Neodymium Hydride Research Articles

Microstructural Investigation of Discarded NdFeB Magnets After Low-Temperature Hydrogenation

Due to continuously increasing demand and limited resources of rare-earth elements (REEs), new solutions are being sought to overcome the supply risk of REEs. To mitigate the supply risk of REEs, much attention has recently been paid to recycling. Despite the more common recycling methods, including hydrometallurgical and pyrometallurgical processes, hydrogen processing of magnetic scrap (HPMS) is still in the development stage. Magnet-to-magnet recycling via hydrogenation of discarded NdFeB magnets provides a fine powder suitable for the production of new magnets from secondary sources. One of the crucial aspects of HPMS is the degree of recovery of the magnetic properties, as the yield efficiency can easily reach over 95%. The amount, morphology, and distribution of the Nd-rich phase are the key parameters to achieve the excellent performance of the magnet by isolating the matrix grain. Therefore, a better insight into the microstructure of the matrix grains and the Nd-rich phase before and after hydrogenation is essential. In this study, a low-temperature hydrogenation process in the range of room temperature to 400 °C was conducted as the first step to recycle NdFeB magnets from discarded hard disk drives (HDDs), and the hydrogenated powder was characterized by electron microscopy and X-ray diffraction. The results show that there are three different morphologies of the Nd-rich phase, which undergo two different transformations through oxidation and hydride formation. While at lower temperatures (below 250 °C) the degree of pulverization is higher and the experimental evidence of hydride formation is less clear, at higher temperatures the degree of pulverization decreases. The formation of neodymium hydride at higher temperatures prevents further oxidation of the Nd-rich phase due to its high stability.Graphical

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.

Multiple recycling of NdFeB-type sintered magnets

Abstract Some fully dense, sintered NdFeB-type magnets (employed in VCM disc drives) have been subjected to a recycling process using the hydrogen decrepitation (HD) process. After a brief milling treatment, the powder was aligned, pressed and re-sintered and this procedure was repeated four times with a progressive fall in the density and in the magnetic properties. The chemical analysis indicated that this was due to the progressive oxidation of the Nd-rich material and to some Nd loss by evaporation. The procedure was then repeated but with the addition (blending) of a fine powder of neodymium hydride after the first cycle. It was found that the addition of 1 at.% of neodymium at each stage was sufficient to maintain the density and the magnetic properties of the recycled magnets up to and including the 4th cycle. Inductively coupled plasma (ICP) and metallographic analysis indicated that the neodymium hydride additions compensated for the neodymium loss due to evaporation and to oxidation so that the proportion of Nd-rich material remained approximately constant. The additional amount of Nd2O3 in the blended recycled magnets appeared to inhibit grain growth on the 3rd and 4th cycles when compared to that of the unblended magnets. The next challenge is to see if the process can be scaled-up to an industrial scale.

Tris-pyrazolylborate complexes of redox inactive lanthanides – the structures of [(Tp Me2) 2NdX] (X=Cl, NPh 2, dpm, H 2BEt 2)

A range of complexes of Ce and Nd have been prepared to compare the chemistry of the non-reducible [(Tp Me2) 2LnX] (Ln=Ce, Nd) with that reported previously for the redox-active samarium ion. The chloride [(Tp Me2) 2NdCl] is a useful starting material for the preparation of the seven coordinate [Nd(Tp Me2) 2OAr] (Ar=C 6H 4-4-X; X=H, Bu t , Br, Ph, OMe, F, Me) and [Nd(Tp Me2) 2NPh 2]. Crystal structures of [Nd(Tp Me2) 2Cl] and [Nd(Tp Me2) 2NPh 2] are isostructural with those of the analogous samarium species. The eight coordinate species [Nd(Tp Me2) 2(β-dike)] (β-dike=(RCO) 2CH, R=Me, Ph; Bu t ) and their cerium analogues (R=Ph, Bu t ) have also been prepared. The complexes are fluxional, the extent of this behaviour depending on the steric bulk of the substituents. The structure of [Nd(Tp Me2) 2(Bu t CO) 2CH] has been determined. Finally, in efforts to prepare a neodymium hydride, the unexpected borohydride [Nd(Tp Me2) 2(H 2BEt 2)] has been isolated serendipitously and structurally characterized.

The use of metal hydride powder blending in the production of NdFeB-type magnets

The ability to vary the composition of NdFeB-type magnets, during or after the milling stage offers a number of advantages. To this end, powder blending processes have been developed. In the present work, the sintering characteristics of a Nd13Fe80.5B6.5 alloy have been improved by powder blending with neodymium hydride. A blending addition of 1 at% of neodymium in the form of Nd-hydride, consistent with a final composition of Nd14Fe79.6B6.4, was sufficient to reduce the required sintering temperature from 1130 to 1070°C, with a resultant improvement in the coercivity due to less grain growth and better grain isolation. The addition of dysprosium hydride to a Nd14Fe79B7 alloy by blending resulted in a further enhancement of the coercivity, a 2 at% addition yielding an approximate doubling in the value of intrinsic coercivity. Examination of the microstructure of dysprosium–hydride blended magnets showed the dysprosium to be concentrated in the outer regions of the matrix grains, with the centres being essentially dysprosium-free. Hydrogen has also been employed in improving the milling characteristics of the high melting point elements niobium and vanadium. Fine powders of niobium and vanadium hydride were prepared and were successfully blended into magnets with an even distribution throughout the microstructure. These studies have shown that powder blending of metal hydrides is an effective way of both promoting liquid-phase sintering in low rare earth composition magnets and of modifying the grain boundary phases and hence the magnetic properties.

Early lanthanide organometallic hydrides: electronic and steric control of the stability

Steric and electronic effects on the stability of early lanthanide, neodymium and samarium, organometallic hydrides are discussed. The instability of (tmp) 2SmH (tmp=tetramethylphospholyl) is attributed to electronic factors and the low stability of ( t BuC 5H 4) 2SmH is related to steric reasons. The analogous neodymium hydride ( t BuC 5H 4) 2NdH, could not be obtained, whereas the more hindered triethylborohydride was formed. Access to bisphospholyl bridged heterobimetallic ruthenium–lanthanide hydrides is also related to the size of the lanthanide atom. The bimetallic structures are accessible for lanthanides of ionic radii smaller than 1 Å. The role of organometallic hydrides versus alkyls as catalysts for olefin or diene polymerization is discussed, a first and unique example of hex-1-ene and isoprene copolymerization is presented.

Recombination of the Nd 2Fe 14B phase after reactive milling under hydrogen of a Nd–Fe–B powder

Mechanical milling under reactive hydrogen atmosphere of a Nd–Fe–B powder, followed by vacuum annealing up to 600°C, was achieved. Both as-milled and annealed powders were investigated by X-ray diffraction and Mössbauer spectrometry. During reactive milling, the Nd 2Fe 14B phase disproportionates into neodymium hydrides (NdH 2± x ) poorly crystallized α-Fe and an amorphous Fe–B phase. Upon annealing, hydrogen desorbs from the neodymium hydride and this is followed by the recombination of the Nd 2Fe 14B phase. The recombination temperature was found to be 520°C. The recombination reaction is incomplete, likely due to oxygen contamination, as neodymium oxides, α-Fe and Nd 1.1Fe 4B 4 phases are detected in the powder annealed at 600°C.

Thermal stability of compression moulded neodymium-iron-boron anisotropic magnets : N.Ishigaki et al. (Sumitomo Special Metals Co Ltd, Osaka, Japan.) J. JpnSoc. Powder Powder Metall., vol 46, no 1, 1999, 62–66. (In Japanese.)

High coercive isotropic Nd-Fe-B powders can be obtained using the hydrogenation-disproportionation-desorption-recombination (HDDR) process. The Nd-Fe-B technological alloy is made of several phases; among them the main one is Nd2Fe14B but with important additions of an eutectic composition rich in neodymium (Nd-rich) and of NdFe4B4. In situ neutron diffraction experiments have been performed on different alloys and Nd-rich compositions. It was possible to follow solid-gas reactions and to record in situ the different steps of the HDDR process, and more particularly, the behaviour of the intergranular phase under vacuum and under hydrogen. Experiments performed on Nd2Fe14B11 during the hydruration process have evidenced the presence of hydrides with composition varying with temperature up to the decomposition of the phase in neodymium hydride and iron. Structural refinements of the phases appearing during the process have been performed and a mechanism for the HDDR process is proposed.

The use of metal hydrides in powder blending for the production of NdFeB-type magnets

A blending process involving neodymium hydride has been used to improve the sintering characteristics of a Nd13Fe80.5B6.5 alloy. An addition of 1 at% of neodymium hydride made by blending of the powders, consistent with a final composition of Nd14Fe79.6B6.4, was sufficient to reduce the required sintering temperature from 1130 to 1070°C; with resultant improvement in the coercivity due to less grain growth and better grain isolation. The addition of dysprosium hydride to a Nd14Fe79B7 alloy by blending resulted in a further enhancement of the coercivity, a 2 at% addition yielding an approximate doubling in the value of intrinsic coercivity. Examination of the microstructure of dysprosium hydride blended magnets showed the dysprosium to be concentrated in the outer regions of the matrix grains, with the centres being essentially dysprosium free. Hydrogen has also been employed in improving the milling characteristics of the high melting point elements niobium and vanadium. Fine powders of niobium and vanadium hydride were prepared and were successfully blended into magnets with an even distribution throughout the microstructure. These studies have shown that powder blending of metal hydrides is an effective way of both promoting liquid-phase sintering in low rare earth composition magnets and of modifying the grain boundary phases and hence the magnetic properties.

Mechanical strength of the solid-HDDR treated Nd–Fe–B-type materials

The hydrogenation, disproportionation, desorption, recombination (HDDR) is known as a unique process through which a noncoercive cast Nd–Fe–B-type material can be easily converted into a coercive one. Although the conventional HDDR process generally leads to a powderlike material, a solid-HDDR material can be realized if the HDDR process is properly modified (solid-HDDR). In the present study, the change of mechanical strength (compressive strength) of the Nd–Fe–B-type material during the solid-HDDR has been investigated using a sintered magnet with composition Nd13.8Dy0.7Fe78.25Si0.15Mn0.6B6.5. It has been found that the low strength of the hydrided material was improved by the subsequent disproportionation. The restoration of the strength was explained by the eutectoidlike disproportionation structure containing fine neodymium hydride rod embedded in tough iron matrix. The high strength of disproportionated material was reduced radically in an earlier stage of recombination, and this was explained by the reduction of the disproportionated phase. The reduced strength was, however, recovered by further recombination, and this was explained by the fact that as the recombination continues the recombined grains become continuous and adhere together. The optimally HDDR processed material has a comparable or even higher strength with respect to the initial sintered material prior to the solid-HDDR. The present study suggested that the mechanical strength of Nd–Fe–B-type material could be retained even after the solid-HDDR.

In situ neutron diffraction study of the HDDR process of NdFeB magnets

High coercive isotropic Nd-Fe-B powders can be obtained using the hydrogenation-disproportionation-desorption-recombination (HDDR) process. The Nd-Fe-B technological alloy is made of several phases; among them the main one is Nd 2Fe 14B but with important additions of an eutectic composition rich in neodymium (Nd-rich) and of NdFe 4B 4. In situ neutron diffraction experiments have been performed on different alloys and Nd-rich compositions. It was possible to follow solid-gas reactions and to record in situ the different steps of the HDDR process, and more particularly, the behaviour of the intergranular phase under vacuum and under hydrogen. Experiments performed on Nd 2Fe 14B 11 during the hydruration process have evidenced the presence of hydrides with composition varying with temperature up to the decomposition of the phase in neodymium hydride and iron. Structural refinements of the phases appearing during the process have been performed and a mechanism for the HDDR process is proposed.

Desorption characteristics of rare earth (R) hydrides (R=Y, Ce, Pr, Nd, Sm, Gd and Tb) in relation to the HDDR behaviour of R–Fe-based-compounds

Vacuum desorption characteristics of binary hydrides of rare earth metals (R=Ce, Pr, Nd, Y, Sm, Gd, Tb) were studied. A complete decomposition of hydrides, confirmed by X-ray diffraction, was achieved at temperatures up to 820 °C. When heating the hydrogenated material in vacuum, two distinct hydrogen desorption events were observed for all the hydrides under investigation. The hydrogen desorption behaviour is different for the two groups of metals, namely group I (Ce, Pr and Nd) and group II (Y, Sm, Gd and Tb). The hydrogenation–disproportionation–desorption–recombination (HDDR) process was shown to take place for the Nd 6Fe 13Ge and Nd 6Fe 12.8Ga 1.2 intermetallic compounds, both with the Nd 6Fe 13Si-type structure. The hydrogen absorption and desorption properties of these compounds were studied in relation to the desorption characteristics of neodymium hydride.

Disproportionation in Nd 16Fe 76B 8-type hydrides

This work has shown that heating Nd 16Fe 76B 8-type materials in a hydrogen atmosphere, with subsequent vacuum annealing at greater than 750°C produces coercive powders which are suitable for the production of polymer-bonded magnets. A study of the material at different stages of the process has confirmed that the Nd-Fe-B alloy disproportionates into iron, Fe 2B and neodymium hydride before recombining during the vacuum annealing stage to produce the fine-grained microstructure necessary for highly coercive material. The grain size of the final microstructure has been found to be very dependent on the temperature at which the hydrogenation-disproportionation-desorption cycle is carried out with optimum properties for the Nd 16Fe 76B 8 alloy being obtained for a desorption temperature of 785°C for 1 h.

Phase relations and hydrogen absorption of neodymium-iron-(boron) alloys

Based on an investigation of the phase correlations in the system Nd-Fe-B, a complete study of the hydrogen absorption of Nd-Fe-B alloys is presented, mainly concerning the permanent magnet material Nd2Fe14 B. Absorption isotherms of the compound Nd2Fe14 B and of the master alloy for permanent magnet production, Nd15Fe77B8, have been recorded and their hydrogenation behaviour is discussed. Thermal desorption spectra support the conclusion that neodymium hydride is the second hydride phase in the hydrogenated master alloy. In the Nd-Fe binary system, Nd2Fe17 was confirmed as the only equilibrium phase at 870 and 1170 K. The properties of a new ferromagnetic ternary hydride Nd2Fe17Hx,x = 0 to 5, with the Th2Zn17 type structure, space group\(R\bar 3m\), are reported.

Cleavage of Tris(cyclopentadienyl)lanthanide Complexes with Alkyllithium Compounds—a New Route to Alkylbis(cyclopentadienyl)lanthanides

Bis(cyclopentadieny)neodymium hydride 3 is a superb hydrogenation catalyst for alkynes. Exchange of a C5H5 ligand in 1 for sec- or tert-butyl (not for n-butyl) at −78°C affords 2a and 2b, respectively, the first alkylbis(cyclopentadienyl)neodymium complexes. 2a decomposes at −30°C to 3 and butene; 2b can be hydrogenated at room temperature to give 3 and isobutane.

Neodymium-neodymium hydride phase system

The phase relationships between neodymium and neodymium hydride have been established. Hydrogen in solution increases the melting point of neodymium and lowers the α−β transformation temperature. A metal-rich intermediate phase was found between 680° and 880°C. The solid immiscibility gap seemed to close above 1150°C.