Dy-free Magnets Research Articles

Unveiling the origin of the large coercivity in (Nd, Dy)-Fe-B sintered magnets

Nd-Fe-B-based permanent magnets are widely used for energy conversion applications. However, their usage at elevated temperatures is difficult due to the relatively low coercivity (Hc) with respect to the anisotropy field (HA) of the Nd2Fe14B compound, which is typically 0.2HA. In this work, we found that the coercivity of an (Nd0.8Dy0.2)-Fe-B sintered magnet could reach 0.4HA, which was twice as high as the Hc/HA of its Dy-free counterpart. Detailed microstructural characterizations, density functional theory and micromagnetic simulations showed that the large value of coercivity, Hc = 0.4HA, originated not only from the enhanced HA of the main phase (intrinsic factor) but also from the reduced magnetization of the thin intergranular phase (extrinsic factor). The latter was attributed to the dissolution of 4 at.% Dy in the intergranular phase that anti-ferromagnetically coupled with Fe. The reduction in the magnetization of the intergranular phase resulted in a change in the angular dependence of coercivity from the Kondorsky type for the Dy-free magnet to the Stoner–Wohlfarth-like shape for the Dy-containing magnet, indicating that the typical pinning-controlled coercivity mechanism began to show nucleation features as the magnetization of the intergranular phase was reduced by Dy substitution.

Design to Reduce Cogging Torque and Irreversible Demagnetization in Traction Hybrid Motor Using Dy-free Magnet

The PMSM (Permanent Magnet Synchronous Motor) is being studied a lot in traction motors for high efficiency and high performance. Usually, magnets containing rare earth elements are used. However, since rare earth elements are not suitable for future industries due to their limited reserves, many studies on magnets excluding rare earth elements are being conducted. Magnets excluding rare earth elements have a low coercive force and are vulnerable to irreversible demagnetization, so their design must be robust. Additionally, the design used for reducing the cogging torque, which is a major variable of traction motors, is also important. Therefore, in this study, a design process for reducing irreversible demagnetization and cogging torque is proposed. There are three methods for this process. The first one is the application of the tapering structure, and the second one is the tapering skew structure. The third one is the application of an asymmetric air hole. As a result of these methods, the target irreversible demagnetization ratio was satisfied and the cogging torque was reduced. This was proven through FEA (Finite Element Analysis) and verified by comparing and analyzing the experimental results and simulation results of actual manufacturing.

Irreversible Demagnetization Improvement Process of Hybrid Traction Motors with Dy-Free Magnets

Permanent magnet synchronous motors (PMSMs), with their high power density, high efficiency, and thermal stability, are widely used nowadays. PMSM magnets are composed of rare earth elements. However, rare earth elements are subject to severe price fluctuations because of their limited availability and the monopoly of some countries. Therefore, extensive research on magnets devoid of rare earth elements has been conducted recently. Although a magnet devoid of rare earth elements has a high irreversible demagnetization rate at high temperatures owing to its low coercive force, an irreversible demagnetization improvement process is proposed in this paper to compensate for this disadvantage. This process analyzes the contribution of the magnet’s back EMF (electromotive force) using the flux linkage equation, which does not change with time. Next, the location of irreversible demagnetization is moved to a position with a low contribution to the back electromotive force. Consequently, even if irreversible demagnetization occurs at the same size, the irreversible demagnetization ratio is reduced. The proposed process can minimize irreversible demagnetization while maintaining performance.

Angular dependence and thermal stability of coercivity of Nd-rich Ga-doped Nd–Fe–B sintered magnet

We studied the angular dependence and thermal stability of coercivity of Nd-rich Ga-doped Nd–Fe–B sintered magnets. Experimentally measured angular dependence of coercivity for the as-sintered magnet agrees well with the micromagnetic simulation results for exchange-coupled polycrystalline model that assumes the magnetization reversal is dominated by the domain wall depinning at the grain boundaries. On the other hand, the measured angular dependence of coercivity of the post-sinter annealed sample with an enhanced coercivity deviates from fully exchange-decoupled model in spite of previous experimental results on inter-grain exchange decoupling. Based on micromagnetic simulations and Kronmüller equation, it is suggested that the entire elimination of magnetic exchange coupling between Nd2Fe14B grains can further improve the coercivity and its thermal stability of the 5 µm-grain-sized Dy-free magnet.

Microstructure and coercivity of grain boundary diffusion processed Dy-free and Dy-containing Nd[sbnd]Fe[sbnd]B sintered magnets

Dy distributions in Dy-free and Dy-containing NdFeB sintered magnets in the course of the Dy-vapor grain boundary diffusion (GBD) process have been investigated in order to understand the origin of the high coercivity of 3.0 T that is reachable only when the initial magnet is alloyed with Dy. We have discovered the formation of a secondary Dy-rich shell within the well-known primary Dy-rich shell, which is a key contributor to the 3.0 T coercivity. The coercivity increment of the Dy-containing magnet after the GBD treatment was only 0.08 T, much lower than 0.87 T for the Dy-free magnet; however, it was substantially enhanced by a post-diffusion annealing. Compared to the Dy-free magnet, a larger amount of Dy atoms were diffused from the Nd-rich grain boundary (GB) phase to the primary Dy-rich shell in the Dy-containing magnet after the annealing, resulting in the formation of a secondary Dy-rich shell with a higher Dy-concentration at the GB phase/secondary shell interfaces.

Effect of Isothermal Aging Time on Magnetic Properties and Microstructure of Sm1-xDyx(Co0.695Fe0.2Cu0.08Zr0.025)7.2 magnets

The ordering process of 2:17R disordered phase with microtwin structure is a very important process during aging in 2:17-type Sm–Co magnets. In this paper, Sm1– x Dy x (Co0.695Fe0.2Cu0.08Zr0.025)7.2 x = 0, 0.5) magnets with different degrees of ordering were prepared. The effect of different isothermal aging times on the magnetic properties of magnets with microtwin structure has been investigated. After isothermal aging for 0, 0.5, 2, 6, and 12 h, the coercivity and remanence of magnets without Dy doped increased gradually, while almost no change in the coercivity and remanence is observed in magnets with x = 0.5. After the slow cooling process, the coercivity about 20 kOe is obtained in all Dy-free magnets with different aging times. On the contrary, with an increase in the ordering degree of 2:17R disordered phase, the coercivity of Dy-doped magnets at room temperature enhances from 5.82 to 7.88 kOe. The temperature coefficient of abnormal coercivity between 20 °C and 100 °C of Sm0.5Dy0.5(Co0.695Fe0.2Cu0.08Zr0.025)7.2 magnets with isothermal aging for 12, 20, and 40 h reduces from +0.176%/°C to +0.065%/°C, which could be due to more content of Dy diffuse from cell boundary to cell phase.

Rediscovery of permanent magnet flux-switching machines applied in EV/HEVs: Summary of new topologies and control strategies

Substantial efforts are currently underway around the world to develop environmentally friendly transportation, of which the development of electric vehicles plays a key role. Stator-permanent magnet (PM) machines have attracted wide attention due to the robust rotor structure and comparable performance in terms of normal permanent magnet synchronous machines (PMSM) having magnets in the rotor. In particular, the flux-switching machines feature a particular magnetic circuit configuration that favors Dy-free magnets in terms of demagnetization risk, and as a result, the cost of the motor can be reduced considerably. With focus on applications in electric vehicles (EVs) and hybrid electric vehicles (HEVs), this paper reviews the latest developments of PM flux switching machines, with particular emphasis on the novel machine topologies and control strategies.

Coercivity enhancement of Dy-free Nd–Fe–B sintered magnets by intergranular adding Ho63.4Fe36.6 alloy

High coercivity Nd–Fe–B sintered magnets serving in high-temperature environments always consume expensive and scarce heavy rare-earth Dy, which has simulated considerable interest to reduce Dy usage. In this work, coercivity of Dy-free magnets was investigated through intergranular adding eutectic Ho63.4Fe36.6 powders. The coercivity increases gradually up to 4wt% Ho63.4Fe36.6 addition, however the remanence starts to deteriorate drastically as the addition is over 2.5wt%. Coercivity above 18.0kOe is obtained at the expense of a slight reduction in remanence through optimizing the addition amount and sintering conditions. The coercivity enhancement is explained through microstructural observations and elemental distribution analysis. (i) (Nd, Ho)2Fe14B shell forms in the outer region of 2:14:1 phase grains, strengthening the local magnetic anisotropy filed, (ii) RE-rich grain boundary phase with low Fe content is thickened, weakening the magnetic coupling between adjacent 2:14:1 phase grains, and (iii) 2:14:1 phase grains are refined upon lowering sintering temperature, reducing the microstructural defects and the stray fields aroused from neighboring grains.

Effect of Magnet Types on Performance of High-Speed Spoke Interior-Permanent-Magnet Machines Designed for Traction Applications

Interior permanent magnet (PM) machines are considered the state of the art for traction motors, particularly in light-duty hybrid and electrical vehicles. These motors usually use neodymium–iron–boron (NdFeB) PMs. These magnets include both light rare-earth materials such as neodymium (Nd) as well as heavy rare-earth materials such as dysprosium (Dy). The main purpose of Dy is to enhance the magnet coercivity to avoid demagnetization under both high temperatures as well as flux weakening. One of the key risks in terms of using these rare-earth magnets is the significant fluctuation/increase in their prices over the past few years. Applications that use large quantities of these magnets, such as traction motors and wind generators, are the most affected by these fluctuations. There has been an ongoing global effort to try to reduce or eliminate the use of rare-earth materials (particularly Dy which is the most expensive) without sacrificing too much performance. This paper will focus on advanced spoke designs targeting traction applications. The goal of this paper is to come up with new spoke designs using various grades of Dy-free magnets as well as ferrites targeting the same set of specifications. This paper will provide a detailed comparison between the various designs highlighting the key tradeoffs in terms of power density, efficiency, flux-weakening capability, and magnet susceptibility to demagnetization. Also, a prototype using ferrites has been built and tested, and the experimental results will be presented.

Development of Dy-Free Hot-Deformed Nd-Fe-B Magnets by Optimizing Chemical Composition and Microstructure

The effects of chemical composition and deformation temperature on the magnetic properties of hot-deformed Nd-Fe-B magnets were studied systematically, with the following results: 1) all magnet samples consisted of single- and multi-domain grains, 2) single-domain grains increased with decreasing crystal grain sizes, and 3) both coercivity and its temperature coefficient were strongly related to crystal grain size and total rare earth content. By optimizing the chemical composition and the hot-deformation conditions, Dy-free magnets with a coercivity of 1600 kA/m, which is 23% higher than that of conventional magnets, were fabricated.

Microstructure evaluation for Dy-free Nd-Fe-B sintered magnets with high coercivity

Nd-Fe-B sintered magnets are used for motors of hybrid or electric vehicles due to their high energy products. Dy is added to Nd-Fe-B sintered magnets to work in a high temperature environment. Although the addition of Dy decreases the magnetization of Nd-Fe-B magnets, it increases coercivity; a decrease in the amount of Dy is strongly required. Recently, Nd-Fe-B sintered magnets with a grain size of 1 μm achieved high coercivity of ∼20 kOe without the addition of Dy or other heavy rare earth elements. In this paper, the microstructure of their magnets was observed and compared to magnets with a grain size of ∼3 μm. The coercivity of magnets consisting of larger particles was 17 kOe. Microstructures were observed by the scanning electron microscope and the shapes of grains and the distribution of the Nd-rich phase were evaluated. The observation was promoted in two directions. One direction is the plane perpendicular to the magnetically aligned direction (c plane side) and the other is the side parallel to the magnetically aligned direction (c axis side). For magnets consisting of smaller particles, the shapes of grains are isotropic for the c plane side and elongated for the c axis side. The angle of minor axis prefers to be parallel to magnetically aligned direction. The distribution of the Nd-rich phase for magnets was also evaluated for both magnets. The distribution of the Nd-rich phase at triple junctions for the magnets with smaller particles becomes homogeneous compared to that for magnets with larger particles. It is considered that Dy-free magnets with high coercivity were realized by the achievement of homogeneous distribution of Nd-rich phase besides decreasing grain size.

Design and fabrication of Dy-free sintered permanent magnets with high coercivity

The purpose of this study was to save heavy rare earth Dy in NdFeB production. By ways of optimizing preparation process and replacing Nd with Pr, Dy-free magnet with high coercivity was successfully prepared, the nominal composition is Pr14B6Co2.2Cu0.2Al1Fe76.7 (at %). The effect of Pr on magnetic performance and microstructure of the magnet were investigated systemically, alloy Nd13.65B6Co2.2Cu0.2Al1Fe76.9 (at %) was prepared by the same preparation process as comparison. The results indicated that magnet Pr14B6Co2.2Cu0.2Al1Fe76.7 has much higher coercivity than that of magnet Nd13.65B6Co2.2Cu0.2Al1Fe76.9. Microstructure investigation showed that the presence of small grains, thin and smooth RE-rich film, associated with higher coercivity.

The origin of unique initial magnetization curves in Dy-substituted (Nd1−x, Dyx)2Fe14B (x = 0−1) sintered magnets

The coercivities (Hc), saturation magnetizations (Js), and anisotropy constants K1 of (Nd1−x, Dyx)2Fe14B sintered magnets (x = 0 to 1.0) are determined using high magnetic field measurements up to about 8 MA/m. The Ha, Js, and K1 values of samples with x < 0.5 are similar to those obtained in previous studies using single crystals; the Hc values for the sintered magnets are reported here for the first time. The magnetization into the single-domain states (SDS) in Dy-substituted sintered magnets is completed in lower fields than with Dy-free magnets (for example, 0.64 MA/m in the x = 1.0 sample and 1.6 MA/m in the x = 0.25 sample). The volume fraction of magnetized SDS grains when applying fields less than the demagnetizing fields of the samples also increases with increasing Dy content, i.e., 22 vol. % for a Dy-free sample and 48 vol. % for a Nd-free sample. A characteristic magnetization behavior of large pinning-like magnetization curves appeared in samples that were initially magnetized in a direction perpendicular to the c-axis and successively magnetized in the parallel direction. The apparent pinning field increased with increasing Dy concentration and reached 2.8 MA/m for the Nd-free samples. The origin of this behavior is the tilting of the c-axis in the crystal grains of sintered magnets and the clear preference for crystal grains in Dy-substituted samples to form SDS grains.

Effect of very small additions on the coercivity of Dy-free Nd–Fe–(Co)–B-sintered magnets

The effect of small substitutions on the coercivity of Nd–Fe–B and Nd–Fe–Co–B-sintered magnets has been investigated. Addition of 0.18 at% Ga was found to be the most effective for improving the coercivity in both Nd–Fe–B and Nd–Fe–Co–B without decreasing much the remanent magnetization. Scanning electron microscopy on Ga-free and Ga-added magnets did not reveal a noticeable difference in phase morphology. However, Fe and especially Co concentrations in the intergranular Nd-rich phase were found to be markedly increased after the small Ga addition. Combined addition of Co with a small amount of Cu did not increase the coercivity of the Dy-free magnets. However, none of the examined very small additives can yet be considered as an alternative to Dy for extending operating temperature range of the high-energy Nd–Fe–B magnets.

Magnetic Properties of Sintered Dy Substituted Nd-Fe-B Magnets

Nd-Fe-B based magnets with Dy additions were investigated using alloys of the compositions 18.5 at.% (Nd+Dy), 75 at.% Fe and 6.5 at.% B. Nd was substituted by Dy in steps of 5 to 10 at.% of rare earth metal up to complete substitution. The variation of the Dy content influences the coercivities in 18.5(Nd 100-x Dy x )-75Fe-6.5B magnets (0≤x≤100) and yielding two maxima with more than three times higher values of the coercivity compared to Dy-free magnets. The maxima occur at Dy amounts of 35% and 85% of rare-earth metal (RE)

Improved corrosion and temperature behaviour of modified Nd-Fe-B magnets

The characteristics of the high energy product NdFeB magnets obtained by the powder metallurgy route have been improved by appropriate vanadium additions. Specific combinations of V and Co lead to an intrinsic coercive force of about 1350 kA/m (17 kOe) in Dy-free magnets, without any large decrease of remanence. If 1.5-at.% Dy is added, load line down to -B/ mu H=0.5 can be achieved at 180 degrees C without irreversible losses. Corrosion resistance is improved by blocking the selective oxidation of the Nd-rich phase. As a consequence, organic-coated magnets are able to withstand 15 days at 115 degrees C-1.75 bars in a H/sub 2/O saturated atmosphere without any degradation. It is concluded that this set of new properties will enlarge the application fields of Nd-F-B permanent magnets. >