Investigation Of Microstructure And Sinterability Of YAG Powders Made By The Microwave Method

Transparent Yttrium aluminum garnet (Y3Al5O12, YAG) ceramics have high thermal conductivity, low thermal expansion coefficient, excellent mechanical properties, excellent optical properties. It holds promising potentials as substrate materials for various optical devices.In this dissertation, inexpensive inorganic salts were used as raw materials for the fabrication of transparent YAG ceramics. The low-agglomerated monophase YAG nano-powders were successfully synthesized by the ammonium bicarbonate co-precipitation method and the microwave-assisted homogeneous precipitation (MAHP) method, respectively. YAG transparent ceramics were obtained by vacuum sintering. For the first time, laser output was realized on Nd:YAG transparent ceramics prepared by microwave-assisted homogeneous precipitation.This finding suggests that the MAHP method is promising for the preparation of Nd:YAG transparent ceramics.The aging effects on the properties of the Nd:YAG nano-powders were studied in details, with emphasis on aging condition, aging time, temperature, and humidity.. The optimum condictions for the synthesis of Nd: YAG nano-powders with good sinterability and dispersity were found. The sintered mechanism of YAG ceramic under vacuum was also discussed. The results obtained are shown as follows:1) We report here the synthesis of yttrium aluminum garnet (YAG) nano-powders by the co-precipitation method through dropping Al(NO3)3·9H2O and Y(NO3)3·6H2O solution into ammonium hydrogen carbonate solution. Three factors that affect the synthesis of YAG nano-powders, including the mother salts concentration, the species of organic solvents, and the dropping speeds of the mother salts solution, were analyzed and discussed. The microstructural aspects of the precursors and the YAG nano-powders were characterized and analyzed. Our results show that the dispersion, the composition and the morphology of the precursors and the YAG nano-powders are affected remarkably by these factors.(1) The YAG nano-powders with excellent dispersion can be obtained when the mother salt concentration ([Y3+]=0.08M, [Al3+]=0.13M) is relatively low.(2) Monophase YAG nano-powders can be obtained more easily by adding some ethanol (the optimum volume proportion of the ethanol and the mother salts solution was found to be 1:5) and by high speed dropping the mother salts solution.(3) Transparent ceramic pellet is obtained by a systematic control of the above mentioned factors, which demonstrates that the YAG nano-powders have excellent sinterability.2) The Nd:YAG (Nd:Y3Al5O12) nano-powders were successfully prepared by the microwave-assisted homogeneous precipitation method. The Nd:YAG nano-powders were analysed by means of XRD and SEM, and effects of various factors on the preparation of the Nd:YAG nano-powders were discussed. Our results show that the composition and morphology of the Nd:YAG precursors and Nd:YAG nano-powders are remarkably affected by these factors.(1) Factors that affect the characteristics of the Nd:YAG nano-powders include the concentrations of the mother salt solution, the amount of. precipitant (urea) and dispersant ((NH4)2SO4). The concentrations of the mother salt solution are [Nd3++Y3+]=0.06M, [Al3+]=0.1M, the molar ratio of urea and metal ions is 25:1, and the quality ratio of dispersant ((NH4)2SO4) and nitrate salts in the solution is 8%. These are the optimum values for the preparation of the Nd:YAG nano-powders.(2) Aging condition and aging time remarkably affect the preparation of the Nd:YAG nano-powders. Aging effect on the preparation of the Nd:YAG nano-powders in vessel is better than that in air. It is more beneficial to obtain the Nd:YAG nano-powders with uniform particle size and approximately elliptical shape. However, some materials exchange still happen during the aging course. It indicates that aging effect on the Nd:YAG precursors is sensitive to air. And aging time can significantly affect sizes of the Nd:YAG nano-powders. The sizes of the Nd:YAG nano-powders decrease and then increase with aging time, and the Nd:YAG nano-powders obtained from the precursors aged for 6 days in vessel have the smallest particle sizes.(3) Effects of environmental humidity and temperature on the preparation of the Nd:YAG nano-powders are dependent on each other, which must be properly controlled for the preparation of the Nd:YAG nano-powders. The Nd:YAG nano-powders with good quality can be obtained with humidity of 30%-50% at 25℃.3) The microstructural characteristics of Nd:YAG nano-powders must include as follows:(1) First of all, pure phase Nd:YAG nano-powders must be prepared for the sintered Nd:YAG transparent ceramics.(2) The Nd:YAG nano-powders must have relative stabilities in chemical and physical properties as well as sintering activities to ensure the successful preparation of the Nd:YAG transparent ceramics. Therefore, the Nd:YAG nano-powders with the smallest crystallite size, a uniform crystallite size distribution, and less density defects are needed to be prepared, which are helpful to the sintering of transparent Nd:YAG ceramics.(3) Besides the characteriatics of the Nd:YAG nano-powders mentioned above, the Nd:YAG nano-powders should have uniform micro-components and the proper ratio of(Y3++Nd3+)and Al3+.(4) The laser output (305 mW) with a slop efficiency of 5.1% was realied on Nd:YAG transparent ceramic obtained from the Nd:YAG nano-powders sintered at 1730℃under vacuum for 10 h..In summary, all the above mentioned factors of the Nd:YAG nano-powders should be carefully and strictly controlled for the fabrication of transparent Nd:YAG ceramics in order to obtain high performance Nd: YAG laser ceramics.