Hydrogenation Disproportionation Desorption Recombination Research Articles

수소파쇄 제어를 통한 HDDR 처리한 Nd-Fe-B계 재료의 보자력 재현성 향상

Practical difficulty in the HDDR (hydrogenation disproportionation desorption recombination) processing of Nd-Fe-B-type alloy is a poor reproducibility of coercivity of the HDDR-treated material. In an attempt to improve the reproducibility of coercivity of the HDDR-treated Nd12.5Fe80.6B6.4Ga0.3Nb0.2 alloy, the hydrogen decrepitation was carefully controlled so as to induce more extensive micro-cracks in the particle. Prior to the hydrogenation and disproportionation reaction of HDDR processing, an additional hydrogen degassing was carried out at an elevated temperature of 600 C under vacuum for the previously hydrogen decrepitated particle. During the additional hydrogen degassing the lattice of hydrogen absorbed Nd2Fe14B phase was further shrunken, hence more microcracks were introduced in the particle due to its brittle nature. Particles with more micro-cracks had more homogeneous hydrogen absorption and desorption reaction during the HDDR-treatment. The improved reproducibility of coercivity of the HDDR-treated material was attributed to the improved homogeneity of the HDDR reactions due to the presence of more micro-cracks.

Coercivity enhancement by grain refinement for anisotropic Nd2Fe14B-type magnetic powders

Anisotropic Nd–Fe–B magnetic powders with high coercivity of 1440kA/m as well as maximum energy products of 296kJ/m3 were prepared. It was clearly observed that hydrogenation disproportionation desorption recombination (HDDR) process was confined in single crystal particles of Nd–Fe–B alloys. The recombined grains can be refined from 400 to 250nm. The grain size refinement leads to a coercivity enhancement from 920 to 1440kA/m. The coercivity enhancement is related to the grain refinement and the resultant formation of uniform Nd-rich grain boundary as strong pinning sites.

Influence of RE-rich phase distribution in initial alloy on anisotropy of HDDR powders**Project supported by the National Natural Science Foundation of China (Grant No. 51101167), the Ningbo Natural Science Foundation, China (Grant No. 2013A610075), the Ningbo Science and Technology Project, China (Grant No. 2013B10004), the Program of International Science and Technology Cooperation of China (Grant No. 2010DFB53770), the China Postdoctoral

The influence of the RE-rich phase distribution in the precursor alloys on the anisotropy of the hydrogenation disproportionation desorption recombination (HDDR) processed powders is investigated. The homogenized ingot alloy and the as-cast strip casting (SC) alloy with a uniform RE-rich grain boundary phase lead to high anisotropy of the refined powders, acquiring degrees of alignment (DOA) of 0.62 and 0.54, respectively. The RE-rich phase aggregation results in a deteriorated DOA of the powders due to the drastic disproportionation rate, while a thin and uniform RE-rich phase distribution is beneficial for DOA. A reaction model of the initial particle microstructure is proposed for optimizing the HDDR powder anisotropy.

Preparation of Nd–Fe–B sintered magnets from HDDR-processed powder

The electric-current sintering technique was used to fully densify hydrogenation–disproportionation–desorption–recombination (HDDR)-processed Nd–Fe–B powder at temperatures below the grain growth temperature in order to produce high-coercive bulk magnets. However, the sintered magnets exhibited anomalous coercivity reduction that depended on sintered density. Reheating examination of the sintered magnets revealed that the reduced coercivity was increased in proportion to the heating temperature, resulting in complete recovery of coercivity. As a result, the combination of electric-current sintering and post-annealing produced sintered magnets with a coercivity of 15kOe. Scanning and transmission electron microscopy revealed no evidence that associated the anomalous coercivity reduction and recovery with grain boundary morphology. On the other hand, various HDDR powders with different particle sizes were sintered, and finer powders yielded lower coercivity after sintering, implying that the anomalous coercivity reduction was associated with particle surface oxides of the raw powder.

Effect Of The Desorption-Recombination Temperature On The Microstructure And Magnetic Properties Of HDDR Processed Nd-Fe-B Powders

Abstract The effect of the desorption-recombination temperature on the microstructure and magnetic properties of hydrogenation-disproportionation-desorption-recombination (HDDR) processed Nd-Fe-B powders was studied. The NdxB6.4Ga0.3Nb0.2Febal (x=12.5-13.5, at.%) casting alloys were pulverized after homogenizing annealing, and then subjected to HDDR treatment. During the HDDR process, desorption-recombination (DR) reaction was induced at two different temperature, 810°C and 820°C. The higher Nd content resulted in enhanced coercivity of the HDDR powder, and which was attributed to the thicker and more uniform Nd-rich phase along grain boundaries. But this uniform Nd-rich phase induced faster grain growth. The remanence of the powder DR-treated at 820°C is higher than that DR-treated at 810°C. In addition, it was also confirmed that higher DR temperature is much more effective to improve squareness.

Simultaneous enhancement in coercivity and remanence of Nd2Fe14B permanent magnet by grain boundary diffusion process using NdHx

We have successfully developed a Dy-free grain boundary diffusion process with neodymium hydride (NdHx) alloy to the permanent magnet Nd2Fe14B powders using hydrogenation – disproportionation – desorption – recombination (HDDR) method. All the diffusion treatments were performed at 700–800 °C for various annealing time under the high vacuum with rotating diffusion method that effectively control the abnormal grain growth. The coercivities of Dy-treated Nd2Fe14B powders were varied from 9.5 kOe to 13.2 kOe but the remanence was decreased to 8.1 kG (10% reduction) depending on dysprosium hydride (DyHx) content and diffusion treated time. However, the coercivity and remanence of Dy-free diffusion treated powder have been increased to 12.2 kOe (28.5% enhancement) and 11.1 kG (22% enhancement) at the optimal diffusion treatment (800 °C for 3 h), respectively. This unique simultaneous enhancement is to isolate the magnetic coupling between Nd2Fe14B grains by creating non-magnetic Nd grain boundaries and enhance the alignment of the Nd2Fe14B hard magnetic phase, fabricated by optimal diffusion conditions.

Effects of Hydrogenation-Disproportionation-Desorption-Recombination Processing Parameters on the Particle Size of Ultrafine Jet-Milled Nd-Fe-B Powders

The effects of variations in the hydrogenation-disproportionation-desorption-recombination (HDDR) processing parameters on the size of Nd-Fe-B ultrafine powders were investigated. The ultrafine powders were fabricated by hydrogen decrepitation and jet milling of the HDDR alloys. Before pulverization, the crystal grain size of the HDDR alloys obtained by changing the HD processing temperature (THD) varied from 0.27 to 0.55µm, and decreased as THD was lowered. The ultrafine powder of the HDDR alloy became finer as THD decreased, with sizes in the range of 0.33 to 0.58µm. [doi:10.2320/matertrans.M2014273]

Mechanism of the texture development in hydrogen-disproportionation–desorption-recombination (HDDR) processed Nd–Fe–B powders

Microstructure evolution in Nd12.8Fe80.1B6.6Ga0.3Nb0.2 alloy powders at different hydrogen pressures during the hydrogen-disproportionation process PH2HD has been studied in order to understand the underlying mechanism of the texture development during the dynamic hydrogen-disproportionation–desorption-recombination (d-HDDR) process. Transmission electron microscopy showed that Fe2B grains “memorize” the crystallographic orientation of the initial Nd2Fe14B phase and transfer it to the recombined Nd2Fe14B grains in the highly textured sample. 3-D tomography of backscattered electron SEM images showed that recombined Nd2Fe14B grains nucleate at the interfaces of Fe2B/NdH2 phases, which grow through the interfaces of NdH2/α-Fe phases during the DR process. Boron segregation was found at the NdH2/α-Fe interfaces, which serve as a boron source for the growth of the recombined Nd2Fe14B grains. The mechanism of the texture development is discussed based on the microstructure observations and the corresponding evolution of magnetic properties.

Coercivity Enhancement of HDDR Hot-Pressed Magnets by NdCu Diffusion Treatment

In order to improve the coercivity of hot-pressed magnets with hydrogenation disproportionation desorption recombination (HDDR) powders being the precursor, diffusion processing of an NdCu eutectic alloy is employed for grain boundary modification, resulting in an increase of coercivity from 15.4 to 17.8 kOe, which leads to a better thermal stability with the temperature coefficient (beta) changing from -0.549%/K to -0.519%/K. It is evidently observed that Nd-rich grain boundary layers are formed between the adjacent matrix phase grains as well as in the junction parts after diffusion treatment, which is responsible for the coercivity enhancement of the HDDR hot-pressing magnets. The influence of the microstructure on the coercivity has been further investigated to reveal that the Nd-rich phases occurred during the boundary modification process of the NdCu eutectic alloy decouple the exchange interactions between the grains.

Microstructure and magnetic properties of anisotropic Nd–Fe–B magnets prepared by spark plasma sintering and hot deformation

Bulk anisotropic Nd–Fe–B magnets were prepared from hydrogen–disproportionation–desorption–recombination (HDDR) powders via spark plasma sintering (SPS) and subsequent hot deformation. The influence of sintering temperature on the structure and magnetic properties of the spark plasma sintered Nd–Fe–B magnets were studied. The remanence Br, intrinsic coercivity Hcj, and the maximum energy product (BH)max, of sintered Nd–Fe–B magnets first increase and then decrease with the increase of sintering temperature, TSPS, from 650 °C to 900 °C. The optimal magnetic properties can be obtained when TSPS is 800 °C. The Nd–Fe–B magnet sinter treated at 800 °C was subjected to further hot deformation. Compared with the starting HDDR powders or the SPS treated magnets, the hot-deformed magnets present more obvious anisotropy and possess much better magnetic properties due to the good c-axis texture formed in the deformation process. The anisotropic magnet deformed at 800 °C with 50% compression ratio has a microstructure consisting of well aligned and platelet-shaped Nd2Fe14B grains without abnormal grain growth and exhibits excellent magnetic properties parallel to the pressing axis.

Effect of the dehydrogenation speed and Nd content on the microstructure and magnetic properties of HDDR processed Nd-Fe-B magnets

The effect of Nd content and dehydrogenation speed on the microstructure and magnetic properties of hydrogenation-disproportionation-desorption-recombination (HDDR) processed Nd-Fe-B magnetic powders was studied. The NdxB6.4Ga0.3Nb0.2Febal (x=12.5–13.5, at.%) mold casting alloys were subjected to HDDR process after homogenization heat treatment. During desorption-recombination stage, dehydrogenation speed and time were systematically changed to control the speed of the desorption-recombination reaction. The higher Nd content resulted in better magnetic properties of the HDDR powder, and this was attributed to the thicker and more uniform Nd-rich phase at grain boundaries. It was also confirmed that the slow dehydrogenation speed could maximize the effect of high Nd content on the magnetic properties of HDDR powder. At the optimized dehydrogenation speed, the coercivity and remanence was 15.3 kOe and 13.0 kG, respectively, at 12.9 at.% Nd content, which resulted in a (BH)max of 36.8 MGOe.

Hydrogenation and disproportionation of SmCo5 compound by high energy ball milling in heptane

Abstract A traditional hydrogenation disproportionation desorption recombination (HDDR) technique succeeds in fabricating bonded Nd–Fe–B magnet while encounters difficulty in the fabrication of Sm–Co magnet, due to the higher thermodynamic stability of the Sm–Co compounds against the disproportionation by hydrogen. To induce the disproporationation of Sm–Co compounds, high pressure hydrogen under high temperature has to be employed. This paper reports a hydrogenation and disproporationation reactions of SmCo 5 compound that does not involve gaseous hydrogen: high energy ball milling in heptane. The H atom comes from heptane. Being milled for more than 600 min, the SmCo 5 phase completely disproportionates into Sm hydride (SmH 2 ± δ ) and cubic Co. Heating the disproportionated powder in vacuum from room temperature to 800 °C, two desorption processes, one between 200 and 400 °C and the other around about 600 °C, were observed. The desorbed gas is proved to be hydrogen by gas chromatography. The Heated product consists of mainly hexagonal SmCo 7 phase with a TbCu 7 structure. The coercivity of SmCo 7 phase is larger than 1 T, being able to meet the demand of permanent magnetic application. Excess high energy ball milling results in the appearance of minor SmCoC 2 and cubic Co phases.

The Influence of Dehydrogenation Speed on the Microstructure and Magnetic Properties of Nd-Fe-B Magnets Prepared by HDDR Process

The influence regarding the dehydrogenation speed, at the desorption-recombination state during the hydrogenation-disproportionation-desorption-recombination (HDDR) process, on the microstructure and magnetic properties of Nd-Fe-B magnetic powders has been studied. Strip cast Nd-Fe-B-based alloys were subjected to the HDDR process after the homogenization heat treatment. During the desorption-recombination stage, both the pumping speed and time of hydrogen were systematically changed in order to control the speed of the desorption-recombination reaction. The magnetic properties of HDDR powders were improved as the pumping speed of hydrogen at the desorption-recombination stage was decreased. The lower pumping speed resulted in a smaller grain size and higher DoA. The coercivity and the remanence of the 200-300 μm sized HDDR powder increased from 12.7 to 14.6 kOe and from 8.9 to 10.0 kG, respectively. In addition, the remanence was further increased to 11.8 kG by milling the powders down to about 25-90 μm, resulting in (BH) max of 28.8 MGOe.

Origin of radical coercivity reduction in fine Nd-Fe-B-type hydrogenation, disproportionation, desorption, recombination particles and its recovery

Coercivity variation of fine Nd-Fe-B-type particles prepared by ball milling of HDDR (hydrogenation, disproportionation, desorption, recombination)-treated Nd-Fe-B-type material was investigated using a HDDR-treated Nd12.5Fe80.6B6.4Ga0.3Nb0.2 material, and it was compared with that of fine particles prepared by mechanical milling of sintered Nd10Pr2.5Fe80.4B6Ga1Cu0.1 magnet. Coercivity of the fine particles prepared from the HDDR-treated Nd-Fe-B-type material was radically reduced as particle size decreased. In order to determine the major cause for the radical coercivity reduction, surface oxidation, structural damage (micro-strain), and crystallographic change (lattice shrinkage due to residual hydrogen desorption) in the milled fine particles were examined. The major contributory factor for the observed radical coercivity reduction was surface oxidation. Chemical etching proved an effective means for the recovery of the reduced coercivity in the milled fine particles prepared from the HDDR-treated Nd-Fe-B-type material. In contrast to fine particles prepared from sintered Nd-Fe-B-type magnet, the fine particles prepared from the HDDR-treated material could have reasonably high coercivity even as a very fine powder.

Effect of Annealing on Magnetic Properties of Ultrafine Jet-Milled Nd-Fe-B Powders

This paper reports the effects of annealing on the magnetic properties of ultrafine Nd-Fe-B powders with an average particle size of 0.67 μm. The powder was fabricated from hydrogenation­disproportionation­desorption­recombination (HDDR)-treated Nd-Fe-B alloys by hydrogen decrepitation and helium jet milling. The coercivity of the ultrafine powders was slightly increased by annealing below 500°C, and was drastically increased by annealing above 600°C. These two stepwise increases in coercivity were attributed to hydrogen desorption and formation of a liquid Nd-rich phase. In addition, after annealing below 500°C, the coercivity (®0Hc) of the ultrafine powder was higher than that of the conventional powder, which was prepared from a strip-cast Nd-Fe-B alloy power with a particle size of 1.12 μm, because of a decrease in particle size. In contrast, after annealing above 600°C, the ®0Hc of the ultrafine powder was smaller than that of the conventional powder. The rare earth element content (Nd + Pr) was lower in the ultrafine powders than in the conventional powder. These results indicate that the rare earth element content was insufficient in the ultrafine powders. [doi:10.2320/matertrans.MAW201402]

Improved HDDR processing route for production of anisotropic powder from sintered NdFeB type magnets

The hydrogenation disproportionation desorption recombination (HDDR) process has been investigated as a possible means of producing bonded magnets from used NdFeB-type sintered magnets with compositions, Nd13.4Dy0.8Al0.7Nb0.3Fe78.5B6.3 and Nd12.5Dy1.8Al0.9Nb0.6Co5.0Fe72.8B6.4 (atomic%). It has been shown that by increasing the processing temperature, an increase in the equilibrium pressure for disproportionation and in the overall reaction time was observed. The magnetic properties of the lower Dy content magnet were affected significantly by the change in processing temperature with a peak in properties observed at 880°C producing magnetic powder with a remanence of 1.08 (±0.02)T, a coercivity of 840 (±17)kAm−1, and a maximum energy product of 175 (±2.5)kJm−3. Further work on magnets with a significantly higher Dy content has shown that simultaneous processing of sintered magnets with varying compositions can be achieved by increasing the hydrogen pressure, however a range of magnetic properties are produced depending on the initial compositions of the samples in the input feed.

Preparation of ultrafine jet-milled powders for Nd-Fe-B sintered magnets using hydrogenation–disproportionation–desorption–recombination and hydrogen decrepitation processes

Dy addition is used to increase the coercivity of Nd-Fe-B sintered magnets. Given that Dy is rare and expensive, a method is needed for reducing the Dy content in such magnets without decreasing their coercivity. Refining Nd2Fe14B grains is a prospective method for increasing the coercivity of Nd-Fe-B magnets. Conventional jet milling, however, cannot crush strip-casted Nd-Fe-B alloys into powders less than 1 μm in size. We report a process for preparing ultrafine jet-milled powders with an average size of 0.33 μm for Nd-Fe-B sintered magnets: a combination of hydrogenation–disproportionation–desorption–recombination, hydrogen decrepitation, and He jet milling.

Low Hydrogen Pressure Induced High Anisotropy in HDDR-Treated Nd<sub>13</sub>Fe<sub>75</sub>Co<sub>4</sub>B<sub>8</sub> Powders

High anisotropic Nd13Fe75Co4B8 magnetic powders are obtained with low desorption H2-pressure of the hydrogenation disproportionation desorption recombination (HDDR) process. Scanning electron microscopy (SEM) indicates a lamella structure in the 20 kPa desorption H2-pressure HDDR products and may be related to the high anisotropy. The magnetic powders with a desorption H2-pressure of 20 kPa have the magnetic properties of Br =0.86 T and Mr/Ms =0.81.This shows that the H2-pressure has great effect on the formation of crystal texture in the HDDR Nd-Fe-B powders.

Suitable Hydrogen Pressure Induced Anisotropy in HDDR-Treated Pr-Fe-B Powders

Anisotropic Pr12.5Fe72.5Co6Cr2B7 magnetic powders are obtained by varying the desorption H2-pressure of the hydrogenation disproportionation desorption recombination (HDDR) process. The magnetic powders with a desorption H2-pressure of 30 kPa have the magnetic properties of Br =0.96 T, iHc=8.2 kOe and Br/Bs =0.78, whereas the magnetic powders obtained with a higher and a lower desorption H2-pressure are isotropic. A distinct lamella crystal is observed in the 30 kPa desorption H2-pressure products and may be related to the anisotropy in the HDDR-treated powders. It is believed that the desorption H2-pressure has great effect on the formation of crystal texture in the HDDR Pr-Fe-B powders.

Anisotropic ternary Ce13Fe80B7 powders prepared by hydrogenation–disproportionation–desorption–recombination process and the diffusion of Ce–Cu eutectic alloys

Anisotropic Ce13Fe80B7 powders were obtained for the first time by a modified hydrogenation– disproportionation–desorption–recombination (HDDR) process. It was found that the formation of anisotropy (the crystal texture) in the HDDR treated powders is related to a rod-like disproportionation microstructure. In order to further improve the coercivity, the HDDR treated powders were mixed with Ce72Cu28 powders and then were heated. As a result, the coercivity of the HDDR powders was enhanced from 400Oe to 1450Oe.