A First-principles Study On Zirconium Carbide As Wellas Its Defect Formation And Diffusion

With high stiffness,B1-ZrC in rocksalt structure is a hard structural material for various applications in harsh environments such as nuclear reactors.The excellent structural role of ZrC in service draws close attention to its performance,surface and other related issues.Whereas the formation and distribution of defects affected by temperature and pressure in the bulk phase of ZrC,a non-strictly stoichiometric compound,lack systematic research.Concurrently,the formation and diffusion of surface defects have not yet been fully understood.Herein,first-principles calculations are carried out to study related properties of ZrC and its defects in this paper.The calculation results are helpful for the design of ZrC and applications of defect engineering in ZrC-based materials,and provide important guidance for the safe and effective utilization of ZrC in nuclear reactors.Using the quasi-harmonic Debye approximation,the thermodynamic properties of ZrC bulk phase and the formation and distribution characteristics of point defects in ZrC over a high temperature and high pressure range are studied,and the stability and concentration of defects were systematically evaluated.The results show that the introduction of point defects usually reduces the thermophysical properties of ZrC,and the formations of different point defects with the effect of temperature and pressure are characteristic and can be classified.In terms of quantity formed,carbon vacancies and their complexes are the main point defects.In the spatial distribution,the accumulation among point defects is ordered,and the nearest-neighbor defect pair is most stable under high pressure.In addition,the Gibbs energies of formation and concentrations of point defects are sensitive to pressure in the lower pressure range,suggesting an effective routine to manipulate point defects in ZrC.In view of the surface issues encountered in the application of cladding,combined with the existing theoretical calculations and experimental results,the surface properties including surface energetics and surface relaxation of ZrC are described and compared in detail.During the process,three general and progressive methods are employed to evaluate the surface energy for each low-index surface.In addition,this paper also systematically analyzes the self-adsorption trends for each low-index surface and points out the cause of nuance for configurations with similar adsorption energies on the {100} surface.The dominant role for self-adsorption and the tendency of spontaneous termination for {111} surface are discovered.Afterwards,adsorption thermodynamic stability and migration calculations for adatom suggest that the growth of ZrC{100} is based on a cluster-dominated mechanism.Finally,the formation of single-atom defects including the fission product Ag for the most stable {100} plane is discussed.The CI-NEB method is used to simulate and compare the lateral and longitudinal diffusion processes of defects.The results show that the formation of surface defects is jointly affected by the atomic arrangement and the stress release of the surface end,the lateral diffusion of defects is dominated by the surface C sublattice,and the ubiquitous extrusion mechanism increases the probability of longitudinal penetration of Zr and Ag with a large atomic radius.