A facile process to optimize performance of regenerated Nd-Fe-B sintered magnets:Chemo-selective dissolution washing

In this work,the recycled Nd-Fe-B powders and regenerated Nd-Fe-B sintered magnets with low impurity content were successfully prepared from Nd-Fe-B magnet sludge via reduction diffusion(RD)method followed by a chemo-selective dissolution washing proc ess.The chemo-selective dissolution effect of various solution(deionized water,dilute acetic acid solution,NH_(4)Cl-methanol solution) was evaluated by impurity content and magnetic properties of the recycled Nd-Fe-B powder.The NH_(4)Cl-methanol solution can selectively remove impurities with minimal damage to the magnetic phase.Besides,the optimal NH_(4)Cl concentration and liquid-to-solid ratio were investigated.As a consequence,the contents of Ca,O,and H after optimal washing process are reduced to 0.07 wt%,0.31 wt% and 0.22 wt%,respectively.Hence,M_(3) Tis increased to 146.72 emu/g,which is 33% higher than that of the initial sludge.Then,the regenerated Nd-Fe-B sintered magnets with properties of B_(r)=11.66 kG,H_(cj)=16.49 kOe,and(BH)_(m)=31.78 MGOe were successfully prepared by mixing with 40 wt% Nd4Fe14B alloy powders.Compared with the corresponding regenerated magnets washed with deionized water,the remanence and coercivity are increased by 18% and 59%,respectively.

Study on preparation and properties of CeO_2/epoxy resin composite coating on sintered NdFeB magnet

The CeO2/epoxy resin composite coating was deposited on NdFeB substrate by cathode electrophoresis method for enhancing the anticorrosion and anti-wear performances. The morphologies and structures were characterized by a scanning electron microscope and an X-ray diffractometer. The micro hardness of the composite coating was evaluated by a microhardness tester. The corrosive behaviors of the coatings were studied by potentiodynamic polarization curve, electrochemical impedance spectroscopy and neutral salt spray tests. The concentration of CeO2 nanoparticles（NPs） in the electrophoresis bath was optimized according to the coating structures and anticorrosion performances. The results show that CeO2 NPs can enhance the microhardness of the composite coatings. Moreover, the nanoparticles disperse uniformly in the matrix when the concentration is lower than 30 g/L. The microhardness of CeO2/epoxy resin（30 g/L） composite coating is about 63% higher than that of the blank epoxy resin coating. And the NSS time of the CeO2/epoxy resin（30 g/L） composite coated sample can reach 1248 h.Meanwhile, the composite coatings possess no deteriorate influence on the magnetic properties of NdFeB substrates. The anticorrosion mechanisms of the composite coatings on the NdFeB substrate are deeply discussed.

Improvement of coercivity and thermal stability of Nd-Fe-B sintered magnets by intergranular addition of Tb80Fe20 alloy

To improve the coercivity and temperature stability of Nd-Fe-B sintered magnets for high-temperature applications,the eutectic Tb_(80)Fe_(20)(wt%)alloy powders were added into the Nd-Fe-B sintered magnets by intergranular method to enhance the coercivity(H_(cj))and thermal stability.The micro structure,magnetic properties and thermal stability of the Nd-Fe-B magnets with different Tb_(80)Fe_(20)contents were studied.The experimental results demonstrate that the coercivity(H_(cj))of the sintered Nd-Fe-B magnet is significantly enhanced from 14.12 to 27.78 kOe,and the remanence(Br)decreases not obviously by introducing 4 wt%Tb_(80)Fe_(20)alloy.Meanwhile,the reversible tempe rature coefficients of coercivity(β)and remanence(α)of the Nd-Fe-B magnets are increased from-0.5634%/℃to-0.4506%/℃and-0.1276%/℃to-0.1199%/℃at 20-170℃,respectively.The Curie temperature(TC)of the Nd-Fe-B magnet is slightly enhanced with the increase of Tb_(80)Fe_(20)content.Moreover,the irreversible flux magnetic loss(hirr)is obviously reduced as Tb80Fe20addition increases.Further analysis of the microstructure reveals that a modified microstructure,i.e.clear and continuous RE-rich grain boundary layer,is acquired in the sintered magnets by introducing Tb_(80)Fe_(20)alloy.The associated mechanisms on improved coercivity and thermal stability were comprehensively researched.

Calcium hydride reduced high-quality Nd-Fe-B powder from Nd-Fe-B sintered magnet sludge

The structural and magnetic properties were studied for recycling Nd-Fe-B powders from Nd-Fe-B sintered magnets sludge via reduction diffusion(RD)with calcium hydride(CaH_(2))particles.For comparison,traditional reducing agent calcium granules were applied to prepare recycled Nd-Fe-B powders.Finer particle size and better size distribution as well as lower impurity content are achieved by using CaH_(2)instead of Ca.In detail,the average particle size of the recycled Nd-Fe-B powder is reduced from 4.66 to 3.43μm,and the bimodal distribution disappears.Moreover,the residual calcium content and oxygen content are reduced to about 0.080 wt%and 0.32 wt%.As a consequence,the roomtemperature magnetization of the CaH_(2)-recycled Nd-Fe-B powder is increased to 146.30 emu/g,6.8%and 33%,respectively,higher than that of Ca-reduced powder and the initial sludge.Further analysis indicates that CaH_(2)is able to reduce the sludge at lower tempe rature to fabricate well-dispersed,unifo rm recycled powder with high magnetization arising from a combination factors of its low melting point,low thermodynamic behavior,and the release of hydrogen during the reaction.

Corrosion resistance and mechanical properties of (Ho,Nd)FeB magnets

(Ho,Nd)FeB magnets with different Ho contents were prepared by Ho substitution for part of Nd during the casting process.Effects of Ho contents on the corrosion resistance and mechanical properties of(Ho,Nd)FeB magnets were analyzed by a highly accelerated aging tester,an electrochemical workstation,a microhardness tester,a bending tester,a scanning electron microscope and an X-ray diffractometer.Results show that the addition of Ho can change the main phase structure,optimize the distribution of rare-earth rich(RE-rich) phases in grain boundary,and improve the corrosion resistance and mechanical properties of NdFeB magnets.When the content of Ho increases from 0 to 21.0 wt%,the weight loss of magnets decreases from 2.672 to 0.933 mg/cm^(2),and the microhardness and bending strength increase from 528.74 HV and 374.92 MPa to 633.84 HV and 459.80 MPa,respectively.