GRAPHITE

Technical Field The technical field of the present invention is production of graphene from graphite. An application of the invention is for production of graphene used in electrical conductors, for example to provide leads for integrated circuits, wires for electronic devices, electrical cables, transmission lines, electrical motors and transformers.

Background to the Invention Graphene is a known to have desirable electrical conductivity properties. As a constituent of graphite intercalation compounds, graphene is a monoatomic layer of carbon with hexagonal lattice in which one atom forms each vertex. As a result of this atomic structure graphene has many unusual properties. It is strong and has the highest known electrical conductivity of natural substances at room temperature. This is because 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 a 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. The linear Dirac dispersion at the K point in the band structure of graphene leads to zero effective mass of the electrons, thus overcoming the phonon scattering and resulting in remarkable electron mobility of 200000 cm ² /V-s with a carrier density of 10 ¹² /cm ² at room temperature. The corresponding resistivity of graphene sheets is 10 ^~6 Ω-cm, which is less than the resistivity of silver and it is the lowest otherwise known at room temperature. In particular, graphene can carry electric current with densities 6 orders of magnitude higher than copper as a conductor of electricity. The weight of graphene is also ~40% lighter than aluminium. Graphene can also efficiently conduct heat and it is nearly transparent.

It should be noted that the high conductivity of graphene discussed above is with reference to conductivity within a single atomic layer of graphene (in the basal plane). Conductivity between single atomic layers (in the normal plane) graphite or between individual particles of graphene (sheets or flakes) is significantly lower than that of single atomic layer graphene, for example as may be formed on a substrate, for example using chemical production techniques such as chemical vapour deposition or epitaxial growth. The relatively poor conductivity of graphene powder is caused by, at an atomic level, relatively large distances between the graphene sheets requiring high kinetic energy for an electron to hop from one graphene sheet to another. The distance between graphene sheets is determined by van der Waals forces, which are distance dependent interactions between atoms or molecules, causing atoms to both attract and repulse each other depending on the distance. In the absence of any other force the van der Waals forces will cause atoms to remain distant from each other. This distance (cause by the van de Walls force) results in individual graphene sheets remaining distant from one another, thereby requiring high kinetic energy for an electron to hop from one graphene sheet to another. Thus, the low conductivity of particulate graphene (graphene powder). Van der Waals forces are also responsible for the bonding between layers in the crystalline form of graphene, graphite. Graphite is electrically conductive in the basal plane but not in the normal direction. This is due to the van der Waals bonds maintaining a distance between individual graphene layers (around 0.335nm).

Most of the graphene produced for industry application is in the powder form with flake shapes or thin film form on substrates. The covalent nature of carbon bonds means the graphene flakes are difficult to join and shape to form products on a bulk scale.

Another barrier to widespread use of graphene conductors relates to limited availability of graphene on a commercial scale. Although there are several known methods for producing graphene there are no currently available methods for producing high quality material including pristine or near pristine graphene, on a large scale in a low cost reproducible manner.

Graphene can be fabricated by micro-mechanical exfoliation of highly ordered pyrolytic graphite, epitaxial growth, chemical vapour deposition, electrochemical exfoliation, and the reduction of graphene oxide. The first three methods can produce graphene with a relatively perfect structure and excellent properties, but the quantity of the production is very limited.

Graphene oxide can be produced using inexpensive graphite as raw material by cost- effective chemical methods with a high yield. However, the graphene oxide produced typically has many chemical and structural defects. Similarly, this also occurs in the electrochemical exfoliated graphene. Although reduction processing of graphene oxide can partly restore the structure and properties of graphene, not all defects can be removed.

Different reduction processes can also result in different properties of the reduced GO (rGO). Therefore, a problem with this technique is the quality of graphene sheets produced, which can be highly variable (for some methods) and display properties currently below the theoretical potential of pristine graphene produced the other methods, such as mechanical exfoliation. To date graphene produced by the mechanical exfoliation exhibits the best physical properties, regardless the specific technology used to produce the graphene. Exfoliation techniques detach single or multi layers of graphene from graphite. Graphite has a layered, planar structure which consists on a number of individual graphene layers. The carbon atoms are arranged in a honeycomb lattice with separation of 0.142 nm, and the distance between planes is 0.335 nm. Atoms in the plane are bonded (sp ² ) covalently, with only three of the four potential bonding sites satisfied. The fourth electron is free to migrate in the plane, making graphite electrically conductive. Bonding between layers is via weak van der Waals bonds. The sp ² bonding in graphite is extremely strong and it has a binding energy of ~284.4 eV while the exfoliation energy of the interlayers is four orders of magnitude lower (42.6 meV), thus allowing the layers of graphite to be separated. Mechanical exfoliation methods take advantage of this property to break the weak van der Walls bonds and separate individual graphene layers. Quality of the graphene produced using mechanical exfoliation techniques is high. Mechanical exfoliation techniques can produce pristine or near pristine graphene. Mechanical exfoliation is the only technique known to be able to produce substrate free pristine graphene so far. But the production rate for mechanical exfoliation techniques is extremely low.

With other graphene production technologies, the interaction between impurities, such as oxygen, nitrogen, organic groups and others, and the graphene causes the reduction of free electrons on the graphene surfaces. These impurity groups can also act as the scattering centres to affect the mobility of the free electrons on the surface, thus resulting in a much lower electric conductivity compared to the pristine graphene. Chemical vapour deposition can produce the monolayed pristine graphene, as a coating on a substrate. However, to achieve high purity graphene using this technique can be high cost and low yield. There is a need for a method capable of producing high quality graphene including pristine or near pristine graphene on a large scale to satisfy the needs of industry and the research community.

Summary of the Invention

A method of manufacturing a graphene composite product comprising the steps of: forming an initial composite structure comprising graphite within a compressing support structure configured to hold the graphite within the compressing support structure during mechanical deformation; and applying to the initial composite structure an iterative mechanical deformation process comprising a plurality of mechanical deformation iterations, each iteration comprising a forming process configured to apply lateral and longitudinal forces to the composite structure to compress the graphite within the compressing support structure, and where iterations of the forming process cause densification of the graphite within compressing support structure and sufficient iterations are performed to cause shearing of graphene sheets from graphite grains of the graphite within compressing support structure to provide a graphene composite product.

Embodiments of the iterative mechanical deformation process can comprise a first phase where the iterative mechanical deformation of the composite structure causes densification of graphite grains within the compressing support structure.

In an embodiment the first phase can be characterised by increase graphite density by lateral and longitudinal forces causing packing and reorienting of graphite grains to a density that inhibits further rotational movement of the graphite grains. The density that inhibits further rotational movement of the graphite grains can be greater than 75%.

Embodiments of the iterative mechanical deformation process can comprise a second phase where the iterative mechanical deformation causes shearing of graphite layers from the graphite grains. Compression forces applied by the forming process can have a shear component parallel to the graphite basal plane sufficient to cause shearing along the basal plane of some of the graphite grains due to the packing density of the graphite grains. For example, the energy produced by the shear component sufficient to cause shearing along the basal plane can be greater than 42.6 meV. In some embodiments in both the first and second phase frictional forces between graphite grains and the support structure can cause shearing in some graphite grains where the frictional force occurs parallel to the graphite basal plane.

In some embodiments the method can have a transition phase where both reorientation and shearing of graphite layers occurs.

In some embodiments the forming process is a drawing process. In such embodiments the compressing support structure can comprise a tube formed of drawable material, the graphite being placed within the tube for the drawing process. Some examples of tube material include metal or polymer materials.

In some alternative embodiments the forming process is a rolling process. In such embodiments the initial composite structure can be a laminate structure comprising alternating substrate and graphite layers, where the substrate layers provides the compressible support structure holding the graphite. Some examples of the substrate materials include metals or polymers.

In some embodiments the method further comprises a step of periodically measuring the conductivity of the composite structure to determine an extent of conversion of graphite to graphene within the composite structure.

In some embodiments the method further comprises a post processing step of separating the graphene from the compressing support structure.

In some embodiments the post processing step includes dissolving the compressing support structure material to extract the graphene. In some embodiments where the compressing support structure is a laminate structure, the post processing step can include delaminating the compressing support structure to extract the graphene.

Brief Description of the Drawings

Figure 1 illustrates a Graphite lattice structure. Figure 2 is a high level flowchart of a process for conversion of graphite to graphene in accordance with embodiments of the present invention.

Figure 3 schematically illustrates the principle of powder-in-tube processing used to produce prototype Al sheathed multifilament graphite/graphene cores.

Figure 4 is a table of sizes of drawings dies available for drawing the prototype AL/graphite composite from φδ.ΟΟ mm to φ1 .00 mm with deformation ratio of ~12%

Figure 5 schematically illustrates the dimension relationship between the number of the filament cores and the inner diameter of the metal sheath tube.

Figure 6 schematically illustrates an arrangement of Al/graphite composite wires packed into the Al tube, having a numerical relationship with a formula of 1 +6n, where n = 1 , 2, 3, 4, Figure 7 schematically illustrates the forces produced in the wire drawing process. Figure 8a schematic illustrates graphene being exfoliated from the graphite grain where basal plane is parallel to the drawing direction.

Figure 8b schematic illustrates rotation of graphite grain (having random orientations) driven by the frictional force, in the initial stage of the wire drawing process. Figure 9a schematically illustrates the forces exerted on the graphite grains in the initial stages of the wire drawing process.

Figure 9b schematically illustrates the forces exerted on the graphite grains in the stages of the wire drawing process where strong mechanical deformation is achieved.

Figure 9c schematically illustrates the forces exerted on the graphite grains in the later stages of the wire drawing process, for example where the Al sheath is work hardened with strong mechanical deformation.

Figure 10a-c schematically illustrates stages of how the graphite slates are split by the mechanical deformation.

Figure 1 1 a shows cross-sectional morphology of an Al sheathed mono graphite core. Figure 1 1 b shows cross-sectional morphology of an Al sheathed multifilament graphite core having 7 filaments.

Figure 1 1 c shows cross-sectional morphology of an Al sheathed multifilament graphite core having 49 filaments.

Figure 12a graphs the current dependence of resistivity measured from the pure Al and the Al sheathed with monofilament, 7 multifilament and 49 multifilament graphite cores.

Figure 12b graphs the current dependence of temperature measured from the pure Al and the Al sheathed with monofilament, 7 multifilament and 49 multifilament graphite cores.

Figure 13 schematically illustrates the principle of using roll milling to produce the pristine graphene using continual mechanical deformation. Figure 14 is a high resolution TEM image showing a graphite grain split into two nano-plates of thickness of ~15 nm by the shearing force between the Al sheath and graphite.

Figure 15 is a high resolution TEM image shows the overlapping carbon nano-flakes with physical contact created by the compression force. Figure 16 is a cross-sectional optical image of a monofilament Cu/graphite composite wire with an outside diameter of Φ0.58 mm.

Figure 17 is a cross-sectional optical image shows the multifilament Cu/graphite composite wire (61 filaments) with an outside diameter of Φ0.58 mm. Figure 18a is a cross-sectional optical image shows the multifilament Cu/graphite composite wire with 3721 (61x61) filaments in an outside diameter of Φ2 mm;

Figure 18b is an enlarged section of the cross-sectional optical image of Figure 18a, the enlarged image shows the shape of individual filament has been changed from round shape to plate shape. Figure 19 is a cross-sectional optical image showing a multifilament (3721 filaments) Cu/graphite composite wire with an outside diameter of Φ0.58 mm;

Figure 20 is a cross-sectional optical image showing a multifilament (226,981 filaments) Cu/graphite composite wire with an outside diameter of Φ0.58 mm

Figure 21 is a cross-sectional SEM image showing a multifilament (226,981 filaments) Cu/graphite composite tape (0.020 mm x 0.378 mm) that was produced from a Cu/graphite composite wire (Φ0.108 mm) using a rolling process.

Figure 22 is an enlarged cross-sectional SEM image of the multifilament Cu/graphite composite tape of Figure 21

Figure 23 is a cross-sectional TEM image showing the individual monolayered graphene flakes are well aligned in the Cu/graphite composite tapes.

Figure 24 is a cross-sectional optical image showing an as-prepared solid Cu wire used as a reference sample for electrical measurement.

Figure 25 is a graph showing plots of the resistivity against the loading current for the Cu/Graphite composite wires having different mechanical deformation extents. Detailed Description

Embodiments of the present invention provide a method of utilising mechanical deformation to cause exfoliation of graphene layers from graphite grains held in a compressing support structure. The mechanical deformation process uses a plurality of mechanical deformation iterations to cause densification of graphite grains within the compressing support structure as this is mechanically deformed during a first phase, and cause shearing of graphite layers from the graphite grains to form graphene within the compressing support structure as this is mechanically deformed in a second phase. The graphene within the compressing support structure may be utilised as a composite graphene product. Alternatively, a further processing phase may be used to separate the graphene from the compressing support structure to recover the graphene. The compressing support structure can be any form of structure capable of holding the graphite throughout the mechanical deformation process and deforming during the mechanical deformation process to convey mechanical deformation forces to the graphite grains. For example, in one embodiment the compressing support structure is a tube within which graphite can be packed for a powder in tube wire drawing or extrusion process. In other embodiments the compressing support structure may be a laminate structure, container or honeycomb structure within which graphite grains can be held during a pressing, rolling process or stamping type mechanical deformation process.

The compressing support structure can be formed from any material that will be permanently mechanically deformed. For example, for a drawing process the compressing support structure may be an aluminium or copper tube. Alternatively, this could be made from a polymer or any other material suitable for drawing or extrusion. Similarly, any material suitable to hold the graphite during a mechanical deformation process and maintain mechanical deformation may be used for processes such as rolling, stamping, pressing etc. In some embodiments the compressible support structure material may be chosen based on properties for an end use graphene composite product (for example an electrical conductor) as well as material requirement for the deformation process to be used. In other embodiments the compressible support structure material may be chosen based on properties desirable for post processing of the graphene composite structure. In one embodiment a desirable property may be solubility for post processing to recover the graphene. In other embodiments the post processing may involve production of a further product and the support structure material chosen for suitability for the further production process.

Graphite is a naturally occurring form of crystalline carbon and the most stable form of carbon. Graphite can also be synthesized. Graphite is abundant and commercially available in a number of forms, such as a power, flakes or solid rods, bars or ingots. As discussed above and illustrated in Figure 1 Graphite 100 has a layered, planar structure which consists on a number of individual graphene layers 1 10, 1 12, 1 14. The carbon atoms are arranged in a honeycomb lattice with separation of 0.142 nm, and the distance between planes 1 10, 1 12, 1 14 is 0.335 nm. Atoms 130 in the plane 1 10, 1 12, 1 14 are bonded (sp ² ) covalently 135, with only three of the four potential bonding sites satisfied. The fourth electron is free to migrate in the plane, making graphite electrically conductive. The monatomic layers of 1 10, 1 12, 1 14 are clued (or held) with van der Waals intermolecular bonding force represented in Figure 1 by lines 140. The sp ² bonding in graphite is extremely strong and it has a binding energy of ~284.4 eV while the exfoliation energy of the interlayers is four orders of magnitude lower (42.6 meV), thus allowing the layers of graphite 1 10, 1 12, 1 14 to be separated, or to slide past each other. Embodiments take advantage of this property of graphite and use mechanical deformation processes to apply sufficient force parallel to the plane of the graphite layers to cause separation of these layers.

Figure 2 is a high-level flowchart of a method 200 of manufacturing a graphene from graphite. The method comprises an initial step 210 of forming an initial composite structure comprising graphite within a compressing support structure configured to hold the graphite within the compressing support structure during mechanical deformation. Then applying to the initial composite structure an iterative mechanical deformation process 220 comprising a plurality of mechanical deformation iterations, each iteration comprising a forming process configured to apply lateral and longitudinal forces to the composite structure to compress the graphite within the compressing support structure, where iterations of the forming process cause densification 222 of the graphite within compressing support structure and sufficient iterations are performed to cause shearing 225 of graphene sheets from graphite grains of the graphite within compressing support structure to provide a graphene composite product 230. The graphene composite structure 230 output from this process may be in a form usable for some applications - for example a graphene composite conductor. However, an optional step 240 may be performed to separate the graphene from the compressing support structure to extract the graphene.

The iterative mechanical deformation process 220 will comprise a plurality of iterations of at least one forming process. The iterative mechanical deformation process may however, be considered to include two main phases, characterised based on the physical effect of the forming process on the graphite grains rather than the nature of the forming process itself. Indeed, the same forming process may be used for all iterations as will come clear from the examples discussed below. However, first we will discuss the characteristics of the two main phases. During the first phase 222 the iterative mechanical deformation of the composite structure causes densification of graphite grains within the compressing support structure. This phase increases the density of the graphite. This phase may be characterised by increase graphite density by lateral and longitudinal forces causing packing and reorienting of graphite grains to a density that inhibits further rotational movement of the graphite grains. During the second phase 225 the iterative mechanical deformation causes shearing of graphite layers from the graphite grains. In this second phase compression forces applied by the forming process have a shear component parallel to the graphite basal plane sufficient to cause sliding of some layers along the basal plane of some of the graphite grains due to the packing density of the graphite grains. This may result in layers of graphene being exfoliated from the graphite grain or breaking of the graphite grain into smaller "slats" comprising several graphene layers. Densification and reorientation of the graphite grains, exfoliated graphene and slats can also occur during this phase of the processing as mechanical deformation continues. For example, further densification and shearing can result in slats being exfoliated into individual graphene layers.

In addition, during both the first and second phase frictional forces between graphite grains and the support structure can cause shearing in some graphite grains where the frictional force occurs parallel to the graphite basal plane.

It should be appreciated that a transition phase can also occur where both reorientation and shearing of graphite layers occurs. The extent to which reorientation and shearing occur during iterations of the forming process can vary due to the type of forming process used and the geometry of the composite structure.

The method will now be discussed in further detail with reference to prototype examples. The inventor has devised methods for fabricating graphene core conductors utilising powder in tube manufacturing processes which are described in co-pending International Patent Application No. PCT/AU2017/050309, priority date 7 April 2016, and laminate structures as described in co-pending Australian Provisional Patent Applications No. 2016903265 and 2017901290, filed 17 August 2016 and 7 April 2017 respectively. These production methods utilise conventional drawing and rolling fabrication processes and equipment. Initial prototype testing has been performed utilising this same manufacturing equipment and fabrication processes to convert graphite to graphene during the manufacturing process for a graphene composite conductor. This prototype production and testing has proved the feasibility of the claimed graphite production method, but the method should not be considered limited to these techniques - any fabrication methodology providing iterative mechanical deformation processes exerting both compressive (lateral) and elongating (longitudinal) forces can be used for this graphene production method.

A first prototype was manufactured using a drawing process for the iterative mechanical deformation, and the process is illustrated in Figure 3. In this prototyping exercise the inventors used a known powder in tube drawing technique to produce severe mechanical deformation to exfoliate the graphene layers from graphite. The graphite input to this process can be bulk graphite or graphite powder. An advantage of utilising iterative mechanical deformation is that the method of the invention can be implemented using known manufacturing techniques and equipment using continual processing methodologies to enable efficient bulk production. The first experiment used for proof of concept utilised a powder in tube fabrication technique as schematically illustrated in Figure 3. In a first step (a) high purity graphite powder 310 with grain size from 10 nm to 500 μηι was packed into an aluminium (Al) tube 320 to provide the compressing support structure 325. The Al tube 120 had an outside diameter of (j)8mm and inner diameter of (j)4mm with the length of 1000 mm. It should be noted that input material and the parameters for the Al tube were chosen based on material availability and equipment available to the inventors for prototyping rather than any process requirement or limitation. This process could also use coarse graphite particles (> 200 μηι) or a solid graphite rod to fill or insert into the tube for processing. The material of the tube may be metal (such as Al, Cu or Ag tubes etc.), polymer or others. Any sheath material that can be deformed to produce shear force on the graphite core may be used. The outside diameter and inner diameter of the tube can be varied in a wide range depending on the tube materials and also the loading capacity of the machine that is used to produce the mechanical deformation on the materials. The tube length can be chosen from a few millimeters to kilometers or longer based on the applications of the as-produced materials. Is should be appreciated that the parameters such as tube material, tube diameter and length may be influenced by the chosen forming process and will therefore vary widely within the scope of the present invention.

Subsequently, the composite of Al/graphite tube 325 was drawn down to an Al sheathed graphite core wire 330 using an iterative drawing process, to form the Al/graphite composite wire with a graphite monofilament (graphite single core). During this drawing process the Al tube (with its graphite core) is iteratively drawn through a set of drawing dies, the drawing dies reducing in diameter for each drawing iteration.

In the example prototype the initial Al sheathed graphite composite structure, having a diameter of (j)8mm, was drawn down to a graphite core wire having a diameter of φ1 .24mm. The monofilament Al/graphite composite wire was then cut into seven pieces having a length of 1200 mm. These wires were bundled together 340 with one wire in the center surrounded by the other six wires (as shown in Figure 5) and then inserted into another Al tube 342 with an outside diameter of (j)8mm and inner diameter of (j)4mm with a length of 1000 mm, for further drawing. For the prototyping exercise this step was necessary due to the limitation of the drawing equipment - having a minimum diameter of φ1 mm. It should be appreciated that the outside diameter of the composite wire depends on inner diameter of the tube that will be used for fabricating the multifilament cores of Al/graphite composite wires and also and the number of filament cores that will be fabricated. For example, to fabricate an Al/graphite composite wire with 7 filament cores of graphite with an Al tube having an outside diameter of (j)8mm and inner diameter of (j)4mm, theoretically the diameter of the wire should be (j)1 .33mm, based on the arrangement illustrated in Figure 5. However, sometimes it also depends on the drawing dies available. On one hand, the diameter of the wire must be smaller than theoretical size in order to easily insert the as-fabricated 7 wires into the tube. It is desirable to use a diameter of the wire close to the theoretical diameter to increase the packing density. Figure 4 shows a table of drawing dies that were available to the inventors during prototyping to draw the materials with an outside diameter of (j)8mm down to φ1 mm. Obviously, the die size of 1 .34mm is over the threshold size and the next available size was φ1 .24mm. In this prototype example, the Al/graphite composite wire with a graphite monofilament (graphite single core) was fabricated with the outside diameter of φ1 .24 mm.

Repeating the aforementioned process, the Al/graphite composite tube is drawn down into a wire with a diameter of φ1 .24 mm to form the Al/graphite composite wire with 7-multifilament graphite. Such a processing can be repeated to form the Al/graphite composite wires with 49 (7 ² ), 343 (7 ³ ), 2401 (7 ⁴ ), 16,807 (7 ⁵ ), 1 17,649 (7 ⁶ ), 823,543 (7 ⁷ ), 5,764,801 (7 ⁸ ) multifilament graphite and so on. Alternatively, the number of multifilament graphite can be arranged using exponentials of 19, 37, 61 , etc. respectively for example based on the geometry shown in Figure 6, it can be 19 , 361 (19 ² ), 6859 (19 ³ ), 130,321 (19 ⁴ ) etc.

depending on the specifications of the products. It should be appreciated that the arrangement of multifilament cores discussed above is one example for a powder in tube methodology, and the arrangement (i.e. number of cores and packing arrangements) may vary between embodiments and need not follow that discussed above. The important characteristic of the processing is extreme mechanical deformation of the graphite containing support structure causing the graphite grains to shear.

During the drawing process, there are three forces, F 720, F _c 730 and Fb 740, exerted on the wire 770 by the drawing die 710 as shown in Figure 7, where F _d 750 is the drawing force, Fc 730 is the compressive force that the die 710 exerts on the wire 770 that is perpendicular to the drawing 750 force. F _p 720 is the friction force that the drawing die 710 exerts on the wire 770. Fd 750 is the drawing force and is equivalent to the Fb 740. In these forces, Fb 740 is the tensile force to elongate the length the materials 760 and F _c 730 is the compression force. Fc 730 is perpendicular to the surface of the tube 760 or wire 770 to reduce the diameter of the materials.

In the initial stage of drawing processing, the mechanical deformation mainly occurs in the Al sheath as the plastic deformation ability of Al is much greater than that of the graphite. Therefore, the flow of Al in drawing processing causes a friction force, F _f , to be exerted on the graphite core surface - this friction occurring between the inner surface of the aluminium sheath and the outer edges of the graphite core resulting from flow of aluminium during the drawing process. During the initial phase of the mechanical deformation process the effect of the compressive force F _c is mainly to increase the packing density of the graphite within the core. For example, in the prototype example the initial packing density of the graphite powder was around 45%, allowing significant movement of particles of the graphite powder during initial mechanical deformation (drawing) iterations. The result of this movement and compression within the early iterations is increasing the packing density of the graphite powder. As discussed above in the first phase, the function of F _c is mainly to increase density of the graphite core, reducing the cross sectional area of the Al/graphite composite by squeezing the hollow space of the Al tube. In this stage, compared to F _f , the force induced by F _c on the individual graphite grains is insignificant due to the low packing density and movement of the grains, compared with the impact of the friction force. There are two different effects that can occur in the graphite grains (crystals) at this early stage due to the friction force. First, where a graphite grain is aligned such that the basal plane is parallel to the direction of F _f , the top graphene layer of the particular graphite grain may be directly exfoliated due to the friction force F _f . This is illustrated in Figure 8a where the friction force 810 and basal plane 820 are parallel, the friction force may be sufficient to overcome the weak van der Waals bonds between two layers 830, 840 of the graphite lattice structure, resulting in shearing and separation of one layer 840 from the other 830.

Second, as illustrated in Figure 8b, for the grains where the basal plane 820 is not aligned with the direction of the frictional force 810 (random orientations) only a component of the frictional force will be acting parallel with the basal plane. This component of the frictional force will be typically insufficient to overcome the weak van der Waals bonds during the early phase processing. However, the frictional force can cause rotation of the grains 850 to reorient the graphite grains and enable increased packing density to be achieved.

Figures 9a to 9c illustrate the forces on the graphite grains in more detail and discuss the behaviour of the graphite grains in conditions of increasing density and compression - as deformation processing continues. Figure 9a shows the forces exerted on the graphite grain in the initial stage processing . As discussed above during this phase the compressive force becomes insignificant due relative movement of the grains within the tube. The friction force Ff 91 0 has an apparent component Ft perpendicular to the basal plane 920 and an apparent component F _s 940 parallel to the basal plane 920. The shear force F _s 940 that results from the friction plays a more important role in determining the grain behaviors compared to the force Ft 930, which is perpendicular to the basal plane of graphite. This is because the sp ² carbon bonding in the layer plane of graphite (284.8 eV) is four orders of magnitude higher than the van der Waals force between the interlayers of graphite (exfoliation energy of 42.6 meV). In the early processing phase the graphite grains having random orientations cannot be broken by the F _s and Ft as the force components 940 exerted on graphite basal plane (a- b plane) 920 is not large enough. However, the frictional force can drive these grains (near the sheath interface) to rotate (as shown in Figure 8b) till the basal or layer planes of these graphite grains are parallel to the direction of the frictional force F _f . As shown in Figure 8a when the basal plane 820 is parallel with the frictional force this becomes the shear force F _s = Ff and the frictional force induced by the further drawing process can cause exfoliation of the top graphene layers from these well-aligned graphite grains as shown in Figure 8a.

As the density of the graphite increases (due to continued mechanical deformation) movement of the graphite grains/crystals is inhibited. The density that inhibits further rotational movement of the graphite grains is in the range >75%. Once the movement of graphite grains becomes inhibited the compression force Fc 730 becomes significant as well as the friction force 910. As shown in Figure 9b, in a higher mechanical deformation stage, the deformation of Al sheath exerts high friction force on the graphite core. The grain rotation is restricted by the surrounding grains. In this case, the majority of F _c 730 is transferred to the particular graphite grain as F _c 950, which can be divided into two force components as shear force, F _s 960 that is parallel to the basal plane of graphite grain and the force, F _t 970 that is perpendicular to the basal plane of graphite grain. The force component in each direction is dependent on the orientation of the grains. The shear components of the frictional and compressive forces are additive (F _s + F _s ) and can become large enough to cause sliding of the ultrathin blocks of graphite, sometimes graphene layers in the graphite grains as shown in Figure 9b.

In some circumstances - such as continued mechanical deformation - work hardening may reduce the flow in the sheath, causing a reduction in the frictional force, making the effect of compression dominant. An example of this is illustrated in Figure 9c, where the dominant force is the compression force F _c 950. The compression force 950 has a component of shear force, F _s 960 that is parallel to the basal plane of graphite grain and a force component F _t 970 perpendicular to the basal plane of graphite grain, and a shear force component of sufficient magnitude will overcome the van der Waals force between layers and cause exfoliation or breaking of the grains into slates, comprising several layers of graphene and then further break down of the slates into single graphene layers through further iterations of mechanical deformation. The energy produced by the shear component sufficient to break the van der Waals intermolecular bonding is in the range of > 42.6 meV.

Further reduction of cross-sectional area of the Al/graphite composite (or the thickness reduction caused by roll milling) rotates the exfoliated ultrathin blocks or graphene layers to make their basal planes close to parallel with the interfaces between Al and graphite cores. The subsequent drawing or roll milling process further exfoliates the multilayered graphene into the single layered graphene. This occurs in the mechanical deformation of Al/graphite composite with multifilament cores. Figures 10a-c is a schematic illustration showing how the graphite slates can be split by mechanical deformation. It should be appreciated that the mechanical deformation could utilise a predominantly compressive mechanical deformation process, such as stamping or pressing. Flow effects achieved using processes such as drawing or rolling can be advantageous for rapid densification during early phase processing due to high friction forces aiding reorientation of graphite grains and early phase shearing at the interface between the sheath and graphite. However, some flow and friction effects are still produced during a stamping or pressing process due to relative movement of the grains during densification. It is envisaged that embodiments of the method may utilise predominantly compressive mechanical deformation processes. Alternatively, a combination of deformation processing encouraging flow effects (such as rolling or drawing) and predominantly compressive mechanical deformation processes may be used. The images shown in Figures 14 and 15 are high resolution transmission electron microscopy (TEM) images of a sample from a prototype AL/graphite composite wire produced as described above. Figure 14 is a high resolution TEM image which shows a graphite grain that was split into two nano-plates of thickness of ~15 nm by the sharing force between the Al sheath and graphite. This image demonstrates that the shearing force induced by the wire drawing process split the large graphite grain (~40 μηι) was split into two nano-plates (nano-flake 1 1410 and nano-flake 2 1420) of thickness of 15 nm. This image also shows part of the aluminium sheath 1430. The force produced by the drawing die to reduce the wire diameter compressed nano-flakes (5 nm) to create physical contact between nanoflakes as shown in Figure 15. Figures 1 1 a to 1 1 c shows the cross-sectional morphologies of the prototype Al/graphite composites, with Figure 1 1 a showing a monofilament core wire, Figure 1 1 b a 7 filament core, and Figure 1 1 c a 49 filament core. The outside diameters of these composite wires all are φ1 .00 mm. The diameters of the graphite cores are decreased significantly from 380 μηι for the monofilament wire by increasing the number of filaments in the composite wires. Such a process causes the graphite grains (crystals) to be split into many slates (comprising several layers of graphene) and then eventually into single atomic graphene layers through the mechanical deformation by wire drawing or roll milling process. In most of the cases, the graphene layers are formed and aligned from the interfaces between Al sheath and graphite cores to the center of the cores through increase of the deformation ratio.

In general, Al has a resistivity of 2.85x10 ^-6 Ω-cm while the resistivity of graphite is 2.5x10 ^-4 Ω-cm to 5.0x10 ^-4 Ω-cm along the basal plane and 3.05.0x10 ^-1 Ω-cm perpendicular to the basal plane. For the graphite consisting of grains with random orientations, its average resistivity 1 .5x10 ^-1 Ω-cm. which is five orders of magnitude higher than the resistivity of Al. Therefore, a graphite core composite wire formed by addition of graphite into an Al tube and drawing will result in a composite wire having a much higher resistivity than that of a pure Al wire. Although the mechanical deformation exerted on the graphite core in the monofilament Al/graphite composite is reasonably strong, the formation of graphene only occurs in an extremely thin layer underneath of the Al sheath and the majority of the carbon core is still the graphite. This phenomenon results in a higher resistivity of the composite wire compared to the pure Al wire as shown in the graph of Figure 12a. For the multifilament wires, the mechanical deformation that the Al sheath exerted on the graphite cores is severe. It causes that the graphite grains to be split, rotated and then exfoliated to form the thicker layers of the single atomic layered graphene underneath of the Al sheath. These graphene layers are aligned along the interface between the Al sheath and "graphite" cores. Further deformation results in the significant reduction of the diameter of the graphite cores, thus producing the full graphene cores in ideal conditions. Figure 12a plots the resistivity for a pure Al wire 1210, single core 1220, 7-multifilament 1230 and 49-multifilament 1240 Al/graphite composite wires. Figure 12b plots the temperature and current dependence for this same set of wires. Figure 12a shows that both 7-multifilament 1230 and 49- multifilament 1240 composite wires, which are fabricated by filling the graphite into a pure Al tube, have lower DC resistivity than the pure Al wire 1210. The single core graphite wire 1220 shows a higher resistivity due to the graphite having a resistivity that is five orders of magnitude higher than the pure Al. These test results demonstrate that the prototype manufacturing formed graphene in Al/graphite composite wires through the severe mechanical deformation. The materials prepared with powder-in-tube technology with more filaments result in lower resistivity because more graphene was exfoliated from the graphite grains due to the stronger mechanical deformation. In addition to the greater extent of mechanical deformation for 49 filament multifilament cores, these cores also experienced a greater number of iterations of mechanical deformation. The method can further comprise a step of periodically measuring the conductivity of the composite structure to determine an extent of conversion of graphite to graphene within the composite structure. This can provide a simple and non-destructive method for testing the extent to which the graphite to graphene conversion has been achieved.

Similar to the powder-in-tube methodology, a continual mechanical deformation produced by roll milling as shown in Figure 13 can also exfoliate the graphite grains efficiently, thus producing high quality graphene on a large scale. In this embodiment the initial composite structure is a laminate structure comprising alternating substrate and graphite layers, where the substrate layers provides the compressible support structure holding the graphite.

Although the example above is discussed using aluminium for forming the compressible support structure, other metals or polymer materials can be used.

In another prototype example graphite was filled into a Cu tube having an outside diameter of Φ8 mm and inside diameter of Φ6 mm. The initial packing density of graphite was ~54%. Subsequently, the Cu/graphite composite tube was drawn to a wire having an outside diameter of Φ0.58 mm, forming a monofilament Cu/graphite composite wire as shown in Figure 16. In the image of Figure 16 clearly shows the graphite core 1610 and copper sheath of the composite wire.

In a next stage 61 -monofilament wires were packed into another Cu tube 1720, which was drawn down to a Cu/graphite composite wire with 61 filaments 1710, and optical image of a cross section of this wire is shown in Figure 17. The cross-sectional optical image in New Figure 17 shows that most of the filaments 1710 remain distinct but some are losing the round shape and some of the filaments have a "tails" 1440. The distortion of filament 1710 shape is a result of the compression and drawing forces. This distortion is not surprising due to the difference in malleability of the copper sheathing material compared to graphite and graphene. Repeating the packing and drawing process, a Cu/graphite composite wire with 3721

(61x61) filaments was produced as shown in Figures 18a and 18b. Figure 18a is a cross- sectional optical image of the 3721 (61 ² ) filament composite wire having a diameter of 02mm. The enlarged image of Figure 18b clearly shows that the shape of individual filaments has been further distorted and ceases to be round but rather are becoming more planar with random orientation. It can be observed in Figures 18a and 18b that some filaments may have merged or have not thinned to the same extent as others, this may be explained by differing originating graphite grain size, this can result in shearing of the graphite crystal into states and/or graphene sheets different rates between filaments. Also some sheath material may be forced from between filaments causing filaments to merge, however further mechanical deformation causing further shearing of graphite crystals and alignment of graphene flakes can reduce the impact of such defects. For example, as shown in Figure 19 the thickness of the planar filaments is reduced significantly by further reducing the diameter of the wire to a diameter of Φ0.58 mm.

Similar processing was used to produce the Cu/graphite composite wire with 226,981 random oriented filaments as shown in Figure 20. Figure 20 is a cross-sectional optical image of a 61 ³ multifilament (226,981 filaments) Cu/graphite composite wire with an outside diameter of Φ0.58 mm. In another prototype a CU/graphite composite wire (in this example the 61 ³ multifilament wire) was rolled into tapes having different thickness. Figure 21 is a cross-sectional SEM image of a tape (0.020 mm x 0.378 mm) 21 10, which was produced from a composite wire of Φ0.108 mm. Figure 22 is an enlarged image of section 2120 and the detail cross- sectional image clearly shows that the normal of the carbon planar filaments were aligned along the normal orientation of the tape surface, forming the texture microstructure. This image evidences that the random orientated multifilament plates in the composite wire were aligned to the normal of the tape surface, forming a carbon texture microstructure. Figure 23 is a cross-sectional TEM image showing the individual monolayered graphene flakes are well aligned in the Cu/graphite composite tapes. The high resolution TEM images shown in Figure 23 evidences the individual graphene sheets were well aligned by the deformation process, and with physical contact (reduced interlayer distance) between graphene sheets.

For comparison, a Cu tube without filling graphite was also drawn into the wire with a diameter of Φ0.108 mm. The cross-sectional image of Figure 24 shows this solid Cu wire used as a reference for the electrical measurements of the Cu/graphite composite wires. The composition of reference Cu wire that we used for this comparison is shown in Table Cu 1 . The Cu wire composition was determined by using Inductively Coupled Plasma

Spectrometry. Table Cu 1 Composition of the reference wire

Figure 25 is a graph showing plots of the resistivity against the loading current for the Cu/Graphite composite wires having different mechanical deformation extents. The relationships between resistivity and loading current for the Cu/graphite composites having different mechanical deformation extents demonstrates the significant improvement of electrical conductivity by the heavy mechanical deformation, evidencing the contribution of graphene produced by this invention. "Pure Cu" 2510 is the reference sample, meaning Cu without carbon products. Figure 25 shows that the resistivity of the composite 2520 wire with 3721 filaments is much higher than that of the wire 2530 with 226,981 filaments. It is clear that both composite wires have higher resistivity than the reference Cu wire 2510. However, the resistivity of the tapes 2540, 2550 with 228,981 filaments is much lower than the Cu wire 2510, demonstrating that the alignment and physical contact of graphene sheets caused by the additional mechanical deformation made significant contribution to the electrical conductivity.

Looking at the plots 2520, 2530 for the CU/graphite composite wires it is clear that the resistivity improves significantly from the 3721 filament wire 2520 to the 226981 filament wire 2530 from this one can infer that the mechanical deformation is causing increasing amounts of graphite to be sheared to graphene and/or improvement in alignment and reduction of interlayer distance between graphene sheets is contributing to reduced resistivity.

Comparing the plot 2530 for the 226981 filament wire with the plots 2540, 2550 for the tapes made from this wire one immediately notes a very significant improvement in conductivity of the tapes compared to the wire 2530 - better than the reference copper wire. Further, comparing the resistivity of the 20 micron tape 2540 to the resistivity of the 17 micron tape 2550 one observes an improvement in the resistivity with the thinner tape. This observation is significant because both of these tapes are produced using the same initial wire structure. The difference between the structure of the 17 micron tape 2550 and 20 micron tape 2540 is the extent of mechanical deformation and resulting physical structural changes in alignment and interlayer distance between graphite/graphene particles caused by the mechanical deformation. The significant difference between the resistivity of the two tapes evidences the improvement in conductivity as the alignment of graphene particles is improved and the interlayer distance reduced.

It should be appreciated that the mechanical deformation process may be chosen so the method provides an output composite product in a form usable as an end product, for example a low resistivity wire or conductive tape.

The method can further comprising a post processing step of separating the graphene from the compressing support structure. This pros processing step may include dissolving the compressing support structure material to extract the graphene. For example in an embodiment the compressing support structure is formed of a polymer material and the post processing step may include dissolving the polymer material. The graphene may then be extracted from the solution, for example by centrifuging and drying. Alternatively a metal compressing support structure may also be mechanically and/or chemically removed to recover the graphene.

Alternatively the graphene may be extracted by a mechanical means. For example, where the compressing support structure is a laminate structure, the post processing may include delaminating the compressing support structure to extract the graphene.

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.