Study On The Electrodeposition Of Ti₅Si₃ Alloy And Its Reaction Mechanism In Chloride Molten Salt

As a typical intermetallic compound,Ti₅Si₃ has received a tremendous amount of attention due to its distinct properties,such as low density(4.32 g cm^–3),low electrical resistivity(50?120??cm),high melting point(2130°C),excellent oxidation resistance and high specific strength,etc.At present,most of the preparation methods suffer from the problems such as high cost,high energy consumption,complex production process,limited product purity,etc.Herein,we report the successful demonstration of Ti₅Si₃ alloy electrodeposition in molten CaCl₂-CaO containing titanium/silicon dioxides at a relatively low temperature(850°C).In this study,high temperature in-situ X-ray diffraction(in-situ XRD)and high temperature in situ Raman spectroscopy(in-situ Raman)were used to analyze the dissolution behavior and the structure of molten salt system.Besides,the formation mechanism of Ti₅Si₃ prepared by electrodeposition was analyzed based on cyclic voltammetry(CV)analysis,which may provide a theoretical guidance for the preparation of Ti₅Si₃ in CaCl₂-CaO-SiO₂-TiO₂ system.In addition,three dimensional atomic probe(3DAP),transmission electron microscope(TEM)and electron backscatter diffraction(EBSD)were also used to analyze the phase,composition and crystal orientation of the products.The main results are as follows:(1)According to the in-situ XRD results,Ca₄OCl₆,Ca₂SiO₄ and Ca₃SiO₄Cl₂ were formed in CaCl₂-CaO-SiO₂ system during the whole heating process,and Raman peaks were corresponding to[SiO₄]^4-.In the CaCl₂-CaO-TiO₂ system,Ca₄OCl₆ and Ca TiO₃were formed,and their characteristic Raman peaks at 850?were related to[TiO₃]^2-.When adding SiO₂ and TiO₂ into the molten CaCl₂-CaO simultaneously,the in-situ XRD and in-situ Raman results showed that the phases and ions appeared in the molten salts were the same as that of CaCl₂-CaO-SiO₂ and CaCl₂-CaO-TiO₂,and no obvious changes have been observed.These three systems indicate that CaO can promote the solution processes of SiO₂ and TiO₂ in molten CaCl₂.When adding SiO₂ and TiO₂ into the molten CaCl₂-CaO simultaneously,the silicate ions and titanate ions coexist in the molten salts.(2)At 850?,the reduction potential of Si(?)in CaCl₂-CaO-SiO₂ system was determined at around-1.08 V,the reaction has been controlled by diffusion mass transfer,and the diffusion coefficient was calculated to be 3.31×10^-5 cm² s^-1.In CaCl₂-Ca₂SiO₄ system,the reaction Si(?)?Si(0)proceeds a two-step reduction process.The reduction potentials were-1.1 V and-1.3 V,respectively,and the reaction processes were also controlled by diffusion mass transfer.Besides,the electrochemical behavior of Ti(IV)was also studied in CaCl₂-CaO-TiO₂ and CaCl₂-Ca TiO₃ systems.The potentials of the two-step reduction preocess of the two systems were about-1.4V,-1.6 V and-1.2 V,-1.4 V respectively,and the reduction processes were also controlled by diffusion mass transfer.The cyclic voltammetry curves of CaCl₂-CaO-SiO₂-TiO₂ showed that the reduction potentials did not change when Ti(?)and Si(?)coexisted at the same time.However,due to the relative low solubility of TiO₂ in CaCl₂,one of the reduction peaks of TiO₂ has not been observed.The reduction potentials of Ti(IV)and Si(IV)in CaCl₂-Ca₂SiO₄-Ca TiO₃ system shifted slightly.In addition,the reduction order of Si(IV)was prior to that of Ti(IV)in both of the systems.(3)The thickness of Ti₅Si₃ alloy film increased with the increasing electrodeposition time.The results also showed that the morphology of the products can be significantly influenced by current density.By controlling current density at the range from 10 m A cm^–2 to 25 m A cm^–2,Ti₅Si₃ products with different morphologies,i.e.,dendritic particles,dense films and porous powders can be obtained in a controlled manner.The general reaction mechanism of the electrodeposition of Ti₅Si₃ from CaCl₂-CaO-SiO₂-TiO₂ molten salt system can be summarized as three periods,i.e.,(i)CaO-assisted dissolution of SiO₂ and TiO₂ in molten CaCl₂ to form silicate ions(SiO₄^4–)and titanate ions(TiO₃^2–),respectively;(ii)the electrodeposition of Si and Ti from silicate ions and titanate ions,respectively;(iii)the formation of stable Ti₅Si₃ through Si,Ti?Ti_xSi_y?Ti₅Si₃.