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Title:
GRAPHENE BASED ELECTRICAL CONDUCTORS
Document Type and Number:
WIPO Patent Application WO/2018/032055
Kind Code:
A1
Abstract:
Provided is a method of producing an electrical conductor and an electrical conductor having a lamellar structure of one or more graphene layers alternating with and compressed between substrate layers, whereby graphene particles compressed between the substrate have a density causing alignment and improved physical contact of the graphene particles within the graphene layers between the substrate layers. The electrical conductor is produced by layering particulate graphene material on substrate layers to provide a sheet of alternating graphene and substrate layers. Then compressing the sheet to compress and align the graphene particles of the graphene layers between the substrate layers by mechanically deforming the sheet using an iterative mechanical deformation process which applies force components both perpendicular and parallel to the graphene layers.

Inventors:
LI SEAN SUIXIANG (AU)
GE CHEN (CN)
Application Number:
PCT/AU2017/050876
Publication Date:
February 22, 2018
Filing Date:
August 17, 2017
Export Citation:
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Assignee:
NEWSOUTH INNOVATIONS PTY LTD (AU)
HANGZHOU CABLE CO LTD (CN)
International Classes:
H01B1/04; B32B9/00; B32B15/04; B32B37/10; H01B13/00
Foreign References:
CN105127197A2015-12-09
US20160168693A12016-06-16
Attorney, Agent or Firm:
GRIFFITH HACK (AU)
Download PDF:
Claims:
CLAIMS

1 . An electrical conductor comprising a lamellar structure of one or more graphene layers alternating with and compressed between substrate layers whereby graphene particles compressed between the substrate have a density causing alignment and improved physical contact of the graphene particles within the graphene layers between the substrate layers.

2. An electrical conductor as claimed in claim 1 wherein the graphene layers compressed between the substrate layers have an inter-particle density within the range of 80-100%.

3. An electrical conductor as claimed in any one of the preceding claims wherein the lamellar structure comprises one graphene layer compressed between two substrate layers.

4. An electrical conductor as claimed in claim 1 wherein the lamellar structure comprises a two or more alternating graphene and substrate layers compressed between substrate layers. 5. An electrical conductor as claimed in any one of the preceding claims wherein the lamellar structure is incorporated into an electrical conductor structure.

6. An electrical conductor as claimed in claim 5 wherein the lamellar structure is incorporated into the electrical conductor structure as a plurality of conductive strips.

7. An electrical conductor as claimed in claim 6 wherein the conductive strips are formed by cutting a sheet of the lamellar structure.

8. An electrical conductor as claimed in claim 6 or claim 7 wherein the conductor structure is a wire and the plurality of conductive strips is encapsulated within a sheath and drawn to form the wire.

9. An electrical conductor as claimed in claim 5 wherein the lamellar structure is incorporated into the electrical conductor structure by rolling a sheet of the lamellar structure.

10. An electrical conductor as claimed in claim 9 wherein the sheet of lamellar structure is rolled and encapsulated within a sheath. 1 1 . An electrical conductor as claimed in claim 10 wherein the rolled lamellar structure and sheath are drawn to form a wire.

12. An electrical conductor as claimed in claim 10 wherein the rolled lamellar structure and sheath are extruded to form a wire.

13. An electrical conductor as claimed in claim 9 wherein the conductor structure is a transmission line bundle and the lamellar structure is rolled around a support structure or one or more conductors within the transmission line bundle. 14. An electrical conductor as claimed in claim 13 wherein the lamellar structure rolled around the support structure or one or more conductors is subject to an extrusion process during formation of the transmission line bundle.

15. An electrical conductor as claimed in any one of the preceding claims wherein the lamellar structure is formed by:

layering one or more layers of graphene particles with one or more substrate layers to form a sheet of alternating substrate and graphene layers; and

compressing the sheet to compress and align the graphene particles of the graphene layers between the substrate layers by mechanically deforming the sheet using an iterative mechanical deformation process applying force components both

perpendicular and parallel to the graphene layers.

16. An electrical conductor as claimed in claim 15 wherein the iterative mechanical deformation process uses deformation ratios for each iteration in the range of 5~25%

17. An electrical conductor as claimed in claim 16 wherein the total mechanical deformation ratio for the iterative mechanical deformation process is in the range of 10 to 80% or more.

18. An electrical conductor as claimed in any one of claims 15 to 17 wherein the mechanical deformation process is any one or more of: roll milling, extrusion or drawing.

5 19. An electrical conductor as claimed in claim 15 wherein the layering step

comprises one or more repetitions of the steps of depositing graphene particles on a substrate layer to form a graphene deposited substrate layer, and layering one or more graphene deposited substrate layers; and layering a final substrate layer on a top graphene deposited substrate layer to from the sheet of alternating substrate and0 graphene layers.

20. An electrical conductor as claimed in claim 19 wherein the graphene layer is deposited on the substrate layer using any one or more of: powder laying, deposition, printing, painting, coating, spraying or tape casting.

5

21 . An electrical conductor as claimed in any one of claims 15 to 20 wherein the compressing step utilises more than one mechanical deformation process.

22. An electrical conductor as claimed in any one of claims 15 to 21 further o comprising a step of annealing the sheet during the iterative mechanical deformations.

23. An electrical conductor as claimed in any one of the preceding claims wherein the graphene comprises graphene flakes. 5 24. An electrical conductor as claimed in any one of the preceding claims wherein the substrate layers comprise any one or more of: metallic, polymer of other materials.

25. A method of producing an electrical conductor, the method comprising the steps of:

0 layering one or more graphene layer with two or more substrate layers to form a sheet of alternating substrate and graphene layers; and

compressing the sheet to compress and align the graphene particles of the graphene layers between the substrate layers and form a compressed lamellar graphene structure.

5

26. A method as claimed in claim 25 wherein the compressing step comprises mechanically deforming the sheet using an iterative mechanical deformation process applying force components both perpendicular and parallel to the graphene layers. 27. A method as claimed in claim 26 wherein the mechanical deformation process is any one or more of: roll milling, extrusion or drawing.

28. A method as claimed in any one of claims 25 to 27 wherein the layering step comprises one or more repetitions of the steps of depositing a graphene layer on a substrate layer, and layering one or more graphene deposited substrate layers; and layering a final substrate layer on a top graphene layer to from the sheet of alternating substrate and graphene layers.

29. A method as claimed in claim 28 wherein the graphene layer is deposited on the substrate layer using any one or more of: powder deposition, printing, painting, coating or tape casting.

30. A method as claimed in any one of claims 26 to 29 wherein the compressing step utilises more than one mechanical deformation process.

31 . A method as claimed in any one of claims 26 to 30 further comprising a step of annealing the sheet during the iterative mechanical deformations.

32. A method as claimed in any one of claims 25 to 31 wherein the graphene comprises graphene flakes.

33. A method as claimed in any one of claims 25 to 32 wherein the substrate layers comprise any one or more of: metallic, polymer of other materials. 34. A method as claimed in any one of claims 25 to 33 further comprising the step of incorporating the compressed lamellar graphene structure into an electrical conductor structure.

35. A method as claimed in claim 34 further comprising the steps of: cutting the compressed lamellar graphene structure into a plurality of conductive strips; and

the incorporating step includes arranging the plurality of conductive strips and forming the electrical conductor structure including the arranged plurality of conductive strips.

36. A method as claimed in claim 35 wherein arranging the plurality of conductive strips comprises any one or more or: stacking, bundling, layering or weaving.

37. A method as claimed in claim 35 or 36 wherein the electrical conductor structure is a wire and the step of forming the electrical conductor structure comprises inserting the arranged plurality of conductive strips into a tubular sheath and drawing into a wire.

38. A method as claimed in claim 37 wherein the step of drawing comprises pulling the tubular sheath through a series of drawing dies.

39. A method as claimed in claim 37 or 38 further comprising the step of bundling a plurality of the wires into a transmission line cable.

40. A method as claimed in claim 37 or 38 further comprising the step of rolling the wire to form an electrically conductive tape.

41 . A method as claimed in claim 40 further comprising wrapping the tape around an electrical conductor of a transmission line bundle.

42. A method as claimed in claim 34 wherein the lamellar structure is incorporated into the electrical conductor structure by rolling a sheet of the laminar structure.

43. A method as claimed in claim 42 wherein incorporating the electrical conductor into an electrical conductor structure includes the steps of:

rolling the sheet of lamellar structure; and

encapsulating the rolled lamellar structure within a sheath.

44. A method as claimed in claim 43 further comprising the step of drawing the rolled lamellar structure and sheath to form a wire.

45. A method as claimed in claim 43 or 44 further comprising the step of rolling the wire to form an electrically conductive tape.

46. A method as claimed in claim 45 further comprising wrapping the tape around an electrical conductor of a transmission line bundle. 47. A method as claimed in claim 42 wherein the conductor structure is a transmission line bundle and comprising the step of rolling the lamellar structure around a support structure or one or more conductors during formation of the transmission line bundle. 48. A method as claimed in claim 47 wherein the lamellar structure rolled around the support structure or one or more conductors is subject to an extrusion process during formation of the transmission line bundle.

Description:
GRAPHENE BASED ELECTRICAL CONDUCTORS

Technical Field The technical field of the present invention is low resistance electrical conductors suitable for use in electrical cables and transmission lines, also transformers and electrical devices. In particular, an application of the electrical conductors is in conductor bundles for electrical transmission lines, for example high power electrical transmission lines for use in electrical power grids.

Background

The transport and distribution of electricity in most urban environments relies on an electric power grid consisting of transmission lines and the distribution network. The electric power transmission is the bulk transfer of electricity from power generation plants to electrical substations located near demand centres.

Resistance of the transmission line materials causes the loss of electric energy during transmission. Reduction in loses during transmission is desirable to enable improved efficiency, allow wider distribution and improve power grid performance. There is a need for improved performance and lower resistance electrical transmission lines.

Summary of the Invention According to one aspect there is provided an electrical conductor comprising a lamellar structure of one or more graphene layers alternating with and compressed between substrate layers whereby graphene particles compressed between the substrate have a density causing alignment and improved physical contact of the graphene particles within the graphene layers between the substrate layers.

In some embodiments the graphene layers compressed between the substrate layers have an inter-particle density within the range of 80~100%.

In some embodiments the lamellar structure comprises one graphene layer compressed between two substrate layers. In some embodiments the lamellar structure comprises a two or more alternating graphene and substrate layers compressed between substrate layers.

The lamellar structure can be incorporated into an electrical conductor structure.

In an embodiment the lamellar structure is incorporated into the electrical conductor structure as a plurality of conductive strips. For example the conductive strips can be formed by cutting a sheet of the lamellar structure. In an embodiment the conductor structure is a wire and the plurality of conductive strips is encapsulated within a sheath and drawn to form the wire.

In an alternative embodiment the lamellar structure is incorporated into the electrical conductor structure by rolling a sheet of the lamellar structure. In an embodiment the sheet of lamellar structure is rolled and encapsulated within a sheath. The rolled lamellar structure and sheath can be drawn to form a wire. Alternatively the rolled lamellar structure and sheath can be extruded to form a wire.

In an embodiment the conductor structure is a transmission line bundle and the lamellar structure is rolled around a support structure or one or more conductors within the transmission line bundle. In an embodiment the lamellar structure rolled around the support structure or one or more conductors is subject to an extrusion process during formation of the transmission line bundle. In some embodiments the lamellar structure is formed by:

layering one or more layers of graphene particles with one or more substrate layers to form a sheet of alternating substrate and graphene layers; and

compressing the sheet to compress and align the graphene particles of the graphene layers between the substrate layers by

mechanically deforming the sheet using an iterative mechanical deformation process applying force components both perpendicular and parallel to the graphene layers. For example, the mechanical deformation process is any one or more of: roll milling, extrusion or drawing.

In some embodiments the iterative mechanical deformation process uses deformation ratios for each iteration in the range of 5~25% In some embodiments the total mechanical deformation ratio for the iterative mechanical deformation process is in the range of 10 to 80% or more. In some embodiments the layering step comprises one or more repetitions of the steps of depositing graphene particles on a substrate layer to form a graphene deposited substrate layer, and layering one or more graphene deposited substrate layers; and layering a final substrate layer on a top graphene deposited substrate layer to from the sheet of alternating substrate and graphene layers. For example, the graphene layer can be deposited on the substrate layer using any one or more of: powder laying, deposition, printing, painting, coating, spraying or tape casting.

In some embodiments the compressing step utilises more than one mechanical deformation process.

Some embodiments further comprise a step of annealing the sheet during the iterative mechanical deformations.

The graphene can comprise graphene flakes or graphene sheets.

The substrate layers can comprise any one or more of: metallic, polymer of other materials.

According to another aspect there is provided a method of producing an electrical conductor, the method comprising the steps of:

layering one or more graphene layer with two or more substrate layers to form a sheet of alternating substrate and graphene layers; and

compressing the sheet to compress and align the graphene particles of the graphene layers between the substrate layers and form a compressed lamellar graphene structure by mechanically deforming the sheet using an iterative mechanical deformation process applying force components both perpendicular and parallel to the graphene layers. For example, the mechanical deformation process can be any one or more of: roll milling, extrusion or drawing. In an embodiment of the method the layering step comprises one or more repetitions of the steps of depositing a graphene layer on a substrate layer, and layering one or more graphene deposited substrate layers; and layering a final substrate layer on a top graphene layer to from the sheet of alternating substrate and graphene layers. For example, the graphene layer can be deposited on the substrate layer using any one or more of: powder deposition, printing, painting or coating.

In an embodiment the compressing step utilises more than one mechanical deformation process. The method can further comprise a step of annealing the sheet during the iterative mechanical deformations.

Embodiments of the method further comprise the step of incorporating the compressed lamellar graphene structure into an electrical conductor structure. This method can further comprise the step of cutting the compressed lamellar graphene structure into a plurality of conductive strips; and the incorporating step includes arranging the plurality of conductive strips and forming the electrical conductor structure including the arranged plurality of conductive strips. For example, arranging the plurality of conductive strips comprises any one or more or: stacking, bundling, layering or weaving.

In an embodiment the electrical conductor structure is a wire and the step of forming the electrical conductor structure comprises inserting the arranged plurality of conductive strips into a tubular sheath and drawing into a wire. For example, the step of drawing can comprise pulling the tubular sheath through a series of drawing dies.

The method can further comprise the step of bundling a plurality of the wires into a transmission line cable.

The method can further comprise the step of rolling the wire to form an electrically conductive tape. The method can further comprise wrapping the tape around an electrical conductor of a transmission line bundle.

In an embodiment of the method the lamellar structure is incorporated into the electrical conductor structure by rolling a sheet of the laminar structure. For example, incorporating the electrical conductor into an electrical conductor structure can include the steps of: rolling the sheet of lamellar structure; and encapsulating the rolled lamellar structure within a sheath.

The method can further comprise the step of drawing the rolled lamellar structure and sheath to form a wire.

The method can further comprise the step of rolling the wire to form an electrically conductive tape. The method can further comprise the step of wrapping the tape around an electrical conductor of a transmission line bundle.

In an embodiment of the method the conductor structure is a transmission line bundle and the method comprises the step of rolling the lamellar structure around a support structure or one or more conductors during formation of the transmission line bundle. In an embodiment the lamellar structure rolled around the support structure or one or more conductors is subject to an extrusion process during formation of the

transmission line bundle.

Brief Description of the Drawings Figure 1 Transmission Electron Microscopy image shows the wrinkle graphene sheets

Figure 2 Schematic illustration showing how a single and multi-layered graphene composite can be produced with a lamellar structure. Figure 3 (a) the electrical resistance between two overlapped wrinkle aluminium foils is 1 .18 Ω; (b) the electrical contact between these two wrinkle aluminium foils can be improved significantly by increasing surface contact area between the two sheets by applying a compressive force on the foils and the electrical resistance between the foils is reduced to 0.0 Ω within the measurement error.

Figure 4 Schematic illustration show (a) multi-point contacts between two wrinkle graphene sheets, which are laid parallel; (b) single point contact between two wrinkle graphene sheets, which are laid with an angle; (3) surface contact between two graphene sheets can be induced by applying the compressive force on the two wrinkle graphene sheets through mechanical deformation. Figure 5 illustrates force components produce by the roll milling process.

Figure 6 Schematic illustration shows how the graphene layer can be

coated/printed/deposited on the substrates with roll to roll printing technology

Figure 7 illustrates the lamellar composite being rolled by a precise roller mill using a series of the optimized deformation ratios from 5% to 25% or higher depending on thicknesses and numbers of the substrate and also the coated graphene/graphene oxide/graphite layers to improve the alignment and also the contact between the graphene sheets.

Figure 8a and 8b Schematic illustration shows that the lamellar structured

graphene/metal belts can be (a) cut into strips or (b) rolled-up into a cylinder shaped electrical conductors

Figure 9a and 9b Schematic illustrations show (a) the strips or (b) roll-up cylinders are filled into the metal tubes, which are subsequently drawn into wires and then roll milled into the tapes. Figure 10 Schematic illustration shows how to use the lamellar graphene/metal composite to improve the newly made or the existing metal core electrical transmission lines.

Figure 1 1 The textural structure of graphene in the composite fabricated without the rolling process can also be produced through the large transverse stress generated by the extrusion process of the insulating polymer materials surround the metal core.

Figure 12a Represents the crystal structure of monolayer graphene, showing how an electron moves in a single atomic layered graphene sheet.

Figures 12b and 12c Illustrate the unit cell structure of single layer graphene. Figure 13a Represents the crystal structure of bilayer graphene in a Bernal stacking configuration showing a top view.

Figure 13b Represents the crystal structure of bilayer graphene showing a side view, with electron hopping direction indicated by arrows. Figure 13c and 13d Illustrate the unit cell structure of bi-layer graphene.

Figure 13e Schematic representation of how the electron moves in a bi-layer graphene sheet.

Figure 14a Illustrates a model configuration for conductance calculation for monolayer graphene.

Figure 14b Illustrates a model configuration for conductance calculation for bilayer graphene.

Figure 15a is a graph of the calculated conductance as a function of energy for monolayer graphene calculated for a 10 x 10 sheet of monolayer graphene in accordance with Figure 10a.

Figure 15b is a graph of the calculated conductance as a function of energy for bilayer graphene calculated for a 10 x 10 sheet of bilayer graphene in accordance with Figure 10b.

Figure 16 shows a representation of two overlapping sheets of monolayer graphene and the lead placement for modelling charge transfer between adjacent sheets of graphene.

Figures 17a to e are graphs of conductance for side contact graphene layers at different interlayer hopping energy.

Figures 18a and 18b are schematic illustrations of design parameters for the modelled interlayer electron transfer in bilayer graphene.

Figure 19a and 19b represent molecule structures used to model systems of graphene for investigating the geometric dependence of interlayer electronic couplings.

Figure 20 graphs scanning of dimer transfer integrals for the model graphene system of Figure 19a, having 3.6 A intermolecular separation with a 180 degree in-plane rotation of one monomer scan of the change in electron transfer integrals (T.I.) Figure 21 graphs scanning of dimer transfer integrals for the model graphene system of Figure 19b, having 3.6 A intermolecular separation with a 180 degree in-plane rotation of one monomer scan of the change in T.I.

Figure 22 graphs scanning of dimer transfer integrals for the model graphene system of Figure 19a as a function of interlayer distances d at selected different in-plane rotation angles.

Figure 23 graphs scanning of dimer transfer integrals for the model graphene system of Figure 19b as a function of interlayer distances d at selected different in-plane rotation angles. Figure 24 graphs Raman spectroscopy of the as-received and the as-annealed reduced graphene oxide (rGO).

Figure 25 graphs X-ray photoelectron spectroscopy (XPS) spectra of the as-received rGO and the as-annealed rGO. Detailed Description

This invention develops high performance electric conductors by using a lamellar structure of one or more graphene layers compressed between substrate layers through iterative mechanical deformation. Iterative mechanical deformation is used to increase density and proximity of individual graphene flakes and improve alignment between graphene flakes to improve electrical properties of particulate graphene. This composite lamellar structure can be used in conjunction with or substituting conventional conductive materials in conductor structures such as transmission lines. Embodiments can also be applied to electrical conductors for microelectronic devices, and other devices including high performance transformers and electric motors. Graphite is an abundant material and graphene is a single atomic layer of graphite.

Graphene is an allotrope of carbon that is made up of very tightly bonded carbon atoms organised into a hexagonal lattice. The atomic thin nature with the unique sp2 hybridisation provides significant advantages for graphene in terms of electrical conductivity, heat conduction and strength. In particular, graphene is a zero-overlap semimetal with very high electrical conductivity. However, although graphite is an abundant material, graphene isolated in its two dimensional lattice form does not occur naturally. Typically graphite is processed to fabricate graphene, for example by micro- mechanical exfoliation and highly ordered pyrolytic or electrochemical exfoliation. Or graphene is fabricated directly, for example using fabrication methods such as epitaxial growth, chemical vapour deposition etc. Chemically produced graphene oxide can also be reduced to provide graphene. The size of graphene particles (flakes or sheets) can vary greatly for different production techniques.

The electrical conductivity within a layer of graphene is 1 .00x10 8 S/m, which is a result of remarkable electron mobility of 200000 cm 2 /V-s and a carrier density of 10 12 /cm 2 . Graphene has the highest electric conductivity of natural substances at room temperature. Table 1 lists the resistivity and conductivity of the most common used highly conductive materials. It shows the electric conductivity of graphene is 37% higher than silver, more than 40% higher than copper, and 62% higher than aluminum. In particular, graphene can carry the electric current with the densities 6 orders of magnitude higher than copper as a conductor of electricity and its weight is also ~40% lighter than aluminum

Table 1 Resistivity and conductivity of various materials

The high conductivity makes graphene conceptually desirable for use in electrical conductors. However, the above conductivity is a measure of the conductivity with an individual single layer of graphene. This may be achieved on a small scale using graphene fabrication methods such as epitaxial growth to produce a single layer of graphene on a substrate, for example in production of integrated circuits etc. For larger scale industrial use, graphene is typically produced in a particulate form as sheets or flakes. For example, by exfoliating graphene sheets from graphite using mechanical or chemical exfoliation methods, or reduction of chemically produced graphene oxide. Such as-produced graphene particles are far less suitable for large scale practical manufacture and use in electrical conductors. The conductance of particulate graphene is far lower than that of a single sheet as charge transfer between particles is less efficient than within graphene sheets. Further, the covalent nature of carbon bonding in graphene makes the graphene difficult to join and shape. Thus to date, application of graphene in high volume production of electrical conductors has been limited.

Attempts have been made to utilise graphene in combination with other materials, aiming to enhance electrical properties of these materials. One known approach is to add graphene or carbon nanotubes into copper or aluminium directly during the physical metallurgy process (e.g. melting and casting) to form materials that consist of a mixture of metal and graphene (or carbon nanotubes). However, a problem with this process is that the formed materials consist of mixture phases of metal and graphene (or carbon nanotubes) and the graphene tends to form clusters within the metal.

Because of the different weight densities between the metals and graphene, it is a very difficult to form a metallic material with homogeneously dispersed graphene using known metallurgy processes, thus making it difficult to achieve enhancement of electrical conductance. In fact, this approach, using commonly used metallurgy processes has been found to be ineffective at improving electrical properties. It is believed that this is due to the graphene and/or carbon nanotubes forming inclusions (clusters) in the material, thus failing to enhance the electrical conductivity effectively. Further, inductance was a problem in transmission lines formed using this material, the inductance believed to be due to the cluster formations causing the conductors to have inconsistent structure. An alternative approach is therefore needed to utilise graphene on an industrial scale for electrical conductors.

Most of the graphene produced for industry applications is in a particulate or powder form, with the particles having sheet or flake shapes from a nanometer to centimetres or larger. However, fabricating and shaping a large scale graphene product is extremely difficult. The covalent nature of carbon bonds means the graphene flakes are extremely difficult to join together and shape to form products, particularly on a bulk scale.

The electrical current is transported in an atomic plane of graphene. In general, each carbon atom has a total of 6 electrons with 2 in the inner shell and 4 in the outer shell. In bulk, these 4 outer shell electrons are available for chemical bonding, but in graphene, each atom is connected to 3 other carbon atoms in the two dimensional atomic layer, freeing 1 electron available in the third dimension for electronic conduction. These highly-mobile electrons are located above and below the graphene sheet. Similar to the free electrons in metals - where electrons can transfer from one metal wire to another at the twisted end of these two wires or from one metal plate to another if these two flat plates are partially overlapped - these free electrons on the surface of a graphene sheet can migrate to another graphene sheet through the electron-electron interaction if these two graphene sheets are in contact with each other. Establishing good contact is however a problem due to the structure of graphene flakes. As a result the conductivity of particulate graphene powder is typically within the range of around 1 .0x10 3 S/m to 1 .0x 1 0 5 S/m significantly below the conductivity of 1 .00x 10 8 S/m within a graphene layer. Further industrially produced particulate graphene may also include lattice defects or impurities which can negatively affect the electrical conductivity.

Graphene is an allotrope of carbon having a two-dimensional, honeycomb lattice form on an atomic scale. In most of the cases, the graphene particles produced for industrial processes have a wrinkled sheet-like or flake-like structure as shown in the Transmission Electron Microscopy Image of Figure 1 . Throughout the following description the expressions graphene particle, graphene sheet or graphene flake or graphene sheets/flakes are used to refer to the two dimensional graphene particles, which are typically formed having a sheet-like or flake-like structure as shown in Figure 1 . Graphene flake and graphene sheet are used to refer to two dimensional graphene particles of differing sizes, flakes for example having an area smaller than sheets the actual size of the graphene flakes or sheets can vary widely depending on production methodology, any of which are suitable for use in embodiments of the present invention. The term graphene particle is used throughout the specification to refer to any two-dimensional graphene particle.

The image of Figure 1 shows a wrinkled and corrugated surface to the graphene particles, which means that adjacent sheets may have only a few contact points, meaning contact between the graphene particles is very poor, thus resulting in a very low electric conductivity (in the scale of ~10000 S/m) and current density in their bulk counterpart. At the time of filing, the highest reported electric conductivity of the graphene buckypaper is about 55088 S/m [C. Liu, F. Hao, X. Zhao, Q. Zhao, S. Luo and H. Lin, Scientific Reports 4, 3965 (2014)], which is far inferior to the electric conductivity of the single atomic layered graphene sheet (1 .00x10 8 S/m). The inventors have developed an industrially applicable technique to improve the contact between graphene particles and thereby electrical conductivity in a composite graphene conductor structure. The composite graphene conductor structure can have improved performance and applicable for a range of practical applications.

Embodiments of the present invention provide an electrical conductor comprising a lamellar structure of one or more graphene layers alternating with and compressed between substrate layers using mechanical deformation to increase density to enhance the physical contact between the individual graphene sheets and thereby improve electrical conductivity within the graphene layers of the lamellar structure. The lamellar structure may be a single graphene layer or multi graphene layer structure. The multi graphene layer structure comprises a two or more alternating compressed graphene and substrate layers. The lamellar structure is formed by layering one or more particulate graphene layer with one or more substrate layers to form a sheet of alternating substrate and graphene layers. The sheet is then compressed using a compression process to compress the graphene layers between the substrate layers to thereby cause increase in density to enhance the physical contact between the individual graphene sheets and alignment of graphene particles. This alignment and compression of the graphene particles improves the contact between the individual graphene particles and thereby improving the conductive properties. Further, the substrate provides a supporting framework enabling further forming of the lamellar structure and incorporation into electrical conductor structures, including traditional structures such as wires and transmission lines.

Figure 2 illustrates an example of the process for forming the lamellar structure. A substrate layer 210, such as a sheet of metallic foil or other material, has a layer of particulate graphene 220 deposited on one surface, as shown in step (a). A further substrate layer 230 is placed on top of the graphene layer as shown instep (b) to provide the alternating substrate and graphene layer structure 240 shown in step (c). If a single graphene layer is required then the layering process can stop at this step. Otherwise further alternating layers of graphene and substrate can be applied to form a structure 250 as shown in step (d). The layered structure 240 or 250 is then compressed to cause alignment of the graphene flakes. The applied mechanical deformation ratio depends on the thickness and number layers 210 and 220 in the lamellar structure. For example, a thicker particulate graphene layer 220 requires a higher mechanical deformation to improve the physical contacts between the individual graphene sheets. But the processing efficiency also needs to be considered. A typical mechanical deformation used for the process is ~5% to 25% per iteration of mechanical deformation (pass) with more than 5 iterations (passes). An example of an embodiment uses a 12% deformation ratio for each mechanical deformation iteration. The result of the compression step is improved crystallographic orientation alignment and also a better contact of the graphene flakes within each layer, thus improving the electrical properties. It should be appreciated that the particulate graphene layers will typically comprise multiple layers of graphene sheets or flakes which are compressed, for example using a mechanical deformation process, to force increased density within the graphene particles in the graphene layers. The mechanical deformation process can also improve alignment of the graphene particles within the graphene layer which can enhance the physical contact and improve electrical conductivity between the graphene particles. Increasing contact and alignment between graphene particles enables improved conductivity for the composite conductor structure. Since the size of graphene sheets/flakes is around nanometre to centimetre scale, it is difficult to experimentally demonstrate the conceptual work of this invention in a visible scale. To clearly outline the underlying principles, a demonstration using aluminium foil is provided and illustrated in Figure 3, to demonstrate the conceptual operation of this invention through understanding how the contacts of the wrinkled graphene sheets/flakes affect the electric transport. Figure 3(a) shows the poor contact of two wrinkled aluminium sheets, which results in the measured resistance between these two sheets of foil being 1 .18 Ω. However, if the contact between the two sheets of foil is improved by exerting a compressive force to cause more surface contact between the two sheets, further applying a force to flatten the wrinkled aluminium sheets, i.e. a force parallel to the sheet to smooth the wrinkles, and also a compressive force to push the two sheets of foil together and hold them in place to produce the surface contact on the overlapping area, as shown in Figure 3(b), the resistance from one foil to another is reduced to around 0.0 Ω, within the measurement precision of the measurement instrument. Figure 4 schematically illustrates the principle of the improving contact between wrinkled graphene sheets. Basically, there are three types of contacts between two wrinkle graphene sheets. Figure 4(a) shows multi-point contacts between two wrinkled graphene sheets, which are laid parallel. A single point contact is formed when the two graphene are laid with an angle as shown in Figure 4(b). Applying a compressive force to the graphene sheets will force the sheets together and improve the surface area contact. Applying a compressive force (a c ) and also a force (σ β ) that is perpendicular to the compressive force (parallel with the lamellar structure) on these wrinkled graphene sheets as shown in Figures 4(a) and (b) will cause the wrinkles in the graphene sheet to flatten and can also facilitate sheets moving to align wrinkles to thus improve surface area contact as shown in Figure 4(c). The compressive

(perpendicular) and sheer (parallel) forces can be applied through a mechanical deformation process, for example rolling. The mechanical deformation causes wrinkles in the graphene sheets to flatten and the particles move or flow for better alignment of any residual wrinkles and improved contact, thus converting their point contacts to the surface contact as shown in Figure 4(c). This surface contact allows the free electrons to transfer from one graphene sheet to another easily with extremely low contact resistance. Compression alone may cause some alignment of particles and flattening of the wrinkles and thus enable improvement in electrical characteristics, but improved performance can be achieved by using a combination of perpendicular and parallel forces on the graphene layers. The mechanical deformation to obtain improved alignment and physical contact between graphene particles is achieved in an embodiment through iterative mechanical deformations.

As shown in the Figure 4(c), the contacts between the graphene sheets play an important role in determining the electric conductivity of the materials in bulk scale. A mechanical deformation to produce the strong compressive force (a c ) and the force, (σ β ), which is perpendicular to the compressive force, exerted on the wrinkled graphene sheets/flakes can improve their contacts significantly. By alternating layers of substrate and graphene the substrate can provide a supporting structure to enable handling of the structure and compression of the graphene flakes in the layers using known mechanical deformation equipment and processes.

In general, there are a number of mechanical deformation methods that will generate both perpendicular and parallel force components. For example, roll milling can provide these two force components on the lamellar structure materials as shown in Figure 5. In this mechanical deformation process, the compression force component (a c ) is produced by the gap between the rollers, which is smaller than the thickness of the lamellar structural materials. While elongation or flow of the substrate materials driven by rolling process induces the force component (σ β ), which is parallel to the surface of the lamellar materials but perpendicular to the compression force (a c ). The effect of the parallel force component is to flatten the wrinkled graphene and also improve the alignment of the particles. The parallel force component can also improve alignment and contact of the graphene particles with the substrate material, and hence improve electrical conductivity at the interface between the graphene and substrate, where the substrate is an electrically conductive material, for example a metal substrate. Although roll milling is provided as an example, other processes may be used, for example extrusion and drawing exert perpendicular and parallel force components. An axial press can also produce these forces but the forces induced by the substrate deformation parallel to the surface are very limited.

In an embodiment the lamellar substrate is subject to multiple rolling iterations each reducing the thickness of the lamellar structure by around 12%, resulting in a total mechanical deformation of around 80%. During each rolling iteration there is a combination of compression from perpendicular forces and flow from parallel forces causing particles within the graphene layers to increase in density. By virtue of the flow effect graphene particles can also be encouraged to reorient to align with each other to improve the physical contact between the particles. This may also include reorientation of the graphene particles to improve alignment between the lattice structures of the individual sheets or flakes. The combination of parallel and perpendicular force components can act to encourage the individual sheets and flakes to lie parallel bringing the planar lattice structures into alignment and closer proximity for improved electrical transport. The deformation ratio for each iteration may vary in some embodiments, the range for each mechanical deformation iteration or pass can be around 5% to 25%. The total mechanical deformation of the lamellar structure may be in the range of 10% to 80% or more. The deformation ratio for each iteration and total mechanical deformation may be chosen based on the initial thickness of the substrate layers and overall lamellar structure. The substrate material is chosen to be deformed during the mechanical deformation process, which also causes compression within the graphene layers, and remain in the deformed state to hold the graphene layers compressed between the substrate layers. Depending on the substrate material the substrate may be deformed (thinned due to mechanical deformation) more than the graphene layers (in which the thinning of the layer is primarily due to densification of the particulate graphene material and some spreading rather than mechanical deformation of the actual material) during this process. For example, where the substrate material is aluminium it may be expected that the deformation of the substrate layers will contribute more to the overall deformation ratio of the lamellar structure than the graphene layers. This may be taken into consideration when choosing the mechanical deformation ratio for each of the iterations. It should also be appreciated that the extent to which the substrate and graphene layers are thinned may vary between mechanical deformation iterations. In an embodiment where the graphene layers are deposited or coated on the substrate having relatively low density, one or more early mechanical deformation iterations may primarily cause increase in packing density of the graphene particles within the lamellar structure and the thinning of the substrate due to this packing be more significant than deformation of the substrate. Later iterations may show relative increase in the thinning of the substrate layers through mechanical deformation which can also cause the reorientation and sliding of graphene flakes once graphene layer density is increased. In some embodiments the density of the graphene layers can be increased from 30 to about 80% to ~100% by iterative deformation.

In some embodiments of this invention, the graphene particles (sheets/flakes) are layered on sheets of substrate material. The graphene can be layered on the substrate by powder laying, powder deposition, coating, printing, painting, spraying, tape casting, or other deposition process. It should be appreciated that the graphene layer can be relatively thick, comprising multilayered graphene flakes or sheets to enable these to be compressed and thinned out during the deformation process to improve the physical contact between the graphene particles to provide a substantive, highly conductive graphene layer within the lamellar structure. The substrate can be foils or sheets of a material. The substrate material can be aluminium, copper, silver, gold, platinum, alloys and other materials. It should be appreciated that although this example uses a metallic substrate that is rolled, any suitable material may be used. In the examples discussed herein, an electrically conductive metal is used but other metals or metal alloys may also be used for the substrate material. However, the substrate material does not need to be metallic. Further, as each graphene layer is electrically conductive, the substrate material does not need to be electrically conductive. The choice if substrate may be based on requirements of the fabrication process, for example the selection of the substrate material may be constrained by mechanical properties required - for example choosing a material suitable for a rolling, drawing or extrusion process. Alternatively or additionally the choice of substrate may be made in consideration of the final product or operating environment for the final conductor product - rather than electrical properties of the material. For example, for conductors applicable for use in marine environments polymer substrate may be utilised to minimise or eliminate corrosion risks. Biocompatible polymer substrates may be used for conductors formed for used in implantable medical devices. The choice of substrate materials may also take into consideration requirements for products produced incorporating the lamellar graphene composite structure, the requirement may be based on further processing techniques and well as end product requirements. The industrial scale production of lamellar graphene composite structures can be realized by using known technologies such as roll-to-roll printing/coating, or tape casting and other technologies. For example, roll-to-roll printing/coating technologies can be implemented using a slot die system, knife system, engraved roller system, commabar system, micro roller system, screen printing system, nanoimprint system and inkjet system etc. various technologies. Figure 6 schematically illustrates an example of roll-to-roll coating using slot die/knife technology. The graphene layered sheets can be treated with UV radiation to remove the solvents and other organic substances, for example where such solvents etc. were required to enable the coating, printing or deposition process. The graphene layers sheets can also be subsequently heat treated at the temperature with the range from 50°C to a suitable temperature, which depends on the materials of the substrates, to remove the unwanted substances including moisture, gases or air, which cannot be removed with UV radiation. For example, for aluminium, the temperature can be from 50°C to 550°C while it can be from 50°C to 850°C for silver.

A plain foil sheet, for example, an aluminium sheet, can be used as a cover on the top of the one layer graphene substrate structure or the multi-layer structure of alternating graphene and substrate foil/sheet layers to form the laminated composite with lamellar structure as shown in Figure 2. The top cover sheet is optional and the lamellar structure may be formed with a graphene layer as an outer layer. This embodiment may not be practical for some applications.

In the example discussed above a process of iteratively building up layers is described. However, alternative processes may also be used. For example, graphene may be deposited on a plurality of substrate layers, which are subsequently stacked one on top of the other to form the alternating layers, and may be finished with a plain substrate layer.

The prepared layered structure is then subject to a process whereby mechanical force is applied to cause compression and alignment of particles within the graphene layers. In an embodiment this is performed by substantive mechanical deformation of the layered structure. The force applied is a compressive force and in some embodiments the mechanical force may include components both perpendicular (compressive) and parallel (shear) to the layers. The lamellar structure serves to maintain the graphene layers compressed within the lamellar structure as the mechanical deformation acts on the substrate structure to cause permanent deformation, and the deformation of the substrate structure holds the graphene compressed within the lamellar structure post the deformation processing. Thus, each mechanical deformation pass can cause thinning of the graphene layers. Each iteration will increase the total mechanical deformation imposed on the lamellar structure.

In an embodiment, the lamellar composite is rolled by a precise roller mill with a series of the optimized deformation ratios as illustrated in Figure 7. The prepared layers structure 710 is input to a roller mill 720. The rollers 720 mechanically deform the input sheet 730 of the layered structure as it is rolled through between the rollers in accordance with a set deformation ratio, such that the output sheet 740 will be thinned relative to the input sheet. It should be appreciated that such a rolling process applies both compresses and elongates the layered structure, using a combination of perpendicular and parallel force components. This will cause the graphene particles to flatten and flow to align within the graphene layers. The rolling process may use several iterations of rolling, using controlled deformation ratios to achieve a target thickness of the final mechanically deformed lamellar structure 750. For example, the deformation ratios can vary from 5% to 25% or more in an embodiment. The deformation ratios depend on the substrate materials and initial thickness of the substrate and graphene layers. The deformation ratios for each iteration of mechanical deformation can be controlled to avoid the formation of "sausage" or "wavy" structures induced by the different flow rates between the graphene and the alternating materials as well as the substrate materials in the rolling process.

During the rolling process, interval annealing can be performed to release the stress or work hardening caused by the mechanical deformation, if necessary. In general, the annealing temperature is selected as 0.35 to 0.4 of the melting temperature of the substrate materials, for example, ~250°C for aluminium. The annealing step may be performed after each iterative mechanical deformation or at alternative intervals between mechanical deformation iterations, for example after a set of iterations. The pattern of mechanical deformation iterations and annealing can vary between embodiments based on the substrate materials, compression process and extent of compression/mechanical deformation (cumulative or iterative deformation ratio). The extent of the mechanical deformation depends on the initial thickness of the materials, which are determined by the number of the alternating layers of the lamellar composite, and also the final thickness of the product after the rolling process. Large mechanical deformation can result in a better crystallographic orientation alignment and also a better contact of the graphene sheets or flakes.

In some embodiments of the invention the mechanical deformation step may be performed in conjunction with further forming of the lamellar structure into an electrical conductor structure. For example, where further processing involves a step whereby sufficient deformation is applied to the lamellar structure to cause alignment of the graphene flakes within the lamellar structure. Examples of processes whereby sufficient compression may be applied include applying an extrusion coating, stamping, rolling or wire drawing.

The laminated composite with the lamellar structure of multi-layer compressed graphene and substrate layers can be further processed for use as electrical conductor applications. For example, a sheet of the lamellar structure can be cut into strips of different widths, based on the intended applications, and then be used as a high performance electrical conductor, either directly as shown in Figure 8(a) or integrated into an electrical conductor structure. For example, the lamellar composite may be inserted into or adhered to a supporting structure to provide an electrical conductor compatible with an existing device. Alternatively further forming may be applied to the lamellar structure to form an electrical conductor structure, for example conductive wires or tapes. For example, a belt of the lamellar composite can be rolled-up into a cylinder shape to act as the wires or rods for the applications of electric conductors as shown in Figure 8(b). The cut lamellar strips or cylinder shaped lamellar composites can also be filled into a metal tube, for example an aluminium tube, as shown in Figure 8. The tubed lamellar strip, Figure 9(a), or cylinder shaped composites, Figure 9(b), can be further mechanically deformed, for example, using wire drawing to form wires. Wires can optionally also be rolled to form conductive tapes. It should be appreciated that such wires and tapes may have many different applications and can vary in size and structure appropriate to the target application.

One application of the lamellar structures of embodiments of the present invention is for the formation of high performance electrical transmission lines.

In an embodiment of this invention, the wired formed incorporating the lamellar graphene composite structure, in accordance with the embodiments discussed above, can be substituted for currently used metal conductive wires, such as copper, and aluminium, to produce high performance electric power grid transmission lines. The lamellar graphene composite wires may be used to replace some or all of the electrical conductors in a transmission line bundle. Such transmission line may be produced using known techniques. For example, known transmission line fabrication techniques include aluminium conductor steel reinforced (ACSR) and all-aluminium alloy conductor (AAAC) techniques bundling strengthening wires around conductive core wires, and composite core conductors such as aluminium conductor composite reinforced (ACCR) and aluminium conductor composite core (ACCC) where the strengthening structure is a conductor core, for example carbon and glass fibre core, and the wires for carrying the electrical power are bundled around the supportive core. In embodiments of the invention the electrically conductive wires (traditionally aluminium or copper) are replaced with the lamellar graphene composite wires fabricated as discussed above. It is envisaged that the lamellar graphene composite wires of the present invention can be incorporated into any known transmission line structure.

Although the above described embodiment utilises lamellar graphene composite wires for all of the conductive wires in the transmission line bundle it is envisaged that hybrid structures may also be used, where a plurality of different types of wires are incorporated in the transmission line bundle. Advantages of electric conductors made with the lamellar graphene composite structure, can include reducing the resistance, limiting the temperature effects and suppressing the skin effects, which may increase the electric energy transmission efficiency of the transmission lines by more than 5%. This translates to significant cost and energy saving on a national scale. For example, the electrical energy

consumption in China was 5500 TWh in 2015. 5% saving of this energy consumption is 275 TWh, which was slightly more than the total electrical energy consumption of Australia (248 TWh) in 2015. Therefore, the impact of this invention is significant. Alternatively embodiments the lamellar graphene composite structure may be applied to existing transmission line structures to improve electrical performance. In one example the as-produced lamellar composite structure can be used to enhance the electric conductivity and current density as well as thermal conductivity of overhead and underground transmission lines. In an embodiment for application to naked or overhead transmission lines, the as-produced belt-shaped composite of the mono- layered graphene/metal (aluminium, copper, silver, alloys and other conductive materials) or the lamellar materials with the alternative-layered graphene/metal (aluminium, copper, silver, alloys and other conductive materials) is used to spirally wrap on the surface of transmission lines as shown in Figure 10. This can be utilised to cost-effectively enhance the existing or newly made overhead or the core of the underground transmission lines. Such a technology can also overcome the adhesive problem of the graphene directly coating on the surface of the overhead transmission lines. In an embodiment for application to underground transmission lines, the composite made with (1) the mono-layered graphene coated on the substrate, which can be conductive or non-conductive materials, or (2) the multiple-alternative layered graphene/substrate, which the lamellar substrate is the conductive materials, can also be spirally wrapped on the surface of the metal core of the transmission lines. In the embodiment the lamellar composite can be fabricated with/without mechanical deformation by roll milling. The textural structure of graphene in the composite fabricated without the rolling process can be formed through the large transverse stress generated by the extrusion process of the insulating polymer materials surround the metal core with the lamellar graphene/metal wrapping as shown in Figure 1 1 . Such a processing can produce the high performance of underground electric transmission lines.

Utilising embodiments of this invention can increase the electrical conductivity of electric power grid transmission lines by using the highly conductive graphene to form the high performance electrical conductors with the graphene/metal composites that have lamellar structures. Such electrical conductors having a belt or tape shape can be used to spirally wrap on the surface of the existing or newly made transmission lines to enhance their electrical and thermal conductivity. The lamellar graphene composite structure has high electrical conductivity and can be incorporated into a variety of electrical conductor structures, such as transmission lines, electrical wires, conductive plates, and other structures depending on the application. The properties of the graphene and metal lamellar composite structure include low overall resistance and higher conductance pathways through the graphene layers. Overall energy loss caused by the electric resistance of the conductors can be reduced. Further by the higher conductance pathways though the graphene layer can also suppress skin effects. The structure can also be less sensitive to temperature effects than conventional conductors. When considering application of such conductors in electrical transmission lines, a few percent of increase in electric energy

transmission processing would translate to significant cost and energy saving on a national scale.

The electrical current is transported in an atomic plane of graphene. However, fabricating and shaping a large scale graphene product is extremely difficult. In general, the as-synthesized graphene sheets are not flat with relatively large wrinkles. This results in a very poor contact between the graphene sheets. By using the technique discussed above, we can produce the highly dense, well aligned and fully contacted graphene structure, thus significantly improving the electrical pathway. The manufacturing technique can also be applied on a large scale.

It should be appreciated that the proposed embodiments are suitable for large scale manufacture of electrical conductors using the proposed laminate graphene composite manufacturing technique. In accordance with this industrial application of the methodology it should be appreciated that particulate graphene material produced in large quantities may vary in purity and structure. Although in theory using pristine graphene, having no defects or impurities, as the input material for manufacturing the lamellar graphene conductors should produce very high quality conductors, this may not be currently practical for some applications. For example, for applications such as wires for transmission lines requiring high volume cost effective production, products produced using pristine graphene may not be an economically viable substitute for current transmission lines, based on the cost of the input materials. Pristine graphene has been produced in highly controlled processes, but is not currently produced in large quantities. The current cost of pristine graphene is also prohibitive for use in large scale cost effective production processes. Although pristine graphene may be produced in a manner currently economically viable for used for some products, for example conductors for medical devices, complex electronic products, sensors, etc. it should be appreciated that for large scale industrial conductor production, for example for transmission lines, it would be currently more cost effective if graphene materials of lower quality than pristine graphene are used. All known current industrial processes for large quantity production of graphene yield graphene having impurities and defects. Particle size and shape may vary for the individual sheets and flakes making up the particulate graphene material. Further, defects can also be present in the lattice structure. The nature and quantity of defects can be a consequence of the method used to produce the particulate graphene material. In embodiments of the present invention the process of compressing the graphene within the laminate structure causes close packing and alignment of the sheets and flakes of the graphene thereby improving charge transport characteristics. Improvement in charge transport characteristics can be achieved using this method even for particulate graphene material having defects and impurities, thus indicating suitability for commercial industrial production of laminate graphene conductors.

For example, in one known method for producing graphene is to chemically produce graphite oxide (and a number of different methods can be used), which is then dispersed in a basic solution to yield monomolecular sheets known as graphene oxide. The graphene oxide is then reduced to provide a graphene product. However, the reduce graphene oxide (rGO) has many chemical and structural defects. The quantity and type of defects vary greatly depending on the production methods used. Types of defects can include binding of oxygen within or to the graphene lattice structure (i.e. C- O, C-O-C, C=0 and others). The variation in quantity and type of defects can affect the properties of the graphene material, for example it is known that the impurities in reduced graphene oxide cause the conductivity and charge motility of reduced graphene oxide to be significantly worse than that of pristine graphene. However, testing by the inventors has shown feasibility of embodiments prepared using reduced graphene oxide. For example, reduced graphene oxide should have more than 35% of C=C bonds. Testing by the inventors has indicated potential feasibility for graphene composite conductors using reduced graphene oxide, despite such defects and impurities.

Some embodiments of the lamellar graphene composite conductor fabrication method can also include graphene pre-processing steps to improve the characteristics of the input particulate graphene material to the laminating and compressing manufacturing process. In an embodiment heat treatment or annealing of the particulate graphene material may be performed before the laminating and compressing steps. For example annealing may reduce the surface area of the particulate graphene (sheets and flakes) and improve the quality of the materials. Annealing the reduce graphene oxide in a reduction atmosphere, such as hydrogen or nitrogen environments can remove the oxygen and other absorbed substances from the surface of the graphene, thus increasing the specific surface area of the materials. This processing can free electrons from the bonding of C-O, C-O-O, C=0, 0-C=0 etc. In turn this can increase the electron density. Therefore, annealing reduced graphene oxides may enhance the surface area of the particulate graphene (sheets and flakes) and improve the electrical conductivity.

Testing by the inventors has indicated that annealing can improve the specific surface area of rGO without any negative impact on the number of defects in the lattice structure. The improved surface area may be related to reduction in oxygen content and other impurities. The improved surface area may translate to improved conductance in the final graphene core wires produced. Initial testing by the inventors performed surface analysis using Brunauer-Emmett-Teller (BET) surface analysis for measuring the specific surface area of for reduced graphene oxide (rGO) comparing as produced rGO and the as produced rGO after an annealing process. In this example rGO was subject to annealing at 1000°C in an N 2 atmosphere for 1 hour. Table 2 shows the effect of heat treatment on the surface area of the reduce graphene oxide (rGO). The BET surface area of the as-received rGO is 404.36 m2/gram and this can be improved to 552.1 m2/gram by annealing the materials at 1000°C in N2 atmosphere for 1 hour. It is believed that the improvement is related to the reduction of oxygen content and removal of other impurities.

Table 2. BET measurement of reduced graphene

Therefore, before application in the laminate manufacturing process, rGO graphene may benefit from being thermally treated to reduce the oxygen content to the lowest level for enhancing the electron density of the materials.

Raman spectroscopy is an important part of graphene study. It is used to study the number of layers, quality of graphene, defects, doping and etc. in carbon-based materials. Figure 24 shows the Raman spectrum of the as-received 2410 and the as- treated 2420 rGO. Details of the positions and intensities in Figure 20 are tabulated in Table 3. The presence of the D- and G-bands is quite distinguishable in this figure. The D-band is known as the disorder band or the defect band it represents a ring breathing mode from sp 2 carbon rings whilst the G-band represents the in-plane vibrational mode involving the sp 2 hybridised carbon atoms that comprise the graphene sheet. The increase of l D /l G ratio suggests a decrease in the average size of sp 2 band or increase of defects.

Table 3. Raman spectroscopy of the reduced graphene sample at different

It is interesting to note that the sample annealed at 1000°C for 1 hour in N 2 atmosphere has almost the same l D /l G ratio (see Table 3). This indicates that annealing at this temperature was effective to increase the surface area but did not result in creating more defects in the reduced graphene oxide lattice structure.

In general, the D-band would not appear in pristine graphene. Therefore, the appearance of D-band in both treated and non-treated rGO indicates the existence of chemical bonds and edges in the rGO sheets. The X-ray photoelectron spectroscopy (XPS) spectra of the as-received 2410 and the as-annealed 2420 rGO are shown in Figure 25, C=C, C-C, C-O, C-O-C, C=0, and O- C=0 characteristic peaks were observed at 284.5, 285, 286.4, 287.8, 289.2 and 290.85 eV, respectively. The percentage of C=C plus C-C bonds was increased from 63.86% to 78.67% by the annealing the materials at 1000°C for 1 hr in N 2 atmosphere. In other words, the intensity of oxygen-related functionalities (organic C-O, C-O-C,

C=0 and etc.) decreased substantially after reduction annealing. Table 4 tabulates the summary of the XPS results, including binding energy peak, and the atomic fraction of molecular bonding.

Table 4. XPS results of the as-received and thermally treated specimens.

As-received Annealed at 1000°C for 1 hr

Name Peak BE Atomic Bonding Name Peak BE Atomic Bonding

% %

C1 s A 284.5 37.1 1 C=C C1 s A 284.5 52.34 C=C

C1 s B 285 26.76 C-C C1 s B 285 26.33 C-C

C1 s C 286.4 9.85 C-O C1 s C 286.4 8.13 C-O

C1 s D 287.8 4.91 C-O-C C1 s D 287.8 2.76 C-O-C

C1 s E 289.2 3.13 c=o C1 s E 289.2 1 .18 c=o

C1 s F 290.85 5.02 o-c=o C1 s F 290.82 5.63 o-c=o

N1 sA 399.7 0.52 C-NH 2 Sb3d5A 531 .86 0.06

Organic

N1 sB 401 .89 0.26 Nitrate 01 sA 533.43 2.33 c=o

Organic Organic

01 sA 533.32 6.83 c=o 01 sB 530.58 1 .07

C-O

Organic Thiol, R-

01 sB 531 .47 5.36 S2p3A 163.81 0.17

C-O SH

Metal

S2p3A 168.41 0.25 — — — —

sulfate The above discussion indicates that reduction annealing can be a useful preprocessing step to improve the properties of at least some types of particulate graphene materials prior to the laminate manufacturing process, potentially improving quality of the output graphene conductors. Annealing or other pre-processing steps may also be used. The pre-processing processes applied to the particulate graphene material may vary between embodiments. The pre-processing processes to apply may be chosen based on the types of defects and impurities present in the input, raw, particulate graphene material. It should be appreciated that the input graphene used to implement embodiments of the present invention can be produced by a variety of different production

methodologies. An advantage of the present invention is that any such industrially produced graphene product may be utilised in embodiments of the present invention. The actual size of the graphene flakes or sheets can vary widely depending on production methodology, any of which are suitable for use in embodiments of the present invention. For example, one production methodology uses a high power ultrasonic probe to exfoliate multilayered graphene particles to produce mono-atomic graphene sheets. Other methodologies may also break a mono-atomic sheet into flakes. Some graphenes are fabricated by depositing the carbon precursors on metallic catalysts, resulting in mono-atomic graphene sheets in the centimetre range. Although such technology is not suitable for industrial production or used for commercial graphene production at the time of filing, such graphene sheets may be used in embodiments of the present invention. In some embodiments multi-layered graphene particles may also be used and shearing force induced by strong mechanical deformation can also exfoliate the multilayered graphene into mono-atomic layers within the graphene layers, which are also aligned and compressed by the mechanical deformation.

As discussed above, by compressing graphene flakes/sheets the charge transport behaviour of the graphene material can be improved. Embodiments of the present invention utilise this property in combination with fabrication techniques utilising mechanical deformation to provide advantageous electrical conductors. To provide a better understanding of embodiments of the invention the following discussion describes charge transport behaviours between graphene layers. The inventors modelled the conductance in the graphene layers using open-sourced Kwant code. The calculations were performed applying an assumption that graphene behaves as a ballistic conductor. Further investigation on the effect of the structural geometry on the inter-layer charge hopping was performed using density functional theory. The modelled results indicate that interlayer charge hopping can significantly affect the overall conductance between the graphene sheets. The modelling and calculated results are discussed in further detail in the following paragraphs.

The conductance modelling method applied is based on a theory that since graphene has very high carrier mobility, it is possible to describe the electrical transport in graphene as ballistic conduction. To explain in more detail the ballistic condition assumes that the electrons can travel almost without resistance and each electron does not interact with other electrons or defects within the graphene sheet.

Furthermore, since the structure of graphene is a single atomic layer, the charge transport behaviour has to be described by the quantum theory. In a normal conductor, the conductance can be described by Ohm's law: G = σΑ/L, where A is the area of the cross section of the conductor, L is the length of the conductor, and σ is its conductivity. However, as the cross-sectional area reaches atomic scale, as in the case of graphene, Ohm's law ceases to apply and the conductance G has to be reformulated based on quantum-mechanical theory. A standard technique to calculate the conductance for nano-scaled device or material is the Landauer-Btittiker formalism

G [Equation 1]

Equation 1 describes the conductance as the sum of amplitude of transmission probability \S nm \ where S nm is the scattering matrix. The scattering matrix S nm describes behaviour of electrons when it is injected from a left-lead (n) to a right lead (m). Since the material is ballistic, it is not expected that there will be scattering inside the material, thus the scattering process occurs at the leads. In quantum theory, electrons can be described as travelling waves, the scattering process will result in incoming and out-going waves at the left-lead (n) and incoming and out-going waves on the right lead (m). The S nm matrix relates to the incoming waves in left and right leads to the out-going waves on the left and right leads.

The KWANT software package for modelling quantum transport was used to calculate the conductance G for the single layer and bi-layer graphene, as well as side contact graphene. It should be appreciated that although the KWANT software package was chosen by the inventor for modelling other quantum transport simulation software may also have been used.

The following section describes the modelling as performed using KWANT. An example of the structure of monolayer graphene as used in the modelling is shown in Figure 12a. A unit-cell refers to the minimalist periodic repeated geometrical configuration and the number of atoms which are used to describe the crystal structure of a material. The unit-cell of graphene contains two Carbon atoms located at two different lattice positions. In graphene, there are two in-equivalent sub-lattices a and b. Figure 12b and 12c show the unit cell of single layer graphene. To perform the conductance calculation, the unit cells of a single layer graphene and of a bilayer graphene were expanded in the horizontal plane to simulate a graphene sheet, for the conductance calculation. The positions of sub-lattices a 1210 and b 1220 are shown in Figures 12b and 12c. Figure 12a shows schematically how an electron can move in a single layer graphene sheet. The nearest neighbour hopping energy of β 0 = 3.16 eV was input in the Kwant program. Hopping energy refers to the energy of an electron when it moved from one carbon (C) atom to another C atom. The nearest neighbour hopping energy refers to the energy of an electron when it moves from sub-lattice a 1210 to the closest sub-lattice b 1220 as shown in Figure 12a. In the case of graphene, there are three closest sub-lattice b 1220, 1230, 1240 surrounding one sub-lattice a 1210. Conductance in a graphene sheet happens when an electron is injected from the left lead 1250 and hops from a sub-lattice a to a sub-lattice b, or sub-lattice b to sub- lattice a until it reaches the right lead 1260. The Kwant code calculates the overall transmission amplitude at the left and right lead to determine the overall conductance G as shown in Equation 1 Figures 13a and 13b represent crystal structure of bi-layer graphene, with Figure 13a representing a Bernal stacking geometry, and Figures 13c and 13d illustrate the unit cell structure of bilayer graphene. For the bilayer graphene structure, the unit-cell consists of 4 inequivalent lattice sites 1310, 1320, a 2 1330 and b 2 1340 within a Bernal stacking geometry as illustrated in Figure 13e. Bernal stacking refers to the geometry in which the b 2 1340 sub-lattice of one of the layer is located directly underneath the 1310 sub-lattice of the other layer, while the a 2 1330 sub-lattice is located directly beneath the centre point of the layer composed of the 1310 and 1320 sub-lattices. Figure 13e shows schematically how an electron can move in a bilayer graphene sheet. The nearest neighbour hopping energy 1350 of β 0 = 3.16 was input in the Kwant program eV, also input was the interlayer hopping energy 1360 of = 0.4 eV as shown in Figure 13e. In this context the nearest neighbour hopping energy is a measure of the kinetic energy required for electrons to travel within the same layer, and interlayer hopping energy is a measure of the kinetic energy required for an electron to move from one layer to the next. Here the interlayer hopping between the layers is only considered for a^→ a 2 and b^→b 2 , the hopping between a^→ b 2 between the layers is ignored, these interlayer hops are illustrated in Figure 13e.

Figures 14a and Figure 14b illustrate the modelled graphene sheets used for conductance calculation in the single layer, bilayer and side-contact graphene. The sheets were modelled by expanding the unit cells of the monolayer (Figures 12b, and 12c), and of the bilayer (13c and 13d) to form the monolayer sheet (Figure. 14a) and bilayer sheet (Figure. 14b). Conductance calculations were performed for sheets modelled having the unit cell expanded ten times in two perpendicular (x and y) horizontal directions (10x10). To calculate the conductance using the Kwant software program, the positions of the leads and the hopping positions need to be defined. The leads were attached to the zigzag edges (1430, 1435, and 1440, 1445 respectively) for the monolayer 1410 in Figure 14a and bilayer 1420 in Figure 14b. In single layer graphene 1410, the leads are attached to the left 1430 and right 1435 side of the 10x10 unit cells of graphene sheet at the zigzag position edges. In the bilayer graphene 1420, the leads 1440, 1445 are attached to both of the top and bottom layers on the left 1445 and right 1440 side with similar electron hopping as in the case of the bilayer region 1020, i.e. a^→ b a 2 →b 2 , a^→ a 2 and b^→b 2 for both of the left and right lead.

Figure 16 illustrates a model for side contact graphene for modelling the charge transport between adjacent overlapping graphene sheets or flakes 1610, 1615 where there is some side-contact or close proximity between the graphene sheets or flakes. The difference between side contact and bilayer graphene is for the bilayer graphene, electrons can move from the left side to right side within the same layer. For side- contact graphene layers, electrons cannot move from the left side to the right side in the same layer. Electrons now move from the left side of the bottom layer, then they jump to the top layer to reach the right side of top layer. For side contact graphene hopping in the leads 1625, 1620 is restricted to, a^→ b^ for the left lead 1625 and a 2 → b 2 for the right lead 1620. In this model, electrons are injected into one layer 1615, and then hop from one layer to the next layer 1610 to reach the lead 1620 on the other side. The hopping in the bi-layer region 1640 is still → b^ a 2 → b 2 , → a 2, and → b 2 . The hopping energies used in the leads 1620, 1625 are similar to those used in graphene mono-layer and bi-layer graphene.

Simulated conductance results in single-layer graphene, bi-layer and overlapping layer (side contact) graphene

Figures 15a and 15b show the respective calculated conductance of monolayer 1410 (Figure 12a-d & Figure 14a) and bilayer 1420 graphene (Figure 13a-e & Figure 14b) as a function of energy, with t=0 indicating the Fermi level and increasing t shows higher band occupancy and therefore higher conductance. As shown in Figures 15a and 15b, the monolayer 1410 and bilayer 1420 graphene shows constant conductance 1510,

1530 at the Fermi level (t = 0 eV). As the energy increases, step increases in conductance 1520, 1540 occur as more bands are filled. The conductance steps in bilayer graphene are twice as large as in monolayer graphene due to the coupling of the two layers. However, when modelling conductance between two overlapping graphene sheets the conductance is dependent on how the electrons move between the layers, i.e. the value of Yi the interlayer hopping energy between the sheets. This model is illustrated in Figure 16, as two overlapped graphene sheets 1610 and 1615, with a lead attached to one sheet 1625 and another electrode 1620 attached to the other sheet at the opposite end 1620, so a current transmission path 1630, 1640, 1650 from one electrode 1625 to the other 1620, hopping 1640 between the two sheets 1615, 1610. The calculations were performed using a model having two 10 x 10 graphene sheets 100% overlapped using Bernal stacking configuration. It should be appreciated that the relative orientation between two independent graphene sheets or flakes can vary. Many different orientations may exist within a sample of graphene material. In practice, a Bernal stacking configuration, as used in the above modelling, may occur but is unlikely to be the only inter-particle orientation found in a real life sample of graphene particles. In practice partial overlapping of sheets is also anticipated. Further, graphene particles (sheets and flakes) will typically vary in size in practice. Thus, effects of particle distance and relative orientation on charge transport between particles need to be considered. The inventors have modelled side by side contact transport between two graphene sheets, this modelling indicated that the interlayer electron hopping energy plays an important role in determining the overall charge transport behaviour. To determine the role of the electron hopping energy y^ on the overall conductance from one graphene sheet to the other graphene sheet, the value of y^ was varied from 0 to 0.4 eV. A value of 0 eV means that there is no conduction from the bottom layer to the top layer, while a value of 0.4 eV represents the maximum conduction from the bottom layer to the top layer. The higher the value of y^ means that electron can easily hop between from one layer to the next so it has high kinetic energy In the case of bi-layer graphene with Bernal stacking, the maximum value of the kinetic energy y^ is 0.4 eV. Generally, y^ decreases exponentially with respect to the increase of interlayer distances. This is because y^ depends strongly upon overlap between the frontier orbitals of two interacting graphene sheets, and these frontier orbitals decay exponentially moving perpendicularly away from the graphene layer. In the modelling method for the side by side contact graphene sheets, the leads are attached only on the left of the bottom sheet and on the right of top graphene sheet, if there is no charge conduction between the top and bottom sheet (y^ = 0), the overall conductance would be zero (as shown in Figure 17a). Figures 17a to e, graph the results of modelling conductance for side contact graphene layers for different values of χ 1 (0, 0.1 , 0.2, 0.3 and 0.4 eV, respectively), this model shows a high dependence between the interlayer hopping energy and overall conductance from one graphene sheet to the other. As there is more conduction between the sheets (higher γ^, the conduction between the two layers increases as y^ reaches the maximum value of 0.4 eV, as illustrated in Figures 17a-e. The conductance increases from 0 to 1 as y^ increases from 0 to 0.4 eV. In a perfect bilayer graphene, the kinetic energy value of electrons y^ is 0.4 eV, which represents a perfect environment which would allow electrons to move easily from one layer to the next. The physical properties which influence the interlayer hopping energy y^ include the physical distance between the two layers and relative planar orientation of the crystalline structures. Further, when there is a defect, it would be more difficult for electrons to move from one layer to the next, which would decrease the value of γι.

Thus, using a method which optimizes the geometrical alignment between the layers can make it easier for electrons to travel from one sheet to the next and thereby enhance current transmission from one side to the other side. The inventor's study is described in further detail below. The effect of structural geometry on the interlayer charge hopping

As shown in Figures 17a-e, for the case of the side-contact transport between the two graphene sheets, the interlayer hopping energy plays an important role in determining the overall charge transport behaviour. Electron transfer integrals (T.I.) show whether there is charge transfer between layers and is an equivalent to hopping energy. The electron transfer integrals (T. I.s ) between the layers were investigated using a DFT (discrete Fourier Transform) method. Two specific parameters were considered in the study as illustrated in Figures 18a and 18b: the interlayer distance d 1810 and the relative rotational angle (twisting angle) a 1820 between the layers 1830, 1840 as shown in Figures 18a and b. Figure 18a and 18b are schematic illustrations of the design parameters investigated, in terms of their influences on the interlayer electron transfer integrals in bilayer discotic systems. One of the molecules, chosen to be investigated in the present study is shown in Figure 18b, which was selected as a structural model to represent graphene bi-layer. The design parameters investigated here include the interlayer stacking distance d between two molecules, and the rotational angle a of one molecule with respect to the other.

Two different geometrical configurations for the bi-layer graphene were considered for a finite structure of graphene flakes, which contain either zigzag edges as illustrated in Figure 19a, or both armchair and zigzag edges as illustrated in Figure 19b. Figures 19a and 19b illustrate molecular structures used to represent model systems of graphene for investigating the geometric dependence of interlayer electronic couplings. Some initial calculations done with benzene and triphylene showed highly irregular angular (a) dependence on the coupling term, most likely due to the fact that the Generalized Gradient approximation (GGA) method failed to create the correct spin density distributions at the frontier orbitals for highly symmetrical molecules (i.e. unable to remove orbital degeneracies in the problem). Therefore, the above two polyaromatic molecules with lower point group symmetries were chosen to represent graphene flakes. Both molecules can be regarded as (3x3) in size with 3 benzene rings along each edge. For the molecule of Figure 19a, all four edges are of zigzag type, whereas for the molecule of figure 19b, two edges are of zigzag type and the others are of armchair type. All edge carbons are capped with hydrogen atoms.

These different terminations of the graphene flakes can have strong influence on the interlayer charge hopping energy when the two graphene layers are twisted at different angle. For zigzag only graphene flakes, the charge transfer integral exhibits an oscillation between 0 and 180 degrees, this is demonstrated in Figures 20 and 21. Figure 20 shows scanning of dimer transfer integrals for the system of Figure 19a at 3.6 A intermolecular separation with a 180 degree in-plane rotation of one monomer scan of the change in T.I. A perfect symmetrical pattern can be observed for this system due to the existence of inversion centre in the monomer. The scan was performed at 2 degree angle interval. Figure 21 shows scanning of dimer transfer integrals for the system of Figure 19b at 3.6 A intermolecular separation with a 180 degree in-plane rotation of one monomer scan of the change in T.I. For this molecule that is lack of a rotational centre of symmetry, there is also a clear lack of symmetry in the pattern of T.I. scan with respect to the in-plane rotation angle.

Figures 20 and 21 show that the two layers exhibit strongest charge coupling when they are completely aligned (0 degrees). As the two layers start to rotate relative to each other, the interlayer couplings exhibit complex oscillatory patterns. This phenomenon originates from the complex shape of the frontier molecular orbitals formed in the graphene models constructed. Alignment between layers can strongly affect overlapping of the wave function which can weaken the interlayer hopping energy, resulting in much weaker side-contact transport. The relative orientations when the interlayer couplings decreased to zero correspond to the cases where the wave functions of the two layers become completely out-of-phase, and thus should be avoided in device design. In both cases (Figures 20 and 21), the maximum couplings are observed when the two layers are completely aligned (0 degrees) and these are the optimum configurations that one should aim for optimum device performance. In Figure 20, another maximum at 180 can be observed, which originated from the existence of 2-fold symmetry in the model chosen, such that the structure at 180 degree rotation is completely identical to the case of 0 degree. As such, this result demonstrates the importance of how the graphene layers are aligned with each other.

In addition to the rotational angles between the graphene layers, the role of the interlayer distance was also investigated for different rotational angles, which is demonstrated in Figures 22 and 23. Figure 22 shows scanning of dimer transfer integrals for the system of Figure 19a as a function of interlayer distances d at a selected different in-plane rotation angles (0° 2210, 30° 2220, and 90° 2230). Figure 23 shows scanning of dimer transfer integrals for the system of Figure 19b as a function of interlayer distances d at a few different in-plane rotation angles (0° 2310, 30° 2320, 60° 2330, 90° 2340, 120° 2350, 150° 2360, and 180° 2370). From Figure 22 a trend can be observed for the zigzag termination in which it shows how maximum charge transfer integral corresponds with the shortest interlayer distance. This indicates that as the layers come closer together, the possibility for conductance between the layers increases, i.e. electrons can move more easily from one layer to next. While as shown in Figure 23 for the mix armchair and zigzag terminations, the interlayer charge hopping increases at around d = 1 .5 A and then decreases monotonically as the interlayer distance increases. Consequently, the overall results suggest that an optimal charge transport between the graphene layers is strongly dependent on the role of the interlayer distance and the rotational angle alignment between the layers so the interlayer hopping can reach its maximum energy similar to the transport in a perfect mono-layer graphene sheet. Perfect rotational alignment may be difficult to achieve. However, Figures 22 and 23 demonstrate that even without rotational angle alignment between the layers reduction of the interlayer distance can significantly improve the charge transport between graphene layers.

The proposed manufacturing technique compresses particulate graphene between substrate layers using mechanical deformation that provides force components both perpendicular and parallel to the graphene layer. The action of the parallel force components on the graphene particles creates a flow effect encouraging alignment of the individual graphene sheets or flakes with the direction of their basal planes in the direction of this force component. The individual graphene sheets or flakes have a two dimensional structure, but initially these particles can be in random orientations relative to one another, simple compression using a perpendicular force will not necessarily alter this orientation for all particles. However, using a process which also includes a parallel component to cause flow effects can act to cause rotation and sliding of particles so that the basal (hexagonal lattice) plane of each particle is more aligned with the parallel force. This results in the individual sheets or flakes aligned parallel with the layers of the lamellar structure. These particles are also forced closer together by the perpendicular (compressive) force component. It should be appreciated that the combination of basal alignment and compression reduces the distance between the individual sheets and flakes. As is demonstrated above reduction in interlayer distance enables improved charge transport between graphene particles irrespective of rotational alignment by the improvement of their physical contacts. Thus, it is advantageous for a manufacturing technique to encourage improved basal alignment between graphene particles, in particular in the layering of graphene sheets and flakes, for, reduction in distance between graphene particles to improve charge transport characteristics.

As discussed above the proposed laminate manufacturing technique encourages alignment and reduction of interlayer distance between graphene particles through the process of mechanical deformation. The rolling process combines a sideways (perpendicular) force with an elongating (parallel or aligned) force to elongate the lamellar structure, this combination of forces applied to the graphene particles encourages tight packing of the graphene particles. Flow effects of the rolling and/or drawing processes encourage reorientation and alignment of graphene sheets, and the compression also forces reduction of interlayer distances. Thus, using a lamellar rolling technique may improve charge transfer properties of graphene materials, through improving alignment and density of the graphene sheets and flakes.

Embodiments of the invention provide graphene composite conductors utilising lamellar manufacturing techniques.

It should be appreciated that the described techniques for producing composite graphene conductors can be utilized for large scale production of graphene composite conductors. For example, the lamellar structure can be used to provide large conductive sheets, or kilometers of conductive tapes or wires. These products may be used directly or further processed, for example bundling for transmission lines.

The commercial applications are not only for the electric power transmission lines and electrical conductors of microelectronic devices but also other devices including high performance transformers and electric motors.

It will be understood to persons skilled in the art of the invention that many modifications may be made without departing from the spirit and scope of the invention.

In the claims which follow and in the preceding description of the invention, except where the context requires otherwise due to express language or necessary implication, the word "comprise" or variations such as "comprises" or "comprising" is used in an inclusive sense, i.e. to specify the presence of the stated features but not to preclude the presence or addition of further features in various embodiments of the invention.

It is to be understood that, if any prior art publication is referred to herein, such reference does not constitute an admission that the publication forms a part of the common general knowledge in the art, in Australia or any other country.