热交换法掺钛蓝宝石晶体生长过程中的热质耦合输运机理研究

Large aperture titanium-doped sapphire (Ti:sapphire) crystal is the prime laser gain medium for the ultra-fast ultra-high-power petawatt scale laser system. At present, heat exchanger method (HEM) is the most promising technique for growing Ti:sapphire with diameter being over 200 mm; but it also faces the problem of nonuniform distribution of titanium dopants in the crystal, which is very detrimental to the optical homogeneity of the crystal and the laser performance. Therefore, in this project we aim to study the mechanism of heat and mass transfer during the HEM Ti:sapphire crystal growth process, and to clarify its influence on the distribution of titanium dopants by means of numerical simulations and experimental investigations. Taking into account the internal radiation heat transfer in the crystal and melt, as well as the segregation and evaporation of the titanium dopants, a comprehensive numerical model is to be established for the coupled heat and mass transfer during the growth process. With the proposed model, the influence mechanism of the internal radiation heat transfer on the heat and mass transfer in the growth process is firstly clarified. Then, the effects of key parameters, such as the crystal growth rate, furnace pressure and power ratio between the top and side heaters, on the thermal and flow fields, solidification interface shape, segregation and evaporation characteristics of titanium dopants are studied. Finally, the distribution of titanium dopants in the melt and crystal are revealed. According to the simulated and experimental results, some preliminary technical solutions will be proposed to improve the uniformity of titanium dopants distribution in the crystal. This project involves heat and mass transfer and material science, which not only improves the theoretical basis research of crystal growth, but also promotes the application and development of engineering thermophysics.

大口径掺钛蓝宝石晶体（Ti:Al2O3，简称钛宝石）是拍瓦级超强超短激光器装置的核心材料。热交换法是目前制备直径大于200mm钛宝石晶体的最佳方法，但面临着晶体内组分分布不均匀的难题，严重影响晶体光学均匀性和激光性能。因此，本项目采用数值模拟与实验研究相结合的方法，探索热交换法钛宝石晶体生长过程中的热质耦合输运机理及其对晶体内钛组分分布的影响机制。建立能够完善考虑晶体、熔体内辐射传热及组分分凝、蒸发特性的热质耦合输运数值模型。基于建立的数值模型，研究内辐射传热对晶体生长过程中热质输运的影响机理。然后，结合晶体生长实验，研究生长速率、炉腔压力、加热器功率配比等工艺参数对熔体热流场、结晶界面形状及组分分凝、蒸发的影响机制，揭示晶体内组分分布规律，提出有效改善组分分布均匀性的初步技术方案。本项目涉及传热传质学、材料科学等学科，既促进了晶体生长的理论基础研究，又推动了工程热物理学科的应用和发展。

大尺寸掺钛蓝宝石晶体是超强超短激光器的核心增益介质。热交换法是制备大尺寸钛宝石晶体的主要方法之一，但其晶体内钛组分分布不均匀，严重影响了激光性能。因此，本项目采用有限体积法辐射模型和基于固定网格下的焓方法，建立了完善考虑热交换法掺钛蓝宝石晶体生长过程中内辐射传热（吸收、发射、散射）的全局传热数值模型，系统研究了内辐射传热对熔体流动、温度分布、固液界面形状及晶体热应力的影响规律。数值模拟结果表明，当考虑内辐射传热时，熔体和晶体区域的热量输运显著增强，熔体对流强度增大，晶体底部狭窄区域内等温线密集分布，温度梯度和热应力显着增加。内辐射导致固液界面严重凸向熔体，造成大尺寸钛宝石晶体中钛浓度沿径向分布极不均匀。此外，还研究了晶体、熔体吸收系数和散射系数对晶体生长过程的影响。结果表明，随着晶体吸收系数的增加，晶体底面辐射热流量逐渐降低，但在内辐射传热和导热传热的耦合作用下，导热热流量、底部温度梯度和热应力呈现先升高后降低的变化规律。固液界面弯曲度和熔体流动强度均随着晶体吸收系数的增加而逐渐下降。随着熔体吸收系数的增加，固液界面弯曲度逐渐增大，但晶体温度及热应力分布变化不大。随着晶体散射系数的增加，固液界面弯曲度、熔体流动强度及晶体底部温度梯度及热应力均逐渐降低。但是只有当散射系数大于吸收系数的时候，上述影响才变得显著。另外，在实验研究方面，初步生长了直径100mm的高浓度高吸收系数钛宝石晶体，检测结果表明晶体质量良好。在本项目的资助下，发表学术论文4篇，培养硕士研究生2名。