| Recently, applying a electric current(electric field), as a new assistive technology on the preparation of materials, has been given more and more attention. For example, when A uniaxial pressure is added simultaneously with pulsed direct current to primarily sintered powders(SPS), liquid phases could always be found in the sinters, which not only make the sinter process carried out at a lower temperature and in a shorter time, but also makes the sinters got more outstanding properties compared with the traditional method. The imposition of a DC current on self-propagating high-temperature synthesis(SHS) might increase the reaction rate and improve the compound efficiency. In addition, soldering interfaces always form high current density and large joule heat in the service process due to the development trend of miniaturization of integrated circuit. High temperature could force solders melt locally. However, the solder will maintain its initial shape, even after melting, due to surface tension and protection from the underfill. It will lead to the failure of the solders at early stage since the diffusivity of atoms in liquid is much higher than that in solid. However, the mechanisms of these effects, so far, have not been well understood.Based on the above processes related to the interactions between the liquid phase and the solid phase, it seems reasonable to conjecture that electric current could improve these applied processes through promoting the wetting behavior, mass transfer and interfacial reaction. To our knowledge, very work has so far been carried out on this issue. In the present work, we studied the effect of the electric current on the wetting behavior and interfacial characteristics between the liquid/solid metal systems systematically for the first time and revealed the law of wettability under a direct current and attempted to establish the theoretical basis for controlling the wetting and interfacial behaviors by applying direct current. The main conclusions are given as follows.1. Take the Sn/(Ni, Cu, Fe) and Sn57Bi/(Cu, Fe) system as the representative for reactive systems, the effects of direct current on the wetting behavior and interfacial morphology between the molten on the substrate were investigated. Electromigration and the complex convection in the liquid droplet can significantly promote the dissolution of the substrate and interfacial reaction. The independent or synergistic action can disrupt the oxide film front of the triple line, so as to improve the wetting. However, when the surface of the metal substrate is "clean", the influence with current has limited.2. Although the wettability was not significantly influence with current, the interfacial behaviors of the reactive systems were changed. Applying a current not only changed the morphology(Sn/Ni), the number and distribution of the interfacial reaction products(Sn/Cu), but also changed the type of the product(Sn/Fe).3. The effect of direct current on the wetting behavior of pure dissolved systems was studied by Bi/Cu system. When the surface of the Cu substrate was oxided, the appropriate experimental temperature and atmosphere can get rid of the oxide film on the surface of the substrate, so as to improve the wettability. In addition, the electromigration and Marangoni convection caused by applying a curret can promote the dissolution of the Cu to the molten Bi, and thus disrupt the oxide film on the surface of Cu and improve the wettability. When the surface of the substrate is "clean", the effect of applying a current on the wetting was limited in pure Ar or Ar–10H2. However, Since Joule heating can not be compensated quickly Because of the lack of heat transfer medium in high vacuum, the strong Marangoni convection broke the equilibrium of the triple line and improved the wetting.4. Electromigration and Marangoni convection were the key factors on the dissolution of Cu substrate with current. When the Joule heating effect could be compensated rapidly(in pure Ar and Ar–10H2), electromigration becomes the dominant factor of the diffusion. The diffusion of Cu was promoted by the electron flow to the molten, but the diffusion was inhibited with current in the opposite direction. When the Joule heating effect could not be compensated rapidly(in high vacuum), Marangoni convection was dominant and promoted the dissolution of Cu regardless of the current polarity.5. In the Sn(Sn57Bi)/Cu systems, the mass transfer and interfacial reaction between Sn(Sn57Bi) and Cu substrate have great influence under current. With the increase of current density, cathode Cu substrate dissolution intensifies and removes to the anode side driving by the electromigration force. It makes the dissolution of the anode Cu is inhibited but a large number of IMCs formed at the anode side.6. The activation energy of the dissolution of Cu, which indicates that the applied current reduce the activation energy of the Cu dissolution significantly. Derivation and calculation of the effective charge number of Cu in the molten Sn and the value of the electromigration force. Accordingly, we considered the electromigration force can be the major driving force for promoting the diffusion of Cu.7. Take the Sn/W and Bi/Fe systems as the representative for the systems with dissolution of trace, the effects of direct current on the wetting behavior and interfacial morphology between the molten on the substrate were investigated. It is found that(1) applying a current could change the solubility of metal solid phase in liquid phase, and promote the dissolution of substrate.(2) Since the current density was small at this work, the dissolution of the trace and the electromagnetic force caused were not enough to break the equilibrium of the triple line, the effect of applying a current on wettability was limited.In a word, the study of different types of systems under current is not only helpful to the wettability and mass transfer theory, but also provides reference and guidance for the connection of the advanced materials and the service behavior of the materials. |
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