超细晶碳化硅包覆层的制备及其尺寸效应研究

High temperature gas-cooled reactor (HTGR) with inherent safety characteristics is considered as one of the attractive and competitive generation IV nuclear reactors in the future energy markets. Tristructural-isotropic (TRISO)-coated particle fuel is the most significant safety aspect in this nuclear reactor, since it relies on the properties of the coatings surrounding the kernel fuel to stop the release of harmful radioactive material. Among these layers, the silicon carbide coating is considered the most important since it not only provides the TRISO particle with structural integrity but also retains fission products at elevated temperatures. In this project, the ultra-fine grain structures are designed to optimize the comprehensive performances of the high density SiC layers coated on micrometer particles. To obtain this unique structure, the idea of gradient is proposed in the preparation system. By applying the temperature or the reactant concentration with gradient or periodic changes, or by addition of other reactants, the nucleation process can be promoted and the growth process can be restrained which is very helpful for the formation of ultra-fine grain structures. To explore the formation mechanism of ultra-fine grains, the nucleation and growth process in the complex chemical vapor deposition system will be simulated. The variation of the physical properties of the coating layers with grain sizes will be investigated. The basic physical mechanism of the size effect will be uncovered from the perspective of the connection between the microstructures and the properties. We hope that, the results achieved in this project can be further used in the very-high temperature gas-cooled reactor (VHTGR) and can be promoted to the preparation of other materials with similar ultra-fine grain structures.

高温气冷核反应堆由于固有安全性被认为是能够适应未来能源市场的第四代先进核反应堆堆型之一，其安全性的第一道保障就是使用了TRISO型包覆燃料颗粒。在四层包覆结构中，SiC层是承受包覆燃料颗粒内压以及阻挡裂变产物释放的关键层。为了优化材料性能指标，本项目将超细晶结构引入包覆于微米颗粒之上的SiC层，将梯度和变化的思想引入SiC包覆层的制备过程。通过梯度或脉冲变化的温度或浓度条件，或者通过外加其它反应物来促进化学气相沉积形核过程，抑制晶粒生长，以期得到高密度均匀化的超细晶SiC包覆层。同时理论研究复杂体系中SiC的晶体形核和生长机理，探索超细晶的形成机制，研究不同尺度SiC包覆层各物理性能的变化规律，建立SiC包覆层的尺寸效应，通过对包覆层精细显微结构的分析揭示尺寸效应的物理本质。本项目所取得的成果能够为超高温气冷堆燃料元件的制备提供技术储备，并能为其它相关的超细晶材料研究提供指导和借鉴。

高温气冷核反应堆是第四代先进核反应堆堆型之一，其安全性的第一道保障为使用了以SiC为主要包覆层的包覆燃料颗粒。在包覆燃料颗粒中，SiC层是承受包覆燃料颗粒内压以及阻挡裂变产物释放的关键层。为了优化包覆燃料颗粒的性能指标，本项目提出以制备具有优异综合性能的超细晶SiC包覆层为研究方向。在项目执行过程中，采用理论分析与实验验证相结合的方法，研究了流化床化学气相沉积条件下制备SiC包覆层前躯体的高温裂解机理及细晶SiC材料的形核和生长规律。根据异质形核机理，提出了脉冲丙烯法制备细晶SiC包覆层的思路，制备了晶粒尺寸小于300nm的超细晶SiC包覆层，并通过控制脉冲的时间和丙烯的浓度，实现了对超细晶SiC包覆层晶粒尺寸的调控。在脉冲丙烯法的基础上，进一步深入研究SiC包覆层的制备规律，在氢气—氩气体系中，建立了SiC的实验相图，在低温条件下制备出晶粒尺寸为100nm的致密SiC包覆层。对所制备的超细晶SiC进行微观结构分析，深入研究了超细晶结构的形成机理。所制备的超细晶SiC包覆层具有优异的力学性能和抗氧化性能。分析了超细晶SiC包覆层的性能与尺寸效应的关联，研究尺寸效应的物理机制。通过拓展研究，还得到了一些其他形式的纳米化的SiC产物，并将制备的SiC材料应用于新型核燃料元件的设计上。本项目所取得的成果有望在下一步超高温气冷堆中获得实际应用。