| Summary: | 碩士 === 國立成功大學 === 資源工程學系碩博士班 === 97 === Al composition diffuses into the Y2O3 structure in the reaction process of YAG synthesis which using the Y2O3 and α-Al2O3 powders as raw materials by solid state reaction. The reaction is limited to the diffusion distance while using the micro-scaled raw materials. Thus, temperatures for synthesizing single phase YAG must reach above 1600oC. In general, size reduction of raw materials could increase contact points among reactants and decrease the diffusion distance by which the reaction would be accelerated. In this study, the size ratio (dY/dAl) effect on YAG synthesis process and formation activation energy were observed by changing sizes of Y2O3 and α-Al2O3 respectively. The relative importance of these two components also discussed. Experiments divided into two parts. The part (A) for changing the size of α-Al2O3 used α-Al2O3 of 0.2μm (AS), 0.35μm (AM), and 0.75μm (AL) to mix with Y2O3 of 0.07μm (YS) ,and noted starting powders as YSAS, YSAM, and YSAL respectively. The part (B) for changing the size of Y2O3 used Y2O3 of 0.07μm (YS), 0.4μm (YM), and 2μm (YL) to mix with α-Al2O3 of 0.2μm (AS), then noted starting powders as YSAS, YMAS, and YLAS, respectively. Thus, there were 5 kinds starting powders with different size ratios.
Results in the part (A) showed that contact points of reactants reduced as the size of α-Al2O3 became larger, and the reaction rate become slower. That resulted in the production of YAM phase became fewer at beginning, the maximum yield and disappeared temperatures for YAM and YAP shifted to higher ones, and various sizes of YAG powders because of the prolonged reaction time and extensive reaction temperatures. Furthermore, temperatures for onset of each Y-Al crystal phase and for the single phase of YAG got closer when three samples were compared. YAG formation mechanism all represented a diffusion-controlled one in this part. Since the diffusion distance was not apparently affected by the increasing size of α-Al2O3, activation energies with slight increased values of 343, 393, and 510 kJ/mol were obtained for three samples.
Results in the part (B) showed that the contact points of reactants reduced when the size of Y2O3 became larger, and the reaction rate become slower. It was the same as part (A), but the varied size ratio affected the diffusion distance of reaction. When the diffusion distance increased to micrometer scale, temperatures for synthesizing YAG increased dramatically. The mechanism of YAG formation was still the diffusion-controlled, but the increment of activation energy was more distinct than that in part (A). The activation energy increased from 343 to 547 kJ/mol. The reaction even could not be completed in YLAS sample. Therefore, this study demonstrated that it would be more effective to synthesize pure YAG at low temperatures when decreased the size of Y2O3 than that of α-Al2O3.
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