Method of producing an electric brush

Abstract

A method of producing an electric brush configured to transmit electric current between a stationary part and its moving counterpart, the method comprising: producing a composite material, including: a) mixing 85 to 99 5 wt% copper powder having a particle size of 10-500 μm with 0.5 to 5 wt% graphene to obtain a material mixture, the weight percentages being based on the composite material as a whole; and b) subjecting the material mixture to hot isostatic pressing at a temperature of between 650 and 950 °C and under a pressure of 1500 to 2000 bar for at least 1 hour to form the composite material. The method further comprises shaping the composite material as an electric brush.

Description

[0001] METHOD OF PRODUCING AN ELECTRIC BRUSH

[0002] TECHNICAL FIELD

[0003] The present disclosure generally relates to a composite material for an electric brush configured to transmit electric current between a stationary part and its moving counterpart.

[0004] BACKGROUND

[0005] Electric brushes are used for conducting current to or from the rotating shaft of electrical machines. In some examples, a rotating contacting component, such as a slip ring or a commutator, is fixedly attached to the rotating shaft. The rotating contacting component, typically made of copper, bronze or stainless steel, is thus in contact with the stationary part via the electric brush, the latter being e.g. made of carbon and graphite or a metal-graphite blend. The electric brush is pressed to the surface of the rotating contacting component by a spring to ensure good electrical contact.

[0006] The graphite in the electric brush has two functions. The first is to provide sufficient conductivity to transfer the needed current to or from the rotating contacting component, and the second is to act as a solid lubricant to provide low friction to keep the rotating contacting component intact.

[0007] The main drawback of existing electric brushes is that they wear out over time and need to be replaced to ensure proper functionality. Depending on the application, electrical brushes usually need to be replaced every 6 to 12 months, generating high maintenance costs during the entire lifetime of electrical machines.

[0008] Moreover, due to the wear of the electric brushes of carbon, dust is created, which can lead to clogging or electrical bridging.

[0009] A common practice to prolong the lifetime of electric brushes is to limit the contact pressure between the electric brush and rotating contacting component to a very low level, for example 20-25 kPa. Because the contact resistance is inversely proportional to the contact pressure, the electrical losses at the contact region are relatively high. Another limitation of the commercial graphite containing electrical brushes is that the frictional performance is sensitive to humidity which limits the application to less humid environments.

[0010] SUMMARY

[0011] A general object of the present disclosure is to provide a method for producing an electric brush that solves or at least mitigates the problems of the prior art.

[0012] There is hence according to a first aspect of the present disclosure provided a method of producing an electric brush configured to transmit electric current between a stationary part and its moving counterpart, the method comprising: producing a composite material, including: a) mixing 85 to 99 wt% copper powder having a particle size of 10-500 pm with 0.5 to 5 wt% graphene to obtain a material mixture, the weight percentages being based on the composite material as a whole; and b) subjecting the material mixture to hot isostatic pressing at a temperature of between 650 and 950 °C and under a pressure of 1500 to 2000 bar for at least 1 hour to form the composite material. The method further comprises: shaping the composite material as an electric brush. The step of shaping the composite material as an electric brush may be performed in step b), or subsequent to step b).

[0013] The composite material exhibits advantageous tribological properties including low friction and high wear resistance in comparison with the known electric brushes such as e.g. commercial graphite containing electrical brushes. Thus, the electric brush produced by the first aspect of the disclosure exhibits well-balanced properties related to conducting electricity effectively while also resisting deterioration from friction and minimizing the material loss due to wear. This contributes to a more reliable and efficient operation of the device in which the electric brush is used. Moreover, the electric brush produced by the first aspect of the present disclosure exhibits advantageous tribological properties in comparison with an electric brush produced by conventional sintering, or even spark plasma sintering. For example, electric brush produced by the first aspect of the present disclosure may exhibit a compact density over 90% relative to the theoretical density of pure copper, and a higher hardness compared to an electric brush produced by conventional sintering, or even spark plasma sintering. Due to the improved properties of the electric brush produced by the first aspect of the present disclosure, the contact pressure between the electric brush and the moving counterpart can be increased, leading to reduced contact resistance and voltage drop, and thereby also reduced electrical loss during use. The amount of copper powder and graphene may be adapted such that the copper powder and the graphene in the material mixture amount to at least 92 wt%, or at least 95 wt%, or at least 97 wt%, or at least 99 wt% such as at least 99.5 wt% of the composite material. Any remaining part may e.g. stem from ceramic nanoparticles and / or one or more additives, as described later. Alternatively, the copper powder is, or is comprised in, a copper-based compound powder (e.g. be bronze powder), wherein the copper-based compound powder comprises at least 85 wt%, at least 90 wt%, at least 95 wt% or at least 99 wt% copper (based on the total weight of the copper -based compound powder), and wherein the copper-based compound powder and the graphene in the material mixture amount to at least 92 wt%, or at least 95 wt%, or at least 97 wt%, or at least 99 wt% such as at least 99.5 wt% of the composite material. As previously described, any remaining part may stem from the ceramic nanoparticles and / or the one or more additives. According to one embodiment, the material mixture comprises: 85 to 99 wt% of the copper powder and 0.5 to 5 wt% of the graphene, wherein the amount of copper powder is adapted such that the copper powder and the graphene amount to at least 92 wt%, or at least 95 wt%, or at least 97 wt%, or at least 99 wt% such as at least 99.5 wt% of the composite material.

[0014] The composite material thus provides low friction, good electrical conductivity and additionally less wear which leads to an extended lifetime of the electric brush. Reduced voltage drop and contact resistance also opens up the possibility to increase the current carrying capacity. Unlike the graphite containing commercial brushes which are sensitive to humidity change, the present composite material is more robust and maybe operated at a broad humidity and temperature range.

[0015] The electric brush produced by the first aspect of the present disclosure may consist of the composite material, i.e. the whole electric brush may be made out of the composite material. As an alternative, the electric brush may comprise a support to which the composite material is attached, e.g. as a composite material layer. Typically, the electric brush produced by the first aspect of the present disclosure comprises a contacting surface configured to be in contact with the moving counterpart during use. Therefore, the composite material may be referred to as a composite contacting material.

[0016] Herein the term graphene is used collectively for carbon atoms in a 2D- honeycomb lattice in the form of mono-layer sheets, bi-layer sheets, few (3-5 layers) layer sheets, or nano-platelets having a thickness of at most 50 nm, e.g., within the range of 1 to 50 nm. Thus, the graphene maybe referred to as 2D material. The graphene may be pure graphene or reduced graphene oxide (rGO). The graphene is preferably pure graphene. However, certain advantages for using rGO over pure graphene exists. In this context, rGO refers to graphene oxide with an oxygen content below 4%, offering a partially restored sp2-bonded carbon network, thus fulfilling the same, or essentially the same functional role as pure graphene in the present composite material, e.g. in providing electrical conductivity and contributing to the lubricating effect. The use of rGO is advantageous in that it retains key electrical and structural properties of graphene while being more economically favourable, thus enabling broader industrial applicability. Accordingly, in at least some embodiments, the term “graphene” as used herein encompasses rGO, unless otherwise explicitly specified.

[0017] Hot isostatic pressing (HIP) typically comprises the following additional steps: placing the material mixture in a holder, e.g. a metal capsule, and placing the holder with the material mixture in a HIP furnace having an inert atmosphere (e.g. a He-atmosphere). Thereafter, the previously mentioned step b) is performed. Finally, the holder (e.g. the metal capsule) is machined away and the composite material is shaped as an electric brush (e.g. during the machining step). Step b) may e.g. be performed for at least 2 hours, such as e.g. between i and 3 hours. During hot isostatic pressing, the material mixture is subjected to the same pressure in all directions compared to a uniaxial sintering process.

[0018] According to one embodiment, the mixing in step a) includes dry-mixing the copper powder with the graphene. Hereby, the dispersity of the graphene in the material mixture may be improved, resulting in advantageous tribological properties of the composite material. By using dry-mixing of the copper powder and the graphene instead of wet-mixing, impurities originating from residues of the solvent used during the wet-mixing can be avoided. The drymixing can be performed in a high-speed shaker, for example, with adequate results. Other mixing methods such as ball milling may also be employed. However, according to at least one embodiment, step a) excludes ball milling. According to one embodiment, the mixing in step a) is a mechanical mixing.

[0019] According to one embodiment, the content of graphene in the material mixture is in a range of 1 to 3 wt%. It has been found that this ranges of graphene provides an advantageous trade-off between cost, and lubrication and wear resistance properties, while accounting for the risk of graphene turning into graphite. The weight percentage is based on the composite material as a whole.

[0020] According to one embodiment, the content of copper powder in the material mixture is between 90 and 99 wt%. Thus, and according to one example, the composite material produced in step b) comprises: 90 to 99 wt% copper and 1 to 3 wt% of graphene. These ranges of copper and graphene provides an advantageous trade-off between cost, and lubrication and wear resistance properties. The weight percentage is based on the composite material as a whole. According to one embodiment, the content of the copper powder in the material mixture is between 92 or 95 and 99 wt%, the weight percentages being based on the composite material as a whole.

[0021] The particle size of the copper powder used in step a) is typically referring to the average particles size. That is, the copper powder used in step a) typically has a particle size of 10-500 pm. According to one embodiment the copper powder has a particle size of at least 15 pm, such as at least 30 pm, at least 50 pm, at least 100 pm, or at least 200 pm, and a particle size of at most 500 pm, such as at most 450 pm, at most 400 pm, at most 350 pm, or at most 300 pm. The particle size of the copper powder maybe determined with quasi spherical morphology. The particle size of the copper powder may be referred to as the grain size of the copper powder. Alternatively, the average size of the copper powder may be determined using an electron microscope according to know methods. For example, the size of the copper powder may be determined by Transmission Electron Microscopy (TEM), Dynamic Light Scattering (DLS) or Scanning Electron Microscopy (SEM). The size determination of the copper powder is typically performed on dry particles. However, the DLS is typically performed for the hydrodynamic particle size.

[0022] According to one embodiment, the copper powder is in the form of bronze powder. The bronze powder typically comprises at least 85 wt% copper. Thus, the composite material produced in step b) may comprise, or consists of, the bronze and the graphene, and possibly one or more additives (as described later).

[0023] According to one embodiment, graphene is graphene nanoplatelets. Graphene nanoplatelets is a low-cost material which suffices for the purpose of making the composite material. Typically, the graphene nanoplatelets are visible under electron microscope in the composite material.

[0024] According to one embodiment, the graphene is in the form of graphene particles each having a surface area in a range of 100-750 m2 / g. For example, the graphene maybe graphene nanoplatelets each having a surface area in a range of 100-750 m2 / g. However, it should be mentioned that the graphene particles may be graphene nanopowder, or graphene flakes. The surface area of the graphene particles may e.g. be measured using the Brunauer- Emmett-Teller (BET) method.

[0025] According to one embodiment, step a) involves mixing a first sub-set of the copper powder having a first average particle size with a second sub-set of the copper powder having a second average particle size, wherein the first and second average particle size differ from each other by at least 50 pm. Hereby, the density of the composite material of the electric brush may be increased. For example, the first and second average particle size differ from each other by at least 75 pm, or at least 100 pm, such as e.g. 150 pm. Additionally or alternatively, the first average particle size is between 50 and 100 pm, and the second average particle size is between 200 and 300 pm. That is, as the material mixture comprises copper powder of at least two sub-sets having distinctly different average particles sizes differing by at least 50 pm, the copper powder may be more densely packed. Moreover, the dispersity of the graphene in the material mixture may be improved, resulting in advantageous tribological properties of the composite material of the electric brush. As previously described, the size or average particle size of the copper powder may be determined by TEM, DLS or SEM. The respective weight of the first sub-set of the copper powder and the second sub-set of the copper powder may e.g. be approximately the same, or differ by at most 10%, or at most 20% or at most 50% . The first sub-set of the copper powder and the second sub-set of the copper powder may e.g. amount to at least 50 wt% of the total copper powder, or at least 75 wt% of the total copper powder, or at least 90 wt% of the total copper powder, or at least 99 wt% (or all of) the copper powder.

[0026] According to one embodiment, the composite material has a density of at least 85% of the theoretical density of pure copper. For example, the composite material of the electric brush has a density of at least 90% of the theoretical density of pure copper, or of at least 95% of the theoretical density of pure copper. Samples of composite material produced by the method according to the first aspect of the disclosure have shown to have a compact density between 80 to 99% of the theoretical density of pure copper. The density of the composite material may e.g. be determined according to the Archimedes’ principle using water as the immersion medium, in accordance with ISO 3369 (e.g. ISO 3369:2006) or equivalent recognized standards. The density maybe calculated based on the measured mass of the sample in air and its apparent mass when immersed in water. The measured density may then be compared to the theoretical density of pure copper (8.96 g / cm3) to express the compactness as a relative percentage.

[0027] According to one embodiment, the composite material has a Vickers Hardness of at least 65 HV for a theoretical density of at least 90 % of pure copper. Thus, the composite material of the electric brush may exhibit a higher hardness compared to a composite material produced by conventional sintering, an even an electric brush produced by spark plasma sintering. For example, the composite material has a Vickers Hardness of at least 70 HV, or at least 75 HV, or at least 80 HV for a theoretical density of at least 90 % of pure copper.

[0028] According to one embodiment, the contact resistance of the composite material is below 20 mohm for a contact load of 10 N, and / or is below 5 mohm for a contact load of 40 N, using the contact resistance method as defined herein. Thus, during use of the electric brush, as the composite material of the stationary part is brought into contact with the moving counterpart, the contact resistance of the composite material is below 20 mohm for a contact load of 10 N, and / or is below 5 mohm for a contact load of 40 N, using the contact resistance method as defined herein. The contact resistance method is defined later in the text. According to one embodiment, the contact resistance of the composite material is below 10 mohm for a contact load of 10 N, and / or is below 3 mohm for a contact load of 40 N, using the contact resistance method as defined herein. According to one embodiment, the contact resistance of the composite material is below 1 mohm for a contact load of above 60 N, and / or below 0.4 mohm for a contact load of 100 N or above.

[0029] According to one embodiment, the method further comprises that step a) in addition to mixing the copper powder with the graphene, involves mixing 0.5 to 3 wt%, preferably 0.5 to 1 wt%, of ceramic nanoparticles having a size of between 5 to 100 nm and / or 0.1 to 5 wt% of one or more additives, with the copper powder and the graphene to obtain the material mixture.

[0030] By including ceramic nanoparticles in the material mixture, the composite material produced in step b) may exhibit advantageous tribological properties in comparison with an electric brush comprising a copper- graphene composite material without the ceramic nanoparticles. For example, the advantageous tribological properties may include low friction and high wear resistance. Due to the low wear nature of the copper- graphene-ceramic nanoparticles composite material, the contact pressure between the electric brush and the moving counterpart may be further increased, leading to reduced contact resistance and voltage drop, and thereby also reduced electrical loss during use. Typically, the mixing of the ceramic nanoparticles with the copper powder and the graphene is included in the previously described dry-mixing.

[0031] According to one embodiment, the material mixture comprises: 92 to 99 wt% copper powder; 0.5 to 5 wt% of graphene; 0.5 to 3 wt% of ceramic nanoparticles having a size of between 5 to 100 nm; wherein the weight percentages are based on the composite material as a whole. According to one embodiment, the material mixture comprises: 85 to 99 wt% of the copper powder; 0.5 to 5 wt% of the graphene; and 0.5 to 3 wt% of the ceramic nanoparticles, wherein the amount of copper powder is adapted such that the copper powder, the graphene and the ceramic nanoparticles amount to at least 95 wt%, or at least 97 wt%, or at least 99 wt% such as at least 99.5 wt% of the composite material. Any remaining part may e.g. stem from the one or more additives. According to one embodiment, the composite material produced in step b) comprises, or consists of, 92 to 99 wt% of a copper-based compound (such as bronze as previously described), 0.5 to 5 wt% of graphene; and optionally 0.5 to 3 wt% of ceramic nanoparticles having a size of between 5 to 100 nm and possibly one or more additives. The copperbased compound may e.g. be bronze and comprise at least 85 wt% copper, or it may be pure copper (e.g. at least 99 wt% copper). The copper-based compound may e.g. comprise between 5 to 12 wt% tin, and optionally small amounts of Al, P, Mn Ni, Fe Zn, Sb or S. According to one embodiment, the composite material consists of the copper-based compound, the graphene, optionally the ceramic nanoparticles and possibly the one or more additives previously described. According to one embodiment, the ceramic nanoparticles are selected from the group consisting of aluminium oxide, silicon oxide, yttrium oxide, silicon carbide and tungsten carbide. Such ceramic nanoparticles may improve the strength of the composite material at the same time showing excellent dry lubricating properties and good electrical conductivity. For example, the ceramic nanoparticle is aluminium oxide. According to one embodiment, the ceramic nanoparticles are selected from the group consisting of aluminium oxide, silicon oxide, yttrium oxide and silicon carbide.

[0032] Typically, the size of the ceramic nanoparticles is referring to the average size, or average diameter of the nanoparticles. The average size may e.g. be determined using an electron microscope according to know methods. For example, the size of the ceramic nanoparticles may be determined by Transmission Electron Microscopy (TEM), Dynamic Light Scattering (DLS) or Scanning Electron Microscopy (SEM). However, there are other ways of determining the size of the ceramic nanoparticles, such as the Brunauer- Emmett-Teller (BET) surface area analysis and subsequent calculation of the average particle size. The size determination of the ceramic nanoparticles is typically performed on dry particles. However, the DLS is typically performed for the hydrodynamic particle size.

[0033] According to one embodiment, the ceramic nanoparticles have a size of between 5 to 50 nm. For example, the ceramic nanoparticle is aluminium oxide having an average particle size of below 50 nm, but above 5 nm. In one embodiment, the composite material comprises: a) 85 to 99 wt% copper; b) 1 to 3 wt% of graphene; c) 0.5 to 3 wt% of ceramic nanoparticles being aluminium oxide having a size of between 5 to 100 nm or between 5 to 50 nm; or a) 85 to 99 wt% copper; b) 0.5 to 5 wt% of graphene; c) 0.5 to 1 wt% of ceramic nanoparticles being aluminon oxide having a size of between 5 to 100 nm or between 5 to 50 nm. These ranges of copper, graphene and ceramic nanoparticles provides an advantageous trade-off between cost, and lubrication and wear resistance properties. As previously described, the amount of the copper in the composite material maybe at least: 90, or 92, or 95, or 98.5 or 99 wt%. According to one embodiment, the material mixture comprises: at least 92 wt% of the copper powder; 1 to 3 wt% or 0.5 to 5 wt% of the graphene; and 0.5 to 1 wt% or 0.5 to 3 wt% of the ceramic nanoparticles. For example, the material mixture may comprise 92 to 98.5 wt % of the copper powder; 1 to 3 wt% of the graphene; and 0.5 to 1 wt% of the ceramic nanoparticles.

[0034] According to one embodiment, the ceramic nanoparticles have a mean aspect ratio of between 1 and 3, preferably between 1 and 2. It has been found that such aspect ratio of the ceramic nanoparticles is preferable for the composite material. For example, the dispersity of the ceramic nanoparticles in the copper powder and graphene is improved, resulting in advantageous tribological properties of the composite material. Stated differently, a majority, e.g. at least 90 wt%, of the ceramic nanoparticles have an aspect ratio of between 1 and 3, preferably between 1 and 2. For example, the ceramic nanoparticles may be spherical, or near spherical, e.g. such that a majority, e.g. at least 90 wt%, of the ceramic nanoparticles have an aspect ratio of between 1 and 1.5. According to one embodiment, the ceramic nanoparticles are not formed as whiskers (e.g. 90 wt% of the ceramic nanoparticles do not have an aspect ratio of between 3 and 7). The aspect ratio refers to the ratio of the longest to the shortest axis of a particle (or ratio of the longest edge by the shortest edge of the particle), and may e.g. be determined using an electron microscope according to know methods, such as measured by electron microscopy techniques such as SEM or TEM.

[0035] The one or more additives which may be included in the material mixture may include a stabilizer and / or a binder. According to one embodiment the one or more additives consist of the stabilizer and the binder. The term additives as used herein may refer to materials added in minor amounts (such as 0.1 to 5 wt%) to facilitate processing or stability. Specifically, additives may include stabilizers to prevent oxidation during processing, and / or binders to enhance sintering.

[0036] According to one embodiment, the composite material consists of the copper and the graphene, and at least one of the ceramic nanoparticles and the one or more additives. Thus, the composite material may consist of a sintered copper, graphene, ceramic nanoparticles mixture including the one or more additives. According to one embodiment, the composite material consists of, or consists essentially of, the copper (or bronze), the graphene, the ceramic nanoparticles selected from aluminium oxide, silicon oxide, yttrium oxide, or silicon carbide, and optionally one or more additives, such as a stabilizer and / or binder. For example, no other constituents are intentionally included in the composite material apart from trace impurities.

[0037] According to one embodiment, the mixing in step a) excludes wet-mixing the copper powder with the graphene.

[0038] According to one embodiment, the electric brush is free of graphite.

[0039] According to one embodiment, the electric brush is free of nickel and / or free of chromium. Thus, according to one embodiment, the composite material is nickel-free and / or chromium-free and / or graphite-free. For example, the absence of graphite combined with HIP processing results in superior mechanical strength and / or contact resistance stability.

[0040] According to one embodiment, the method further comprises that step b) is the only sintering step performed for producing the composite material of the electric brush. By only performing one sintering step being hot isostatic pressing, the density of the composite material will be high enough to be used as an electric brush.

[0041] There is according to a second aspect of the disclosure provided an electric brush configured to transmit electric current between a stationary part and its moving counterpart, the electric brush being produced by the method of the first aspect of the disclosure.

[0042] There is according to a third aspect of the disclosure provided an electric motor or generator comprising a stationary part and a moving counterpart, and an electric brush of the second aspect of the disclosure, wherein the electric brush is configured to transmit electric current between the stationary part and the moving counterpart. Thus, the electric brush typically forms part of the stationary part of the motor or generator, and the moving counterpart may form part of a rotating shaft. Effects and features of the second and third aspects of the disclosure are largely analogous to those described above in connection with the first aspect of the disclosure. Embodiments mentioned in relation to the first aspect of the invention are largely compatible with the second and third aspects of the invention, of which some are exemplified below.

[0043] The motor or generator may e.g. be a wind turbine generator such as a doubly fed wind turbine generator, a slip ring modular motor, a (large) synchronous motor, and a non-magnet based brushed synchronous motor. The motor or generator may e.g. be a stationary motor or generator, or may be used in moving applications, such as e.g. in electric vehicles (EVs).

[0044] According to one embodiment, the moving counterpart is a slip ring or a commutator.

[0045] Generally, all terms used in the claims are to be interpreted according to their ordinary meaning in the technical field, unless explicitly defined otherwise herein. All references to "a / an / the element, apparatus, component, means, etc. are to be interpreted openly as referring to at least one instance of the element, apparatus, component, means, etc., unless explicitly stated otherwise.

[0046] BRIEF DESCRIPTION OF THE DRAWINGS

[0047] The specific embodiments of the inventive concept will now be described, by way of example, with reference to the accompanying drawings, in which:

[0048] Fig. 1 is a side view of an electric motor comprising an electric brush configured to transmit electric current between a stationary part of the motor and its moving counterpart according to one example;

[0049] Fig. 2 is a flowchart of a method of producing an electric brush according to one example;

[0050] Fig. 3 is a graph showing contact resistance vs contact force for various samples of electric brushes;

[0051] Fig. 4 is a graph showing normalized volume wear rate vs current for various samples of electric brushes;

[0052] Fig. 5 is a graph showing Vickers hardness vs theoretical density for various samples of electric brushes; and

[0053] Fig. 6 shows two SEM images of composite material produced using hot isostatic pressing according to one example.

[0054] DETAILED DESCRIPTION

[0055] The inventive concept will now be described more fully hereinafter with reference to the accompanying drawings, in which exemplifying embodiments are shown. The inventive concept may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided by way of example so that this disclosure will be thorough and complete, and will fully convey the scope of the inventive concept to those skilled in the art. Like numbers refer to like elements throughout the description.

[0056] Fig. 1 depicts an electric motor 1 comprising a stationary part io and a moving counterpart 20. The moving counterpart 20 comprises a rotating shaft 22 and a plurality of slip rings 24 mounted in a support 26. The support 26 is attached to the shaft 22. Correspondingly, the stationary part 10 comprises a plurality of associated electric brushes 12. The electric brushes 12 are in Fig. 1 mounted to a holder 14, and are pushed against the slip rings 24 by means of a force member 16, such as one or more mechanical springs. The force maybe higher than 25 kPa, such as 30 kPa or higher than 30 kPa, for example equal to or higher than 35 kPa or equal to or higher than 40 kPa, such as equal to or higher than 45 kPa. According to some examples, the force maybe equal to or higher than 100 MPa, for example equal to or higher than 250 MPa.

[0057] Thus, each electric brush 12 is arranged in mechanical and electrical contact with a respective slip ring 24. Thus, the electric brushes 12 are arranged stationarily and the slip rings 24 are rotated concurrently with rotation of the shaft 22. The electric brushes 12 are pressed radially inwards towards its associated slip ring 24 by means of the force member 16. The electric brushes 12 are configured to transmit electric current between the stationary part 10 and the moving counterpart 20 via the slip rings 24. For example, the electric brushes 12 are configured to transmit electric current from the stationary part 10 to the moving counterpart 20 via the slip rings 24. However, it should be mentioned that the electric motor 1 maybe operated in reverse, and thus function as a generator.

[0058] A method for producing an electric brush comprising a composite material, such as the electric brush 12 of Fig. 1, will now be described with reference to Fig. 2.

[0059] The method generally comprises producing a composite material, which is shaped or formed as an electric brush.

[0060] In step a), or a first step, S10, copper powder is mixed with graphene to obtain a material mixture. In more detail, 85 or 90 to 99 wt% copper powder having a particle size of 10-500 pm is mixed with 0.5 to 5 wt% graphene to obtain the material mixture, the weight percentages being based on the composite material as a whole. The mixing in the first step S10 is preferably dry-mixing, and preferably mechanical mixing. For example, the material mixture is achieved by vigorously mixing, e.g. using a high-speed shaker. According to one example, the shaking speed maybe 700 rpm and the mixing time may be 150 seconds. The mixing time may vary depending on the amount of material mixed. The mixing in the first step S10 may exclude wetmixing the copper powder with the graphene.

[0061] The average particles size of the copper powder maybe between 30 and 200 pm, such as between 40 or 50 pm and 100 or 150 pm.

[0062] The content of graphene in the material mixture is preferably in a range of 1 to 3 wt%. The graphene, which is a 2D material, maybe in the form of graphene particles each having a surface area in a range of 100-750 m2 / g. The graphene is preferably in the form of graphene nanoplates. The first step Sio may comprise the sub-step S15 of mixing a first sub-set of the copper powder having a first average particle size with a second sub-set of the copper powder having a second average particle size, wherein the first and second average particle size differ from each other by at least 50 pm. Hereby, copper powder, and other components of the material mixture, may be more densely packed.

[0063] According to one embodiment, and in addition to mixing the copper powder with the graphene, the first step Sio additionally includes mixing 0.5 to 3 wt%, preferably 0.5 to 1 wt%, of ceramic nanoparticles having a size of between 5 to 100 nm and / or 0.1 to 5 wt% of one or more additives, with the copper powder and the graphene to obtain the material mixture. The weight percentages are here based on the composite material as a whole.

[0064] The ceramic nanoparticles are preferably selected from the group consisting of aluminium oxide, silicon oxide, yttrium oxide, silicon carbide and tungsten carbide. The ceramic nanoparticles may be defined by an aspect ratio of between 1 and 3, preferably between 1 and 2. For example, the ceramic nanoparticles may be spherical, or near spherical having an aspect ratio of between 1 and 1.5.

[0065] In a step b), or a second step, S20, occurring subsequent to the first step Sio, the material mixture (with or without the ceramic nanoparticles and / or the one or more additives) is subjected to hot isostatic pressing (HIP) at a temperature of between 650 and 950 °C and under a pressure of 1500 to 2000 bar for at least 1 hour to form the composite material. Thus, in the second step S20, the material mixture is sintered by hot isostatic pressing.

[0066] During the hot isostatic pressing in the second step S20, the material mixture is placed in a holder, typically a metal capsule, whereafter the holder with the material mixture is subjected to the hot isostatic pressing, typically for between 1 and 3 hours. The sintering is typically performed in a HIP furnace with inert atmosphere (e.g. a He-atmosphere). Finally, the holder (e.g. the metal capsule) is machined away. It should be mentioned that the sintering of the second step S20 is preferably the only sintering step performed to obtain the composite material or the electric brush.

[0067] The composite material may be shaped as an electric brush during the second step S20, or it may be shaped as an electric brush in a third step S30 after the second step S20. The electric brush thus obtained is composed of the composite material obtained according to the method. The composite material may consist of the copper and the graphene, and optionally at least one of the ceramic nanoparticles and the one or more additives.

[0068] Turning back to Fig. 1, each one of the electric brushes 12 may be produced by the method described with reference to Fig. 1. Thus, each one of the electric brushes 12 may be made of a composite material comprising: a) 85 or 90 to 99 wt% copper; and b) 0.5 to 5 wt% of graphene, e.g. 1 to 3 wt% graphene, and / or c) 0.5 to 3 wt% of ceramic nanoparticles having a size of between 5 to 100 nm; and / or d) 0.1 to 5 wt% of one or more additives, wherein the weight percentages are based on the composite material as a whole, here being the electric brush 12 as a whole. Typically, no other material (except for traces of below 0.1 wt %) is included in the composite material. The one or more additive may e.g. be a stabilizer and / or a binder. In other words, the composite material of each electric brush 12 is a sintered composite material which consists of the previously described material mixture.

[0069] The composite material of the electric brush 12 is, during use, arranged in contact with the moving counterpart 20 and the slip rings 24 as previously described.

[0070] The electric brush 12 is preferably free of nickel, chromium and / or graphite.

[0071] Examples

[0072] In the following, various samples were prepared and analyzed with regards to specific material parameters. Each sample correspond to the previously described electric brush. The samples were prepared by mixing copper powder with graphene powder. The graphene powder was graphene nanoplatelets, and the copper powder had an average particle size of 75 pm. The mixing was dry-mixing. For the mixing, a paint shaker model SK35 from Fast & Fluid was used.

[0073] The copper-graphene powder mixture was then sintered by hot isostatic pressing (Quintus Technologies, HIP QIH9 with graphite furnace) into a composite material to be used as an electric brush. Prior to hot isostatic pressing, the copper-graphene powder mixture was placed in a holder, here a metal capsule (tube-shaped), whereafter the holder with the copper-graphene powder mixture was placed in a HIP furnace with inert atmosphere (He- atmosphere) and subjected to hot isostatic pressing (1800 bar) under elevated temperature (740 °C) for approximately 3 hours. Finally, the holder was machined away and the composite material shaped into an electric brush.

[0074] Example 1.

[0075] In a first set of samples, a first sample (IE1) was prepared by dry-mixing 99 wt% copper powder with 1 wt% graphene powder using hot isostatic pressing as previously described, a second sample (IE2) was prepared by dry-mixing 97 wt% copper powder with 3 wt% graphene powder using hot isostatic pressing as previously described.

[0076] With reference to Fig. 3, the contact resistance vs contact force was evaluated for the first and second samples, together with a comparison with an electrographite brush (CEi) and a graphite-copper brush (CE2) not produced by hot isostatic pressing. As shown in Fig. 3, the contact resistance of IE1 is below 20 mohm for a contact load of 10 N, below 8 mohm for a contact load of 20 N, below 3 mohm for a contact load of 30 N, below 2 mohm for a contact load of 40 N, and even below 1 mohm for a contact load of 50 N. The contact resistance of IE2 is below 10 mohm for a contact load of 10 N, below 4 mohm for a contact load of 20 N, below 3 mohm for a contact load of 30 N, and below 2 mohm for a contact load of 40 N and 50 N. This is vastly better performance as compared to CEi and CE2, for which the contact resistance is above 20 mohm for all contact loads 0-50 N. Thus, for both of IE1-IE2, the contact resistance is below 20 mohm for a contact load of 10 N, and is below 6 mohm for a contact load of 40 N. For higher contact forces, both IE1-IE2 exhibit a contact resistance of below 1 mohm for a contact load of above 60 N, and even below 0.4 mohm for a contact load of 100 N.

[0077] Example 2.

[0078] With reference to Fig. 4, the normalized wear rate vs current was evaluated for IE2 of example 1, a comparative sample (CE3) prepared by dry-mixing 99 wt% copper powder with 1 wt% graphene powder using spark plasma sintering (SPS), and the graphite-copper brush (CE2) not produced by hot isostatic pressing of example 1. As shown in Fig. 4, the wear rates for 1 hour were compared. It is evident that the normalized wear rate (mm3 / km) for IE2 is vastly better than CE3, and in parity with CE2. For IE2, the normalized wear rate is below 1 mitf / km for all measured currents 0-30 A, as compared to CE3 exhibiting a normalized wear rate of over 1 mitf / km, and even over 10 mm3 / km, for all measured currents except that of 6 A. Thus, the wear rate is significantly reduced, by more than one order of magnitude (mm3 / km), owing to the hot isostatic pressing.

[0079] Example 3.

[0080] With reference to Fig. 5, the Vickers hardness was evaluated for the previously described samples IE1, IE2 and CEi, together with and an additional comparative sample (CE4) prepared by dry-mixing 97 wt% copper powder with 3 wt% graphene powder using spark plasma sintering (SPS). As shown in Fig. 5, the samples prepared by spark plasma sintering method (CEi and CE4) both showed lower hardness values compared to their corresponding inventive samples prepared by hot isostatic pressing (IE1 and IE2) at the same graphene loading (1 wt% or 3 wt%). For all samples (IE1, IE2, CEi, CE4) the densities were at least 90 % of the theoretical density of pure copper. Both IE2 and IE2 exhibit a Vickers hardness of above 65 HV, even above 70 HV or above 75 HV. Typically, the Coefficient of Friction (COF) is below 0.3 as measured by a standard pin-on-disk tribology test (using stainless steel or bronze counterface).

[0081] Example 4.

[0082] With reference to Fig. 6, two SEM images are shown for a composite material produced using hot isostatic pressing as previously described and comprising 99 wt% copper and 1 wt% graphene (left) and 97 wt% copper and 3 wt% graphene. As shown in Fig. 6, the copper-graphene composite material produced through hot isostatic pressing retains the structure of graphene material and is almost free from aggregation. The homogenous distribution of graphene flakes at the grain boundary revealed in Fig. 6 is believed to contribute to the increased strength of the composite material produced by hot isostatic pressing. That is, the increased strength could be partially due to grain-boundary strengthening of graphene sheets that are distributed around the grain boundary, serving as a barrier for dislocations.

[0083] Methodology

[0084] The previously mentioned contact resistance vs contact force was performed according to the method as described in the following.

[0085] The contact resistances were measured between a corresponding flat surface of the sample materials against an Ag contact pin having a half spherical tip with a diameter of 10 mm. For the contact resistance measurements, a microohmmeter MR 300 C-A from Schuetz-Messtechnik was used. The method used was a 4-point probe measurement method. Certain contact forces were applied during the CR measurement with a spring load measured by a load cell from Nobel Elektronik. The contact resistance was measured by the 4- point probe technique involving four equally spaced probes around the contact region. A DC current was applied between the outer two probes and a voltmeter measured the voltage difference between the two inner probes. The contact force between the contact surfaces (between flat surface of the sample and the half spherical tip of the Ag contact pin having a diameter of 10 mm) was manipulated with a screw connected to a spring load and monitored with force measuring transducer.

[0086] The previously mentioned normalized wear rate vs current was performed according to the method as described in the following.

[0087] The sample was prepared in a test rig including a stationary part and a moving counterpart. The sample was used an electric brush contacting the moving counterpart during rotation of the latter. The applied power / contact pressure of the sample relative to the moving counterpart was 6-8 N. The current was varied from o to 26 A. The normalized wear rate was measured by weight loss of the sample after a certain traveling distance of the moving counterpart (i.e. the product of circumference of the moving counterpart, the rpm and the time).

[0088] It is noted that the wear rate of the electric brush depends on many parameters, such as contact pressure, electrical load, speed of the moving counterpart, state of the collector, ambient conditions etc. However, by applying the above method for the different samples and ensuring that the conditions and parameters are corresponding, comparison between the wear rates of the samples can be achieved.

[0089] The previously mentioned Vickers hardness was performed according to the method as described in the following.

[0090] Vickers hardness test according to ISO 6507 for a load of 500 g (HV 0.5) valid on the date of priority for the present patent application.

[0091] The inventive concept has mainly been described above with reference to a few examples. However, as is readily appreciated by a person skilled in the art, other embodiments than the ones disclosed above are equally possible within the scope of the inventive concept, as defined by the appended claims.

Claims

CLAIMS1. A method of producing an electric brush configured to transmit electric current between a stationary part and its moving counterpart, the method comprising: producing a composite material, including: a) mixing 85 to 99 wt% copper powder having a particle size of 10- 500 pm with 0.5 to 5 wt% graphene to obtain a material mixture, the weight percentages being based on the composite material as a whole; and b) subjecting the material mixture to hot isostatic pressing at a temperature of between 650 and 950 °C and under a pressure of 1500 to 2000 bar for at least 1 hour to form the composite material; and shaping the composite material as an electric brush.

2. The method of claim 1, wherein the mixing in step a) includes drymixing the copper powder with the graphene.

3. The method of any of the preceding claims, wherein the content of graphene in the material mixture is in a range of 1 to 3 wt%.

4. The method of any of the preceding claims, wherein the graphene is graphene nanoplatelets.

5. The method of any of the preceding claims, wherein the graphene is in the form of graphene particles each having a surface area in a range of 100- 750 m2 / g.

6. The method of any of the preceding claims, wherein step a) involves mixing a first sub-set of the copper powder having a first average particle size with a second sub-set of the copper powder having a second average particle size, wherein the first and second average particle size differ from each other by at least 50 pm.

7. The method of any of the preceding claims, wherein the composite material has a density of at least 85 % of the theoretical density of pure copper.

8. The method of any of the preceding claims, wherein the composite material has a Vickers Hardness of at least 65 HV for a theoretical density of at least 90 % of pure copper.

9. The method of any of the preceding claims, wherein the contact resistance of the composite material is below 20 mohm for a contact load of 10 N, and / or is below 6 mohm for a contact load of 40 N, using the contact resistance method as defined herein.

10. The method of any of the preceding claims, wherein step a) in addition to mixing the copper powder with the graphene, involves mixing 0.5 to 3 wt%, preferably 0.5 to 1 wt%, of ceramic nanoparticles having a size of between 5 to 100 nm and / or 0.1 to 5 wt% of one or more additives, with the copper powder and the graphene to obtain the material mixture.

11. The method of claim 10, wherein the composite material consists of the copper and the graphene, and at least one of the ceramic nanoparticles and the one or more additives.

12. The method of any of the preceding claims, wherein the mixing in step a) excludes wet-mixing the copper powder with the graphene.

13. The method of any of the preceding claims, wherein the electric brush is free of graphite.

14. The method of any of the preceding claims, wherein step b) is the only sintering step performed for producing the composite material of the electric brush.

15. An electric brush configured to transmit electric current between a stationary part and its moving counterpart, the electric brush comprising a composite material obtainable by means of the method of any of the preceding claims.

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