硅酸盐学报

以商业高纯Al_2O₃粉和自制纳米Y_2O₃粉为原料，采用真空固相反应烧结法成功制备出了高透光的钇铝石榴石(YAG)透明陶瓷。研究了反应烧结过程中物相演变和显微结构变化，烧结温度和保温时间对YAG透明陶瓷光学透过率的影响以及YAG透明陶瓷在深海装备领域的应用性能等。结果表明：高温条件下，Al_2O₃和Y_2O₃反应先依次形成中间相YAM和YAP，最终在1 500℃完全形成YAG；随着烧结温度的升高，YAG陶瓷内气孔不断排出且晶粒不断长大，光学透过率先增加后降低，在1 780℃透过率达到最高；随着保温时间的延长，YAG陶瓷的透过率逐渐提高，但超过6 h后，再延长保温时间，对提高透过率的效果不显著。采用优化工艺制备的YAG透明陶瓷球罩经受住127 MPa抗压模拟试验，并成功应用在深海领域装备大功率LED照明灯和摄像机保护罩上，为拍摄“奋斗者”号万米海底作业以及央视现场直播做出了贡献。

Introduction Deep sea equipment is an important guarantee for the exploration, development, and protection of deep-sea resources, including various types of deep-sea submersibles, landers, gliders, and their auxiliary equipment. In the deep-sea equipment above, the transparent protective cover for observation windows, cameras, and lighting must use high-strength transparent materials. The main transparent materials currently used are ultra-thick organic glass(observation window), inorganic glass, and sapphire(single crystal aluminum oxide). Their application in deep-sea equipment is possible due to the superior transparency of yttrium aluminum garnet(YAG) transparent ceramics like single crystals, as well as the characteristics of optical isotropy, high strength, high thermal conductivity, and stable physical and chemical properties. At present, there are many kinds of transparent ceramics, among which YAG transparent ceramics are the most representative. The existing researches on YAG transparent ceramics mainly focus on doping rare-earth elements to achieve optical functional applications, such as laser output, white LED luminescence, etc.. There are a few reports on the research of large-sized and complex shaped YAG transparent ceramics. In this paper, large-sized and complex shaped YAG transparent ceramics were prepared by a solid-state reaction sintering method, which could be applied to deep-sea equipment at 10 000 meter. Methods High-purity powders of α-Al_2O₃(99.99% purity, Taimei Co., Japan), Y_2O₃(self-synthesized by co-precipitation method) were used as starting materials. The starting powders were weighed according to a stoichiometric ratio of YAG, and then mixed with 0.5% tetraethyl orthosilicate(TEOS, 99.99%) and 0.05% MgO powder(analytical pure) as a sintering additive in ethanol by ball milling for 12 h. The mixtures were dried in an oven at 60 ℃ for 24 h and then sieved through 200-mesh screen. A portion of the dried powder was calcined at 900–1 500 ℃, and another part of the powder was pressed into circular discs with a diameter of 20 mm using a steel mold at 30 MPa, and then subjected to cold isostatic pressing at 200 MPa to obtain ceramic billets. The green body pellets were then sintered in a vacuum furnace with tungsten meshes as the heating elements under 10-3 Pa at 1 200–1 800 ℃ for 2–20 h. After sintering in vacuum, the pellets were further annealed in air at 1 400 ℃ for 10 h. Finally, the both surfaces of the as-prepared ceramic samples were mirror-polished with different grade of diamond slurries. The phase compositions of all the samples were qualitatively identified by a model D/Max-2550V X-ray diffractometer(RigakuCo., Japan). The optical transmittances of all the samples of ceramics were measured by a model Carry 5000 UV-VIS-NIR spectrometer(Varian Co., USA). The surface microstructures of each group of samples after hot corrosion were characterized by a model JSM-6360 scanning electron microscope(JEOL Co., Japan). Results and discussion YAG transparent ceramics were fabricated by a solid-phase reaction sintering method. The phase evolution and microstructural changes during sintering were investigated. At high temperatures, Al_2O₃ and Y_2O₃ react to form intermediate phases YAM and YAP, eventually forming YAG. The effects of sintering temperature and holding time on the optical transmittance of YAG transparent ceramics were investigated. As the sintering temperature increases, the pores in YAG ceramics continue to discharge and the grains continue to grow. The optical transmittance firstly increases and then decreases, reaching its maximum value at 1 780 ℃. As the holding time prolongs, the transmittance of YAG ceramics gradually increases, but after more than 6 h, extending the holding time does not have a significant effect on improving the transmittance. The YAG ceramic dome is formed using a specially designed rubber mold and a stainless steel hemispherical mold by a cold isostatically press. The YAG ceramic dome is then vacuum sintered in the optimized process. The optical window under 127 MPa was analyzed by finite element analysis(i.e., a software named ANSYS). The analysis results show that the maximum compressive stress of the 120 mm diameter dome is 773.7 MPa and the maximum tensile stress is 21.9 MPa, both of which are less than the maximum allowable stress of YAG transparent ceramic material(i.e., approximately 1 000 MPa). The YAG transparent ceramic dome and titanium alloy cylinder are sealed and packaged. The transparent ceramic dome is undamaged without cracking or leakage at 127 MPa for 1.5 h. The high-power deep-sea LED lighting and deep-sea camera, using YAG transparent ceramics as protective covers, are installed on a deep-sea equipment “Canghai” lander. In November 2020, the lander sank to the depths of 10 000 meters in the Mariana Trench(the deepest part of the oceans) for several times, and a large amount of valuable video data were recorded. Conclusions Highly transparent YAG transparent ceramics were fabricated in vacuum via solid-state reaction sintering. Under high temperature conditions, Al_2O₃ and Y_2O₃ reacted to form intermediate phases YAM and YAP in sequence, and finally formed YAG at 1 500 ℃. As the sintering temperature increased, the pores in YAG ceramics continued to discharge and the grains continued to grow. The optical transmittance firstly increased and then decreased, reaching its maximum value at 1 780 ℃. As the holding time prolonged, the transmittance of YAG ceramics gradually increased. The transmittance of YAG ceramic samples sintered in vacuum at 1 780 ℃ for 20 h was 84.7% at 1 100 nm and 82.8% at 400 nm, respectively. The YAG transparent ceramic dome was prepared via optimization. It could be applied to high-power LED lighting and camera protection covers in the deep-sea equipment at 10 000 meters.