| Yttrium aluminum garnet (Y3Al5O12, YAG) is an important laser material due to its excellent optical properties. Comparing with single crystal YAG, polycrystalline YAG ceramic exhibits many advantages. The ability of polycrystalline YAG ceramic incorporated with very high concentrations of Nd provides the possibility of high power laser. The first transparent Nd: YAG ceramic with nearly equivalent refractive index and thermal conductivity with that of a YAG single crystal was reported in 1995 by Ikesue et al, who demonstrated the potential of substituting YAG single crystal with polycrystalline YAG ceramic if proper synthesis methods are adopted. Although polycrystalline Nd:YAG laser ceramic has been produced and 67kW continuous wave laser operation was obtained in America, the fundamental relationship among the sintering process, microstructure and the optical properties of transparent laser ceramics has not been systematically studied yet.In this dissertation, YAG nanopowders were synthesized by a modified co-precipitation technique, and the fabrication of transparent YAG ceramics using the as synthesized nanopowders was investigated. A hydrate precursor of yttrium aluminum garnet (YAG) was synthesized by the co-precipitation method, in which a distillation process was used with n-amyl alcohol as dehydration solvent. By preventing the connection among particles, hard agglomerates were avoided and well dispersed nanopowders were obtained. The average particle size of the nanocrystalline YAG powders calcined at 1100℃was about 50nm. The sintering regression equation and the kinetic equation for the densification of YAG ceramic were obtained by manipulate the densification behavior and the grain growth behavior of YAG compacts during the sintering process. The kinetic equation revealed that both the sintering mechanism and the grain growth of YAG ceramic were dominated by grain boundary diffusion. Transparent YAG ceramics were prepared by vacuum sintering at 1750℃for 5~18hours. The transmittance of the YAG ceramics heated at 1450℃for 2 hours under flowing air reached 54~79.7% at a wavelength of 800nm, which was 19~32% higher than that without the heat-treatment.A two-step sintering method was used to fabricate transparent YAG ceramics, the YAG pellets were rapidly heated to a peak temperature (1750℃or 1800℃) without holding time, and then immediately cooled down to a lower temperature (1550℃or 1600℃) and held for 10 hours. The maximum transmittance of the YAG ceramics by the two step sintering method can reach 74%. When the sample was sintered at 1800℃as the first sintering step and held at 1550℃for 10 hours as the second step, the grain-boundary migration was effectively suppressed while the grain-boundary diffusion was enhanced. The mean grain size of the densified YAG ceramic was 6μm and the transmittance was 71%.Translucent YAG ceramics were fabricated by reactive hot pressure sintering at 1650℃for 90 minutes using micro-sized Al2O3 and Y2O3 powders as reactants. The grain size of the hot- pressed YAG ceramic is much smaller than that of the samples prepared by vacuum sintering. This work reveals that YAG nanopowder is at least not the exclusive starting material for preparing transparent YAG ceramic. In contrast, the final microstructure is the most important factor to determine the transmittance of the YAG ceramic.
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