The low coercivity is the major factor inhibiting the large-scale commercial utilization of Nd-Ce-Fe-B sintered magnets.In this work,we achieved a record-high coercivity of 15.04 kOe in Ga-doped Nd-Ce-Fe-B sintered ma...The low coercivity is the major factor inhibiting the large-scale commercial utilization of Nd-Ce-Fe-B sintered magnets.In this work,we achieved a record-high coercivity of 15.04 kOe in Ga-doped Nd-Ce-Fe-B sintered magnets with 30 wt%Ce replacing Nd,demonstrating enormous potential.The Ga-doped Nd-Ce-Fe-B magnets with higher boron(HB)and lower boron(LB)content are designed.The coercivity of the HB magnet increases slightly from 10.80 to 12.26 kOe after annealing,attributed to the optimized distribution of grain boundary(GB)phases.In contrast,the coercivity of the LB magnet remarkably increases from 8.13 to 15.04 kOe after annealing.Microstructural observations indicate that the narrow GB phase in the as-sintered magnet is rich in Fe,and the strong exchange coupling of adjacent grains resulted in low coercivity.The evolution of Ga-rich phases reveals a potential formation mechanism of the RE_(6)Fe_(13)Ga phase,that is the RE-Fe amorphous phase and REGa phase in the as-sintered magnet combine to form the RE_(6)Fe_(13)Ga phase and RE-Ga amorphous phase during post-sinter annealing(RE:rare earth).Moreover,the GB phase of the annealed magnet transforms into a Fe-lean phase with a thickness of 16.4 nm.Magnetization and demagnetization behavior characterizations reveal that the exchange decoupling of adjacent grains induced by the optimized GB phases is the main reason for the remarkable coercivity enhancement,which is also validated by micromagnetic simulations.展开更多
Nanocrystalline Ce-based rare earth(RE)-Fe-B alloys exhibit relatively good hard magnetic properties and high performance-cost ratio,but their properties deteriorate seriously after hot deformation(HD).Here,we present...Nanocrystalline Ce-based rare earth(RE)-Fe-B alloys exhibit relatively good hard magnetic properties and high performance-cost ratio,but their properties deteriorate seriously after hot deformation(HD).Here,we present a simplified one-step HD process for preparing anisotropic Ce-based Ce_(25.88)La_(2.85)Y_(4.56)Fe_(65.73)B_(0.98)(wt%)magnets.The precursor of nanocrystalline powders is first compacted inside a copper tube,and then the powders with the tube are deformed together to achieve magnetic anisotropy.Compared with the conventional two-step HD magnet,i.e.,hot pressing followed by HD,one-step HD significantly increased the coercivity from 1.6 to 3.0 kOe,and the maximum magnetic energy product was improved from 3.7 to 4.8 MGOe.The microstructure characterization indicates that one-step HD can not only produce a more desirable microstructure,characterized by well-aligned platelet-shaped grains with reduced aspect ratio but also greatly inhibit the formation of coarse grain(CG)region.Both of them have been confirmed to be beneficial to enhancing coercivity by micromagnetic simulations.Our results thus demonstrate that the simplified one-step HD process offers a promising approach to developing high-performance anisotropic Ce-based magnets.展开更多
In this study,to enhance the coercivity and high-temperature stability of hot-deformed PrNd-Fe-B magnets,the NdHoAlGa alloy was utilized as a diffusion source and a dual-alloy diffusion process was employed to enhance...In this study,to enhance the coercivity and high-temperature stability of hot-deformed PrNd-Fe-B magnets,the NdHoAlGa alloy was utilized as a diffusion source and a dual-alloy diffusion process was employed to enhance the room temperature coercivity from 1.72 to 2.28 T.For the magnet doped with7 wt%Nd_(72.3)Ho_(13).8Al_(2.3)Ga_(11.7),within the temperature range from 20 to 200℃,the remanence temperature coefficient a increases from-0.16%/℃to-0.14%/℃,and the coercivity temperature coefficientβincreases from-0.49%/℃to-0.43%/℃.By controlling grain boundary(GB)phases and optimizing the main phase simultaneously,Ga was induced to motivate the formation of non-ferromagnetic GB phases,reducing the size of grains and intergranular exchange coupling.Additionally,Ho was diffused into the main phase,forming(Pr,Nd,Ho)-Fe-B phase,which enhances the magnetic crystalline anisotropy field of the main phase grains at high temperatures.展开更多
We investigate a class of elliptic equations with an L^(1)source in the framework of variable exponent spaces.A key characteristic of these equations is the coexistence of a degenerate coercivity term and a lower-orde...We investigate a class of elliptic equations with an L^(1)source in the framework of variable exponent spaces.A key characteristic of these equations is the coexistence of a degenerate coercivity term and a lower-order convection term.By employing innovative integralbased test functions,we derive the necessary a priori estimates.To prove the convergence of solutions to the degenerate coercivity problem,we adopt a method that combines monotonicity and truncation techniques.This approach allows us to demonstrate that the gradient sequences converge almost everywhere.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
MM85Cu15(MM=La,Ce,Pr,Nd)eutectic alloys were added into the hot-deformed Nd-Fe-B magnets to enhance the coercivity.It is found that three endothermic peaks occur on the differential scanning calorimetry curve of the M...MM85Cu15(MM=La,Ce,Pr,Nd)eutectic alloys were added into the hot-deformed Nd-Fe-B magnets to enhance the coercivity.It is found that three endothermic peaks occur on the differential scanning calorimetry curve of the MM-Cu melt-spun ribbons at 432.2,451.1 and 516.5℃.The peaks substantially correspond to three types of MM-Cu low-melting eutectic phase.The coercivity of magnets increases when the MM-Cu content is lower than 4 wt%,and then keeps almost no change with the content further increasing to 5 wt%.The coercivity of the hot-deformed magnets with 4 wt%and without MM-Cu addition is 948 and 683 kA/m,respectively.Nearly all the platelet-shaped grains are isolated by the thickened intergranular phase after MM-Cu addition.Moreover,the average grain size of the magnets with MM-Cu addition decreases compared with that of the magnet without MM-Cu addition.Scanning electron microscopy images show that the areal fraction of the RE-rich grain boundary phase increases from 8.6%to 15.1%after MM-Cu addition.The La,Ce together with Cu and Ga aggregate at the grain boundary regions separating neighboring grains and smoothing the grain boundaries.Therefore,both the thickened grain boundary and decreased mean grain size result in the enhancement of coercivity after MM-Cu eutectic alloy addition.展开更多
It is confirmed that phase homogenization is very important for improving the magnetic properties of 2:17-type Sm-Co sintered magnets,In this work,the influence of solid solution process on microstructure and magnetic...It is confirmed that phase homogenization is very important for improving the magnetic properties of 2:17-type Sm-Co sintered magnets,In this work,the influence of solid solution process on microstructure and magnetic properties of the Sm(CobalFe0.233Cu0.073Zr0.024)7.6 sintered magnets was systematically studied.With the solid-solution treating duration(tS)increasing from 0 to 4 h,intrinsic coercivity(Hcj)increases from 12.83 to 36.54 kOe,magnetic field at knee-point(Hknee)increases from2.76 to 19.14 kOe,and the maximum energy product increases from 19.79 to 29.48 MGOe.The electron probe microanalyzer results reveal that there mainly exist gray and dark regions besides"white"rare earth-rich phase,and the conte nt of Sm,Fe and Cu elements for the two kinds of regions changes a lot for the specimens,Furthermore,with tS increasing up to 4 h,the elements content deviation between the gray and dark regions becomes small gradually from 3.94 at%to 0.27 at%,7.66 at%to 0.21 at%and 7.27 at%to 0.16 at%for Sm,Fe and Cu elements,respectively.Moreover,transmission electron microscopy results show that the distribution of cell size is much more concentrated for aged specimens when tS is 4 h.It is also found that the Cu concentration at cell boundaries for the 4 h solid-solution treatment case shows relatively higher values and greater concentration gradient(1.94 at%/nm).It is verified that sufficient solution treatment duration is prerequisite to form these homogeneous microstructural features,which are the key points for obtaining both high Hcj and Hknee.展开更多
The double-alloy powder mixed method is very proper for developing new small-mass products by changing the composi- tion of sintered Nd-Fe-B magnets, and there is little research on this aspect. The variation on magne...The double-alloy powder mixed method is very proper for developing new small-mass products by changing the composi- tion of sintered Nd-Fe-B magnets, and there is little research on this aspect. The variation on magnetic and mechanical properties of high intrinsic coercivity Nd-Fe-B magnets prepared by double-alloy powder mixed method was discussed, which is a method blend- ing two-type main phase alloy powders with different components. The results showed that the intrinsic coercivity and density of sin- tered Nd-Fe-B magnets increased gradually with the increase in Dy content, and the double-alloy powder mixed method could obtain high intrinsic coercivity Nd-Fe-B magnets with good crystallographic alignment and microstructure. The bending strength of sintered Nd-Fe-B magnets declined, and the Rockwell hardness of sintered Nd-Fe-B magnets first declined, and then increased with the in- crease in Dy content. The microstructure showed that there existed the phenomenon that the Dy element diffused into main phase dur- ing sintering process, and the distribution of Dy content in main phase had some variation in homogeneity as a result of incomplete reaction between the double-alloy powder types.展开更多
Nd-Fe-B permanent magnets with a small amount of A1 nano-particles doping were prepared by conventional sintered method. Effect of AI content on magnetic property, corrosion resistance and oxidation properties of the ...Nd-Fe-B permanent magnets with a small amount of A1 nano-particles doping were prepared by conventional sintered method. Effect of AI content on magnetic property, corrosion resistance and oxidation properties of the magnets were studied. Inves- tigation showed that the coercivity rose gradually, while the remanence decreased simultaneously with increase of A1 doping amount. Further investigation revealed that most A1 element diffused into the main phase and some A1 element diffused into the Nd-rich phase The autoclave test results showed that the corrosion rate of the magnets decreased with A1 content increasing. After oxidation, the maximum energy product losses of the magnets with 0.0 wt.% and 0.2 wt.% AI nano-particles doping were 6.13% and 3.99%, respec- tively. Therefore, A1 nano-particles doping was a promising way to enhance the coercivity and corrosion resistance of sintered Nd-Fe-B magnet.展开更多
The effects of Tb addition on the microstructure and magnetic properties of the NdFeB magnets prepared by HD method were investigated by X-ray diffraction(XRD) and BH magnetometers.The results of the microstructure sh...The effects of Tb addition on the microstructure and magnetic properties of the NdFeB magnets prepared by HD method were investigated by X-ray diffraction(XRD) and BH magnetometers.The results of the microstructure showed that both the Tb-doped and undoped permanent magnets were composed mostly of Tetragonal phase Nd2Fe14B(space group P42/mnm) and a trace amount of Nd-rich phase.Accordingly,addition of Tb led to a decrease of the pole density factor of(004),(006) and(008) crystal plane of the Nd2Fe14B phase calculated by Horta formula,but the coercivity of the magnets increased from 2038 kA/m up to 2302 kA/m as a consequence of Tb addition.The study of the Hc(T)/Ms(T) versus/Ms(T) behavior showed that the nucleation was the dominating mechanism for the magnetization reversal in both sintered magnets,and the microstructural parameters of αk and Neff were obtained also.The Kronmüller-plot showed an increase of the αk responsible for an increase of the coercivity.展开更多
Nd-Fe-B permanent magnets with a small amount of Cu nano-particles doping have been prepared by con-ventional sintered method. Effects of Cu content on magnetic properties, corrosion resistance, and oxidation properti...Nd-Fe-B permanent magnets with a small amount of Cu nano-particles doping have been prepared by con-ventional sintered method. Effects of Cu content on magnetic properties, corrosion resistance, and oxidation properties of the magnets have been studied. It shows that the coercivity rises gradually, while the remanence decreases simultaneously with increasing Cu doping amount. Microstructure observation reveals that Cu ele- ment enriches mainly the Nd-rich phase. Autoclave test results show that the corrosion rate of the magnets decreases with increasing Cu content. After oxidation, the maximum energy product loss of the magnets with 0 and 0.2 wt% Cu nano-particles doping are 6.13% and 0.g9%, respectively. Therefore, it is concluded that Cu nano-particles doping is a promising way to enhance the coercivity and corrosion resistance of sintered Nd-Fe-B magnets.展开更多
An N38SH-grade magnet with low oxygen content was used to study the evolution of magnetic properties upon post-sin-tering annealing. Phase transformation of as-sintered magnet was investigated by differential scanning...An N38SH-grade magnet with low oxygen content was used to study the evolution of magnetic properties upon post-sin-tering annealing. Phase transformation of as-sintered magnet was investigated by differential scanning calorimetry （DSC）. Three low temperature eutectic transition points were detected. Little change could be found when annealed below the lowest eutectic transition point. A wide annealing temperature range （460–560 oC） between the lowest and highest eutectic transition point was available for this magnet to achieve a relatively high coercivity （~1671 kA/m） at a relatively low Dy content （~3 wt.%）. However, squareness fac-tor （SF） of the demagnetizing curve and its temperature stability were found to decrease after annealing above the highest eutectic transition point. This was attributed to the change of Cu content in the Nd-rich phase under different annealing temperatures.展开更多
The effects of Nb on the thermal stability and impact toughness of ultra-high intrinsic coercivity of Nd-Fe-B magnets were investigated.The results showed that the addition of Nb could improve the thermal stability,an...The effects of Nb on the thermal stability and impact toughness of ultra-high intrinsic coercivity of Nd-Fe-B magnets were investigated.The results showed that the addition of Nb could improve the thermal stability,and obviously increased the impact toughness of sintered Nd-Fe-B magnets.The optimum thermal stability of sintered Nd-Fe-B magnets was obtained when the content of Nb was 1.0 at.%.The maximum impact toughness of sintered Nd-Fe-B magnets was obtained when the content of Nb was 1.5 at.%,but the magnetic properties of sintered Nd-Fe-B magnets drastically deteriorated when the content of Nb increased from 1.0 at.% to 1.5 at.%.The microstructure showed that overfull Nb addition made many ultra-fine grains get together,which led to the density of sintered Nd-Fe-B magnets decline and drastically deteriorated the magnetic properties of sintered Nd-Fe-B magnets.展开更多
We propose a hybrid coercivity mechanism for exchange-coupled hard/soft multilayers, which incorporates elements of both the traditional nucleation and pinning mechanisms based on both threedimensional(3 D) and one-di...We propose a hybrid coercivity mechanism for exchange-coupled hard/soft multilayers, which incorporates elements of both the traditional nucleation and pinning mechanisms based on both threedimensional(3 D) and one-dimensional(1 D) micromagnetic calculations. The magnetic reversal starts with the nucleation of the domain wall near the defects or soft phases, which ends by the pinning usually in the same place. Therefore, pinning near the nucleation centers are the dominant coercivity mechanism for both exchange-coupled nanocomposites and so-called single-phased permanent magnets. Our proposed coercivity mechanism and calculated results agree very well with available experimental data,especially the recently reported high energy products achieved in NdFeB and SmCo based hard/soft multilayers. The hybrid coercivity mechanism can be readily extended to single-phased permanent magnets with defects and other magnetic systems.展开更多
Two models are established to reveal the underlying coercivity mechanism for SmCo/Fe films,where one model considers a transition layer between hard and soft layers,while the other model does not consider this layer.B...Two models are established to reveal the underlying coercivity mechanism for SmCo/Fe films,where one model considers a transition layer between hard and soft layers,while the other model does not consider this layer.Based on the two models,hysteresis loops,nucleation fields and coercivity are obtained by one-dimensional(1 D)and three-dimensional(3 D)micromagnetic methods.In particular,the calculated nucleation fields(H_N)and coercivity(H_C)match very well with the experimental data.It is found that the increase in the soft phase thickness(Ls)leads to a transition of the coercivity mechanism from nucleation to pinning.Such a pinning is inherently related to nucleation and has both attributes of traditional nucleation and pinning,called as a hybrid coercivity mechanism here.It is general for all hard/soft composites and can be extended to single-phased permanent magnets where defects are inevitable.展开更多
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 intergran...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.展开更多
Usually the improved coercivity of rare earth(RE)based 2:14:1-type permanent magnets via RE-rich intergranular additives is achieved at the cost of more corrosion channels and deteriorated corrosion resistance,which r...Usually the improved coercivity of rare earth(RE)based 2:14:1-type permanent magnets via RE-rich intergranular additives is achieved at the cost of more corrosion channels and deteriorated corrosion resistance,which remains a challenging hurdle in the RE-Fe-B community.Distinctly,here we report the concurrent improvements of corrosion resistance and coercivity in 40 wt.%Ce-substituted Nd-Ce-Fe-B sintered magnets through engineering the intergranular phase using simple(Nd,Pr)H_(x)additive.The dehydrogenated Nd/Pr changes the RE concentration gradients between 2:14:1 matrix and intergranular phases during sintering and enlarges the fraction of corrosion-resistant REFe_(2) phase,rather than the conventionally assumed Nd/Pr-rich intergranular phase with high chemical vulnerability.The spontaneous formation of REFe_(2) intergranular phase after(Nd,Pr)H_(x) addition generates the uniquely enhanced corrosion resistance against the hot/humid and acidic environments,and counts as one peculiar feature of Nd-Ce-Fe-B magnets at high Ce substitution level,being distinct from previously reported Ce-free/lean RE-Fe-B.Simultaneously,the formation of continuous grain boundaries enhances the coercivity from 8.7to 12.5 k Oe with trace addition of(Nd,Pr)H_(x).Above findings may spur progress towards developing a high-performance Nd-Ce-Fe-B permanent magnet.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
The microstructure of twinning as well as the phase boundary between 1:5 H and 2:17 R phase in Fe-rich Sm_(2)Co_(17)-type magnets was characterized at atomic scale using nanobeam diffraction and highresolution STEM-HA...The microstructure of twinning as well as the phase boundary between 1:5 H and 2:17 R phase in Fe-rich Sm_(2)Co_(17)-type magnets was characterized at atomic scale using nanobeam diffraction and highresolution STEM-HAADF imaging,and the reason for the dramatic increase of coercivity during slow cooling was investigated based on the microchemistry analysis.The twinning relationship in the 2:17 R phase originates from ordered substitution of Sm atoms by Co-Co atomic pairs on every three(3033)and(3033)planes,leading to formation of two corresponding equivalent twin variants.The basal plane of the 2:17 R phase,the 1:3 R platelet phase across the 2:17 R cell and the 1:5 H cell boundary phase between two adjacent 2:17 R cells all can act as effective twin boundary.The cell boundary phase is precipitated along the pyramidal habit plane,and a fully coherent phase boundary(PB)is formed between the 1:5 H and 2:17 R phases with the orientation relationship to be PB//(1121)1:5 H//(1011)_(2):17 R.The phase boundary may either be parallel to or intersect with the pyramidal planes occupied by Co-Co atomic pairs.The substantial increase of coercivity during slow cooling is ascribed to the development of large gradient of the elements concentration within the cell boundary phase,resulting in large gradient of domain wall energy,and thus the pinning strength of the cell boundary phase against magnetic domain wall motion is significantly enhanced.展开更多
Coercivity mechanism in permanent magnets has been debated for many years.In this paper, various models of the coercivity mechanism are classified and re-examined by the comparison and contrast.Coherent rotation and c...Coercivity mechanism in permanent magnets has been debated for many years.In this paper, various models of the coercivity mechanism are classified and re-examined by the comparison and contrast.Coherent rotation and curling models can reveal the underlying reversal mechanism clearly based on isolated grains with elliptic shapes.By contrast, the numerical methods consider inter-grain interactions while simulating the evolution of the spins and hysteresis loops with complicated shapes.However, an exact simulation of magnetic reversal in permanent nanomagnets requires many meshes to mimic the thin domain wall well.Nucleation and pinning are the two main coercivity mechanisms in permanent magnets.The former signifies the beginning of the magnetic reversal, whilst the latter completes it.Recently, it is proposed that the large difference between the intrinsic magnetic properties of the nucleation centers and those of the main phase can result in a large pinning field(self-pinning), which has the attributes of both traditional nucleation and pinning.Such a pinning explains the experimental data of permanent magnets very well, including the enhancement of the coercivity by the grain boundary pinning.展开更多
基金supported by the National Natural Science Foundation of China(Nos.52261037,52088101)the Key research project of Jiangxi Province(No.20203ABC28W006)the Double-Thousand Plan of Jiangxi Province(No.jxsq2023101057).
文摘The low coercivity is the major factor inhibiting the large-scale commercial utilization of Nd-Ce-Fe-B sintered magnets.In this work,we achieved a record-high coercivity of 15.04 kOe in Ga-doped Nd-Ce-Fe-B sintered magnets with 30 wt%Ce replacing Nd,demonstrating enormous potential.The Ga-doped Nd-Ce-Fe-B magnets with higher boron(HB)and lower boron(LB)content are designed.The coercivity of the HB magnet increases slightly from 10.80 to 12.26 kOe after annealing,attributed to the optimized distribution of grain boundary(GB)phases.In contrast,the coercivity of the LB magnet remarkably increases from 8.13 to 15.04 kOe after annealing.Microstructural observations indicate that the narrow GB phase in the as-sintered magnet is rich in Fe,and the strong exchange coupling of adjacent grains resulted in low coercivity.The evolution of Ga-rich phases reveals a potential formation mechanism of the RE_(6)Fe_(13)Ga phase,that is the RE-Fe amorphous phase and REGa phase in the as-sintered magnet combine to form the RE_(6)Fe_(13)Ga phase and RE-Ga amorphous phase during post-sinter annealing(RE:rare earth).Moreover,the GB phase of the annealed magnet transforms into a Fe-lean phase with a thickness of 16.4 nm.Magnetization and demagnetization behavior characterizations reveal that the exchange decoupling of adjacent grains induced by the optimized GB phases is the main reason for the remarkable coercivity enhancement,which is also validated by micromagnetic simulations.
基金supported by the National Natural Science Foundation of China(Nos.U21A2052,52071143 and 52301237)Guangdong Basic and Applied Basic Research Foundation(Nos.2023A1515010431 and 2022A1515011453)+1 种基金China Postdoctoral Science Foundation funded project(No.2022M720845)GDAS Project of Science and Technology Development(Nos.2022GDASZH-2022010104 and 2023GDASZH-2023010104)。
文摘Nanocrystalline Ce-based rare earth(RE)-Fe-B alloys exhibit relatively good hard magnetic properties and high performance-cost ratio,but their properties deteriorate seriously after hot deformation(HD).Here,we present a simplified one-step HD process for preparing anisotropic Ce-based Ce_(25.88)La_(2.85)Y_(4.56)Fe_(65.73)B_(0.98)(wt%)magnets.The precursor of nanocrystalline powders is first compacted inside a copper tube,and then the powders with the tube are deformed together to achieve magnetic anisotropy.Compared with the conventional two-step HD magnet,i.e.,hot pressing followed by HD,one-step HD significantly increased the coercivity from 1.6 to 3.0 kOe,and the maximum magnetic energy product was improved from 3.7 to 4.8 MGOe.The microstructure characterization indicates that one-step HD can not only produce a more desirable microstructure,characterized by well-aligned platelet-shaped grains with reduced aspect ratio but also greatly inhibit the formation of coarse grain(CG)region.Both of them have been confirmed to be beneficial to enhancing coercivity by micromagnetic simulations.Our results thus demonstrate that the simplified one-step HD process offers a promising approach to developing high-performance anisotropic Ce-based magnets.
基金Project supported by the National Key Research and Development Program of China(2022YFB3503403)the Inner Mongolia Major Technology Project(2021ZD0035)+1 种基金the Ningbo Natural Science Foundation(2023J343)the Science and Technology Innovation 2025 Major Project of Ningbo(2022Z106)。
文摘In this study,to enhance the coercivity and high-temperature stability of hot-deformed PrNd-Fe-B magnets,the NdHoAlGa alloy was utilized as a diffusion source and a dual-alloy diffusion process was employed to enhance the room temperature coercivity from 1.72 to 2.28 T.For the magnet doped with7 wt%Nd_(72.3)Ho_(13).8Al_(2.3)Ga_(11.7),within the temperature range from 20 to 200℃,the remanence temperature coefficient a increases from-0.16%/℃to-0.14%/℃,and the coercivity temperature coefficientβincreases from-0.49%/℃to-0.43%/℃.By controlling grain boundary(GB)phases and optimizing the main phase simultaneously,Ga was induced to motivate the formation of non-ferromagnetic GB phases,reducing the size of grains and intergranular exchange coupling.Additionally,Ho was diffused into the main phase,forming(Pr,Nd,Ho)-Fe-B phase,which enhances the magnetic crystalline anisotropy field of the main phase grains at high temperatures.
基金Supported by the National Natural Science Foundation of China(Grant No.11901131)。
文摘We investigate a class of elliptic equations with an L^(1)source in the framework of variable exponent spaces.A key characteristic of these equations is the coexistence of a degenerate coercivity term and a lower-order convection term.By employing innovative integralbased test functions,we derive the necessary a priori estimates.To prove the convergence of solutions to the degenerate coercivity problem,we adopt a method that combines monotonicity and truncation techniques.This approach allows us to demonstrate that the gradient sequences converge almost everywhere.
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the National Natural Science Foundation of China(51771055,51331003)Plan of National Key Research and Development(2016YFB0700903)。
文摘MM85Cu15(MM=La,Ce,Pr,Nd)eutectic alloys were added into the hot-deformed Nd-Fe-B magnets to enhance the coercivity.It is found that three endothermic peaks occur on the differential scanning calorimetry curve of the MM-Cu melt-spun ribbons at 432.2,451.1 and 516.5℃.The peaks substantially correspond to three types of MM-Cu low-melting eutectic phase.The coercivity of magnets increases when the MM-Cu content is lower than 4 wt%,and then keeps almost no change with the content further increasing to 5 wt%.The coercivity of the hot-deformed magnets with 4 wt%and without MM-Cu addition is 948 and 683 kA/m,respectively.Nearly all the platelet-shaped grains are isolated by the thickened intergranular phase after MM-Cu addition.Moreover,the average grain size of the magnets with MM-Cu addition decreases compared with that of the magnet without MM-Cu addition.Scanning electron microscopy images show that the areal fraction of the RE-rich grain boundary phase increases from 8.6%to 15.1%after MM-Cu addition.The La,Ce together with Cu and Ga aggregate at the grain boundary regions separating neighboring grains and smoothing the grain boundaries.Therefore,both the thickened grain boundary and decreased mean grain size result in the enhancement of coercivity after MM-Cu eutectic alloy addition.
基金Project supported by the National Key Research and Development Program of China(2016YFB0700903)the National Natural Science Foundation of China(5159088251401054)。
文摘It is confirmed that phase homogenization is very important for improving the magnetic properties of 2:17-type Sm-Co sintered magnets,In this work,the influence of solid solution process on microstructure and magnetic properties of the Sm(CobalFe0.233Cu0.073Zr0.024)7.6 sintered magnets was systematically studied.With the solid-solution treating duration(tS)increasing from 0 to 4 h,intrinsic coercivity(Hcj)increases from 12.83 to 36.54 kOe,magnetic field at knee-point(Hknee)increases from2.76 to 19.14 kOe,and the maximum energy product increases from 19.79 to 29.48 MGOe.The electron probe microanalyzer results reveal that there mainly exist gray and dark regions besides"white"rare earth-rich phase,and the conte nt of Sm,Fe and Cu elements for the two kinds of regions changes a lot for the specimens,Furthermore,with tS increasing up to 4 h,the elements content deviation between the gray and dark regions becomes small gradually from 3.94 at%to 0.27 at%,7.66 at%to 0.21 at%and 7.27 at%to 0.16 at%for Sm,Fe and Cu elements,respectively.Moreover,transmission electron microscopy results show that the distribution of cell size is much more concentrated for aged specimens when tS is 4 h.It is also found that the Cu concentration at cell boundaries for the 4 h solid-solution treatment case shows relatively higher values and greater concentration gradient(1.94 at%/nm).It is verified that sufficient solution treatment duration is prerequisite to form these homogeneous microstructural features,which are the key points for obtaining both high Hcj and Hknee.
基金Project supported by the Natural Science Foundation of Hubei Province(2014CFB626,2015CFC785)the Research Project of Hubei Provincial Department of Education(D20151801)the Opening Foundation of Hubei Key Laboratory of Automotive Power Train and Electronic Control(ZDK1201404)
文摘The double-alloy powder mixed method is very proper for developing new small-mass products by changing the composi- tion of sintered Nd-Fe-B magnets, and there is little research on this aspect. The variation on magnetic and mechanical properties of high intrinsic coercivity Nd-Fe-B magnets prepared by double-alloy powder mixed method was discussed, which is a method blend- ing two-type main phase alloy powders with different components. The results showed that the intrinsic coercivity and density of sin- tered Nd-Fe-B magnets increased gradually with the increase in Dy content, and the double-alloy powder mixed method could obtain high intrinsic coercivity Nd-Fe-B magnets with good crystallographic alignment and microstructure. The bending strength of sintered Nd-Fe-B magnets declined, and the Rockwell hardness of sintered Nd-Fe-B magnets first declined, and then increased with the in- crease in Dy content. The microstructure showed that there existed the phenomenon that the Dy element diffused into main phase dur- ing sintering process, and the distribution of Dy content in main phase had some variation in homogeneity as a result of incomplete reaction between the double-alloy powder types.
基金Project supported by National Natural Science Foundation of China(51001002)Research Fund for the Doctoral Program of Higher Education of China(20091103120024)+3 种基金National High Technology Research and Development Program of China(2012AA063201)the Key Program of Science and Technology Development Project of Beijing Municipal Education Commission(KZ201110005007)the Importation and Development of High-Caliber Talents Project of Beijing Municipal Institutions211 Project of Beijing University of Technology and Rixin Talents of Beijing Universityof Technology for financial support
文摘Nd-Fe-B permanent magnets with a small amount of A1 nano-particles doping were prepared by conventional sintered method. Effect of AI content on magnetic property, corrosion resistance and oxidation properties of the magnets were studied. Inves- tigation showed that the coercivity rose gradually, while the remanence decreased simultaneously with increase of A1 doping amount. Further investigation revealed that most A1 element diffused into the main phase and some A1 element diffused into the Nd-rich phase The autoclave test results showed that the corrosion rate of the magnets decreased with A1 content increasing. After oxidation, the maximum energy product losses of the magnets with 0.0 wt.% and 0.2 wt.% AI nano-particles doping were 6.13% and 3.99%, respec- tively. Therefore, A1 nano-particles doping was a promising way to enhance the coercivity and corrosion resistance of sintered Nd-Fe-B magnet.
基金Project supported by the Key Project of the International Cooperation and Exchanges of the Zhejiang Province (2006C14014)the Provincial Science and Technology Project (2009C21010)+2 种基金the Provincial Natural Science Foundation (Z4090462 and Y6100640)the National Natural Science Foundation (51001092)the Foundation for University Young Teachers from the Ministry of Education of Zhejiang Province
文摘The effects of Tb addition on the microstructure and magnetic properties of the NdFeB magnets prepared by HD method were investigated by X-ray diffraction(XRD) and BH magnetometers.The results of the microstructure showed that both the Tb-doped and undoped permanent magnets were composed mostly of Tetragonal phase Nd2Fe14B(space group P42/mnm) and a trace amount of Nd-rich phase.Accordingly,addition of Tb led to a decrease of the pole density factor of(004),(006) and(008) crystal plane of the Nd2Fe14B phase calculated by Horta formula,but the coercivity of the magnets increased from 2038 kA/m up to 2302 kA/m as a consequence of Tb addition.The study of the Hc(T)/Ms(T) versus/Ms(T) behavior showed that the nucleation was the dominating mechanism for the magnetization reversal in both sintered magnets,and the microstructural parameters of αk and Neff were obtained also.The Kronmüller-plot showed an increase of the αk responsible for an increase of the coercivity.
基金supported by the National Natural Science Foundation of China (No. 51001002)the Research Fund for the Doctoral Program of Higher Education of China (No. 20091103120024)+2 种基金the National High Technology Research and Development Program of China (No.2012AA063201)the Key Program of Science and Technology Development Project of Beijing Municipal Education Commission (No. KZ201110005007)211 Project of Beijing University of Technology and Rixin Talents of Beijing University of Technology
文摘Nd-Fe-B permanent magnets with a small amount of Cu nano-particles doping have been prepared by con-ventional sintered method. Effects of Cu content on magnetic properties, corrosion resistance, and oxidation properties of the magnets have been studied. It shows that the coercivity rises gradually, while the remanence decreases simultaneously with increasing Cu doping amount. Microstructure observation reveals that Cu ele- ment enriches mainly the Nd-rich phase. Autoclave test results show that the corrosion rate of the magnets decreases with increasing Cu content. After oxidation, the maximum energy product loss of the magnets with 0 and 0.2 wt% Cu nano-particles doping are 6.13% and 0.g9%, respectively. Therefore, it is concluded that Cu nano-particles doping is a promising way to enhance the coercivity and corrosion resistance of sintered Nd-Fe-B magnets.
基金Project supported by the National Natural Science Foundation of China(51171111)
文摘An N38SH-grade magnet with low oxygen content was used to study the evolution of magnetic properties upon post-sin-tering annealing. Phase transformation of as-sintered magnet was investigated by differential scanning calorimetry （DSC）. Three low temperature eutectic transition points were detected. Little change could be found when annealed below the lowest eutectic transition point. A wide annealing temperature range （460–560 oC） between the lowest and highest eutectic transition point was available for this magnet to achieve a relatively high coercivity （~1671 kA/m） at a relatively low Dy content （~3 wt.%）. However, squareness fac-tor （SF） of the demagnetizing curve and its temperature stability were found to decrease after annealing above the highest eutectic transition point. This was attributed to the change of Cu content in the Nd-rich phase under different annealing temperatures.
基金supported by the National High Technology Research and Development Program of China(2007AA03Z438)the National Natural Science Foundations of China(50871032)
文摘The effects of Nb on the thermal stability and impact toughness of ultra-high intrinsic coercivity of Nd-Fe-B magnets were investigated.The results showed that the addition of Nb could improve the thermal stability,and obviously increased the impact toughness of sintered Nd-Fe-B magnets.The optimum thermal stability of sintered Nd-Fe-B magnets was obtained when the content of Nb was 1.0 at.%.The maximum impact toughness of sintered Nd-Fe-B magnets was obtained when the content of Nb was 1.5 at.%,but the magnetic properties of sintered Nd-Fe-B magnets drastically deteriorated when the content of Nb increased from 1.0 at.% to 1.5 at.%.The microstructure showed that overfull Nb addition made many ultra-fine grains get together,which led to the density of sintered Nd-Fe-B magnets decline and drastically deteriorated the magnetic properties of sintered Nd-Fe-B magnets.
基金Project supported by National Natural Science Foundation of China(51771127,51571126,51772004)
文摘We propose a hybrid coercivity mechanism for exchange-coupled hard/soft multilayers, which incorporates elements of both the traditional nucleation and pinning mechanisms based on both threedimensional(3 D) and one-dimensional(1 D) micromagnetic calculations. The magnetic reversal starts with the nucleation of the domain wall near the defects or soft phases, which ends by the pinning usually in the same place. Therefore, pinning near the nucleation centers are the dominant coercivity mechanism for both exchange-coupled nanocomposites and so-called single-phased permanent magnets. Our proposed coercivity mechanism and calculated results agree very well with available experimental data,especially the recently reported high energy products achieved in NdFeB and SmCo based hard/soft multilayers. The hybrid coercivity mechanism can be readily extended to single-phased permanent magnets with defects and other magnetic systems.
基金financially supported by the National Natural Science Foundation of China(Nos.51771127,51571126 and 51772004).
文摘Two models are established to reveal the underlying coercivity mechanism for SmCo/Fe films,where one model considers a transition layer between hard and soft layers,while the other model does not consider this layer.Based on the two models,hysteresis loops,nucleation fields and coercivity are obtained by one-dimensional(1 D)and three-dimensional(3 D)micromagnetic methods.In particular,the calculated nucleation fields(H_N)and coercivity(H_C)match very well with the experimental data.It is found that the increase in the soft phase thickness(Ls)leads to a transition of the coercivity mechanism from nucleation to pinning.Such a pinning is inherently related to nucleation and has both attributes of traditional nucleation and pinning,called as a hybrid coercivity mechanism here.It is general for all hard/soft composites and can be extended to single-phased permanent magnets where defects are inevitable.
基金Project partly supported by the Natural Science Foundation of Shanxi Province,China(201801D121100)the Program for the Outstanding Innovative Teams of Higher Learning Institutions of Shanxi(OIT)the Scientific and Technological Innovation Programs of Higher Education Institutions in Shanxi(STIP)(201802033)。
文摘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.
基金financially supported by the National Natural Science Foundation of China(Nos.52171192 and 51801181)the Zhejiang Province Public Welfare Technology Application Research Project(No.LGG20E010007)the Key Research and Development Program of Zhejiang Province(Nos.2020C01008,2021C01192 and 2021C01023)。
文摘Usually the improved coercivity of rare earth(RE)based 2:14:1-type permanent magnets via RE-rich intergranular additives is achieved at the cost of more corrosion channels and deteriorated corrosion resistance,which remains a challenging hurdle in the RE-Fe-B community.Distinctly,here we report the concurrent improvements of corrosion resistance and coercivity in 40 wt.%Ce-substituted Nd-Ce-Fe-B sintered magnets through engineering the intergranular phase using simple(Nd,Pr)H_(x)additive.The dehydrogenated Nd/Pr changes the RE concentration gradients between 2:14:1 matrix and intergranular phases during sintering and enlarges the fraction of corrosion-resistant REFe_(2) phase,rather than the conventionally assumed Nd/Pr-rich intergranular phase with high chemical vulnerability.The spontaneous formation of REFe_(2) intergranular phase after(Nd,Pr)H_(x) addition generates the uniquely enhanced corrosion resistance against the hot/humid and acidic environments,and counts as one peculiar feature of Nd-Ce-Fe-B magnets at high Ce substitution level,being distinct from previously reported Ce-free/lean RE-Fe-B.Simultaneously,the formation of continuous grain boundaries enhances the coercivity from 8.7to 12.5 k Oe with trace addition of(Nd,Pr)H_(x).Above findings may spur progress towards developing a high-performance Nd-Ce-Fe-B permanent magnet.
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Project supported by Zhejiang Province Public Welfare Technology Application Research Project(LGC20E010002)National Natural Science Foundation of China(51877094)。
文摘The microstructure of twinning as well as the phase boundary between 1:5 H and 2:17 R phase in Fe-rich Sm_(2)Co_(17)-type magnets was characterized at atomic scale using nanobeam diffraction and highresolution STEM-HAADF imaging,and the reason for the dramatic increase of coercivity during slow cooling was investigated based on the microchemistry analysis.The twinning relationship in the 2:17 R phase originates from ordered substitution of Sm atoms by Co-Co atomic pairs on every three(3033)and(3033)planes,leading to formation of two corresponding equivalent twin variants.The basal plane of the 2:17 R phase,the 1:3 R platelet phase across the 2:17 R cell and the 1:5 H cell boundary phase between two adjacent 2:17 R cells all can act as effective twin boundary.The cell boundary phase is precipitated along the pyramidal habit plane,and a fully coherent phase boundary(PB)is formed between the 1:5 H and 2:17 R phases with the orientation relationship to be PB//(1121)1:5 H//(1011)_(2):17 R.The phase boundary may either be parallel to or intersect with the pyramidal planes occupied by Co-Co atomic pairs.The substantial increase of coercivity during slow cooling is ascribed to the development of large gradient of the elements concentration within the cell boundary phase,resulting in large gradient of domain wall energy,and thus the pinning strength of the cell boundary phase against magnetic domain wall motion is significantly enhanced.
基金Project supported by the National Natural Science Foundation of China(Grant Nos.11074179,51771127,51571126,and 51772004)the Scientific Research Fund of Sichuan Provincial Education Department,China(Grant Nos.18TD0010 and 16CZ0006)
文摘Coercivity mechanism in permanent magnets has been debated for many years.In this paper, various models of the coercivity mechanism are classified and re-examined by the comparison and contrast.Coherent rotation and curling models can reveal the underlying reversal mechanism clearly based on isolated grains with elliptic shapes.By contrast, the numerical methods consider inter-grain interactions while simulating the evolution of the spins and hysteresis loops with complicated shapes.However, an exact simulation of magnetic reversal in permanent nanomagnets requires many meshes to mimic the thin domain wall well.Nucleation and pinning are the two main coercivity mechanisms in permanent magnets.The former signifies the beginning of the magnetic reversal, whilst the latter completes it.Recently, it is proposed that the large difference between the intrinsic magnetic properties of the nucleation centers and those of the main phase can result in a large pinning field(self-pinning), which has the attributes of both traditional nucleation and pinning.Such a pinning explains the experimental data of permanent magnets very well, including the enhancement of the coercivity by the grain boundary pinning.
