Tb-rich Shells Research Articles

Construction of Tb-rich shells via novel interdiffusion approach to tackle coercivity-remanence trade-off

The strategic introduction and efficient utilization of heavy rare earth elements within Nd-Fe-B-based permanent magnets are instrumental in achieving an optimal balance between remanence (Br) and high coercivity (Hcj). Herein, we propose a novel interdiffusion approach based on hydrogen decrepitation (HD) for the cost-effective and high-performance sintered Nd-Fe-B magnets. With the aim of generating continuous Tb-rich shells on the grain surface, which favors high Br and high Hcj, strip cast (SC) flakes homogeneously mixed with 0.9 wt% Tb were processed by the HD. It is shown that the formation of (Nd, Tb)-Fe-B by the interdiffusion of Nd-Fe-B and Tb during de-hydrogenation, as well as the interdiffusion of Nd-Fe-B and (Nd, Tb)-Fe-B during sintering and tempering, is the key to the construction of Tb-rich shells. As a result, a magnet (Mag-Tb&HD) with maximum magnetic energy product ((BH)max) of 52.21 MGOe was achieved, which is superior to magnet prepared form SC flakes containing 0.9 wt% Tb (Mag-Tb&SC, (BH)max=49.36 MGOe) and the magnet prepared by adding 0.9 wt% Tb to the jet milling (Mag-Tb&JM, (BH)max=50.41 MGOe).

Role of (Pr20Nd80)60Tb15Cu15Ga10 intergranular addition on the magnetic properties of Nd-Fe-B sintered magnet

Intergranular addition using heavy rare earth (HRE) Dy/Tb containing alloys has been an effective way to modulate the microstructure and chemical distribution towards fabricating high-coercivity Nd-Fe-B sintered magnets. In this work, (Pr20Nd80)60Tb15Cu15Ga10 (at%) intergranular alloy powders were designed and introduced with dual purposes of forming Tb-rich matrix shell and antiferromagnetic RE6Fe13Ga grain boundary phase (GBP). With 3 wt% (Pr20Nd80)60Tb15Cu15Ga10 addition, the coercivity Hcj is significantly increased from 11.1 kOe to 17.9 kOe, accompanied with a high utilization efficiency of Tb (11.8 kOe/%) and a superior temperature coefficient of coercivity β(−0.531%/K). Microstructural analysis suggests that the improved wettability of grain boundary by (Pr20Nd80)60Tb15Cu15Ga10 addition is beneficial to form the Tb-rich matrix shell and antiferromagnetic RE6Fe13Ga GBP throughout the bulk magnets. Micromagnetic simulation is implemented to study the effect of formed ferromagnetic/antiferromagnetic GBP and Tb-rich matrix shell on the magnetization reversal process. Results show that the synergy of Tb-rich shell surrounding Nd/Pr-rich core and antiferromagnetic Nd/Pr-rich RE6Fe13Ga GBP is an effective way to promote the coercivity with high utilization efficiency of HRE Tb.

Experimental and calculational analysis about the influence of the grain boundary diffusion depth on the magnetic properties of a sintered Nd-Fe-B magnet

The grain boundary diffusion process (GBDP) has been widely applied to increase the coercivity of Nd-Fe-B magnets. After GBDP with Dy/Tb-rich diffusion sources the thickness of the Dy/Tb-rich shell formed on the epitaxial layer of the 2:14:1 main phase grain decreases from the magnet surface to the center. However, the influence of the Dy/Tb-rich shell gradient distribution on magnetic properties has not been thoroughly studied. In this work, a sintered Nd-Fe-B magnet was subjected to GBDP with Pr60Tb10Cu30 alloy at 860°C for various diffusion times (3 h, 6 h and 9 h). The coercivity improves rapidly from 884 kA/m (without GBDP) to 1533 kA/m after GBDP of 3 h. The coercivity further increases to 1741 kA/m with increased diffusion time to 6 h. But only marginal coercivity enhancement (rising to 1803 kA/m) can be obtained by further prolonging the diffusion time to 9 h. Microstructure analysis indicates that the long diffusion time leads to the surface grain coarsening, which degrades the diffusion efficiency. Meanwhile, micromagnetic simulation indicates that if the thickness of the Tb-rich shell in magnet center is less than 4 nm, the coercivity increases significantly with the enhanced thickness uniformity of the Tb-rich shell. But if the thickness of the Tb-rich shell in magnet center is higher than 4 nm, the coercivity cannot be improved effectively by further increasing the thickness uniformity of the Tb-rich shell. The results in this work clarify the mechanism of the magnetic property dependence on the diffusion time and help to optimize the GBDP parameters in the future.

Comprehensively improved magnetic performance of Nd-Fe-B magnets with high-efficiency grain boundary diffusion of heavy rare earth

Comprehensively improved magnetic performance Nd-Fe-B magnets with extremely limited amount of Heavy Rare Earth (HRE) elements are obtained to meet the increasingly demanding applications. In this work, Tb–Co thin film with uniform distribution, flat surface and strong adhesion is successfully prepared by magnetron sputtering, and the effects of Tb–Co film thickness on the magnetic properties are investigated. The magnets with optimal magnetic properties are achieved when the film thickness is about 3.5 μm, the coercivity of 1284.81 kA/m, the remanence of 1.48 T, and the maximum magnetic energy product of 437.32 kJ/m3, compared with the original magnets, enhancing by 47.9 %, 0.4 % and 5.7 %, respectively, so the magnetic properties have been comprehensively improved using little amount of HRE and Co elements. HRTEM and EPMA results revealed that Tb-rich shell distributed around the Nd2Fe14B grain. The mechanisms of the remanence and coercivity simultaneous enhancement of Nd-Fe-B magnets is elaborated in detail. Besides, the diffusion of Tb–Co thin film can enhance thermal stability of the magnets.

The magnetic hardening and exchange-decoupling strengthening of combined diffusion Nd-Fe-B magnet

Microstructure and magnetic properties of single-sided diffused magnet by TbF3 following Pr-Cu-Al alloy were investigated. It was confirmed that the core–shell structure of TbF3 diffused magnets was developed at the depth of about two hundred micrometers from coated-surface and the coercivity enhancement was mainly resulted from magnetic hardening of Tb-rich shells. However, microstructure and magnetic properties indicated obvious gradient characteristics along diffusion depth and Tb element was partially blocked at near surface of magnet. It was also verified that the following diffusion treatment by Pr80Cu10Al10 alloy for TbF3-diffused magnets can obviously promote grain boundary diffusion of Tb element into deeper regions and optimize uniformity of Tb-rich shells. Meanwhile, additional Pr-Cu-Al phase was introduced and continuously distributed at intergranluar regions. Therefore, magnetic hardening of Tb-rich shells and exchange-decoupling strengthening of adjacent 2:14:1 grains isolated by intergranluar phases contributed to further coercivity increment ΔHci of magnet simultaneously. Furthermore, it was demonstrated that the Hci of diffused magnets was enhanced with a reduction of magnet thickness and an increase in Pr80Cu10Al10 source proportion and then closetosaturation due to the limitation of diffusion depth.

Modifying Tb-Cu grain boundary diffusion behavior in Nd-Fe-B magnets by Gd or other rare earth elements

The grain boundary diffusion using mixed rare earths (REs) as the diffusion source is very effective in enhancing the coercivity of Nd-Fe-B magnets. Here, we firstly investigated the Gd added Tb-Cu diffusion alloys and found that the coercivity of Nd-Fe-B magnets can be improved from 1094 kA/m to 1704, 1677 and 1593 kA/m by Tb70Cu30, Tb60Gd10Cu30 and Tb50Gd20Cu30 diffusion, respectively. The material cost of diffusion source in grain boundary diffusion process can be reduced from 1091 $/kg of Tb70Cu30 to 788 $/kg of Tb50Gd20Cu30. Binary Gd-Cu alloy source has no positive effect on the coercivity enhancement. The enhanced thermal stability with increased temperature coefficient of coercivity from −0.62 %/°C for the initial magnet to −0.55 %/°C and −0.56 %/°C is also obtained by Tb70Cu30 and Tb50Gd20Cu30 diffusion, respectively. Tb exhibits a higher diffusion coefficient than Gd, but the introduction of Gd decreases the melting point of Tb-Cu alloy and promotes the infiltration of Tb. The typically core-shell structure with Tb-rich shell is observed after diffusion, which enhances the coercivity. The effects of RE elements in the Tb-RE-Cu diffusion source on the diffusion behavior of Tb has also been discussed. Y and Gd elements tend to enter into RE2Fe14B grain, resulting in the reduction of thickness for the Tb-rich shell, but Pr, La and Ce elements prefer to stay in the grain boundaries, which provides the diffusion channel for Tb, leading to the increased diffusion depth. This work provides a design idea for the RE-based diffusion sources of Nd-Fe-B magnets.

Coercivity saturation in Nd-Fe-B sintered magnets treated by grain boundary diffusion process of Tb-Ni-Al alloy

Microstructures of Nd-Fe-B sintered magnets treated by a grain boundary diffusion (GBD) process with different Tb coating amounts are investigated by scanning electron microscopy (SEM) and scanning transmission electron microscopy (STEM). Tb-substituted shell structures are formed in the outer portion of all Nd2Fe14B grains in the magnets. However, thick Tb-rich shells are formed on a micrometer scale on Nd2Fe14B grains in the region up to about 0.5 mm from the coating surface of the magnet, and thin Tb-rich shells several tens of nanometers in size are also formed in the inner region. As the Tb coating amount increases, the Tb concentration and thickness of the thick Tb-rich shells increase. At the same time, the Nd replaced by Tb forms Nd-rich grain boundary (GB) phases, and a large amount of those Nd-rich GB phases are discharged to the coating surface. The coercivity enhancement in GBD is mostly due to the formation of the thin Tb-rich shells, whereas the formation of thick Tb-rich shells wastes a large amount of Tb, causing coercivity saturation. Therefore, it is important to suppress the formation of thick Tb-rich shells and form thin Tb-rich shells with a high Tb concentration uniformly throughout the magnets.

Inhibiting the abnormal grain growth for grain boundary diffused Nd-Fe-B magnets through Tb3O4 grain boundary construction

The grain boundary diffusion (GBD) process has been widely applied to effectively improve the coercivity of sintered Nd-Fe-B magnets. However, the abnormal grain growth and shallow diffusion depth were not conducive to coercivity enhancement. In this work, 0.2wt.% of Tb3O4 powder addition during the sintering process was applied to inhibit the abnormal grain growth and increase the continuous GB phase, and thus a superior coercivity of 26.22kOe was achieved after Tb GBD by comparing with that of 24.15kOe for the GBD magnet without Tb3O4 powder addition. Correlated with the electron probe microanalyzer (EPMA) and transmission electron microscopy (TEM) characterizations, a deeper diffusion depth, a thicker Tb-rich shell, and finer Nd2Fe14B phase grains make synergetic contributions to this coercivity enhancement. Furthermore, the Nd6Fe13Ga phase is formed surrounding the Tb3O4 intergranular phase, which is conductive to reduce magnetic coupling. The relationships among the coercivity, grain size, and core-shell structure are further confirmed by micromagnetic simulation. This work provides a dependable approach to further coercivity enhancement by grain boundary regulation.

Coercivity enhancement of sintered Nd-Fe-B magnets by diffusing TbHx and Tb86Al7Co7 alloy

TbHx and Tb86Al7Co7 alloys were used as diffusion sources to enhance the coercivity of Nd-Fe-B magnets. The coercivity enhancement and microstructure of the magnets were examined after diffusion, and the results showed that the coercivity increased from the original 1525.1 to 2382.4 kA/m and 2274.9 kA/m in Tb86Al7Co7 and TbHx diffused magnets, respectively. This improvement in coercivity was attributed to the formation of a Tb-rich shell and a new NdTb-AlCo-grain-boundary phase. The Tb86Al7Co7 alloy had a higher diffusion efficiency and diffused deeper than the TbHx alloy due to the low melting point of the NdTb-AlCo-grain-boundary phase. The coercivity temperature coefficient of the magnets improved, increasing from the original −0.55%/K to −0.42%/K and −0.44%/K in Tb86Al7Co7 and TbHx diffused magnets, respectively. The Tb86Al7Co7 diffused magnet exhibited better thermal stability than the TbHx diffused magnet, which suggests that Nd-Fe-B magnets could potentially be used at considerably higher temperatures than they are now.

Effect of diffusing TbH3 on magnetic, corrosion and mechanical properties of NdFeB magnet

Grain boundary diffusion can enhance the coercivity of NdFeB magnets with low consumption of heavy rare earths Dy/Tb. This work systematically studied the effects of diffusing TbH3 on the magnetic, corrosion and mechanical properties of NdFeB magnet. The results show that the remanence Br and maximum energy product (BH)max of magnets slightly decreased after diffusion, while coercivity increased from 14.47 kOe to 22.08 kOe with 0.20 wt% Tb usage. The formation of Tb-rich shell and RE-(FeO) metallic-type grain boundary phases attribute to the enhancement of coercivity. The fracture and corrosion behavior of NdFeB magnet are discussed before and after diffusion. This study provides an experiment and theoretical basis for the industry fabrication of high-performance magnet with low heavy rare earth.

Pr–Tb synergistic strengthening on coercivity of Nd-Ce-Fe-B magnets grain boundary diffused with Pr-Tb-Al-Ga alloys

Sintered Nd-Ce-Fe-B magnets were grain boundary diffused (GBDed) with PrxTb80–xAl10Ga10 (at%) (x = 0, 20, 40, 60, 80) alloys. The effect of Pr/Tb content in diffusion source on magnetic properties, microstructure and elements distribution of GBDed magnets was investigated. When Pr is used to substitute for 75% Tb in diffusion source, Tb consumption per unit coercivity improvement of GBDed magnet reduces by 77%, compared with the Tb80Al10Ga10 diffused magnet. Tb element diffuses into magnets and then forms Tb-rich shell with high magneto-crystalline anisotropy field surrounding main phase grains, resulting in substantial coercivity improvement. Pr with low melting point diffuses deeply along liquid grain boundary phase during GBD process. It can eliminate some sharp defects of main phase grains and make grain boundaries smooth, which provides diffusion channels for further diffusion of Tb element. Therefore, there are more diffusion channels for Tb and less Tb enriched at surface region, making Tb diffuse more deeply and improving Tb utilization efficiency. This method significantly improves the coercivity, and realizes the green, efficient and high-quality utilization of heavy rare earth (HRE) elements.

Evolution of Tb-shell homogeneity in Nd-Fe-B sintered magnets by long-term Tb grain boundary diffusion

Grain boundary diffusion process has been applied to effectively facilitate the coercivity enhancement of Nd-Fe-B magnets by the formation of core-shell structure with less heavy rare earth consumption. However, the excessively thick Tb-rich shells and asymmetric shells formed near the shallow diffused surface, not only have minor contribution to the coercivity enhancement but consume more Tb resources. In this work, the influences of diffusion time on coercivity changes and the evolution of asymmetric to symmetric Tb-rich shell have been investigated for the Tb-diffused Nd-Fe-B magnets. With prolonging the diffusion time at 890 ℃, the coercivity rapidly increases from 17.66 kOe to 24.41 kOe when the diffusion time is less than 18 h and keeps nearly unchanged after a longer diffusion time due to the trade-off effect of Tb-rich shell homogeneity and grain growth. Besides, magnetic domain structure and its dynamic evolution of different core-shell structures in Nd-Fe-B magnet further illustrate the positive effects of homogeneous symmetric Tb-rich shell on coercivity enhancement, which is directly observed by in-situ Lorentz TEM and magneto-optical Kerr microscope, and further proved by micromagnetic simulation. This work thus provides a new insight for tuning the Tb-rich shell distribution in the Nd-Fe-B sintered magnets with high performance by grain boundary diffusion.

Microstructure and magnetic properties of grain boundary diffused Nd-Fe-B magnets with Tb-Al-Ga alloy

The coercivity of Nd-Fe-B magnets can be effectively improved by grain boundary diffusion using low melting point alloy as diffusion source. In this work, Tb-Al-Ga alloy was used as diffusion source for grain boundary diffused Nd-Fe-B magnets. Tb-rich shell structure, potential synergistic effect of Al-Ga, interface between diffusion source and the magnets were analyzed. When the magnets were processed at 875 °C for 10 h, the coercivity increased by 123 % from 10.38 kOe to 23.15 kOe and the remanence and the maximum magnetic energy product only decreased by 3 %. Tb element mainly formed shell structure with high magneto-crystalline anisotropy field surround the main phase grains. Non-magnetic Al element was distributing around the main phase grains as network structure, which could well isolate the main phase grains. The Tb-rich shell with high magneto-crystalline anisotropy field and the non-magnetic phase with magnetic exchange decoupling between the adjacent grains contributed to substantial increase in the coercivity of the grain boundary diffused magnets. The grain boundary phase was wetted by Al element, which promoted Tb element diffused into the magnets more deeply.

Microstructure regulation and optimizing magnetic properties of (La, Ce, Pr)17Fe78B6 alloy via short-range grain boundary diffusion

Increasing the utilization ratio of La and Ce in rare earth permanent magnets is an effective measure to solve the balance utilization of rare earth resources. In this work, nanocrystalline melt spun [(Ce0.7La0.3)0.3Pr0.7]17Fe78B6 powder and low melting point PrTbCu alloy were used as the initial material and the diffusion source for short-range grain boundary diffusion (SRGBD) to improve the coercivity. High coercivity of up to 18.21 kOe can be achieved, enhancing by 108.82% compared with the initial powder, and meanwhile, the remanence was only slightly reduced by 2.13%. After that, nanocrystalline bulk magnets with excellent magnetic performance were produced through spark plasma sintering (SPS). TEM observation reveals that the bcc-RE2O3 intergranular phase that induced by short-range grain boundary diffusion is distributed between adjacent matrix phase grains, which is conductive to the improvement of magnetic isolation effect. EDS results show that the Tb-rich shell with high magnetocrystalline anisotropy field has formed. Moreover, it is found that Tb diffuses into the 2:14:1 phase more easily than Pr element during the initial stage of diffusion. Enhanced magnetic isolation effect and the formed Tb-rich shell in grain surface contributed to high coercivity.

Microstructure evolution and metallurgical behavior of (Y/RE)2Fe14B sintered magnets (RE = Nd, Tb)

The magnetically hardened shells constructed by the spontaneous core–shell structure due to the enrichment of Y in the core region of (Y/RE)-Fe-B facilitate the enhancement of coercivity. In this work, we systematically compared the metallurgical behaviors of Y in R2Fe14B (RE = Nd, Tb) to investigate the modulation effect of Y on the distribution of REs in the matrix phase. During the strip casting, Y was depleted in the RE-rich phase in Y1/3Nd2/3-Fe-B while enriched in Y1/3Tb2/3-Fe-B. Under the sintering and annealing, Y was simultaneously enriched in the core of matrix phase grain, resulting in the spontaneously formation of Y-rich core and Nd-rich shell structure as well as the Y-rich core and Tb-rich shell structure. Further statistics on the concentration distribution of Y in the matrix phase indicated that the degree of aggregation for Y in Y1/3Nd2/3-Fe-B is higher than that of Y1/3Tb2/3-Fe-B, due to the different formation energy of Re2Fe14B phases. Notably, the spontaneous Y-rich core and Tb-rich shell structure demonstrates the modulation effect of Y on the distribution of Tb, inspiring an innovative scheme for construction of magnetically hardened shells.

Microstructure modification and coercivity enhancement of sintered NdFeB magnet by vacuum diffusion 70Tb–20Cu–10Ga (at%) alloys

In this work, the eutectic Tb70Cu20Ga10 alloy was applied to high magnetic performance NdFeB magnet. The coercivity of magnets effectively improved of 60.61% without reduction in maximum energy product and remanence after diffusion. The magnetic vector potential of magnet was analyzed by general finite element method. The diffusion mechanism of eutectic had been discussed at various diffusion depth. The Tb diffuses into the magnet formation continuous Tb-rich shells and thin grain boundary phase, which strongly enhance coercivity. The Cu, Ga and O enrich into grain boundary form thin RE2(Fe, CuGaO)3 phases, which increase diffusion channel and recover microcrack near surface of magnet. This Tb70Cu20Ga10 diffusion alloys has a promising potential to develop high magnetic performance and low cost sintered NdFeB magnet.

Highly thermally stable Nd0.6Y0.4-Fe-B magnet with a spontaneous shell-strengthened structure by Tb alloying

Thermally stable Nd0.6Y0.4-Fe-B magnet with 0.83 at% Tb alloying was yielded with a coercivity of 1.58 T and a temperature coefficient (β20–120 ℃) of − 0.531%/°C. Microstructure analyses showed that the aggregation of Y in the core region essentially dominated the heterogeneous distribution of rare earth (RE) elements in the matrix phase grains, leading to the enrichment of Pr, Nd and Tb in the outer layers of matrix grains. The preferred regulation of Y for Tb distribution contributed to the spontaneous Tb-rich shells with strengthened anisotropy field, significantly mitigating the inevitable degradation of coercivity caused by the high Y-substitution. Continuous non-ferromagnetic grain boundary phases were also constructed to isolate the adjacent grains. The synergistic effect of above microstructural modification presents a reliable new strategy to compensate for the magnetic dilution effect of Y and promote the effective utilization of Tb.

On the anisotropic grain boundary diffusion of sintered Nd-Fe-B magnets

• Anisotropic diffusion of heavy rare earth can be avoided for sintered Nd-Fe-B magnets. • The alloying of diffusion source distinctly enhance diffusion efficiency of Tb along a -axis. • The alloying of diffusion source inhibits the excessive lattice diffusion of Tb. Grain boundary diffusion (GBD) efficiency of heavy rare earths (HREs) for sintered Nd-Fe-B magnets along the easy magnetization direction ( c -axis) was reported to be higher than that along the difficult one ( a -axis). However, this conclusion is challenged by the present results, which indicate that the anisotropic diffusion does not exist, at least for some low-melting HRE-based diffusion sources. The alloying of Tb by Pr, Al and Cu can distinctly enhance the diffusion efficiency of Tb along the a -axis, resulting in quite similar coercivity enhancement to that diffused along the c -axis. The diffusion kinetic analysis and microstructure characterizations demonstrate that the alloying lowers the melting point of diffusion source and wets the intergranular phase as effective diffusion paths for Tb. This suppresses the excessive lattice diffusion of Tb and the formation of over-thick Tb-rich shells, which is beneficial to enhancing the Tb diffusion efficiency along the a -axis and reducing the total Tb consumption.

Diffusion behavior and coercivity enhancement of Tb-containing NdFeB magnet by dip-coating TbH3

There is a growing demand for high coercivity of NdFeB magnets with low heavy rare earth in the emerging applications. In this work, dip-coating 0.02 g and 0.09 g TbH3 on the Tb-containing NdFeB magnet enhance coercivity at room temperature，respectively. The highest coercivity achieve about 30 kOe at room temperature after diffusion, which can apply to traction motors of electric vehicles. The magnetic field intensity distribution of magnet was analyzed by finite element method. Microstructures show TbH3 coat contents and original composition of NdFeB magnet influences the formation of Tb-rich shell and grain boundary phase, and determine finally diffusion depth and Tb concentration. The Tb atoms diffuse into the interior of the magnet, forming a network Tb-rich shell existed around Nd2Fe14B grains, resulting in the coercivity enhancement.

Coercivity enhancement of the sintered Nd-Ce-Fe-B magnet via grain boundary diffusion with Tb70Fe30 alloy

Commercial N45 sintered Nd-Ce-Fe-B magnets were processed by the grain boundary diffusion (GBD) with Tb and Tb70Fe30 alloys. Although the substitution of Tb (30 at.%) by Fe in the Tb70Fe30 alloy, the coercivity was still remarkably enhanced from 12.71 kOe to 22.17 kOe, nearly as well as the Tb GBD magnet. Especially, the Tb70Fe30 GBD magnet possessed a higher remanence of 13.01 kGs than that of 12.72 kGs for the Tb GBD magnet. The thickness of the Tb-rich shell was reduced in the Tb70Fe30 GBD magnet. Moreover, the antiferromagnetic RE6Fe13Cu phase was found in grain boundary in both diffused magnets, while more Fe atoms provided in the Tb70Fe30 alloy decreased the decomposition of the matrix grains due to the formation of the RE6Fe13Cu phase, thereby suppressed the remanence degradation. This work provides new prospects for the development of the Nd-Ce-Fe-B magnets processed by grain boundary diffusion.