A method for rapid sintering of rare-earth magnet powders based on thermal radiation and / or convection

Abstract

The present invention is a method for rapid sintering of rare-earth comprising powders based on thermal radiation and / or convection for the manufacture of rare-earth permanent magnets. The method is performed in a vacuum of at least 10-1 mbar or in an inert atmosphere and comprises the following steps: a) providing at least one powder compact or porous body made from rare-earth comprising magnet powders and placing said powder compact or porous body in an electrically conductive crucible, wherein said powder compact is insulated from the crucible with a suitable insulator, b) connecting the crucible to electrodes for generating an electrical current resulting in heating of the crucible and through radiation and / or convection also the powder compact, c) heating the crucible up to the sintering temperature, d) optionally maintaining the sintering temperature for at least 1 minute, and e) cooling the crucible and thus the magnet to room temperature.

Description

Field of the invention

[0001] The present invention belongs to the field of devices and methods for manufacturing permanent magnets. The invention relates to a method for rapid sintering of rare-earth comprising powders based on thermal radiation and / or convection for the manufacture of rare-earth permanent magnets.Background of invention, prior art and the technical problem

[0002] A rare-earth magnet is a strong permanent magnet based on rare-earth containing materials. There are three types: Neodymium-iron-boron (Nd-Fe-B), samarium-cobalt (Sm-Co), and samarium-iron nitride (Sm-Fe-N) magnets. Nd-Fe-B magnets are most commonly used rare-earth magnets. They are polycrystalline, multiphase materials predominantly composed of the Nd 2 Fe 14 B hard-magnetic phase, accompanied by secondary phases enriched in neodymium. Their magnetic performance is intricately tied to their microstructural specifics and chemical composition. The nominal chemical composition is often complex and tailored for specific applications. The general term "Nd-Fe-B" encompasses a diverse array of Nd-Fe-B-like materials, wherein Nd can be partially substituted by other rare-earth elements such as Pr, Dy, Tb, and Ce. Additionally, other alloying elements, for example, Al, Cu, Ni, and Co are frequently incorporated to refine the material's microstructure and enhance its magnetic performance. The phase composition of Nd-Fe-B magnets is primarily influenced by the nominal chemical composition of the Nd-Fe-B alloy and by the thermal processing history of the material. Additionally, it can be tailored by modifying the phase composition of the initial powders. One approach to achieve this involves introducing secondary powders, for example, metal hydrides, rare-earth fluorides, etc., to the primary Nd-Fe-B powder through powder blending or mixing before sintering (Mottram et al., 1999: The use of metal hydrides in powder blending for the production of NdFeB-type magnets, Journal of Alloys and Compounds 283(1-2): 282-288). In a different approach, the deposition of tantalum thin film on Nd-Fe-B powders by magnetron sputtering was demonstrated (Kimura et al., 2019: Fabrication of Anisotropic Nd-Fe-B Powders by Ta Sputtering, Materials Transactions 60(5): 830-836). To achieve Nd 2 Fe 14 B stoichiometry in an Nd-Fe-B powder, selective leaching by weak organic acids was used to remove the secondary phases from fresh jet-milled powders or recycled Hydrogen Processing of Magnet Scrap (HPMS) powders (Mishra et al., 2023: Short-Loop Recycling of Nd-Fe-B Permanent Magnets: A Sustainable Solution for the RE2Fe14B Matrix Phase Recovery, Materials 16(19): 6565). Similarly, a selective electrochemical leaching approach has been demonstrated to remove the secondary phases and reclaim Nd 2 Fe 14 B grains from bulk-sintered magnets.

[0003] Nd-Fe-B permanent magnets find extensive applications in various fields such as hard disk drives, magnetic resonance imaging (MRI), air-conditioning systems, energy-efficient home appliances, sensors, and play integral roles in renewable energy and future mobility systems, including generators (wind turbines, hydroelectric) and traction motors for electric vehicles (EVs). An important figure of merit for a magnet's performance is maximum energy product - (BH) max which is associated with the energy stored in the stray field that a magnet generates. The highest (BH) max values are attained in fully dense, metallic, magnetically anisotropic magnets prepared through the sintering of finely milled anisotropic powders (Brown, et al., 2002: Developments in the processing and properties of NdFeb-type permanent magnets, Journal of Magnetism and Magnetic Materials 248(3): 432-440). Driven by the global shift towards sustainability, there is a rapid increase in the demand for sintered Nd-Fe-B magnets. However, conventional sintering poses challenges due to its time-consuming and energy-intensive nature. This process involves slow heating to a sintering temperature typically within the range of 1000-1100 °C, with prolonged dwell times at these elevated temperatures, wherein the total heating time can exceed 20 hours (Zakotnik et al., 2016). In the production of sintered Nd-Fe-B magnets, milling and sintering stand as the second and third largest energy consumers, respectively, collectively rivaling the energy consumption of the casting process (Zakotnik et al., 2016: Analysis of energy usage in Nd-Fe-B magnet to magnet recycling, Environmental Technology & Innovation 5: 117-126). To address this issue and align with eco-friendly manufacturing practices, it becomes crucial to devise a sintering strategy characterized by short sintering cycles. Such a strategy should also be adaptable for consolidating coarser powders, thereby reducing the energy consumption associated with the production of high-performance sintered Nd-Fe-B magnets.

[0004] The manufacturing process of bulk Nd-Fe-B magnets encompasses the production of powders and their subsequent consolidation into solid bulk components. Various techniques for powder production and consolidation have been developed to achieve specific magnetic properties. Nanostructured polycrystalline powders are created through the rapid solidification of the melt using a melt-spinning process. These powders are crystallographically untextured and consequently magnetically isotropic, thereby limiting their remanent magnetization - M r to around 0.8 T, which is half of the saturation magnetization - M S of the Nd 2 Fe 14 B hard-magnetic phase. To address this limitation, the hydrogenation-disproportionation-desorption-recombination (HDDR) process was introduced. This method produces magnetically anisotropic, nanostructured, polycrystalline powders from microcrystalline precursors, such as cast ingots or end-of-life (EOL) sintered Nd-Fe-B magnets. HDDR powders can achieve M r values exceeding 1.3 T. For most applications, both melt-spun powders and HDDR powders are typically mixed with a binder, which may be a resin or a polymer. Subsequently, they are pressed into either isotropic bonded magnets for melt-spun powders or anisotropic bonded magnets for HDDR powders. The inclusion of a binder in the process reduces the M r values, limiting the (BH) max values of bonded magnets to around 140 kJ / m 3< (Brown et al., 2002).

[0005] In the conventional powder metallurgy approach, microcrystalline powders are generated through strip casting. Subsequently, a hydrogen decrepitation process is employed to break the large strip-cast flakes into a coarse and friable powder. This is followed by jet-milling to reduce the particle size, typically below 10 µm, resulting in a highly anisotropic, near-monocrystalline powder (Brown et al., 2022). The mean particle size of these powders is up to two orders of magnitude smaller than that of melt-spun or HDDR powders. Following jet-milling, the powders are magnetically aligned in an external magnetic field and then pressed into a powder compact using either uniaxial or isostatic pressing. The conventional sintering process for powder compacts into fully dense, metallic, textured Nd-Fe-B magnets is typically conducted in a vacuum. Sintering involves extended dwell times at sintering temperatures, usually two hours or more, with several isothermal steps at intermediate temperatures. The heating rate from room temperature to sintering temperature is typically close to 2 °C / min. Notably, the M r values of advanced sintered Nd-Fe-B magnets surpass 1.4 T, resulting in (BH) max values exceeding 400 kJ / m 3< .

[0006] The growing demand for Nd-Fe-B magnets underscores the need for sustainable magnet production paradigms, including advancements in recycling strategies for end-of-life (EOL) magnets and production waste, which constitutes 20-30% of the starting alloy. HPMS presents a straightforward process involving the pulverization of bulk Nd-Fe-B scrap with hydrogen gas. However, HPMS powders initially have a coarse particle size in the range of hundreds of micrometers, rendering them unsuitable for conventional sintering. To address this, these recycled powders undergo further milling to achieve a particle size comparable to that of fresh powders prepared through the conventional powder metallurgy approach. Only after this refinement process can the HPMS powders be re-sintered effectively (Zakotnik and Tudor, 2015: Commercial-scale recycling of NdFeB-type magnets with grain boundary modification yields products with 'designer properties' that exceed those of starting materials, Waste management 44: 48-54).

[0007] In comparison to established conventional sintering methods, the reduction of heating times is achieved by employing electrical fields or currents to enhance densification. Modern sintering methods demonstrating this approach include ultrafast high-temperature sintering (Wang et al., 2020: A general method to synthesize and sinter bulk ceramics in seconds, Science 368(6490): 521-526), flash sintering (Cologna et al., 2010: Flash sintering of nanograin zirconia in< 5 s at 850 C, Journal of the American Ceramic Society 93(11):3556-3559), and spark-plasma sintering (SPS) (Guillon et al., 2014: Field-assisted sintering technology / spark plasma sintering: mechanisms, materials, and technology developments, Advanced Engineering Materials 16(7): 830-849). SPS is an electrical current-activated, pressure-assisted technique operating in a high-current, low-voltage regime. In SPS, the sample-containing die heats up through a pulsed DC, enabling direct Joule heating of electrically conductive materials. With SPS, Nd-Fe-B magnets based on nanostructured melt-spun powders and HDDR powders have been successfully prepared within 15 minutes of heating time ( Saito et al., 2005: Magnetic properties of Nd-Fe-Co-Ga-B magnets produced by spark plasma sintering method, Journal of applied physics 97(10); Suresh et al., 2009: Consolidation of hydrogenation-disproportionation-desorption-recombination processed Nd-Fe-B magnets by spark plasma sintering, Journal of magnetism and magnetic materials 321(22): 3681-3686). However, the sintering of anisotropic near-monocrystalline jet-milled powders via SPS approach has presented challenges. Firstly, direct Joule heating of the jet-milled Nd-Fe-B material can facilitate the decomposition of the Nd 2 Fe 14 B hard-magnetic phase due to localized overheating at the particle-particle contacts. Secondly, the uniaxial pressure applied on the powder compact during SPS degrades crystallographic texture, which decreases the M r by more than 10% compared to conventional sintering (Tomse et al., 2020: A spark-plasma-sintering approach to the manufacture of anisotropic Nd-Fe-B permanent magnets, Journal of Magnetism and Magnetic Materials 502: 166504). Concerning the rapid heating of an electrically conductive die by the Joule effect, it was demonstrated on ceramic materials that the powder compact can be heated indirectly through thermal radiation if it is thermally and electrically insulated from the die. Materials sintered via thermal radiation include ceramics, namely zirconia (Salamon et al., 2012: Rapid sintering of crack-free zirconia ceramics by pressure-less spark plasma sintering, Scripta Materialia 66(11): 899-902), alumina (Hofer et al., 2022: High-strength lithography-based additive manufacturing of ceramic components with rapid sintering, Additive Manufacturing 59: 103141), potassium sodium niobate (Kuscer et al., 2019: Evolution of phase composition and microstructure of sodium potassium niobate-based ceramic during pressure-less spark plasma sintering and post-annealing, Ceramics International 45(8): 10429-10437), CrB 2 ( Sairam et al., 2016: Pressureless sintering of chromium diboride using spark plasma sintering facility, International Journal of Refractory Metals and Hard Materials 58:165-171), SiC (Li and Shen, 2015: Sintering by intense thermal radiation (SITR): A study of temperature distribution by simulation and experiments, Journal of the European Ceramic Society 35(12): 3303-3309), Si 3 N 4 foams (Li et al., 2014: Rapid sintering of silicon nitride foams decorated with one-dimensional nanostructures by intense thermal radiation, Science and Technology of Advanced Materials), and porous 3D printed titanium structures (Kashimbetova et al., 2023: Pressure-less spark plasma sintering of 3D-plotted titanium porous structures, Journal of Materials Research Technology 22: 2147-2157). A similar thermal-radiation-sintering approach enabling fast consolidation of rare-earth comprising magnet powders under pressureless conditions has not been developed thus far.

[0008] In summary, the conventional sintering approach for producing anisotropic rare-earth comprising magnet powders and particularly Nd-Fe-B permanent magnets is a time-consuming and energy-intensive process that is well-suited for fine-milled powders. While contemporary sintering strategies like SPS offer the potential for reducing heating times and consolidating coarser powders, the combination of direct Joule heating and external pressure applied on the powder compact proves unsuitable for processing anisotropic microcrystalline rare-earth comprising magnet powders, including Nd-Fe-B powders.Description of the invention

[0009] Given the challenges associated with the manufacture of sintered rare-earth magnets, particularly anisotropic Nd-Fe-B magnets, it is an object of the present invention to provide a faster and more energy-efficient sintering procedure for rare-earth comprising magnet powders, preferably Nd-Fe-B-type powders or powder mixtures of any nominal chemical composition, including powders that cannot be sintered conventionally. The technical problem is solved as defined in the independent claim, wherein the preferred embodiments are defined in dependent claims.

[0010] In the method according to the invention, a fast-sintering strategy is employed to enable fast sintering of rare-earth comprising magnet powders, particularly Nd-Fe-B-type powders or powder mixtures based on thermal radiation and / or convection. The method according to the invention is performed in a vacuum or in an inert atmosphere, wherein the method essentially comprises the following steps: a) providing at least one powder compact or porous body made from rare-earth comprising magnet powders and placing said powder compact or porous body in an electrically conductive crucible, wherein said powder compact is insulated from the crucible with a suitable insulator, b) connecting the crucible to electrodes for generating an electrical current resulting in heating of the crucible and through radiation and / or convection also the powder compact, c) heating the crucible up to the sintering temperature from 600 to 1300 °C, d) optionally maintaining the sintering temperature for at least 1 minute, preferably up to 180, more preferably up to 60 minutes, most preferably from 1 to 30 minutes, and e) cooling the crucible and thus the magnet, preferably by terminating the heating and leaving the crucible and the resulting magnet to cool to room temperature.

[0011] A low-density powder compact is prepared by compressing an Nd-Fe-B-type powder or a powder mixture under external pressure or through 3D printing to form the porous body. Before compaction, the powders can be aligned in an external magnetic field. The powder compact or the porous body is placed into an electrically conductive crucible. The material of the crucible is arbitrary, as long as it is electrically conductive. Preferably, graphite is used due to its low price. Other materials such as tungsten carbide or steel may also be used. No mechanical pressure is applied on the powder compact during heating; however, pressure can be applied on the device so as to improve contacts between the electrodes and the crucible.

[0012] Insulating material is provided between the crucible and the powder compact or the porous body. The insulating material should have low thermal and / or electrical conductivity. Preferably, graphite felt is used. The insulating material, in case it is not inert, may also be provided with a protective layer or coating, such as a thermally-stable and inert coating or foil. Tantal, volfram or steel foil may be used or a boron nitride coating, which does not react with Nd-Fe-B and shows required stability at sintering temperatures.

[0013] The crucible is heated through resistive heating by the Joule effect. Joule heating is the physical effect by which the pass of current through an electrical conductor produces thermal energy, which results a rise in the conductor material temperature. The powder compact or the porous body is electrically insulated from the crucible in a way that the electrical current flows through the crucible. The intensity of the electrical current and electric potential difference can be preset. The applied power (P) is controlled by the electrical current (I) and voltage (U) / potential difference. (P=U*I). Alternatively, the intensity of the electrical current and electric potential difference is temperature-controlled by measuring the temperature of the crucible or the sample. The crucible acts as a heat source, i.e., a radiator. The heat is transferred from the crucible to the powder compact or the porous body mainly through thermal radiation. To achieve this, the heating process is performed in a vacuum of at least 10 -1< mbar. However, in an inert atmosphere, such as any noble gases or nitrogen, in addition to thermal radiation heat may also be transferred through thermal convection. This highly efficient heat transfer enables high heating rates, which preferably range between 10 to 500 °C / min (for example up to several 100s °C / min) from room temperature to the maximum temperature of the radiator, i.e., sintering temperature, and short dwell times (in the order of minutes), i.e., isothermal steps, at sintering temperatures. The heating rate, sintering temperature, and dwell time are freely varied. The heating rate may also be changed during the heating, for example by reducing the heating rate towards the desired sintering temperature.

[0014] The cooling stage in the above-mentioned step e) may be controlled or uncontrolled. The latter is achieved by simply terminating heating of the crucible, wherein the crucible and the powder compact or the sintered porous body cool at their own speed. Alternatively, the crucible may be cooled at any desired cooling rate, which preferably range between 10 to 500 °C / min, by reducing the electrical current flowing through the crucible. The cooling rate may be constant or variable, similarly as has been described above for the heating rate.

[0015] Potential powders can be i) anisotropic microcrystalline powders, for example, standard powders prepared using a conventional powder-metallurgy method based on strip casting, hydrogen decrepitation, and jet milling, ii) HPMS-type powders, iii) HDDR-type powders, iv) HPMS-type or HDDR-type powders further processed by milling to reduce the mean particle size, v) any of the above further processed to modify their phase composition, for example, by selective acid leaching to obtain powders with stoichiometric Nd 2 Fe 14 B-like composition or deposition of metallic phases via magnetron sputtering or similar techniques, vi) mixture of one or several Nd-Fe-B-type powders and one or several non-magnetic powders.

[0016] A device for performing the above-described method according to the invention comprises at least the following: a housing or a chamber ensuring vacuum or inert atmosphere, inside which at least the following is provided: ∘ at least one electrically conductive crucible having a space arranged to receive at least one powder compact or porous body, wherein said crucible is connectable to suitable electrodes arranged to allow heating of the crucible, ∘ optionally spacers, preferably graphite spacers, installed between the electrodes and the crucible, ∘ an insulating material with low electric and / or thermal conductivity placed on the bottom of the space arranged to receive at least one powder compact or porous body, wherein said insulating material is preferably a graphite felt, said insulating material provided with a protective coating, such as a boron-nitride coating for ensuring that the materials of the powder compact or the porous body are not contaminated with the insulating material or any components thereof.

[0017] The device may further comprise a measuring device for measuring temperature of the crucible, for example a pyrometer.

[0018] Optionally, pressure can be applied on the device for ensuring better contacts between the crucible and the electrodes.

[0019] The magnets may be prepared in any shape, which allows a variety of possible uses.Brief description of the drawings

[0020] The method will be described in further detail based on exemplary embodiments, examples and figures, which show: Figure 1A schematic illustration of an experimental setup allowing thermal-radiation-based heating to ensure fast and effective heating of a powder compact according to the invention Figure 2Scanning electron microscope images of (a) jet-milled Nd-Fe-B-type powder prepared by conventional powder metallurgy method and (b) bulk sintered magnet prepared from the jet-milled powder, (c) HPMS powder and (d) bulk sintered magnet prepared from the HPMS powder, (e) jet-milled HPMS powder and (f) bulk sintered magnet prepared from the jet-milled HPMS powder mixed with 1 wt% addition of Nd-hydride powder and 1 wt% addition of TbF 3 powder Figure 3Example of a sintering cycle with an average heating rate 150 °C / min from room temperature to 1100 °C and 2 minutes dwell time at 1100 °C, showing the electrical current intensity profile (1) and measured temperature of the heater (2) Figure 4Second quadrant demagnetization curves of bulk sintered Nd-Fe-B magnets prepared from different types of powders Detailed description of the invention

[0021] In the following examples of the method according to the invention bulk Nd-Fe-B-type magnets are manufactured from different powder types by employing a thermal-radiation-based sintering approach. As mentioned above, the methods according to the invention may be performed in vacuum or in an inert atmosphere. In the examples only methods performed in vacuum are discussed, which does not limit the possibility to replace vacuum with an inert atmosphere.POWDER MATERIALS

[0022] Three types of Nd-Fe-B-type powders were used.

[0023] The first powder (powder 1) was anisotropic powder with the composition Nd 30.0 Pr 0.6 Dy 1.0 Fe 63.8 Co 3.0 Ga 0.2 Cu 0.1 Al 0.1 B 0.9 O 0.3 (wt.%), supplied by Magneti Ljubljana d.d (Ljubljana, Slovenia). It was prepared using a conventional powder-metallurgy method: the alloy underwent strip casting, and the resulting flakes were subsequently decrepitated with hydrogen to yield a coarse and friable powder, followed by jet milling with nitrogen.

[0024] The second powder (powder 2) was an HPMS-type powder with the composition Nd 24.3 Pr 0.2 Dy 3.7 Fe 70.1 Al 0.3 B 0.9 O 0.5 (wt.%), provided by Hochschule Pforzheim. It was prepared from end-of-life magnets originating from wind turbine generators.

[0025] The third powder (powder 3) was prepared from the second powder by milling it in a jet mill using argon gas to reduce its particle size. Terbium(III) fluoride powder (TbFs, >99%) was purchased from Sigma-Aldrich, USA. Neodymium(III) hydride powder (NdHs, >99%) was purchased from American Elements, USA.PREPARATION OF POWDER COMPACTS

[0026] To prepare powder compacts for sintering experiments, an Nd-Fe-B-type powder or a powder mixture was placed into a cylindrical silicon mold under a protective argon atmosphere and vacuum sealed. Powder mixtures were prepared through a powder blending approach by weighing and homogenizing the powders under an inert atmosphere using mortar and pestle. In the next step, a vacuum-sealed Nd-Fe-B-type powder or a powder mixture was magnetically aligned in an external pulsed magnetic field with 6 T magnitude and isostatically pressed under 800 MPa to produce a magnetically aligned powder compact with dimensions 15 (diameter) × 12 (height) mm and approx. 55% theoretical density.EXPERIMENTAL SETUP

[0027] Concerning the schematic illustration in Figure 1 (cross-section), sintering was performed in a commercially available laboratory-scale furnace (SPS-632LxEx, Dr. Sinter, SPS Syntex Inc., Japan) under vacuum. A powder compact 1 was placed in a custom-made graphite crucible 2 with dimensions 30.0 mm (inner diameter) × 23.4 mm (height) on a 6 mm thick graphite felt 3. Graphite felt was covered with ≈0.2 mm thick boron nitride (BN) coating 4 to prevent reaction between Nd-Fe-B and graphite. Graphite spacers 5 and 6 were placed between the crucible and electrodes 7. 1 kN force was applied uniaxially on the setup to ensure good electrical contact. Electrical current (DC) with a 12:2 on-off pulse sequence (12 × 3.3 ms ≈0.04 s of current, followed by 2 × 3.3 ms ≈0.007 s of pause) was applied to heat the crucible through Joule heating. The powder compact was smaller than the inner diameter of the crucible and was not in thermal contact with the walls of the crucible. Due to the low thermal and electrical conductivity of the graphite felt, it is assumed that thermal radiation is the only factor influencing the sample temperature. The crucible acted as a heat source, i.e., the radiator. The heating of the crucible was initiated at room temperature (RT). The temperature was measured on the external surface of the wall of the crucible at the sample height 8 with a pyrometer. Below the visible range of the pyrometer at ≈575 °C, the electrical current increase was preset. Above the visible range, the heating was temperature-controlled. The sintering temperature was 1100 °C. Considering the limitations of the furnace, the heating rate from room temperature to sintering temperature was varied from 10 to 500 °C / min. Depending on the heating rate and choice of the initial Nd-Fe-B powder type, dwell time at sintering temperature was needed to fully densify the compact. After the dwell time, the heating was terminated and the experimental setup was left to cool to room temperature.Examples EXAMPLE 1:

[0028] Powder 1 was sintered using the setup in Fig. 1. As shown in Fig. 2a (BSE-SEM image), the powder is near-monocrystalline, consisting of Nd 2 Fe 14 B matrix phase (grey) and Nd-rich secondary phases (bright contrast). The mean powder particle size is 6.5 µm. Fig. 2b (BSE-SEM image) shows the microstructure of a bulk sintered magnet prepared from powder 1. The magnet was prepared at 1100 °C of sintering temperature by employing 150 °C / min of heating rate from room temperature to sintering temperature and 2 minutes of dwell time. Like the initial powder, the microstructure of the magnet consists of the Nd 2 Fe 14 B matrix phase (grey) and Nd-rich secondary phases (bright contrast). The magnet's density exceeds 99% of the theoretical density. Fig. 3 shows the electrical current intensity profile (1) and the measured temperature of the crucible (2) for the corresponding heating cycle. The electrical current continuously increases from the start of heating to the sintering temperature, with a minor setback at approx. 600 °C when the pyrometer starts recording data and the heating process becomes temperature-controlled. The current peaks at 1450 A at the beginning of the short two-minute dwell time. The total heating time was 9.2 minutes. Several bulk magnets were prepared at 1100 °C of sintering temperature while the heating rate varied from 10 to 500 °C / min. Fig. 4 shows the corresponding demagnetization curves of magnets, prepared with 10 °C / min (A), 50 °C / min (B), 150 °C / min (C), and 500 °C / min (D) heating rate. Magnetic measurements were performed by applying a closed-loop hysteresisgraph (Permagraph, Magnet-Physik) at room temperature. The magnetic properties of the magnets were as follows: 1.32-1.35 T of remanent magnetization, 342-352 kJ / m 3< of maximal energy product, and 575-710 kA / m of intrinsic coercivity.EXAMPLE 2

[0029] Powder 2 was sintered using the setup in Fig. 1. As seen in Fig. 2c (SEM image), the powder is polycrystalline, with a broad particle size ranging up to ≈500 µm. The microstructure of an Nd-Fe-B bulk sintered magnet prepared from powder 2 is shown in Fig. 2d (BSE-SEM image). The magnet was prepared at 1100 °C by employing 50 °C / min of heating rate and 15 minutes of dwell time. The total heating time was 36.5 minutes. The magnet's density exceeds 99% of the theoretical density. Its microstructure consists of Nd 2 Fe 14 B matrix phase (grey) and Nd-rich secondary phases (bright contrast). Magnetic measurement of the magnet was performed by applying a closed-loop hysteresisgraph (Permagraph, Magnet-Physik) at room temperature. Fig. 4 shows the corresponding demagnetization curve (E). The magnet's magnetic properties are 1.04 T of remanent magnetization, 180 kJ / m 3< of maximal energy product, and 890 kA / m of intrinsic coercivity.EXAMPLE 3

[0030] Powder 3 was sintered using the setup in Fig. 1. As seen in Fig. 2e (SEM image), milling of powder 2 broke down the large polycrystalline particles shown in Fig. 2c into individual grains via intergranular fracturing. Three powder compacts were prepared from powder 3 with and without the addition of secondary powders. The first powder compact was prepared without the addition of secondary powders. The second powder compact was prepared with 1 wt% addition of NdH 3 powder. The third powder compact was prepared with a 1 wt% addition of NdH 3 powder and a 1 wt% addition of TbF 3 powder. The powder compacts were sintered at 1100 °C by employing 50 °C / min of heating rate and 5 minutes of dwell time. The total heating time was 26.5 minutes. The microstructure of a magnet prepared from a powder compact with 1 wt% addition of NdH 3 powder and 1 wt% addition of TbF 3 powder is shown in Fig. 2f (BSE-SEM image). The magnet's density is more than 99% of the theoretical density. Its microstructure consists of Nd 2 Fe 14 B matrix phase (grey) and Nd-rich secondary phases (bright contrast). Fig. 4 shows the corresponding demagnetization curve (F). The magnet's magnetic properties are 1.27 T of remanent magnetization and 295 kJ / m 3< of maximal energy product. Magnetic measurement was performed by applying a closed-loop hysteresis graph (Permagraph, Magnet-Physik) at room temperature. Intrinsic coercivity value was not obtained with this measurement due to measuring limitations of the hysteresisgraph.

Examples

example 1

[0028]Powder 1 was sintered using the setup in Fig. 1. As shown in Fig. 2a (BSE-SEM image), the powder is near-monocrystalline, consisting of Nd 2 Fe 14 B matrix phase (grey) and Nd-rich secondary phases (bright contrast). The mean powder particle size is 6.5 µm. Fig. 2b (BSE-SEM image) shows the microstructure of a bulk sintered magnet prepared from powder 1. The magnet was prepared at 1100 °C of sintering temperature by employing 150 °C / min of heating rate from room temperature to sintering temperature and 2 minutes of dwell time. Like the initial powder, the microstructure of the magnet consists of the Nd 2 Fe 14 B matrix phase (grey) and Nd-rich secondary phases (bright contrast). The magnet's density exceeds 99% of the theoretical density. Fig. 3 shows the electrical current intensity profile (1) and the measured temperature of the crucible (2) for the corresponding heating cycle. The electrical current continuously increases from the start of heating to the sintering temperatu...

example 2

EXAMPLE 2

[0029]Powder 2 was sintered using the setup in Fig. 1. As seen in Fig. 2c (SEM image), the powder is polycrystalline, with a broad particle size ranging up to ≈500 µm. The microstructure of an Nd-Fe-B bulk sintered magnet prepared from powder 2 is shown in Fig. 2d (BSE-SEM image). The magnet was prepared at 1100 °C by employing 50 °C / min of heating rate and 15 minutes of dwell time. The total heating time was 36.5 minutes. The magnet's density exceeds 99% of the theoretical density. Its microstructure consists of Nd 2 Fe 14 B matrix phase (grey) and Nd-rich secondary phases (bright contrast). Magnetic measurement of the magnet was performed by applying a closed-loop hysteresisgraph (Permagraph, Magnet-Physik) at room temperature. Fig. 4 shows the corresponding demagnetization curve (E). The magnet's magnetic properties are 1.04 T of remanent magnetization, 180 kJ / m 3< of maximal energy product, and 890 kA / m of intrinsic coercivity.

example 3

EXAMPLE 3

[0030]Powder 3 was sintered using the setup in Fig. 1. As seen in Fig. 2e (SEM image), milling of powder 2 broke down the large polycrystalline particles shown in Fig. 2c into individual grains via intergranular fracturing. Three powder compacts were prepared from powder 3 with and without the addition of secondary powders. The first powder compact was prepared without the addition of secondary powders. The second powder compact was prepared with 1 wt% addition of NdH 3 powder. The third powder compact was prepared with a 1 wt% addition of NdH 3 powder and a 1 wt% addition of TbF 3 powder. The powder compacts were sintered at 1100 °C by employing 50 °C / min of heating rate and 5 minutes of dwell time. The total heating time was 26.5 minutes. The microstructure of a magnet prepared from a powder compact with 1 wt% addition of NdH 3 powder and 1 wt% addition of TbF 3 powder is shown in Fig. 2f (BSE-SEM image). The magnet's density is more than 99% of the theoretical densi...

Field of the invention

[0001] The present invention belongs to the field of devices and methods for manufacturing permanent magnets. The invention relates to a method for rapid sintering of rare-earth comprising powders based on thermal radiation and / or convection for the manufacture of rare-earth permanent magnets.Background of invention, prior art and the technical problem

[0002] A rare-earth magnet is a strong permanent magnet based on rare-earth containing materials. There are three types: Neodymium-iron-boron (Nd-Fe-B), samarium-cobalt (Sm-Co), and samarium-iron nitride (Sm-Fe-N) magnets. Nd-Fe-B magnets are most commonly used rare-earth magnets. They are polycrystalline, multiphase materials predominantly composed of the Nd 2 Fe 14 B hard-magnetic phase, accompanied by secondary phases enriched in neodymium. Their magnetic performance is intricately tied to their microstructural specifics and chemical composition. The nominal chemical composition is often complex and tailored for specific applications. The general term "Nd-Fe-B" encompasses a diverse array of Nd-Fe-B-like materials, wherein Nd can be partially substituted by other rare-earth elements such as Pr, Dy, Tb, and Ce. Additionally, other alloying elements, for example, Al, Cu, Ni, and Co are frequently incorporated to refine the material's microstructure and enhance its magnetic performance. The phase composition of Nd-Fe-B magnets is primarily influenced by the nominal chemical composition of the Nd-Fe-B alloy and by the thermal processing history of the material. Additionally, it can be tailored by modifying the phase composition of the initial powders. One approach to achieve this involves introducing secondary powders, for example, metal hydrides, rare-earth fluorides, etc., to the primary Nd-Fe-B powder through powder blending or mixing before sintering (Mottram et al., 1999: The use of metal hydrides in powder blending for the production of NdFeB-type magnets, Journal of Alloys and Compounds 283(1-2): 282-288). In a different approach, the deposition of tantalum thin film on Nd-Fe-B powders by magnetron sputtering was demonstrated (Kimura et al., 2019: Fabrication of Anisotropic Nd-Fe-B Powders by Ta Sputtering, Materials Transactions 60(5): 830-836). To achieve Nd 2 Fe 14 B stoichiometry in an Nd-Fe-B powder, selective leaching by weak organic acids was used to remove the secondary phases from fresh jet-milled powders or recycled Hydrogen Processing of Magnet Scrap (HPMS) powders (Mishra et al., 2023: Short-Loop Recycling of Nd-Fe-B Permanent Magnets: A Sustainable Solution for the RE2Fe14B Matrix Phase Recovery, Materials 16(19): 6565). Similarly, a selective electrochemical leaching approach has been demonstrated to remove the secondary phases and reclaim Nd 2 Fe 14 B grains from bulk-sintered magnets.

[0003] Nd-Fe-B permanent magnets find extensive applications in various fields such as hard disk drives, magnetic resonance imaging (MRI), air-conditioning systems, energy-efficient home appliances, sensors, and play integral roles in renewable energy and future mobility systems, including generators (wind turbines, hydroelectric) and traction motors for electric vehicles (EVs). An important figure of merit for a magnet's performance is maximum energy product - (BH) max which is associated with the energy stored in the stray field that a magnet generates. The highest (BH) max values are attained in fully dense, metallic, magnetically anisotropic magnets prepared through the sintering of finely milled anisotropic powders (Brown, et al., 2002: Developments in the processing and properties of NdFeb-type permanent magnets, Journal of Magnetism and Magnetic Materials 248(3): 432-440). Driven by the global shift towards sustainability, there is a rapid increase in the demand for sintered Nd-Fe-B magnets. However, conventional sintering poses challenges due to its time-consuming and energy-intensive nature. This process involves slow heating to a sintering temperature typically within the range of 1000-1100 °C, with prolonged dwell times at these elevated temperatures, wherein the total heating time can exceed 20 hours (Zakotnik et al., 2016). In the production of sintered Nd-Fe-B magnets, milling and sintering stand as the second and third largest energy consumers, respectively, collectively rivaling the energy consumption of the casting process (Zakotnik et al., 2016: Analysis of energy usage in Nd-Fe-B magnet to magnet recycling, Environmental Technology & Innovation 5: 117-126). To address this issue and align with eco-friendly manufacturing practices, it becomes crucial to devise a sintering strategy characterized by short sintering cycles. Such a strategy should also be adaptable for consolidating coarser powders, thereby reducing the energy consumption associated with the production of high-performance sintered Nd-Fe-B magnets.

[0004] The manufacturing process of bulk Nd-Fe-B magnets encompasses the production of powders and their subsequent consolidation into solid bulk components. Various techniques for powder production and consolidation have been developed to achieve specific magnetic properties. Nanostructured polycrystalline powders are created through the rapid solidification of the melt using a melt-spinning process. These powders are crystallographically untextured and consequently magnetically isotropic, thereby limiting their remanent magnetization - M r to around 0.8 T, which is half of the saturation magnetization - M S of the Nd 2 Fe 14 B hard-magnetic phase. To address this limitation, the hydrogenation-disproportionation-desorption-recombination (HDDR) process was introduced. This method produces magnetically anisotropic, nanostructured, polycrystalline powders from microcrystalline precursors, such as cast ingots or end-of-life (EOL) sintered Nd-Fe-B magnets. HDDR powders can achieve M r values exceeding 1.3 T. For most applications, both melt-spun powders and HDDR powders are typically mixed with a binder, which may be a resin or a polymer. Subsequently, they are pressed into either isotropic bonded magnets for melt-spun powders or anisotropic bonded magnets for HDDR powders. The inclusion of a binder in the process reduces the M r values, limiting the (BH) max values of bonded magnets to around 140 kJ / m 3< (Brown et al., 2002).

[0005] In the conventional powder metallurgy approach, microcrystalline powders are generated through strip casting. Subsequently, a hydrogen decrepitation process is employed to break the large strip-cast flakes into a coarse and friable powder. This is followed by jet-milling to reduce the particle size, typically below 10 µm, resulting in a highly anisotropic, near-monocrystalline powder (Brown et al., 2022). The mean particle size of these powders is up to two orders of magnitude smaller than that of melt-spun or HDDR powders. Following jet-milling, the powders are magnetically aligned in an external magnetic field and then pressed into a powder compact using either uniaxial or isostatic pressing. The conventional sintering process for powder compacts into fully dense, metallic, textured Nd-Fe-B magnets is typically conducted in a vacuum. Sintering involves extended dwell times at sintering temperatures, usually two hours or more, with several isothermal steps at intermediate temperatures. The heating rate from room temperature to sintering temperature is typically close to 2 °C / min. Notably, the M r values of advanced sintered Nd-Fe-B magnets surpass 1.4 T, resulting in (BH) max values exceeding 400 kJ / m 3< .

[0006] The growing demand for Nd-Fe-B magnets underscores the need for sustainable magnet production paradigms, including advancements in recycling strategies for end-of-life (EOL) magnets and production waste, which constitutes 20-30% of the starting alloy. HPMS presents a straightforward process involving the pulverization of bulk Nd-Fe-B scrap with hydrogen gas. However, HPMS powders initially have a coarse particle size in the range of hundreds of micrometers, rendering them unsuitable for conventional sintering. To address this, these recycled powders undergo further milling to achieve a particle size comparable to that of fresh powders prepared through the conventional powder metallurgy approach. Only after this refinement process can the HPMS powders be re-sintered effectively (Zakotnik and Tudor, 2015: Commercial-scale recycling of NdFeB-type magnets with grain boundary modification yields products with 'designer properties' that exceed those of starting materials, Waste management 44: 48-54).

[0007] In comparison to established conventional sintering methods, the reduction of heating times is achieved by employing electrical fields or currents to enhance densification. Modern sintering methods demonstrating this approach include ultrafast high-temperature sintering (Wang et al., 2020: A general method to synthesize and sinter bulk ceramics in seconds, Science 368(6490): 521-526), flash sintering (Cologna et al., 2010: Flash sintering of nanograin zirconia in< 5 s at 850 C, Journal of the American Ceramic Society 93(11):3556-3559), and spark-plasma sintering (SPS) (Guillon et al., 2014: Field-assisted sintering technology / spark plasma sintering: mechanisms, materials, and technology developments, Advanced Engineering Materials 16(7): 830-849). SPS is an electrical current-activated, pressure-assisted technique operating in a high-current, low-voltage regime. In SPS, the sample-containing die heats up through a pulsed DC, enabling direct Joule heating of electrically conductive materials. With SPS, Nd-Fe-B magnets based on nanostructured melt-spun powders and HDDR powders have been successfully prepared within 15 minutes of heating time ( Saito et al., 2005: Magnetic properties of Nd-Fe-Co-Ga-B magnets produced by spark plasma sintering method, Journal of applied physics 97(10); Suresh et al., 2009: Consolidation of hydrogenation-disproportionation-desorption-recombination processed Nd-Fe-B magnets by spark plasma sintering, Journal of magnetism and magnetic materials 321(22): 3681-3686). However, the sintering of anisotropic near-monocrystalline jet-milled powders via SPS approach has presented challenges. Firstly, direct Joule heating of the jet-milled Nd-Fe-B material can facilitate the decomposition of the Nd 2 Fe 14 B hard-magnetic phase due to localized overheating at the particle-particle contacts. Secondly, the uniaxial pressure applied on the powder compact during SPS degrades crystallographic texture, which decreases the M r by more than 10% compared to conventional sintering (Tomse et al., 2020: A spark-plasma-sintering approach to the manufacture of anisotropic Nd-Fe-B permanent magnets, Journal of Magnetism and Magnetic Materials 502: 166504). Concerning the rapid heating of an electrically conductive die by the Joule effect, it was demonstrated on ceramic materials that the powder compact can be heated indirectly through thermal radiation if it is thermally and electrically insulated from the die. Materials sintered via thermal radiation include ceramics, namely zirconia (Salamon et al., 2012: Rapid sintering of crack-free zirconia ceramics by pressure-less spark plasma sintering, Scripta Materialia 66(11): 899-902), alumina (Hofer et al., 2022: High-strength lithography-based additive manufacturing of ceramic components with rapid sintering, Additive Manufacturing 59: 103141), potassium sodium niobate (Kuscer et al., 2019: Evolution of phase composition and microstructure of sodium potassium niobate-based ceramic during pressure-less spark plasma sintering and post-annealing, Ceramics International 45(8): 10429-10437), CrB 2 ( Sairam et al., 2016: Pressureless sintering of chromium diboride using spark plasma sintering facility, International Journal of Refractory Metals and Hard Materials 58:165-171), SiC (Li and Shen, 2015: Sintering by intense thermal radiation (SITR): A study of temperature distribution by simulation and experiments, Journal of the European Ceramic Society 35(12): 3303-3309), Si 3 N 4 foams (Li et al., 2014: Rapid sintering of silicon nitride foams decorated with one-dimensional nanostructures by intense thermal radiation, Science and Technology of Advanced Materials), and porous 3D printed titanium structures (Kashimbetova et al., 2023: Pressure-less spark plasma sintering of 3D-plotted titanium porous structures, Journal of Materials Research Technology 22: 2147-2157). A similar thermal-radiation-sintering approach enabling fast consolidation of rare-earth comprising magnet powders under pressureless conditions has not been developed thus far.

[0008] In summary, the conventional sintering approach for producing anisotropic rare-earth comprising magnet powders and particularly Nd-Fe-B permanent magnets is a time-consuming and energy-intensive process that is well-suited for fine-milled powders. While contemporary sintering strategies like SPS offer the potential for reducing heating times and consolidating coarser powders, the combination of direct Joule heating and external pressure applied on the powder compact proves unsuitable for processing anisotropic microcrystalline rare-earth comprising magnet powders, including Nd-Fe-B powders.Description of the invention

[0009] Given the challenges associated with the manufacture of sintered rare-earth magnets, particularly anisotropic Nd-Fe-B magnets, it is an object of the present invention to provide a faster and more energy-efficient sintering procedure for rare-earth comprising magnet powders, preferably Nd-Fe-B-type powders or powder mixtures of any nominal chemical composition, including powders that cannot be sintered conventionally. The technical problem is solved as defined in the independent claim, wherein the preferred embodiments are defined in dependent claims.

[0010] In the method according to the invention, a fast-sintering strategy is employed to enable fast sintering of rare-earth comprising magnet powders, particularly Nd-Fe-B-type powders or powder mixtures based on thermal radiation and / or convection. The method according to the invention is performed in a vacuum or in an inert atmosphere, wherein the method essentially comprises the following steps: a) providing at least one powder compact or porous body made from rare-earth comprising magnet powders and placing said powder compact or porous body in an electrically conductive crucible, wherein said powder compact is insulated from the crucible with a suitable insulator, b) connecting the crucible to electrodes for generating an electrical current resulting in heating of the crucible and through radiation and / or convection also the powder compact, c) heating the crucible up to the sintering temperature from 600 to 1300 °C, d) optionally maintaining the sintering temperature for at least 1 minute, preferably up to 180, more preferably up to 60 minutes, most preferably from 1 to 30 minutes, and e) cooling the crucible and thus the magnet, preferably by terminating the heating and leaving the crucible and the resulting magnet to cool to room temperature.

[0011] A low-density powder compact is prepared by compressing an Nd-Fe-B-type powder or a powder mixture under external pressure or through 3D printing to form the porous body. Before compaction, the powders can be aligned in an external magnetic field. The powder compact or the porous body is placed into an electrically conductive crucible. The material of the crucible is arbitrary, as long as it is electrically conductive. Preferably, graphite is used due to its low price. Other materials such as tungsten carbide or steel may also be used. No mechanical pressure is applied on the powder compact during heating; however, pressure can be applied on the device so as to improve contacts between the electrodes and the crucible.

[0012] Insulating material is provided between the crucible and the powder compact or the porous body. The insulating material should have low thermal and / or electrical conductivity. Preferably, graphite felt is used. The insulating material, in case it is not inert, may also be provided with a protective layer or coating, such as a thermally-stable and inert coating or foil. Tantal, volfram or steel foil may be used or a boron nitride coating, which does not react with Nd-Fe-B and shows required stability at sintering temperatures.

[0013] The crucible is heated through resistive heating by the Joule effect. Joule heating is the physical effect by which the pass of current through an electrical conductor produces thermal energy, which results a rise in the conductor material temperature. The powder compact or the porous body is electrically insulated from the crucible in a way that the electrical current flows through the crucible. The intensity of the electrical current and electric potential difference can be preset. The applied power (P) is controlled by the electrical current (I) and voltage (U) / potential difference. (P=U*I). Alternatively, the intensity of the electrical current and electric potential difference is temperature-controlled by measuring the temperature of the crucible or the sample. The crucible acts as a heat source, i.e., a radiator. The heat is transferred from the crucible to the powder compact or the porous body mainly through thermal radiation. To achieve this, the heating process is performed in a vacuum of at least 10 -1< mbar. However, in an inert atmosphere, such as any noble gases or nitrogen, in addition to thermal radiation heat may also be transferred through thermal convection. This highly efficient heat transfer enables high heating rates, which preferably range between 10 to 500 °C / min (for example up to several 100s °C / min) from room temperature to the maximum temperature of the radiator, i.e., sintering temperature, and short dwell times (in the order of minutes), i.e., isothermal steps, at sintering temperatures. The heating rate, sintering temperature, and dwell time are freely varied. The heating rate may also be changed during the heating, for example by reducing the heating rate towards the desired sintering temperature.

[0014] The cooling stage in the above-mentioned step e) may be controlled or uncontrolled. The latter is achieved by simply terminating heating of the crucible, wherein the crucible and the powder compact or the sintered porous body cool at their own speed. Alternatively, the crucible may be cooled at any desired cooling rate, which preferably range between 10 to 500 °C / min, by reducing the electrical current flowing through the crucible. The cooling rate may be constant or variable, similarly as has been described above for the heating rate.

[0015] Potential powders can be i) anisotropic microcrystalline powders, for example, standard powders prepared using a conventional powder-metallurgy method based on strip casting, hydrogen decrepitation, and jet milling, ii) HPMS-type powders, iii) HDDR-type powders, iv) HPMS-type or HDDR-type powders further processed by milling to reduce the mean particle size, v) any of the above further processed to modify their phase composition, for example, by selective acid leaching to obtain powders with stoichiometric Nd 2 Fe 14 B-like composition or deposition of metallic phases via magnetron sputtering or similar techniques, vi) mixture of one or several Nd-Fe-B-type powders and one or several non-magnetic powders.

[0016] A device for performing the above-described method according to the invention comprises at least the following: a housing or a chamber ensuring vacuum or inert atmosphere, inside which at least the following is provided: ∘ at least one electrically conductive crucible having a space arranged to receive at least one powder compact or porous body, wherein said crucible is connectable to suitable electrodes arranged to allow heating of the crucible, ∘ optionally spacers, preferably graphite spacers, installed between the electrodes and the crucible, ∘ an insulating material with low electric and / or thermal conductivity placed on the bottom of the space arranged to receive at least one powder compact or porous body, wherein said insulating material is preferably a graphite felt, said insulating material provided with a protective coating, such as a boron-nitride coating for ensuring that the materials of the powder compact or the porous body are not contaminated with the insulating material or any components thereof.

[0017] The device may further comprise a measuring device for measuring temperature of the crucible, for example a pyrometer.

[0018] Optionally, pressure can be applied on the device for ensuring better contacts between the crucible and the electrodes.

[0019] The magnets may be prepared in any shape, which allows a variety of possible uses.Brief description of the drawings

[0020] The method will be described in further detail based on exemplary embodiments, examples and figures, which show: Figure 1A schematic illustration of an experimental setup allowing thermal-radiation-based heating to ensure fast and effective heating of a powder compact according to the invention Figure 2Scanning electron microscope images of (a) jet-milled Nd-Fe-B-type powder prepared by conventional powder metallurgy method and (b) bulk sintered magnet prepared from the jet-milled powder, (c) HPMS powder and (d) bulk sintered magnet prepared from the HPMS powder, (e) jet-milled HPMS powder and (f) bulk sintered magnet prepared from the jet-milled HPMS powder mixed with 1 wt% addition of Nd-hydride powder and 1 wt% addition of TbF 3 powder Figure 3Example of a sintering cycle with an average heating rate 150 °C / min from room temperature to 1100 °C and 2 minutes dwell time at 1100 °C, showing the electrical current intensity profile (1) and measured temperature of the heater (2) Figure 4Second quadrant demagnetization curves of bulk sintered Nd-Fe-B magnets prepared from different types of powders Detailed description of the invention

[0021] In the following examples of the method according to the invention bulk Nd-Fe-B-type magnets are manufactured from different powder types by employing a thermal-radiation-based sintering approach. As mentioned above, the methods according to the invention may be performed in vacuum or in an inert atmosphere. In the examples only methods performed in vacuum are discussed, which does not limit the possibility to replace vacuum with an inert atmosphere.POWDER MATERIALS

[0022] Three types of Nd-Fe-B-type powders were used.

[0023] The first powder (powder 1) was anisotropic powder with the composition Nd 30.0 Pr 0.6 Dy 1.0 Fe 63.8 Co 3.0 Ga 0.2 Cu 0.1 Al 0.1 B 0.9 O 0.3 (wt.%), supplied by Magneti Ljubljana d.d (Ljubljana, Slovenia). It was prepared using a conventional powder-metallurgy method: the alloy underwent strip casting, and the resulting flakes were subsequently decrepitated with hydrogen to yield a coarse and friable powder, followed by jet milling with nitrogen.

[0024] The second powder (powder 2) was an HPMS-type powder with the composition Nd 24.3 Pr 0.2 Dy 3.7 Fe 70.1 Al 0.3 B 0.9 O 0.5 (wt.%), provided by Hochschule Pforzheim. It was prepared from end-of-life magnets originating from wind turbine generators.

[0025] The third powder (powder 3) was prepared from the second powder by milling it in a jet mill using argon gas to reduce its particle size. Terbium(III) fluoride powder (TbFs, >99%) was purchased from Sigma-Aldrich, USA. Neodymium(III) hydride powder (NdHs, >99%) was purchased from American Elements, USA.PREPARATION OF POWDER COMPACTS

[0026] To prepare powder compacts for sintering experiments, an Nd-Fe-B-type powder or a powder mixture was placed into a cylindrical silicon mold under a protective argon atmosphere and vacuum sealed. Powder mixtures were prepared through a powder blending approach by weighing and homogenizing the powders under an inert atmosphere using mortar and pestle. In the next step, a vacuum-sealed Nd-Fe-B-type powder or a powder mixture was magnetically aligned in an external pulsed magnetic field with 6 T magnitude and isostatically pressed under 800 MPa to produce a magnetically aligned powder compact with dimensions 15 (diameter) × 12 (height) mm and approx. 55% theoretical density.EXPERIMENTAL SETUP

[0027] Concerning the schematic illustration in Figure 1 (cross-section), sintering was performed in a commercially available laboratory-scale furnace (SPS-632LxEx, Dr. Sinter, SPS Syntex Inc., Japan) under vacuum. A powder compact 1 was placed in a custom-made graphite crucible 2 with dimensions 30.0 mm (inner diameter) × 23.4 mm (height) on a 6 mm thick graphite felt 3. Graphite felt was covered with ≈0.2 mm thick boron nitride (BN) coating 4 to prevent reaction between Nd-Fe-B and graphite. Graphite spacers 5 and 6 were placed between the crucible and electrodes 7. 1 kN force was applied uniaxially on the setup to ensure good electrical contact. Electrical current (DC) with a 12:2 on-off pulse sequence (12 × 3.3 ms ≈0.04 s of current, followed by 2 × 3.3 ms ≈0.007 s of pause) was applied to heat the crucible through Joule heating. The powder compact was smaller than the inner diameter of the crucible and was not in thermal contact with the walls of the crucible. Due to the low thermal and electrical conductivity of the graphite felt, it is assumed that thermal radiation is the only factor influencing the sample temperature. The crucible acted as a heat source, i.e., the radiator. The heating of the crucible was initiated at room temperature (RT). The temperature was measured on the external surface of the wall of the crucible at the sample height 8 with a pyrometer. Below the visible range of the pyrometer at ≈575 °C, the electrical current increase was preset. Above the visible range, the heating was temperature-controlled. The sintering temperature was 1100 °C. Considering the limitations of the furnace, the heating rate from room temperature to sintering temperature was varied from 10 to 500 °C / min. Depending on the heating rate and choice of the initial Nd-Fe-B powder type, dwell time at sintering temperature was needed to fully densify the compact. After the dwell time, the heating was terminated and the experimental setup was left to cool to room temperature.Examples EXAMPLE 1:

[0028] Powder 1 was sintered using the setup in Fig. 1. As shown in Fig. 2a (BSE-SEM image), the powder is near-monocrystalline, consisting of Nd 2 Fe 14 B matrix phase (grey) and Nd-rich secondary phases (bright contrast). The mean powder particle size is 6.5 µm. Fig. 2b (BSE-SEM image) shows the microstructure of a bulk sintered magnet prepared from powder 1. The magnet was prepared at 1100 °C of sintering temperature by employing 150 °C / min of heating rate from room temperature to sintering temperature and 2 minutes of dwell time. Like the initial powder, the microstructure of the magnet consists of the Nd 2 Fe 14 B matrix phase (grey) and Nd-rich secondary phases (bright contrast). The magnet's density exceeds 99% of the theoretical density. Fig. 3 shows the electrical current intensity profile (1) and the measured temperature of the crucible (2) for the corresponding heating cycle. The electrical current continuously increases from the start of heating to the sintering temperature, with a minor setback at approx. 600 °C when the pyrometer starts recording data and the heating process becomes temperature-controlled. The current peaks at 1450 A at the beginning of the short two-minute dwell time. The total heating time was 9.2 minutes. Several bulk magnets were prepared at 1100 °C of sintering temperature while the heating rate varied from 10 to 500 °C / min. Fig. 4 shows the corresponding demagnetization curves of magnets, prepared with 10 °C / min (A), 50 °C / min (B), 150 °C / min (C), and 500 °C / min (D) heating rate. Magnetic measurements were performed by applying a closed-loop hysteresisgraph (Permagraph, Magnet-Physik) at room temperature. The magnetic properties of the magnets were as follows: 1.32-1.35 T of remanent magnetization, 342-352 kJ / m 3< of maximal energy product, and 575-710 kA / m of intrinsic coercivity.EXAMPLE 2

[0029] Powder 2 was sintered using the setup in Fig. 1. As seen in Fig. 2c (SEM image), the powder is polycrystalline, with a broad particle size ranging up to ≈500 µm. The microstructure of an Nd-Fe-B bulk sintered magnet prepared from powder 2 is shown in Fig. 2d (BSE-SEM image). The magnet was prepared at 1100 °C by employing 50 °C / min of heating rate and 15 minutes of dwell time. The total heating time was 36.5 minutes. The magnet's density exceeds 99% of the theoretical density. Its microstructure consists of Nd 2 Fe 14 B matrix phase (grey) and Nd-rich secondary phases (bright contrast). Magnetic measurement of the magnet was performed by applying a closed-loop hysteresisgraph (Permagraph, Magnet-Physik) at room temperature. Fig. 4 shows the corresponding demagnetization curve (E). The magnet's magnetic properties are 1.04 T of remanent magnetization, 180 kJ / m 3< of maximal energy product, and 890 kA / m of intrinsic coercivity.EXAMPLE 3

[0030] Powder 3 was sintered using the setup in Fig. 1. As seen in Fig. 2e (SEM image), milling of powder 2 broke down the large polycrystalline particles shown in Fig. 2c into individual grains via intergranular fracturing. Three powder compacts were prepared from powder 3 with and without the addition of secondary powders. The first powder compact was prepared without the addition of secondary powders. The second powder compact was prepared with 1 wt% addition of NdH 3 powder. The third powder compact was prepared with a 1 wt% addition of NdH 3 powder and a 1 wt% addition of TbF 3 powder. The powder compacts were sintered at 1100 °C by employing 50 °C / min of heating rate and 5 minutes of dwell time. The total heating time was 26.5 minutes. The microstructure of a magnet prepared from a powder compact with 1 wt% addition of NdH 3 powder and 1 wt% addition of TbF 3 powder is shown in Fig. 2f (BSE-SEM image). The magnet's density is more than 99% of the theoretical density. Its microstructure consists of Nd 2 Fe 14 B matrix phase (grey) and Nd-rich secondary phases (bright contrast). Fig. 4 shows the corresponding demagnetization curve (F). The magnet's magnetic properties are 1.27 T of remanent magnetization and 295 kJ / m 3< of maximal energy product. Magnetic measurement was performed by applying a closed-loop hysteresis graph (Permagraph, Magnet-Physik) at room temperature. Intrinsic coercivity value was not obtained with this measurement due to measuring limitations of the hysteresisgraph.

Examples

example 1

[0028]Powder 1 was sintered using the setup in Fig. 1. As shown in Fig. 2a (BSE-SEM image), the powder is near-monocrystalline, consisting of Nd 2 Fe 14 B matrix phase (grey) and Nd-rich secondary phases (bright contrast). The mean powder particle size is 6.5 µm. Fig. 2b (BSE-SEM image) shows the microstructure of a bulk sintered magnet prepared from powder 1. The magnet was prepared at 1100 °C of sintering temperature by employing 150 °C / min of heating rate from room temperature to sintering temperature and 2 minutes of dwell time. Like the initial powder, the microstructure of the magnet consists of the Nd 2 Fe 14 B matrix phase (grey) and Nd-rich secondary phases (bright contrast). The magnet's density exceeds 99% of the theoretical density. Fig. 3 shows the electrical current intensity profile (1) and the measured temperature of the crucible (2) for the corresponding heating cycle. The electrical current continuously increases from the start of heating to the sintering temperatu...

example 2

EXAMPLE 2

[0029]Powder 2 was sintered using the setup in Fig. 1. As seen in Fig. 2c (SEM image), the powder is polycrystalline, with a broad particle size ranging up to ≈500 µm. The microstructure of an Nd-Fe-B bulk sintered magnet prepared from powder 2 is shown in Fig. 2d (BSE-SEM image). The magnet was prepared at 1100 °C by employing 50 °C / min of heating rate and 15 minutes of dwell time. The total heating time was 36.5 minutes. The magnet's density exceeds 99% of the theoretical density. Its microstructure consists of Nd 2 Fe 14 B matrix phase (grey) and Nd-rich secondary phases (bright contrast). Magnetic measurement of the magnet was performed by applying a closed-loop hysteresisgraph (Permagraph, Magnet-Physik) at room temperature. Fig. 4 shows the corresponding demagnetization curve (E). The magnet's magnetic properties are 1.04 T of remanent magnetization, 180 kJ / m 3< of maximal energy product, and 890 kA / m of intrinsic coercivity.

example 3

EXAMPLE 3

[0030]Powder 3 was sintered using the setup in Fig. 1. As seen in Fig. 2e (SEM image), milling of powder 2 broke down the large polycrystalline particles shown in Fig. 2c into individual grains via intergranular fracturing. Three powder compacts were prepared from powder 3 with and without the addition of secondary powders. The first powder compact was prepared without the addition of secondary powders. The second powder compact was prepared with 1 wt% addition of NdH 3 powder. The third powder compact was prepared with a 1 wt% addition of NdH 3 powder and a 1 wt% addition of TbF 3 powder. The powder compacts were sintered at 1100 °C by employing 50 °C / min of heating rate and 5 minutes of dwell time. The total heating time was 26.5 minutes. The microstructure of a magnet prepared from a powder compact with 1 wt% addition of NdH 3 powder and 1 wt% addition of TbF 3 powder is shown in Fig. 2f (BSE-SEM image). The magnet's density is more than 99% of the theoretical densi...

Claims

1. A method for rapid sintering of rare-earth magnet powders based on thermal radiation and / or convection, wherein said method is performed in a vacuum of at least 10-1 mbar or in an inert atmosphere and comprises the following steps: a) providing at least one powder compact or porous body made from rare-earth comprising magnet powders and placing said powder compact or porous body in an electrically conductive crucible, wherein said powder compact is insulated from the crucible with a suitable insulator, b) connecting the crucible to electrodes for generating an electrical current resulting in heating of the crucible and through radiation and / or convection also the powder compact, c) heating the crucible up to a sintering temperature, which is preferably from 600 to 1300 °C, d) optionally maintaining the sintering temperature for at least 1 minute, and e) cooling the crucible and thus the magnet to room temperature.

2. The method according to claim 1, wherein step e) of maintaining the sintering temperature for at least 1 minute is performed.

3. The method according to claim 2, wherein step e) lasts for up to 180 minutes, preferably up to 60 minutes, more preferably from 1 to 30 minutes.

4. The method according to any of the preceding claims, wherein the powder compact is prepared by compressing a rare-earth comprising powder or a powder mixture under external pressure or the porous body is prepared by 3D printing.

5. The method according to any of the preceding claims, wherein the powders are before compaction aligned in an external magnetic field.

6. The method according to any of the preceding claims, wherein the insulating material is an inert material with low thermal and / or electrical conductivity, preferably graphite felt.

7. The method according to any of the preceding claims, wherein the insulating material is provided with a protective layer or coating, such as a thermally-stable and inert boron nitride coating or tantalum, wolfram or steel foil.

8. The method according to any of the preceding claims, wherein the method is performed in an inert atmosphere, such as any noble gases or nitrogen.

9. The method according to any of the preceding claims, wherein heating rates range between 10 to 500 °C / min from room temperature to the sintering temperature.

10. The method according to any of the preceding claims, wherein the cooling stage in step e) may be controlled or uncontrolled, wherein the latter is achieved by simply terminating heating of the crucible, or the crucible may be cooled at any desired cooling rate, which preferably range between 10 to 500 °C / min, by reducing the electrical current flowing through the crucible.

11. The method according to any of the preceding claims, wherein the rare-earth comprising powders are Nd-Fe-B-type powders or powder mixtures comprising Nd-Fe-B.

12. The method according to claim 11, wherein the magnetic powder is any Nd-Fe-B-type powder or a powder mixture of the following: - anisotropic microcrystalline powders, for example, standard powders prepared using a conventional powder-metallurgy method based on strip casting, hydrogen decrepitation, and jet milling; - Hydrogen Processing of Magnet Scrap (HPMS)-type powders; - Hydrogenation-Disproportionation-Desorption-Recombination (HDDR)-type powders; - HPMS-type or HDDR-type powders further processed by milling to reduce the mean particle size; - any of the above further processed to modify their phase composition, for example, by selective acid leaching to obtain powders with stoichiometric Nd2Fe14B-like composition or deposition of metallic phases via magnetron sputtering or similar techniques; - mixture of one or several Nd-Fe-B-type powders and one or several non-magnetic powders.

13. The method according to any of the preceding claims, wherein more than one magnetic powder compacts or 3D printed porous bodies are placed in crucible or more than one magnetic powder compacts or 3D printed porous bodies in a set of crucibles.

14. A device for carrying out the method according to any of the preceding claims, wherein said device comprises at least the following: - a housing or a chamber ensuring vacuum or inert atmosphere, inside which at least the following is provided: - at least one electrically conductive crucible having a space arranged to receive at least one powder compact or porous body, - wherein said crucible is connectable to suitable electrodes arranged to allow heating of the crucible, - and wherein said crucible is preferably made from graphite, tungsten carbide or steel, - optionally spacers, preferably graphite spacers, installed between the electrodes and the crucible, and - an insulating material with low electric and / or thermal conductivity placed on the bottom of the space arranged to receive at least one powder compact or porous body, wherein said insulating material is preferably a graphite felt, said insulating material provided with a protective coating, such as a boron-nitride coating for ensuring that the materials of the powder compact or the porous body are not contaminated with the insulating material or any components thereof.

15. The device according to the preceding claim, which further comprises a measuring device for measuring temperature of the crucible, for example a pyrometer.

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