Structures Of WxNy Crystals And Their Intrinsic Properties:First-Principles Calculations

Nuclear fusion is an important way to solve the problem of energy needs of human beings,which is of great significance to the energy-scarce modern society.The international community attaches great importance to this project.In 2006,the project of ITER was launched.The thermonuclear fusion reaction emits a large amount of high-energy plasma particles,and there is a high temperature field around the high-flux neutron flux.Therefore,Plasma Facing Materials(PFMs)are exposed to higher temperatures and energetic particles.Tungsten W was chosen as the first wall material in the ITER project,more impressive,its high melting point,low thermal expansion,high strength and low retention of deuterium and tritium are better than other materials.In recent years,the Institute of Plasma Physics of the Chinese Academy of Sciences has also replaced W as the first wall material in the fusion reactor assembly,Eastern Super Ring(EAST).It has been experimentally found that during the operation of the fusion reactor,N in the plasma edge region interacts with the first wall material W to form a localized WN compounds on the W-based first wall material.When W surface is irradiated by high energy particles,a number of hydrogen bubbles are formed,which seriously affect the mechanical properties and radiation resistance of the material.However,there are almost no hydrogen bubbles in the surface area of WN or WN amorphous materials.The above experimental phenomena strongly indicate that H in the surface region of the WN material does not agglomerate effectively to form H bubbles.Moreover,the existing experimental data show that the WN compound is not a single stoichiometric ratio of W to N,and many structures of WxNy having different stoichiometry have not yet been determined.This shows that the early WN crystal structure deduced from a small number of experimental results needs to be re-evaluated.For the above content,this article is divided into four chapters;the specific content is as follows:The first chapter mainly introduces the research background of the first wall radiation materials,describes the advantages of the first tungsten-based materials,and puts forward the problems to be solved.The second chapter introduces the main theoretical methods used in this study,including a brief introduction of CALYPSO structure prediction method and VASP software package.In the third chapter,we introduce the structure of WxNy(x = 1-6,y = 1-6)predicted by CALYPSO in detail,combining with the first-principles calculation and binding energy,selecting in the ratio of the most stable structure.Next,we used PHONOPY software to calculate phonon dispersion curves of these stable structures.In the meantime,we also give the Electron structure and XRD patterns of the dynamically stable structure.In the fourth chapter,we calculate the mechanical properties of the dynamic stability structure,including the bulk modulus,shear modulus,Young’s modulus,etc.,and choose the best alternative mechanical structure.In order to understand the intrinsic causes of better mechanical properties,we analyzed the charge density of the structure.The results show that not only ionic bonds but also covalent bonds exist in these structures,which explains the reasons for the better mechanical properties of these structures from a microscopic perspective.