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Hydrogenation process of Nd-Dy-Fe-B alloy
Abstract
The influence of the alloy granulation grade, temperature and surface oxidation on the kinetics and the course of the fracture processes during the hydrogenation of Nd16-xDyxFe76B8 (x=0–3) alloy was studied. The average particle sizes of the alloy were 1 mm, 5 mm and 3 cm and the temperature range between 20 and 200 °C. Pre-oxidation times of the alloy were 0, 5, 10, 20, 30, 1 20 and 1200 min. No influence of the alloy composition on the kinetics and fracture processes during the hydrogenation was observed. An influence of the granulation grade of the alloy particles on the reaction rate was established and an influence of increasing temperature on the shortening of the incubation period was observed. It was also shown that the incubation time is influenced by surface oxidation. A tentative model of the hydrogenation-decrepitation process is suggested.
... First, the Nd-rich grain boundary phase absorbs the hydrogen in an exothermic reaction forming neodymium tri-hydride NdH2.7 3 (Harris et al., 1985;Moosa & Nutting, 1988;K. Oesterreicher & Oesterreicher, 1984a;Saje et al., 1992), followed by the matrix phase forming an interstitial hydride of Nd2Fe14BHx (where x ≈ 2.7) (K. Oesterreicher & Oesterreicher, 1984a). ...
... The increases in lattice volume (ΔV), for NdH 2-3 and Nd 2 Fe 14 BH 2-3 are 16% and 5%, respectively [14]. Strain generated by lattice expansion promotes crushing of magnets into powder (decrepitation) [15]. ...
An effective and complete processing route for the recycling of sintered Nd-Fe-B scrap magnets was proposed. Sintered Nd-Fe-B magnets were subjected to the Hydrogen Decrepitation (HD) process at various temperatures in the range of 50-300°C, at two different pressures, 50 kPa and 200 kPa, followed by vacuum dehydrogenation in the range of 720-820°C. The structure refinement efficiency and magnetic properties of the powders obtained were characterized. Low hydrogen pressure (50 kPa) was found to increase the magnetic properties at each temperature for all powder fractions. It was also shown that particle refinement occurs at low (50°C, 100°C) temperatures for low (50 kPa) pressure and higher temperatures (200°C, 300°C) for high pressure (200 kPa), respectively. High pressure also accelerates the initialization of the HD process at each temperature. No correlation was found between hydrogenation temperature and magnetic properties of the powders, except for the small increase in coercivity increase with growing temperature. The coarse fraction (400-500 µm) was found to have the highest magnetic properties for almost each HD process. The best magnetic properties Hci = 497 kA/m, Br = 1.1 T, and (BH)max = 121 kJ/m³ were obtained for hydrogenation at a temperature of 50°C, pressure 50 kPa and dehydrogenation at 780°C.
To reduce the Nd-Fe-B material cost and balance the utilization of rare earth sources, the misch metal (MM) substitution for Nd in the sintered magnet attracts interest. Under the processing of fabricating Nd-Fe-B magnet, hydrogen decrepitation (HD) is widely used to obtain magnetic powder; however, the HD behaviors of RE-Fe-B strips based on MM may be complex. The HD behaviors of the [(Pr, Nd)
1-x
MM
x
]
30.8
FebalCo
1.4
B
0.97
(x = 0-1) strips were studied under 0.2 MPa pressure. We find that it is similar to the Nd-Fe-B, 2:14:1 tetragonal structure of RE-Fe-B strips based on MM which is not broken during hydrogen decrepitating. But the REFe
2
phase in MM
30.8
FebalCo
1.4
B
0.97
disappears dramatically as a result of hydrogenation. For the samples with MM, the HD process is quicker and the particle size gets smaller than that for free MM, especially for samples with 35 wt.% MM, which may be attributed to La and Ce elements.
Scrap containing NdFeB is a valuable resource for the production of NdFeB magnets as the demand for these materials grows. One of the challenges is to recover the rare earths or the NdFeB alloy powder in a clean and cost effective manner so that it can be re-processed into new magnets rather than becoming lost to landfill. Work on using hydrogen to process scrap magnets (HPMS) has been shown to be successful when targeting hard disk drives. Currently, there is a lack of information on reliable methods to separate out NdFeB from other scrap sources such as automotive drives. In the near future, with increasing sales and electrification of cars, the automotive sector could be an important source for Dy containing magnets. In this paper, the hydrogen processing of scrap magnets has been demonstrated as an extraction method for NdFeB from automotive rotors for the first time, with the aim to examine the viability of this recycling process and learn lessons for design for recycling. Thus leading to the sustainable production of these components. Significant challenges were outlined when applying the hydrogen process to rotors with embedded magnets. After the extraction, further process steps may also be needed to separate epoxy coatings, as sieving could only reduce the carbon content to 1420 ppm, compared to 770 ppm in the base alloy. The gravimetric measurements also confirmed that Dy additions increase both the initiation and absorption time for hydrogen decrepitation. Hence, a higher hydrogen pressure will be required to speed up the process.
We present results of experiments on hydrogenation of Nd16Fe75.8B8.2 controlled by two parameters: the amount of hydrogen and the rate of feeding the sample with hydrogen at low pressures. By proper combination of these parameters four different schemes for the hydrogenation process were generated: (1) selective hydrogenation of the Nd-rich phase, (2) hydrogenation of the Nd-rich phase followed by diffusion of hydrogen into the Nd2Fe14B phase, (3) simultaneous partial hydrogenation of both phases, (4) simultaneous complete hydrogenation of both phases. For the first two cases the results of measurements of equilibrium pressure versus hydrogen concentration at room temperature are presented in the form of a p–c diagram with three phase regions: (1) Nd+NdHy+Nd2Fe14B, (2) NdHy+Nd2Fe14B+Nd2Fe14BHx2.7
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[`3]mR\bar 3m
, are reported.
A new compound composed of Nd, Fe, and a small quantity of B (about 1 wt. %) has been found, which has a tetragonal structure with lattice constants a=0.880 nm and c=1.221 nm. This phase, which has the approximate composition, 12 at. % Nd, 6 at. % B and balance Fe, possesses remarkable magnetic properties. From the approach to saturation an anisotroy constant of about 3.5 MJ/m3 can be calculated, while saturation magnetization amounts to 1.35 T. The magnetization versus temperature curve shows a Curie temperature of 585 K, which is much higher than those of the Fe and light rare earth binary compounds. Based on the new compound, sintered permanent magnets have been developed which have a record high energy product. Permanent magnet properties and physical properties of a typical specimen which has the composition Nd 1 5 B 8 Fe 7 7 are as follows: B r =1.23 T, H cB =880 kA/m, H cI =960 kA/m, (BH) max =290 kJ/m3, temperature coefficient of B r =-1260 ppm/K, density=7.4 Mg/m3, specific resistivity=1.4 μΩm, Vickers hardness=600, flexual strength=250 MPa.
A combination of hydrogen decrepitation (HD) and jet milling (JM) has been used to produce powder for the processing of permanent magnets. The procedure has proved to be very successful for both NdFeB (Neomax) alloys and the NdDyFeNbB high coercivity alloys. The magnets produced by the HD/JM process showed excellent coercivities when sintered between 980 and 1040 °C. At higher temperatures, excessive grain growth reduced the coercivity significantly. The hydrogen absorption/desorption (HAD) behaviors of alloys based on NdââFeââBâ and the stoichiometric phase NdâFeââB have been examined in an attempt to relate these effects to the microstructures of these alloys. The HAD processes were studied by microbalance, differential thermal analysis (DTA), differential scanning calorimetry (DSC), and mass spectroscopy, and these techniques showed that the absorption process in the NdââFeââBâ alloy are multistage and occur readily at room temperature. The NdâFeââB alloy, on the other hand, could not be activated at room temperature and hydrogen absorption could only be achieved at elevated temperatures. The mass spectroscopy/vacuum degassing studies showed a two-stage desorption process whereby hydrogen was first desorbed from the matrix (NdâFeââB) phase and then from the phase(s) at the grain boundaries.
The kinetics and mechanism of titanium hydride formation were studied by measurements of overall reaction rates combined with metallographic examinations of partially hydrided samples. The main part of the reaction involves the formation of a continuous protective hydride layer which progresses into the sample. Paralinear rate equations fit the kinetic behaviour over a pressure range of 30–450 Torr H2 and temperatures of 100–250 °C. The reaction rate is controlled by the diffusion of hydrogen through the protective thickening product layer which finally reaches an apparently constant thickness for diffusion as a result of cracking on the outer surface. This maximum thickness is temperature dependent and increases abruptly above approximately 150 °C. A corresponding apparent deviation from Arrhenius dependence is observed in the linear rate constants. Morphological aspects of hydride development are described. The effect of the metal microstructure on the hydriding kinetics is discussed.
The nature of hydrogen decrepitation when applied to a cast Nd-Fe-B permanent magnet alloy has been studied by following the microstructural changes on polished surfaces of the material exposed to hydrogen at a pressure of 4 bar. The milling of the material decrepitated at a pressure of 10 bar has also been studied by determining the particle size as a function of milling time. The milled particles have also been examined by scanning electron microscopy.It is concluded that preferential swelling of the neodymium-rich phase occurs and that this gives rise to 1.(i) shear stresses leading to fracture at the interface between the particles and the matrix, and2.(ii) cracks in the iron-rich matrix phase which show 90° intersections characteristic of tensile stress relief.
Though there is continuing interest in basic research on hydrogen in, and on, metals, applied research on metal hydrides has known many ups and downs. About ten years ago it was thought that hydrides of intermetallic compounds would become important in the mobile and stationary storage of hydrogen as a synthetic fuel. Subsequently it became evident that hydrogen would not be an economic fuel in the immediate future, unless the cost of removing pollution was added to the price of fuels. Today applied research on metal hydrides is more oriented towards the use of hydrogen in closed systems, e.g., thermal compressors or heat pumps [1].The aim of the present contribution is to show that metal hydrides are exciting materials allowing the study of many topics of solid state science.
A bulk ingot of a Nd-Fe-B alloy has been powdered by a combination of hydrogen decrepitation and attritor milling. The powder
was aligned and pressed in the hydrided condition and the green compact sintered at 1080‡ C for 1 h after an appropriate heating
rate. Excellent densities were achieved after this procedure and the magnets produced by this method exhibited energy products
in the region of 250 kJm−3 (32 M GOe).
Oxide layers on metal surfaces impede or prevent hydrogen absorption at temperatures below 400 °C. Passivation can be caused by slow dissociation of hydrogen molecules on the surface, slow transfer of chemisorbed hydrogen atoms into the oxide or slow permeation of hydrogen atoms through the oxide layer. Experiments on the hydrogen absorption kinetics of film samples with known oxygen precoverage yield direct information on reaction models if relevant results of other techniques are taken into account together with inherent thermochemical and atomistic limits. Typical examples are discussed to elucidate this approach for the investigation of complex reaction mechanisms in hydrogen absorption kinetics at room temperature. They indicate that hydrogen dissociation on the surface is a very important partial step in the reaction and plays a substantial role in activation and passivation of hydrogen absorption reactions at room temperature.
In this article, the application of the hydrogen decrepitation (HD) process to the production of rare earth containing permanent magnets is discussed. It is shown that the HD process can be applied successfully to the production of SmCo5−, Sm2(Co, Fe, Cu, Zr)17− and Nd2Fe14B-type magnets. The particular HD conditions for each category of permanent magnet are described. The characteristics of the HD powder of all three alloy types are compared with those of the normal milled material and it is seen that significant advantages accrue from the use of the HD powder.