| The application field of rare earth metal alloy is expanding continuously during the past 20 years.the development of medium and heavy rare earth alloy is particularly rapid.It is urgent to explore efficient and green alloy preparation technology and purification technology,among which the related research of variable valence rare earth Yb alloy has attracted wide attention.Based on this,this paper proposes to use LiFCaF2 as the supporting medium to analyze the behavior of Yb2O3 in the process of electrochemical reduction,so as to provide theoretical support for the preparation of Yb-Ni master alloy by molten salt electrolysis and deep deoxidation and impurity removal purification.The main research work is divided into the following three parts:(1)Basic thermodynamic properties of LiF-CaF2-Yb2O3 molten salt systemThe phase equilibrium data of LiF-CaF2 molten salt system were optimized,and the LiF-CaF2=78:22(molar ratio)was confirmed to be the eutectic composition point(LiF:CaF2=78:22,molar ratio)of LiF-CaF2 system by thermal analysis.The variation rules of thermal conductivity and thermal expansion coefficient temperature decreasing with the increase of temperature were quantified at the same time.Based on the thermodynamic data of phase equilibrium,the dominant zone diagram of Li-Ca-Yb-NiO-F system is calculated,and the way of removing oxygen in Yb-Ni alloy is obtained.The solubility of Yb2O3 in LiF-CaF2 eutectic system was measured by isothermal saturation method.The solubility equilibrium period(≥90 min)was determined,the solubility-time regression equation was fitted,the solubility ΔfusGYb2O30 and γYb2O3 were determined,and the diffusion kinetic model was established.The mathematical models were measured and evaluated by rotation method,Archimedes method,pull cylinder method and continuously changing conductivity constant method,and the temperature-component regression equations of viscosity,density,surface tension and conductivity of LiF-CaF2 and LiF-CaF2-Yb2O3 molten salt systems were fitted.(2)Analysis of LiF-CaF2-Yb2O3 melting system structureThe freezing point drop method is used to analyze that the main existing forms of ions are Ca2+,F-and[CaF3]-complex ion clusters in low component LiF-CaF2 system.The structure and charge distribution of LiF,CaF2 and Yb2O3 are optimized by quantum chemistry,and the valence bond structure of Li-F,Ca-F and Yb-O is optimized.The valence bond structure and bond strength of Li-F,Ca-F and Yb-O are compared by bond order,bond length,parameters at the BCP point on the bond and the flexible force constant of the bond.The dissociation and combination of LiF and CaF2 are analyzed.The structures of LiF-CaF2 and LiF-CaF2-Yb2O3 were optimized by quantum molecular dynamics.ELF and energy analysis were carried out.The stability of bond length,coordination number of ion pairs and diffusion properties of Li-F,Ca-F,Li-O and YbO were analyzed.The results show that the strength of Yb-O bond is the strongest in LiF-CaF2-Yb2O3 system.(3)Analysis of electrode reaction Mechanism of Yb2O3 in LiF-CaF2 molten Salt systemThe electrochemical behavior of Yb(Ⅲ)and the nucleation and growth mode of Yb-Ni crystal were analyzed by transient electrochemical techniques(cyclic/square wave voltammetry and chronoamperometry)on W and Ni working electrodes.It was determined that Yb(Ⅲ)could complete the reduction process of Yb3+→Yb2+→Yb under the catalysis of Ni active electrode in(LiF-CaF2)eut.-Yb2O3 system,and alloyed on the surface of Ni electrode to form NixYby.The deposition and crystallization process of Ni electrode accords with the "mixed nucleation model",and the whole electrolysis process is controlled by Yb(Ⅲ)ion diffusion.The scanning electrochemical microscopy(SECM)was used to analyze the current response of Ni cathode surface under different Yb(Ⅲ)ion concentrations and potentials by means of positive and negative current feedback and generation collection mode.The results show that the active sites of Ni cathodes are increased with the increase of Yb(Ⅲ)concentration.With the negative potential,the banded two-dimensional active regions on the electrode surface expand and the three-dimensional active sites increase.(4)Yb-Ni alloy was prepared by LiF-CaF2-Yb2O3 molten salt system.Yb-Ni master alloy was prepared by electrolysis of LiF-CaF2-Yb2O3 molten salt system with nickel rod as cathode and graphite as anode according to different electrolysis temperature and voltage.Through the changes of appearance,microstructure and composition of the alloy products obtained by electrolysis under different variables,the optimum process parameters for the preparation of Yb-Ni master alloy by electrolysis in this system were obtained.The alloying evolution process of NixYby was analyzed by analyzing the alloy products obtained by constant voltage electrolysis at 1473 K,1523 K and 1573 K and the alloy phase distribution on the surface of nickel rod after electrolysis.The results showed that Ni and Ni2Yb were easily formed at 1473K and 1523K,while Ni,Ni17Yb2 and NisYb were easily formed at 1573K. |
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