| The Y3Al5O12 (YAG) is a promising ceramic applied as high-thermal structural materials, reinforcements in ceramic matrix composites and a host for rare-earth based phosphor, for its outstanding chemical stability, mechanical and optical properties. Single crystal of neodymium-doped YAG prepared by Czochralski technique has been most widely used as a four-level solid-state laser host. In 1995, A.Ikesue et al demonstrated the feasibility to fabricate polycrystalline Nd:YAG transparent ceramics with optical quality and lasing efficiency parallel to Nd:YAG single crystal. Moreover, YAG ceramics have advantages as such:ease of fabrication, low manufacturing cost, no strict limitation in size and shape, high doping concentration of activator ions, and multi-functionality of the resultant ceramics. Owing to such potential application of YAG ceramics, the researches related with synthesis of YAG powders and sintering of YAG transparent ceramics become a hot-issue.High-thermal solid-reaction between Y and Al oxides to synthesize YAG powders is traditionally adopted for a long time. However, the process requires a high temperature up to 1600℃to eliminate the intermediate phase (YAM and YAP) and the as-prepared YAG powders usually has a poor control of particle size and make a dismal performance in sintering process. In this work, the chemical additives of AIF3 and NH4F, often used as mineralizer, were introduced as one of the raw materials to mix with AlOOH, respectively. AlOOH is also a active intermediate product occurring at the transformation form Al(OH)3 of to stableα-Al2O3. An amorphous mixture of fine particle size by ball-crushing of certain amount AIF3 and AlOOH was substituted for conventionalα-Al2O3 as the starting materials, aiming for reduce the crystallization temperature of YAG. The specific routes for ball-crushing technique such as dispersant portion and ball-crushing duration were determined by particle size characterization of precursor powders; After the heat-treatment of precursor, the as-calcined powders were characterized by XRD analysis to detect the phase composition, form which we determined the ideal amount of mineralizers; The SEM was performed to analyze the microstructure of YAG powders. From the TG-DSC, XRD, SEM and Particle size analysis results, we can conclude the effect of the each process and finally determine a proper way to synthesize pure YAG powders.The results show that, mono-phase YAG powder can be obtained after a ball-crushing process of the mixture between amorphous Al compound and Y2O3 with certain amount mineralizer at 1200℃for 3 hours'heat-treatment. Such a technique reduce the YAG crystallization temperature by approximate 400℃by using solid-reation method.The sintering of YAG polycrystalline ceramic was also carried out by the as-synthesized YAG powders at different temperature and duration.Intact garnet crystals of YAG were spotted onto the fracture of the greenbody treated at 1550℃by solid-reative sintering, while the transluecent effect of YAG ceramic was begun at 1550℃by liquid-phase sintering. Throughout the characterization of as-sintered ceramics by Archimedes measurement and SEM analysis, we determined the thermodynamic mechanism of YAG ceramic-sintering as follows:the growth of YAG crystallites dominates the initial stage(below 1400℃); the overwhelming majority of densification was performed in middle stage (between 1400℃and 1550℃) by surface diffusion and boundary diffusion process and the apparent activation energy was also determined; Over 1550℃, the grainboundary growth dominates the final stage by evaporation and condensation process, the mechanism of grainboundary growth correspond well to the normal isothermal dynamic relationship, which was proved to be beneficient to the transparency of the ceramics. |
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