| Molybdenum and Molybdenum-rhenium al oys,for their great high temperature performance,great resistance to neutron irradiation and good compatibility with liquid sodium,are widely used as sodium heat pipe structural materials.Thearatically,molybdenum aloys have great corrosion resistance to alkali metals.But in fact,inherent impurity atoms(such as carbon,nitrogen,oxygen)that existed in molybdenum and molybdenum-rhenium aloys will interact with liquid sodium,thereby affect the properties of structural metal,so as to reduce the service life of the heat pipe.Therefore,it is of great significance to study the behavior of impurity atoms in molybdenum and the influence of impurity atoms to the behavior of sodium atoms.In this work,we have investigated the behavior of impurity atoms inside and on the surface of the metallic molybdenum bulk and their influence on the behavior of sodium atoms by a first-principle study.By calculating the formation energies,adsorption energies and diffusion barriers of impurity atoms on molybdenum,molybdenum-rhenium bulk and their surfaces,we investigated the most stable occupancy of impurity atoms as well as their lowest diffusion barrier.We also calculated the adsorption energy and diffusion barrier of sodium atoms on the surface of molybdenum and molybdenum-rhenium aloys,and investigated the interaction between sodium atoms and impurity atoms by investigating the behavior of sodium on the surface under the condition of pre-adsorption of impurity atoms.In addition,the effects of impurity atoms and sodium atoms on the dissolution of both surfaces were investigated by the calculation of vacancy formation energy under different configurations.Our calculation results show that:(1)Interstitial atoms are easier to form but harder to diffuse in Molybdenumrhenium bulk,and impurity oxygen atoms have the lowest interstitial formation energy and the lowest diffusion barrier,so interstitial oxygen atoms are the most easily formed interstitial atoms,followed by carbon atoms,and interstitial nitrogen atoms are the most difficult to be formed.(2)On the surfaces of molybdenum and molybdenum-rhenium aloys,the most stable adsorption sites of impurity atoms are the hollow sites.Oxygen atoms have the lowest adsorption energy on these two surfaces,which is most likely to form a stable adsorption structure,followed by nitrogen atoms.It is difficult for carbon atoms to form a stable adsorption structure.Besides,rhenium atoms can slightly reduce the adsorption energy of these impurity atoms on the surface of molybdenum,making it easier to form a more stable adsorption.In addition,the diffusion barrier of impurity oxygen atoms on the surface is higher than that in the bulk,so oxygen atoms are more inclined to diffuse in the bulk;but impurity carbon and nitrogen atoms are more inclined to diffuse on the surface.(3)Sodium atoms are easily adsorbed on both surfaces,and the adsorption energy is lower on the molybdenum-rhenium surface.Impurity atoms have little effect on the adsorption of sodium atoms.Sodium atoms cannot exist stably on the subsurface,therefore they prefer to diffuse on the surface rather than diffuse into the subsurface.(4)Whether inside the bulk or on the surface,the vacancy formation energy of nearby molybdenum atoms is reduced where exists interstitial impurity atoms,sodium atoms or substitutional rhenium atoms,especially on the bulk where oxygen atoms and on the surface where sodium-oxygen atoms coexist,molybdenum atoms are easily dissolved to form vacancies,but the presence of rhenium atoms can inhibit the dissolution of molybdenum.Therefore,it can be speculated that in the Molybdenumrhenium aloy sodium heat pipe,the impurity oxygen has the greatest influence on its performance,which will accelerate the dissolution of the heatpipe wall material,but the addition of rhenium atoms can slow down this dissolution phenomenon. |
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