An electric brush

Abstract

An electric brush configured to transmit electric current between a stationary part and its moving counterpart, the electric brush comprising a composite material for contacting the moving counterpart, the composite material comprising: a) 85 to 99 wt% copper; b) 0.5 to 5 wt% of graphene; c) 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.

Description

[0001] 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 a 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 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 an electric brush configured to transmit electric current between a stationary part and its moving counterpart. The electric brush comprises a composite material for contacting the moving counterpart, the composite material comprising: a) 85 to 99 wt% copper; b) 0.5 to 5 wt% of graphene; c) 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.

[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 of 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 of the first aspect of the present disclosure exhibits advantageous tribological properties in comparison with an electric brush comprising a copper- graphene composite material without the ceramic nanoparticles. Due to the low wear nature of the copper-graphene-ceramic nanoparticles composite, 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 may be adapted such that the compounds a), b) and c) 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 of the composite material may e.g. stem from the one or more additives as later described. Alternatively, the copper is comprised in a copper-based compound (e.g. bronze), wherein the copperbased compound 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), and wherein the copper-based compound, 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. As previously described, any remaining part of the composite material may e.g. stem from the one or more additives.

[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 may be operated at a broad humidity and temperature range.

[0015] The electric brush 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 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] According to one embodiment, the composite material comprises: a) 92 to 99 wt% copper; b) 0.5 to 5 wt% of graphene; c) 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 composite material comprises a) 85 to 99 wt% of the copper; b) 0.5 to 5 wt% of the graphene; c) 0.5 to 3 wt% of the ceramic nanoparticles, wherein the amount of copper is adapted such that the compounds a), b) and c) amounts 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 of the composite material may e.g. stem from the one or more additives.

[0018] According to one embodiment, the composite material comprises: a) 85 to 99 wt% copper; b) 1 to 3 wt% of graphene; c) 0.5 to 1 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. It has been found that these ranges of copper, graphene and ceramic nanoparticles provides an advantageous trade-off between cost, and lubrication and wear resistance properties, while maintaining excellent electrical properties. According to 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 having a size of between 5 to 100 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 having a size of between 5 to 100 nm; wherein the weight percentages are based on the composite material as a whole. These ranges of copper, graphene and ceramic nanoparticles provides an advantageous trade-off between cost, and lubrication and wear resistance properties.

[0019] According to one embodiment, the amount of copper in the composite material is between 90 and 99 wt%. Thus, and according to one example, the composite material comprises: a) 90 to 99 wt% copper; b) 1 to 3 wt% of graphene; c) 0.5 to 1 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. These ranges of copper, graphene and ceramic nanoparticles provides an advantageous trade-off between cost, and lubrication and wear resistance properties.

[0020] According to one embodiment, the composite material comprises: a) at least 92 wt% of the copper; b) 1 to 3 wt% or 0.5 to 5 wt% of the graphene; c) 0.5 to 1 wt% or 0.5 to 3 wt% of the ceramic nanoparticles. For example, the composite material comprises a) 92 to 98.5 wt % of the copper; b) 1 to 3 wt% of the graphene; c) 0.5 to 1 wt% of the ceramic nanoparticles.

[0021] According to one embodiment, the 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.

[0022] 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 may be 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.

[0023] 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.

[0024] 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.

[0025] 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. As previously described, the amount of a) copper in the composite material maybe at least: 90, or 92, or 95, or 98.5 or 99 wt%.

[0026] 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 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 i and 2. For example, the ceramic nanoparticles maybe 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.

[0027] According to one embodiment, the composite material further comprises: d) 0.1 to 5 wt% of one or more additives. The one or more additives 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.

[0028] According to one embodiment, the composite material consists of the copper, the graphene, the ceramic nanoparticles and 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.

[0029] According to one embodiment, the composite material is nickel-free and / or chromium-free and / or graphite-free.

[0030] According to one embodiment, the contact resistance of the composite material is below 5 mohm, such as below 2 mohm, for a contact load of 20 N, and / or is below 5 mohm, such as below 1 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 5 mohm for a contact load of 20 N, and / or is below 5 mohm for a contact load of 40 N, using the contact resistance method as defined herein. Additionally or alternatively, 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, using the contact resistance method as defined herein. The contact resistance method is defined later in the text.

[0031] According to one embodiment, the composite material consists of a sintered copper, graphene, ceramic nanoparticles mixture. In other words, the composite material has been produced by sintering the copper-grapheneceramic nanoparticles mixture. That is, the composite material may be referred to as a sintered composite material. Hereby, the material can be advantageously densified, even though small particles are used as ingoing component, while avoiding coarsening which accompanies other densification processes. For example, powders, such as dry powders, of the copper graphene and ceramic nanoparticles are used in the copper-grapheneceramic nanoparticles mixture. Preferably, the composite material of the sintered copper, graphene, ceramic nanoparticles mixture has a compact density of between 80 to 99% of the theoretical density of pure copper.

[0032] According to one embodiment, the composite material has been produced by dry-mixing the copper, graphene and ceramic nanoparticles mixture prior to sintering the copper, graphene, ceramic nanoparticles mixture. For example, the dispersity of the ceramic nanoparticles and the graphene in the copper- graphene-ceramic nanoparticles mixture may hereby be improved, resulting in advantageous tribological properties of the composite material. By using dry-mixing of the copper, graphene and ceramic nanoparticles mixture instead of wet-mixing, impurities originating from residues of the solvent used during the wet-mixing can be avoided. Thus, according to one embodiment, the composite material has been produced by excluding wetmixing the copper, graphene and ceramic nanoparticles mixture. The dry- mixing can be performed in a high-speed shaker, for example, with adequate results. Other mixing methods such as ball milling may also be employed.

[0033] According to one embodiment, the composite material has been produced by using a copper powder with a particle size of 10-500 pm. That is, the copper, or copper powder, used in the composite material may have 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 may be determined with quasi spherical morphology. The particle size of the copper powder may be referred to as the grain size of the copper. The particle size of the copper powder is typically the average particle size. 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.

[0034] According to one embodiment, the composite material has been produced by using copper powder in the form of bronze powder. Thus, the copper in the composite material may be comprised in bronze. The bronze typically comprises at least 85 wt% copper. Thus, the composite material may comprise, or consists of, the bronze, the graphene, the ceramic nanoparticles and possibly the one or more additives. Thus, and according to one embodiment, the composite material comprises: a) 92 to 99 wt% of a copperbased compound; b) 0.5 to 5 wt% of graphene; c) 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. 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, the ceramic nanoparticles and the one or more additives previously described.

[0035] According to one embodiment, the composite material has been produced by using 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 may be increased. That is, as the copper, graphene and ceramic nanoparticles 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 and / or the ceramic nanoparticles in the copper, graphene and ceramic nanoparticles mixture may be improved, resulting in advantageous tribological properties of the composite material. As previously described, the size or average particle size of the copper powder maybe 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.

[0036] For example, and 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 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 may be 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.

[0037] According to one embodiment, the composite material has been produced by spark plasma sintering or hot isostatic pressing. For example, the composite material may have been produced by only employing one sintering step being spark plasma sintering or hot isostatic pressing. By only performing one sintering step being spark plasma sintering or hot isostatic pressing, the density of the composite material will be high enough to be used as an electric brush.

[0038] For example, in case spark plasma sintering (SPS) is used for producing the composite material, the high speed of the spark plasma sintering process ensures that it can densify powders with small particle size while avoiding coarsening which accompanies standard densification processes, such as non-spark plasma sintering techniques. Samples of composite material produced by this method have shown to have a compact density between 80 to 99% of the theoretical density of pure copper. Due to the rapid nature of the spark plasma sintering process, which is characterised by a high heating rate and short sintering time, typically only a couple of minutes, the composite material retains the structure of the graphene material, and it is free from aggregation compared to other sintering techniques which take much longer time, with a risk of causing the graphene sheets to oxidise or agglomerate, deteriorating the lubricating and electrical properties of graphene. The spark plasma sintering used for producing the composite material may utilize a sintering temperature of 65O-95O°C and a pressure of 10-iooMPa for at least 2 minutes, such as at least 3 minutes, or at least 5 minutes, and for at most 20 minutes, such as at most 15 minutes, such as at most 10 minutes. Typically, an inert atmosphere is used during the spark plasma sintering for eliminating the risk of oxidation of the composite material. Oxidation reduces the electrical conductivity of materials.

[0039] For example, in case hot isostatic pressing (HIP) is used for producing the composite material, the copper-graphene-ceramic nanoparticles mixture is typically placed in a holder, e.g. a metal capsule, and thereafter placed in a HIP furnace having an inert atmosphere (e.g. a He-atmosphere). Thereafter, the copper-graphene-ceramic nanoparticles mixture and the holder is subjected to hot isostatic pressing. The hot isostatic pressing for producing the composite material may utilize a sintering temperature of 650 and 950 °C and a pressure of 1500 to 2000 bar for at least 1 hour, such as at least 2 hours, e.g. between 1 and 3 hours. During hot isostatic pressing, the copper- graphene-ceramic nanoparticles mixture is subjected to the same pressure in all directions compared to a uniaxial sintering process. In case the composite material has been produced by hot isostatic pressing, the composite material may exhibit 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, or even an electric brush produced by spark plasma sintering.

[0040] However, the composite material may be produced using other technologies than spark plasma sintering and hot isostatic pressing, e.g. extrusion and powder metallurgy consolidation methods.

[0041] There is according to a second aspect of the disclosure provided an electric motor or generator comprising a stationary part and a moving counterpart, and an electric brush of the first aspect of the disclosure, wherein the electric brush is configured to transmit electric current between the stationary part and the moving counterpart.

[0042] Effects and features of the second aspect 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 aspect of the invention, of which some are exemplified below. 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.

[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; and

[0051] Fig. 4 is a graph showing volume wear resistance vs sliding distance for various samples of electric brushes. DETAILED DESCRIPTION

[0052] 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.

[0053] 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 rotating contacting components, here exemplified as 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 may be 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.

[0054] 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.

[0055] 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 i maybe operated in reverse, and thus function as a generator.

[0056] Each one of the electric brushes 12 in Fig. 1 is made of a composite material comprising: a) 85 or 90 to 99 wt% copper; b) 0.5 to 5 wt% of graphene; c) 0.5 to 3 wt% of ceramic nanoparticles having a size of between 5 to 100 nm; and optionally 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.

[0057] The amount of graphene in the composite material is preferably 1 to 3 wt%. Additionally or alternatively, the amount of ceramic nanoparticles in the composite material is preferably 0.5 to 1 wt%. The composite material of the electric brush 12 is, during use, arranged in contact with the moving counter part 20 and the slip rings 24 as previously described.

[0058] The graphene in the composite material is preferably graphene nanoplatelets, each having a surface area in a range of 100-750 m2 / g. 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 maybe defined by an aspect ratio of between 1 and 3, preferably between 1 and 2. For example, the ceramic nanoparticles maybe spherical, or near spherical having an aspect ratio of between 1 and 1.5.

[0059] Typically, the composite material is a sintered composite material. For example, the composite material may consist of a sintered copper, graphene, ceramic nanoparticles mixture, optionally including the previously mention one or more additives. For example, the composite material may have been produced by spark plasma sintering (SPS) or hot isostatic pressing (HIP). Typically, during such process, the composite material is produced by drymixing the copper, graphene and ceramic nanoparticles mixture (and optionally the one or more additives) prior to sintering the copper, graphene, ceramic nanoparticles mixture. 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.

[0060] The method generally comprises producing a composite material, which is shaped or formed as an electric brush.

[0061] In a first step, S10, copper powder is mixed with graphene and ceramic nanoparticles to obtain a copper-graphene-ceramic nanoparticle mixture. As previously mentioned, one or more additives may be added to the mixture. The mixing in the first step S10 is preferably dry-mixing, and preferably mechanical mixing. For example, the copper-graphene-ceramic nanoparticle 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 graphene and ceramic nanoparticles.

[0062] The first step S10 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 copper-grapheneceramic nanoparticle mixture, may be more densely packed.

[0063] Typically, the amount of copper powder is between 85 or 90 and 99 wt%, the amount of graphene is between 0.5 and 5 wt%, such as between 1 and 3 wt%, and the amount of ceramic nanoparticles is between 0.5 and 3 wt %, such as between 0.5 and 1 wt%. In case one or more additives is added to the mixture, the amount of additives is between 0.1 and 5 wt%. The weight percentages are here based on the composite material as a whole.

[0064] The ceramic nanoparticles typically have an average particles size of between 5 and 100 nm, and the copper powder may have an average particles size of between 10 and 500 pm. For example, the average particles size of the ceramic nanoparticles is between io and 90 nm, or between 20 and 80 nm, such as between 30 and 70 nm. Additionally, or alternatively, the average particles size of the copper powder is between 15 and 300 pm, or between 30 and 200 pm, such as between 40 or 50 pm and 100 or 150 pm. The graphene, which is a 2D material, may preferably be in the form of graphene nanoplates, each having a surface area in a range of 100-750 m2 / g.

[0065] In a second step, S20, occurring subsequent to the first step S10, the copper- graphene-ceramic nanoparticles mixture (with or without one or more additives) is sintered. The composite material is thus obtained.

[0066] The sintering in the second step S20 maybe spark plasma sintering, S22, or hot isostatic pressing, S24.

[0067] For spark plasma sintering in step S22, the copper-graphene-ceramic nanoparticles powder mixture is subjected to the spark plasma sintering at 100 to 600 bar under an elevated temperature of between 650 and 950 °C for between 2 and 20 minutes. The sintering is preferably achieved in an inert atmosphere (e.g. a He-atmosphere).

[0068] For hot isostatic pressing in step S24, the copper-graphene-ceramic nanoparticles powder mixture is placed in a holder, typically a metal capsule, whereafter the holder with the copper-graphene-ceramic nanoparticles powder mixture is subjected to hot isostatic pressing at 1500 to 2000 bar under an elevated temperature of between 650 and 950 °C 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.

[0069] The composite material may be shaped as an electric brush during the second step S20, or it maybe 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 sintered copper-graphene-ceramic nanoparticles mixture. 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.

[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.

[0073] 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. Subsequently, ceramic nanoparticles (A12O3) with an average diameter of 50 nm were added to the copper-graphene powder mixture (also by dry-mixing). For the mixing, a paint shaker model SK35 from Fast & Fluid was used.

[0074] The copper-graphene-ceramic nanoparticles powder mixture was then sintered using either spark plasma sintering (Fuji Electronics, Dr. Sinter SPS530ET with graphite furnace) or hot isostatic pressing (Quintus Technologies, HIP QIH9 with graphite furnace) into a composite material to be used as an electric brush. For the spark plasma sintering, the copper- graphene-ceramic nanoparticles powder mixture was added to the graphite furnace, heated up to a predetermined temperature of 660 °C (at a rate of about 75 °C per minute) and subjected to the spark plasma sintering (620 bar) under the elevated temperatures (660 °C) for approximately 10 minutes. For hot isostatic pressing, the copper-graphene-ceramic nanoparticles powder mixture was placed in a holder, here a metal capsule, whereafter the holder with the copper-graphene-ceramic nanoparticles 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. Example 1.

[0075] In a first set of samples, a first sample (IE1) was prepared by dry-mixing 98 wt% copper with 1 wt% graphene and 1 wt% ceramic nanoparticles, and using spark plasma sintering as previously described, and a second sample (IE2) was prepared by dry-mixing 98.5 wt% copper with 1 wt% graphene and 0.5 wt% ceramic nanoparticles, and using spark plasma sintering as previously described. Moreover, in the first set of samples a third sample (CEi) was prepared by dry-mixing 99 wt% copper with 1 wt% graphene (i.e. no ceramic nanoparticles), and using spark plasma sintering as previously described.

[0076] With reference to Fig. 3, the contact resistance vs contact force was evaluated for the first, second and third samples, together with a comparison with an electrographite brush (CE2) and a graphite-copper brush (CE3) not produced by spark plasma sintering or hot isostatic pressing. As shown in Fig. 3, the contact resistance of IE1 is below 10 mohm for contact loads over 6 N, and more specifically below 1 mohm for a contact load of 20 N (approximately 0.55 mohm), below 0.5 mohm for a contact load of 30 N (approximately 0.4 mohm) and below 0.4 mohm for a contact load of 40 N and 50 N (approximately 0.35 mohm). The contact resistance for IE2 is below 10 mohm for a contact loads over 2 N, and more specifically below 2 mohm for a contact load of 20 N (approximately 1.5 mohm), approximately 1 mohm for a contact load of 30 N and below 1 mohm for a contact load of 40 N and 50 N (approximately 0.7 mohm and 0.5 mohm respectively). This is vastly better performance as compared to CE2 and CE3, for which the contact resistance is above 10 mohm for all contact loads 0-50 N. IE1 and IE2 are also an improvement compared to CEi. For example, the lowest value for the contact resistance of CEi is just above 1 mohm for a contact load of 50 N, and is above 2 mohm for a contact load of 20 N and above 1 mohm for a contact load of 40 N. Thus, the contact resistance of IE1 and IE2 are below 5 mohm, and even below 2 mohm, for a contact load of 20 N, and is below 5 mohm, and even below 1 mohm, for a contact load of 40 N.

[0077] Example 2. With reference to Fig. 4, the volume wear resistance vs sliding distance was evaluated for IE2, CE1, CE2 and CE3 of Example 1. As shown in Fig. 4, the wear resistance was compared. With reference to Fig. 4, it is evident that the volume wear resistance (mm3 / km) for IE2 is vastly better than CE1, and in parity with CE2 and CE3. For IE2, the volume wear resistance is below 1 mm3 / km for a sliding distance of approximately 30 km, as compared to CE1 exhibiting a volume wear resistance of close to 100 mm3 / km for a sliding distance of approximately 20 km. Thus, the wear rate is significantly reduced, by more than one order of magnitude (mm3 / km), owing to the additional ceramic nanoparticles.

[0078] Typically, the Vickers Hardness of each one of IE1 and IE2 is above 50, and the Coefficient of Friction (COF) below 0.45 as measured by a standard pin- on-disk tribology test (using stainless steel or bronze counterface).

[0079] Methodology

[0080] The previously mentioned contact resistance vs contact force was performed according to the method as described in the following.

[0081] 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. The previously mentioned volume wear resistance vs sliding distance was performed according to the method as described in the following.

[0082] The sample was prepared in a test rig including a stationary part and a moving counterpart. The sample was used as an electric brush contacting the moving counterpart during rotation of the latter. The applied contact force (N) multiplied with surface line speed (m / s) of the sample relative to the moving counterpart was 0.3 N*m / s. No current was applied. The volume 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).

[0083] 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.

[0084] 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. An electric brush configured to transmit electric current between a stationary part and its moving counterpart, the electric brush comprising a composite material for contacting the moving counterpart, the composite material comprising: a) 85 to 99 wt% copper; b) 0.5 to 5 wt% of graphene; c) 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.

2. The electric brush of claim 1, wherein the composite material comprises: a) 85 to 99 wt% copper; b) 1 to 3 wt% of graphene; c) 0.5 to 1 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.

3. The electric brush of any of the preceding claims, wherein the amount of copper in the composite material is between 90 and 99 wt%.

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

5. The electric brush 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 electric brush of any of the preceding claims, wherein the ceramic nanoparticles are selected from the group consisting of aluminium oxide, silicon oxide, yttrium oxide, silicon carbide and tungsten carbide.

7. The electric brush of any of the preceding claims, wherein the ceramic nanoparticles have a mean aspect ratio of between i and 3, preferably between 1 and 2.

8. The electric brush of any of the preceding claims, wherein the composite material further comprises: d) 0.1 to 5 wt% of one or more additives.

9. The electric brush of claim 8, wherein the composite material consists of the copper, the graphene, the ceramic nanoparticles and the one or more additives.

10. The electric brush of any of the preceding claims, wherein the contact resistance of the composite material is below 5 mohm, such as below 2 mohm, for a contact load of 20 N, and / or is below 5 mohm, such as below 1 mohm, for a contact load of 40 N, using the contact resistance method as defined herein.

11. The electric brush of any of the preceding claims, wherein the composite material consists of a sintered copper, graphene and ceramic nanoparticles mixture.

12. The electric brush of any claim 11, wherein the composite material has been produced by dry-mixing the copper, graphene and ceramic nanoparticles mixture prior to sintering the copper, graphene and ceramic nanoparticles mixture.

13. The electric brush of any of claims 11-12, wherein the composite material has been produced by spark plasma sintering or hot isostatic pressing.

14. An electric motor or generator comprising a stationary part and a moving counterpart, and an electric brush of any of claims 1-13 configured to transmit electric current between the stationary part and the moving counterpart.

15. The electric motor or generator of claim 14, wherein the moving counterpart is a slip ring or a commutator.

Images