Optimization of core-shell structure distribution in sintered Nd-Fe-B magnets by titanium addition

In view of the uneven distribution of the core-shell structure of sintered Nd-Fe-B magnets after grain boundary diffusion,this study proposes to use high-melting-point and reactive element titanium(Ti) as an additive to increase the diffusion channels and to enhance the diffusion of heavy rare earth elements along the grain boundary phase.By adding Ti element,the diffusion depth and hence the intrinsic coercivity of magnets are increased significantly.The addition of Ti increases the coercivity ...     

Sandwiched structure of hot-deformed Nd-Fe-B permanent magnets processed by Nd-Cu eutectic alloys diffusion

A series of sandwiched structures with different near-surface mass fractions x(x=3 wt%,4 wt%,5 wt%)was employed to develop high-coercivity hot-deformed Nd-Fe-B magnets by the addition of 2 wt%Nd-Cu eutectic alloys via adjusting the middle thickness and near-surface thickness.The designed magnet with a pronounced composite structure shows a 23% increase in coercivity with a 6% loss of remanence by adjusting the sandwiched structure at 4 wt% Nd-Cu eutectic alloys in the near-surface regions.The results indicate that the near-surface Nd-Cu-rich "shell" structure can effectively suppress the magnetization reversal of overall magnets,enhancing the coercivity.With the help of loading stress,Nd-Cu liquid enriched at the near-surface regions of the sample is infiltrated into the Nd-Cu-lean middle region,resulting in a concentration gradient.Microstructure characterizations further demonstrate that the infiltrated Nd-Cu eutectic plays a critical role in inhibiting grain growth and intergranular magnetic interaction.The optimized microstructure features suppress the reversed magnetization process,which makes a positive contribution to coercivity.     

Magnetovisual method for monitoring thermal demagnetization of permanent magnets used in magnetostrictive actuators

The design and measuring potential of the latest generation of the magnetic scanner called Magscanner-Maglab System （MMS） was presented. It enabled the fast acquisition of 3D signals from magnetic sensors and their visualization as digitalized mag- netic images. This system was used for monitoring of a thermal demagnetization process of permanent magnets. The original method and measurement devices were capable for examination of magnetic, mechanical and thermal defects in cylindrical rods made of NdFeB and non-rare earth components. Effectiveness of the method and device was tested for the reference demagnetized magnet dedicated for magnetostrictive actuators.     

Effects of grain boundary ternary alloy doping on corrosion resistance of(Ce,Pr,Nd)-Fe-B permanent magnets

To satisfy the application of different environments,grain boundary doping is commonly used in the preparation of sintered magnets to improve the coercivity and the corrosion resistance.In this paper,the alloys were prepared by mixing different ratios of the master alloy(Ce,Pr,Nd)-Fe-B and the sintering aid(Pr,Nd)-Al.The coercivity of sintered(Ce,Pr,Nd)-Fe-B magnet is substantially enhanced by doping 2 wt%of(Pr,Nd)-Al,while the maximum energy product decreases slightly.We systematically investigated the corrosion behavior and micro structure of the sintered magnets in order to determine the mechanism of the degradation.The sintered(Ce,Pr,Nd)-Fe-B magnets with 2 wt% of(Pr,Nd)-Al addition exhibit the decreasing corrosion rate compared with others,due to the distribution of intergranular phases.The electrode potential difference between the main phase and the RE-rich phase is reduced by the addition of Al,improving the potential and stability of RE-rich phase due to the higher electrode potential of Al than that of Nd,Pr or Ce.In addition,the element distribution of the magnets doped by(Pr,Nd)-Al indicates that the Al-rich shell formed at the marginal area of the Ce-rich phase improves its stability.Therefo re,intergranular adding te rnary(Pr,Nd)-Al alloy powders results in both high coe rcivity and good corrosion resistance synchronously.     

Magnetic properties and microstructures of terbium coated and grain boundary diffusion treated sintered Nd-Fe-B magnets by magnetron sputtering

Tb coating on the surface of commercial sintered Nd-Fe-B magnet was prepared by DC magnetron sputtering.The secondary heat treatment was used to regulate the microstructure for the enhancement of coercivity,namely diffusion treatment and annealing treatment.The coercivity increases significantly from 18.3 to 28.0 kOe,the remanence decreases slightly from 14.1 to 14.0 kGs,and the comprehensive magnetic properties are higher than 75(Hcj+(BH)_max=76.7).SEM results indicate that,on the one hand,950℃is the optimal diffusion temperature.Lower diffusion temperature results in insufficient diffusion of Tb element.Higher diffusion temperature can lead to the main phase grain growth,the decrease of Nd-rich phase,and forming holes in the magnet.On the other hand,500℃is the optimal annealing temperature.Lower annealing temperature can result in the reduction of Nd-rich phase.Higher annealing temperature can generate the non-defined Nd-rich thin layer between grains.     

Effect of aluminum on microstructure and magnetic properties of sintered Nd-Fe-B magnets processed by grain boundary diffusion of Tb-Al

The grain boundary diffusion process(GBDP) of Tb can improve the coercivity of sintered Nd-Fe-B magnets.In this study,the effect of AI on the diffusion of Tb in the GBDP was investigated.The content of diffused Tb-Al was precisely controlled by adjusting the magnetron sputtering process.The Tb equivalent of Al was also studied.Results show that AI promotes the diffusion of Tb deeper into the magnet,reducing the thickness of the shell in the core-shell structure.This study is helpful for further ...     

Effect of Na~+ on catalytic performance of CoCe/ZSM-5 catalysts for oxidation of toluene

A CoCe/ZSM-5 catalyst was prepared by ultrasonic-assisted impregnation for the catalytic combustion of toluene.To study the effect of Na+on catalytic performance of CoCe/ZSM-5 catalysts,a series of different Si/Al ZSM-5 zeolites and catalysts doped with Na were synthesized.The experimental results show that 0.71 wt%Na+can inhibit active growth,generate more active small crystal grains,and promote improvement of the catalytic activity by the grain boundary segregation block mechanism,and the catalyst with 0.71 wt%of Na shows toluene conversion of 90 vol%at 250℃.Over 0.71 wt%of Na+content will neutralize the acid centre of the catalyst,lowering the specific surface area of the catalyst and resulting in a gradual decrease in the catalytic activity.     

Coercivity enhancement mechanism of grain boundary diffused Nd-Fe-B sintered magnets by magnetic domain evolution observation

The grain boundary diffusion(GBD) technology was used to prepare high performance Nd-Fe-B sintered magnets by NdH_3 and TbH_3 nanoparticle diffusion.The factors affecting the coercivity of GBD magnets include distribution of rare earth rich grain boundary phase and substitution of the heavy rare earth.In order to distinguish the influence of various factors on the coercivity,the microstructure and magnetic domain evolution of the original,reference,Nd-diffused,and Tb-diffused magnets were analyzed.The core-shell structure formed by heavy rare earth substitution is the main factor of coercivity enhancement,and it can transform the magnetic domain reversal mode from easy-nucleation(EN) to difficultnucleation(DN).With increasing the diffusion depth,the shell of the core-shell structure gradually becomes thinner,DN grains gradually decrease while the EN grains gradually increase,indicating that the magnetic domain reversal mode is directly related to the core-shell structure.     

Investigation on mechanism of magnetization reversal for nanocrystalline Pr-Fe-B permanent magnets by micromagnetic finite element methods

Magnetization configurations were calculated under various magnetic fields for nanocrystalline Pr-Fe-B permanent magnets by micromagnetic finite element method. According to the configurations during demagnetization process, the mechanism of magnetization reversal was analyzed. For the Pr2Fe14B with 10 nm grains or its composite with 10vol.% α-Fe, the coercivity was determined by nucleation of reversed domain that took place at grain boundaries. However, for Pr2Fe14B with 30 nm grains, coercivity was controlled by pinning of the nucleated domain. For Pr2Fe14B/α-Fe with 30vol.% α-Fe, the demagnetization behavior was characterized by continuous reversal of α-Fe moment.