Hot-pressed Magnets Research Articles

Effect of Rare-Earth Dopant on Coercivity of Hot-Pressed Nd–Fe–B Magnet Doped With RF3

An RF3 (R = La, Ce, Pr, Nd, and Dy)-doped Nd13.6Fe73.6Co6.6Ga0.6B5.6 hot-pressed magnet was prepared using melt-spun flakes. The substitution of Nd in Nd–Fe–B flakes by rare-earth dopant in the hot-pressed magnets doped with various RF3 and its effect on coercivity were investigated. In the RF3-doped Nd13.6Fe73.6Co6.6Ga0.6B5.6 magnet hot pressed at the modest temperature (735 °C), the dopants Ce, Pr, Nd, and Dy actively substituted Nd in the region near the flake surface to form the thin shell in the flake. RF3 (R = Dy, Nd, and Pr) doping achieved significant coercivity enhancement in the hot-pressed magnet, and it was attributed to the enhanced anisotropy field of the Nd2Fe14B-type grains (for Dy and Pr doping) and grain boundary restructuring (for Nd doping) in the shell. The most profound coercivity enhancement as high as 5 kOe was achieved in the DyF3-doped magnet. Doping with the light rare-earth elements using RF3 (R = La and Ce) led to poor coercivity enhancement with respect to the RF3 (R = Dy, Nd, and Pr) doping.

Decomposition of DyF3 and its effect on magnetic performance of DyF3-doped Nd-Fe-B-type hot-deformed magnet

Decomposition of DyF3 and its effect on the magnetic performance of the hot-pressed compact and die-upset magnet of melt-spun Nd-Fe-B-type material were investigated. DyF3 was thermally decomposed above 660 °C, and this decomposition was linked closely to the coercivity enhancement. When the DyF3 doped flakes were hot-pressed above the decomposition temperature of DyF3, the diffusion of Dy into the flakes was promoted, and leading to profound coercivity enhancement. Coercivity of the hot-pressed magnet was further enhanced by post-hot-press annealing, and coercivity as high as 24.5 kOe was obtained after the optimum annealing. The DyF3 doped hot-deformed magnet exhibited enhanced magnetic performance (iHc = 17.5 kOe, Br = 12.8 kG, (BH)max = 37.6 MGOe) with respect to the un-doped magnet without sacrificing significant remanence. Coercivity was improved by 30%. In magnet in which the decomposition of DyF3 and Dy diffusion were fully accomplished, the region originally occupied by added DyF3 was completely replaced by NdF3.

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.

Corrosion behavior of hot-pressed nanocrystalline NdFeB magnet in a simulated marine atmosphere

The corrosion behavior of hot-pressed nanocrystalline NdFeB magnet in a NaCl-induced atmosphere with relative humidity of 80% was investigated using SEM/EDS and XPS techniques. The NaCl deposits significantly increased the initiation and development of the corrosion of NdFeB, especially that of Nd. During corrosion process, Nd2Fe14B decomposed into several phases with different Nd contents. The preferentially corroded Nd came from the Nd-rich phase and the decomposition product of Nd2Fe14B which had high Nd content. Moreover, NaCl dramatically accelerated the formation of Nd(OH)3, making it rich in the corrosion films. Nd(OH)3 was unstable in the system. It gradually decomposed into Nd2O3.

Impact of Nd–Cu diffusion on microstructure and coercivity in hot-pressed and die-upset nanocomposite magnets

The microstructure evolution in hot-pressed and die-upset magnets was studied to clarify the impact of Nd-Cu diffusion on microstructure and coercivity. It was found that Nd-Cu diffusion mainly occurs at the interface of the raw powders during the hot-pressing process. In the die-upsetting process, grain boundary diffusion is main reason for the homogeneous distribution of Nd-Cu. Moreover, Nd-Cu grain boundary diffusion decoupling the hard phases, grain morphology and the presence of Nd2O3 contribute to the higher coercivity in die-upset magnets with low height reduction. (C) 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Dysprosium-free melt-spun permanent magnets

Melt-spun NdFeB powders can be formed into a number of different types of permanent magnet for a variety of applications in electronics, automotive and clean technology industries. The melt-spinning process produces flake powder with a fine uniform array of nanoscale Nd2Fe14B grains. These powders can be net-shape formed into isotropic polymer-bonded magnets or hot formed into fully dense magnets. This paper discusses the influence of heavy rare earth elements and microstructure on the magnetic performance, thermal stability and material cost of NdFeB magnets. Evidence indicates that melt-spun nanocrystalline NdFeB magnets are less dependent on heavy rare earth elements for high-temperature performance than the alternative coarser-grained sintered NdFeB magnets. In particular, hot-pressed melt-spun magnets are an attractive low-cost solution for applications that require thermal stability up to 175–200 °C.

Structure and magnetic properties of bulk nanocrystalline Nd-Fe-B permanent magnets prepared by hot pressing and hot deformation

Structure and magnetic properties were studied for bulk nanocrystalline Nd-Fe-B permanent magnets that were prepared at 650 °C for 3 min under 300 MPa using the SPS-3.20-MK-V sintering machine and the hot pressed magnets were then submitted to hot deformation with height reduction of 50%, 60%, 70%, 80%, and 85%. Effects of height reduction (HR) and deformation temperature on the structure and magnetic properties of the magnets were investigated. The crystal structure was evaluated by means of X-ray diffraction (XRD) and the microstructure was observed by transmission electron microscopy (TEM). The magnetic properties of the magnets were investigated by vibrating sample magnetometer (VSM). As the height reduction increased, the remanence (Br) of the magnets increased first, peaks at 1.3 T with HR=60%, then decreased again, and the coercivity (Hci) of the magnets decreased monotonically. On the other hand, as the deformation temperature increased, the Br of the magnets increased first, peaks at 1.36 T with HR=60%, then decreased again, and the Hci of the magnets decreased monotonically. Under optimal conditions, the hot deformed magnet possessed excellent magnetic properties as Br=1.36 T, Hci=1143 kA/m, and (BH)max=370 kJ/m3, suggesting the good potential of the magnets in practical applications.

Mechanism Analysis of Coercivity Enhancement of Hot Deformed Nd-Fe-B Magnets by ${\rm DyF}_{3}$ Diffusion

Isotropic and anisotropic Nd-Fe-B magnets with 2 wt% DyF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> addition are successfully prepared. With the addition of 2 wt% , the coercivity of the hot-pressed magnets increases from 20.52 kOe to 23.13 kOe by increasing the hot-pressed temperature. BSEM results show that most of the DyF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> additions exist in triple junctions and on the surface of ribbons even though the hot-pressed temperature is as high as 983 K. The coercivity of the die-upset magnets increases from 15.18 kOe to 19.14 kOe when 2 wt% DyF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> added. The BSEM results of the hot-deformed magnets show that the content of the DyF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> between ribbons in the die-upset magnets is reduced which indicates that most DyF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> decomposes into Dy and F elements. The Dy atoms penetrate into the Nd <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">14</sub> B phases and (Nd, Dy) <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">14</sub> B phases are formed. The improvement of coercivity of the die-upset magnets is caused by the higher magnetocrystalline anisotropy of (Nd, Dy) <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">14</sub> B.

Structural and Magnetic Study of Hot-Pressed and Hot-Deformed $\hbox{Nd}_{13.5-{\rm x}}\hbox{Ce}_{\rm x}\hbox{Fe}_{80.4}\hbox{Ga}_{0.5}\hbox{B}_{5.6}$ (${\rm x}=0, 0.5, 1$) Prepared by Spark Plasma Sintering

Bulk isotropic and anisotropic Nd-Fe-B magnets were synthesized by hot pressing and subsequent hot deforming melt-spun ribbons with addition of Ce using spark plasma sintering (SPS). It was found that Ce addition had a prominent effect on the bulk magnets by changing the structures of the matrix and boundary phase. With Ce-contents increasing, the magnetic properties of hot-pressed magnets increased first, then dropped and peaked at 0.5at.% Ce addition: B <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">r</sub> = 0.83 T, H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">cj</sub> = 1621 kA/m, (BH) <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">m</sub> = 116 kJ/m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> . However, the magnetic properties of anisotropic magnets deteriorated with the Ce-contents increasing.

Microstructure and Magnetic Properties of Hot-Pressed and Hot-Deformed Composite Magnets Produced by Spark Plasma Sintering Method

Bulk isotropic and anisotropic Nd13.5Fe80.4Ga0.5B5.6 and Nd13.5Fe80.4Ga0.5B5.6/Fe magnets were synthesized by applying spark plasma sintering (SPS) technique. The effect of hot-pressing temperature on the magnetic properties of hot-pressed (HP) and hot-deformed (HD) magnets without additive and with 5% Fe addition was investigated. With increasing sintering temperature for HP magnets, the grain grew gradually. For HD magnets, the optimal magnetic properties could be obtained at hot-pressing temperature 680°C due to the development of desired c-axis texture and uniform microstructure, which resulted from the appropriate and uniform grain size in HP magnets. Fe addition could enhance remanence (Br) and magnetic energy products ((BH)m) of HP and HD magnets. However, the maximum magnetic energy product of HD magnets decreased when hot-pressing temperature was higher than 650°C.

Magnetic domain observation of Nd-Fe-B magnets with submicron-sized grains by high-resolution Kerr microscopy

A Kerr microscope that uses UV light for high-resolution observation of magnetic domains was developed, and the domain structure and magnetization process of hot-pressed magnets prepared from hydrogenation-disproportionation-desorption-recombination (HDDR) processed powders were examined. This microscope is capable of distinguishing nanometer-sized domain patterns. The coercivity of the hot-pressed HDDR magnet depends on the area of magnetization reversal during a reversal process. Magnetization reversal occurs simultaneously in a few grains in a low-coercivity magnet because the Nd-rich phase along the grain boundaries is absent. In contrast, in a high-coercivity magnet, magnetization reversal in a grain independently occurs because of an adequate Nd-rich phase along the grain boundaries. It follows that the high coercivity of an HDDR powder compacted by hot pressing is due to domain wall pinning at the grain boundaries.

Microstructures of Mischmetal–Fe-B Hot-pressed Magnets with Ti Addition

Microstructures of MM-Fe-B hot-pressed magnets with and without Ti addition were investigated. Both materials showed hard magnetic behaviours with more or less same (BH)max of ∼4MGOe. Microstructures were, however, quite different to each other. Grain size of specimen without Ti were ∼500 nm while MM-Fe-B(Ti) showed drastically reduced grain sizes of ∼40nm. Increases in coercivity were found in specimens with Ti. DSC results indicated that Ti triggers and speeds the crystallization of Nd2Fe14B phase when pressure of >1MPa is applied.

Magnetic Properties and Microstructure Studies of Hot-Deformed Nd-Fe-B Magnets With Zr Addition

We synthesized bulk isotropic and anisotropic Nd <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">13.5</sub> Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">77.5</sub> Co <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> B <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> and Nd <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">13.5</sub> Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">76.5</sub> Co <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Zr <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sub> B <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> magnets by spark plasma sintering (SPS). For hot-pressed magnets, the values of remanence (B <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">r</sub> ) and coercivity (H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ci</sub> ) decreased with addition of Zr. After hot-deformation, it was difficult to obtain the desired alignment for Nd <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">13.5</sub> Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">76.5</sub> Co <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Zr <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sub> B <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> magnets, which had a lower degree of texture compared with Nd <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">13.5</sub> Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">77.5</sub> Co <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> B <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> even at a higher than 65% deformation ratio. The research on the microstructure of hot-pressed and hot-deformed magnets shows that Zr addition made the grain finer for hot-pressed magnets and after hot-deforming there were still plenty of areas of poor texture where the content of Zr was higher than 1 at% according to the results of energy-dispersive spectroscopy. It reveals that Zr addition could retard the grain growth and the development of texture during hot-pressing and hot-deforming.

Fluoride-added Pr–Fe–B die-upset magnets with increased electrical resistivity

This work reports the effect of NdF3, DyF3, and CaF2 additions on the electrical resistivity and magnetic properties of Pr–Fe–B hot-pressed and die-upset permanent magnets. Composite magnets were synthesized from ground Pr14.5Fe79.5B6 melt-spun ribbons blended with 5wt% of fluoride fine powders and consolidated by hot pressing at 650°C, followed by die upsetting at 800°C. While CaF2 is stable at the processing temperatures, the rare earth atoms separate from their fluorides to a certain degree with the assistance of the Pr-rich phase from the magnet matrix. Addition of fluorides increased the resistivity of the hot-pressed specimens by more than 200%. The resistivity of the die-upset specimens measured perpendicularly to the direction of the applied pressure, which is also the direction of magnetization, is, however, only slightly increased compared to the magnet counterparts without the fluoride addition. The intrinsic coercivity of Pr14.5Fe79.5B6 die-upset specimens is increased from 14.5kOe to 15.3, 17.1, and 17.7kOe for the addition of CaF2, DyF3, and NdF3, respectively, at a slight expense of the residual flux.

Bulk anisotropy Nd‐Fe‐B/α‐Fe nanocomposite permanent magnets prepared by sonochemistry and spark plasma sintering

AbstractNdFeB/α‐Fe nanocomposite magnetic powders were prepared by sonochemical process. The powders were then submitted to a hot press and subsequent hot deformation process by spark plasma sintering (SPS) technique. Effect of α‐Fe content on structure and magnetic properties of both isotropic and anisotropic magnets was investigated. For hot pressed magnets, the remanence increases with the content of α‐Fe, while the coercive force drops simultaneously. After hot deformation, the magnets with no more than 2 vol% α‐Fe exhibit obvious anisotropic characteristic. For the magnets with more α‐Fe, however, the magnetic anisotropy disappears due to the absence of (00l) crystal texture after deformation. It is, therefore, expected that α‐Fe content plays an important role in the formation of C‐axis crystal texture during hot deformation process. (© 2008 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

High electrical resistance hot-pressed NdFeB magnet for low loss motors

Fluoride-coated hot-pressed NdFeB magnets with electrical resistivity of 1.4mΩcm, which is ten times larger than that of a noncoated sintered NdFeB magnet, were prepared using fluoride coating powders. The high resistive NdFeB magnets consist of coated powders and are sintered with a neodymium fluoride layer, which was grown using a fluoride solution. No degradation of coercivity was observed in the fluoride coated NdFeB powders. The increase in magnet temperature caused by an alternating magnetic field (Eddy currents) was eightfold with the use of a fluoride-treated magnet. Furthermore, the increase in temperature at the magnet’s rotor was reduced by 50% when the high resistive hot-pressed magnet was substituted for the conventional commercial magnet.

Possible methods of recycling NdFeB-type sintered magnets using the HD/degassing process

With the rapid growth in the use of NdFeB-type magnets and with the growing environmental need to conserve both energy and raw materials, the recycling of these magnets is becoming an ever important issue. In this paper it is demonstrated that hydrogen could play a vital role in this process. Fully dense sintered NdFeB-type magnets have been subjected to the hydrogen decrepitation (HD) process. The resultant powder has been subsequently processed in one of two ways in order to produce permanent magnets. Firstly, the powder was subjected to a vacuum degassing treatment over a range of temperatures up to 1000 °C in order to produce powder that would be suitable for the production of anisotropic bonded or hot pressed magnets. Secondly, the HD-powder has been used to produce fully dense sintered magnets; in which case optimisation of the milling time, sintering temperature and time was carried out. The optimum degassing temperature for coercive powder was found to be 700 °C, giving powder with a remanence (Br) of ∼1350 mT (±50 mT) and an intrinsic coercivity (Hcj) of ∼750 kA m −1 (±50 kA m −1). The best sintered magnet was produced by very lightly milling the powder (30 min, roller ball mill), aligning, pressing and vacuum sintering at 1080 °C for 1 h. The magnetic properties of this magnet were: (BH) max = 290 kJ m −3 (±5 kJ m −3), Br = 1240 mT (±50 mT) and Hcj = 830 kA m −1 (±50 kA m −1); representing decreases of 15%, 10% and 20%, respectively, from the properties of the initial magnet.

Hydrogen Decrepitation and Recycling of NdFeB-type Sintered Magnets

With the rapid growth in the use of NdFeB-type magnets and with the growing environmental need to conserve both energy and raw materials, the recycling of these magnets is becoming an ever important issue. In this paper it is demonstrated that hydrogen could play a vital role in this process. Fully dense, sintered NdFeB-type magnets have been subjected to the hydrogen decrepitation (HD) process. The resultant powder has been subsequently processed in one of two ways in order to produce permanent magnets. Firstly, the powder was subjected to a vacuum degassing treatment over a range of temperatures up to 1000°C in order to produce powder that would be suitable for the production of anisotropic bonded or hot pressed magnets. Secondly, the HD-powder has been used to produce fully dense sintered magnets; in which case optimisation of the milling time, sintering temperature and time was carried out. The optimum degassing temperature for coercive powder was found to be 700°C, giving powder with a remanence ( B r) of ∼1350mT (±10 mT) and an intrinsic coercivity ( H cj) of ∼750kAm −1 (±10 kAm −1). The best sintered magnet was produced by very lightly milling the powder (30 min, roller ball mill), aligning, pressing and vacuum sintering at 1080°C for 1 hour. The magnetic properties of this magnet were: ( BH) max = 290 kJm −3 (±5 kJm −3), Sr = 1240mT (±5 mT) and H cj = 830 kAm −1 (±5 kAm −1); representing decreases of 15%, 10%, and 20% respectively, from the properties of the initial magnet.

Magnetic Alignment Study of Hybrid Magnet Consisting of R-Rich and R-Lean Nd-Fe-B Alloys

A hybrid magnet was prepared by the hot-pressing and die-upsetting of the mixture of the R-rich Nd xFe 93.5–xGa 0.5B 6 ( x= 13.5 and 11.8) alloy and the R-lean Nd xFe 93–xNb 1B 6 (x = 6, 9) alloy melt-spun ribbons. The microstructure and magnetic properties of the hybrid magnet were investigated. In the hot-pressed or die-upset hybrid magnet the R-rich and R-lean alloy regions existed independently without alloying between them. The two alloy regions in the die-upset hybrid magnet were coupled effectively via a magnetostatic interaction. A texture was developed only in the R-rich Nd 2Fe 14B single phase alloy region in the die-upset hybrid magnet, and this led to an anisotropic nature in die-upset hybrid magnet. The die-upset hybrid magnets containing higher Nd-content (13.5 at%) host alloy shows consistently a better magnetic alignment with respect to the magnets with lower Nd-content (11.8 at%) host alloy.

Magnetic properties of nd-fe-b HDDR thin hot pressed magnet

Fully dense magnets prepared by hot pressing of the hydrogenation-decomposition-desorption-recombination (HDDR)-processed powders have very fine microstructures. We investigated the magnetic properties and the surface morphology of the machined HDDR thin hot pressed magnets. In the performance of the machined thin hot pressed magnets, it is hard to get the influence of thickness and almost not get the influence of damage by surface roughness. The long-term stabilities were investigated and the total irreversible flux loss of a high coercivity hot pressed magnet at 423 K was -1.3% after 1000 h. The HDDR hot pressed magnets make it possible to stabilize the magnetic properties of thin Nd-Fe-B magnets.