A comparative study of NdY-Fe-B magnet and NdCe-Fe-B magnet

Low cost and high abundance rare earth elements Y and Ce have attracted tremendous interests of the industrial and scientific societies for fabricating the highly cost-performance efficient rare earth permanent magnets. However, the effect of separate replacement of Nd by Y or Ce on the performances of NdFeB-type magnet under the same atomic ratio and preparation conditions is still unclear. In this work, we systematically investigated the magnetic properties, thermal stabilities and service performances of (Nd_0.8Y_0.2)_13.80Fe_balAl_0.24Cu_0.1B_6.04 (atomic fraction, denoted as 20Y) and (Nd_0.8Ce_0.2)_13.80Fe_balAl_0.24Cu_0.1B_6.04 (atomic fraction, denoted as 20Ce) magnets. The results demonstrate that the μ₀M_r, μ₀H_c and (BH)_max of 20Y magnet are respectively 1.325 T, 1.173 T and 343.467 kJ/m³, which are comprehensively higher than those of 20Ce magnet (μ₀M_r = 1.310 T, μ₀H_c = 0.948 T, (BH)_max = 321.105 kJ/m³). Moreover, the 20Y magnet has higher thermal stability compared with 20Ce magnet which is favorable for the magnetic performances at elevated temperatures. The investigation of microstructure and elemental distribution indicates that the excellent magnetic performances of NdY-Fe-B magnet can be attributed not only to the preferable intrinsic properties 4πM_s, H_a and T_c of Y₂Fe₁₄B, but also to the in-situ core–shell structure of the 2:14:1 matrix phase grain with Y-rich core and Nd-rich shell, along with the thicker grain boundary layer between the adjacent matrix phase grains in NdY-Fe-B magnet. Furthermore, the 20Y magnet exhibits better mechanical property and higher corrosion resistance than 20Ce magnet, which are helpful for prolonging the service life of the magnet in practical application.     

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

To improve the coercivity and temperature stability of Nd-Fe-B sintered magnets for high-temperature applications, the eutectic Tb₈₀Fe₂₀ (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 microstructure, magnetic properties and thermal stability of the Nd-Fe-B magnets with different Tb₈₀Fe₂₀ 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 (B_r) decreases not obviously by introducing 4 wt% Tb₈₀Fe₂₀ alloy. Meanwhile, the reversible temperature 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 (T_C) of the Nd-Fe-B magnet is slightly enhanced with the increase of Tb₈₀Fe₂₀ content. Moreover, the irreversible flux magnetic loss (h_irr) is obviously reduced as Tb₈₀Fe₂₀ addition 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 Tb₈₀Fe₂₀ alloy. The associated mechanisms on improved coercivity and thermal stability were comprehensively researched.     

Magnetic Alignment Study of Hybrid Magnet Consisting of R-Rich and R-Lean Nd-Fe-B Alloys

A hybrid magnet was prepared by the hot-pressing and die-upsetting of the mixture of the R-rich Nd_xFe_93.5–xGa_0.5B₆ (x= 13.5 and 11.8) alloy and the R-lean Nd_xFe_93–xNb₁B₆ (x = 6, 9) alloy melt-spun ribbons. The microstructure and magnetic properties of the hybrid magnet were investigated. In the hot-pressed or die-upset hybrid magnet the R-rich and R-lean alloy regions existed independently without alloying between them. The two alloy regions in the die-upset hybrid magnet were coupled effectively via a magnetostatic interaction. A texture was developed only in the R-rich Nd₂Fe₁₄B single phase alloy region in the die-upset hybrid magnet, and this led to an anisotropic nature in die-upset hybrid magnet. The die-upset hybrid magnets containing higher Nd-content (13.5 at%) host alloy shows consistently a better magnetic alignment with respect to the magnets with lower Nd-content (11.8 at%) host alloy.     

Effects of grain boundary diffusion process on magnetic properties enhancement and microstructure evolution of hot-deformed Nd-Fe-B magnets

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 prope...     

Influence of Pr-Al-Co alloy diffusion source on magnetic properties and microstructure of sintered Nd-Fe-B magnets processed by grain boundary diffusion

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 Pr₇₀Al_30–xCo_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 Pr₇₀Al₂₀Co₁₀ diffused magnet is the highest, increased from 1.62 to 2.24 T, higher than 2.01 T of the Pr₇₀Al₃₀ diffused magnet. With further increase of Co content in the diffused source, the coercivity of the diffused magnet decreases gradually, the coercivity of Pr₇₀Al₁₅Co₁₅, Pr₇₀Al₂₀Co₁₀ and Pr₇₀Co₃₀ diffused magnet is 2.15, 1.99 and 1.81 T, respectively. Microstructural analysis shows that plenty of continuous grain boundary phases (CGBPs) can be formed in the Pr₇₀Al₂₀Co₁₀ 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.     

Effects of Nd–Ga intergranular addition on microstructure and magnetic properties of heavy-rare-earth-free Nd-Fe-B sintered magnets

In this study, we propose an approach of grain boundary modification that can significantly increase the coercivity of the B-lean Nd-Fe-B sintered magnets by intergranular addition of Nd–Ga. The coercivity is substantially enhanced from 1.51 to 2.04 T through optimizing the microstructure and adjusting the phase composition for the grain boundary phase in the annealed magnets. The matrix grains are covered by a continuous thin grain boundary phase accompanying the formation of intermetallic Nd₆Fe₁₃Ga phases. The analysis of magnetic behaviors above Curie temperature confirms that the grain boundary phase of annealed Nd–Ga doped magnets appears to be non-ferromagnetic, facilitating the intergrain exchange decoupling. Microstructure observation in grain boundary area indicates that some surface of the matrix grain is dissolved in the formation process of the Nd₆Fe₁₃Ga phase. It gives rise to a decrease in the proportion of matrix grains and saturation magnetization of the magnet. The detailed relationship between magnetic properties and microstructure is discussed based on these results.     

Microstructure evolution and coercivity mechanism of hydrogenation-disproportionation-desorption-recombination (HDDR) treated Nd-Fe-B strip cast alloys

In this paper, we systematically investigated the microstructure evolution and coercivity mechanism of hydrogenation-disproportionation-desorption-recombination (HDDR) treated Nd-Fe-B strip cast alloys by transmission electron microscopy (TEM) and three-dimensional atom probe (3DAP) analyses. The rod-like NdH_2+x phases with diameters of 10–20 nm are embedded into α-Fe matrix, which hereditarily leads to textured grains in HDDR alloy. The migration of NdH_2+x from Nd-rich region to α-Fe matrix during hydrogen absorption process contributes to the uniform redistribution of Nd-rich phases after HDDR treatment. The HDDR alloy with single domain grain sizes of 200–300 nm exhibits relatively low coercivity of 1.01 T that arises from pinning magnetic domain motion. The weak c-axis orientation of HDDR alloy results in a lower reverse magnetic field (coercivity) to reduce remanence to 0. Moreover, the direct contact of Nd₂Fe₁₄B grains and the high concentration of ferromagnetic elements (Fe content ≈ 66.06 at%, Co content ≈ 0.91 at%) in Nd-rich grain boundary layer lead to strong magnetostatic coupling effect among Nd₂Fe₁₄B grains. The nano-sized α-Fe inside Nd₂Fe₁₄B matrix makes the magnetization reversal easily and decreases the coercivity of HDDR alloy.     

Effect of Rare Earth Substitution on Machinability of R-Fe-B Sintered Magnets

The machinability of sintered Nd-Fe-B magnets with nominal compositions of (Nd_1–xDy_x)₁₆Fe₇₈B₆ (x = 0, 0.05, 0.10, 0.15) and (Nd_1–yPr_y)₁₆Fe₇₈B₆ (y = 0, 0.33, 0.67, 1), has been investigated. The bending strength, fracture toughness and the Vickers Hardness were measured. It shows that the Dy substituted magnets shows higher Vickers Hardness than the Pr substituted magnets. The brittle index for investigated magnets improves monotonously with increasing Dy content, reduces with increasing Pr content, respectively, which seems to relate closely to the change of the lattice parameters.     

Phase precipitation and magnetic properties of melt-spun ternary Gd2Fe14B alloy and advantages of gadolinium substitution in Y2Fe14B alloy

To take the advantage of gadolinium(Gd) in developing and manufacturing RE-permanent magnets,the magnetic properties and phase precipitation behavior of Gd₂Fe₁₄B alloys prepared by melt spinning were investigated in this work.The results show that optimally direct quenched nanocrystalline Gd₂Fe₁₄B alloy exhibits the magnetic properties with remanence J_r of 0.51 T,coercivity H_c of 187 kA/m,and maximum energy product(BH)_max     

Effect of La–Y co-substitution on magnetic properties and microstructure of NdFeB alloy ribbons

Committed to obtaining cost-effective NdFeB based permanent magnets, Nd_27–xLa₃Y_xFe_balAl_0.1Cu_0.1B₁ (x = 0–3) alloys were fabricated to detect the magnetic properties and microstructure. When x = 1.8, coercivity of 1004 kA/m and the magnetic remanence of 0.75 T are obtained, which are close to those of the original Nd₃₀Fe_balAl_0.1Cu_0.1B₁ alloy ribbons. The temperature coefficient of coercivity (β) and the temperature coefficient of remanence (α) of the LaY-substituted alloys are better than those of the original alloys. The research results manifest that La is mainly distributed in the grain boundary phases and plays the role of refining the main grains, optimizing the grain boundary phases and improving the wettability between the main phases and the grain boundary phases, while Y tends to enrich in the main phases and enhances the short-range exchange coupling.     

Application of Magnetic Field in Fabrication Process of Anisotropic Bonded Nd-Fe-B Magnet

This work studied the application of the different magnetic field used in the compaction process for die fabrication of anisotropic Nd-Fe-B bonded magnet. The static field made from Nd-Fe-B permanent magnets was used in the blending process to separate the particles each other. The SEM observation gave intuitionistic results about it. The anisotropic Nd-Fe-B bonded magnets were fabricated with warm-compaction under the electromagnetic field about 2.5 T. It is known that magnetic field is necessary for anisotropic materials fabrication for alignment. And warm compaction was used to decrease the viscousness of binder, to enhance alignment magnetic particle while press, and to get high density materials. For coercivity of Nd-Fe-B magnets decrease largely with the temperature increasing, press in proper temperature and oriented field is benefit to the magnetic characteristics and the mechanical properties of the anisotropic bonded Nd-Fe-B magnets. Finally solidifying process was performed under the pulse field of 4 T. The increment for solidifying in the field was about 15% for maximum energy product of the bonded magnet. The magnetic properties of anisotropic bonded Nd-Fe-B magnets from d-HDDR powders compact at 90 °C in alignment field of 2.5 T were: B_r=8.55 kGs, _iH_c=12.0 kOe, (BH)_max=14.57 MGOe.     

Influence of gadolinium and dysprosium substitution on magnetic properties and magnetocaloric effect of Fe78−xRExSi4Nb5B12Cu1 amorphous alloys

Amorphous Fe_78−xRE_xSi₄Nb₅B₁₂Cu₁ (RE = Gd, Dy) ribbons with different RE contents were prepared by melt spinning to investigate the effect of heavy rare earth (Gd, Dy) substitution on the hyperfine structure, magnetic properties and magnetocaloric effect. The Curie temperature of RE substituted alloys, hyperfine field and magnetic moments of Fe atoms initially increase up to 1 at% RE content and then decrease monotonously for increasing RE content up to 10 at%. The dependence of magnetic entropy change (–ΔS_M) and refrigeration capacity (RC) of the alloys on RE contents displays the same tendency. The RC_AREA values of the alloys substituted with 1 at% Gd and Dy are similar to those of recently reported Fe-based metallic glasses with enhanced RC values compared with those of Gd₅Ge_1.9Si₂Fe_0.1. Enhanced –ΔS_M and RC values, negligible coercive force and hysteresis commonly make these Fe_78−xRE_xSi₄Nb₅B₁₂Cu₁ amorphous alloys as low-cost candidates for high-temperature magnetic refrigeration.     

Progress of Research on Bonded Nd-Fe-B Magnets

Some progress of research on bonded Nd-Fe-B magnets in National University of Defense Technology(NUDT) is presented in this paper. The contents include B-rich R₂Fe₁₄B-based nano composite with good performance; a model to determine of the least amount of binder; resin for high temperature application; resin encapsulating magnetic powders for long-term storage; thermoplastic polymer used for mold-pressing magnets; hybrid bonded Nd-Fe-B/Sm₂Co₁₇ magnet with a potentially useful improvements in remanence and magnetic energy product.     

Preparation and magnetic properties of anisotropic Nd_2Fe_(14)B/Sm_2Co_(17) hybrid-bonded magnets

In the present work, anisotropic Nd_2 Fe_(14) B/Sm_2 Co_(17) hybrid-bonded magnets were prepared with different Nd-Fe-B contents. It is found that the particle distributions and ratios between the two magnetic phases have important roles in the magnetic properties, microstructures and thermal stability of the magnets. With increase of Nd-Fe-B content, the saturation magnetization of the anisotropic hybrid magnet increases significantly, however, coercivity decreases, and the demagnetization curves show magnetically single-phase behavior. The anisotropic Nd_2 Fe_(14) B/Sm_2 Co_(17) hybrid-bonded magnets exhibit a maximum energy product and remanence of 14.15 MGOe and 99.53 A·m~2/kg, respectively, when the NdFe-B content is 70 wt% at room temperature. Furthermore, the hybrid magnets also have better thermal stability at elevated temperatures due to the interaction between the two magnetic particles.     

Superior corrosion resistance and corrosion mechanism of dual-main-phase (Ce15Nd85)30FebalB1M magnets in different solutions

The electrochemical corrosion behavior of both(Ce₁₅Nd₈₅)₃₀Fe_balB₁ M sintered magnets prepared with dual-main-phase method and N45-type magnets was studied in 3.5 wt% NaCl,1.1 wt% NaH₂ PO₄,and2.5 wt% NaOH solutions,respectively.The(Ce₁₅Nd₈₅)₃₀Fe_balB₁ M sintered magnets perform superior corrosion resistance than N45-type magnets in the tested solutions.In general,two ...     

Relationship between controllable preparation and microstructure of NdFeB sintered magnets

The double hard magnetic phase magnets with nominal compositions of Nd_30–xDy_xFe₆₉B₁(x=2, and 4) (wt.%) were prepared. The magnetic properties of the magnets were measured with a NIM-2000H hysteresigraph. The crystalline structures of the magnets were identified by X-ray diffraction (XRD). The Rietveld refinement was carried out using the FULLPROF software. The scanning electron microscopy (SEM) and transmission electron microscopy (TEM) analyses were carried out in order to investigate the microstructure of the magnets. It showed that the magnets consisted mainly of Nd₂Fe₁₄B phase, and some Nd-rich phase. Two types of matrix-phase grains in dark grey and light grey were found in the magnets with x=2 and 4. The Dy content was obviously different in the two types of grains, which proved that the double hard magnetic phases (Dy-rich and Dy-lean phases) coexisted in the magnet. It revealed that the Nd-rich phases in junction regions had fcc structure, with the unit cell parameter of about 0.52–0.56 nm. The weak superlattice spots were found in the SAD patterns of the junction Nd-rich phases with large scale. The double hard magnetic phase structure seemed to improve the magnetic properties of NdFeB magnets with high coercivity, while decrease the consumption of Dy element, compared with the single alloy magnet.     

Effect of surface Nd-rich phase and oxygen content of melt-spun flakes on formation of coarse grains in hot-pressed Nd-Fe-B magnet

Nd-Fe-B hot-pressed (HP) magnet prepared from melt-spun MQU-F flakes features coarse grains (CG) with the average size of both 200 nm (CG_S) and 700 nm (CG_L) at flake boundary. The grain growth at the flake boundary of Nd₂Fe₁₄B/α-Fe composite HP magnet before and after diffusion of low-melting-point Pr₈₂Cu₁₈ phase was investigated, revealing the indispensable role of surface RE-rich phase of melt-spun flakes in the formation of CG in HP magnet. The dominant role of surface oxygen content of melt-spun flakes in the formation of CG_L has been clarified with etching method. The HP magnet prepared from the etched flakes with dramatically decreased oxygen content exhibits the CG regions merely with homogeneous equiaxed CG_S at flake boundary. Consequently, the coercivity (μ₀H_c) shows significant increase while remanent magnetization (μ₀M_r) inappreciable change. Further investigation with sieving method reveals the elimination of CG_L via removal of the fine Nd-Fe-B flakes smaller than 54 μm due to their much higher oxygen content, confirming the dominant role of oxygen content in the formation of CG_L. The quantitative analysis on the magnetic properties of the above HP magnets reveals the monotonic increase of coercivity (μ₀H_c) and negligible change of remanent magnetization (μ₀M_r) with decreased oxygen contents of Nd-Fe-B flakes. The maximum value of coercivity (μ₀H_c) increases from 2.26 to 2.47 T as the oxygen content decreases from 0.1692 wt% to 0.079 wt%.     

Optimization of both coercivity and knee-point magnetic field of Sm2Co17-type magnets via solid solution process

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(Co_balFe_0.233Cu_0.073Zr_0.024)_7.6 sintered magnets was systematically studied. With the solid-solution treating duration (t_s) increasing from 0 to 4 h, intrinsic coercivity (H_cj) increases from 12.83 to 36.54 kOe, magnetic field at knee-point (H_knee) increases from 2.76 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 content of Sm, Fe and Cu elements for the two kinds of regions changes a lot for the specimens. Furthermore, with t_s 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 t_s 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 H_cj and H_knee.     

Effect of niobium substitution on microstructures and thermal stability of TbCu7-type Sm–Fe–N magnets

This paper reports crystal structures, magnetic properties and thermal stability of TbCu7-type Sm_(8.5)Fe_((85.8-x)Co_(4.5)Zr_(1.2)Nb_x(x = 0-1.8) melt-spun compounds and their nitrides, investigated by means of X-ray diffraction, vibrating sample magnetometer, flux meter and transmission electron microscope. It is found that the lattice parameter ratio c/a of TbCu_7-type crystal structure increases with Nb substitution, which indicates that the Nb can increase the stability of the metastable phase in the Sm-Fe ribbons. Nb substitution impedes the formation of magnetic soft phase a-Fe in which reversed domains initially form during the magnetization reversal process. Meanwhile, Nb substitution refines grains and leads to homogeneous micro structure with augmented grain boundaries. Thus the exchange coupling pining field is enhanced and irreversible domain wall propagation gets suppressed. As a result, the magnetic properties are improved and the irreversible flux loss of magnets is notably decreased. A maximum value 771.7 kA/m of the intrinsic coercivity H_(cj) is achieved in the 1.2 at% substituted samples.The irreversible flux loss for 2 h exposure at 120 ℃ declines from 8.26% for Nb-free magnets to 6.32% for magnets with 1.2 at% Nb substitution.     

Multilayer Magnets

After experimental evidence of intergrain exchange coupling was reported, nanocomposite magnets with high remanence and large energy products were predicted. However, the experimental values of the maximum magnetic energy product of nanocomposite bulk magnets have been much less than the theoretically predicted ones. We gave a brief review on advances in multilayer magnets. The exchange coupling and remanence enhancement were realized in nanocomposite (Nd,Dy)(Fe,Co,Nb,B)_5,5/α-Fe thin films prepared by sputtering and heat treatments. Well-designed multilayer films consist of magnetically hard Nd₂Fe₁₄B-type phase with the grain size of 40 nm and magnetically soft α-Fe phase existing in the form of the continuous layers. Furthermore, we reported the structural and magnetic properties of anisotropic Nd-Fe-B thin films. The effects of thickness, deposition rates, substrate temperature, annealing temperature were studied. A high maximum energy product of (BH)_max = 270 kJ/m³ was obtained for anisotropic Nd-Fe-B thin films.