| High-tech industry applications require that the sintered Nd-Fe-B magnets has the characteristics of high coercivity,high magnetic energy product and high stability under the premise of high remanence.As a method for preparing high-coercivity sintered Nd-Fe-B magnet,the grain boundary diffusion process(GBDP)can significantly increase the coercivity of the magnet without reducing the remanence.It also can significantly reduce the amount of heavy rare earth elements(HREE)added to reduce production costs.However,the optimal heat treatment process for grain boundary diffusion Tb element is still uncertain,and no large-scale industrial application has been seen.In addition,Tb element distribution in the magnet under different processing conditions still needs to be further studied.In this paper,metallic Tb was deposited on the surface of sintered Nd-Fe-B by magnetron sputtering method.The effects of different heat treatment processes and the thickness of metallic Tb film on the microstructure and magnetic properties of the magnets were studied by pulse magnetic field magnetometer(PFM),scanning electron microscope(SEM),glow discharge spectrum analyzer(GDOES)and fluxmeter.The mechanism of grain boundary diffusion process and the coercivity improvement was further clarified to provide technical parameters for large-scale preparation of high-performance and low-HREE sintered Nd-Fe-B magnets.The main results are summarized as follows:1.After the grain boundary diffusion treatment,Tb element on the surface of the magnets permeated into the inside along the grain boundary,and formed the(Nd,Tb)2Fe14B shells with high magnetic crystal anisotropy on the surface of the main phase grains,which enhanced the anisotropy field of the magnetic phase grains and inhibited the nucleation of anti-magnetization domain.At the same time,the Nd-rich phase was uniformly and continuously distributed along the grain boundary.This distribution effectively reduced the coupling effect of magnetic exchange between magnetic phase particles and significantly improved the coercivity of the magnet.Since Tb was concentrated in the surface region of the grains without entering the interior,which avoided the anti-ferromagnetic coupling between Tb and Fe.That was the main reason why the GBDP magnet still has high remanence.With the diffusion heat treatment temperature increasing and the time extending,the depth and concentration of the heavy rare earth element diffused into the magnet was gradually increased,and the ratio of(Nd,Tb)2Fe14B in the magnet was increased.Thereby the coercivity was improved.However,excessive diffusion temperature and time would lead to the growth of grain,which also leaded to the change of structure and distribution of Nd-rich phase.These changes reduced the increase in coercivity of the magnet.2.After 925?×10 h diffusion treatment,Tb element diffused into the magnet center,that improved the proportion of the core-shell structure and the uniformity of the microstructure and chemical composition.After tempering at 500?,the morphology and distribution of the Nd-rich phase in the diffusion magnet was obviously optimized.The thin and continuous grain boundary phase obstructed the movement of the magnetization reversal domain wall,which increased the coercivity from 1077.25 kA/m to 1630.9 kA/m,with an increase of 51.39%.Besides,the squareness of the demagnetization curve stayed above 90%.The remanence and energy product of the GBDP magnet almost unchanged compared to the initial magnet.The analysis concluded that 925?×10 h+500?×2 h was the best grain boundary diffusion process parameters for the N52 Nd-Fe-B sintered magnets with Tb metallic film.3.When the Tb film deposited on the magnet is relatively thin,there was no(Nd,Tb)2Fe14B grain layer formed in the surface area of the GBDP magnet,and the diffusion distance of Tb element did not exceed 300 ?m.With the thickening of Tb film layer on the surface of the magnet,the diffusion distance and depth of Tb in the magnet was significantly increased.The thickness of(Nd,Tb)2Fe14B grain layer and the ratio of the core-shell structure also was gradually increased.For the GBDP magnet with the Tb film thickness of 13 ?m,the concentration of Tb element with the diffusion depth tended to be linear.4.With the increase of Tb film thickness,the coercivity of single-side deposited magnet increased significantly,but the remanence and magnetic energy product decreased.Compared with the single-side Tb film deposited magnet,the coercivity of the double-side magnet was significantly increased,and the remanence and magnetic energy product were close to the original magnet.At the same time,the magnet had a high demagnetization curve squareness.5.After grain boundary diffusion treatment,the thermal stability of the N52 sintered Nd-Fe-B magnet magnetic was significantly improved and the highest temperature up to 120 ?.The results shown that magnetron sputtering method with grain boundary diffusion process is an effective way to improve the thermal stability of Nd-Fe-B sintered magnets. |
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