Molecular Dynamics Study On Thermodynamic Properties Of Fe-Ni Nanoparticles

Iron(Fe)is one of the most abundant metals on earth.As the size lowers to the nanometer level,Fe nanoparticles exhibit many excellent properties and have broad application prospects in industrial production and civilian usage.Fe nanoparticles have certain reducing properties,they will still be oxidized and corroded under long-term exposure to natural conditions.In order to prevent Fe nanoparticles from being oxidized,a strategy often used is to add nickel(Ni)into Fe nanoparticles to form core-shell structures or mixed-phase(solid-solution)nanoparticles.The performance of Fe-Ni bimetallic nanoparticles is closely related to their structural stability,based on this consideration,the structural evolution of core-shell and mixed-phase structures Fe-Ni bimetallic nanoparticles during heating is studied by molecular dynamics simulations in this thesis.With temperature increasing,the solid-solid phase transitions from bcc to fcc are observed in the Fe cores of Fe-Ni core-shell(Fe@Ni)nanoparticles,which are absent in the corresponding pure Fe and Ni counterparts.The transition occurs through an approximate Nishiyama-Wassermann path rather than the Bain path.The phase transition temperature increases with the thickness of Ni shell.Afterwards,the pre-melting is found to simultaneously happen at the surface and the core-shell interface.It then gradually develops to other regions and finally leads to the overall melting.When the radius of Fe@Ni nanoparticles is less than 3.6 nm,the melting point increases as the particle radius increases;otherwise,no appreciable increase in the melting point is observed until the particle size becomes significantly larger.The simulations were also carried out on the mixed-phase Fe-Ni alloyed nanoparticles with different composition ratios.In the temperature range of 10 K-1350 K,both bcc and fcc lattices were considered as the initial structures.The results show that Fe-Ni mixed-phase nanoparticles with bcc structure are more stable when the Ni content is less than 40%.However,the coexistence of local bcc and fcc structures may occur when Ni content is in the range of 40%-60%.For the Ni content higher than 60%,the Fe-Ni mixed-phase nanoparticles with fcc structure are more stable.Meanwhile,the study of Fe-Ni mixed-phase nanoparticles with different initial structures and Fe-Ni ratios reveal that the melting point and average atomic potential energy of Fe-Ni mixed-phase nanoparticles rely heavily on the proportion of Ni content while are irrelevant with the initial lattice structure(fcc or bcc).