Two-step Sintering Process for Lutetium Oxide Transparent Ceramics
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Two-step Sintering Process for Lutetium Oxide Transparent Ceramics Xiaomei Guo*, Kewen K. Li, Yanyun Wang, Yingyin K. Zou and Hua Jiang Boston Applied Technologies Inc., Woburn, MA ABSTRACT During a two-step sintering practice, important factors such as final grain sizes and residual pore status can be controlled through adjusting the first and second step sintering temperatures and durations. Moreover, the sintering temperatures (both the first and the second step) can be several hundred degrees lower than those in a traditional sintering process to obtain fully dense ceramics. Therefore, it is a potentially cost-effective preparation procedure for ceramics with fine grains. In this work, we successfully demonstrated the synthesis of aggregate-free sesquioxide nanometer-sized powders with a narrow size distribution through a modified chemical co-precipitation process. Subsequently, ytterbium-doped Lu2O3 ceramics of near full density were obtained through a two-step sintering process. INTRODUCTION In recent years, there has been an increasing interest in developing polycrystalline dielectric ceramics with full density due to their superior performance compared to single crystals in many applications such as high power laser systems [1], scintillators, and missile domes, etc.. Yb-doped crystalline sesquioxides of Yb:Y2O3, Yb:Sc2O3, and Yb:Lu2O3 are very attractive gain media for high power lasers due to their excellent thermal properties [2]. In the case of Lu2O3 single crystal, the thermal conductivity was reported to be 12.5 W/mK [3]. In Yb-doped sesquioxides, though the thermal conductivity depends on the concentration of the Yb3+ ions, they are in general higher than that of Yb:YAG with the same doping concentration. Among them, Yb:Lu2O3 single crystal has the highest thermal conductivity of 11 W/mK with a 2.7 at.% Ybdoping concentration, while that of Yb:YAG at the same doping level is about 6.8 W/mK. While sesquioxides are promising high power laser host materials, they generally have high melting points of more than 2400°C. It is therefore extremely difficult to grow large-size single crystals with good optical quality. By using ceramic fabrication technology, it is possible to obtain transparent optical sesquioxides ceramic laser hosts with large sizes and complex structures. One of the crucial factors in densification of ceramics is the suppression of the final-stage grain growth, which is directly related to undesired pores. Instead of a “brute force” approach of high pressure consolidation at elevated temperatures employed in a conventional transparent ceramics process [4-6], Chen and Wang reported a two-step sintering mechanism for full density nano-ceramics fabrication of Y2O3 [7]. Fully dense *
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cubic Y2O3 (melting point 2,439ºC) with a grain size of 60 nm was prepared by a simple two-step sintering method, at temperatures of about 1,300ºC without any applied pressure. In a two-step sintering process, the powder compact is first heated to a higher t
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