METHOD FOR PREPARING MAGNETIC ALPHA-MNBI BULK MATERIAL

Abstract

 The present invention relates to a method of preparing a rare-earth-free magnetic α-MnBi-based bulk material, the method comprising a first procedural stage, the first procedural stage comprising the steps of: (Step A) consolidating under oxygen-free inert gas atmosphere a powder mixture consisting of Manganese powder and Bismuth powder; (Step B1) deforming the consolidated powder mixture obtained in Step A by a high-pressure torsion (HPT) deformation process at a temperature of up to 262°C and at a pressure of at least 0.5 GPa, to obtain a bulk material; and (Step C1) performing a magnetic field annealing process on the bulk material obtained in Step B1 under vacuum or oxygen-free gas atmosphere at an annealing temperature of up to 262 °C for at least 1 hour and under a magnetic field of at least 0.5 T to obtain an α-MnBi-based bulk material comprising a quantity of α-MnBi phase therein.

Description

Field of the Invention

[0001] The invention relates to a method of preparing a magnetic rare-earth-free α-MnBi-based bulk material. The α-MnBi-based bulk material obtained in the method according to the invention is a magnetic material useful for incorporation in permanent magnet-containing devices.

Background

[0002] As current high-performance permanent magnets contain a significant amount of rare-earth elements, an important goal is to find a rare-earth-free alternative. One candidate material is α-MnBi, which is an intermetallic low temperature phase with exceptional hard magnetic properties. It has a positive temperature coefficient of intrinsic coercivity for temperature T < 500 K, which means its intrinsic coercivity increases with rising temperatures. Thus, a large and peaking magnetocrystalline anisotropy of about 2.2 MJ/m³ above 400 K is reached, which makes this material interesting to be used as permanent magnet for high temperature applications.

[0003] The main problem lies in the difficult fabrication of the ferromagnetic α-MnBi phase, and, in particular, high volume fractions are a specific challenge. α-MnBi is described in the literature with an equal atomic fraction of Mn and Bi, and according to the current state of the Mn-Bi phase diagram it exists below a temperature of about 355°C but is thermodynamically stable only below about 262°C [Okamoto H, J. Phase Equilib. Diffus., 2015, 36, 10-21, https://doi.org/10.1007/s11669-014-0341-7]. Due to this physical existence limitation, but also because of a large number of other Mn-Bi phases above said temperatures, a direct fabrication of the α-MnBi phase with classical metallurgical methods is only possible to a limited extent.

[0004] Currently, high volume fractions of the α-MnBi phase are obtained, for example, by a sequence of different processing steps including non-equilibrium processing techniques. In methods known in the art, first, extremely rapid cooling of a Mn-Bi melt is conducted, followed by a milling process to produce a fine powder. This powder is magnetically sieved and then compressed into a pellet. The choice of chemical composition associated with annealing treatments allows the formation of an intermediate phase (β-MnBi phase) and a transformation to the α-MnBi phase [Gabay et al, Journal of Magnetism and Magnetic Materials, Vol. 495, 2020, 165860, https://doi.org/10.1016/j.jmmm.2019.165860].

[0005] US 2021/0304933A1 describes the synthesis of mass quantities of α-MnBi magnetic powder and the fabrication of a bulk permanent magnet, comprising a number of complex and tedious processing steps.

[0006] The magnetic field effect on the reactive sintering process of α-MnBi has been investigated and described by Mitsui et al. [Mitsui et al, 2018, Journal of Magnetism and Magnetic Materials 453, 231-235, https://doi.org/10.1016/j.jmmm.2018.01.026]. Miyazaki et al. described the preparation of α-MnBi by in-field solid-phase reactive sintering to investigate the origin of magnetic field effects [Miyazaki et al, 2017, Materials Transactions 58, 720-723, https: / / doi:10.2320/ matertrans.MBW201609] .

[0007] US10109418B2 describes a system and process for friction consolidation fabrication of an exchange coupled nanocomposite magnet, wherein a soft magnetic and a hard magnetic metal are processed by means of friction consolidation fabrication and extrusion at forming temperatures of > 500°C caused by friction.

[0008] US9418779B2 describes a process for production of α-MnBi-based materials, the process requiring a number of complex process steps, including co-melting Mn and Bi powders to obtain a MnBi alloy which is heat treated, milled, screened and further processed.

[0009] US9373433B2 discloses exchange coupled nanocomposite permanent magnets and methods for making the same.

[0010] The extraordinary complexity of the necessary processing steps of the above-described known manufacturing methods is a huge disadvantage. Each step is subject to a certain variance of the process parameters. Furthermore, the pronounced tendency of oxidation of the starting material Mn but also of the resulting α-MnBi phase is problematic. Oxidation during the process steps inevitably leads to a reduction in the volume of the α-MnBi phase, making inert atmosphere during processing necessary, especially in the grinding and powdering steps. For industrial utilization a simplified processing route is highly needed.

[0011] It is an object of the invention to provide a novel and simplified method for preparing a magnetic α-MnBi-based bulk material, which meets the above-mentioned needs and solves the above-mentioned technical problems and challenges.

Summary

[0012] Said object is solved by the present invention which is directed to a novel method of preparing a rare-earth-free magnetic α-MnBi-based bulk material as defined in the appended claims.

Detailed description of the invention

[0013] In the following the method of the present invention and preferred embodiments will be described in more detail. The description of the method of the present invention and preferred embodiments is also accompanied by Fig. 1 which shows a flow chart schematically illustrating the individual processing steps of preparing the rare-earth-free magnetic α-MnBi-based bulk material.

[0014] Further scope of applicability of the present invention will become more apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples are given by way of illustration only, since various changes and modifications within the scope of the appended claims will become apparent to those skilled in the art from the detailed description.

[0015] As used herein, the term "about" means within a statistically meaningful range of a value, such as a stated amount, time frame, particle size, or temperature. That is, the term "about" is used herein to extend the boundaries of the stated value above and below by a meaningful variance. Such variance can be within an order of magnitude, typically within 10 percent, more typically within 5 percent, above or below the stated value. Sometimes, the variance can be within the experimental error typical of standard methods used for the measurement and/or determination of a given value. The allowable variation encompassed by the term "about" will be readily appreciated by those skilled in the art.

[0016] Generally, the present invention relates to a method of preparing a rare-earth-free magnetic α-MnBi-based bulk material, the method comprising a first procedural stage, wherein the first procedural stage comprises the steps of:

• Step A: consolidating under oxygen-free inert gas atmosphere a powder mixture consisting of Manganese (Mn) powder and Bismuth (Bi) powder;
• Step B1: deforming the consolidated powder mixture obtained in Step A by a high-pressure torsion (HPT) deformation process at a temperature of up to 262°C and at a pressure of at least 0.5 GPa, to obtain a bulk material; and
• Step C1: performing a magnetic field annealing process on the bulk material obtained in Step B1 under vacuum or oxygen-free gas atmosphere at an annealing temperature of up to 262 °C, preferably at an annealing temperature of about 240°C, for at least 1 hour, preferably for at least 4 hours, and under a magnetic field of at least 0.5 T, preferably at least 2 T, to obtain an α-MnBi-based bulk material comprising a quantity of α-MnBi phase therein.

[0017] Accordingly, the present invention relates to a solid-state processing route, wherein in a first procedural stage a bulk magnetic α-MnBi material comprising a quantity of α-MnBi therein is obtained. The first procedural step comprises three major steps: a step of consolidating a mixture of Mn and Bi powders (Step A), an HPT deformation step (Step B1), and a magnetic field annealing step (Step C1). The invention is based on the surprising finding that by HPT deformation (Step B1), a severe plastic deformation process, a bulk material which exhibits grain and phase refinement is produced, introducing a high crystallographic defect density, wherein said high crystallographic defect density was found to be crucial for accelerated diffusion processes in the subsequent magnetic field annealing step (Step C1) which lead to the formation of the α-MnBi phase in the bulk material. Furthermore, the magnetic field applied during annealing has a positive effect on the α-MnBi phase formation and allows an adjustable anisotropic orientation of the phase.

[0018] The advantages and beneficial technical effects of the method according to the present invention are listed below. The technical effects are also demonstrated and evidenced by the experimental data given in the experimental chapter below.

• The present invention provides an advantageous process of manufacturing magnetic α-MnBi-based bulk materials, which comprises significantly fewer and less tedious process steps than the previously known processes of manufacturing magnetic α-MnBi based materials. High-pressure torsion (HPT) deformation as used in the present invention allows to combine completely different processing steps, namely powder compaction, grain refinement and texture formation, in one process.
• HPT deformation provides further specific process-inherent advantages compared to other non-equilibrium bulk processing techniques: it is based on a simple shear process, which can be easily implemented and adjusted to industrial standards and needs. HPT deformation decreases microstructural feature sizes of the Mn-Bi-composites, which gives an additional increase in coercivity. Because of the shear deformation, anisotropic bulk permanent magnets on basis of α-MnBi phase can be processed. Anisotropy is highly desired as it allows material fabrication with a certain polarity, reducing the amount of material while the magnetic field strength remains constant.
• The present invention provides for the manufacture of hard magnetic material, thereby filling the gap currently existing between ferrites and the strongest Nd-Fe-B magnets.
• The present invention also provides for the manufacture of hard magnetic materials with positive temperature coefficients of coercivity suitable for applications at elevated temperature (e.g. application in electric motors).
• Another advantage is that the α-MnBi-based bulk materials fabricated by the method of the invention are rare-earth-free magnetic materials, thereby meeting the important goal of providing a rare-earth element-free alternative to known magnetic materials used in current high-performance permanent magnets.

[0019] The steps of the method according to the invention, namely, the first procedural stage covering three major steps (A, B1, C1), will be described in the following in more detail, wherein reference is also made to the flow chart of Fig. 1.

Step A:

[0020] In Step A, a powder mixture consisting of Manganese (Mn) powder and Bismuth (Bi) powder in various Mn:Bi ratios is consolidated under oxygen-free inert gas atmosphere, thereby forming a solid bulk body having a suitable shape, e.g. a disk shape, to be processed in the following manufacturing steps. In preferred embodiments it can be provided that a powder mixture consisting of Mn powder and Bi powder in a Mn:Bi ratio ranging from 45:55 at% to 55:45 at% is consolidated. In a particular preferred embodiment, a powder mixture consisting of Mn powder and Bi powder in a Mn:Bi ratio of about 50:50 at% is consolidated as it is the chemical composition of α-MnBi phase according to the Mn-Bi phase diagram.

[0021] Mn and Bi powders as used in the present invention are well known and widely used for many different applications. Preferably, the Mn and Bi powders described herein have a purity of at least 99.3 and 99.5 wt-%, respectively, unless otherwise indicated, with the remainder being unavoidable impurities. More preferably, the Mn and Bi powders described herein have a purity of at least 99.95 and 99.999 wt-%, respectively. The term "unavoidable impurities" as used herein refer to impurities that cannot be controlled, or controlled with difficulty, during manufacture of the powders. These can come from the raw materials used and also from the process of manufacturing the powders. These unavoidable impurities present in the Mn and Bi powders may include metal impurities of different kind.

[0022] In preferred embodiments the Mn and Bi powders used in Step A have a particle size of smaller than 18 mesh (1mm), preferably smaller than 200 mesh (74 µm); the mesh sizes may be determined according to DIN EN933-1:2012-03. Micrometre-sized powders are preferred to nanometre-sized powders because micrometre-sized powders are easier to handle and produce.

[0023] To prevent oxidation, the consolidation under Step A is carried out under oxygen-free inert gas atmosphere, e.g. under Ar. Preferably, Step A is performed at room temperature.

[0024] The term "room temperature" as used in the present disclosure, refers to a temperature of between 20 - 30°C, unless otherwise indicated.

Step B1:

[0025] In Step B1 the consolidated powder mixture obtained in Step A is processed by HPT deformation at a temperature of up to 262°C and at a pressure of at least 0.5 GPa, to obtain a bulk material. As mentioned above, HPT deformation represents a severe plastic deformation process, resulting in a bulk material which exhibits grain and phase refinement and a high crystallographic defect density. The HPT deformation process of Step B1 may be performed at a temperature of up to 262°C, i.e. a temperature below the melting point of Bi.

[0026] HPT deformation devices are known in the art. The HPT deformation device used in the present invention is described in Example 1 below and schematically illustrated in Fig. 2.

[0027] According to preferred embodiments, the HPT deformation process of Step B1 is performed at a temperature of below 40°C, most preferably at room temperature. Room temperature deformation has the benefit that there is no need for a heating device. Room temperature deformation further has economic benefits, e.g. energy saving.

[0028] In certain embodiments it can be provided that the HPT deformation process of Step B1 is performed at a pressure of at least 2 GPa, preferably 2-5 GPa, most preferably about 2 GPa.

[0029] In certain embodiments it can be provided that the deforming by the respective HPT deformation process in Step B1 comprises at least a 1/4 rotation, preferably at least 1 rotation, most preferably at least 100 rotations.

[0030] In certain embodiments it can be provided that the deforming by the respective HPT deformation process in Step B1 is performed at a rotational speed of up to 1.4 rpm, most preferred at a rotational speed of 0.6 rpm or less.

Step C1:

[0031] In Step C1 a magnetic field annealing process on the bulk material obtained in Step B1 is performed under vacuum or oxygen-free gas atmosphere at an annealing temperature of up to 262 °C, for at least 1 hour, preferably for at least 4 hours, and under a magnetic field of at least 0.5 T, preferably at least 2 T, to obtain an α-MnBi-based bulk material comprising a quantity of α-MnBi phase therein. Preferably, the magnetic field annealing process is performed at a temperature of at least 100 °C, as temperatures below 100 °C would result in slow diffusion processes. Thus, in preferred embodiments, the magnetic field annealing process of Step C1 is performed at a temperature of between 100°C and 262°C. In a particularly preferred embodiment, the magnetic annealing process is performed at an annealing temperature of about 240°C.

[0032] In certain embodiments, the magnetic annealing process of Step C1 is performed for a period of at least 4 hours. In preferred embodiments, the magnetic annealing process is performed under a magnetic field of at least 2 T, preferably at higher magnetic fields up to 15 T.

[0033] Devices for magnetic field annealing are known in the art. For example, such devices may comprise a custom-made vacuum chamber, which is placed into the homogeneous field region provided by the conical pole pieces of an electromagnet operated at a constant magnetic field. A constant temperature can be monitored by a thermocouple mounted next to the samples. The applied external magnetic field can be recorded using a Hall-probe, which is placed outside of the vacuum chamber. As a non-limiting example, a device for magnetic field annealing is also in its principle described in Example 1 below and schematically illustrated in Fig. 2.

[0034] In particularly preferred embodiments it can be provided that, following the first procedural stage, the method further comprises a second procedural stage to increase the quantity of α-MnBi phase in the α-MnBi-based bulk material, the second procedural stage comprising the steps of:

• Step B2: deforming the α-MnBi-based bulk material by a high-pressure torsion (HPT) deformation process at a temperature of up to 262°C and at a pressure of at least 0.5 GPa;
• Step C2: performing a magnetic field annealing process on the α-MnBi-based bulk material obtained in Step B2 under vacuum or oxygen-free gas atmosphere at an annealing temperature of up to 262°C, preferably at an annealing temperature of about 240°C, for at least 1 hour, preferably for at least 4 hours, and under a magnetic field of at least 0.5 T, preferably at least 2 T, thereby increasing the quantity of the α-MnBi phase in the α-MnBi-based bulk material; and
• optionally repeating the sequence of Steps B2 and C2 at least once.

[0035] Preferably, the sequence of Steps B2 and C2 is repeated at least once. After each cycle of performing Steps B2 and C2 the quantity of α-MnBi phase in the α-MnBi-based bulk material is increased.

[0036] With reference to Example 1 given below, performing the first procedural stage leads to a quantity of α-MnBi phase in the α-MnBi-based bulk material of about 20-30 wt%. After performing the second procedural stage with one cycle of HPT deformation (Step B2) and magnetic field annealing (Step C2) the amount of α-MnBi phase in the α-MnBi-based bulk material could be increased to 70 wt%. Thus, the second procedural stage advantageously improves homogeneity of the α-MnBi-based bulk material with each cycle of Steps B2 and C2, leading to microstructural refinement of the previously formed α-MnBi phase comprised in the α-MnBi-based bulk material. Thus, α-MnBi phase is enhanced, which is accompanied by an increase of coercivity due to grain refinement.

[0037] The steps of the second procedural stage covering Steps B2 and C2 for increasing the quantity of α-MnBi phase in the α-MnBi-based bulk material will be described in the following in more detail, wherein reference is also made to the flow chart of Fig. 1.

Step B2:

[0038] In Step B2 the α-MnBi-based bulk material is processed by HPT deformation at a temperature of up to 262°C and at a pressure of at least 0.5 GPa. The HPT deformation process of Step B2 may be performed at a temperature of up to 262°C, i.e. a temperature below the melting point of Bi.

[0039] According to preferred embodiments, the HPT deformation process of Step B2 is performed at a temperature of below 40°C, most preferably at room temperature. Room temperature deformation has the benefit that there is no need for a heating device. Room temperature deformation further has economic benefits, e.g. energy saving.

[0040] In certain embodiments it can be provided that the HPT deformation process of Step B2 is performed at a pressure of at least 2 GPa, preferably 2-5 GPa, most preferably about 2 GPa.

[0041] In certain embodiments it can be provided that the deforming by the respective HPT deformation process in Step B2 comprises at least a 1/4 rotation, preferably at least 1 rotation, more preferably at least 10 rotations, most preferably at least 100 rotations.

[0042] In certain embodiments it can be provided that the deforming by the respective HPT deformation process in Step B2 is performed at a rotational speed of up to 1.4 rpm, most preferred at a rotational speed of 0.6 rpm or less.

Step C2:

[0043] In Step C2 a magnetic field annealing process on the bulk material obtained in Step B2 is performed under vacuum or oxygen-free gas atmosphere at an annealing temperature of up to 262 °C, for at least 1 hour, preferably for at least 4 hours, and under a magnetic field of at least 0.5 T, preferably at least 2 T, thereby increasing the quantity of the α-MnBi phase in the α-MnBi-based bulk material. Preferably, the magnetic field annealing process is performed at a temperature of at least 100 °C, as temperatures below 100 °C would result in slow diffusion processes. Thus, in preferred embodiments, the magnetic field annealing process of Step C2 is performed at a temperature of between 100°C and 262°C. In a particularly preferred embodiment, the magnetic annealing process is performed at an annealing temperature of about 240°C.

[0044] In certain embodiments, the magnetic annealing process of Step C2 is performed for a period of at least 4 hours. In preferred embodiments, the magnetic annealing process is performed under a magnetic field of at least 2 T, preferably at higher magnetic fields up to 15 T.

[0045] Those skilled in the art will readily understand the method described herein, where appropriate, may include heating or cooling steps between the process steps described herein, in order to reach a predetermined temperature for performing the next respective process step. For example, a cooling step for cooling down the material to a certain temperature, e.g. to room temperature, may be performed after a process step requiring a higher temperature (e.g. for performing HPT deformation Step B2 at room temperature after magnetic field annealing Step C1 or Step C2 performed at about 240 °C). In heating steps, for example, a heating rate of 1-20 °Cmin^-1, preferably about 5 °Cmin^-1, may be applied to reach the desired predetermined temperature necessary for performing a step requiring a higher temperature, e.g. the annealing temperature in the first and second magnetic field annealing process of Steps C1 and C2, respectively. In the light of the present disclosure, those skilled in the art, because of their technical knowledge, will be readily able to include respective heating and cooling steps in the method according to the inventive.

[0046] Preferably, the method of preparing the α-MnBi-based bulk material is terminated either after the first procedural stage or after the second procedural stage, once a desired quantity of the α-MnBi phase in the α-MnBi-based bulk material is reached. In one embodiment, the method of preparing the α-MnBi-based bulk material is terminated after the first procedural stage. In preferred embodiments, the method of preparing the α-MnBi-based bulk material is terminated either after the second procedural stage, since, as described above, higher quantities of α-MnBi phase comprised in the α-MnBi-based bulk material may be obtained when performing the second procedural stage.

[0047] The quantity of α-MnBi phase comprised in the α-MnBi-based bulk material may be determined by the following methods: The α-MnBi content in weight percent is calculated using a linear approximation with the measured saturation magnetization (Ms) and with the theoretical Ms of 79 emu/ g of the α-MnBi phase as the paramagnetic Mn and the diamagnetic Bi phases within the sample do not contribute significantly to Ms [Park et al, 2014, Metals 4(3), 455-464, https://doi.org/10.3390/met4030455].

[0048] Preferably the quantity of the α-MnBi phase comprised in the α-MnBi-based bulk material is at least 30 wt%, more preferably at least 70 wt%. A quantity of at least 30 wt% can be beneficial to achieve exchange coupling in nanocomposites. By combining the α-MnBi phase with a magnetic soft phase, anisotropic exchange spring magnetic material might be processed by HPT deformation on the basis of the α-MnBi phase.

[0049] The rare-earth-free magnetic α-MnBi-based bulk material prepared by the method of the present invention represents a useful hard magnetic material and, hence, can be suitably applied as anisotropic bulk permanent magnets. Anisotropy is highly desired because it allows material fabrication with a certain polarity, reducing the amount of material while the magnetic field strength remains constant. Furthermore, the rare-earth-free magnetic α-MnBi-based bulk material prepared by the method of the present invention also provides for hard magnetic materials with positive temperature coefficients of coercivity suitable for applications at elevated temperature (e.g. application in electric motors). Moreover, the α-MnBi-based bulk materials fabricated by the method of the invention are rare-earth-free magnetic materials, thereby meeting the important goal of providing a rare-earth element-free alternative to known magnetic materials used in current high-performance permanent magnets.

[0050] Therefore, in embodiments, the method of the present invention may comprise further processing steps, wherein the α-MnBi-based bulk material obtained after the first or second procedural stage, respectively, is processed to form a permanent magnet structure having a selected shape and size or is processed for incorporation as a component of a permanent magnet-containing device. Permanent magnets and permanent magnet devices are well-known in the art, which is why a detailed description is dispensed with. Due to their technical skills those skilled in the art will be readily able, without undue experimental burden and without applying inventive skill, to form a permanent magnet structure having a selected shape and size from the α-MnBi-based bulk material obtained by the present invention or to process the same for incorporation as a component of a permanent magnet-containing device.

[0051] Accordingly, in embodiments the prepared α-MnBi-based bulk material may be processed for incorporation as a component of a permanent magnet-containing device selected from one of the following: a device for converting electrical energy to mechanical energy (e.g. electromechanical drives, loudspeakers, actuators), devices for converting mechanical energy to electrical energy (e.g. generators, alternators, sensors), devices to direct, shape and/or control electron or ion beams (e.g. magnetic focused cathode-ray tubes, ion pumps).

[0052] The present invention is further demonstrated and illustrated by the following examples and drawings yet without being restricted thereto. The accompanying drawings being part of the present disclosure provide a further understanding of the present invention and, together with the description, serve to further explain the present invention.

Brief Description of the Drawings

[0053] 

Fig. 1 shows a flow chart schematically illustrating the general processing steps of preparing a rare-earth-free magnetic α-MnBi-based bulk material by a method according to the invention.

Fig. 2 shows a schematic overview of the processing steps with processing details of the method according to the invention described in Example 1.

Fig. 3 shows the microstructure of the Mn-Bi composites after the processing steps performed in the method according to the invention described in Example 1. Due to Z-contrast, the Mn phase appears darkest, Bi phase brightest and the α-MnBi phase has a medium contrast.

Fig. 4a shows the XRD pattern after processing steps performed in the method according to the invention described in Example 1. References for expected peak positions are taken from the Crystallography Open Database (Mn: COD 9011108, Bi: COD 9008576, α-MnBi: COD 9008899).

Fig. 4b shows the crystallite size of the α-MnBi phase after processing steps performed in the method according to the invention described in Example 1.

Fig. 5 shows the SQUID magnetometry hysteresis loops for the bulk material obtained in the last processing step performed in the method according to the invention described in Example 1.

[0054] In the following example an exemplary, non-restrictive embodiment of a method according to the invention for preparing a rare-earth-free magnetic α-MnBi-based bulk material is described in detail.

Example 1: Manufacture of a rare-earth-free magnetic α-MnBi-based bulk material by a method according to the invention

[0055] A schematic overview of the processing steps of an exemplary embodiment according to the invention as performed in this example is shown in Fig. 2. For the first procedural stage, Mn-Bi composites were processed using high-pressure torsion (HPT). Mn (99.95% - 325 mesh, Alfa Aesar) and Bi powders (99.999% - 200 mesh, Alfa Aesar), blend and mixed at a ratio of 50 at. %, were subsequently compressed (2 GPa pressure, room temperature) into disk-shaped samples (8 mm diameter, ~0.7 mm thickness) in an in-house custom-built device (Fig. 2/1^st stage/Step A). Sufficient thickness for the subsequent deformation was ensured by filling the powder into a copper ring, which was fully removed after the compression step. To prevent oxidation, the above-described blending and compression process steps were carried out in an Ar-atmosphere. The compacted disks were at first HPT deformed at room temperatures to 100 rotations (Fig. 2/1^st stage/Step B1). The rotational speed and the applied pressure were 0.6 rpm and 2 GPa.

[0056] After HPT deformation (Step B1), the samples were annealed in a magnetic field at 240°C for 4h (Fig. 2/1^st stage/Step C1). For this process step, a custom-made vacuum chamber was used, which was placed into the homogeneous field region provided by the conical pole pieces (diameter of 176 mm) of an electromagnet (Type B-E 30, Bruker). The electromagnet operated at a constant magnetic field of 2 T. Two copper blocks with cartridge heating elements (Keller, Ihne & Tesch Ges.m.b.H.; HHP, ∅10 x L50 mm, 100 W) were inside the vacuum chamber. The previously HPT deformed samples were heated by the direct heat transfer through the copper stock. A constant temperature was monitored by a thermocouple mounted next to the samples. The applied external magnetic field was recorded using a Hall-probe (Model 475 DSP, Lakeshore), which was placed outside of the vacuum chamber.

[0057] The annealed disks obtained in Step C1 were then cooled down to a temperature of below 40°C (Fig. 2/1^st stage/cooling step D1). In the following second procedural stage, the annealed disks obtained in the first procedural stage were HPT deformed at room temperature for 10 rotations with a rotational speed of 0.6 rpm and an applied pressure of 2 GPa (Fig. 2/2^nd stage/Step B2). After the second HPT deformation step (Step B2), the same annealing procedure as described above was applied (Fig. 2/2^nd stage/Step C2); after completion of annealing step C2, the annealed disks were cooled down to room temperature (cooling step D2).

[0058] Composite microstructures were analysed using scanning electron microscopy (field emission gun SEM, Zeiss Leo 1525, Carl Zeiss Microscopy, Germany) using backscattered electrons. Fig. 3 displays representative microstructures of the Mn-Bi composites after each processing step (for detailed description of the processing steps, see Fig. 2 and pertaining description). Due to Z-contrast, the Mn phase appears darkest, Bi phase brightest and the α-MnBi phase has a medium contrast. The second HPT deformation step (Fig. 3/Step B2) does not only affect overall homogeneity of the Mn and Bi phases in the composite, but also the grain/phase size of the α-MnBi phase. After the last processing step (Fig. 3/Step C2, cooled down in Step D2), a high amount of α-MnBi phase is visible in the micrographs obtained by SEM in this composite.

[0059] X-ray diffraction (XRD) results confirmed, that the α-MnBi phase was formed in the material after the first annealing step (Step C1; cooled down in Step D1) and its amount increases in the subsequent processing steps (Fig. 4a; the diffraction pattern relating to the respective processing steps B1, C1+D1, B2, and C2+D2 is marked with arrows). References for expected peak positions are taken from the Crystallography Open Database (Mn: COD 9011108, Bi: COD 9008576, α-MnBi: COD 9008899). Measured XRD patterns were analysed by using the Rietveld refinement method and the determined crystallite size of the α-MnBi phase is plotted in Fig. 4b (the crystallite size bar relating to the respective processing steps C1+D1, B2, and C2+D2, is marked in the bar chart with arrows).

[0060] Magnetic properties were measured with a SQUID-magnetometer (Quantum Design MPMS-XL-7, Quantum Design, Inc., San Diego, CA, USA) operated with the manufacturer's software MPMSMultiVu Application. All hysteresis loops are measured between ±70 kOe at 300 K measurement temperature. Fig. 5 shows hysteresis loops of the sample after the last processing step (Step C2, cooled down in Step D2) and the magnetic properties are summarized in Table 1 below. Values for the saturation magnetization (Ms) are acquired by linearly extrapolating the magnetization against H^-1 and determining the intercept at M(H^-1) = 0 [Bean and Jacobs, 1956, J. Appl. Phys. 27, 1448-1452, https://doi.org/10.1063/1.1722287. Chen et al, 2009, J. Appl. Phys. 105, 083924, https://doi.org/10.1063/1.3117512.]. The α-MnBi content in weight percent is further given in Table 1. It is calculated using a linear approximation with the measured saturation magnetization (Ms) and with the theoretical Ms of 79 emu/ g of the α-MnBi phase as the paramagnetic Mn and the diamagnetic Bi phases within the sample should not contribute significantly to Ms [Park et al, 2014, Metals 4(3), 455-464, https://doi.org/10.3390/met4030455].

Table 1: Magnetic properties of hysteresis loops presented in Fig 5. The determined coercivity Hc, maximum energy product BHmax, saturation magnetization Ms, remnant magnetization Mr and a-MnBi phase amount are listed.

Rotations (N)	Hc (kOe)	BH (kJ/m3)	Mr (emu/g)	Ms (emu/g)	α-MnBi (wt%)
100+10	3,1	4,4	17,7	55,1	69,7

[0061] To conclude, bulk Mn-Bi composite material, which exhibits grain and phase refinement, was produced from an elemental Mn and Bi powder mixture by severe plastic deformation. The high crystallographic defect density introduced by the severe plastic deformation process is crucial for accelerated diffusion processes during the subsequent thermal treatment with an additional applied magnetic field, which lead to the formation of the α-MnBi phase. A second severe plastic deformation step on these bulk Mn-Bi composite material with subsequent magnetic field annealing improved homogeneity of the Mn-Bi composites and lead to microstructural refinement of the previously formed α-MnBi phase. As a consequence, a significant increase of the α-MnBi phase in the bulk material during the second annealing step to ~70 wt% was obtained.

[0062] The foregoing example and described embodiments of this disclosure have been set forth merely to illustrate the invention and are not intended to be limiting. Since modifications of the disclosed embodiments incorporating the substance of the invention may occur to those skilled in the art, the invention should be construed to include everything within the scope of the appended claims.

Claims

1. A method of preparing a rare-earth-free magnetic α-MnBi-based bulk material, the method comprising a first procedural stage, the first procedural stage comprising the steps of:

• Step A: consolidating under oxygen-free inert gas atmosphere a powder mixture consisting of Manganese (Mn) powder and Bismuth (Bi) powder;

• Step B1: deforming the consolidated powder mixture obtained in Step A by a high-pressure torsion (HPT) deformation process at a temperature of up to 262°C and at a pressure of at least 0.5 GPa, to obtain a bulk material; and

• Step C1: performing a magnetic field annealing process on the bulk material obtained in Step B1 under vacuum or oxygen-free gas atmosphere at an annealing temperature of up to 262 °C, preferably at an annealing temperature of about 240°C, for at least 1 hour, preferably for at least 4 hours, and under a magnetic field of at least 0.5 T, preferably at least 2 T, to obtain an α-MnBi-based bulk material comprising a quantity of α-MnBi phase therein.

2. The method according to claim 1, wherein in Step A a powder mixture consisting of Mn powder and Bi powder in a Mn:Bi ratio ranging from 45:55 at% to 55:45 at% is consolidated.

3. The method according to claim 1 or 2, wherein in Step A a powder mixture consisting of Mn powder and Bi powder in a Mn:Bi ratio of about 50:50 at% is consolidated.

4. The method according to any one of claims 1 to 3, characterized in that the Mn and Bi powders used in Step A have a particle size of smaller than 18 mesh, preferably smaller than 200 mesh.

5. The method according to any one of claims 1 to 4, characterized in that, following the first procedural stage, the method further comprises a second procedural stage to increase the quantity of α-MnBi phase in the α-MnBi-based bulk material, the second procedural stage comprising the steps of:

• Step B2: deforming the α-MnBi-based bulk material by a high-pressure torsion (HPT) deformation process at a temperature of up to 262°C and at a pressure of at least 0.5 GPa;

• Step C2: performing a magnetic field annealing process on the α-MnBi-based bulk material obtained in Step B2 under vacuum or oxygen-free gas atmosphere at an annealing temperature of up to 262°C, preferably at an annealing temperature of about 240°C, for at least 1 hour, preferably for at least 4 hours, and under a magnetic field of at least 0.5 T, preferably at least 2 T, thereby increasing the quantity of the α-MnBi phase in the α-MnBi-based bulk material; and

• optionally repeating the sequence of Steps B2 and C2 at least once.

6. The method according to claim 5, wherein the sequence of Steps B2 and C2 is repeated at least once.

7. The method according to any one of claims 1 to 6, characterized in that the HPT deformation process of Step B1 and Step B2, respectively, is performed at a temperature of below 40°C, preferably at room temperature.

8. The method according to any one of claims 1 to 7, characterized in that the HPT deformation process of Step B1 and Step B2, respectively, is performed at a pressure of at least 2 GPa, preferably 2-5 GPa, most preferably about 2 GPa.

9. The method according to any one of claims 1 to 8, characterized that in Step B1 and Step B2, respectively, the deforming by the respective HPT deformation process comprises at least a 1/4 rotation, preferably at least 1 rotation, most preferably at least 100 rotations.

10. The method according to any one of claims 1 to 9, characterized that in Step B1 and Step B2, respectively, the deforming by the respective HPT deformation process is performed at a rotational speed of up to 1.4 rpm, most preferred at a rotational speed of 0.6 rpm or less.

11. The method according to any one of claims 1 to 10, characterized that the magnetic field annealing process of Step C1 and Step C2, respectively, is performed at an annealing temperature of about 240°C.

12. The method according to any one of claims 1 to 11, wherein the method of preparing the α-MnBi-based bulk material is terminated either after the first procedural stage or after the second procedural stage, once a desired quantity of the α-MnBi phase in the α-MnBi-based bulk material is reached.

13. The method according any one of claims 1 to 12, wherein the quantity of the α-MnBi phase comprised in the α-MnBi-based bulk material is at least 30 wt%, preferably at least 70 wt%.

14. The method according to any one of claims 1 to 13, characterized in that the prepared α-MnBi-based bulk material is processed to form a permanent magnet structure having a selected shape and size or is processed for incorporation as a component of a permanent magnet-containing device.

15. The method according to claim 14, wherein the prepared α-MnBi-based bulk material is processed for incorporation as a component of a permanent magnet-containing device selected from one of the following: a device for converting electrical energy to mechanical energy (e.g. electromechanical drives, loudspeakers, actuators), devices for converting mechanical energy to electrical energy (e.g. generators, alternators, sensors), devices to direct, shape and/or control electron or ion beams (e.g. magnetic focused cathode-ray tubes, ion pumps).

Drawing