嬗变产物Mg在SiC中的扩散机制研究

Silicon carbide (SiC) has been considered as a promising candidate structural and functional material for fusion power plants because of its perfect high temperature resistance, corrosion resistance, irradiation resistance, small neutron capture cross section, low permeation of tritium and other attractive features. However, Mg generated by nuclear transmutation will potentially change the electrical, oxidation and other properties of SiC, which could influence the engineering application of SiC-based materials in fusion reactor. The previous studies show that the electrical transport property and oxidation behavior are associated with the diffusion of impurities. Thus, understanding the diffusion behavior of Mg in SiC is crucial to evaluate the performance degradation of SiC-based materials induced by Mg. However, the related researches are still in the beginning stages and the diffusion mechanism of Mg in SiC is unknown yet. Therefore, ion implantation and irradiation will be used in this project to simulate the transmutation products and displacement damage induced by fusion neutron irradiation. Combined with transmission electron microscope (TEM), time-of-flight secondary ion mass spectrometry (TOF-SIMS) and other advanced characterization techniques, the influence of grain boundary, helium bubble and irradiation defects on the diffusion behavior of Mg will be studied. Meanwhile, the chemical reaction of Mg in SiC and its role in the diffusion will also be clarified. The diffusion mechanism of Mg in SiC under fusion neutron irradiation environment will be revealed through above studies, which could help to evaluate the feasibility of SiC-based materials for engineering application in fusion reactor.

碳化硅（SiC）具有耐高温、耐腐蚀、耐辐照、中子俘获截面小和氚渗透性低等优点，是聚变堆中极具潜力的候选结构材料和功能材料。然而，核嬗变反应产生的金属Mg会改变SiC的电学及氧化等性质，影响SiC材料在聚变堆中的工程应用。研究表明，SiC的电输运性质和氧化行为与杂质的扩散有着密切的联系。因此，明确Mg在SiC中的扩散行为是评估其导致SiC材料性能退化的关键。但目前为止，相关研究尚处于起步阶段,Mg在SiC中的扩散机制还不清楚。基于此，本项目拟采用离子注入及离子辐照技术来模拟聚变中子辐照产生的嬗变产物和离位损伤，同时结合透射电镜和飞行时间-二次离子质谱仪等先进表征技术来研究晶界、氦泡和辐照缺陷对Mg扩散行为的影响规律，并阐明Mg在SiC中的反应及其在扩散中的作用。通过本项目的研究，可揭示聚变中子辐照环境下Mg在SiC中的扩散机制，为评估SiC材料在聚变堆中的工程应用可行性提供依据。

碳化硅（SiC）是聚变堆中流道插件和包层结构的理想候选材料，但理论计算发现高能聚变中子将与SiC发生(n,α)嬗变反应，生成高含量的金属嬗变产物Mg，它将影响SiC的电学及氧化等性能。本项目以β-SiC为研究对象，利用离子辐照向SiC中引入嬗变产物和离位损伤，采用聚焦离子束（FIB）、透射电镜（TEM）、纳米压痕仪（Nanoindentation）和飞行时间二次离子质谱仪（ToF-SIMS）并结合理论模拟计算系统地研究了Mg在SiC中的扩散行为。. 50keV的常温Mg离子辐照导致SiC形成了约127nm厚的非晶层。利用500keV的He离子在650℃下辐照SiC，经表征发现：辐照缺陷导致材料的纳米硬度上升，且随辐照剂量增加而增大；SiC在高剂量辐照下还形成了氦泡和位错环。. 对Mg及Mg/He离子注入后的SiC在1300℃下进行退火实验，以研究辐照非晶层及He对Mg扩散行为的影响。ToF-SIMS结果表明：经20小时退火后，Mg在SiC中发生了显著的扩散，并且He明显拟制了Mg的扩散。TEM结果表明：辐照非晶层在退火过程中均发生了再结晶；而在Mg/He离子注入的样品中，He原子扩散至表面高辐照损伤区域形成了氦泡，并提高了非晶层的再结晶速率。利用基于分子动力学的均方位移法计算了1300℃下，Mg在非晶及单晶β-SiC中的扩散系数，首次从理论上证实了Mg在非晶SiC中扩散更快。这表明氦泡促进非晶层再结晶是拟制Mg在SiC中扩散的原因之一。. 利用第一性原理计算了β-SiC单晶基体及Σ5晶界中空位及Mg缺陷的形成能。结果表明，基体中哑铃状间隙型Mg缺陷的形成能最低，而C空位型缺陷比Si空位型缺陷更稳定。Σ5晶界处的空位形成能以及间隙、置换型Mg缺陷的最小形成能均比基体中的小2-3eV，这表明：Mg更易在晶界处偏析，同时，Mg沿着Σ5晶界的间隙和置换扩散速率要显著高于基体。. 通过本项目的研究，揭示了Mg离子辐照对SiC造成的离位损伤，阐明了辐照非晶化、He以及晶界对Mg在SiC中扩散行为的影响规律及机制，为SiC材料在聚变堆中的工程应用提供理论基础和指导。