Electrochemical Synthesis And Mechanism Of Rare Earth-iron Magnetic Materials In Deep Eutectic Solvent

Rare earth（RE）/transition metallic（TM）thin film materials have huge potential application value duo to their unique structure and magnetic properties.Fe-Co-Ni coating materials have attracted a lot of attention,because of their unique characteristics,such as prominent magnetism performance,outstanding thermal stability,high permeability,low hysteresis loss,good anisotropy energy,and super physical and chemical properties.In addition,Tb-Fe-Co thin films are considered as representative materials for magneto-optical recording optical disc media owing to their low saturation magnetization and high vertical magnetization anisotropy.Traditionally,Fe-Co-Ni and Tb-Fe-Co thin films are mainly prepared by vacuum evaporation and magnetron sputtering.However,these methods have disadvantages such as high cost,difficult operation and poor coating quality.While,electrodeposition method can overcome the above shortcomings and realize production for large scale.Electrodeposition of Fe-Co-Ni and Tb-Fe-Co thin films are usually implemented in aqueous solutions and non-aqueous solutions including molten salts,organic solvents and ionic liquids（ILs）.However,it is difficult to electrodeposit in aqueous solutions due to the negatively reductive potential of RE metals.In addition,there are some problems in organic solvents,molten salts and ILs.Deep eutectic solvents（DESs）have such advantages as easy preparation,water stability,fine ionic conductivity,supernal thermal stability and strong solubility,which provides a new way for preparation of RE-TM thin film materials by electrodeposition.Fe-Co-Ni and Tb-Fe-Co film material was prepared in ChCl/2Urea DES in this dissertation,providing a novel theoretical foundation for the electrodeposition of RE-TM film material.ChCl/2Urea DES was prepared in the ratio of 1:2 and used to prepare Fe-Co-Ni coating via three-electrode potentiostatic electrodeposition.The technological conditions of electrodeposition were obtained by experimental as follows:deposition temperature 30～60 ^oC,deposition time 600～4200 s,deposition potential-0.8～-1.5 V,working electrode Cu sheet electrode.The CV results showed that the process of reduction of Fe-Co-Ni in ChCl/2Urea DES was irreversible,which was diffusion-controlled.The nucleation of Fe-Co-Ni on the Cu electrode in ChCl/2Urea DES was three-dimensional instantaneous nucleation demonstrated by CA result.The effect of deposition time,deposition temperature,deposition potential,concentration of metal salts,ultrasonic power and additives on the microstructure,chemical composition,crystal structure,corrosion resistance and magnetic properties of Fe-Co-Ni coatings was investigated by SEM,XRD,potentiodynamic polarization and VSM.SEM results showed that the surface morphology,microstructure,composition and grain size of Fe-Ni-Co alloy were influenced by the above deposition conditions.XRD results showed that the Fe-Co-Ni coatings were composed of face-centered cube（fcc,Ni Fe）and body-centered cube crystals（bcc,Fe Co）.The tafel and VSM results showed that Fe-Co-Ni coating was prepared in ChCl/2Urea DES under different deposition conditions,which had good corrosion resistance and magnetic anisotropy.After that,Tb-Fe-Co thin film material was prepared on Cu substrate in ChCl/2Urea DES by electrodeposition with three electrodes.The microtopography,composition of coating and crystal structure of Tb-Fe-Co thin films were systematically studied by SEM,EDS and XRD.ChCl/2Urea DES electrolytes containing different metal ions were analyzed by FT-IR.The electrochemical behavior and electrocrystallization mechanism of ChCl/2Urea DES electrolytes containing Tb³⁺,Fe²⁺and Co²⁺were studied.The effect of deposition time,deposition temperature,working electrode,deposition potential,concentration of Tb³⁺,ultrasonic and additives on the corrosion resistance and magnetic properties of Tb-Fe-Co coatings was also studied.CV results revealed that the reduction process of Tb-Fe-Co thin films in ChCl/2Urea EDS containing Tb³⁺,Fe²⁺and Co²⁺was an irreversible diffusion-controlled reaction.The CA result demonstrated that the nucleation of Tb-Fe-Co on the Cu electrode in ChCl/2Urea DES was three-dimensional instantaneous nucleation.SEM and EDS analysis displayed that the microtopography and elemental content of Tb-Fe-Co coating.In addition,the microtopography and elemental content of Tb-Fe-Co coating were also influenced by electrodeposition potential and additive.XRD showed that the Tb-Fe-Co coating gradually converted to amorphous with the increase of Tb content in the Tb-Fe-Co coating.The corrosion resistance of the Tb-Fe-Co coating was evidently enhanced as Tb content increased in Tb-Fe-Co coating.The Tb-Fe-Co thin films prepared by electrodeposition had high coercivity and obvious parallel magnetic anisotropy,which could be used as magnetic recording medium materials.The coordination structures of Fe²⁺,Co²⁺and Ni²⁺in ChCl/2Urea DES electrolyte was analyzed by Fourier transform infrared spectroscopy and UV-Vis.It was found that the common coordination structures of Ni²⁺ions in ChCl/2Urea DES were tetrahedral or octahedral.The coordination structures of Fe²⁺and Co²⁺were tetrahedral structures,and there might also be complexes between Urea and H₂O and metal ions.On the basis of experimental verification,the method of density functional theoretical（DFT）calculation was used to research the structure of Fe²⁺,Ni²⁺,Co²⁺stable complex and its frontier orbital information.The results showed that[FeCl·3H₂O]⁺,[CoCl·3H₂O]⁺and[NiCl₆]^4-had the highest reactivity on the electrode surface,which may be reduced at the cathode.