Electrodeposition Of Rare Earth-Iron Group Alloy From An Ionic Liquid

Rare earth-iron group metal alloys possess much high magnetic anisotropy and high coercivity, which have been widely used for high density magnetic recording media. Rare earth-iron group metal alloy films were mainly prepared by physical methods, In order to overcome the disadvantage of physical methods, electrodeposition was used for the preparation of rare earth-iron group metal alloy films in organic solutions and in aqueous solutions, but these electrolytes have a lot of shortcomings, such as the negative reduction potential of rare earth ions, low solubility of rare earth salts and environmental pollution. Due to the wide electrochemical window of ionic liquid, electrodeposition of rare-earth metals in various ionic liquids have been reported. In this study, electrodeposition of rare earth-iron group metal alloy films from an ionic liquid were investigated, which provided a new method to prepare rare earth-iron group metal alloys magnetic recording materials.The electrodeposition of metallic cobalt from a 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM]BF₄) ionic liquid was investigated. The optimal electrolyte composition and technological conditions were as follows: Co(BF₄)2 0.5¹.5mol/L, deposition potential -1.2^-1.8V and bath temperature 40⁸0℃. X-ray diffraction (XRD) pattern indicated that there was a preferred orientation direction at 60℃. The electrochemical behavior of CO²⁺ in the ionic liquid on a platinum electrode has been studied by cyclic voltammetry and chronoamperometry. The results show that the diffusion coefficient D0 of CO²⁺ was calculated to be 1.76×10-8cm2/s at 60℃, the electrodeposition of metallic Co in the ionic liquid followed the mechanism of instantaneous nucleation and three-dimensional growth with diffusion controlled.Sm-Co alloy films were electrodeposited on copper coils in [BMIm]BF₄ ionic liquid by constant current. Sm-Co alloy films obtained were silvery, compact and adhesive. Electrolyte composition and technological conditions were optimized by systemic experiment: [Sm³⁺]/[CO²⁺] molar ratio 0.5:1.0².0:1.0, LiClO4 60g/L, 2-Butyne-1.4-diol 1.5g/L, bath temperature 40⁸0℃and current density 0.6⁰.8A/dm2. The content of Sm in Sm-Co alloy coatings varied in the range from 0% to 55%. The amorphous Sm-Co alloys are obtained, when the Sm content in the coating exceeds 21 percent by weight. The electrochemical behavior of Sm³⁺ and CO²⁺ in the ionic liquid on a platinum working electrode at 60℃has been studied by cyclic voltammetry (CV) and liner sweep voltammetry (LSV). In the absence of cobalt, Sm³⁺ was not reduced to Sm alone in the ionic liquid, but it could be inductively codeposited with CO²⁺. Cobalt coatings were electrodeposited at the potential of -0.92V -1.51V, Sm-Co coatings were electrodeposited at lower potential of -1.51 V.Tb-Fe-Co alloy films prepared were electrodeposited by pulse technique and constant potential on copper coils in an ionic liquid at 50℃. The good alloy films with high Tb content were obtained by pulse technique. The optimized technics parameters are as follows. Some factors could increase Tb content in Tb-Fe-Co alloy film, such as high average voltage, low duty cycle moderate pulse frequency, high temperatures and low the stirring speed. The content of Tb in the Tb-Fe-Co alloy film obtained was 50% at 50℃. The X-ray diffraction pattern of the alloy film showed that the alloy film was composed of Tb2Co17, Fe-Co intermetallic compound and the amorphous Tb-Fe-Co alloys. An increase in 2-butyne-1.4-diol concentration could not only improve the content of Tb in Tb-Fe-Co alloy coating and refine the crystal grains, but also enhance cathodic polarization and promote the deposition of rare earth metal.The magnetic properties of the Co, Sm-Co and Tb-Fe-Co films were measured by vibrating sample measurement (VSM). Cobalt films electrodeposited by constant potential on copper substrates possess obvious magnetic anisotropy and the high coercivity at 60℃, the coercivity was 248.1Oe. Sm-Co alloy films prepared by constant current have obvious magnetic anisotropy and were used as longitudinal magnetic recording medium. When the content of Sm was 12.6%, Sm-Co alloy film possesses the best magnetic properties, the squareness and coercivity were 0.413 and 243.7Oe, respectively. With the increase of Sm content, Sm-Co alloy films obtained possess lower coercivity and squareness resulting from the amorphous Sm-Co alloy, the hard magnetic properties turned to soft magnetic. Tb-Fe-Co alloy film electrodeposited by pulse technique possesses higher squareness and coercivity, which was used as the longitudinal magnetic recording medium, the squareness and coercivity were up to 0.568 and 1522Oe, respectively. Tb-Fe-Co alloy film possesses the best magnetic properties with the frequency of 3.0kHz and the duty cycle of 60%.