Fundamental Study On Preparation Of Sm₂Fe₁₇ Alloy By Molten Salt Electrolysis

Sm2Fe17Nx is considered to be one of the most visible high-performance permanent-magnet materials and the potential magnet in electromagnetic industry,which has the higher Curie temperature and greater resistance to corrosion,is widely used in electromagnetic field.The Sm2Fe 17 alloy is a precursor of Sm2Fe17Nxx materials,which dominate the magnetic performance of Sm2Fe17Nx.But impurities of ?-Fe,SmFe2,SmFe3 or CaO are unavoidable in Sm2Fe17 alloy during the traditional production processes.In this work,a molten salt electrolysis is used to prepare Sm2Fe17 alloy,in which the preparation of electrolytic raw materials,electrochemical behavior of Sm3+ in molten salt,preparation of Sm2Fe17 alloy by molten salt electrolysis and revealing of the preparation mechanism of Sm2Fe17 alloy.Firstly,SmCl3 and SmF3 were prepared by using Sm2O3,respectively.The phase transition process of SmF3 was investigated.It was understood that the SmF3 crystal with smaller size appeared at low temperature,which should have a high surface energy,resulting in decrease of transformation temperature of SmF3 from hexagonal to orthorhomic.Meanwhile,the preparation process is optimized by adding a low-temperature fluorination(LTF)process for decreasing of the volatilization rate of NH4HF2.Secondly,CaCl2-CaF2-SmCl3 and LiF-CaF2-SmF3 systems were used as electrolyte to investigate the electrochemical behavior from Sm3+ to Sm0 on the cathode.The reduction on the iron electrode includes two steps.The electroreduction from Sm3+ to Sm2+ is controlled by the diffusion of Sm3+ in the molten salt and then Sm2+ is reduced to form the Sm2Fe17 alloy by an underpotential reation with iron.In addition,the preparation of Sm2Fe 17 alloy was carried out by the chronopotentiometry to find that the thickness of Sm2Fe 17 layer increased with increase of the initial concentration of Sm3+ when it was less than 1.74×10-5 mol·cm-3,while it did not increased obviously when the initial concentration of Sm3+ exceeded 1.74×10-5 mol cm-3,which indicates the reaction is controlled by the diffusion of atoms in Sm2Fe17 layer in the late stage.The mechanism of Sm2Fe17 alloy preparation on iron electrode in the electrolysis process was proposed.Finally,the diffusion of atoms in Sm2Fe17 layer was studied by Fe/Sm diffusion couple,which indicate the rate of reaction is mainly affected by the diffusion of iron in the Sm2Fe17 layer at the late stage of electrolysis process.It makes clearer the mechanism of Sm2Fe17 alloy preparation.