Theoretical Investigation On Structure And Thermodynamic Properties Of W/Cu- And WN- Based Materials

The energy issue concerns national economy and people's livelihood.As a kind of clean energy,nuclear fusion energy is expected to be the main energy source for the international community in the future.The ITER(International Thermonuclear Experimental Reactor)project under development is the largest and most far-reaching nuclear fusion reactor project to date.This project carries the good wishes of human for the peaceful use of nuclear fusion energy.Due to its high melting point,low sputtering rate and low retention rate of hydrogen(H)isotopes,tungsten(W)is selected as plasma facing materials(PFMs)in fusion reactors.However,during the service of W-PFMs,the materials are irradiated by high-energy particles and impacted by high-density heat flow,which seriously affects the physical performance of PFMs.Nitrogen(N2)gas would be implanted into the plasma to reduce the temperature of W-PFMs,and prevent the materials from overheating and melting during the operation of fusion reactors.Since nitrogen is implanted into the plasma,N ions will stay in the surface of W-PFMs,affecting the sputtering rate of the materials.In addition,tungsten nitride(WN)compounds are also detected in the surface of W-PFMs.The formation of WN compounds in the surface of W-PFMs also causes corrosion in the materials' surface to some extent.Copper(Cu)and its alloys are selected as heat sink materials in fusion reactors because of their excellent heat transport properties.Yet,due to the large difference in thermodynamic properties between Cu and W,they cannot be directly connected together.And W/Cu functional gradient materials can play a role in connecting the Cu heat sink materials and W-PFMs.Both the Cu system and W/Cu gradient materials are bombarded by various high-energy particles during service.The radiation damage will occur inside the materials,thus degrading their thermodynamic properties.Moreover,different ratios of Cu and W as well as different sizes of W and Cu particles in the W/Cu composite lead to a large difference in the transport performance of the composite.In this thesis,we have carried out a series of theoretical studies on the thermodynamic properties of W-PFMs,Cu materials and W/Cu composites under the fusion reactor environment.In chapter 1,we briefly review the current research progress of W-PFMs,Cu heat sink materials and W/Cu functionally gradient materials in the service process of nuclear fusion reactors.In chapter 2,the tight binding(TB)potential model and the Kubo method used for electronic transport properties calculations are mainly introduced.Staring from chapter 3,we present the details of our research on the concerns mentioned above.In chapter 3,we discuss the thermodynamic properties of Cu materials and W/Cu composites.We systematically study the component segregation behavior of W/Cu composites based on the TB method.The mechanical properties of W/Cu composites with various ratios of Cu and W are calculated.An empirical law for evaluating the elastic moduli of W/Cu composites is proposed.This empirical law can provide guidance for synthesis W/Cu composites possessing specific mechanical properties in experiments.In addition,by combining TB potential model with Kubo method,we systematically study the thermal transport properties of defective Cu materials,and find that intrinsic defects in Cu materials generally degrade the thermal transport performance of the system.In chapter 4,the non-thermal equilibrium state occurring in materials is introduced firstly.After considering the intense electronic excitation effect,the non-thermal melting and non-thermal phase transition behavior of W material have been systematically investigated.It is found that W system undergoes phase transition from bcc phase to hcp phase at high electronic temperature.Besides,the melting point of W system decreases sharply with the increase of electronic temperature,and non-thermal melting behavior occurs in the W film under intense electronic excitations.In chapter 5,we systematically study the flat band in a three-dimensional tungsten nitride compound,as well as the interaction between WN and H.After theoretically studying the nature of flat band in tungsten nitride compound,we find that the flat band in tungsten nitride compound is predominantly originated from the coupling of the N element.Furthermore,with hole doping,the compound exhibits ferromagnetism and becomes half-metallic.Through theoretical research on the retention mechanism of H in WN composites,it is found that WN layer can reduce the retention of H in W-PFMs,which is conducive to solving the retention and corrosion problems of H in W-PFMs.