Hydrogenation Disproportionation Desorption Research Articles

Effects of Misalignment of c-axis on the Properties of Hydrogenation-Disproportionation-Desorption-Recombination Particles.

Hydrogenation-Disproportionation-Desorption-Recombination (HDDR) Nd2Fe14B particles have excellent magnetic properties, but the magnetic properties of powder are not uniform across different particle sizes. The remanence and maximum magnetic energy products of samples with a particle size of 120 μm are 14.0 kGs and 41.35 MGOe, while the products of samples with a particle size of 60 μm are only 13.3 kGs and 36.31 MGOe. The macroscopic morphology of HDDR Nd2Fe14B particles and the gradient distribution of microstructures in different micro-regions were observed. By modifying the macroscopic morphology of the particles, the poorly oriented clusters on the surface of the particles were precisely eliminated, and the remanence and maximum magnetic energy products of the particles increased to 14.5 kGs and 45 MGOe, respectively. Compared with the original particles, the samples after mechanical grinding had better grain arrangement. The effects of the nanocrystalline c-axis and field misalignment angle θ on the magnetic properties of HDDR Nd2Fe14B particles were investigated through micromagnetic simulation. The targeted removal of macroscopic defects on the particle surface contributed to a 3.6% increase in remanence and an 8.8% increase in the maximum magnetic energy product, offering a promising approach to enhance the microstructure of high-performance HDDR Nd2Fe14B particles.

Pre-orientation sintering: an alternative process to prepare high-performance anisotropic Nd–Fe–B HDDR magnets

Hydrogen–disproportionation–desorption–recombination (HDDR) Nd–Fe–B magnetic powders are promising to prepare bulk anisotropic magnets, but high magnetic performance has not been achieved due to the absence of an Nd-rich phase in the powder and the low degree of orientation of the bulk magnets. In this study, an alternative process of pre-orientation sintering via magnetic alignment followed by spark plasma sintering was proposed to prepare the precursor of hot-deformation (HD) magnets, and a high maximum energy product of 294 kJ m−3 was achieved in the HD magnet with a relatively low height reduction of 35%, then an improved coercivity of 1107 kA m−1 could be obtained followed by a grain boundary diffusion of Pr40Tb30Al20Cu10. Microstructure analysis indicates that pre-orientation of HDDR powders facilitates grain rotation and alignment during the HD process, thereby reducing the minimum deformation ratio. It helps to obtain the deformed grains with lower shape anisotropy and smaller grain size, enabling a good compatibility of magnetic and mechanical behaviors. In-situ Lorentzian transmission electron microscopy results show that the magnetic domains have been strongly fixed by the thick intergranular RE-rich phase and the fully Tb-diffused grains, which contributes to the improved coercivity after grain boundary diffusion. This study provides a guiding significance for the preparation of the anisotropic Nd–Fe–B HDDR magnets with optimized performance.

Effect of Phosphate Treatment on the Corrosion Resistance of Nd–Fe–B Anisotropic Magnetic Powder

Anisotropic Nd–Fe–B hydrogenation disproportionation desorption recombination powder with high corrosion resistance was prepared by phosphoric acid treatment. The treated samples baked at temperatures higher than 250 °C exhibited high corrosion resistance. In addition, the change ratio of coercivity before and after the anticorrosion test was within 5%. Microstructural observations indicated the formation a uniform phosphate layer with a small amount of Nd oxide at high baking temperature.

Anisotropy optimization of HDDR magnetic powders by precursor alloy annealing

The anisotropy of hydrogenation–disproportionation–desorption–recombination (HDDR) method to form Nd2Fe14B powders was effectively improved by annealing the strip casting (SC) alloy precursors at 1080 °C. According to the analysis of the evolution of microstructure, the excellent anisotropy was attributed to the nearly consistent orientation of the nanoscale Nd2Fe14B grains in the HDDR magnetic powders, which perfectly inherited that of the coarse grains in the treated SC alloy flakes. In contrast, for the untreated specimen, more nanoscale clusters of different orientations were generated after smashing and HDDR treatment of fine columnar Nd2Fe14B grains with different orientations in the original SC alloy, severely affecting the anisotropy of the magnetic powders. This study provides new insights into the development of high-anisotropy and high-performance Nd-Fe-B magnets.

Coercivity enhancement of hydrogenation disproportionation desorption recombination Nd-Fe-B powders by grain boundary diffusion process using Tb70Cu15Al15 alloy

The hydrogenated disproportionation desorption recombination (HDDR) Nd-Fe-B powders mixed thoroughly with Tb70Cu15Al15. The samples were subjected to a grain boundary diffusion process using various heat treatment conditions. The results show that the coercivity of the magnetic powders is significantly influenced by the diffusion conditions. Under the optimum treatment process, the coercivity of treated powders increased from 1074.3 kA/m to 1424.4 kA/m. Furthermore, the irreversible flux loss of the treated powders was also dramatically decreased by 50% at 150 °C for 100 hrs, demonstrating a significant improvement in temperature stability. The nanoscale core–shell structure leading to a notable improvement in magnetic properties was observed using transmission electron microscopy (TEM). Micromagnetic simulations were used to explain how diffusion processes affected the properties of magnetic powders.

Grain boundary infiltration in HDDR processed Nd2Fe14B magnets

We investigate the grain boundary infiltration process of various low melting eutectic alloys for the coercivity enhancement of hydrogenation disproportionation desorption recombination (HDDR) processed Nd-Fe-B powders. Nd-based as well as heavy rare earth (Tb) and light rare earth (La, Ce) containing alloys were systematically studied: Nd70Cu30, Nd90Al10, Nd80Ga15Cu5, Nd62Fe14Ga20Cu4, Nd60Tb10Cu30, La71Cu29 and Ce72Cu28. Moreover, the Fe content in the quaternary Nd-Fe-Ga-Cu system was varied to investigate the effect of grain boundary phase magnetism on the resultant coercivity.The largest coercivity enhancement, from 0.42 T in the as-HDDR powder to 1.88 T after infiltration was observed in the case of ternary Nd80Ga15Cu5 composition. Furthermore, it also shows the best temperature stability with the infiltrated sample still exhibiting a coercivity of 0.58 T at 200 °C. Infiltration of light rare earth (La, Ce) based alloy did not increase coercivities due to poor wetting at the grain boundaries. Adding Fe to the grain boundary alloys was shown to enhance magnetization up to a certain extent without significant loss in coercivity. These findings demonstrate the effectiveness of grain boundary infiltration in HDDR-processed magnets without using heavy rare earths. The infiltration with non-magnetic material strongly decreases the intergranular interaction, reflected in a different magnetic domain evolution during the magnetization reversal process.

The character and role of grain boundaries in NdFeB-type alloys and magnets

Abstract The character and role of grain boundaries in NdFeB alloys and magnets are described and discussed. The role of the grain boundaries in the hydrogen decrepitation process is considered and the influence of the character and mobility of grain boundaries on the properties of fully dense sintered magnets is described. Grain boundaries also play an important role in the hydrogenation disproportionation desorption and recombination process whereby the coarse grain, cast material is converted to very fine grain particles.

Study on Magnetization Reversal Processes of Anisotropic HDDR Pr2Fe14B-Type Magnetic Materials

The magnetization reversal processes of hydrogenation–disproportionation–desorption–recombination (HDDR) anisotropic Pr–Fe–B magnetic powders were systematically investigated. It is found that with the increase of external magnetic field, the magnetization first increases rapidly, and then increases slowly, and finally increases rapidly again. The increasing trend of coercivity with the increase of external magnetic field is just opposite to that mentioned above. Further micromagnetic simulation studies suggest that the coercivity is mainly controlled by the nucleation of reversed domain, and this is also indicated by the subsequent investigation for the evolution of the magnetic domain under the magnetic field.

Coercivity Enhancement of Hydrogenation Disproportionation Desorption Recombination Nd-Fe-B Powders by Grain Boundary Diffusion Process Using Tb70cu15al15 Alloy

Coercivity Enhancement of Hydrogenation Disproportionation Desorption Recombination Nd-Fe-B Powders by Grain Boundary Diffusion Process Using Tb70cu15al15 Alloy

Effect of Co and Zr content on the magnetic properties of SmFe11Ti-based alloys produced by hydrogenation-disproportionation-desorption-recombination

The recent increasing demand for magnetic materials for applications such as electric motors in electric vehicles, as well as issues with restrictions to the supply of certain rare-earth elements, has led to renewed interest in SmFe-based alloys. The SmFe12-type crystal structure is especially desirable due to its high saturation magnetisation, but it is difficult to obtain this metastable phase on its own. Co, Zr, and Ti substitutions have been found to improve the magnetic properties and stabilise the hard magnetic Sm1Fe11Ti1 phase, while melt spinning and the hydrogenation disproportionation desorption recombination (HDDR) process have been found to produce fine grain size, and thus high coercivity. Therefore, in this work the effect of Co and Zr content on the magnetic properties of melt-spun and HDDR processed (Sm1-xZrx)(Fe1-yCoy)11.2Ti0.8 samples has been examined, where 0 ⩽ x ⩽ 0.3 and 0 ⩽ y ⩽ 0.25. Melt-spun, HD, and DR samples were analysed to determine the effect of varying the Co or Zr content. Amorphous Sm1Fe11Ti1 phase was found in all melt-spun samples. Co was found to exist in equal amounts in the α-(Fe, Co) and (Fe, Co)2(Ti, Zr) phases in the HD treated samples, while Zr existed mostly within the (Fe, Co)2(Ti, Zr) phase. The crystal size of the α-Fe or α-(Fe, Co) phase was shown to increase with increasing Co content but decreased with increasing Zr content. Increasing Co content generally improved Mr and Ms in the DR treated samples, whereas increasing Zr content was generally detrimental to Mr, Ms, and Hcj. However, each sample had a different fraction of the Sm1Fe11Ti1 phase, and higher Sm1Fe11Ti1 phase fractions were found to increase the Mr, Ms, and Hcj. Values of Mr, Ms, and Hcj corrected for the fraction of Sm1Fe11Ti1 phase were highest in the (Sm0.8Zr0.2)(Fe0.8Co0.2)11.2Ti0.8 sample. This composition should be investigated further to determine if superior magnetic properties can be produced, by optimising the DR treatment conditions to obtain 100 % Sm1Fe11Ti1 phase.

Low-Temperature Reduction of Graphene Oxide Using the HDDR Process for Electrochemical Supercapacitor Applications

In the present work, attempts of reducing a graphene oxide powder using a low temperature hydrogenation disproportionation desorption and the recombination process (L-HDDR) has been carried out. A lower processing temperature in large scale production is significant when costs are concerned. Graphite oxide was prepared using a modified Hummers’ method dispersed in ethanol and exfoliated using ultrasonication to produce Graphene Oxide (GO). Investigations have been carried out by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The experimental results of L-HDDR processing graphene oxide powder, using unmixed hydrogen at 400°C and relatively low pressures (<2 bars) have been reported. X-ray diffraction patterns showed a reduction of graphene oxide with the L-HDDR process. The results showed that both processes, the L-HDDR as well as the standard HDDR, may be applied to the reduction of graphene oxide in order to produce supercapacitor materials. The advantage of employing the L-HDDR process is a relatively low temperature reducing the cost of treatment, what is a very important factor for producing a large amount of material. Thus, the L-HDDR process has been considered a promising alternative method of reducing graphene oxide with efficiency, with the possibility of large scale production.

Spark Plasma Sintering as an Effective Texturing Tool for Reprocessing Recycled HDDR Nd-Fe-B Magnets with Lossless Coercivity

The low-pressure hot-deformation methodology was applied to reprocess the nanocrystalline hydrogenation–disproportionation–desorption–recombination (HDDR) Nd-Fe-B powders from end-of-life (EOL) permanent magnets’ waste to determine the mechanism of texture development and the resultant improvement in remanence (and BHmax) in the recycled material. Both the hot-pressed and hot-deformed magnets produced via spark plasma sintering (SPS) were compared in terms of their magnetic properties with respect to forging pressures. Also, a comparison was established with the microstructure to cite the effectiveness of texture development at low deformation rates and pressures which is pivotal for retaining high coercivity. The hot-pressed magnets maintain the high coercivity (better than 100%) of the original recycled powder due to the control of SPS conditions. The hot deformation pressure was varied from 100–150 MPa at 750 °C processing temperature to identify the optimal texture development in the sintered HDDR Nd-Fe-B magnets. The effect of post-hot-deformation thermal treatment was also investigated, which helped in boosting the overall magnetic properties and better than the recycled feedstock. This low-pressure hot deformation process improved the remanence of the hot-pressed magnet by 11% over the starting recycled powder. The Mr/MS ratio which was 0.5 for the hot-pressed magnets increased to 0.64 for the magnets hot-deformed at 150 MPa. Also, a 55% reduction in height of the sample was achieved with the c-axis texture, indicating approximately 23% higher remanence over the isotropic hot-pressed magnets. After hot deformation, the intrinsic coercivity (HCi) of 960 kA/m and the remanence (Br) value of 1.01 T at 150 MPa is indicative that the controlled SPS reprocessing technique can prevent microstructure related losses in the magnetic properties of the recycled materials. This route also suggests that the scrap Nd-Fe-B magnets can be treated with recoverable magnetic properties subsequently via HDDR technique and controlled hot deformation with a follow-up annealing.

Development of high coercivity anisotropic Nd-Fe-B/Fe nanocomposite powder using hydrogenation disproportionation desorption recombination process

Based on a modified hydrogenation disproportionation desorption recombination (HDDR) process we propose and realize a novel top-down processing route to synthesize anisotropic nano-composite magnet powders. Selection of alloy compositions with Nd content lower than the stoichiometry of Nd2Fe14B phase led to the formation of spherical shaped nano-sized α-Fe phase within the Nd2Fe14B matrix after HDDR process. Next anisotropic nanocomposite Nd-Fe-B/α-Fe powders with substantial coercivity were developed with optimized HDDR conditions and subsequent grain boundary engineering which remedied the initial lack of Nd-rich intergranular phase. Specifically, the infiltration of Nd70Cu30 alloy increased the coercivity from 0.0 to 0.85 T. Note that low coercivity and the absence of texture in nanocomposite magnets have been the main challenges to realize the high (BH)max postulated for many years for anisotropic nanocomposite magnets. We employed micromagnetic simulations for optimum microstructure design of a nanocomposite Nd2Fe14B/α-Fe magnet that gives a large maximum energy product, (BH)max. The simulations are then correlated with macroscopic hysteresis properties and high-resolution electron microscopy as well as atom probe tomography. Another very remarkable result is the observation that the formation of α-Fe phase with a size up to 200 nm within the matrix of Nd2Fe14B grains can still result in a significant coercivity of 0.85 T. This is in contrast to common understanding of exchange-coupled systems and we explain this observation with sharp and defect free α-Fe/Nd2Fe14B interface, the latter a result of the disproportionation and recombination reactions.

Coercivity Increase of the Recycled HDDR Nd-Fe-B Powders Doped with DyF3 and Processed via Spark Plasma Sintering & the Effect of Thermal Treatments.

The magnetic properties of the recycled hydrogenation disproportionation desorption recombination (HDDR) Nd-Fe-B powder, doped with a low weight fraction of DyF3 nanoparticles, were investigated. Spark plasma sintering (SPS) was used to consolidate the recycled Nd-Fe-B powder blends containing 1, 2, and 5 wt.% of DyF3 grounded powder. Different post-SPS sintering thermal treatment conditions (600, 750, and 900 °C), for a varying amount of time, were studied in view of optimizing the magnetic properties and developing characteristic core-shell microstructure in the HDDR powder. As received, recycled HDDR powder has coercivity (HCi) of 830 kA/m, and as optimally as SPS magnets reach 1160 kA/m, after the thermal treatment. With only 1–2 wt.% blended DyF3, the HCi peaked to 1407 kA/m with the thermal treatment at 750 °C for 1 h. The obtained HCi values of the blend magnet is ~69.5% higher than the starting recycled HDDR powder and 17.5% higher than the SPS processed magnet annealed at 750 °C for 1 h. Prolonging the thermal treatment time to 6 h and temperature conditions above 900 °C was detrimental to the magnetic properties. About ~2 wt.% DyF3 dopant was suitable to develop a uniform core-shell microstructure in the HDDR Nd-Fe-B powder. The Nd-rich phase in the HDDR powder has a slightly different and fluorine rich composition i.e., Nd-O-F2 than in the one reported in sintered magnets (Nd-O-F). The composition of reaction zone-phases after the thermal treatment and Dy diffusion was DyF4, which is more abundant in 5 wt.% doped samples. Further doping above 2 wt.% DyF3 is ineffective in augmenting the coercivity of the recycled HDDR powder, due to the decomposition of the shell structure and formation of non-ferromagnetic rare earth-based complex intermetallic compounds. The DyF3 doping is a very effective single step route in a controlled coercivity improvement of the recycled HDDR Nd-Fe-B powder from the end of life magnetic products.

Application of Hydrogen in the Production of Sintered Anisotropic Nanostructured Magnets from Alloys of Rare-Earth and Transition Metals

The main stages of the procedure of hydrogen treatment of ferromagnetic materials based on Nd2Fe14B, SmCo5, and Sm2Co17 compounds of rare-earth and transition metals aimed at manufacturing of nanostructured anisotropic sintered magnets are discussed. It is shown that, by using combined hydrogen treatment based on the modified hydrogenation–disproportionation–desorption–recombination (HDDR) method, we refine the microstructure of ferromagnetic alloys down to the nanolevel. We experimentally establish the mechanism of getting magnetically anisotropic materials by the HDDR method: the materials are anisotropic if they contain residues of the initial phase after disproportionation. We prove the possibility of application of hydrogen treatment by the HDDR method for the purposes of sintering of ferromagnetic nanostructured materials.

Magnetic anisotropy and crystallographic alignment in Fe and NdH2 during d-HDDR process of Nd-Fe-B-Ga-Nb powders

Hydrogen disproportionation (HD) treatments at 820°C for 10 h with different hydrogen pressures (PHD) were performed for Nd-Fe-B-Ga-Nb magnetic powder to clarify the relationship between the structural differences and resultant magnetic anisotropy as a function of PHD during dynamic hydrogen disproportionation desorption recombination. When PHD was 30 kPa, the anisotropy was the highest after recombination, and coarsened lamellar and spherical structures comprising Fe and NdH2 were observed after HD treatment. In the coarse lamellar structures, the crystallographic orientations of adjacent Fe and NdH2 grains were the same. The diameters of such regions were approximately 1100 nm, and their total area fraction was approximately 70% of the Fe and NdH2 phases. In contrast, when PHD was 100 kPa at which the anisotropy was lower, the sample consisted of mainly spherical grains, and the total area fraction of the regions where adjacent Fe and NdH2 grains exhibited the same crystallographic orientations was small (25%). This result indicates that the presence of coarsened lamellar comprising crystallographically aligned Fe and NdH2 grains contributed significantly to the induction of anisotropy after recombination.

Development of ultra-fine grain sized SmFe12-based powders using hydrogenation disproportionation desorption recombination process

SmFe12-based compounds possess promising intrinsic magnetic properties, even superior to those of Nd2Fe14B phase at elevated temperatures. However, the remaining challenge is to demonstrate that those intrinsic properties can be further developed into practically relevant extrinsic ones by proper processing. Here we have employed hydrogenation disproportionation desorption recombination (HDDR) process for preparation of ultrafine-grained SmFe12-based alloys with the tetragonal ThMn12-type crystal structure. The effect of alloying elements and grain boundary phase on disproportionation rate is investigated and experimental HDDR process phase diagram is proposed. Due to the high vapor pressure of Sm as well as the competing phases, recombination into phase-pure samples can be realized only in a narrow processing parameters window. The resultant phase constitutions, chemical composition, microsctructure and crystallographic relations at different process steps are discussed.

Hydrogen Reaction with SmCo Compounds: Literature Review

The Hydrogen Decrepitation and the Hydrogen Disproportionation Desorption Recombination processes for the preparation and the recycling of the two industrially produced phases of SmCo alloys, SmCo5 and Sm2Co17, are reviewed. The effects of the chemical composition, the microstructure, the exposure time, the hydrogen pressure, and the temperature on the hydrogen absorption are discussed. The magnetic properties for the SmCo powder after hydrogen absorption are discussed in both the powder form and after preparing sintered or plastic-bonded magnets.

Coercivity Enhancement of Nd-Fe-B HDDR Powder by Grain Boundary Diffusion Process with Rare-Earth Hydride

The grain boundary diffusion (GBD) process with rare-earth hydride was performed to increase the coercivity of hydrogenation–disproportionation–desorption–recombination (HDDR) powder. Before the GBD process, we investigated the effect of post-annealing of the initial HDDR powder on its magnetic properties. Low-temperature annealing reduced the coercivity of the HDDR powder. However, the coercivity decline decreased with increasing annealing temperature, becoming similar to that of the initial powder at 900°C. After the GBD process at 850°C for 1 h, the coercivity increased by about 4 kOe with 4 wt.% NdH x -Cu, forming a thick and continuous grain boundary phase. In addition, the coercivity and remanence of the HDDR powder produced by the GBD process with NdH x -Cu were higher when using NdH x in spite of the same amount of diffusion as at 2 wt.%.

Crystallographic alignment of Fe2B and Nd2Fe14B for texture memory in hydrogenation–disproportionation–desorption–recombination-processed Nd–Fe–B powders

Texture development in hydrogenation–disproportionation–desorption–recombination (HDDR)-processed NdFeB magnets is discussed with a focus on the crystallographic alignment of the Nd2Fe14B and Fe2B phases. By careful control of the dynamic HDDR-processing parameters, shape changes in the HDDR-treated samples were minimized by suitable modifications to the sample dimensions to trace the orientation relation between the c-axes of the Nd2Fe14B and Fe2B phases. The magnetic anisotropy was significantly dependent on the hydrogen pressure during the HD process and highly aligned Fe2B phases, with the same c-axis orientation as the initial Nd2Fe14B phase, were formed under the dynamic HDDR treatment at a hydrogen pressure of 30 kPa under controlled HD conditions. During the DR reaction following the HD process, the orientation of the initial Nd2Fe14B was recovered by a highly aligned Fe2B phase. It was demonstrated that the Fe2B phase could mediate texture during the HDDR processing owing to the atomic configuration and crystallographic relation between the Nd2Fe14B and Fe2B phases.