The grain boundary diffusion process(GBDP)has proven to be an effective method for enhancing the coercivity of sintered Nd-Fe-B magnets.However,the limited diffusion depth and thicker shell struc-ture have impeded the...The grain boundary diffusion process(GBDP)has proven to be an effective method for enhancing the coercivity of sintered Nd-Fe-B magnets.However,the limited diffusion depth and thicker shell struc-ture have impeded the further development of magnetic properties.Currently,the primary debates re-garding the mechanism of GBDP with Tb revolve around the dissolution-solidification mechanism and the atomic substitution mechanism.To clarify this mechanism,the microstructure evolution of sintered Nd-Fe-B magnets during the heating process of GBDP has been systematically studied by quenching at different tem peratures.In this study,it was found that the formation of TbFe_(2) phase is related to the dis-solution of _(2)Fe_(14)B grains during GBDP with Tb.The theory of mixing heat and phase separation further confirms that the Nd_(2)Fe_(14)B phase dissolves to form a mixed phase of Nd and TbFe_(2),which then solidifies into the(Nd,Tb)_(2)Fe_(14)B phase.Based on the discovery of the TbFe_(2) phase,the dissolution-solidification mechanism is considered the primary mechanism for GBDP.This is supported by the elemental content of the two typical core-shell structures observed.展开更多
Grain boundary diffusion technology is pivotal in the preparation of high-performance NdFeB magnets.This study investigates the factors that affect the efficiency of grain boundary diffusion,starting from the properti...Grain boundary diffusion technology is pivotal in the preparation of high-performance NdFeB magnets.This study investigates the factors that affect the efficiency of grain boundary diffusion,starting from the properties of the diffusion matrix.Through the adjustment of the sintering process,we effectively prepared magnets with varied densities that serve as the matrix for grain boundary diffusion with TbH,diffusion.The mobility characteristics of the Nd-rich phase during the densification stage are leveraged to ensure a more extensive distribution of heavy rare earth elements within the magnets.According to the experimental results,the increase in coercivity of low-density magnets after diffusion is significantly greater than that of relatively high-density magnets.The coercivity values measured are 805.32 kA/m for low-density magnets and 470.3 kA/m for high-density magnets.Additionally,grain boundary diffusion notably enhances the density of initial low-density magnets,addressing the issue of low density during the sintering stage.Before the diffusion treatment,the Nd-rich phases primarily concentrate at the triangular grain boundaries,resulting in an increased number of cavity defects in the magnets.These cavity defects contain atoms in a higher energy state,making them more prone to transition.Consequently,the diffusion activation energy at the void defects is lower than the intracrystalline diffusion activation energy,accelerating atom diffusion.The presence of larger cavities also provides more space for atom migration,thereby promoting the diffusion process.After the diffusion treatment,the proportion of bulk Nd-rich phases significantly decreases,and they infiltrate between the grains to fill the cavity defects,forming continuous fine grain boundaries.Based on these observations,the study aims to explore how to utilize this information to develop an efficient technique for grain boundary diffusion.展开更多
A sintered Nd-Y-Fe-B magnet was designed and manufactured by the multi-main-phase process.Unevenly distributed Y in the magnet decreases the adverse magnetic weakening effect of Y on the coercivity.Grain boundary diff...A sintered Nd-Y-Fe-B magnet was designed and manufactured by the multi-main-phase process.Unevenly distributed Y in the magnet decreases the adverse magnetic weakening effect of Y on the coercivity.Grain boundary diffusion process(GBDP)was conducted to further enhance the coercivity of the Nd-Y-Fe-B magnet.The coercivity increases significantly from 884 to 1741 kA/m after GBDP with Pr_(60)Tb_(10)Cu_(30)alloy.The mechanism of the coercivity enhancement is discussed based on the microstructure analysis.Micromagnetic simulation reveals that when the diffused Tb-rich shell thickness is lower than 12 nm the c-plane shell(perpendicular to the c-axis)is much more effective in enhancing the coercivity than the side plane shell(parallel to the c-axis).But when the Tb-rich shell thickness is above12 nm the side plane shell contributes more to the coercivity enhancement.The results in this work can help to design and manufacture Nd-Fe-B magnets with low cost and high magnetic properties.展开更多
The compositional design of diffusion source plays a crucial role in improving magnetic properties of Nd-Fe-B magnet.In this work,Dy_(80)-_(x)Ce_(x)Al_(20)(x=0-50,in at%)alloy was employed as the diffusion source for ...The compositional design of diffusion source plays a crucial role in improving magnetic properties of Nd-Fe-B magnet.In this work,Dy_(80)-_(x)Ce_(x)Al_(20)(x=0-50,in at%)alloy was employed as the diffusion source for grain boundary diffusion.The results show that Dy-Ce co-diffusion can effectively enhance the infiltration ability of diffusion source.The maximum coercivity increment of up to 795 kA/m can be achieved when x=10 due to the deeper diffusion depth and higher Dy content in the shell,while a significant degradation of remanence is also exhibited due to the formation of a larger number of Dy-rich grains.Thus,Ce content is regulated to inhibit the deterioration of remanence.Increasing Ce content to above x=30,it is found that the formed CeFe_(2)phase near the surface can regulate the infiltration ability of diffusion source,reducing the area fraction of Dy-rich grain region that is detrimental to the rema-nence,and eventually,when x=50,the remanence is recovered to be comparable to that of as-prepared magnet,with a coercivity increment of 430 kA/m concurrently.This suggests that rationally designing the diffusion source composition enables the preparation of cost-effective magnets.展开更多
The substitution of Fe by Co in the 2:14:1 phase is an effective method to increase the Curie temperature and enhance the thermal stability of the Nd-Fe-B magnets.However,the accumulation of Co ele ment at the grain b...The substitution of Fe by Co in the 2:14:1 phase is an effective method to increase the Curie temperature and enhance the thermal stability of the Nd-Fe-B magnets.However,the accumulation of Co ele ment at the grain boundaries(GBs) changes the GBs from nonmagnetic to ferromagnetic and causes the thinlayer GBs to become rare,In this paper,the method of diffusing Tb element was chosen to improve the microstructure and temperature stability of high-Co magnets.Three original sintered Nd_(28.5)Dy_(3)-CO_(x)e_(bal)M_(0.6)B_(i)(x=0,6 wt%,12 wt%;M = Cu,Al,Zr) magnets with different Co contents were diffused with Tb by grain boundary diffusion(GBD).After GBD,high-Co magnets exhibit more continuously distributed thin-layer GBs,and their thermal stability is significantly improved.In high-Co magnets(x=6 wt%),the absolute value of the temperature coefficient of coercivity decreases from 0.603%/K to0.508%/K in the temperature range of 293-413 K,that of remanence decreases from 0.099%/K to 0.091%/K,and the coercivity increases from 18.44 to 25.04 kOe.Transmission electron microscopy(TEM)characterization reveals that there are both the 1:2 phase and the amorphous phase in the high-Co magnet before and after GBD,EDS elemental analysis shows that Tb element is more likely to preferentially replace the rare earth elements in the 2:14:1 main phase than in the 1:2 phase and the amorphous phase.The concentration of Tb at the edge of the main phase is much higher than that in the 1:2phase and amorphous phase,which is beneficial to the improvement of the microstructure.The preferential replacement of Tb elements at the edge of the 2:14:1 phase and thin-layer GBs with a more continuous distribution are synergistically responsible for improving the thermal stability of high-Co magnets.The study indicates that GBD is an effective method to improve the microstructure and thermal stability of high-Co magnets.展开更多
Grain boundary diffusion process(GBDP)has been developed as an effective approach to increase the coercivity of sintered Nd-Fe-B magnets by regulating the compositions and phase distributions near grain boundaries.Thi...Grain boundary diffusion process(GBDP)has been developed as an effective approach to increase the coercivity of sintered Nd-Fe-B magnets by regulating the compositions and phase distributions near grain boundaries.This work aims to explore how to select the optimum annealing temperature after GBDP.In this work GBDP was performed on a sintered Nd-Fe-B magnet using Dy_(70)Cu_(30) alloy.After GBDP the low eutectic temperature of the grain boundary phases decreases from the initial 492 to 451℃.The magnetic property dependent on different annealing temperatures near the low eutectic temperature was studied.The magnetic properties,especially the squareness factor of demagnetization curve show a strong dependence on the annealing temperature.After GBDP the optimal magnetic properties can be obtained after annealing just above the low eutectic temperature of the grain boundary phases.The mechanism is discussed based on the microstructure analysis.展开更多
Grain boundary diffusion technique with TbH3 nanoparticles was applied to fabricate Tb-less sintered NdFe-B permanent magnets with high coercivity. The magnetic properties and microstructure of magnets were systematic...Grain boundary diffusion technique with TbH3 nanoparticles was applied to fabricate Tb-less sintered NdFe-B permanent magnets with high coercivity. The magnetic properties and microstructure of magnets were systematically studied. The coercivity and remanence of grain boundary diffusion magnet are improved by 112% and reduced by 26% compared with those of the original magnet, respectively. Meanwhile, both the remanence temperature coefficient（α） and the coercivity temperature coefficient（β） of the magnets are improved after diffusion treatment. Microstructure shows that Tb element enriches in the surface region of Nd2Fe（14）B grains and is expected to exist as（Nd,Tb）2Fe（14）B phase. Thus, the magneto-crystalline anisotropy field of the magnet improves remarkably. As a result, the sintered Nd-FeB magnets by grain boundary diffusion with TbH3 nanoparticles exhibit enhanced coercivity.展开更多
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 w...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 developing the process,reducing the consumption of heavy rare earth elements(Tb),and improving the coercivity of sintered Nd-Fe-B magnets.展开更多
Grain boundary diffusion process(GBDP)was first proposed for sintered Nd-Fe-B magnets to achieve the high utilization efficiency of heavy rare earth elements.Recent success of fabricating high performance nanocomposit...Grain boundary diffusion process(GBDP)was first proposed for sintered Nd-Fe-B magnets to achieve the high utilization efficiency of heavy rare earth elements.Recent success of fabricating high performance nanocomposite magnets by GBDP indicates that this method also exerts huge applicable potential on hot-deformed Nd-Fe-B magnets.In this review,the development and magnetic property enhancement mechanisms of different diffusion methods proposed on hot-deformed magnets were thoroughly elucidated.Moreover,the improve room for further property enhancement and the accompanying problems of GBDP on hot-deformed magnets are also discussed in this article.展开更多
To high-power permanent magnetic motors,it is critical for Nd-Fe-B magnets to maintain the desirable coercivity at high-temperature operating conditions.To address this,two approaches have been proven effective:(1)enh...To high-power permanent magnetic motors,it is critical for Nd-Fe-B magnets to maintain the desirable coercivity at high-temperature operating conditions.To address this,two approaches have been proven effective:(1)enhancing the room temperature coercivity;(2)reducing the eddy current loss.However,these two items are difficult to be simultaneously achieved.Here,the grain boundary diffusion(GBD)of the Pr-Tb-Al-Cu-based source is applied to enhance the coercivity and electric resistivity at room temperature from 1101 kA m-1 and 2.13×10–6Ωm to 1917 kA m-1 and 2.60×10–6Ωm,and those at 120°C from 384 kA m-1 and 4.31×10–6Ωm to 783 kA m-1 and 4.86×10–6Ωm,respectively.Such optimization is ascribed to the improved formation depth of Tb-rich 2:14:1 shells with large magnetocrystalline anisotropy and the increased intergranular Pr-based oxides with high electric resistivity,induced by the coordination effects of Tb and Pr,as proven by the atomic-scale observations and the first principles calculations.It thus results in the simultaneously improved output power and energy efficiency of the motor because of the combination of magnetic thermal stability enhancement and eddy current loss reduction,as theoretically confirmed by electromagnetic simulation.展开更多
Grain boundary diffusion(GBD)process is an important approach for producing Nd-Fe-B magnets with high coercivity and high thermal stability.The GBD for hot-deformed Nd-Fe-B magnets with nanocrystalline micro structure...Grain boundary diffusion(GBD)process is an important approach for producing Nd-Fe-B magnets with high coercivity and high thermal stability.The GBD for hot-deformed Nd-Fe-B magnets with nanocrystalline micro structure is more complicated compared to sintered magnets.Here,we investigated the effects of different GBD methods,i.e.,intergranular addition(in-situ GBD 1#),in-situ GBD from magnet surface during hot pressing and hot deformation(in-situ GBD 2#),and conventional GBD,on the magnetic properties and microstructure of hot deformed magnets.After the treatment by these three GBD approaches using 2 wt%Pr_(40)Tb_(30)Cu_(30)diffusion source,the coercivity of the hot-deformed magnet increases from 1281 to 1567,1412 and 2022 kA/m,respectively.The coercivity enhancement is attributed to the formation of local(Nd,Tb)2Fe14B phase with stro ng magnetic anisotropy.Reduced grain orientation is found in both in-situ GBD 1#and conventional GBD treated samples mainly due to the local stress state variation and the rotation of platelet grains.Interestingly,the in-situ GBD 2#processed sample has a high orientation at diffusion surface,which may be caused by the modified surface state of the magnet by the diffusion source.Compared with the in-situ GBD processes,the conventional GBD exhibits a higher utilization efficiency of Tb.Since the in-situ GBD is effective to treat thick hot-deformed magnets,further effort should be aimed at enhancing its diffusion efficiency.展开更多
In this study,the influence of the content of Al and Co in the diffusion source on the magnetic performance and microstructure of the diffused magnet was studied by grain boundary diffusion treatment with Pr_(70)Al_(3...In this study,the influence of the content of Al and Co in the diffusion source on the magnetic performance and microstructure of the diffused magnet was studied by grain boundary diffusion treatment with Pr_(70)Al_(30-x)Co_(x)(x=0 at%,10 at%,15 at%,20 at%,30 at%)alloys.When the Co content in the diffusion source increases from 0 at%to 10 at%,the coercivity enhancement in the Pr_(70)Al_(20)Co_(10)diffused magnet is the highest,increased from 1.62 to 2.24 T,higher than 2.01 T of the Pr_(70)Al_(30)diffused magnet.With further increase of Co content in the diffused source,the coercivity of the diffused magnet decreases gradually,the coercivity of Pr_(70)Al_(15)Co_(15),Pr_(70)Al_(20)Co_(10)and Pr_(70)Co_(30)diffused magnet is 2.15,1.99 and1.81 T,respectively.Microstructural analysis shows that plenty of continuous grain boundary phases(CGBPs)can be formed in the Pr_(70)Al_(20)Co_(10)diffused magnet under the synergistic effect of Al and Co,which leads to the enhancement of magnetic isolation between more adjacent grains.However,the amount of CGBP in the diffused magnets gradually decreases with the further increase of Co content in the diffusion source.展开更多
It is approved that grain boundary diffusion is an effective method to increase the coercivity of hot-deformed NdFeB magnet.In this paper,a new rare earth-free grain boundary diffusion source of hot-deformed magnet wa...It is approved that grain boundary diffusion is an effective method to increase the coercivity of hot-deformed NdFeB magnet.In this paper,a new rare earth-free grain boundary diffusion source of hot-deformed magnet was studied.AlCuZn powders blended with commercial NdFeB powders were hot-compacted to obtain fully dense magnets,hot-deformed into anisotropic magnets and finally annealed to gain better homogeneity.Initially,the influences of annealing temperature and time on the magnetic properties of the specimens were studied and the optimal parameters of 600℃ and 60 min were achieved.Then,by changing the proportions of AlCuZn grain boundary diffusion,the coercivity,remanence and maximum energy product of the hot-deformed NdFeB magnets were examined.The result showed that with 1.0 wt%AlCuZn grain boundary diffusion and annealing at 600℃ for 60 min,the coercivity rose from 828 to 987 kA·m^(-1) without deteriorating the remanence.Microstructural analysis confirmed that AlCuZn diffused into the intergranular boundaries and the magnet diffused with AlCuZn possessed finer grains than that of without AlCuZn grain boundary diffusion.展开更多
The microstructure and magnetic properties of Co/Cr bilayer films were examined before and after postdeposition annealing by using transmission electron microscopy （TEM）, X-ray diffraction （XRD） technique and vibr...The microstructure and magnetic properties of Co/Cr bilayer films were examined before and after postdeposition annealing by using transmission electron microscopy （TEM）, X-ray diffraction （XRD） technique and vibrating sample magnetometer （VSM）. A model of grain boundary （GB） Cr-rich phase growth involving GB diffusion derived from the Cr underlayer was proposed to elucidate the kinetics of the paramagnetic Cr-rich phase growth along Co GBs within the Co layer. The correlation of the GB Cr-rich phase formation with the magnetic Co grain isolation and accordingly, improvement of magnetic properties was experimentally investigated and discussed in detail. Our analysis results are well consistent with previous micromagnetic simulations on the improvement of magnetic properties by the magnetic grain isolation. The results provide some insights into the processing-structure-property relationships of the Co/Cr bilayer films, and thus suggest that the magnetic grain isolation be feasible not only in longitudinal recording media, but also be effective in tuning the exchange coupling of magnetic grains in perpendicular recording media via the GB diffusion from underlayer and/or overlayer.展开更多
A grain boundary diffusion(GBD)process with Pr_(80-x)Al_(x)Cu_(20)(x=0,10,15,20)low melting point alloys was applied to commercial 42M sintered Nd–Fe–B magnets.The best coercivity enhancement of a diffused magnet wa...A grain boundary diffusion(GBD)process with Pr_(80-x)Al_(x)Cu_(20)(x=0,10,15,20)low melting point alloys was applied to commercial 42M sintered Nd–Fe–B magnets.The best coercivity enhancement of a diffused magnet was for the Pr_(65)Al_(15)Cu_(20)GBD magnet,from 16.38 kOe to 22.38 kOe.Microstructural investigations indicated that increase in the Al content in the diffusion source can form a continuous grain boundary(GB)phase,optimizing the microstructure to enhance the coercivity.The coercivity enhancement is mainly due to the formation of a continuous GB phase to separate the main phase grains.Exchange decoupling between the adjacent main phase grains is enhanced after the GBD process.Meanwhile,the introduction of Al can effectively promote the infiltration of Pr into the magnet,which increases the diffusion rate of rare-earth elements within a certain range.This work provides a feasible method to enhance coercivity and reduce the use of rare-earth resources by partial replacement of rare-earth elements with non-rare-earth elements in the diffusion source.展开更多
As the channel for grain boundary diffusion(GBD)in Nd–Fe–B magnets,grain boundary(GB)phases have a very important effect on GBD.As doping elements that are commonly used to regulate the GB phases in Nd–Fe–B sinter...As the channel for grain boundary diffusion(GBD)in Nd–Fe–B magnets,grain boundary(GB)phases have a very important effect on GBD.As doping elements that are commonly used to regulate the GB phases in Nd–Fe–B sintered magnets,the influences of Ga and Zr on GBD were investigated in this work.The results show that the Zr-doped magnet has the highest coercivity increment(7.97 kOe)by GBD,which is almost twice that of the Ga-doped magnet(4.32 kOe)and the magnet without Ga and Zr(3.24 kOe).Microstructure analysis shows that ZrB_(2)formed in the Zr-doped magnet plays a key role in increasing the diffusion depth.A continuous diffusion channel in the magnet can form because of the presence of ZrB_(2).ZrB_(2)can also increase the defect concentration in GB phases,which can facilitate GBD.Although Ga can also improve the diffusion depth,its effect is not very obvious.The micromagnetic simulation based on the experimental results also proves that the distribution of Tb in the Zr-doped magnet after GBD is beneficial to coercivity.This study reveals that the doping elements Ga and Zr in Nd–Fe–B play an important role in GBD,and could provide a new perspective for researchers to improve the effects of GBD.展开更多
Three types of NdFeB magnets with the same composition and different grain sizes were prepared,and then the grain boundary diffusion was conducted using metal Tb under the same technical parameters.The effect of grain...Three types of NdFeB magnets with the same composition and different grain sizes were prepared,and then the grain boundary diffusion was conducted using metal Tb under the same technical parameters.The effect of grain size on the grain boundary diffusion process and properties of sintered NdFeB magnets was investigated.The diffusion process was assessed using X-ray diffractometer,field emission scanning electron microscope,and electron probe microanalyzer.The magnetic properties of the magnet before and after diffusion were investigated.The results show that the grain refinement of the magnet leads to higher Tb utilization efficiency and results in higher coercivity at different temperatures.It can be attributed to the formation of a deeper and more complete core-shell structure,resulting in better magnetic isolation and higher anisotropy of the Nd_(2)Fe_(14)B grains.This work may shed light on developing high coercivity with low heavy rare earth elements through grain refinement.展开更多
The combination of dual-main-phase(DMP)(Nd,Ce)-Fe-B magnets and grain boundary diffusion process(GBDP)is currently a research topic for obtaining high-cost performance materials in rare earth permanent magnet fields.T...The combination of dual-main-phase(DMP)(Nd,Ce)-Fe-B magnets and grain boundary diffusion process(GBDP)is currently a research topic for obtaining high-cost performance materials in rare earth permanent magnet fields.The novel structural features of GBDP(Nd,Ce)-Fe-B magnets give a version of different domain reversal processes from those of non-diffused magnets.In this work,the in-situ magnetic domain evolution of the DMP magnets was observed at elevated temperatures,and the temperature demagnetization and coercivity mechanism of the GBDP dual-main-phase(Nd,Ce)-Fe-B magnets are discussed.The results show that the shell composition of different types of grains in DMP magnets is similar,while the magnetic microstructure results indicate the Ce-rich grains tend to demagnetize first.Dy-rich shell with a high anisotropic field caused by GBDP leads to an increase in the nucleation field,which enhances the coercivity.It is found that much more grains exhibit single domain characteristics in the remanent state for GBDP dual-main-phase(Nd,Ce)-Fe-B magnets.In addition,the grains that undergo demagnetization first are Ce-rich or Nd-rich grains,which is different from that of non-diffused magnets.These results were not found in previous studies but can be intuitively characterized from the perspective of magnetic domains in this work,providing a new perspective and understanding of the performance improvement of magnetic materials.展开更多
In this study,DyF_(3)powder was sprayed onto the polar and side surfaces of the magnets to determine the anisotropic diffusion mechanism of Dy in the sintered Nd-Fe-B magnet.The coercivity and squareness of the magnet...In this study,DyF_(3)powder was sprayed onto the polar and side surfaces of the magnets to determine the anisotropic diffusion mechanism of Dy in the sintered Nd-Fe-B magnet.The coercivity and squareness of the magnet in which the diffusion of Dy is perpendicular to the c-axis(a-magnet)are lower than those of the magnet with the diffusion of Dy parallel to the c-axis(c-magnet).Compared with the c-magnet,the a-magnet has a longer Dy-enrichment region from the diffusion surface,where Dy is enriched in the 2:14:1 grain.By contrast,the Dy concentration in the grain boundaries beyond the Dy enrichment region is lower in the a-magnet.Moreover,the Dy shells beyond the Dy enrichment region in the a-magnet are distributed on the side surfaces of the 2:14:1 grains but not on the polar surfaces.Based on the micromagnetic simulation,the Dy shells on the polar surfaces of the grains are more effective in enhancing coercivity.According to first-principle calculations,Dy migrating through 001 into the Nd vacancy in the Nd_(2)Fe_(14)B crystal has a higher diffusion barrier,thus indicating that the lattice diffusion of Dy parallel to the c-axis is more difficult.展开更多
The microstructure of(Nd,Ce)-Fe-B sintered magnets with different diffusion depths was characterized by a magnetic force microscope,and the relationship between the magnetic properties and the local structure of grain...The microstructure of(Nd,Ce)-Fe-B sintered magnets with different diffusion depths was characterized by a magnetic force microscope,and the relationship between the magnetic properties and the local structure of grain boundary diffused magnets is discussed.The domains perpendicular to the c-axis(easy magnetization direction)show a typical maze-like pattern,while those parallel to the c-axis show the characte ristics of plate domains.The significant gradient change is shown in the concentration of Dy with the direction of diffusion from the surface to the interior.Dy diffuses along grain boundaries and(Dy,Nd)_(2)Fe_(14)B layer with a high anisotropy field formed around the grains.Through in-situ electron probe micro-analysis/magnetic force microscopy(EPMA/MFM),it is found that the average domain width decreases,and the proportion of single domain grains increases as diffusion depth increases.This is caused by both the change of concentration and distribution of Dy.The grain boundary diffusion process changes the microstructure and microchemistry inside the magnet,and these local magnetism differences can be reflected by the configuration of the magnetic domain structure.展开更多
基金supported by the National Key Research and Development Program of China(2022YFB3505503)the National Natural Science Foundation of China(52201230)+2 种基金the Key R&D Program of Shandong Province(2022CXGC020307)the China Postdoctoral Science Foundation(2022M71204)the Beijing NOVA Program(Z211100002121092).
文摘The grain boundary diffusion process(GBDP)has proven to be an effective method for enhancing the coercivity of sintered Nd-Fe-B magnets.However,the limited diffusion depth and thicker shell struc-ture have impeded the further development of magnetic properties.Currently,the primary debates re-garding the mechanism of GBDP with Tb revolve around the dissolution-solidification mechanism and the atomic substitution mechanism.To clarify this mechanism,the microstructure evolution of sintered Nd-Fe-B magnets during the heating process of GBDP has been systematically studied by quenching at different tem peratures.In this study,it was found that the formation of TbFe_(2) phase is related to the dis-solution of _(2)Fe_(14)B grains during GBDP with Tb.The theory of mixing heat and phase separation further confirms that the Nd_(2)Fe_(14)B phase dissolves to form a mixed phase of Nd and TbFe_(2),which then solidifies into the(Nd,Tb)_(2)Fe_(14)B phase.Based on the discovery of the TbFe_(2) phase,the dissolution-solidification mechanism is considered the primary mechanism for GBDP.This is supported by the elemental content of the two typical core-shell structures observed.
基金Project supported by the National Natural Science Foundation of China(52361033)National Key Research and Development Program(2022YFB3505400)+3 种基金Ministry of Industry and Information Technology Heavy Rare Earth Special Use of Sintered NdFeB Project(TC220H06J)Academic and Technical Leaders in Major Disciplines in Jiangxi Province(2022BCJ23007)Jiangxi Province Science and Technology Cooperation Key Project(20212BDH80007)Jiangxi Graduate Student Innovation Special Fund Project(YC2023-B213)。
文摘Grain boundary diffusion technology is pivotal in the preparation of high-performance NdFeB magnets.This study investigates the factors that affect the efficiency of grain boundary diffusion,starting from the properties of the diffusion matrix.Through the adjustment of the sintering process,we effectively prepared magnets with varied densities that serve as the matrix for grain boundary diffusion with TbH,diffusion.The mobility characteristics of the Nd-rich phase during the densification stage are leveraged to ensure a more extensive distribution of heavy rare earth elements within the magnets.According to the experimental results,the increase in coercivity of low-density magnets after diffusion is significantly greater than that of relatively high-density magnets.The coercivity values measured are 805.32 kA/m for low-density magnets and 470.3 kA/m for high-density magnets.Additionally,grain boundary diffusion notably enhances the density of initial low-density magnets,addressing the issue of low density during the sintering stage.Before the diffusion treatment,the Nd-rich phases primarily concentrate at the triangular grain boundaries,resulting in an increased number of cavity defects in the magnets.These cavity defects contain atoms in a higher energy state,making them more prone to transition.Consequently,the diffusion activation energy at the void defects is lower than the intracrystalline diffusion activation energy,accelerating atom diffusion.The presence of larger cavities also provides more space for atom migration,thereby promoting the diffusion process.After the diffusion treatment,the proportion of bulk Nd-rich phases significantly decreases,and they infiltrate between the grains to fill the cavity defects,forming continuous fine grain boundaries.Based on these observations,the study aims to explore how to utilize this information to develop an efficient technique for grain boundary diffusion.
基金Project supported by the National Natural Science Foundation of China(51901087)China Postdoctoral Science Foundation(2021M701504)。
文摘A sintered Nd-Y-Fe-B magnet was designed and manufactured by the multi-main-phase process.Unevenly distributed Y in the magnet decreases the adverse magnetic weakening effect of Y on the coercivity.Grain boundary diffusion process(GBDP)was conducted to further enhance the coercivity of the Nd-Y-Fe-B magnet.The coercivity increases significantly from 884 to 1741 kA/m after GBDP with Pr_(60)Tb_(10)Cu_(30)alloy.The mechanism of the coercivity enhancement is discussed based on the microstructure analysis.Micromagnetic simulation reveals that when the diffused Tb-rich shell thickness is lower than 12 nm the c-plane shell(perpendicular to the c-axis)is much more effective in enhancing the coercivity than the side plane shell(parallel to the c-axis).But when the Tb-rich shell thickness is above12 nm the side plane shell contributes more to the coercivity enhancement.The results in this work can help to design and manufacture Nd-Fe-B magnets with low cost and high magnetic properties.
基金the National Natural Science Foundation of China(52161031,52361034)the 2023 Ganzhou"Leading the Charge with Open Competition"project+4 种基金the Key Project of Jiangxi Provincial Natural Science Foundation(20224ACB204005)the Jiangxi Provincial Natural Science Foundation(20224BAB214016)the National Key Research and Development Program of China(2022YFB3503400)the Major Science&Technology Specific Project of Jiangxi Province(20203ABC28W006)the Key Project of Science and Technology to Promote Mongolian Development(XM2021BT03)。
文摘The compositional design of diffusion source plays a crucial role in improving magnetic properties of Nd-Fe-B magnet.In this work,Dy_(80)-_(x)Ce_(x)Al_(20)(x=0-50,in at%)alloy was employed as the diffusion source for grain boundary diffusion.The results show that Dy-Ce co-diffusion can effectively enhance the infiltration ability of diffusion source.The maximum coercivity increment of up to 795 kA/m can be achieved when x=10 due to the deeper diffusion depth and higher Dy content in the shell,while a significant degradation of remanence is also exhibited due to the formation of a larger number of Dy-rich grains.Thus,Ce content is regulated to inhibit the deterioration of remanence.Increasing Ce content to above x=30,it is found that the formed CeFe_(2)phase near the surface can regulate the infiltration ability of diffusion source,reducing the area fraction of Dy-rich grain region that is detrimental to the rema-nence,and eventually,when x=50,the remanence is recovered to be comparable to that of as-prepared magnet,with a coercivity increment of 430 kA/m concurrently.This suggests that rationally designing the diffusion source composition enables the preparation of cost-effective magnets.
基金supported by the National Key R&D Program of China (2021YFB3502902,2021YFB3503100,2022YFB3503300,2022YFB3505200)。
文摘The substitution of Fe by Co in the 2:14:1 phase is an effective method to increase the Curie temperature and enhance the thermal stability of the Nd-Fe-B magnets.However,the accumulation of Co ele ment at the grain boundaries(GBs) changes the GBs from nonmagnetic to ferromagnetic and causes the thinlayer GBs to become rare,In this paper,the method of diffusing Tb element was chosen to improve the microstructure and temperature stability of high-Co magnets.Three original sintered Nd_(28.5)Dy_(3)-CO_(x)e_(bal)M_(0.6)B_(i)(x=0,6 wt%,12 wt%;M = Cu,Al,Zr) magnets with different Co contents were diffused with Tb by grain boundary diffusion(GBD).After GBD,high-Co magnets exhibit more continuously distributed thin-layer GBs,and their thermal stability is significantly improved.In high-Co magnets(x=6 wt%),the absolute value of the temperature coefficient of coercivity decreases from 0.603%/K to0.508%/K in the temperature range of 293-413 K,that of remanence decreases from 0.099%/K to 0.091%/K,and the coercivity increases from 18.44 to 25.04 kOe.Transmission electron microscopy(TEM)characterization reveals that there are both the 1:2 phase and the amorphous phase in the high-Co magnet before and after GBD,EDS elemental analysis shows that Tb element is more likely to preferentially replace the rare earth elements in the 2:14:1 main phase than in the 1:2 phase and the amorphous phase.The concentration of Tb at the edge of the main phase is much higher than that in the 1:2phase and amorphous phase,which is beneficial to the improvement of the microstructure.The preferential replacement of Tb elements at the edge of the 2:14:1 phase and thin-layer GBs with a more continuous distribution are synergistically responsible for improving the thermal stability of high-Co magnets.The study indicates that GBD is an effective method to improve the microstructure and thermal stability of high-Co magnets.
基金Project supported by the National Natural Science Foundation of China(51901087)Natural Science Foundation of Jiangsu Province(BK20190975,BK20201007)+1 种基金China Postdoctoral Science Foundation(2021M701504)Natural Science Foundation for Colleges and Universities in Jiangsu Province(20KJD470002)。
文摘Grain boundary diffusion process(GBDP)has been developed as an effective approach to increase the coercivity of sintered Nd-Fe-B magnets by regulating the compositions and phase distributions near grain boundaries.This work aims to explore how to select the optimum annealing temperature after GBDP.In this work GBDP was performed on a sintered Nd-Fe-B magnet using Dy_(70)Cu_(30) alloy.After GBDP the low eutectic temperature of the grain boundary phases decreases from the initial 492 to 451℃.The magnetic property dependent on different annealing temperatures near the low eutectic temperature was studied.The magnetic properties,especially the squareness factor of demagnetization curve show a strong dependence on the annealing temperature.After GBDP the optimal magnetic properties can be obtained after annealing just above the low eutectic temperature of the grain boundary phases.The mechanism is discussed based on the microstructure analysis.
基金financially supported by the National Natural Science Foundation of China(Nos.51001002 and 51371002)the National High Technology Research and Development Program of China(No.2012AA063201)+3 种基金the Key Program of Science and Technology Development Project of Beijing Municipal Education Commission(No.KZ201110005007)Jinghua Talents of Beijing University of TechnologyRixin Talents of Beijing University of Technologythe Importation and Development of High-Caliber Talents Project of Beijing Municipal Institutions
文摘Grain boundary diffusion technique with TbH3 nanoparticles was applied to fabricate Tb-less sintered NdFe-B permanent magnets with high coercivity. The magnetic properties and microstructure of magnets were systematically studied. The coercivity and remanence of grain boundary diffusion magnet are improved by 112% and reduced by 26% compared with those of the original magnet, respectively. Meanwhile, both the remanence temperature coefficient（α） and the coercivity temperature coefficient（β） of the magnets are improved after diffusion treatment. Microstructure shows that Tb element enriches in the surface region of Nd2Fe（14）B grains and is expected to exist as（Nd,Tb）2Fe（14）B phase. Thus, the magneto-crystalline anisotropy field of the magnet improves remarkably. As a result, the sintered Nd-FeB magnets by grain boundary diffusion with TbH3 nanoparticles exhibit enhanced coercivity.
基金supported by the National Key R&D Program of China(2021YFB3502902)Ningbo Major Special Projects of Science and Technology Innovation 2025 Plan(2020Z045)Key Research and Development Program of Zhejiang Province(2020C05013)。
文摘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 developing the process,reducing the consumption of heavy rare earth elements(Tb),and improving the coercivity of sintered Nd-Fe-B magnets.
基金Project supported by the Pioneer and Leading Goose R&D Program of Zhejiang(2020C01190)the Science and Technology Innovation 2025Major Project of Ningbo(2020Z064)+1 种基金the Zhejiang Province Postdoctoral Science Foundation(ZJ2021080)the Inner Mongolia Major Technology Project(2021ZD0035)。
文摘Grain boundary diffusion process(GBDP)was first proposed for sintered Nd-Fe-B magnets to achieve the high utilization efficiency of heavy rare earth elements.Recent success of fabricating high performance nanocomposite magnets by GBDP indicates that this method also exerts huge applicable potential on hot-deformed Nd-Fe-B magnets.In this review,the development and magnetic property enhancement mechanisms of different diffusion methods proposed on hot-deformed magnets were thoroughly elucidated.Moreover,the improve room for further property enhancement and the accompanying problems of GBDP on hot-deformed magnets are also discussed in this article.
基金financially supported by the National Natu-ral Science Foundation of China(Nos.U21A205251774146,and 52171175)Jiangxi Provincial Key Science and Technology R&D Project(No.20203ABC28W006)Natural Science Founda-tion of Zhejiang Province(No.LY20E010002,2021C01023,and 2021C01033).
文摘To high-power permanent magnetic motors,it is critical for Nd-Fe-B magnets to maintain the desirable coercivity at high-temperature operating conditions.To address this,two approaches have been proven effective:(1)enhancing the room temperature coercivity;(2)reducing the eddy current loss.However,these two items are difficult to be simultaneously achieved.Here,the grain boundary diffusion(GBD)of the Pr-Tb-Al-Cu-based source is applied to enhance the coercivity and electric resistivity at room temperature from 1101 kA m-1 and 2.13×10–6Ωm to 1917 kA m-1 and 2.60×10–6Ωm,and those at 120°C from 384 kA m-1 and 4.31×10–6Ωm to 783 kA m-1 and 4.86×10–6Ωm,respectively.Such optimization is ascribed to the improved formation depth of Tb-rich 2:14:1 shells with large magnetocrystalline anisotropy and the increased intergranular Pr-based oxides with high electric resistivity,induced by the coordination effects of Tb and Pr,as proven by the atomic-scale observations and the first principles calculations.It thus results in the simultaneously improved output power and energy efficiency of the motor because of the combination of magnetic thermal stability enhancement and eddy current loss reduction,as theoretically confirmed by electromagnetic simulation.
基金supported by the National Natural Science Foundation of China(U21A2052,51774146)。
文摘Grain boundary diffusion(GBD)process is an important approach for producing Nd-Fe-B magnets with high coercivity and high thermal stability.The GBD for hot-deformed Nd-Fe-B magnets with nanocrystalline micro structure is more complicated compared to sintered magnets.Here,we investigated the effects of different GBD methods,i.e.,intergranular addition(in-situ GBD 1#),in-situ GBD from magnet surface during hot pressing and hot deformation(in-situ GBD 2#),and conventional GBD,on the magnetic properties and microstructure of hot deformed magnets.After the treatment by these three GBD approaches using 2 wt%Pr_(40)Tb_(30)Cu_(30)diffusion source,the coercivity of the hot-deformed magnet increases from 1281 to 1567,1412 and 2022 kA/m,respectively.The coercivity enhancement is attributed to the formation of local(Nd,Tb)2Fe14B phase with stro ng magnetic anisotropy.Reduced grain orientation is found in both in-situ GBD 1#and conventional GBD treated samples mainly due to the local stress state variation and the rotation of platelet grains.Interestingly,the in-situ GBD 2#processed sample has a high orientation at diffusion surface,which may be caused by the modified surface state of the magnet by the diffusion source.Compared with the in-situ GBD processes,the conventional GBD exhibits a higher utilization efficiency of Tb.Since the in-situ GBD is effective to treat thick hot-deformed magnets,further effort should be aimed at enhancing its diffusion efficiency.
基金Project supported by Key R&D Program of Zhejiang Province(2021C01190)Major Project of"Science and Technology Innovation 2025"in Ningbo City(2020Z046)+4 种基金Ningbo Natural Science Foundation(202003N4353)Key Research and Development Program of Shandong Province(2019JZZY010321)Inner Mongolia Major Technology Project(2019ZD020)National Natural Science Foundation of China(52101238)Kunpeng Project in Zhejiang Province。
文摘In this study,the influence of the content of Al and Co in the diffusion source on the magnetic performance and microstructure of the diffused magnet was studied by grain boundary diffusion treatment with Pr_(70)Al_(30-x)Co_(x)(x=0 at%,10 at%,15 at%,20 at%,30 at%)alloys.When the Co content in the diffusion source increases from 0 at%to 10 at%,the coercivity enhancement in the Pr_(70)Al_(20)Co_(10)diffused magnet is the highest,increased from 1.62 to 2.24 T,higher than 2.01 T of the Pr_(70)Al_(30)diffused magnet.With further increase of Co content in the diffused source,the coercivity of the diffused magnet decreases gradually,the coercivity of Pr_(70)Al_(15)Co_(15),Pr_(70)Al_(20)Co_(10)and Pr_(70)Co_(30)diffused magnet is 2.15,1.99 and1.81 T,respectively.Microstructural analysis shows that plenty of continuous grain boundary phases(CGBPs)can be formed in the Pr_(70)Al_(20)Co_(10)diffused magnet under the synergistic effect of Al and Co,which leads to the enhancement of magnetic isolation between more adjacent grains.However,the amount of CGBP in the diffused magnets gradually decreases with the further increase of Co content in the diffusion source.
基金This study was financially supported by the National Key Research and Development Program(No.2016YFB0700902).
文摘It is approved that grain boundary diffusion is an effective method to increase the coercivity of hot-deformed NdFeB magnet.In this paper,a new rare earth-free grain boundary diffusion source of hot-deformed magnet was studied.AlCuZn powders blended with commercial NdFeB powders were hot-compacted to obtain fully dense magnets,hot-deformed into anisotropic magnets and finally annealed to gain better homogeneity.Initially,the influences of annealing temperature and time on the magnetic properties of the specimens were studied and the optimal parameters of 600℃ and 60 min were achieved.Then,by changing the proportions of AlCuZn grain boundary diffusion,the coercivity,remanence and maximum energy product of the hot-deformed NdFeB magnets were examined.The result showed that with 1.0 wt%AlCuZn grain boundary diffusion and annealing at 600℃ for 60 min,the coercivity rose from 828 to 987 kA·m^(-1) without deteriorating the remanence.Microstructural analysis confirmed that AlCuZn diffused into the intergranular boundaries and the magnet diffused with AlCuZn possessed finer grains than that of without AlCuZn grain boundary diffusion.
基金supported by the National Natural Sci-ence Foundation of China (No 50671020)the Chinese Ministry of Education (Nos 108039 and IRT0713)
文摘The microstructure and magnetic properties of Co/Cr bilayer films were examined before and after postdeposition annealing by using transmission electron microscopy （TEM）, X-ray diffraction （XRD） technique and vibrating sample magnetometer （VSM）. A model of grain boundary （GB） Cr-rich phase growth involving GB diffusion derived from the Cr underlayer was proposed to elucidate the kinetics of the paramagnetic Cr-rich phase growth along Co GBs within the Co layer. The correlation of the GB Cr-rich phase formation with the magnetic Co grain isolation and accordingly, improvement of magnetic properties was experimentally investigated and discussed in detail. Our analysis results are well consistent with previous micromagnetic simulations on the improvement of magnetic properties by the magnetic grain isolation. The results provide some insights into the processing-structure-property relationships of the Co/Cr bilayer films, and thus suggest that the magnetic grain isolation be feasible not only in longitudinal recording media, but also be effective in tuning the exchange coupling of magnetic grains in perpendicular recording media via the GB diffusion from underlayer and/or overlayer.
基金Project supported by the National Key Research and Development Program of China (Grant No.2021YFB3502802)Major Science and Technology Research and Development Project of Jiangxi Province,China (Grant No.20203ABC28W006)+2 种基金the Key Research and Development Program of Shandong Province,China (Grant No.2019JZZY010321)Major Project of“Science and Technology Innovation 2025”in Ningbo City (Grant No.2020Z046)the K.C.Wong Magna Fund in Ningbo University。
文摘A grain boundary diffusion(GBD)process with Pr_(80-x)Al_(x)Cu_(20)(x=0,10,15,20)low melting point alloys was applied to commercial 42M sintered Nd–Fe–B magnets.The best coercivity enhancement of a diffused magnet was for the Pr_(65)Al_(15)Cu_(20)GBD magnet,from 16.38 kOe to 22.38 kOe.Microstructural investigations indicated that increase in the Al content in the diffusion source can form a continuous grain boundary(GB)phase,optimizing the microstructure to enhance the coercivity.The coercivity enhancement is mainly due to the formation of a continuous GB phase to separate the main phase grains.Exchange decoupling between the adjacent main phase grains is enhanced after the GBD process.Meanwhile,the introduction of Al can effectively promote the infiltration of Pr into the magnet,which increases the diffusion rate of rare-earth elements within a certain range.This work provides a feasible method to enhance coercivity and reduce the use of rare-earth resources by partial replacement of rare-earth elements with non-rare-earth elements in the diffusion source.
基金Project supported by the National Natural Science Foundation of China(Grant No.52261037)self-deployed Projects of Ganjiang Innovation Academy,Chinese Academy of Sciences(Grant No.E055B002)+2 种基金the Project of Baotou City Science and Technology(Grant No.XM2022BT04)the Key Research Program of the Chinese Academy of Sciences(Grant No.ZDRW-CN-2021-3)the Key Research Project of Jiangxi Province(Grant No.20203ABC28W006)。
文摘As the channel for grain boundary diffusion(GBD)in Nd–Fe–B magnets,grain boundary(GB)phases have a very important effect on GBD.As doping elements that are commonly used to regulate the GB phases in Nd–Fe–B sintered magnets,the influences of Ga and Zr on GBD were investigated in this work.The results show that the Zr-doped magnet has the highest coercivity increment(7.97 kOe)by GBD,which is almost twice that of the Ga-doped magnet(4.32 kOe)and the magnet without Ga and Zr(3.24 kOe).Microstructure analysis shows that ZrB_(2)formed in the Zr-doped magnet plays a key role in increasing the diffusion depth.A continuous diffusion channel in the magnet can form because of the presence of ZrB_(2).ZrB_(2)can also increase the defect concentration in GB phases,which can facilitate GBD.Although Ga can also improve the diffusion depth,its effect is not very obvious.The micromagnetic simulation based on the experimental results also proves that the distribution of Tb in the Zr-doped magnet after GBD is beneficial to coercivity.This study reveals that the doping elements Ga and Zr in Nd–Fe–B play an important role in GBD,and could provide a new perspective for researchers to improve the effects of GBD.
基金Key Research and Development Program of Shandong Province(2021CXGC010310)Shandong Province Science and Technology Small and Medium Sized Enterprise Innovation Ability Enhancement Project(2023TSGC0287,2024TSGC0519)+1 种基金Shandong Provincial Natural Science Foundation(ZR2022ME222)National Natural Science Foundation of China(51702187)。
文摘Three types of NdFeB magnets with the same composition and different grain sizes were prepared,and then the grain boundary diffusion was conducted using metal Tb under the same technical parameters.The effect of grain size on the grain boundary diffusion process and properties of sintered NdFeB magnets was investigated.The diffusion process was assessed using X-ray diffractometer,field emission scanning electron microscope,and electron probe microanalyzer.The magnetic properties of the magnet before and after diffusion were investigated.The results show that the grain refinement of the magnet leads to higher Tb utilization efficiency and results in higher coercivity at different temperatures.It can be attributed to the formation of a deeper and more complete core-shell structure,resulting in better magnetic isolation and higher anisotropy of the Nd_(2)Fe_(14)B grains.This work may shed light on developing high coercivity with low heavy rare earth elements through grain refinement.
基金supported by the National Key Research and Development Program of China(Nos.2021YFB3503003,2021YFB3503100,and 2022YFB3505401).
文摘The combination of dual-main-phase(DMP)(Nd,Ce)-Fe-B magnets and grain boundary diffusion process(GBDP)is currently a research topic for obtaining high-cost performance materials in rare earth permanent magnet fields.The novel structural features of GBDP(Nd,Ce)-Fe-B magnets give a version of different domain reversal processes from those of non-diffused magnets.In this work,the in-situ magnetic domain evolution of the DMP magnets was observed at elevated temperatures,and the temperature demagnetization and coercivity mechanism of the GBDP dual-main-phase(Nd,Ce)-Fe-B magnets are discussed.The results show that the shell composition of different types of grains in DMP magnets is similar,while the magnetic microstructure results indicate the Ce-rich grains tend to demagnetize first.Dy-rich shell with a high anisotropic field caused by GBDP leads to an increase in the nucleation field,which enhances the coercivity.It is found that much more grains exhibit single domain characteristics in the remanent state for GBDP dual-main-phase(Nd,Ce)-Fe-B magnets.In addition,the grains that undergo demagnetization first are Ce-rich or Nd-rich grains,which is different from that of non-diffused magnets.These results were not found in previous studies but can be intuitively characterized from the perspective of magnetic domains in this work,providing a new perspective and understanding of the performance improvement of magnetic materials.
基金supported by the National Natural Science Foundation of China(52361033)National Key Research and Development Program(2022YFB3505400)+1 种基金the Main Discipline and Technology Leaders Training Plan of Jiangxi Province(2022BCJ23007)the Jiangxi Province Postgraduate Innovation Project(YC2022-S693)。
文摘In this study,DyF_(3)powder was sprayed onto the polar and side surfaces of the magnets to determine the anisotropic diffusion mechanism of Dy in the sintered Nd-Fe-B magnet.The coercivity and squareness of the magnet in which the diffusion of Dy is perpendicular to the c-axis(a-magnet)are lower than those of the magnet with the diffusion of Dy parallel to the c-axis(c-magnet).Compared with the c-magnet,the a-magnet has a longer Dy-enrichment region from the diffusion surface,where Dy is enriched in the 2:14:1 grain.By contrast,the Dy concentration in the grain boundaries beyond the Dy enrichment region is lower in the a-magnet.Moreover,the Dy shells beyond the Dy enrichment region in the a-magnet are distributed on the side surfaces of the 2:14:1 grains but not on the polar surfaces.Based on the micromagnetic simulation,the Dy shells on the polar surfaces of the grains are more effective in enhancing coercivity.According to first-principle calculations,Dy migrating through 001 into the Nd vacancy in the Nd_(2)Fe_(14)B crystal has a higher diffusion barrier,thus indicating that the lattice diffusion of Dy parallel to the c-axis is more difficult.
基金Project supported by the National Key Research and Development Program of China(2021YFB3503003,2021YFB3503100,2022YFB3505401)。
文摘The microstructure of(Nd,Ce)-Fe-B sintered magnets with different diffusion depths was characterized by a magnetic force microscope,and the relationship between the magnetic properties and the local structure of grain boundary diffused magnets is discussed.The domains perpendicular to the c-axis(easy magnetization direction)show a typical maze-like pattern,while those parallel to the c-axis show the characte ristics of plate domains.The significant gradient change is shown in the concentration of Dy with the direction of diffusion from the surface to the interior.Dy diffuses along grain boundaries and(Dy,Nd)_(2)Fe_(14)B layer with a high anisotropy field formed around the grains.Through in-situ electron probe micro-analysis/magnetic force microscopy(EPMA/MFM),it is found that the average domain width decreases,and the proportion of single domain grains increases as diffusion depth increases.This is caused by both the change of concentration and distribution of Dy.The grain boundary diffusion process changes the microstructure and microchemistry inside the magnet,and these local magnetism differences can be reflected by the configuration of the magnetic domain structure.
