SiC纳米包覆颗粒烧结行为的分子动力学模拟

Molecular dynamics simulation of sintering behavior of SiC nanocoated particles

碳化硅(SiC)制备在核燃料研究中具有重要意义,例如新型事故容错核燃料采用SiC作为关键基体材料。研究SiC纳米包覆颗粒的烧结行为对优化新型核燃料基体材料制备工艺具有指导意义。该文根据纳米颗粒熔点变化规律,验证了Tersoff势函数进行SiC分子动力学模拟的可行性和模型参数的准确性;考察了纯相SiC、富硅(SiC@Si)和富碳(SiC@C)这3种典型SiC纳米颗粒的烧结演化过程;并对烧结过程进行了定量描述,通过烧结颈生长、能量演变和原子扩散等参量分析了烧结机制,重点关注包覆层结构对SiC烧结行为的影响,从而获得包覆颗粒烧结机理。研究结果表明:包覆层的原子扩散性会促进颗粒原子整体迁移,从而加速整体烧结行为。SiC@Si颗粒比SiC@C颗粒更易发生包覆层原子扩散,因而SiC@Si颗粒更易发生烧结;较低的加热速率在一定程度上有利于烧结进行,但并不影响包覆颗粒的原子扩散模式。研究结果对SiC纳米颗粒烧结机制给出了定量解释,有助于理解SiC烧结制备过程的规律。

[Objective] The preparation of SiC materials is significant in nuclear fuel research.Presently,SiC materials are applied as a key material in coating layers of tri-structural isotropic(TRISO)-type coated particles and as fully ceramic microencapsulated accident-tolerant fuel(FCM-ATF)matrix materials.The matrix SiC materials of FCM-ATF are sintered from SiC powder or nanoparticles.The SiC nanocoated particles are a kind of important SiC nanoparticles.The study of the sintering behavior of SiC nanocoated particles aims to develop a framework for optimizing the sintering preparation process of the FCM-ATF matrix material.[Methods]In this paper,the melting points of pure SiC nanoparticles of different sizes were first investigated.The feasibility of the Tersoff potential function for molecular dynamics simulations of SiC materials was confirmed using the nanoparticle melting point variation law.Then,the sintering evolution processes of three typical SiC nanoparticles,pure SiC,SiC@Si,and SiC@C,were examined to investigate the influence of the coating layer structure on the sintering behavior of SiC.The sintering process was quantitatively described using variables such as the sintering neck width,atomic number in the neck region,shrinkage ratio,and degree of system densification.The sintering mechanism was described by the ratio of grain boundary energy to surface energy,mean square displacement,atomic displacement vector,and atomic diffusion coefficient.[Results]The study of the structure of SiC nanocoated particles showed that SiC@Si particles were more prone to sintering than SiC@C particles.Vulnerability to sintering was mainly reflected in the faster neck growth and higher densification during the sintering process.The results were closely related to energy evolution and atomic diffusion phenomena.Regarding energy evolution,the grain boundary energy of SiC@Si particles was rapidly converted to surface energy during the sintering process,but the conversion of grain boundary energy to surface energy of SiC@C particles was very slow.According to classical sintering theory,the sintering driving force was mainly provided by the surface energy of the particles.High surface energy catalyzed the surface diffusion of particle atoms during the sintering process.The evidence was corroborated by an analysis of the atomic diffusion aspect.The coating layer had as high surface energy as the surface of the coated particles.Thus,the overall atomic diffusivity of the particles was partially affected by the atomic diffusivity of the coating layer.The overall sintering behavior of the particles was catalyzed by the high atomic diffusivity of the coating layer.The atomic diffusivity of the silicon coating layer was better than that of the carbon coating layer,and the coated layer of the SiC@Si particles was more prone to atomic diffusion than that of the SiC@C particles;hence,sintering and atomic diffusion were more probable in the SiC@Si than in the SiC@C.The study of the heating rate showed that a lower heating rate was somewhat beneficial for sintering but did not affect the atomic diffusion pattern of the coated particle.[Conclusions] The results give a quantitative explanation of the sintering mechanism of SiC nanoparticles.It helps to understand the laws of the SiC sintering preparation process for FCM-ATF matrix materials and also provides a good reference for raw material design,sintering regime,and process optimization of SiC materials preparation.