Study on fabrication and spectral of yttria transparent ceramics for 2 μm mid-infrared laser

Y2O3透明陶瓷具有优秀的光学、热学、物理和化学性质。作为基质材料，Y2O3具有高的热导率（是YAG的2倍）、宽透过带、良好的化学稳定性、强Stark能级分裂以及低声子能量等特性。但是Y2O3熔点高达2430 ℃，并且在2280 ℃会产生相变，故得到其单晶非常困难。与单晶相比, 透明陶瓷具有易于制造、成本低、制备尺寸大、掺杂浓度高以及形状可控性好等特点，已成为固体激光材料领域的研究热点之一。 作为在人眼安全、光电对抗、激光遥感等领域有重要应用的 2 μm 波段的激活离子，Ho3+具有丰富的能级结构，受激发射截面大(9×10-21cm2)，荧光寿命长(8 ms)；Tm3+具有交叉弛豫作用，具有非常高的量子效率(～2)。因此，制备烧结Tm3+、Ho3+:Y2O3透明陶瓷是一项非常有意义的应用基础研究，这不论是对国防还是对民用都具有深远的影响。 我们分别用化学共沉淀法制备了Ho3+掺杂的Y2O3透明陶瓷粉末，对纳米粉体合成中的分散性、颗粒度等进行了实验分析；采用固相球磨法制备了Tm3+、Ho3+等离子掺杂的Y2O3透明陶瓷粉末，并通过真空烧结得到了两种制备工艺下的透明陶瓷，基本上解决了高透明性的关键问题。实验中发现，在共沉淀法中，粉末质量对pH滴定终点很敏感，适当的pH终点有利于获得高分散性的纳米粉末；在固相球磨法中，PEG的引入能极大的提升陶瓷的透明性。 通过对不同烧结温度和保温时间的不断摸索，得到了1850 ℃保温20～25h是获得高质量Y2O3透明陶瓷的关键；通过场发射扫描电子显微镜对腐蚀后陶瓷界面进行了分析，发现ZrO2的钉扎和溶质拖拽作用有效的抑制了晶粒的异常长大，成功的控制了晶粒尺寸在20～30 nm；通过测量Tm3+:Y2O3透明陶瓷透光及荧光光谱，研究了Tm3+在Y2O3基质中的掺杂极限，发现7 at.%为最优化的掺杂浓度。 Tm3+/Ho3+双掺能够有效的发挥两种离子各自的优点。实验证明，理想的Tm3+-Ho3+离子掺杂比例应该在2.5:1和5:1之间，这样就可以有效的实现Tm3+-Ho3+离子间的能量转移，又可以从整体上提高荧光强度。La3+掺杂能够有效的降低Y2O3烧结的温度，降低烧结成本，防止晶粒异常长大。实验证明，在低于常规烧结温度300 ℃左右的温度下仍然能够获得透明陶瓷，并且其荧光强度较同比例浓度掺杂Tm3+样品要强5-6倍。 进一步从长远角度考虑透明陶瓷的发展，2 inch甚至更大尺寸的Tm3+、Ho3+:Y2O3透明陶瓷具有非常大的意义。初期的探索表明，制备大尺寸Tm3+:Y2O3透明陶瓷具有极大的可行性。Y2O3 is an ideal laser host media for its good optical, thermal, chemical, and mechanical properties, such as its high thermal conductivity (2 times than YAG), wide transmittance region, good chemical stability, strong Stark energy split and low phonon energy and so on. However, it is extremely difficult to grow a high-quality Y2O3 crystal because of its high melting point (about 2430 °C) and phase transition at about 2350 °C. Compared with single crystals, transparent ceramics have many advantages, such as their easy preparation, low cost, convenient, mass production, high doping concentration, and composite or multifunctional structure. Thus, as a further new solid laser gain medium, it is a promising material. As the active ions of 2 μm region, which possesses many features such as eye-safe region、electrooptical countermeasures、laser remote sensing and medical application, Ho3+ is a kind of valuable active ion for its plenty energy levels, large stimulated emission cross section (9×10−21 cm2), long fluorescence lifetime (8 ms); Tm3+ is also a kind of valuable active ion for its high quantum efficiency (～2). Therefore, it must be a far-reaching application basic research for fabricating the Tm3+、Ho3+:Y2O3 transparent ceramics, not only for national defense, but also for civil. We synthesized the Yb3+，Ho3+ co-doped yttria nanopowders by chemical co-precipitation method, and analyzed its disperstiveness and granularity. We also obtained the Tm3+，Ho3+ doped yttria nanopowders by solid state reaction method. And we realized the transparent ceramics by vacuum sintering using the nanopowders obtained by two different ways. We found that appropriate pH value is a key element for high disperstiveness with chemical co-precipitation method and doping PEG would promote the transmittance significantly. Though sintering under different temperature and holding time, we found that high quality Tm3+、Ho3+:Y2O3 would be obtained at 1850 ℃ for 20～25 h. Though the analysis of interface of ceramics after corrosion by field emission scan electron microscope, we found that ZrO2 tends to impede grain boundary mobility and occurrence of secondary recrystallization due to the solute drag mechanism and limit the grains size at 20～30 nm. Though testing the transmittance and emission spectral of Tm3+:Y2O3 transparent ceramics, we found that 7 at.% is the optimum concentration for high doping. Tm3+/Ho3+ co-doped is easy to develop the both of their merits, and the optimum doping proportion is between 2.5:1 and 5:1, which could improve the emission intensity and realize the energy transformation efficiently. Doping La3+ would decrease the sintering temperature significantly. We found that transparent ceramics were still obtained even at 1550 ℃ and the emission intensity of samples of doped La3+ is 5-6 times larger than undoped samples. As the development of transparent ceramics by now, fabricating the 2 inch or even large scale Tm3+、Ho3+:Y2O3 transparent ceramics is meaningful. And our experiments show that large scale Tm3+:Y2O3 transparent ceramics are totally realizable