodynamic stable state.The crystallization process of YAG occurs in the range of 900-1100℃.When the temperature is between 1100-1200℃,γ-Al₂O₃ transforms toα-Al₂O₃.The crystal-lization process of YAG can be divided into two types,one is direct crystallization,the other is phased crystallization.In order to explore the crystallization process of metastable amorphous powder,heat treatment was carried out at different temperatures and atmospheres.When the metastable powder is completely crystallized,the phase will be transformed to the thermodynamic stable phase YAG and the corresponding componentα-Al₂O₃.For the powder near the eutectic point,the YAH intermediate phase is precipitated firstly during the crystallization process,and then the YAH metastable phase is rapidly transformed into YAG.For the system whose component is far away from eutectic point,metastable powder is generally crystallized into YAG crystal phase directly from amorphous state.When the powder contains amorphous Al₂O₃,the amorphous Al₂O₃ will be crystallized toγ-Al₂O₃ at low temperature,and thenγ-Al₂O₃ will be converted toα-Al₂O₃ at 1100-1200℃.The transition from YAH to YAG will be inhibited during the heat treatment in the reducing atmosphere,but heat treatment atmosphere have little influence on the direct crystallization of amorphous YAG and the transition of Al₂O₃ crystal.When metastable powder is heat-treated at 1100-1200℃,Ce element will be precipitated from YAG lattice in the form of Ce O₂.YAG based ceramics were prepared by vacuum hot-pressed sintering of metastable amorphous powder.In the sintering process of metastable powder,amorphous Al₂O₃ formsγ-Al₂O₃ in many places of the powder particle by"nucleating and growing",andγ-Al₂O₃ transforms toγ-Al₂O₃ at high temperature,which is distributed in YAG matrix in bars and irregular blocks.In addition,in the high-temperature sintering process,part of Ce O₂ reacts withγ-Al₂O₃ to form Ce Al O₃,and the remaining Ce O₂ dissolves in the YAG lattice.After annealing in air,YAG lattice constant increases,and Ce Al O₃ decomposes into Al₂O₃ and Ce O₂.The YAG:Ce³⁺phosphor prepared by combustion synthesis with water atomi-zation method combined with amorphous crystallization has two excitation peaks at340nm and 460nm,corresponding to ²F_5/2→5D and ²F_7/2→5D electron transitions of Ce³⁺respectively.When the excitation wavelength is 460nm,the emission range of phosphor powder is between 500 nm and 700nm.Compared with the fluorescence spectra before crystallization,the luminescence intensity of phosphor powder is greatly improved,and the fluorescence spectra is closer to the red direction.