低粘度硅酸铋熔体结构及其晶体生长研究

Bismuth Silicate (Bi4Si3O12, BSO) crystal, with similar scintillation properties and only half cost of Bi4Ge3O12, can detect the Cerenkov and scintillation signals simultaneously, which made it one of the most suitable candidate for the future Homogeneous HCAL (HHCAL) detector. However, it is very difficult to obtain large-size BSO single crystal for its higher viscosity melt and much different components, which may usually bring about the heterogeneities and macroscopic defects in BSO crystals. This project was proposed to make innovations by reducing the melt viscosity of BSO. According to the structure of the silicate system, the oxide of Sn and Pb were induced to reduce the viscosity of BSO melt, for their 18 electron structure and belonging to the same family with Si. In this project, the melt structure and the crystal growth habit will be investigated to explore the lower viscosity system Bi2O3-SiO2-SnO2(PbO). Besides this, further studies will be carried out to learn the Bridgman crystal growth technologies and scintillation properties of BSO crystals, which were grown through the lower viscosity melt structure. By this project, the melt viscosity might be lowered down to increase the solute convection, and the root problems of nonuniform components and segreation defects might be resolved effectively. Consequently, the macro-defects of BSO crystals would be decreased and the crystal uniformity would be improved. The bottleneck of preparation of large cross section BSO crystals might be break though accordingly.

Bi4Si3O12(BSO)晶体具有与Bi4Ge3O12晶体相似的闪烁性能，成本只有Bi4Ge3O12的一半，并且可同时探测切伦科夫辐射和闪烁信号，因而是未来同质强子量能器探测用的最佳候选材料，但由于BSO熔体粘度大和组分差异大等问题，容易导致组分不均匀和各种宏观缺陷的形成，因而很难得到大尺寸的BSO晶体。本项目提出低粘度BSO熔体结构的创新思路，通过掺入与Si同族同时具有18电子层结构的Sn和Pb元素来降低BSO的熔体粘度；通过对熔体结构和相应晶体的结晶习性的系统研究，探索低粘度Bi2O3-SiO2-SnO2(PbO)体系，同时对低粘度熔体结构BSO晶体的坩埚下降法生长工艺和闪烁性能展开深入系统的研究。通过本项目的研究，降低熔体粘度，加强溶质对流，从根本上解决BSO晶体组分不均匀、组分偏析等生长难题，从而进一步减少晶体的宏观缺陷，提高晶体均匀性，因而有望突破大截面 BSO晶体的制备瓶颈。

Bi4Si3O12(BSO)晶体具有与Bi4Ge3O12晶体相似的闪烁性能，成本只有Bi4Ge3O12的一半，并且可同时探测切伦科夫辐射和闪烁信号，因而是未来同质强子量能器探测用的最佳候选材料，但由于BSO熔体粘度大和组分差异大等问题，容易导致组分不均匀和各种宏观缺陷的形成，因而很难得到大尺寸的BSO晶体。本项目提出低粘度BSO熔体结构的创新思路，通过掺入与Si同族同时具有18电子层结构的Sn和Pb元素来降低BSO的熔体粘度。本项目采用坩埚下降法成功制备了各种浓度PbO、SnO2、Bi2Sn2O7和PbSiO3掺杂的BSO晶体，并对其相应的BSO晶体的结晶习性与偏析规律进行研究。发现PbO:BSO内部偏析较少，但是辐照后明显着色；SnO2:BSO晶体呈浅黄色，内部大量散射颗粒。但是二者共掺NaF之后，晶体内部和表面偏析明显减少，晶体透明无色，晶体透过和光致发光性能也明显改善。分析研究了PbO:BSO晶体的辐照损伤来源于掺杂导致少量铅空位和氧空位的生成。通过高温拉曼光谱研究，分析了不同掺杂的BSO拉曼振动模变化和熔体结构，显示 PbO和SnO2的掺入使Si-O振动模强度减弱，预示着BSO熔体粘度的下降。氮气退火有利于改进BSO晶体300～400nm波段的透过率，而该波段透过率的提升主要源于氮气漂白晶体中氧离子杂质，从而减弱了270nm和290nm附近的吸收，提高300～400nm波段的透过率。通过本项目的研究，发现PbO/NaF和SnO2/NaF共掺有利于减少BSO晶体中偏析缺陷的生成，可应用于大尺寸（长度≥250mm，截面≥50mm）BSO晶体的生长研究。