The Coercivity And Microstructure Improved By Grain Boundary Diffusion Process In Nd(Ce)-Fe-B Thin Films

It is important that the application of NdFeB magnets in electric vehicles and wind power generators.The coercivity should be higher than 0.8T at 120-180℃ in these work conditions.To meet such application,the coercivity should be around 3T at room temperature.However,the coercivity in commercial NdFeB magnets is about 1.2T and far from the target.An industrial approach is addition heavy rear-earth Dy and Tb to partially substitute Nd atoms in Nd₂Fe₁₄B and substantially improve the coercivity to nearly 3T.But the addition of too much heavy rear-earth will cause the reduce of saturation magnetization and the increase of cost.Grain boundary diffusion process was considered to be an effective approach which can improve the coercivity substantially using less heavy rear-earth.So,in this paper,the coercivity and microstructure was studied in Dy-based alloys grain boundary diffusion process.Ta（20 nm）/Nd₁₄Fe₇₇B₉（100 nm）/diffusion layer（10 nm）/Ta（20 nm）thin films(diffusion layer=Dy,Dy₇₀Cu₃₀和Dy₈₀Ag₂₀)was prepared by magnetron sputtering.The coercivity was improved to 4T from 1.06T after diffusion process.And the Nd-rich GBs distribute more continuous and more uniform.This distribution of Nd-rich GBs have a more effective decoupling between neighboring Nd₂Fe₁₄B grains.The structure of Nd₂Fe₁₄B-core/(Nd_1-xDy_x)₂Fe₁₄B-shell was achieved because Dy atoms partially substitute Nd atoms.Inaddtiton,with the demand of NdFeB magnets increasing rapidly,and the Nd,Dy and Tb inadequate storage,we need an alternative magnets.The intrinsic magnetic properties of CeFeB is between ferrites and NdFeB.The coercivity and energy product can be improved at the same time.So the coercivity and microstructure was studied in Dy-based alloys grain boundary diffusion process.Ce₁₃Fe₇₉B₈ ribbons were prepared by melt-spinning and diffusion-processing with minor amounts of Nd₇₀Cu₃₀ and Nd₈₀Ag₂₀ alloys.Low coercivities of 0.2T were obtained in the as-melt-spun ribbons and annealed ribbons although Ce-rich grain boundaries were observed.By diffusion process,the coercivity was increased to be over 0.7T,due to the formation of magnetically hardened(Nd_xCe_1-x)₂Fe₁₄B shell.The chemistry of Ce-rich grain boundary phase was studied and face-center-cubic crystallographic structure was determined by nano-beam diffraction.The Henkel plots confirmed the enhanced magnetocrystalline anisotropy due to the formation of core/shell microstructure.Cr（20nm）/Ce₁₃Fe₇₉B₈（100nm）/diffusion layer（11nm）/Cr（20nm）thin films(diffusion layer=Ce₇₀Cu₃₀,Nd and Nd₇₀Cu₃₀)were prepared by magnetron sputtering.The coercivities were still as low as 0.6 T with poor squareness even after diffusion process.By Nd substitution for Ce in Ce_13-xNd_xFe₇₉B₈ magnetic layer,squareness was greatly improved.The coercivity was tunable between 0.6 T and 2.34 T and maximum energy product was ranged from 83.4 kJ/m³ to 204.6 kJ/m³ depending on the Nd composition.Transmission electron microscopy observation showed the columnar 2:14:1 phase grains with typical core/shell microstructure and rare-earth-rich grain boundaries.Our results demonstrated the promising commercialization potential of CeFeB based magnets with performance between ferrites and NdFeB magnets.