Study On Process And Properties Of High Coercivity Nd-Fe-B Magnets By Electrophoretic Deposition

At present,studies on the preparation of high coercivity Nd-Fe-B magnets by electrophoretic deposition by grain boundary diffusion mainly focus on the enhancement of magnetic properties by HREE,but there are few studies on the correlation between grain size,deposition parameters and deposition rate,coating adhesion properties and subsequent grain boundary diffusion effects in electrophoretic deposition.In fact,the investigation of electrophoretic deposition law can guide the determination of appropriate deposition parameters to realize the quantitative deposition of HREE,which is of great significance for the efficient utilization of heavy rare earth resources.This paper combined Biesheuvel equation to study the sedimentary time,voltage and slurry concentration on the deposition rate and coating morphology and the influence law of combining performance systematically,explored the addition Mgl2 to the electrophoretic deposition process and the influence of grain boundary diffusion.An electrophoretic deposition coating with stable deposition,good adhesion performance and stable improvement of coercivity of sintered magnets by grain boundary diffusion was obtained.The deposition characteristics of TbF3 powders with two particle sizes were compared.The fastest deposition rate of submicron TbF3 powders was 3.28 mg/(cm2·min)at a concentration of 12 g/L and a voltage of 60 V,while the deposition rate of nano TbF3 powder was 6.41 mg/(cm2·min)under the same conditions.The deposition law of TbF3 powders with two particle sizes is in good agreement with the Biesheuvel deposition equation,which means the deposition amount is proportional to the deposition voltage,suspension concentration and deposition time.The deposition of submicron TbF3 powders is easy to settle due to its large particle size and less proportion of deposition particles.With the extension of deposition time,the deposition rate decreases significantly.The impurity ions(NH4+,NO3-)in the nano TbF3 powders make the coating have poor adhesion and easy to fall off.The impurity ions can be removed by heat treatment.The adhesion property of nanopowders was improved after removing impurity ions,but the deposition rate decreased significantly due to the reduction of charged ions.In order to improve the ion concentration in the suspension,5 wt.%MgCl2 was added into the nano TbF3 powders.The results showed that the deposition rate and coating adhesion could be significantly improved by this method.After the addition of MgCl2,the deposition rate of TbF3 coating increased from 1.10 mg/(cm2·min)to 2.95 mg/(cm2·min),and the vibration weight loss rate decreased from 12.87%to 4.21%.SEM results show that the coating surface shows regular gully morphology and uniform distribution of microcracks after adding MgCl2,which improves the bonding properties of the coating.After heat treatment at 875?×10 h+500?×1h,the coercivity of the sample with 0.3 wt.%TbF3 coated increased from 18.5 kOe to 22.4 kOe,increasing by 21.1%.The core-shell structure observed by BSE-SEM and the increase of Tb content at the grain boundary determined by EDS are the key reasons for the improvement of coercivity.