A graphene m o nolayer is grown on both sides of a copper-comprising foil in a CVD c o ating facility.

The coated foil is then wrapped around so that a tube or rod is created with a circular concentric structure of alternating c o pper and bi-layer graphene layers.

The rod is then compacted, such as by metal hot rolling pressing, creating a flat co pper/graphene multilayer c ompo site band containing a multitude of graphene bi-layers 22.

The resulting c ompo site band is then etched o n b o th surfaces to pr o vide a pristine copper surface.

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 c o ating facility.

If the counter reaches the threshold value, the copper/graphene composite band is extracted from the c o ating and r ol ling cycle. The pr o cess circle as described above may be c alled "CWH-Ci rcl e" (c oating, wrapping, h o t-rolling). The pr o cess circle may result in a multilayer composite structure in which the volume fraction of graphene is significantly enhanced compared to the single-layer compo site structure 10 of Fig. 1. In particular, the pr o cess 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/o r the mechanical strength of the c o pper/graphene c ompo site material to a desired value. Fig. 1 4 is a schematic illustration of a system 54 f o r f o rming a multilayer composite structure employing the techniques described above. The system 5 4 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 dep o siti o n unit 60 may c o mprise a cold plasma CVD apparatus 3 8, 38 ' as described with reference to Figs. 7 and 8 above, and may deposit pairs of graphene layers i4 a, 14b on o pposing first and second surface sides of the sheet 5 8, resulting in a c o ated sheet 62 with a composite structure 10 as described above with reference to Figs. 1 and 2. In an example, the dep o siti o n unit 6 0 may c o mprise an annealing unit, providing an annealing zone 3 0 upstream of the deposition growth zone as described above with reference to Fig. 5. The transpo r t unit 56 transpo r ts the coated sheet 6 2 from the deposition unit 6 0 to a wrapping unit 64 to wrap the c o ated sheet 62 into a rod 66. The wrapping unit 64 may employ any technique to wrap the c o ated sheet, such as rolling up the c o ated sheet o n a thin cylindrical roll, and removing the roll. The resulting rod 66 is illustrated schematically in Fig. 1 6. 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 i4 a, 1 4b are in immediate and direct contact, as described 5 above with reference to Fig. 3, and therefore has all the conductivity advantages described above. With further reference to Fig. 14, the transport unit 5 6 may transport the rod 66 to a compacting unit 6 8 downstream of the wrapping unit 64. The compacting unit 6 8 may compact the rod 66 into a compacted sheet 70. Fo r instance, the compacting unit 6 8 may be a rolling press, in particular a h o t rolling press, as illustrated schematically in a front view in Fig. 17 a and in a side view in Fig. 1 7b. As can be taken from Figs. 17 a and 17 b, the h o t rolling press 6 8 may c o mprise a plurality of press units 72 a, 72 b, 72c in a staggered configuration, so that the rod 66 passes subsequently through the press units 7 2a, 72c in a transport direction (illustrated by the central arrow in Figs. i a and 17 b) and is thereby converted into the compacted sheet 70, such as a thin long band. Figs. i a and 17 b sh o w a c o mpacting unit 68 with three staggered press units 7 2a, 7 2b, 7 2c. However, this is f o r illustration only, and in other configurations the compacting unit 68 may comprise a smaller or larger number of press units. The compacting unit 6 8 may further comprise a heating unit (not shown) to heat the rod 66 during the c o mpacting. H o t r o lling 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, h o t r o lling may f o ster the c o pper (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. Fo r grain growth to happen effectively, the processing temperature should be sufficiently higher than the c o pper re-c r ystallizati o n temperature of approximately 227" C. However, in order to avoid any nano-cracks of the copper surface, in some examples the hot rolling temperature should be chosen below 6 5o* C. In particular, the h o t r o lling temperature may be chosen between 45 0° C and 550"C. The sequence of Figures i 8a to i 8c sh o w the rod 6 6 in a cross-sectional schematic view and its transformation into the compacted sheet 7 0 as it progresses through the press units 7 2a, 72 b, 72c of the compacting unit 68. The techniques described above take advantage of the fact that graphene layers are sort of "slippery", and generally d o 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 C u (i11) atomic spacing, the graphene layer is 5 strongly mechanically b o nded to the c o pper surface o n which it is gr own. Due to these reasons, graphene/copper layers can slide relatively easily with respect to each other, making the abo ve-described conversion from a round shape of the rod 66 to a flat band possible without sacrificing the excellent mechanical connection between the individual graphene layers i4a, 14b and the copper layers 12 on which they are grown. At the same time, the c o pper surfaces are protected by the mechanically extremely robust graphene coa t ing, keeping any mechanical stress away from the copper during the severe plastic deformation. As illustrated in the insert of Fig. 1 8c, the result is a multilayer compo site structure 2 0 corresponding to a stacking of the c ompo site structure 10 as described above with reference to Fig. 3. As can be further taken from Figs. i 8b and 18 c, the plurality 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 c o mpactificati o n 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 fu r ther reference to Fig. 14, the system 5 4 additionally comprises a back-feeding unit 74 which may comprise transfer rollers to back feed the compacted sheet 70 to the dep o siti o n unit 6 o fo r another round of chemical vapor dep o siti o n, wrapping, and 0 compacting, as described above with reference to the circle diagram of Fig. 1 3. Fig. 1 5 is a schematic illustration of a system 5 4' 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. 1 4 above, and the same reference numerals are employed to denote corresponding components. However, the system 5 4' 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 7 6 upstream of the dep o siti on unit

The etching unit 7 6 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, s o as to provide a pristine c o pper surface f o r a subsequent dep o siti o n in the deposition unit 60. 5 F o r instance, the etching unit 7 6 may comprise a quartz tube furnace, which may expose the sheet 58 to a hydrogen (70 vol.-%) and argon (3 0 vol.-%) gas flow at temperatures in the range of 4 50 " C f o r 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 c o mprise a bypass unit 7 8 configured to bypass the sheet 58 o r compacted sheet 70 past the deposition unit 60. In particular, after the circle pr o cess 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 7 8 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 SVG 15691072.08-30-2017.J₆ZGHKVVRXEAPX0.SPEC.28.15.865.1396.930.1435.svg 0.13 0.217 Chemistry Black and white as will be described in additional detail with reference to Figs. 19 to 24 below. Wire drawing in a wire-drawing system 8o 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. 1 5, the system 54 may c o mprise a slicing unit 8 4 downstream of the compacting unit 6 8 and adapted to slice the compacted sheet in a longitudinal direction into a plurality of elongated slices 86 in a final pr o cess 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 S y stem As described above with reference to Fig. i5, 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 S30, 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 fo rm a spiral o r helix. In a third step S 34, a wire is formed by feeding the spiral through an opening of a die unit. Fig. 2 0 is a schematic illustration of a corresponding wire-drawing system 8 a that may be configured to perform the steps of the method illustrated in the fl o w diagram of Fig.

The die unit 9 4 comprises a plurality of dies 9 6a, 9 6b, 9 6c in a staggered c o nfigurati o n. Each of the dies 9 6a, 9 6b, 9 6c c om prises a respective opening 9 8a, 9 8b, 98 c, wherein a diameter or surface area of the openings 9 8a, 98 b, 9 8c decreases in a transport direction (to the right in Fig. 22a) of the spiral 90 as it passes through the die unit 9 4. The openings 9 8a, 9 8b, 9 8c may be openings with a circular o r elliptic cr o ss- section, depending on the desired cross-section of the wire 8 2. The openings 98 a, 98b, 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 1o 2a, 10 2b that pull the spiral 90 through the openings 9 8a, 98 b, 9 8c of the die unit 9 4. The pulling unit 100 may f o rm part of the feeding unit 9 2, o r may be a separate component. As can be further taken from Fig. 22b, the pulling rolls 1o 2a, 1 0 2b may be configured to rotate o r orbit around the drawn wire with a pre-selectable spacing in between to create a wire 8 2 with a desired outer diameter and shape. The c o nfigurati o n of Figs. 22 a and 22b sh ow s a die unit 9 4 with three dies 9 6a, 9 6b, 96c with openings of decreasing diameter in the transport direction of the spiral 90. However, in general, the die unit 9 4 may comprise any number of dies in a staggered c o nfigurati on. 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 m ono layers 10 4a, 10 4b are stacked with a twist angle 0 between them, may exhibit unique electronic, thermal, and magnetic properties. A rotational twist of the graphene monolayers 10 4a, 10 4b with respect to one another can have a profound effect o n the electrical properties of the b i-layer structure. Controlling the twist angle 0 of b i-layer graphene films hence allows for the preparation of twisted b i-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 b i-layer graphene: AB-stacking (also called Bernal stacking) is illustrated in Fig. 23a. The second graphene layer 4b is displaced by one half of the diameter of the hexagonal ring structure with respect to the hexagons of the first graphene layer 1o 4a. This results in a parallel shift without a twist. Fig. 23b schematically illustrates the so-called AA-stacking (also called non-Bernal stacking), in which the hexagons of the two layers 1o 4a, 10 4b lie right on top of each other (no shift, n o twist). Fig. 23 c illustrates AA'-stacking, which is similar to AA-stacking, but in which the crystallographic axes of the tw o layers io 4a, 10 4b are twisted by an angle 0 between o* 0 and 6 a" with respect to one another (no shift, twist angle 0). By means of twisting the layers by an angle of 0 = 600 SVG 15691072.08-30-2017.J₆ZGHKVVRXEAPX0.SPEC.31.19.1602.1910.1629.1950.svg 0.133 0.09 Chemistry Black and white an AB configuration can betransformed into an AA configuration and vice-versa, as illustrated in Figs. 24 a and 24b. In order to control the electrical, thermal, and magnetic properties of a stacked copper graphene b i-layer composite material, and in particular the wire 82, the twist angle 0 between the layers io 4a, 10 4b may be adjusted by means of a die unit 9 4 in which the openings 98 a, 98 b, 9 8c have a non-zero inclination angle with respect to a feeding direction of the spiral 90. As illustrated in Fig. 22a, a main twist angle O may be determined by means of the inclination angle of the openings 9 8a, 9 8b, 9 8c of the die unit 9 4. Different openings 9 8a, 98 b, 9 8c 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

The description of the embodiments and the Figures merely serve to illustrate the techniques of the disclosure, but should not be underst oo d to imply any limitation. The scope is to be determined o n 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 f o llowing claims) are to be c o nstrued to cover b o th 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 fro m the listed items (A o r B) o r any c o mbinati o n 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 c o nstrued 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 sh or thand 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 o r otherwise clearly c o ntradicted 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. N o language in the specification should be c o nstrued 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 m ode known to the inventors f o r 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 f o r 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 c o mbinati o n of the ab o ve- described elements in all possible variations thereof is encompassed by the invention unless otherwise indicated herein or otherwise clearly c o ntradicted by context. Claims What is claimed is:

A graphene m o nolayer is grown on both sides of a copper-comprising foil in a CVD c o ating facility.

The coated foil is then wrapped around so that a tube or rod is created with a circular concentric structure of alternating c o pper and bi-layer graphene layers.

The rod is then compacted, such as by metal hot rolling pressing, creating a flat co pper/graphene multilayer c ompo site band containing a multitude of graphene bi-layers 22.

The resulting c ompo site band is then etched o n b o th surfaces to pr o vide a pristine copper surface.

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 c o ating facility.

If the counter reaches the threshold value, the copper/graphene composite band is extracted from the c o ating and r ol ling cycle. The pr o cess circle as described above may be c alled "CWH-Ci rcl e" (c oating, wrapping, h o t-rolling). The pr o cess circle may result in a multilayer composite structure in which the volume fraction of graphene is significantly enhanced compared to the single-layer compo site structure 10 of Fig. 1. In particular, the pr o cess 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/o r the mechanical strength of the c o pper/graphene c ompo site material to a desired value. Fig. 1 4 is a schematic illustration of a system 54 f o r f o rming a multilayer composite structure employing the techniques described above. The system 5 4 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 dep o siti o n unit 60 may c o mprise a cold plasma CVD apparatus 3 8, 38 ' as described with reference to Figs. 7 and 8 above, and may deposit pairs of graphene layers i4 a, 14b on o pposing first and second surface sides of the sheet 5 8, resulting in a c o ated sheet 62 with a composite structure 10 as described above with reference to Figs. 1 and 2. In an example, the dep o siti o n unit 6 0 may c o mprise an annealing unit, providing an annealing zone 3 0 upstream of the deposition growth zone as described above with reference to Fig. 5. The transpo r t unit 56 transpo r ts the coated sheet 6 2 from the deposition unit 6 0 to a wrapping unit 64 to wrap the c o ated sheet 62 into a rod 66. The wrapping unit 64 may employ any technique to wrap the c o ated sheet, such as rolling up the c o ated sheet o n a thin cylindrical roll, and removing the roll. The resulting rod 66 is illustrated schematically in Fig. 1 6. 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 i4 a, 1 4b are in immediate and direct contact, as described 5 above with reference to Fig. 3, and therefore has all the conductivity advantages described above. With further reference to Fig. 14, the transport unit 5 6 may transport the rod 66 to a compacting unit 6 8 downstream of the wrapping unit 64. The compacting unit 6 8 may compact the rod 66 into a compacted sheet 70. Fo r instance, the compacting unit 6 8 may be a rolling press, in particular a h o t rolling press, as illustrated schematically in a front view in Fig. 17 a and in a side view in Fig. 1 7b. As can be taken from Figs. 17 a and 17 b, the h o t rolling press 6 8 may c o mprise a plurality of press units 72 a, 72 b, 72c in a staggered configuration, so that the rod 66 passes subsequently through the press units 7 2a, 72c in a transport direction (illustrated by the central arrow in Figs. i a and 17 b) and is thereby converted into the compacted sheet 70, such as a thin long band. Figs. i a and 17 b sh o w a c o mpacting unit 68 with three staggered press units 7 2a, 7 2b, 7 2c. However, this is f o r illustration only, and in other configurations the compacting unit 68 may comprise a smaller or larger number of press units. The compacting unit 6 8 may further comprise a heating unit (not shown) to heat the rod 66 during the c o mpacting. H o t r o lling 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, h o t r o lling may f o ster the c o pper (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. Fo r grain growth to happen effectively, the processing temperature should be sufficiently higher than the c o pper re-c r ystallizati o n temperature of approximately 227" C. However, in order to avoid any nano-cracks of the copper surface, in some examples the hot rolling temperature should be chosen below 6 5o* C. In particular, the h o t r o lling temperature may be chosen between 45 0° C and 550"C. The sequence of Figures i 8a to i 8c sh o w the rod 6 6 in a cross-sectional schematic view and its transformation into the compacted sheet 7 0 as it progresses through the press units 7 2a, 72 b, 72c of the compacting unit 68. The techniques described above take advantage of the fact that graphene layers are sort of "slippery", and generally d o 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 C u (i11) atomic spacing, the graphene layer is 5 strongly mechanically b o nded to the c o pper surface o n which it is gr own. Due to these reasons, graphene/copper layers can slide relatively easily with respect to each other, making the abo ve-described conversion from a round shape of the rod 66 to a flat band possible without sacrificing the excellent mechanical connection between the individual graphene layers i4a, 14b and the copper layers 12 on which they are grown. At the same time, the c o pper surfaces are protected by the mechanically extremely robust graphene coa t ing, keeping any mechanical stress away from the copper during the severe plastic deformation. As illustrated in the insert of Fig. 1 8c, the result is a multilayer compo site structure 2 0 corresponding to a stacking of the c ompo site structure 10 as described above with reference to Fig. 3. As can be further taken from Figs. i 8b and 18 c, the plurality 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 c o mpactificati o n 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 fu r ther reference to Fig. 14, the system 5 4 additionally comprises a back-feeding unit 74 which may comprise transfer rollers to back feed the compacted sheet 70 to the dep o siti o n unit 6 o fo r another round of chemical vapor dep o siti o n, wrapping, and 0 compacting, as described above with reference to the circle diagram of Fig. 1 3. Fig. 1 5 is a schematic illustration of a system 5 4' 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. 1 4 above, and the same reference numerals are employed to denote corresponding components. However, the system 5 4' 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 7 6 upstream of the dep o siti on unit

The etching unit 7 6 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, s o as to provide a pristine c o pper surface f o r a subsequent dep o siti o n in the deposition unit 60. 5 F o r instance, the etching unit 7 6 may comprise a quartz tube furnace, which may expose the sheet 58 to a hydrogen (70 vol.-%) and argon (3 0 vol.-%) gas flow at temperatures in the range of 4 50 " C f o r 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 c o mprise a bypass unit 7 8 configured to bypass the sheet 58 o r compacted sheet 70 past the deposition unit 60. In particular, after the circle pr o cess 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 7 8 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 SVG 15691072.08-30-2017.J₆ZGHKVVRXEAPX0.SPEC.28.15.865.1396.930.1435.svg 0.13 0.217 Chemistry Black and white as will be described in additional detail with reference to Figs. 19 to 24 below. Wire drawing in a wire-drawing system 8o 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. 1 5, the system 54 may c o mprise a slicing unit 8 4 downstream of the compacting unit 6 8 and adapted to slice the compacted sheet in a longitudinal direction into a plurality of elongated slices 86 in a final pr o cess 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 S y stem As described above with reference to Fig. i5, 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 S30, 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 fo rm a spiral o r helix. In a third step S 34, a wire is formed by feeding the spiral through an opening of a die unit. Fig. 2 0 is a schematic illustration of a corresponding wire-drawing system 8 a that may be configured to perform the steps of the method illustrated in the fl o w diagram of Fig.

The die unit 9 4 comprises a plurality of dies 9 6a, 9 6b, 9 6c in a staggered c o nfigurati o n. Each of the dies 9 6a, 9 6b, 9 6c c om prises a respective opening 9 8a, 9 8b, 98 c, wherein a diameter or surface area of the openings 9 8a, 98 b, 9 8c decreases in a transport direction (to the right in Fig. 22a) of the spiral 90 as it passes through the die unit 9 4. The openings 9 8a, 9 8b, 9 8c may be openings with a circular o r elliptic cr o ss- section, depending on the desired cross-section of the wire 8 2. The openings 98 a, 98b, 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 1o 2a, 10 2b that pull the spiral 90 through the openings 9 8a, 98 b, 9 8c of the die unit 9 4. The pulling unit 100 may f o rm part of the feeding unit 9 2, o r may be a separate component. As can be further taken from Fig. 22b, the pulling rolls 1o 2a, 1 0 2b may be configured to rotate o r orbit around the drawn wire with a pre-selectable spacing in between to create a wire 8 2 with a desired outer diameter and shape. The c o nfigurati o n of Figs. 22 a and 22b sh ow s a die unit 9 4 with three dies 9 6a, 9 6b, 96c with openings of decreasing diameter in the transport direction of the spiral 90. However, in general, the die unit 9 4 may comprise any number of dies in a staggered c o nfigurati on. 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 m ono layers 10 4a, 10 4b are stacked with a twist angle 0 between them, may exhibit unique electronic, thermal, and magnetic properties. A rotational twist of the graphene monolayers 10 4a, 10 4b with respect to one another can have a profound effect o n the electrical properties of the b i-layer structure. Controlling the twist angle 0 of b i-layer graphene films hence allows for the preparation of twisted b i-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 b i-layer graphene: AB-stacking (also called Bernal stacking) is illustrated in Fig. 23a. The second graphene layer 4b is displaced by one half of the diameter of the hexagonal ring structure with respect to the hexagons of the first graphene layer 1o 4a. This results in a parallel shift without a twist. Fig. 23b schematically illustrates the so-called AA-stacking (also called non-Bernal stacking), in which the hexagons of the two layers 1o 4a, 10 4b lie right on top of each other (no shift, n o twist). Fig. 23 c illustrates AA'-stacking, which is similar to AA-stacking, but in which the crystallographic axes of the tw o layers io 4a, 10 4b are twisted by an angle 0 between o* 0 and 6 a" with respect to one another (no shift, twist angle 0). By means of twisting the layers by an angle of 0 = 600 SVG 15691072.08-30-2017.J₆ZGHKVVRXEAPX0.SPEC.31.19.1602.1910.1629.1950.svg 0.133 0.09 Chemistry Black and white an AB configuration can betransformed into an AA configuration and vice-versa, as illustrated in Figs. 24 a and 24b. In order to control the electrical, thermal, and magnetic properties of a stacked copper graphene b i-layer composite material, and in particular the wire 82, the twist angle 0 between the layers io 4a, 10 4b may be adjusted by means of a die unit 9 4 in which the openings 98 a, 98 b, 9 8c have a non-zero inclination angle with respect to a feeding direction of the spiral 90. As illustrated in Fig. 22a, a main twist angle O may be determined by means of the inclination angle of the openings 9 8a, 9 8b, 9 8c of the die unit 9 4. Different openings 9 8a, 98 b, 9 8c 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

The description of the embodiments and the Figures merely serve to illustrate the techniques of the disclosure, but should not be underst oo d to imply any limitation. The scope is to be determined o n 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 f o llowing claims) are to be c o nstrued to cover b o th 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 fro m the listed items (A o r B) o r any c o mbinati o n 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 c o nstrued 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 sh or thand 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 o r otherwise clearly c o ntradicted 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. N o language in the specification should be c o nstrued 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 m ode known to the inventors f o r 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 f o r 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 c o mbinati o n of the ab o ve- described elements in all possible variations thereof is encompassed by the invention unless otherwise indicated herein or otherwise clearly c o ntradicted by context. Claims What is claimed is:

A graphene m o nolayer is grown on both sides of a copper-comprising foil in a CVD c o ating facility.

The coated foil is then wrapped around so that a tube or rod is created with a circular concentric structure of alternating c o pper and bi-layer graphene layers.

The rod is then compacted, such as by metal hot rolling pressing, creating a flat co pper/graphene multilayer c ompo site band containing a multitude of graphene bi-layers 22.

The resulting c ompo site band is then etched o n b o th surfaces to pr o vide a pristine copper surface.

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 c o ating facility.

If the counter reaches the threshold value, the copper/graphene composite band is extracted from the c o ating and r ol ling cycle. The pr o cess circle as described above may be c alled "CWH-Ci rcl e" (c oating, wrapping, h o t-rolling). The pr o cess circle may result in a multilayer composite structure in which the volume fraction of graphene is significantly enhanced compared to the single-layer compo site structure 10 of Fig. 1. In particular, the pr o cess 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/o r the mechanical strength of the c o pper/graphene c ompo site material to a desired value. Fig. 1 4 is a schematic illustration of a system 54 f o r f o rming a multilayer composite structure employing the techniques described above. The system 5 4 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 dep o siti o n unit 60 may c o mprise a cold plasma CVD apparatus 3 8, 38 ' as described with reference to Figs. 7 and 8 above, and may deposit pairs of graphene layers i4 a, 14b on o pposing first and second surface sides of the sheet 5 8, resulting in a c o ated sheet 62 with a composite structure 10 as described above with reference to Figs. 1 and 2. In an example, the dep o siti o n unit 6 0 may c o mprise an annealing unit, providing an annealing zone 3 0 upstream of the deposition growth zone as described above with reference to Fig. 5. The transpo r t unit 56 transpo r ts the coated sheet 6 2 from the deposition unit 6 0 to a wrapping unit 64 to wrap the c o ated sheet 62 into a rod 66. The wrapping unit 64 may employ any technique to wrap the c o ated sheet, such as rolling up the c o ated sheet o n a thin cylindrical roll, and removing the roll. The resulting rod 66 is illustrated schematically in Fig. 1 6. 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 i4 a, 1 4b are in immediate and direct contact, as described 5 above with reference to Fig. 3, and therefore has all the conductivity advantages described above. With further reference to Fig. 14, the transport unit 5 6 may transport the rod 66 to a compacting unit 6 8 downstream of the wrapping unit 64. The compacting unit 6 8 may compact the rod 66 into a compacted sheet 70. Fo r instance, the compacting unit 6 8 may be a rolling press, in particular a h o t rolling press, as illustrated schematically in a front view in Fig. 17 a and in a side view in Fig. 1 7b. As can be taken from Figs. 17 a and 17 b, the h o t rolling press 6 8 may c o mprise a plurality of press units 72 a, 72 b, 72c in a staggered configuration, so that the rod 66 passes subsequently through the press units 7 2a, 72c in a transport direction (illustrated by the central arrow in Figs. i a and 17 b) and is thereby converted into the compacted sheet 70, such as a thin long band. Figs. i a and 17 b sh o w a c o mpacting unit 68 with three staggered press units 7 2a, 7 2b, 7 2c. However, this is f o r illustration only, and in other configurations the compacting unit 68 may comprise a smaller or larger number of press units. The compacting unit 6 8 may further comprise a heating unit (not shown) to heat the rod 66 during the c o mpacting. H o t r o lling 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, h o t r o lling may f o ster the c o pper (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. Fo r grain growth to happen effectively, the processing temperature should be sufficiently higher than the c o pper re-c r ystallizati o n temperature of approximately 227" C. However, in order to avoid any nano-cracks of the copper surface, in some examples the hot rolling temperature should be chosen below 6 5o* C. In particular, the h o t r o lling temperature may be chosen between 45 0° C and 550"C. The sequence of Figures i 8a to i 8c sh o w the rod 6 6 in a cross-sectional schematic view and its transformation into the compacted sheet 7 0 as it progresses through the press units 7 2a, 72 b, 72c of the compacting unit 68. The techniques described above take advantage of the fact that graphene layers are sort of "slippery", and generally d o 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 C u (i11) atomic spacing, the graphene layer is 5 strongly mechanically b o nded to the c o pper surface o n which it is gr own. Due to these reasons, graphene/copper layers can slide relatively easily with respect to each other, making the abo ve-described conversion from a round shape of the rod 66 to a flat band possible without sacrificing the excellent mechanical connection between the individual graphene layers i4a, 14b and the copper layers 12 on which they are grown. At the same time, the c o pper surfaces are protected by the mechanically extremely robust graphene coa t ing, keeping any mechanical stress away from the copper during the severe plastic deformation. As illustrated in the insert of Fig. 1 8c, the result is a multilayer compo site structure 2 0 corresponding to a stacking of the c ompo site structure 10 as described above with reference to Fig. 3. As can be further taken from Figs. i 8b and 18 c, the plurality 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 c o mpactificati o n 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 fu r ther reference to Fig. 14, the system 5 4 additionally comprises a back-feeding unit 74 which may comprise transfer rollers to back feed the compacted sheet 70 to the dep o siti o n unit 6 o fo r another round of chemical vapor dep o siti o n, wrapping, and 0 compacting, as described above with reference to the circle diagram of Fig. 1 3. Fig. 1 5 is a schematic illustration of a system 5 4' 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. 1 4 above, and the same reference numerals are employed to denote corresponding components. However, the system 5 4' 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 7 6 upstream of the dep o siti on unit

The etching unit 7 6 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, s o as to provide a pristine c o pper surface f o r a subsequent dep o siti o n in the deposition unit 60. 5 F o r instance, the etching unit 7 6 may comprise a quartz tube furnace, which may expose the sheet 58 to a hydrogen (70 vol.-%) and argon (3 0 vol.-%) gas flow at temperatures in the range of 4 50 " C f o r 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 c o mprise a bypass unit 7 8 configured to bypass the sheet 58 o r compacted sheet 70 past the deposition unit 60. In particular, after the circle pr o cess 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 7 8 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 SVG 15691072.08-30-2017.J₆ZGHKVVRXEAPX0.SPEC.28.15.865.1396.930.1435.svg 0.13 0.217 Chemistry Black and white as will be described in additional detail with reference to Figs. 19 to 24 below. Wire drawing in a wire-drawing system 8o 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. 1 5, the system 54 may c o mprise a slicing unit 8 4 downstream of the compacting unit 6 8 and adapted to slice the compacted sheet in a longitudinal direction into a plurality of elongated slices 86 in a final pr o cess 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 S y stem As described above with reference to Fig. i5, 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 S30, 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 fo rm a spiral o r helix. In a third step S 34, a wire is formed by feeding the spiral through an opening of a die unit. Fig. 2 0 is a schematic illustration of a corresponding wire-drawing system 8 a that may be configured to perform the steps of the method illustrated in the fl o w diagram of Fig.

The die unit 9 4 comprises a plurality of dies 9 6a, 9 6b, 9 6c in a staggered c o nfigurati o n. Each of the dies 9 6a, 9 6b, 9 6c c om prises a respective opening 9 8a, 9 8b, 98 c, wherein a diameter or surface area of the openings 9 8a, 98 b, 9 8c decreases in a transport direction (to the right in Fig. 22a) of the spiral 90 as it passes through the die unit 9 4. The openings 9 8a, 9 8b, 9 8c may be openings with a circular o r elliptic cr o ss- section, depending on the desired cross-section of the wire 8 2. The openings 98 a, 98b, 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 1o 2a, 10 2b that pull the spiral 90 through the openings 9 8a, 98 b, 9 8c of the die unit 9 4. The pulling unit 100 may f o rm part of the feeding unit 9 2, o r may be a separate component. As can be further taken from Fig. 22b, the pulling rolls 1o 2a, 1 0 2b may be configured to rotate o r orbit around the drawn wire with a pre-selectable spacing in between to create a wire 8 2 with a desired outer diameter and shape. The c o nfigurati o n of Figs. 22 a and 22b sh ow s a die unit 9 4 with three dies 9 6a, 9 6b, 96c with openings of decreasing diameter in the transport direction of the spiral 90. However, in general, the die unit 9 4 may comprise any number of dies in a staggered c o nfigurati on. 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 m ono layers 10 4a, 10 4b are stacked with a twist angle 0 between them, may exhibit unique electronic, thermal, and magnetic properties. A rotational twist of the graphene monolayers 10 4a, 10 4b with respect to one another can have a profound effect o n the electrical properties of the b i-layer structure. Controlling the twist angle 0 of b i-layer graphene films hence allows for the preparation of twisted b i-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 b i-layer graphene: AB-stacking (also called Bernal stacking) is illustrated in Fig. 23a. The second graphene layer 4b is displaced by one half of the diameter of the hexagonal ring structure with respect to the hexagons of the first graphene layer 1o 4a. This results in a parallel shift without a twist. Fig. 23b schematically illustrates the so-called AA-stacking (also called non-Bernal stacking), in which the hexagons of the two layers 1o 4a, 10 4b lie right on top of each other (no shift, n o twist). Fig. 23 c illustrates AA'-stacking, which is similar to AA-stacking, but in which the crystallographic axes of the tw o layers io 4a, 10 4b are twisted by an angle 0 between o* 0 and 6 a" with respect to one another (no shift, twist angle 0). By means of twisting the layers by an angle of 0 = 600 SVG 15691072.08-30-2017.J₆ZGHKVVRXEAPX0.SPEC.31.19.1602.1910.1629.1950.svg 0.133 0.09 Chemistry Black and white an AB configuration can betransformed into an AA configuration and vice-versa, as illustrated in Figs. 24 a and 24b. In order to control the electrical, thermal, and magnetic properties of a stacked copper graphene b i-layer composite material, and in particular the wire 82, the twist angle 0 between the layers io 4a, 10 4b may be adjusted by means of a die unit 9 4 in which the openings 98 a, 98 b, 9 8c have a non-zero inclination angle with respect to a feeding direction of the spiral 90. As illustrated in Fig. 22a, a main twist angle O may be determined by means of the inclination angle of the openings 9 8a, 9 8b, 9 8c of the die unit 9 4. Different openings 9 8a, 98 b, 9 8c 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

The description of the embodiments and the Figures merely serve to illustrate the techniques of the disclosure, but should not be underst oo d to imply any limitation. The scope is to be determined o n 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 f o llowing claims) are to be c o nstrued to cover b o th 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 fro m the listed items (A o r B) o r any c o mbinati o n 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 c o nstrued 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 sh or thand 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 o r otherwise clearly c o ntradicted 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. N o language in the specification should be c o nstrued 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 m ode known to the inventors f o r 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 f o r 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 c o mbinati o n of the ab o ve- described elements in all possible variations thereof is encompassed by the invention unless otherwise indicated herein or otherwise clearly c o ntradicted by context. Claims What is claimed is:

A graphene m o nolayer is grown on both sides of a copper-comprising foil in a CVD c o ating facility.

The coated foil is then wrapped around so that a tube or rod is created with a circular concentric structure of alternating c o pper and bi-layer graphene layers.

The rod is then compacted, such as by metal hot rolling pressing, creating a flat co pper/graphene multilayer c ompo site band containing a multitude of graphene bi-layers 22.

The resulting c ompo site band is then etched o n b o th surfaces to pr o vide a pristine copper surface.

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 c o ating facility.

If the counter reaches the threshold value, the copper/graphene composite band is extracted from the c o ating and r ol ling cycle. The pr o cess circle as described above may be c alled "CWH-Ci rcl e" (c oating, wrapping, h o t-rolling). The pr o cess circle may result in a multilayer composite structure in which the volume fraction of graphene is significantly enhanced compared to the single-layer compo site structure 10 of Fig. 1. In particular, the pr o cess 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/o r the mechanical strength of the c o pper/graphene c ompo site material to a desired value. Fig. 1 4 is a schematic illustration of a system 54 f o r f o rming a multilayer composite structure employing the techniques described above. The system 5 4 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 dep o siti o n unit 60 may c o mprise a cold plasma CVD apparatus 3 8, 38 ' as described with reference to Figs. 7 and 8 above, and may deposit pairs of graphene layers i4 a, 14b on o pposing first and second surface sides of the sheet 5 8, resulting in a c o ated sheet 62 with a composite structure 10 as described above with reference to Figs. 1 and 2. In an example, the dep o siti o n unit 6 0 may c o mprise an annealing unit, providing an annealing zone 3 0 upstream of the deposition growth zone as described above with reference to Fig. 5. The transpo r t unit 56 transpo r ts the coated sheet 6 2 from the deposition unit 6 0 to a wrapping unit 64 to wrap the c o ated sheet 62 into a rod 66. The wrapping unit 64 may employ any technique to wrap the c o ated sheet, such as rolling up the c o ated sheet o n a thin cylindrical roll, and removing the roll. The resulting rod 66 is illustrated schematically in Fig. 1 6. 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 i4 a, 1 4b are in immediate and direct contact, as described 5 above with reference to Fig. 3, and therefore has all the conductivity advantages described above. With further reference to Fig. 14, the transport unit 5 6 may transport the rod 66 to a compacting unit 6 8 downstream of the wrapping unit 64. The compacting unit 6 8 may compact the rod 66 into a compacted sheet 70. Fo r instance, the compacting unit 6 8 may be a rolling press, in particular a h o t rolling press, as illustrated schematically in a front view in Fig. 17 a and in a side view in Fig. 1 7b. As can be taken from Figs. 17 a and 17 b, the h o t rolling press 6 8 may c o mprise a plurality of press units 72 a, 72 b, 72c in a staggered configuration, so that the rod 66 passes subsequently through the press units 7 2a, 72c in a transport direction (illustrated by the central arrow in Figs. i a and 17 b) and is thereby converted into the compacted sheet 70, such as a thin long band. Figs. i a and 17 b sh o w a c o mpacting unit 68 with three staggered press units 7 2a, 7 2b, 7 2c. However, this is f o r illustration only, and in other configurations the compacting unit 68 may comprise a smaller or larger number of press units. The compacting unit 6 8 may further comprise a heating unit (not shown) to heat the rod 66 during the c o mpacting. H o t r o lling 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, h o t r o lling may f o ster the c o pper (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. Fo r grain growth to happen effectively, the processing temperature should be sufficiently higher than the c o pper re-c r ystallizati o n temperature of approximately 227" C. However, in order to avoid any nano-cracks of the copper surface, in some examples the hot rolling temperature should be chosen below 6 5o* C. In particular, the h o t r o lling temperature may be chosen between 45 0° C and 550"C. The sequence of Figures i 8a to i 8c sh o w the rod 6 6 in a cross-sectional schematic view and its transformation into the compacted sheet 7 0 as it progresses through the press units 7 2a, 72 b, 72c of the compacting unit 68. The techniques described above take advantage of the fact that graphene layers are sort of "slippery", and generally d o 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 C u (i11) atomic spacing, the graphene layer is 5 strongly mechanically b o nded to the c o pper surface o n which it is gr own. Due to these reasons, graphene/copper layers can slide relatively easily with respect to each other, making the abo ve-described conversion from a round shape of the rod 66 to a flat band possible without sacrificing the excellent mechanical connection between the individual graphene layers i4a, 14b and the copper layers 12 on which they are grown. At the same time, the c o pper surfaces are protected by the mechanically extremely robust graphene coa t ing, keeping any mechanical stress away from the copper during the severe plastic deformation. As illustrated in the insert of Fig. 1 8c, the result is a multilayer compo site structure 2 0 corresponding to a stacking of the c ompo site structure 10 as described above with reference to Fig. 3. As can be further taken from Figs. i 8b and 18 c, the plurality 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 c o mpactificati o n 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 fu r ther reference to Fig. 14, the system 5 4 additionally comprises a back-feeding unit 74 which may comprise transfer rollers to back feed the compacted sheet 70 to the dep o siti o n unit 6 o fo r another round of chemical vapor dep o siti o n, wrapping, and 0 compacting, as described above with reference to the circle diagram of Fig. 1 3. Fig. 1 5 is a schematic illustration of a system 5 4' 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. 1 4 above, and the same reference numerals are employed to denote corresponding components. However, the system 5 4' 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 7 6 upstream of the dep o siti on unit

The etching unit 7 6 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, s o as to provide a pristine c o pper surface f o r a subsequent dep o siti o n in the deposition unit 60. 5 F o r instance, the etching unit 7 6 may comprise a quartz tube furnace, which may expose the sheet 58 to a hydrogen (70 vol.-%) and argon (3 0 vol.-%) gas flow at temperatures in the range of 4 50 " C f o r 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 c o mprise a bypass unit 7 8 configured to bypass the sheet 58 o r compacted sheet 70 past the deposition unit 60. In particular, after the circle pr o cess 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 7 8 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 SVG 15691072.08-30-2017.J₆ZGHKVVRXEAPX0.SPEC.28.15.865.1396.930.1435.svg 0.13 0.217 Chemistry Black and white as will be described in additional detail with reference to Figs. 19 to 24 below. Wire drawing in a wire-drawing system 8o 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. 1 5, the system 54 may c o mprise a slicing unit 8 4 downstream of the compacting unit 6 8 and adapted to slice the compacted sheet in a longitudinal direction into a plurality of elongated slices 86 in a final pr o cess 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 S y stem As described above with reference to Fig. i5, 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 S30, 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 fo rm a spiral o r helix. In a third step S 34, a wire is formed by feeding the spiral through an opening of a die unit. Fig. 2 0 is a schematic illustration of a corresponding wire-drawing system 8 a that may be configured to perform the steps of the method illustrated in the fl o w diagram of Fig.

The die unit 9 4 comprises a plurality of dies 9 6a, 9 6b, 9 6c in a staggered c o nfigurati o n. Each of the dies 9 6a, 9 6b, 9 6c c om prises a respective opening 9 8a, 9 8b, 98 c, wherein a diameter or surface area of the openings 9 8a, 98 b, 9 8c decreases in a transport direction (to the right in Fig. 22a) of the spiral 90 as it passes through the die unit 9 4. The openings 9 8a, 9 8b, 9 8c may be openings with a circular o r elliptic cr o ss- section, depending on the desired cross-section of the wire 8 2. The openings 98 a, 98b, 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 1o 2a, 10 2b that pull the spiral 90 through the openings 9 8a, 98 b, 9 8c of the die unit 9 4. The pulling unit 100 may f o rm part of the feeding unit 9 2, o r may be a separate component. As can be further taken from Fig. 22b, the pulling rolls 1o 2a, 1 0 2b may be configured to rotate o r orbit around the drawn wire with a pre-selectable spacing in between to create a wire 8 2 with a desired outer diameter and shape. The c o nfigurati o n of Figs. 22 a and 22b sh ow s a die unit 9 4 with three dies 9 6a, 9 6b, 96c with openings of decreasing diameter in the transport direction of the spiral 90. However, in general, the die unit 9 4 may comprise any number of dies in a staggered c o nfigurati on. 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 m ono layers 10 4a, 10 4b are stacked with a twist angle 0 between them, may exhibit unique electronic, thermal, and magnetic properties. A rotational twist of the graphene monolayers 10 4a, 10 4b with respect to one another can have a profound effect o n the electrical properties of the b i-layer structure. Controlling the twist angle 0 of b i-layer graphene films hence allows for the preparation of twisted b i-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 b i-layer graphene: AB-stacking (also called Bernal stacking) is illustrated in Fig. 23a. The second graphene layer 4b is displaced by one half of the diameter of the hexagonal ring structure with respect to the hexagons of the first graphene layer 1o 4a. This results in a parallel shift without a twist. Fig. 23b schematically illustrates the so-called AA-stacking (also called non-Bernal stacking), in which the hexagons of the two layers 1o 4a, 10 4b lie right on top of each other (no shift, n o twist). Fig. 23 c illustrates AA'-stacking, which is similar to AA-stacking, but in which the crystallographic axes of the tw o layers io 4a, 10 4b are twisted by an angle 0 between o* 0 and 6 a" with respect to one another (no shift, twist angle 0). By means of twisting the layers by an angle of 0 = 600 SVG 15691072.08-30-2017.J₆ZGHKVVRXEAPX0.SPEC.31.19.1602.1910.1629.1950.svg 0.133 0.09 Chemistry Black and white an AB configuration can betransformed into an AA configuration and vice-versa, as illustrated in Figs. 24 a and 24b. In order to control the electrical, thermal, and magnetic properties of a stacked copper graphene b i-layer composite material, and in particular the wire 82, the twist angle 0 between the layers io 4a, 10 4b may be adjusted by means of a die unit 9 4 in which the openings 98 a, 98 b, 9 8c have a non-zero inclination angle with respect to a feeding direction of the spiral 90. As illustrated in Fig. 22a, a main twist angle O may be determined by means of the inclination angle of the openings 9 8a, 9 8b, 9 8c of the die unit 9 4. Different openings 9 8a, 98 b, 9 8c 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

The description of the embodiments and the Figures merely serve to illustrate the techniques of the disclosure, but should not be underst oo d to imply any limitation. The scope is to be determined o n 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 f o llowing claims) are to be c o nstrued to cover b o th 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 fro m the listed items (A o r B) o r any c o mbinati o n 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 c o nstrued 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 sh or thand 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 o r otherwise clearly c o ntradicted 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. N o language in the specification should be c o nstrued 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 m ode known to the inventors f o r 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 f o r 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 c o mbinati o n of the ab o ve- described elements in all possible variations thereof is encompassed by the invention unless otherwise indicated herein or otherwise clearly c o ntradicted by context. Claims What is claimed is: