Research On Micro-structure Of LiF-DyF₃-MeO Melts And The Preparation Of Dy-Cu Master Alloy

Dy-Cu alloy is an excellent additive,which can significantly improve the coercive force,corrosion resistance and high temperature stability of NdFeB magnets.It can also improve the physical and mechanical properties of copper.Thus,dysprosium-copper alloy has broad application prospects.However,the existing methods for preparing dysprosium copper alloys have the disadvantages that the alloy is easy to burn,the composition is severely segregated,the energy consumption is large,and continuous production cannot be performed.Therefore,it is of great practical significance to seek a stable,economical,environmentally-friendly,and industrially-available method for preparing dysprosium-copper master alloys.So,Dy-Cu master alloy was prepared by the molten salt electrolytic method with Dy₂O₃ and Cu₂O as electrolytic raw materials and LiF-DyF₃ as molten salt electrolyte in this paper.Differential thermal analysis,quantum chemical calculations,and Raman spectroscopy was applied to analyze the ionic structure of molten salt systems.The dissolution behavior of Dy₂O₃ and Cu₂O in molten salt was studied by isothermal saturation method and transparent cell observation method.The dissolution mechanism and dissolution kinetics of metal oxides were deeply analyzed in the electrolytic process.It was expected to provide technical support and theoretical basis for the large-scale production of Dy-Cu alloys by molten salt electrolysis.First of all,differential thermal analysis was used to determine the initial crystal temperatures of the related LiF-DyF₃,LiF-DyF₃-Dy₂O₃ and LiF-DyF₃-Cu₂O molten salts.On this basis,the change law of the initial crystal temperature of the molten salt system was analyzed,and the ionic structure in the molten salt was studied by combining the XRD pattern.Results have shown that for LiF-DyF₃ dilute molten salts,as the molar concentration of DyF₃in the molten salt increases,the decrease in the initial crystal temperature of the molten salt and the molar fraction of DyF₃ decreased linearly.The general formula of Dy-F ion group in molten salt was DyF_x^?3-x?.The most likely complex ions were[DyF₄]^-.For LiF-DyF₃-Dy₂O₃and LiF-DyF₃-Cu₂O molten salt systems,with the increase of Dy₂O₃ content,the primary crystal temperature of molten salt increases gradually,while the change of Cu₂O content has little effect on the primary crystal temperature of molten salt.Then,the microstructure of LiF-DyF₃ binary system,LiF-DyF₃-Dy₂O₃ and LiF-DyF₃-Cu₂O ternary system,LiF-DyF₃-Dy₂O₃-Cu₂O quaternary system were investigated by quantum chemical theoretical calculation simulation and in-situ Raman spectroscopy experiments.Then,the dissolution mechanism of MeO in molten salt were clearly analyzed.It was found that the microstructure of the molten salt in the molten state was significantly different from the microstructure at room temperature.The crystal structure of LiF,DyF₃,Dy₂O₃,and Cu₂O is completely destroyed at high temperature,and the Raman strength of the melt gradually decreased with increasing temperature.However,in the molten state,the vibration characteristic peak position of the ion group was basically unchanged.LiF-DyF₃melt existed in the molten state as[DyF₄]^-with Raman characteristic peaks at 350cm^-11 and450cm-1 and[DyF₆]^3-complex anion with characteristic peaks at 440cm^-1.LiF-DyF₃-Dy₂O₃melts may had Dy-O-F ion groups in the molten state,such as[DyOF₄]^3-,[DyOF₅]^4-,[Dy₂OF₆]^2-,and[Dy₂OF₇]^3-,and possible Dy-F ion groups are[DyF₄]^-,[DyF₆]^3-.LiF-DyF₃-Cu₂O may had[DyF₄]^-,[DyF₆]^3-,and Dy-O-F ion groups[DyOF₄]^3-,[DyOF₅]^4-,[Dy₂OF₇]^3-in the molten state.LiF-DyF₃-Dy₂O₃-Cu₂O melts may had[DyOF₄]^3-,[DyOF₅]^4-,[Dy₂OF₇]^3-,Dy-O-F ion groups that may exist in the molten state,and the possible Dy-F ion groups are[DyF₄]-,[DyF₆]^3-.Moreover,the dissolution behaviors of Dy₂O₃ and Cu₂O in LiF-DyF₃ molten salt were studied by isothermal saturation method and transparent electrolytic cell observation,respectively.It was found that the dissolution equilibrium time of Dy₂O₃ and Cu₂O in LiF-DyF₃ molten salt is 110min and 120min,respectively.For different components of LiF-DyF₃ molten salt electrolyte(x_DyF3=15mo%⁴0mol%),the solubility of Dy₂O₃ was0.55wt%³.45wt%,and the solubility of Cu₂O was 0.39wt%⁰.52wt%.With the increase of DyF₃ concentration in molten salt,the solubility of Dy₂O₃ gradually increased,while the solubility of Cu₂O changed little.The solubility of MeO under the same LiF-DyF₃ molten salt composition increased with increasing temperature.There was a linear correlation between1/T in molten salt.The dissolution of MeO in molten salt was a complex process in which both physical and chemical reactions coexist.The dissolution of MeO was an endothermic reaction.The dissolution rate of Dy₂O₃ and Cu₂O in LiF-DyF₃ molten salt was mainly affected by mass transfer and diffusion.Finally,the formation mechanism of Dy-Cu master alloy was discussed by thermodynamic and electrochemical analysis of molten salt system.It was found that Cu₂O is easier to be reduced than Dy₂O₃ and the reduction process of Dy₂O₃ and Cu₂O was completed in one step.The main reduction product precipitated on graphite anode is CO.The reduction potential of Dy?III?and Cu?I?on tungsten electrode was about-0.6-0.4V and-0.6V,respectively.Dy-Cu binary master alloy was prepared by molten salt electrolysis in LiF-DyF₃-Dy₂O₃-Cu₂O molten salt system.SEM,EDS,XRD and other analytical testing methods were used to characterize the structure of Dy-Cu alloys prepared under better process conditions.The results show that Dy-Cu alloys were mainly composed of CuDy and Cu₂Dy intermetallic compounds,the primary crystal temperature of the alloy is about 805?.The alloy contained only a small amount of C,O,and F impurities.It could be seen that the electrolytic preparation of Dy-Cu alloys with LiF-DyF₃-Dy₂O₃-Cu₂O molten salt system had better advantages and development prospects.