Patent Publication Number: US-10784024-B2

Title: Wire-drawing method and system

Description:
TECHNICAL FIELD 
     The disclosure relates to a method and system for wire-drawing a wrapped sheet comprising a plurality of copper layers and a plurality of graphene layers, in particular to form a wire of enhanced electric conductivity with tailored dimensions. 
     BACKGROUND 
     Copper is known as a cost-effective and reliable electrically conductive material, second only to silver in its ability to conduct electricity. Increasing demand for electrical power and the need to generate and transmit electricity more efficiently, and more environmentally friendly, has spurred the search for conductors with an increased electric conductivity. 
     Intensive investigations have been made for improving the electrical performance of copper nanowires compared to copper thin films, such as described in T. Gao et al., Journal of Applied Physics 2013, 114, 063107. However, the effective conductivity of these nanowires is usually not significantly larger than its ideal bulk value, and may be highly dependent on technological constraints. 
     Co-pending U.S. application Ser. No. 15/290,865 describes techniques for increasing the electric conductivity in a composite structure comprising first and second graphene layers sandwiching a copper layer. 
     BRIEF SUMMARY OF THE DISCLOSURE 
     Known techniques for producing conductive elements may be enhanced by forming multilayer composite structures comprising wires with a plurality of copper layers and a plurality of graphene layers. 
     These objectives are achieved with a wire-drawing method according to independent claim  1  and a wire-drawing system according to independent claim  16 . The dependent claims relate to preferred embodiments. 
     The disclosure relates to a wire-drawing method, comprising: providing a rod comprising a wrapped sheet, wherein the sheet comprises a plurality of copper layers and a plurality of graphene layers; extracting an inner layer of the wrapped sheet from the rod to form a spiral; and forming a wire by feeding the spiral through an opening of a die unit. 
     The wire-drawing technique allows to efficiently form a wire of a desired shape and diameter from a wrapped multilayer sheet, in particular by controlling a shape and/or diameter of the opening of the die unit. 
     Extracting an inner layer of the wrapped sheet may comprise pushing and/or pulling the wrapped sheet through the opening of the die unit. 
     In an example, the opening has an elliptic cross section, in particular a circular cross section. 
     The spiral may be fed through the opening in a feeding direction. 
     In an example, the opening has an inclination angle with respect to the feeding direction of the spiral. 
     Feeding the spiral through an opening having an inclination angle allows to twist the layers of the spiral with respect to one another, which may assist in tuning the mechanical and/or electrical properties of the resulting wire. 
     In an example, the inclination angle is no smaller than 5°, in particular no smaller than 10° or 15°. 
     In an example, the inclination angle is no larger than 30°, in particular no larger than 25° or 20°. 
     Forming the wire may comprise feeding the spiral through a first opening of the die unit, and subsequently feeding the spiral through a second opening of the die unit downstream of the first opening, wherein the second opening is smaller than the first opening. 
     A multi-stage drawing through openings with increasingly smaller dimensions may allow a careful tailoring of a size and diameter of the resulting wire. 
     In an example, a diameter and/or a surface area of the second opening is smaller than the diameter and/or surface area of the first opening. 
     In an example, the first opening and/or the second opening has an elliptic cross section, in particular a circular cross section. 
     The first opening may have a first inclination angle with respect to a feeding direction of the spiral, and/or the second opening may have a second inclination angle with respect to a feeding direction of the spiral. 
     In an example, the first inclination angle and/or the second inclination angle is no smaller than 5°, in particular no smaller than 10° or 15°. 
     In an example, the first inclination angle and/or the second inclination angle is no larger than 30°, in particular no larger than 25° or 20°. 
     In some examples, the second inclination angle is equal to the first inclination angle. 
     In other examples, the second inclination angle differs from the first inclination angle. 
     Varying the inclination angle allows to fine-tune the twist angle, and hence the mechanical and/or electrical properties of the resulting wire. 
     The spiral may be fed through one or more additional openings to further fine-tune the mechanical and/or electrical properties of the wire. 
     In particular, forming the wire may comprise feeding the spiral through a third opening of the die unit downstream of the second opening, wherein the third opening is smaller than the second opening. 
     In particular, a diameter and/or a surface area of the third opening may be smaller than a diameter and/or a surface area of the second opening. 
     The third opening may have an elliptic cross section, in particular a circular cross section. 
     The third opening may have a third inclination angle with respect to a feeding direction of the spiral. 
     The third inclination angle may be no smaller than 5°, in particular no smaller than 10° or 150° The third inclination angle may be no larger than 30°, in particular no larger than 25° or 20°. 
     In some examples, the third inclination angle is equal to the second inclination angle and/or the first inclination angle. 
     In other examples, the third inclination angle differs from the second inclination angle and/or the first inclination angle. 
     In an example, the feeding the spiral comprises pushing and/or pulling the spiral through the opening. 
     Pushing and/or pulling the spiral through the opening may simultaneously provide the necessary pulling and fusing forces to form a multilayer composite wire structure. 
     In an example, the feeding the spiral comprises pulling the spiral through the opening by means of a plurality of pulling rolls, in particular a plurality of rotating pulling rolls. 
     In an example, the pulling rolls may in addition orbit around the spiral, in particular around a longitudinal axis of the spiral. 
     Orbiting the pulling rolls around the spiral with a pre-selectable spacing between them may allow to provide a wire with a desired and carefully controlled outer diameter and optimum round shape. 
     A copper layer, in the sense of the present disclosure, maybe understood to denote a layer comprising copper. In some examples, the copper layer is a pure copper layer, in particular a copper layer comprising at least 90% copper by weight. In other examples, the copper layer may comprise copper and other components and materials in addition to the copper. 
     In an example, at least part of at least one copper layer of the plurality of the copper layers has a Cu (111) crystallographic orientation, which, as used herein, refers to the lattice plane of Copper having 1, 1, and 1, as its Miller Indices. 
     The sheet according to the disclosure may comprise any number of copper layers. 
     According to an example, at least one copper layer of the plurality of copper layers may have a thickness of no more than 500 μm, and in particular no more than 100 μm or no more than 50 μm. 
     In an example, at least one copper layer of the plurality of copper layers has a thickness of no larger than 25 μm or no larger than 10 μm, and in particular no larger than 5 μm. 
     Thin copper layers reduce the volume percentage of copper in favor of graphene in the resulting multi-composite wire structure, which may enhance the electric conductivity, thermal conductivity and/or mechanical strength of the resulting wires. 
     In addition, reducing the thickness of the copper layers may reduce the amount of inelastic scattering of electrons within the copper, which may further enhance the electric conductivity. 
     In an example, at least one copper layer of the plurality of copper layers has a thickness of no larger than 2 μm, and in particular no larger than 1 μm. 
     In an example, at least one copper layer of the plurality of copper layers has a grain size of at least 1 μm, and in particular at least 5 μm. 
     Enhancing the grain size of the copper layer may further reduce inelastic scattering of electrons within the copper layer, which enhances the electric conductivity. 
     In an example, at least one copper layer of the plurality of copper layers has a grain size of at least 10 μm, and in particular at least 50 μm. 
     Grain size, in the sense of the present disclosure, may refer to an average distance between grain boundaries. 
     In an example, the sheet comprises a first copper layer on a first surface side of the sheet, and a second copper layer on a second surface side of the sheet opposite from the first surface side. 
     In this configuration, the windings of the spiral comprise copper on an outer surface layer, which prevents the windings from fusing together homogeneously and provides optimum conditions for connecting the resulting wire to sockets, plugs, etc., such as by means of soldering, clamping, crimping, etc. 
     Graphene layers may be provided in the interior of the sheet to enable easy sliding of the copper layers, which may assist in extracting the inner layer of the wrapped sheet and forming the spiral. 
     In an example, the sheet comprises a copper layer and first and second graphene layers sandwiching the copper layer. 
     In general, the sheet may comprise any number of graphene layers. 
     At least part of the graphene layers may alternate with at least part of the copper layers in the sheet. 
     The resulting multilayer composite structure of copper and graphene may demonstrate enhanced electrical conductivity, thermal conductivity and mechanical strength. 
     Graphene, in the sense of the present disclosure, may refer to a carbon-comprising layer in which the carbon atoms are, as in graphite, hexagonally arranged and generally sp 2  hybridized. The resulting structure may be a honeycomb-shaped hexagonal pattern or fused six-member carbon rings. 
     The term graphene may refer to a configuration with a carbon monolayer, but in practice a small number of layers may also be denoted as graphene. 
     In some examples, the graphene comprises only carbon. In other examples, the graphene may be doped to comprise atoms and/or molecules different from carbon atoms. 
     In an example, at least one graphene layer of the plurality of graphene layers has a thickness of no larger than 5 nm, and in particular no larger than 2 nm. 
     In an example, at least one graphene layer of the plurality of graphene layers may comprise at most 20 graphene monolayers, and in particular at most 10 graphene monolayers. 
     In particular, at least one graphene layer of the plurality of graphene layers may be a monolayer or a bi-layer. 
     In an example, at least one graphene layer of the plurality of graphene layers comprises metal atoms, in particular metal atoms ring-centered on graphene rings. 
     A functionalization of graphene with ring-centered delocalized metal atoms may create anchoring points for firmly connecting the graphene layers to the copper matrix without changing the hexagonal lattice properties of the graphene structure. 
     In an example, the metal atoms comprise silver atoms. 
     The disclosure further relates to a wire-drawing system, comprising: a die unit comprising an opening; an extracting unit to extract an inner layer of a wrapped sheet to form a spiral, wherein the sheet comprises a plurality of copper layers and a plurality of graphene layers; and a feeding unit to feed the spiral through the opening of the die unit. 
     In some examples, the die unit and/or the extracting unit and/or the feeding unit may be separate units. 
     In other examples, some or all of the die unit and/or extracting unit and/or feeding unit may be combined into a common unit. 
     In particular, the extracting unit may comprise the feeding unit. A combined extracting-feeding unit may comprise a pushing unit to push the wrapped sheet towards and against the die unit and through the opening of the die unit, thereby extracting an inner layer of small diameter and forming a spiral. 
     The opening may have a diameter smaller than a diameter of the wrapped sheet or spiral, but larger than a diameter of the wrapped inner layer. In particular, this may allow extracting the inner layer of the wrapped sheet by pushing the wrapped sheet against the boundary of the opening, and thereby extracting the inner layer through the opening. 
     A pulling unit pulling the inner layer from the opposite side of the opening may assist in the extracting of the inner layer and the feeding of the spiral through the opening. 
     The opening may have an elliptic cross section, in particular a circular cross section. 
     The opening may have an inclination angle with respect to a feeding direction of the spiral. 
     In an example, the die unit comprises a first opening and a second opening downstream of the first opening in a feeding direction, wherein the second opening is smaller than the first opening. 
     The die unit may additionally comprise a third opening downstream of the second opening in the feeding direction, wherein the third opening is smaller than the second opening. 
     The first opening may have a first inclination angle with respect to a feeding direction of the spiral. 
     The second opening may have a second inclination angle with respect to a feeding direction of the spiral. 
     The third opening may have a third inclination angle with respect to a feeding direction of the spiral. 
     The first, second, and third inclination angles may be identical, or may differ from one another. 
     In an example, the first inclination angle and/or the second inclination angle and/or the third inclination angle is no smaller than 5°, in particular no smaller than 10° or 15°. 
     In an example, the first inclination angle and/or the second inclination angle and/or the third inclination angle is no larger than 30°, in particular no larger than 25° or 20°. 
     In an example, the feeding unit comprises a pushing unit to push the spiral through the opening. 
     The feeding unit may alternatively or additionally comprise a pulling unit to pull the spiral through the opening. 
     In an example, the pulling unit comprises a plurality of pulling rolls, in particular a plurality of rotating pulling rolls. 
     In an example, the pulling rolls are configured to orbit around the spiral, in particular around a longitudinal axis of the spiral. 
     The features of the wire-drawing method and wire-drawing system have been described above in isolation. However, in some examples some or all of the features as described above may be combined. 
     Wire-drawing method and systems according to the disclosure will now be described with reference to composite structures comprising copper layers sandwiched by graphene layers, and multilayer structures formed by stacking such composite structures. 
     However, this is merely an example, and the disclosure relates to drawing a wire from any wrapped sheet comprising a plurality of copper layers and a plurality of graphene layers. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       The effects and numerous advantages of the present disclosure will become clearer from a detailed description of examples with reference to the accompanying drawings, in which: 
         FIG. 1  is a schematic cross-section of a composite structure according to an example; 
         FIG. 2  is a schematic cross-section illustrating electron tunneling in a composite structure according to an example; 
         FIG. 3  is a cross-sectional schematic illustration of a composite bi-layer structure according to an example; 
         FIG. 4  is a flow diagram illustrating a method for forming a composite structure according to an example; 
         FIG. 5  is a schematic illustration of a system for forming a composite structure according to an example; 
         FIG. 6  is a schematic illustration of a transport unit with counter-rotating rollers according to an example; 
         FIG. 7  is a schematic illustration of a system for forming a composite structure employing plasma tubes according to an example; 
         FIG. 8  is a schematic illustration of a system for forming a composite structure according to an example; 
         FIG. 9  is a schematic cross-sectional illustration to illustrate the effect of grain boundary scattering in a composite structure according to an example; 
         FIG. 10  is a schematic drawing illustrating the effect of coating and grain coarsening according to an example; 
         FIGS. 11 a  and 11 b    illustrate a hexa-hepta functionalization of graphene layers with metal atoms according to an example; 
         FIG. 12  is a flow diagram illustrating a method for forming a multilayer composite structure according to an example; 
         FIG. 13  is a cycle diagram illustrating a method for forming a multilayer composite structure according to an example; 
         FIG. 14  is a schematic illustration of a system for forming a multilayer composite structure according to an example; 
         FIG. 15  is a schematic illustration of a system for forming a multilayer composite structure according to another example; 
         FIG. 16  illustrates a wrapped coated sheet according to an example; 
         FIGS. 17 a  and 17 b    schematically illustrate a compacting unit according to an example; 
         FIGS. 18 a  to 18 c    schematically illustrate the effect of compacting a wrapped coated sheet according to an example; 
         FIG. 19  is a flow diagram illustrating a wire-drawing method according to an example; 
         FIG. 20  is a schematic illustration of a wire-drawing system according to an example; 
         FIG. 21  is a perspective schematic illustration of a composite wrap extracted to form a band spiral according to an example; 
         FIGS. 22 a  and 22 b    illustrate the functioning of a wire-drawing system according to an example; 
         FIGS. 23 a  to 23 c    illustrate different stacking modes in bi-layer graphene structures according to an example; and 
         FIGS. 24 a  and 24 b    illustrate the transformation between different stacking modes of bi-layer graphene structures according to an example. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is a schematic illustration of the composite structure  10  according to an example in a cross-sectional view. The composite structure  10  comprises a copper layer  12  sandwiched by a first (lower) graphene layer  14   a  and a second (upper) graphene layer  14   b.    
     The copper layer  12  may be formed of pure copper (Cu), in particular in a Cu (111) crystallographic orientation. However, in other examples, the copper layer comprises additional material other than copper, such as doped atoms or nanoparticles, as will be described in more detail below. 
     The copper layer  12  as shown in  FIG. 1  extends at a width or thickness d 1  between the first graphene layer  14   a  and the second graphene layer  14   b , wherein d 1  is generally no larger than 25 μm. For instance, the copper layer  12  may be formed at a thickness of 20 μm or 10 μm. In other examples, the copper layer  12  may be formed at a thickness of no larger than 2 μm or no larger than 1 μm. 
     The first graphene layer  14   a  and second graphene layer  14   b  are generally much thinner than the copper layer  12 , and may in particular be graphene layers comprising a single graphene monolayer, or a plurality of graphene monolayers. In  FIG. 1 , the width or thickness of the graphene layers  14   a  and  14   b  is denoted by d a  and d b , respectively. For instance, d a  and d b  may both amount to 5 nm, 2 nm, or even less. 
     In a graphene monolayer configuration, the conduction band touches the valence band in single points, the so-called Dirac points. The infinitesimal small band gap explains the superior electric conductivity of graphene monolayer structures as opposed to graphite. 
     The first graphene layer  14   a  and the second graphene layer  14   b  may be chemically connected or bound to the copper layer  12 , such as deposited on the copper layer  12  by means of chemical vapor deposition, as will be described in additional detail below. 
     The schematic illustration of  FIG. 1 , the width directions d 1 , d a , and d b  define the z direction. The copper layer  12  and first and second graphene layers  14   a ,  14   b  may spatially extend widely in the planar spatial directions x, y perpendicular to the width direction z, so that the composite structure  10  forms a thin foil, as will be described in additional detail below. 
     Graphene coatings on copper are known to have remarkable in-plane stiffness to prevent mechanical deformation, such as surface distortions. Moreover, graphene coatings show impermeability to protect copper against reactive chemical or gaseous species, along with low density-of-states to avoid perturbation of the copper surface potential after coating. All of these superior physical properties make graphene an ideal non-interacting barrier for copper samples, prevent oxidation and surface contamination in general, and contribute to a composite structure with superior parameters. 
       FIG. 1  further illustrates schematically the path  16  of an electron e traveling through the copper layer  12 . As can be taken from  FIG. 1 , the electron e is repeatedly scattered off the boundaries between the copper layer  12  and first graphene layer  14   a  and second graphene layer  14   b , respectively. The graphene layers  14   a ,  14   b  protect the surface of the copper layer  12  to result in a largely smooth, pristine and wrinkle-free layer. In particular, graphene grown on Cu (111) surfaces is only weakly interacting with the Cu surface states. The relatively weak coupling between the carbon  2   p  orbitals of graphene and the Cu  4   p  orbitals leads to predominantly elastic surface scattering, which reduces the electric conductivity of the composite structure  10  in the planer direction x, y. 
     The graphene layers  14   a ,  14   b  in addition improve the thermal conductivity of the composite structure  10 . In general, thermal device design is becoming an important part of electric and microprocessor engineering. The high thermal conductivity of the graphene layers  14   a ,  14   b  can provide a parallel phonon component for heat conduction. 
     In addition, the graphene layers  14   a ,  14   b  enhance the mechanical strength of the composite structure  10  in the planer direction x, y. 
       FIG. 2  is a schematic cross-sectional drawing of the composite structure  10  that generally corresponds to  FIG. 1 , but illustrates a tunneling mechanism in which electrons tunnel through part of the graphene layers  14   a ,  14   b  to result in even higher electric conductivity. A composite structure  10  with a thin copper layer  12 , in particular a copper layer  12  whose width d 1  is in the range of only a few electron mean free path lengths in copper, presents a waveguide-like structure where electrons experience only a fraction of the bulk electrical resistance. This effect exploits the wave nature of the electrons in a quantum electrodynamic description. In particular, electrons traveling through the copper layer  12  may tunnel through electron tunnels  18  that form in the graphene layers  14   a ,  14   b . Since the electrons travel through an ideal graphene monolayer  14   a ,  14   b  close to the speed of light while their speed in the copper layer  12  is much lower in general, and also limited due to various scattering effects, the electron wave approaching the copper/graphene interface appears to be back-scattered not at the location of the impact but further down in the flow direction (to the right in  FIG. 2 ). As shown in the electron path  16  in  FIG. 2 , the electron is still back-scattered from the graphene layer  14   b  into the copper layer  12 , but with a displacement corresponding to the distance it traveled (tunneled) through the graphene layer  14   b . Because of the large difference in electron speed between copper and graphene in a quantum approach, an electron e travels a certain distance within the composite structure  10  much faster than in a classical particle approach. Given that a higher electron speed is generally equivalent to a better electrical conductivity, the possibility of electron tunneling in a composite structure  10  with a thin copper layer  12  sandwiched by graphene layers  14   a ,  14   b  leads to an enhanced electric conductivity. 
     The effect may be explained in additional detail in terms of the Klein effect: A peculiar property of the Dirac Hamiltonian is that charge carriers (like electrons) cannot be confined by electrostatic potentials. In conventional conductors, if an electron strikes an electrostatic barrier that has a potential height above the electron&#39;s kinetic energy, the electron wave function becomes evanescent within the barrier, and exponentially decays with increasing distance into the barrier. This means that the taller and wider the barrier is, the more the electron wave function will decay before reaching the other side. Thus, usually, the taller and wider the barrier is, the lower the probability of the electron quantum tunneling through the barrier. 
     However, if the particles are governed by the Dirac equation, their transmission probability actually increases with increasing barrier height. A Dirac electron that hits a tall barrier may turn into a hole and propagate through the barrier until it reaches the other side, where it may turn back into an electron. This phenomenon is sometimes called Klein tunneling. 
     One possible explanation for this phenomenon is that increasing barrier height may lead to an increased degree of mode matching between the wave functions of the holes within the barrier and the electrons outside of it. When the modes are perfectly matched (which may correspond to the case of an infinitely tall barrier), there may be perfect transmission through the barrier. 
     Applying these considerations to the composite structure  10  as shown in  FIG. 2 , the graphene layers  14   a ,  14   b  may be thin enough to appear electrically invisible, allowing an electron to “see” through the graphene the copper layer on the other side of the tunnel. As soon as the electron is inside the electron tunnel  18 , it experiences the much higher conductivity and speeds up in the direction of the main current flow. 
     As illustrated in the conceptual cross-sectional diagram of  FIG. 3 , several composite structures  10  as described above with reference to  FIGS. 1 and 2  may be stacked to form a multilayer composite structure  20 . In the stacked multilayer composite structure  20 , a first graphene layer  14   a  of a first composite structure  10  is positioned on and in direct contact with the second graphene layer  14   b ′ of another composite structure  10 ′. 
       FIG. 3  shows a multilayer composite structure  20  with two elementary composite structures  10 ,  10 ′, but in general a large number of composite structures may be stacked in the same way. 
     As can be taken from  FIG. 3 , the multilayer composite structure  20  comprises a graphene bi-layer  22  formed of a pair of graphene layers  14   a ,  14   b ′ of neighboring composite structures  10 ,  10 ′. 
     The graphene bi-layer  22  takes particular advantage of the Klein effect as described above, since it allows electrons to tunnel back and forth between copper layers  12  of neighboring composite structures  10  through the electron tunnels  18 ,  18 ′ formed in the graphene layers  14   a ,  14   b ′. Figuratively speaking, the adjacent graphene layers  14   a ,  14   b ′ of the graphene bi-layer  22  form a high-speed tunnel path through the multilayer composite structure  20 , which results in a superior electric conductivity. 
     The effect may be particularly pronounced in multilayer composite structures  20  with only exactly two adjacent graphene layers  14   a ,  14   b ′. With three or more layers, at least one graphene layer would not be directly connected to copper. 
     The composite structure  10  and multilayer composite structure  20  as described with reference to  FIGS. 1 to 3  above may be employed to form electrical cables or wires with enhanced electrical conductivity. 
       FIG. 4  is a schematic flow diagram illustrating a method for forming a composite structure. 
     In a first step S 10 , a copper foil having a first surface side and a second surface side opposite the first surface side is provided, wherein the copper foil has a thickness of no larger than 25 μm. 
     In a second step S 12 , a first graphene layer is deposited on the first surface side of the copper foil, and a second graphene layer is deposited on the second surface side of the copper foil. 
     A plurality of coating techniques may be employed to deposit the first graphene layer and the second graphene layer on the copper foil. For instance, the depositing may comprise chemical vapor deposition techniques. A chemical vapor deposition (CVD) apparatus  24  is schematically illustrated in  FIG. 5 . A corresponding apparatus is described in greater detail by Polson, McNerney, Viswanath, et al. in Nature Scientific Reports, May 2015, 10.1038/srep 10257. 
     The CVD apparatus  24  as shown in  FIG. 5  comprises a transport unit with a plurality of transport rolls  26   a ,  26   b  to transport a thin copper foil  28  subsequently through an annealing zone  30  and a growth zone  32  downstream of the annealing zone  30 . 
     In the annealing zone  30 , the copper foil  28  may be heated to temperatures in the range between 500° C. and 1000° C. in the presence of a pure argon gas flow (indicated by arrows in  FIG. 5 ). By keeping the copper foil  28  for 20 to 30 minutes at elevated temperatures, the copper foil  28  is annealed, resulting in a considerable surface smoothening. Moreover, grain growth can be initiated. 
     Chemical vapor deposition takes place in the growth zone  32 , where carbon atoms supplied by a precursor gas such as methane are deposited as the surface of the copper foil  28 . The flow of the precursor gas is likewise illustrated by arrows in  FIG. 5 . Heating wires  34   a ,  34   b  are arranged along the length of the annealing zone  30  and growth zone  34  to provide the annealing temperature and break up the molecules of the precursor gas. 
     Graphene synthesis under atmospheric CVD conditions may be performed in the growth zone  32  at temperatures between 500° C. and 1000° C. using a gas mixture of argon and methane. Monolayer graphene growth up to 96% of the total area could be observed when a methane gas concentration varying between 0.2% and 1% by volume was employed. Higher methane concentrations may lead to the formation of multi-layer graphene structures. 
     It was also observed that the formation of the graphene monolayer on the copper foil  28  at elevated temperatures led to the preferred formation of a Cu (111) crystallographic orientation structure on the copper surface. In some instances, the graphene layer may act as a sort of template to the copper surface of the copper foil  28 , which promotes the formation of a Cu (111) crystallographic orientation or copper structure because of the close lattice size match between copper and graphene in this configuration. 
     The CVD apparatus  24  as illustrated in  FIG. 5  may be employed to deposit a graphene layer on a single surface side of the copper foil  28 . In order to deposit graphene layers  14   a ,  14   b  on both sides of the copper foil  28  to arrive at a composite structure  10  as described with reference to  FIGS. 1 and 2 , one may pass the copper foil  28  through the CVD apparatus  24  twice, with opposite surface orientations of the copper foil  28 . 
     The surface orientation may be changed by means of a transport unit comprising a pair of counter-rotating transport rolls  36   a ,  36   b  as shown in  FIG. 6 . For instance, a first transport roll  36   a  may rotate in a first (clockwise) direction and may pass the copper foil  28  to a second transport roll  36   b  rotating in an opposite (counter-clockwise) direction to change the orientation of the copper foil  28 . 
     Optionally, the copper foil  28  may be etched before being fed into the CVD apparatus  24  to remove surface contaminants, such as by exposing the samples at 450° C. for 90 minutes to a hydrogen (70 vol.-%) and argon (30 vol.-%) gas flow. 
       FIG. 7  is a schematic illustration of a chemical vapor deposition (CVD) apparatus according to another example that may be employed for forming the composite structures of the present disclosure. The CVD apparatus  38  illustrated in  FIG. 7  is generally similar to the CVD apparatus  24  described above with reference to  FIG. 5 , but employs cold plasma-enhanced chemical vapor deposition rather than electrical heating by means of heating wires  34   a ,  34   b.    
     In greater detail, the cold plasma CVD apparatus  38  comprises a transport unit with transport rolls  26   a ,  26   b  to transport a thin copper foil  28  through a growth zone  32 . A supply unit (not shown in the schematic illustration of  FIG. 7 ) directs a precursor gas comprising carbon into the growth zone  32  so that the precursor gas passes and circulates around the copper foil  28  on both of its opposing surface sides, as indicated by the arrows in  FIG. 7 . For instance, the precursor gas may comprise methane and/or butane and/or propane. 
     As can be further taken from  FIG. 7 , a plurality of plasma units  40  comprising plasma tubes are provided in the growth zone  32  on both sides of the copper foil  28  to generate a cold plasma discharge that breaks up the molecules of the precursor gas and allows the carbon atoms to coat both surface sides of the copper foil to form the first and second graphene layers  14   a ,  14   b . As can be taken from  FIG. 7  and the above description, the cold plasma CVD apparatus  38  allows for the graphene layers  14   a ,  14   b  to be deposited on both surface sides of the copper foil  28  simultaneously, thereby enhancing the process efficiency and reducing cycle times. 
     Preceding etching and/or annealing may take place in the same way as described above with reference to  FIG. 5 . 
     A cold plasma CVD apparatus  38 ′ according to another example is schematically illustrated in the perspective view of  FIG. 8 . 
     As can be taken from  FIG. 8 , the cold plasma CVD apparatus  38 ′ comprises a transport unit with a plurality of transport rolls  42   a  to  42   e  that transport a copper foil  28  through a growth zone  32 , similarly to the configuration of  FIG. 7 . A supply unit  44  supplies a precursor gas such as methane and directs it through the growth zone  32  so that it passes and circulates around the copper foil  28  on both surface sides, as indicated by the arrows in  FIG. 8 . A deposition unit comprising a plurality of plasma units  40  with plasma tubes provides a cold plasma discharge to break up the molecules of the precursor gas so that the carbon is deposited on both surface sides of the copper foil as first and second graphene layers  14   a ,  14   b.    
     However, unlike the configuration of  FIG. 7  in which the copper foil  28  is transported horizontally through the growth zone  32 , in the configuration of  FIG. 8  the transport rolls  42   a  to  42   e  transport the copper foil  28  through the growth zone  32  in a vertical direction, i.e. in a direction that corresponds to the direction of the gravitational field in which the CVD apparatus  38 ′ is placed. This allows to coat both surface sides of the copper foil  28  more uniformly. 
     Preceding etching and/or annealing may take place in the same way as described above with reference to  FIG. 5 . 
     As can be further taken from  FIG. 8 , the cold plasma CVD apparatus  38 ′ additionally comprises a charging unit  46  electrically connected to the transport rolls  42   b ,  42 C to electrically charge the copper foil  28 . Moreover, the charging unit  46  is electrically coupled to the plasma units  40  to charge the precursor gas with a charge opposite from the charge of the copper foil  28 . The electrostatic charging of the copper foil  28  facilitates the growth of the graphene layers  14   a ,  14   b  on the copper foil  28 , and may lead to graphene layers  14   a ,  14   b  that are more homogeneous and uniform. 
     In contrast to more conventional graphene growth in a furnace that reaches temperatures between 900 and 1000® C., the cold plasma coating technology enables graphene deposition at a much lower temperature, which may be in the range of approximately 650° C. The reduced temperature leads to reduced thermal stress of the copper surface, and prevents damage and crystallographic reconfiguration of the surface of the copper foil that could be detrimental to an efficient graphene coating. 
     At the same time, the temperatures reached in the cold plasma coating technology are sufficiently high to provide for a controlled and thorough grain coarsening of the copper layer  12  of the composite structure  10 . It may be advantageous to start with a copper layer  12  that already has a majority Cu (111) crystallographic orientation or grain orientation, which may subsequently transform into complete Cu (111) crystallographic orientation or coarse grain orientation after graphene deposition, supported by the good lattice match between the Cu atomic spacing and the graphene lattice constant. 
       FIG. 9  is a schematic illustration of a composite structure  10  that generally correspond to the composite structure illustrated in  FIGS. 1 and 2  above, but in addition depicts the grain boundaries  48  that may exist in the copper layer  12  and may lead to inelastic electron scattering that can hamper the electric conductivity of the composite structure  10 . The thermal energy supplied in the course of the cold plasma coating process may lead to a grain coarsening, i.e. to a larger grain size of at least 10 μm or greater, as illustrated schematically in  FIG. 10 . The larger grain size may entail a reduction of inelastic scattering of electrons inside the copper layer  12 , and may hence contribute to a higher electric conductivity. 
     The relatively low bonding strength between individual layers of the composite structure  10  or multilayer composite structure  20  may be enhanced by chemically functionalizing graphene, such as by hexa-hepta functionalization. In particular, plasmonic metal particles, such as silver nanoparticles, may be added to the graphene layers  14   a ,  14   b  without changing the hexagonal lattice properties. Instead of adding molecules to the individual carbon atoms of graphene (covalent functionalization), hexa-hepta functionalization or ring-centered functionalization adds metal atoms, such as silver nanoparticles to the center of the graphene ring. The bond is delocalized inside the graphene ring, which keeps the hexagonal arrangement undistorted, so that graphene retains its unique electrical properties. At the same time, the metal atoms may serve as anchoring points for thermally connecting the graphene layers  14   a ,  14   b  to the underlying copper layer  12 . 
       FIGS. 11 a  and 11 b    are a plane view and a perspective view, respectively of a graphene monolayer  50  with the characteristic hexagonal carbon rings and show the doped ring-centered silver nanoparticles  52  delocalized inside the carbon rings. 
     The hexa-hepta functionalization can be performed after CVD growing a graphene monolayer on a pristine copper Cu (111) crystallographic orientation surface as described above. It may consist of a two-step process; a synthesis of η 6 -graphene Cr(Co) 3  followed by silver nanoparticles attachment. Corresponding techniques have been described by S. Che et al. in “Retained Carrier-Mobility and Enhanced Plasmonic-Photovoltaics of Graphene via Ring-Centered Functionalization and Nano-Interfacing”, Nano Letters, Jun. 6, 2017. 
     Forming Multilayer Composite Structures 
     The manufacturing techniques described above provide a composite structure  10  with a single copper layer  12  sandwiched by first and second graphene layers  14   a ,  14   b , as illustrated in  FIGS. 1 and 2 . In order to provide the multilayer composite structures  20  as illustrated in  FIG. 3  with their enhanced electrical conductivity, the coated copper foil  28  may be stacked. 
     In an example, the coated copper foil  28  may be cut into pieces or stripes, and several of these pieces may be stacked under pressure and heat. 
     In an example, pressing may involve sintering, such as hot sintering, microwave sintering or field-assisted sintering involving alternating current or direct current. 
     For instance, sintering temperatures may be in the range between 500° C. and 1000° C. By a suitable choice of the sintering temperature and sintering duration, the copper grain size may be further enhanced. Moreover, sintering allows the copper layers to better adapt to the graphene crystal structure, and hence fosters the formation of a crystallographic Cu (111) crystallographic orientation surface structure in the copper of the copper foil  28 . 
     The stacking may be performed in a combined sintering and pressing apparatus under mechanical pressures in the range of 10 MPa to 300 MPa. 
     In an alternative configuration that will now be described in additional detail with reference to  FIGS. 12 to 18 , a multilayer composite structure  20  may be provided by wrapping and compacting the coated copper foil  28 . 
       FIG. 12  is a flow diagram illustrating a method for forming a multilayer composite structure  20  comprising a stack of composite structures  10  according to an example. 
     In a first step S 20 , a first sheet comprising a copper-comprising layer sandwiched by first and second graphene layers is provided, such as the coated copper foil  28 . 
     In a second step S 22 , the first sheet is wrapped to form a first rod. 
     In a third step S 24 , the first rod is compacted to form a first multilayer composite structure. 
     The sequence of steps as illustrated in  FIG. 12  may be repeated several times. In particular, the multilayer composite structure resulting from step S 24  may again be coated on both sides by back feeding the resulting multilayer foil into the cold plasma CVD apparatus  38 ,  38 ′ as described above with reference to  FIGS. 7 and 8 , respectively, followed by a second round or several additional rounds of wrapping and compacting. 
     An exemplary process cycle is illustrated in  FIG. 13 . 
     In step 1, graphene layers are deposited by means of chemical vapor deposition on both sides of a copper-comprising foil, such as by employing a cold plasma CVD apparatus  38 ,  38 ′. 
     In a subsequent step 2, the resulting foil is rolled or wrapped to form rolled graphene bi-layers. 
     In a subsequent step 3, the resulting rod is roll-milled to convert the bi-layer roll to a bi-layer foil. 
     In a subsequent (optional) step 4, the outer surfaces of the bi-layer foil may be etched to provide a pristine copper surface, before the foil is back fed into the graphene CVD facility in step 5. 
     The process cycle can be described by the following sequence of steps:
         0. A counter is set to zero.   1. A graphene monolayer is grown on both sides of a copper-comprising foil in a CVD coating facility.   2. The coated foil is then wrapped around so that a tube or rod is created with a circular concentric structure of alternating copper and bi-layer graphene layers.   3. The rod is then compacted, such as by metal hot rolling pressing, creating a flat copper/graphene multilayer composite band containing a multitude or plurality of graphene bi-layers  22 .   4. The resulting composite band is then etched on both surfaces to provide a pristine copper surface.   5. The counter is now increased by 1. If the counter is smaller than a pre-defined threshold, the process proceeds with step 1 by back-feeding the resulting structure into the CVD coating facility.   6. If the counter reaches the threshold value, the copper/graphene composite band is extracted from the coating and rolling cycle.       

     The process circle as described above may be called “CWH-Circle” (coating, wrapping, hot-rolling). The process circle may result in a multilayer composite structure in which the volume fraction of graphene is significantly enhanced compared to the single-layer composite structure  10  of  FIG. 1 . In particular, the process circle allows to vary the volume fraction of graphene in the matrix by selecting the number of circle iterations. This may allow to increase and/or tailor the electrical conductivity, the thermal conductivity, and/or the mechanical strength of the copper/graphene composite material to a desired value. 
       FIG. 14  is a schematic illustration of a system  54  for forming a multilayer composite structure employing the techniques described above. 
     The system  54  comprises a transport unit  56 , such as a plurality of driven transport rolls, to transport a sheet  58 , such as the copper foil  28 , to a deposition unit  60 . 
     The deposition unit  60  may comprise a cold plasma CVD apparatus  38 ,  38 ′ as described with reference to  FIGS. 7 and 8  above, and may deposit pairs of graphene layers  14   a ,  14   b  on opposing first and second surface sides of the sheet  58 , resulting in a coated sheet  62  with a composite structure  10  as described above with reference to  FIGS. 1 and 2 . 
     In an example, the deposition unit  60  may comprise an annealing unit, providing an annealing zone  30  upstream of the deposition growth zone as described above with reference to  FIG. 5 . 
     The transport unit  56  transports the coated sheet  62  from the deposition unit  60  to a wrapping unit  64  to wrap the coated sheet  62  into a rod  66 . The wrapping unit  64  may employ any technique to wrap the coated sheet, such as rolling up the coated sheet on a thin cylindrical roll, and removing the roll. 
     The resulting rod  66  is illustrated schematically in  FIG. 16 . As can be taken from  FIG. 16 , due to the wrapping or rolling, the rod  66  comprises graphene bi-layer structures in which two graphene layers  14   a ,  14   b  are in immediate and direct contact, as described above with reference to  FIG. 3 , and therefore has all the conductivity advantages described above. 
     With further reference to  FIG. 14 , the transport unit  56  may transport the rod  66  to a compacting unit  68  downstream of the wrapping unit  64 . The compacting unit  68  may compact the rod  66  into a compacted sheet  70 . 
     For instance, the compacting unit  68  may be a rolling press, in particular a hot rolling press, as illustrated schematically in a front view in  FIG. 17 a    and in a side view in  FIG. 17   b.    
     As can be taken from  FIGS. 17 a  and 17 b   , the hot rolling press  68  may comprise a plurality of press units  72   a ,  72   b ,  72   c  in a staggered configuration, so that the rod  66  passes subsequently through the press units  72   a ,  72  in a transport direction (illustrated by the central arrow in  FIGS. 17 a  and 17 b   ) and is thereby converted into the compacted sheet  70 , such as a thin long band. 
       FIGS. 17 a  and 17 b    show a compacting unit  68  with three staggered press units  72   a ,  72   b ,  72   c . However, this is for illustration only, and in other configurations the compacting unit  68  may comprise a smaller or larger number of press units. 
     The compacting unit  68  may further comprise a heating unit (not shown) to heat the rod  66  during the compacting. Hot rolling may provide two advantageous effects: On the one hand, it further increases the copper grain size, which leads to a reduction of the grain scattering and hence to superior electric conductivity, as described above with reference to  FIGS. 9 and 10 . On the other hand, hot rolling may foster the copper Cu (111) crystallographic orientation, which may increase the lattice match between the copper surface and the grown-on graphene layers, thereby providing a multilayer composite structure with enhanced mechanical properties. 
     For grain growth to happen effectively, the processing temperature should be sufficiently higher than the copper re-crystallization temperature of approximately 227° C. However, in order to avoid, reduce or minimize nano-cracks of the copper surface, in some examples the hot rolling temperature should be chosen below 650° C. In particular, the hot rolling temperature may be chosen between 450° C. and 550° C. 
     The sequence of  FIGS. 18 a  to 18 c    show the rod  66  in a cross-sectional schematic view and its transformation into the compacted sheet  70  as it progresses through the press units  72   a ,  72   b ,  72   c  of the compacting unit  68 . 
     The techniques described above take advantage of the fact that graphene layers are sort of “slippery”, and generally do not stick to each other well. The reason behind this effect is that graphene has strong covalent bonding between its atoms in the horizontal direction (in-plane with the monolayers) but only relatively weak van der Waals forces in a direction vertical to the in-plane direction, which keeps it from mechanically attaching vertically to the next layer. On the other hand, because of the excellent match between the graphene lattice constant and the atomic spacing of the Cu (111) crystalline orientation, the graphene layer is strongly mechanically bonded to the copper surface on which it is grown. 
     Due to these reasons, graphene/copper layers can slide relatively easily with respect to each other, making the above-described conversion from a round shape of the rod  66  to a flat band  70  possible without sacrificing the excellent mechanical connection between the individual graphene layers  14   a ,  14   b  and the copper layers  12  on which they are grown. At the same time, the copper surfaces are protected by the mechanically extremely robust graphene coating, keeping any mechanical stress away from the copper during the severe plastic deformation. As illustrated in the insert of  FIG. 18 c   , the result is a multilayer composite structure  20  corresponding to a stacking of the composite structure  10  as described above with reference to  FIG. 3 . 
     As can be further taken from  FIGS. 18 b  and 18 c   , the plurality of graphene layers in the compacted sheet  70  are still interconnected with one another, which is due to the fact that the compacted sheet  70  results from compactification of the rod  66  having a single wrapped graphene layer. The connection fosters the electric conductivity in a direction perpendicular to the in-plane direction. 
     With further reference to  FIG. 14 , the system  54  additionally comprises a back-feeding unit  74  which may comprise transfer rollers to back feed the compacted sheet  70  to the deposition unit  60  for another round of chemical vapor deposition, wrapping, and compacting, as described above with reference to the circle diagram of  FIG. 13 . 
       FIG. 15  is a schematic illustration of a system  54 ′ for forming a multilayer composite structure according to another example. The system  54 ′ is similar in design and functionality to the system  54  described with reference to  FIG. 14  above, and the same reference numerals are employed to denote corresponding components. However, the system  54 ′ comprises one or a plurality of optional units, which are illustrated in broken lines in  FIG. 15 . 
     In particular, the system  54  may comprise an etching unit  76  upstream of the deposition unit  60 . The etching unit  76  may be employed to etch a first surface side and/or a second surface side of the sheet  58  and/or the back-fed compacted sheet  70  by means of chemical etching, so as to provide a pristine copper surface for a subsequent deposition in the deposition unit  60 . 
     For instance, the etching unit  76  may comprise a quartz tube furnace, which may expose the sheet  58  to a hydrogen (70 vol.-%) and argon (30 vol.-%) gas flow at temperatures in the range of 450° C. for approximately 90 minutes. This allows to efficiently remove surface contaminants from the surfaces of the sheet  58 . 
     As can be further taken from  FIG. 15 , the system  54  may alternatively or additionally comprise a bypass unit  78  configured to bypass the sheet  58  or compacted sheet  70  past the deposition unit  60 . 
     In particular, after the circle process has reached the desired number of iterations, the resulting multilayer compacted sheet  70  may be fed back to the circle by means of the back-feeding unit  74  one last time, but skipping the graphene coating step. In particular, the bypass unit  78  may pass the compacted sheet to the wrapping unit  64  so that it does not traverse the deposition unit  60 . This may ensure that in the final wrapped rod  66  produced by the wrapping unit  64 , the single layers are not separated by graphene layers. In particular, this may facilitate layer fusing in a subsequent wire drawing technique in a wire-drawing system  80 , as will be described in additional detail with reference to  FIGS. 19 to 24  below. Wire drawing in a wire-drawing system  80  may be a final step to convert the rod  66  into a wire  82  with enhanced electrical, thermal and mechanical properties. 
     Alternatively, as further illustrated in  FIG. 15 , the system  54  may comprise a slicing unit  84  downstream of the compacting unit  68  and adapted to slice the compacted sheet  70  in a longitudinal direction into a plurality of elongated slices  86  in a final process step, after the maximum number of iterations has been reached. This may provide wires with a rectangular or square cross-section. 
     Wire-Drawing Method and System 
     As described above with reference to  FIG. 15 , wire drawing techniques may be employed to convert a multilayer composite rod  66  comprising the multilayer composite structure  20  into a wire. 
       FIG. 19  is a flow diagram illustrating a wire-drawing method according to an example. 
     In a first step S 30 , a rod comprising a wrapped sheet is provided, wherein the sheet comprises a plurality of copper layers and a plurality of graphene layers. 
     In a second step S 32 , an inner layer of the wrapped sheet is extracted from the rod to form a spiral or helix. 
     In a third step S 34 , a wire is formed by feeding the spiral through an opening of a die unit. 
       FIG. 20  is a schematic illustration of a corresponding wire-drawing system  80  that may be configured to perform the steps of the method illustrated in the flow diagram of  FIG. 19 . 
     The wire-drawing system  80  comprises an extracting unit  88  adapted to extract an inner layer of a wrapped sheet to form a spiral, wherein the sheet comprises a plurality of copper layers and a plurality of graphene layers. 
     In particular, the wrapped sheet may be the rod  66  comprising the multilayer composite structure as described above with reference to  FIG. 18 a   . The extracting unit  88  may be adapted to extract an innermost layer of the rod  66 , such as by grabbing the innermost layer of the rod  66  and pulling it to form a helix or spiral  90  as illustrated in the perspective drawing of  FIG. 21 . In the spiral  90 , each individual graphene bi-layer  22  is connected on both sides directly to the copper substrate where they are grown, so that the enhanced conductivity occurs in a largely increased volume fraction of the wire, providing for both ultra-high conductivity and high current capacity. 
     As described above with reference to  FIG. 15 , it may be beneficial that the windings of the spiral  90  do not have graphene-coating on the outside, since this might prevent them from fusing together homogeneously. In addition, a graphene-free surface provides optimum conditions for connecting the resulting wire  82  to sockets or plugs, such as by means of soldering, clamping, crimping, etc. 
     With additional reference to  FIG. 20 , the wire-drawing system  80  further comprises a feeding unit  92  downstream of the extracting unit  88 , and adapted to feed the spiral  90  to a die unit  94 , which converts the spiral  90  into a wire  82 . 
     In some examples, the feeding unit  92  may comprise a pushing unit configured to push the spiral  90  towards and through the die unit  94 . 
     In some examples, the extracting unit and the feeding unit are separate units. 
     In other examples, the feeding unit may be part of the extracting unit. For instance, in a configuration in which the feeding units comprises a pushing unit to push the spiral towards and through the die unit  94 , the pushing may extract an inner layer of the wrapped sheet as a result of the pushing force. 
       FIG. 22 a    is a schematic side view of a die unit  94  as it may be used in the wire-drawing system  80 . The die unit  94  comprises a plurality of dies  96   a ,  96   b ,  96   c  in a staggered configuration. Each of the dies  96   a ,  96   b ,  96   c  comprises a respective opening  98   a ,  98   b ,  98   c , wherein a diameter or surface area of the openings  98   a ,  98   b ,  98   c  decreases in a transport direction (to the right in  FIG. 22 a   ) of the spiral  90  as it passes through the die unit  94 . The openings  98   a ,  98   b ,  98   c  may be openings with a circular or elliptic cross-section, depending on the desired cross-section of the wire  82 . The openings  98   a ,  98   b ,  98   c  may have a diameter smaller than a diameter of the rod  66  or spiral  90 , but larger than the wrapped innermost layer and thereby subsequently deform the rod  66  and spiral  90  into a wire under the pulling and/or pushing force. 
       FIG. 22 a    further illustrates a pulling unit  100  comprising a plurality of pulling rolls  102   a ,  102   b  that pull the spiral  90  through the openings  98   a ,  98   b ,  98   c  of the die unit  94 . The pulling unit  100  may form part of the feeding unit  92 , or may be a separate component. 
     As can be further taken from  FIG. 22 b   , the pulling rolls  102   a ,  102   b  may be configured to rotate or orbit around the drawn wire with a pre-selectable spacing in between to create a wire  82  with a desired outer diameter and shape. 
     The configuration of  FIGS. 22 a  and 22 b    shows a die unit  94  with three dies  96   a ,  96   b ,  96   c  with openings of decreasing diameter in the transport direction of the spiral  90 . However, in general, the die unit  94  may comprise any number of dies in a staggered configuration. 
     The wire-drawing techniques according to the disclosure may also be employed for additional conductivity tuning of the composite multilayer structures, as will now be described with reference to  FIGS. 23 and 24 . 
     Stacked graphene, in which at least two graphene monolayers  104   a ,  104   b  are stacked with a twist angle Θ between them, may exhibit unique electronic, thermal, and magnetic properties. A rotational twist of the graphene monolayers  104   a ,  104   b  with respect to one another can have a profound effect on the electrical properties of the bi-layer structure. Controlling the twist angle Θ of bi-layer graphene films hence allows for the preparation of twisted bi-layer graphene films with defined stacking orientations, and in turn the tailoring and fine-tuning of their electronic, thermal, and magnetic properties. 
       FIGS. 23 a  to 23 c    illustrate three different basic stacking modes in bi-layer graphene: 
     AB-stacking (also called Bernal stacking) is illustrated in  FIG. 23 a   . The second graphene layer  104   b  is displaced by one half of the diameter of the hexagonal ring structure with respect to the hexagons of the first graphene layer  104   a . This results in a parallel shift without a twist. 
       FIG. 23 b    schematically illustrates the so-called AA-stacking (also called non-Bernal stacking), in which the hexagons of the two layers  104   a ,  104   b  lie right on top of each other (no shift, no twist). 
       FIG. 23 c    illustrates AA′-stacking, which is similar to AA-stacking, but in which the crystallographic axes of the two layers  104   a ,  104   b  are twisted by an angle Θ between 0° and 60° with respect to one another (no shift, twist angle Θ). 
     By means of twisting the layers by an angle of Θ=60°, an AB configuration can be transformed into an AA configuration and vice-versa, as illustrated in  FIGS. 24 a    and  24   b.    
     In order to control the electrical, thermal, and magnetic properties of a stacked copper graphene bi-layer composite material, and in particular the wire  82 , the twist angle Θ between the layers  104   a ,  104   b  may be adjusted by means of a die unit  94  in which the openings  98   a ,  98   b ,  98   c  have a non-zero inclination angle with respect to a feeding direction of the spiral  90 . 
     As illustrated in  FIG. 22 a   , a main twist angle Θ may be determined by means of the inclination angle of the openings  98   a ,  98   b ,  98   c  of the die unit  94 . Different openings  98   a ,  98   b ,  98   c  may have different inclination angles with respect to a feeding direction of the spiral  90 , which may lead to modifications and fine-tuning of the twist angle between the individual layers of the spiral  90 . 
     The description of the embodiments and the Figures merely serve to illustrate the techniques of the disclosure, but should not be understood to imply any limitation. The scope is to be determined on the basis of the appended claims. 
     All references, including publications, patent applications, and patents, cited herein are hereby incorporated by reference to the same extent as if each reference were individually and specifically indicated to be incorporated by reference and were set forth in its entirety herein. 
     The use of the terms “a” and “an” and “the” and “at least one” and similar referents in the context of describing the invention (especially in the context of the following claims) are to be construed to cover both the singular and the plural, unless otherwise indicated herein or clearly contradicted by context. The use of the term “at least one” followed by a list of one or more items (for example, “at least one of A and B”) is to be construed to mean one item selected from the listed items (A or B) or any combination of two or more of the listed items (A and B), unless otherwise indicated herein or clearly contradicted by context. The terms “comprising,” “having,” “including,” and “containing” are to be construed as open-ended terms (i.e., meaning “including, but not limited to,”) unless otherwise noted. Recitation of ranges of values herein are merely intended to serve as a shorthand method of referring individually to each separate value falling within the range, unless otherwise indicated herein, and each separate value is incorporated into the specification as if it were individually recited herein. All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. The use of any and all examples, or exemplary language (e.g., “such as”) provided herein, is intended merely to better illuminate the invention and does not pose a limitation on the scope of the invention unless otherwise claimed. No language in the specification should be construed as indicating any non-claimed element as essential to the practice of the invention. 
     Preferred embodiments of this invention are described herein, including the best mode known to the inventors for carrying out the invention. Variations of those preferred embodiments may become apparent to those of ordinary skill in the art upon reading the foregoing description. The inventors expect skilled artisans to employ such variations as appropriate, and the inventors intend for the invention to be practiced otherwise than as specifically described herein. Accordingly, this invention includes all modifications and equivalents of the subject matter recited in the claims appended hereto as permitted by applicable law. Moreover, any combination of the above-described elements in all possible variations thereof is encompassed by the invention unless otherwise indicated herein or otherwise clearly contradicted by context.