HDDR Process Research Articles

Applied Magnetic Field Increases Magnetic Anisotropy in HDDR-Processed Nd-Fe-B Alloy

We investigate the effect of an applied magnetic field on the entire HDDR process using a customized reactor vessel and a warm-bore superconducting magnet. We analyzed the resulting properties produced at both a 0 applied field and a 2 Tesla applied field. We show that the application of a magnetic field throughout the HDDR process results in powders that exhibit a greater level of anisotropy compared to their ambient field counterparts.

Relationship between Hydrogen Pressure and Temperature for Recombination Reactions in the HDDR Processing of Ce–Fe–B Alloys

Relationship between Hydrogen Pressure and Temperature for Recombination Reactions in the HDDR Processing of Ce–Fe–B Alloys

Peculiarities of conventional HDDR process in (Nd,Pr)-Fe-B alloy powders under low hydrogen pressure

The relationship between the temperature of the start, peak and finish of conventional disproportionation and recombination, in hydrogen, reactions and the low starting hydrogen pressure of 10, 20, 30, 40, 50, 60, 70 and 80 kPa and maximum processing temperature of 950–1015 °C was studied by DTA and X-ray diffraction measurements for 10 g of (Nd,Pr)-Fe-B ferromagnetic powder batches. It was shown that the starting temperature of the disproportionation reaction increases when hydrogen pressure decreases. A similar dependence took place for finishing temperature of the disproportionation when pressure was in the range of 40–80 kPa. At a hydrogen pressure of 10–30 kPa, finishing temperature of the disproportionation decreased with decreasing of pressure. The temperature of the recombination reaction in hydrogen increased with increasing of the hydrogen pressure. The non-equilibrium pressure–composition–temperature schema with four zones for the (Nd,Pr)-Fe-B alloy–hydrogen system was built. The remnants of the (Nd,Pr)2Fe14B phase and disproportionation products – (Nd,Pr)H2±x, α-Fe and Fe2B were in zone I – zone of the disproportionation reaction and in zone II – interstitial zone. The recombined (Nd,Pr)2Fe14B phase, (Nd,Pr)H0–2±x, α-Fe and Fe2B phases were in the zone III – zone of the recombination reaction. And the (Nd,Pr)2Fe14B phase, recombined in hydrogen, existed in the zone IV – zone of the existence of (Nd,Pr)2Fe14B phase.

Effect of Hydrogen Pressure in the Stage of Recombination of HDDR Process on Phase Equilibrium in Nd – Fe – Co – B – Zr-Base Alloys

X-ray diffractometry with mathematical processing of the diffraction patterns by the Tikhonov method of regularization, which enhances the resolution, is used to study the phase state in Nd – Fe – Co – B – Zr-base alloys during the reverse reaction of HDDR process at different hydrogen pressures. The range of hydrogen pressures in which the reverse reaction (recombination) of the HDDR process develops fully is determined experimentally.

The Effect of Vacuum Annealing and HDDR Processing on the Electrochemical Characteristics of Activated Carbon and Graphene Oxide for the Production of Supercapacitors Electrodes

Electric double-layer capacitors prepared using activated carbons have been subjected to vacuum heat treatments at low and high temperatures (200, 400, 600, 800 and 1000°C). The electrodes have been tested at a potential of 1.1 V employing a KOH electrolyte (1.0and 6.0 mol.L-1). The effect of or HDDR upon the electrical properties has been investigated by cyclic voltammetry. It has been shown that the specific capacitance at 5 msV-1 increases from 50 Fg-1 to 130 Fg-1 after a heat treatment at 400°C for 1 hour under back pump vacuum. At 400°C the diminution in the specific capacitance with higher scanning rate (10 msV-1) was much less pronounced (from 130 Fg-1 to 100 Fg-1). Equivalent series resistance (ESR) and equivalent parallel resistance of supercapacitors electrodes have also been investigated. Internal resistances of the supercapacitors were calculated using the galvanostatic curves at current densities (100 mAg-1).A compositional and morphological evaluation of these electrodes showed no significant change on the activated carbon structure.

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.

Special Features of Formation of Phase State in Nd – Fe – Co – B – Zr-Base Alloys in Early Stages of the HDDR Process

X-ray diffractometry and mathematical processing of diffraction patterns by the A. N. Tikhonov regularization method, which enhances the resolution, are used to study the kinetics of formation of phases in the forward reaction of the HDDR process in its early stages (the first minutes) in alloys based on Nd – Fe – Co – B – Zr.

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.

Ultrafine nanocrystalline NdFeB prepared by cryomilling with HDDR process

In this work, the NdFeB ribbons (Nd13.5Fe73Co7·5B6) were first subjected to the hydrogenation disproportionation (HD) to prepare disproportionation NdFeB precursor and subsequently cryomilled for different times. The submicron NdFeB particles with ultrafine nanocrystalline were finally obtained after desorption recombination (DR) process. The results indicated that with the increase of the cryomilling time, Nd2Fe14B grains were refined to nanoscale, but the coercivity decreased due to the strong exchange coupling between the nano-grains. The optimized nanocrystalline NdFeB particles have a size of 40 nm or less and present a coercivity of 6.63 kOe after cryomilling for 10 h followed by DR treatment at 800 °C for 10 min. To further improve the magnetic properties, Ca was added into HD powders in the course of cryomilling process to protect the powders from oxidation and a noticeable coercivity of 9.49 kOe was obtained. Finally, the mechanism of magnetic interactions and coercivity enhancement were discussed.

Boron behavior induced lamellar structure and anisotropic magnetic properties of Nd2Fe14B during HDDR process

The anisotropy of the Nd2Fe14B powder is originated during the creation of a fine Fe2B lamellar structure in the disproportionation step. The aspect ratio (A/R) of Fe2B structure increased from 3.37 ± 1.5 to 6.69 ± 3.2 during phase decomposition for 0 ~ 60 min at 820 ◦ C (P H2 = 10 kPa). The Fe2B having high A/R ratio recombined Nd2Fe14B, which is close to the single domain, and the magnetic properties are also improved with increasing A/R ratio.

MgCo2-D2 and MgCoNi-D2 systems synthesized at high pressures and interaction mechanism during the HDDR processing

MgCo2 and MgNiCo crystallize with hexagonal Laves type intermetallic structures of the C14 type and do not form hydrides at ambient hydrogen pressures. However, applying high hydrogen pressures in the GPa range forces the hydrogen absorption and leads to the formation of multi-phase compositions, which contain approximately 2.5 atoms H per formula unit of MgCo2 or MgNiCo and remain thermally stable under normal conditions.The hydrogenation of MgCo2 resulted in its decomposition to a ternary Mg2CoD5 deuteride and metallic cobalt. Phase-structural transformations accompanying the vacuum desorption of deuterium in the temperature range of 27–500°C were studied using in situ neutron powder diffraction. The investigation showed a complete recovery of the initial MgCo2 intermetallic via a Hydrogenation-Disproportionation-Desorption-Recombination process. At 300°C, the Mg2CoD5 deuteride first decomposed to elementary Mg and hexagonal Co. At 400°C, a MgCo phase was formed by interaction between Mg and Co. At the highest processing temperature of 500°C, a solid-state interaction of MgCo and Co resulted in the recovery of the initial MgCo2.The interaction of MgNiCo with deuterium under the synthesis conditions of 2.8GPa and 200°C proceeded in a more complex way. A very stable ternary deuteride MgNi2D3 was leached away while Co was separated in the form of Mg2CoD5 and the remaining nickel formed a solid solution with Co with the approximate composition Ni0.7Co0.3.The thermal desorption of deuterium from MgCo2D2.5 and from MgNiCoD2.5 has been studied by Thermal Desorption Spectroscopy with deuterium released into a closed volume. The observed effects nicely correlate with changes in the phase structural composition of the hydrides formed.MgCo2 is a new example of the hydrogen storage alloy, in which a successful HDDR processing results in the reversible formation of the initial intermetallic at much lower temperatures than in the equilibrium phase diagram of the Mg-Co system.

Microstructure evolution and nano-crystalline production of Mg-9Al-Zn alloy during HDDR processing

The microstructure evolution and phase transition of as-cast AZ91 Mg alloy powders during HDDR processing were investigated. The effects of temperature, hydrogen pressure and process time on the hydrogenation and dehydrogenation were also analyzed. The whole process strongly depended on temperature. Disproportionation reaction was activated at higher temperature than hydrogenation temperature and was very difficult to complete. The appropriate higher hydrogen pressure had a positive effect on hydrogenation-disproportionation and grain refinement, however excessive pressure had opposite effect. An optimized HDDR technique for getting the nanocrystalline AZ91 Mg alloys powder is considered as follows: hydriding at 450 °C for 12 h under 4 MPa and then dehydriding at 350 °C for 4 h under 3.5E-3Pa. As a result, the average grain size of AZ91 alloy powders was refined from 100 to 300 μm to about 20 nm.The grain refinement sustained throughout the whole HDDR process, but remarkably occured in dehydrogenation stage instead of in disproportionation and recombination stages. A plate model was suggested to explain the grain refinement mechanism:hydriding cracking and dehydring craking.

Hydrogen disproportionation phase diagram and magnetic properties for Nd15Fe79B6 alloy

Transformation-temperature-hydrogen pressure phase diagram was constructed for a Nd15Fe79B6 alloy in order to estimate appropriate conditions for hydrogenation, disproportionation, desorption and recombination reaction (the HDDR). Optimised recombination time (the highest coercivity) was found to be 10 min. for 5 g samples processed at 740 °C. Several HDDR processes were carried out at 30 kPa of hydrogen pressure at various temperatures. No correlation between magnetic propertiec and a direction of measurement was observed for the samples processed at 740 °C. Remanence anisotropy was induced along an alignment direction when the temperature of the HDDR process was increased up to 800 °C and 850 °C for <100 μm and 100–160 μm particles, respectively. Simultaneously, a small drop in coercivity was observed in the direction of alignment for <100 μm particles, but no for 100–160 μm particles. Furthermore, probably an ordered phase was found by TEM microstructure analysis in the bulk sample disproportionated at 850 °C under 150 kPa of hydrogen. Grains with antiphase domains were observed and corresponding electron diffraction patterns were resolved, likely indicating superlattice structures.

Crystallographic alignment in the recombination stage in d-HDDR process of Nd-Fe-B-Ga-Nb powders

Nd-Fe-B-Ga-Nb magnetic powder was subjected to the dynamic hydrogen disproportionation desorption recombination treatment. For samples disproportionated at both 30 and 100 kPa of hydrogen pressure, the changes in the microstructure and grain orientation during recombination process were investigated. It was observed that even during the recombination process, the orientation relationship was maintained between α-Fe and NdH2+x grains formed after the disproportionation treatment at 30 kPa of hydrogen pressure, [110]α-Fe // [110]NdH2+x, (-110)α-Fe // (-220)NdH2+x. Additionally, the alignment of recombined Nd2Fe14BHy grains became clear after 30 min of DR treatment showing following orientation relationship: (001)Nd2Fe14BHy // (110)α-Fe and (110)NdH2+x. In contrast, such a relationship was not observed in the sample disproportionated at 100 kPa of hydrogen pressure. This difference in the degree of alignment was also confirmed by measuring the magnetic property of the respective samples.

Hydrogen Pressure and Temperature Dependence of the Disproportionated State and Magnetic Anisotropy in the ${d}$ -HDDR Process of Nd–Fe–B–Ga–Nb Powders

Nd–Fe–B-based alloy powders were subjected to the dynamic hydrogen disproportionation desorption recombination treatment and the effect of the hydrogen pressure and temperature conditions in the disproportination step on magnetic anisotropy as well as the state of disproportionated samples was investigated. Magnetic anisotropy was maximized when the sample was hydrogen disproportionated at 30 kPa and 800 °C–820 °C. The analyses of the X-ray diffraction (XRD) patterns showed that the iron boride phase formed during the hydrogen disproportionation step crystallized in the tetragonal structure. The lattice parameters and the peak intensity of this iron boride phase in XRD patterns varied depending on the disproportionation conditions. Both magnetic anisotropy and iron boride peak intensity became stronger when the disproportionation condition was close to the boundary of disproportionation and recombination reactions. These results suggest that there is some kind of relationship between magnetic anisotropy and the state of iron boride phase in disproportionated samples.

Coercivity enhancement in HDDR near-stoichiometric ternary Nd–Fe–B powders

Anisotropic HDDR near-stoichiometric ternary Nd–Fe–B powders have been prepared. The coercivity of the powders was improved from 208.6 to 980.1kA/m by the subsequent diffusion treatment using the Pr–Cu alloy. For comparison, Nd11.5Fe80.7B6.1Pr1.2Cu0.5 alloy, in which Pr and Cu elements were directly added into the original Nd–Fe–B alloy, was also treated by the same HDDR process and the coercivity was only 557.3kA/m. Microstructural investigations showed that a large area of (Nd, Pr)-rich phases concentrated at triangle regions in the HDDR Nd11.5Fe80.7B6.1Pr1.2Cu0.5 powders, while the (Nd, Pr)-rich phases distributed uniformly in the diffusion treated powders. The uniform grain boundary layer can pin the motion of domain wall more effectively, resulting in a higher coercivity in diffusion treated HDDR Nd–Fe–B powders.

Anisotropy of Ternary and (Zr,Ga) Addition Magnetic Powders

Both non-homogenization treatment Zr, Ga addition and ternary Strip Cast- ing alloy flakes can be used to prepare anisotropy HDDR NdFeB magnetic powders. This illustrates that anisotropy formation of magnetic powders neither depends on element addition nor depends on homogenization heat treatment of SC flakes. But the HDDR process procedure also plays an important role in anisotropy inducement. Highly anisotropic magnetic powders are attributed to rapid disproportionation reac- tion course, slow desorption-recombination reaction course, and optimum recombina- tion hydrogen pressure during HDDR procedure process. This paper will provide an important guidance for preparing highly anisotropy magnetic powders with low cost.

Study of novel hard magnetic materials

In this work we systematically review our results about the permanent magnetic materials. We have investigated the structural and magnetic properties of the rare earth-iron based nitrides. Our investigations indicate that the hard magnetic properties of rare earth-iron nitrides are controlled by the domain nucleation. The high performance magnetic powders with maximum energy product of 43 MGOe can be obtained by creating single crystal particles, which could decrease the domain nucleation field. We also find the formation of the texture in HDDR NdFeB is related to the dissipative structure during the disproportional process. Highly textured HDDR magnetic powders can be achieved by controlling the HDDR process. The stability of the HDDR powders could be further improved through the microstructure modification. Finally, we also investigate Mn based hard magnetic materials, which show abnormal temperature coefficeient for coercivity. All these results may pave the road to produce bonded magnets with high performance.

MS-HDDR 프로세스의 DR 속도 제어를 통한 초미세 결정립 Nd2Fe14B계 자성분말 제조

MS-HDDR 프로세스의 DR 속도 제어를 통한 초미세 결정립 Nd2Fe14B계 자성분말 제조

Thermal Aging of Reprocessed NdFeB Powders

Microstructural and magnetic properties of thermal aged NdFeB-based powders, recycled by means of the HDDR process, are studied. The time aging causes a reduction of the remanent magnetic moment per unit mass and intrinsic coercivity of the magnetic powders. On the other hand, it has been verified an increase of the magnetic polarization atµ0H= 2 T after aging during 16 hours. The oxygen concentration is superior in the grain boundary region (Nd-rich) compared to that verified in the hard magnetic phase.