Hydrogenation-decomposition-desorption-recombination Research Articles

Development of a Prototype Thermodynamic Database for Nd-Fe-B Permanent Magnets

For the Nd-Fe-B permanent magnets, a prototype thermodynamic database of the 8-element system (Nd, Fe, B, Al, Co, Cu, Dy, Ga) was constructed based on literature data and assessed parameters in the present work. The magnetic excess Gibbs energy of the Nd2Fe14B compound was reassessed using thoroughly measured heat capacity data. The Dy-Nd binary system was reassessed based on formation energies estimated from ab initio calculations. The constructed database was applied successfully for estimations of phase equilibria during the grain boundary diffusion processes (GBDP) and the reactions in the hydrogenation decomposition desorption recombination (HDDR) processes.

Electrical Resistivity and Magnetic Performance of Ceramics-Bonded Nd-Fe-B-Type Magnet Consolidated Using Dielectric Oxide Binder

Anisotropic ceramics-bonded Nd-Fe-B-type magnet with high electrical resistivity was fabricated with the intention of suppressing induction of eddy current thus lowering operating temperature of the magnet used as rotor magnet in high-speed motor. The ceramics-bonded Nd-Fe-B-type magnet was fabricated by consolidating mixture of hydrogenation decomposition desorption recombination powder and oxide ceramics with the low melting point. Anisotropic ceramics-bonded (15 vol%) Nd-Fe-B-type magnet had remarkably enhanced electrical resistivity ( $\sim 730~\mu \Omega \cdot \text {cm}$ ) with respect to the magnet without ceramics binder ( $\sim 220~\mu \Omega \cdot \text {cm}$ ). Thanks to low-temperature consolidation of Nd-Fe-B-type particles using oxide ceramic binder with low melting point, the detrimental reaction between the magnetic particle surface and oxide was profoundly suppressed, thus retaining the high coercivity of initial magnetic particles even in the ceramics-bonded magnets. Ceramics-bonded (15 vol%) magnet, which had good room temperature magnetic performance ( $_{\mathrm {i}}H_{\mathrm {c}}= 12.7$ kOe, $M_{\mathrm {r}}= 9.4$ kG, and (BH) $_{\mathrm {max}}= 17.0$ MGOe) still had reasonably good performance [ $_{\mathrm {i}}H_{\mathrm {c}}= 5.0$ kOe, $M_{\mathrm {r}}= 8.7$ kG, and (BH) $_{\mathrm {max}}= 10.2$ MGOe] at 150 °C.

Effects of Anisotropy Field and Easy Axis Dispersions on Squareness Ratio for HDDR-Processed Nd-Fe-B Powders

NdFeB magnetic powders that are produced by a hydrogenation decomposition desorption recombination (HDDR) process consist of small grains with highly anisotropic energy [1]–[3]. Therefore, HDDR-processed NdFeB magnet powders are expected to have a high squareness ratio and high coercivity $(H_{c})$ to obtain the high maximum energy product $(BH _{max})$ that is required in highly efficient motors of hybrid or electric vehicles. The squareness ratio is defined as the value of the magnetic field at 90% of the remanent magnetization divided by $H_{c}$. However, the squareness ratio is much lower than the expected value of 1.0, and the $H_{c}$ is lower than a third of the average anisotropy fields. A previous study using a micromagnetic simulator has shown that when an anisotropy field $(H_{k})$ dispersion of grains was assumed to be a Gaussian distribution with a coefficient variation $(\sigma H_{k} {/\lt \mathrm {H}} _{k} \gt )$ of 30%, the squareness ratio corresponded with an experimental value [4]. However, Nishio et al. showed that the $H_{k}$ of a single crystal was 7600 kA/m [5]. Therefore, when the $\sigma H_{k} {/\lt \mathrm {H}} _{k} \gt$ is 30%, the $\lt H_{k} \gt + 3 \sigma H_{k}$ is unrealistically higher than the $H_{k}$ of a single crystal. In this study, the $H_{k}$ dispersion of the grains was assumed to be a horizontally flipped lognormal distribution, and the effects of the $H_{k}$ dispersion of the grains on the squareness ratio were investigated by using a micromagnetic simulator. Moreover, Nishio et al. showed that an easy axis (c-axis) inclination angle was distributed within ± 20° [6], so we also investigated the effects of the c-axis dispersion on the squareness ratio.

Crystallographic orientation analysis in HDDR process of anisotropic Nd-Fe-B magnet powders

Microstructural changes and crystallographic orientation information in the hydrogenation-decomposition-desorption-recombination (HDDR) process of Nd-Fe-B alloy were investigated using electron backscatter diffraction (EBSD) and precession electron diffraction (PED) in order to understand the mechanism of anisotropy inducement in the HDDR process. Recombined Nd2Fe14B grains were found to nucleate at the interfaces between NdH2 and α-Fe grains and to have a [001]-oriented texture from the beginning of the recombination reaction. The Fe grains form with alignment of one of the 〈113〉 directions at decomposed stage. This suggests that α-Fe most likely induces texture development of recombined Nd2Fe14B.

Effect of substituting Pr for Nd on magnetic properties of Nd–Fe–B HDDR powder

The effect of partially substituting Nd by Pr on magnetic properties of HDDR (hydrogenation-decomposition-desorption-recombination)-processed Nd–Fe–Co–B–Ga–Zr powder was studied. It was found that coercivity HcJ of the (Nd1−x,Prx)12.9Febal.Co5.8B6.3Ga0.5Zr0.1 (x = 0–0.40) powder at room temperature is increased with increasing Pr content x, while residual induction was kept almost constant. The increase in HcJ is larger than that expected from the increase in magnetocrystalline anisotropy field Ha of the magnetic phase (Nd,Pr)2Fe14B. This can be attributed to change in grain-boundary microstructures. Temperature-dependence of HcJ in the injection-molded-bonded magnets made from the powders was also examined. It was confirmed that the decrease in HcJ of the magnet with x = 0.24 is kept almost the same as the magnet with no Pr addition in the temperatures ranging from room temperature to 150 °C.

Coercivity enhancement of anisotropic Dy-free Nd–Fe–B powders by conventional HDDR process

Coercivity enhancement of Dy-free Nd–Fe–Co–B–Ga–Zr powders was studied using the conventional hydrogenation–decomposition–desorption–recombination (HDDR) process. It was found that the addition of Al together with the proper Nd content and the slow hydrogen desorption of the HDDR treatment can induce high coercivity in the powder. For example, the 14.0at% Nd–2.0at% Al powder exhibits HcJ of 1560kA/m, Br of 1.22T, and (BH)max of 257kJ/m3. The high coercivity inducement of the powder is thought to be attributed to the formation of Nd-rich phase, which continuously surrounds fine Nd2Fe14B grains.

組織制御に向けた高性能Nd-Fe-B系磁石材料のナノ構造解析

Changes in microstructure of Nd-Fe-B based magnetic materials were investigated with advanced electron microscopes to study the Tb diffusion treatment for high coercivity for sintered magnets and the hydrogenation-decomposition-desorption-recombination (HDDR) process for high anistropic magnet powders. In sintered magnets, it was found that morphology of a Nd-rich wetting-layer phase changes to be continuous in shape and uniform in thickness in association with diffusion of Tb. The formation of the continuous Nd-rich wetting-layer phase containing a small amount of Tb leads to an effective increase in coercivity. In the HDDR magnet powder, a recombined Nd2Fe14B phase is formed first as a rim phase on spherical NdH2 grains during the DR-process. Small grain size of NdH2 phase and large grain size of α-Fe matrix after the HD-process can produce small grains of highly aligned Nd2Fe14B that lead to high remanence and high coercivity. These results indicate that a nano-dimensional analysis is essentially important to develop the high-performance of magnetic materials.

Anisotropic Nd–Fe–B nanocrystalline magnets processed by spark plasma sintering and in situ hot pressing of hydrogenation–decomposition–desorption–recombination powder

An anisotropic full density Nd–Fe–B nanocrystalline magnet was produced by spark plasma sintering of hydrogenation–decomposition–desorption–recombination processed powder and subsequent in situ hot deformation. The in situ hot deformation caused the Nd2Fe14B grains elongated normal to the applied pressure with an enhanced c-axis texture. Accordingly, the remanence (Br) and maximum energy product were improved from 1.07 to 1.35 T and 192 to 224 kJ m−3, respectively, with a slightly reduced coercivity (Hc) of ∼1000 kA m−1.

Relationship between Coercivity and Microstructural Changes During DR Treatment in the HDDR‐processed NdFeCoB Alloy

AbstractRelationship between coercivity and microstructural changes during desorption‐recombination (DR) treatment in the hydrogenation‐decomposition‐desorption‐recombination (HDDR) processed NdFeCoB alloy has been studied by a combination of different experimental methods. In the early DR stage, coercivity was very small and after a certain period of the DR treatment, the coercivity abruptly increased. The corresponding changes in positron lifetime spectrum, X‐ray diffraction (XRD) spectrum, and thermal desorption spectrum (TDS) were measured. The changes suggest that an Nd‐rich phase formed among recombined Nd2(Fe,Co)14B grains in the DR treatment plays an important role for the onset of coercivity in the Nd‐Fe‐Co‐B HDDR‐processed alloy. Copyright © 2008 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.

Magnetic properties of nd-fe-b HDDR thin hot pressed magnet

Fully dense magnets prepared by hot pressing of the hydrogenation-decomposition-desorption-recombination (HDDR)-processed powders have very fine microstructures. We investigated the magnetic properties and the surface morphology of the machined HDDR thin hot pressed magnets. In the performance of the machined thin hot pressed magnets, it is hard to get the influence of thickness and almost not get the influence of damage by surface roughness. The long-term stabilities were investigated and the total irreversible flux loss of a high coercivity hot pressed magnet at 423 K was -1.3% after 1000 h. The HDDR hot pressed magnets make it possible to stabilize the magnetic properties of thin Nd-Fe-B magnets.

機械加工したＮｄ‐Ｆｅ‐Ｂ系ＨＤＤＲ薄型ホットプレス磁石の磁気特性

Fully dense magnets were prepared by hot pressing of the hydrogenation-decomposition-desorption-recombination (HDDR)-processed powders, and they had very fine microstructures. The typical magnetic properties of the HDDR hot pressed magnet with the composition of Nd13.0Fe62.9Co17.4B6.2Zr0.1Ga0.4 were Br=1.26 T, HcJ=969 kA/m and (BH)max=279 kJ/m3. The magnetic properties and the surface morphology of the machined HDDR thin hot pressed magnets were investigated. The performance of the machined thin hot pressed magnets hardly depended on their thickness, and that of high coercivity magnets with the composition of Nd12.4Dy0.6Fe74.5Co5.8B6.2Zr0.1Ga0.4 is the same because of their fine microstructures. Furthermore, the thin hot pressed magnets hardly depended on damage by surface roughness. We investigated the long-term stabilities. As a result, the total irreversible flux loss of high coercivity hot pressed magnet with the composition of Nd12.6Dy0.6Fe58.8Co20B6.2Zr0.1Ga0.4Al1.5 at 423 K was -1.3% after 1000 hours. The HDDR hot pressed magnets make it possible to stabilize the magnetic properties of thin Nd-Fe-B magnets because of their very fine microstructure.

Ｎｄ‐Ｆｅ‐Ｂ系ＨＤＤＲ磁石材料の高性能化と実用化に関する研究

Recent progress in hydrogenation-decomposition-desorption-recombination (HDDR) process technologies is described with regard to the commercial production of Nd-Fe-B magnet powders. The applications of the powders to resin-bonded magnets and hot-pressed magnets are also presented. The HDDR reaction kinetics is an essential factor for improvements in magnetic properties of the Nd-Fe-B powders. Modified HDDR process, i.e., annealing the decomposed alloys under an Ar atmosphere, and then evacuating them with the correct desorption rate, can enhance magnetic anisotropy and coercivities of the powders simultaneously. Utilizing the rapidly solidified alloy flakes as a starting material of the HDDR process induces higher anisotropy in the final powders than using the conventional book-mold cast ingots. This can be ascribed to the complete elimination of α-Fe before the HDDR process. By these improved process, the following powders can be now commercially produced: high Br-type with (BH)max of 302-342 kJ/m3, high Hc-type with HcJ of 1430-1590 kA/m, and super high Br-type with (BH)max of 350-390 kJ/m3. Compression-molded resin-bonded magnets with (BH)max up to 199 kJ/m3 can be fabricated from the super high Br-type powders. Fully dense magnets produced by hot pressing the oriented high Hc-type powders exhibit excellent thermal stabilities.

Anisotropic HDDR-treated Nd–Fe–B alloy flakes for mechanically oriented composite magnets

To explore the possibilities of producing Nd–Fe–B materials for the application of mechanically oriented composite magnets, Nd 12.6Fe 62.3Co 17.4Al 0.6Zr 0.1B 7.0 alloy flakes were prepared by hydrogenation–decomposition–desorption–recombination (HDDR)-treating the melt-spun ribbons, and magnetic properties of the resin-bonded compacts made from them were examined. The as-spun ribbons of 230 μm in thickness exhibit a texture with the c-axes of Nd 2Fe 14B grains oriented perpendicularly to the ribbon planes, and possess low coercivities ( H cJ's) of 90 kA/m. HDDR treatment can induce high H cJ's of approximately 880 kA/m in these ribbons maintaining the original crystallographic orientation. The flakes obtained by crushing the HDDR-treated ribbons can be mechanically aligned by compaction molding with no applied magnetic field. The resin-bonded compact prepared from the flakes with the highest magnetic anisotropy exhibits B r of 0.78 T and ( BH) max of 106 kJ/m 3 in the pressing direction (easy magnetization direction), when its density is 6.0 Mg/m 3. These magnetic properties are superior to those of the isotropic resin-bonded magnets produced from the commercially available rapidly quenched Nd–Fe–B powders.

Magnetic properties of anisotropic Nd–Fe–B HDDR powders prepared from strip cast alloys

Anisotropic hydrogenation–decomposition–desorption–recombination (HDDR)-treated powders were prepared from strip cast (SC) Nd 10.1Pr 2.9Fe 74.6Co 5.8B 6.2Zr 0.1Ga 0.3 alloy flakes. The alloy flakes were annealed at various temperatures before the HDDR treatment. B r and ( BH) max of the powder are increased as the annealing temperature increases from 1000 to 1160 °C, which can be ascribed to growth of the original Nd 2Fe 14B grains. The powder obtained from the optimally annealed alloy flakes, exhibits B r of 1.46 T, H cJ of 1210 kA/m, and ( BH) max of 388 kJ/m 3, which are superior to those of the powder prepared using the conventional book-mold cast (BMC) ingots. This would be related to the complete elimination of α-Fe phase in the strip cast flakes.

Preparation of fully dense Nd–Fe–B magnets using semi-processed HDDR powders

Anisotropic fully dense Nd–Fe–B magnets can be produced by compacting the hydrogenation-decomposition-desorption–recombination (HDDR)-processed powder under an orienting magnetic field, and then hot pressing the green compacts. In this study, magnetic properties of the hot pressed magnets made from Nd 12.7Dy 0.3Fe 64.3Co 16.0B 6.2Zr 0.1Ga 0.4 powders in the different stages of the HDDR process were investigated with regard to the degree of powder particle orientation. It was found that the powder in which small amounts of the decomposed NdH 2 and α-(Fe, Co) phases remain (semi-processed powder), can be more highly oriented in applied magnetic fields less than 1190 kA/m as compared to the fully HDDR-processed powder. The magnet with B r=1.13 T, H cJ =1280 kA/m, and ( BH) max=230 kJ/m 3 can be obtained by dehydrogenating the green compact of the semi-processed powder completely before hot pressing.

Magnetic monitoring of the hydrogenation–decomposition–desorption–recombination process in SmFe-based alloys

A clear understanding of the reactions and phase changes that occur during the processing of a material is vital if we want to be able to achieve the best properties. In this series of experiments, we have used a vibrating-sample magnetometer (VSM) to monitor the magnetic state of compacted powder samples of Sm 13.7Fe 86.3 and Sm 13.8Fe 82.2Ta 4.0 during a standard hydrogenation–disproportionation–desorption–recombination (HDDR) cycle. The samples were mounted in the VSM and heated at 4°C/min in 1 bar of flowing hydrogen to 750°C, they were then held for 60 min under hydrogen and approximately 120 min under vacuum before being cooled to room temperature under vacuum at 4°C/min. The results show that the initial absorption of hydrogen by the Sm 2Fe 17-type phase results in an increase in magnetization as well as T C. At higher temperatures, the disproportionation reactions of the Sm 13.7Fe 86.3 and Sm 13.8Fe 82.2Ta 4.0 materials were observed to proceed at a faster rate for the second and third cycles of the HDDR procedure than for the first. The results are in agreement with our previous transmission electron microscopy studies, which showed that much of the Ta that was initially dissolved in the Sm 2Fe 17-type phase of the cast Sm 13.8Fe 82.2Ta 4.0 material was reformed as Ta-based precipitates after a single HDDR cycle leaving the Sm 2Fe 17-type phase with much less dissolved Ta.

磁性材料 Ｎｄ‐Ｆｅ‐Ｂ系等方性ＨＤＤＲ磁石粉末の磁気特性に及ぼす出発合金組織の影響

We investigated influences of original alloy microstructure on magnetic properties of the isotropic HDDR (hydrogenation-decomposition-desorption-recombination)-treated Nd126Fe79.0Co2.4B6.0 powder by comparing two types of starting materials, i.e., the strip cast (SC) alloy and the conventional book-mold cast (BMC) alloy. It was found that utilizing the SC alloy can achieve better I-H loop squareness, and consequently higher remanence in the final powder. The powder prepared from the SC alloy exhibits a more uniform grain structure of the recombined Nd2Fe14B phase, which is thought to be closely related to its good loop squareness. The effect of using the SC alloy is supposed to be attributed to the complete elimination of α-Fe particles before the HDDR treatment.

Hydrogen-assisted processing of materials

Under certain conditions, hydrogen can degrade the mechanical properties and fracture behavior of most structural alloys; however, it also has some positive effects in metals. Several current and potential applications of hydrogen for enhancing the production and processing of materials are reviewed. These include thermohydrogen processing (THP) and forming of refractory alloys, processing of rare earth-transition metal magnets by hydrogen decrepitation (HD) and hydrogenation–decomposition–desorption–recombination (HDDR), hydrogen-induced amorphization (HIA) and microstructural refinement, extraction of elements from ores and alloys, and the use of hydrogen as a reducing gas for welding and brazing. Hydrogen is found to enhance the formability, microstructure and properties of a large variety of materials, including steels, Ti-based alloys and metal matrix composites (MMCs), refractory metals and alloys, rare earth-transition metal alloys, metalloid-containing metallic glasses, etc.

Anisotropic Nd/sub 2/Fe/sub 14/B-based magnet powder with high remanence produced by modified HDDR process

The hydrogenation-decomposition-desorption-recombination (HDDR) process is a unique method to produce anisotropic Nd/sub 2/Fe/sub 14/B-based magnet powder for bonded magnet application. In this study, we studied the effect of appending the intermediate Ar (IA) treatment, i.e., hydrogenated materials are annealed under an Ar atmosphere before the evacuation treatment in the HDDR process, on the magnetic properties of magnet powder. It was found that the IA treatment is very effective to enhance magnetic anisotropy of the powder. The optimum magnetic properties of the anisotropic bonded magnet made from the IA treatment processed Nd/sub 12.6/Fe/sub bal./Co/sub 17.4/B/sub 6.5/Zr/sub 0.1/Ga/sub 0.3/ powder are as follows: B/sub r/=1.06 T, H/sub cJ/=992 kA/m, and (BH)/sub max/=193 kJ/m/sup 3/. It was observed that the IA treatment induces rapid growth of a /spl alpha/-(Fe,Co) grains in the decomposed mixture of the alloy. This microstructural change of the alloy is considered to be strongly related to the preferred crystallographic orientation of the final magnet powder.

TEM Study of Microstructural Changes in an Anisotropic Nd–Fe–Co–B–Zr Magnet Alloy during HDDR Process

Changes in microstructure of Nd-Fe-Co-B-Zr alloy powder during the hydrogenation-decomposition-desorption-recombination (HDDR) process have been investigated in detail by transmission electron microscopy (TEM). It was found that the microstructural changes in the HDDR process start with the formation of cellular NdH2 and α-(Fe, Co) at a temperature between 933 and 973 K. The starting temperature is about 50 K higher than that of the Nd-Fe-B ternary system. By further annealing at 973 K, colonies of rod-shaped NdH2 and spherical grains of NdH2 grow, embedded in the α-(Fe, Co) matrix. It was confirmed by the selected area electron diffraction (SAED) analysis that there is no specific crystallographic orientation relationship between the original Nd2(Fe, Co) 14 B and the decomposed phases; NdH2 and α-(Fe, Co). (Fe, Co) 2 B grains appear above 973 K, but the number density of the grains is very small. These features suggest that none of the NdH2, the α-(Fe, Co) and the (Fe, Co)2B transfers the c-axis of the original Nd2(Fe, Co) 14 B to the recombined one. The microstructure formed at 1073-1103 K consists mainly of fine spherical NdH2 particles and α-(Fe, Co) matrix. Most of the NdH2 particles are covered with rims. The SAED and EDX analyses confirmed that the rim regions are made of Nd2 (Fe, Co) 14 B crystalline grains which are strongly correlated with one another in the crystal orientation. After a short treatment of hydrogen desorption, the rim-like phases grow up to form a microcrystalline structure of Nd 2 (Fe, Co) 14 B.