| In future fusion devices,the continuous flow of liquid lithium(Li)on the substrate of plasma-facing materials(PFMs)could effectively reduce the hydrogen retention and impurities,thereby significantly improving the plasma and the stability of the fusion reaction.As the liquid first wall,it is needed to form steady-state film when flowing on PFMs substrate.Tungsten(W)is the most promising candidate PFMs,which can be used as the substrate for liquid Li.However,the low surface wettability of liq uid Li on the substrate surface limits the potential application of liquid Li first-wall in fusion,making it difficult for liquid Li to uniformly spread on the substrate.Improving the wettability of liquid Li on the W surface helps to form steady-state film on the substrate.The interactions between liquid Li and W materials need to be considered,such as the corrosion and brittleness.In addition,during the safe operation of the fusion reactor,the liquid Li flows on the PFMs and does not directly contact the heat sink material which uses copper(Cu)as the main alloy element.However,in the event of an accident,the liquid Li may be in direct contact with Cu.Therefore,the interaction between liquid Li and Cu materials needs to be considered.The corrosion and wetting between liquid lithium and solid materials are related to the property of solid-liquid interface.In this paper,the microstructure and diffusion characteristics of Li-Cu and Li-W solid-liquid interfaces are studied using molecular dynamics methods.In addition,the wettability of Li on the W surface have been studied,and a method to enhance the wettability of Li on the W surface is proposed.This paper reports the development of modified analytical embedded atom method(MAEAM)interatomic potentials for pure Li,Cu and W,and the alloy potential for Cu-Li and W-Li systems.These elemental potentials based on the MAEAM model can not only describe the physical properties of BCC crystals(Li,W),but also describe the physical properties of the FCC crystals(Cu).And the pure Li potential can accurately describe the structure and transport properties of liquid Li.Due to the lack of corresponding experimental data,the reference values in the fitting process of the Li-Cu and Li-W alloy potential parameters are based on first-principle first-principle calculated value.The newly developed Li-Cu and Li-W alloy potentials can well describe the thermodynamic properties of the respective alloys.It is proved that these potentials are reliable for the study of the interaction between Li-Cu and Li-W.The microstructure of Cu-Li solid-liquid interfaces and W-Li solid-liquid interfaces was studied using the newly developed potentials.At temperature of 470K,interfacial alloying is found only at the Cu(110)-Li solid-liquid interface but not in other systems.The thermal vibration of solid Cu and W atoms near the sol id-liquid interfaces is enhanced.The liquid Li close to the interfaces is ordered in a layered structure.The structural correlation length were used to quantify the amount of order of liquid Li at the interface.The results show that for the same system and at the same temperature,the degree of ordering of the liquid Li is proportional to the crystal plane spacing of the substrate,but the relationship is not true for the difference systems.That is to say,the degree of structural ordering of the liqui d close to the solid-liquid interface system is not only related to the crystal plane spacing,but also related to the substrate material in the system.In addition,the ordered structure within the layer is also observed in the Li liquid layer adjacent to the substrate,and its inter-layer structure is similar to that of the neighboring substrate.Combining with the research results of the microstructure of the solid-liquid interfaces,the influence of the liquid ordered structure on the diffusion mechanis ms of Li atoms at the solid-liquid interface was also studied.The results show that there are many diffusion modes of Li atoms on the surface of Cu and W,which is different from the diffusion mechanism of Li atoms in pure liquid Li.According to the clas sification of the motion range,the diffusion mechanism of Li atoms at the interfaces can be divided into two categories,one is short-range diffusion,and the other is long-range diffusion.The mechanism of long distance motion is the desorption-mediated diffusion,where a liquid atom desorbs from the surface,and diffuses through the adjacent liquid phase,and then readsorbs on the surface.At the same time,the probability of Li desorbing from the W surface is higher than Cu surface.The mechanisms of short distance motion shows remarkable anisotropy.For the Cu(100)-Li,Cu(110)-Li and W(100)-Li solid-liquid interfaces,the short-range diffusion mode of Li atom is vacancy jump diffusion,in which Li atoms vibrate at the equilibrium site and then diffuse through the nearest neighbor site by the vacancy diffusion mechanism.Another short-range diffusion mode has been found at Cu(110)-Li solid-liquid interface,that is inter-layer diffusion mechanism.For the Cu(111)-Li,W(110)-Li and W(111)-Li solid-liquid interfaces,the short-range diffusion mode is consistent with the two-dimensional random walk model.The short-range diffusion coefficients of liquid Li atoms are equal to the long-range diffusion coefficients of liquid Li atoms for W(110)-Li and W(111)-Li solid-liquid interfaces.The short-range diffusion coefficients of liquid Li atoms are clearly lower than the long-range diffusion coefficients of liquid Li atoms for other interfaces.The short-range diffusion coefficient of liquid Li atoms ranges from 53.5~615.2?~2/ns.Finally,the wetting behaviors between solid W and liquid Li are examined using molecular dynamics simulations.The simulation results illustrate that the Li-W potential can be used to investigate the surfaces wettability of liquid Li on W substrate.We examine the stability of Li film and the wetting behavior of Li droplet on the W surfaces.The results show that liquid Li tends to agglomerate on W surfaces,which is consistent with relative experimental phenomenon.Due to the higher potential energy of isolated Li atoms than the aggregated ones,the Li film is unstable and trend to aggregate on W substrate.The low surface wettability of liquid Li on W substrate is a key factor impeding the formation of steady-state liquid film.So we etched the W surface to change the wettability of Li droplets on the W surface.The results show that the the wettability of Li can be significantly improved along grooves,but the opposite effect in perpendicular direction.The pinning effect at the edge of the groove is an important factor that hinders the spreading of droplet Li,and the mutual attraction between Li atoms in the groove plane enhances this effect.By pre-filling liquid Li in the groove,the pinning effect can be effectively eliminated,the wettability of Li o n the W surface can be enhanced,so that the adverse effect of the groove can be eliminated,which is favorable for the liquid Li to form steady-state film on the W surface. |
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