| Sintered Nd-Fe-B is currently the most widely used and strongest magnetic permanent magnet material,which is widely used in wind power generation,power motors,computers and other fields.However,the coercivity of sintered Nd-Fe-B manufactured now cannot reach the ideal state,therefore,in recent years,the research goal of researchers is to further improve the coercivity without reducing the remanence and magnetic energy product.At the beginning,researchers generally used alloying to improve coercivity,its defects are low utilization rate of heavy rare earth,and greatly reduced remanence and maximum magnetic energy product of magnets.In recent years,researchers have discovered a novel process-grain boundary diffusion method,grain boundary diffusion process can greatly improve the coercivity of magnets without significantly reducing the remanence,and the utilization rate of heavy rare earths is high,thus reducing the production cost.In this paper,Tb films were deposited on the upper and lower surfaces of sintered Nd-Fe-B magnets by magnetron sputtering,the effects of different heat treatment processes on the magnetic properties of magnets were researched,the change rule of microstructure of magnets before and after grain boundary diffusion was analyzed,and the internal mechanism of coercivity improvement was explored.The main research results are as follows:(1)The addition of Pr element in the original magnet can reduce the melting point of neodymium-rich phase in grain boundary diffusion,and promote the diffusion of Tb into the magnet.On the one hand,the accumulation of Tb near the surface is reduced,and a thin Tb shell structure is formed,which significantly improves the coercivity of the magnet,and does not significantly reduce the remanent and magnetic energy product.On the other hand,the diffusion depth of Tb is also increased,the main phase grain boundary is clear at the center of the magnet,and the magnetic coupling effect of the magnet is greatly reduced.Moreover,the addition of Tb elements in the original magnet can increase the Tb concentration of the shell and increase the magnetocrystalline anisotropy field of the main phase.Due to the optimization of the above related microstructure,ultra-high performance Nd-Fe-B magnets with comprehensive magnetic properties close to 80[Hcj/k Oe)+(BH)max/MGOe]are obtained.(2)The influence of temperature on diffused Nd-Fe-B magnet was systematically studied.The magnets were diffused at 900 ～ 940℃ for 10 h,and it was concluded that 930℃ was an ideal diffusion temperature.The coercivity of the 930℃ diffused magnet increased from 32.76 k Oe to 42.12 k Oe,coercivity increased by 28.57%,the coercivity is obviously improved,and the remanence and magnetic energy product are reduced very little.The improvement of coercivity of the magnet after grain boundary diffusion treatment is mainly due to the Tb shell and Nd-rich grain boundary phase.Compared with the 900℃diffused magnet,the 930℃ diffused magnet has a higher diffusion temperature,which helps to form more Tb shell,improve the magnetocrystalline anisotropy of the magnet,and further increase the coercivity.After Tb diffused,the temperature stability is improved.(3)The influence of time on the diffused Nd-Fe-B magnet was systematically studied.The magnets were diffused at 900℃ for 10～20 h,and it was concluded that 20 h was an ideal diffusion time.The coercivity of the 20 h diffused magnet increased from 32.88 k Oe to 42.22 k Oe,coercivity increased by25.64%,the coercivity is obviously improved.By observing the microstructure of 10 h and 20 h diffused magnets,it is concluded that long-term diffusion is beneficial to the diffusion of Tb into the magnet to form more Tb shell structures,which can effectively improve the magnetocrystalline anisotropy field of the magnet,and further increase the coercivity.(4)The influence of coating thickness on the diffused Nd-Fe-B magnet was systematically studied.Under the same heat treatment state,compared with the 15 μm coated magnet,the coercivity of the 25 μm coated magnet is obviously improved,remanence and magnetic energy product is reduced littlely,and the performance of the 25 μm coated magnet is more excellent.Microstructure analysis shows that the Nd-rich phase of 25 μm coated magnet is more uniform,consecutive and clear,it can be known that the grain boundary can be optimized by increasing the thickness of the diffused magnet coating in a certain range,and improve the magnetic properties.The corrosion resistance descended after Tb diffused treatment,and the possibility of corrosion can be decreased by coating protective layer in the later stage. |
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