Graphene production

Abstract

Apparatus for processing a bulk material, such as graphite, to convert the bulk material into a laminar form, such as graphene, comprises a containment vessel, in the form of a cylindrical drum (1), for containing the bulk material during processing. Pairs of arms (14) and pads (13) are located within the drum (1) and are mounted for rotation with a shaft (3). An outer surface (16) of the pads (13) opposes the internal surface (15) of the drum (1) to form a gap (17) between the outer surface (16) and the internal surface (15). The arms (14) move the pads (13) across the internal surface (15) of the drum (1) and thereby capture bulk material in the gap (17). A shear force (20) is generated across the bulk material in the gap (17) by the relative movement of the outer and internal surfaces, which acts to separate layers of the bulk material. The apparatus can process the bulk material in air without a carrier liquid, which provides for simpler and more environmentally friendly processing than known techniques.

Description

GRAPHENE PRODUCTIONTECHNICAL FIELD

[0001] The present invention relates to apparatus and methods for the production of graphene from bulk graphite. However, the invention is of application to the processing of any bulk material to produce a laminar form of the bulk material.BACKGROUND

[0002] Graphene is a form of carbon composed of a single layer of carbon atoms arranged in a hexagonal lattice nanostructure. The term "graphene" as used herein may also refer to a stack of several layers of graphene, which is a commercially useful form of graphene and may be less arduous to produce than pure, single layer graphene. Such a stack is also referred to as a graphene platelet.

[0003] Graphene can be produced from graphite by mechanical exfoliation, i.e. the mechanical removal of graphene layers from the surface of the graphite. Famously, Andre Geim and Konstantin Novosolev were awarded the 2010 Nobel Prize in Physics for their work on graphene, which included separating graphene layers from graphite using adhesive tape. Since then, many methods for producing graphene have been disclosed, some of which use a liquid carrier to transport the graphene and / or graphite during processing.

[0004] For example, WO 2019 / 095012 discloses the production of nanostructure materials, such as graphene, in a thin film reactor from a starting material of layered or two-dimensional structure and a solvent or liquid phase. The reactor can be a vortex fluidic device or a device with spaced first and second fluid contact surfaces, which can be conical, for relative rotation to generate shear stress in the thin film therebetween. A liquid supply means delivers a liquid between the first and second fluid contact surfaces.

[0005] The present invention seeks to provide an alternative apparatus and method suitable for the processing of graphite to produce graphene layers. Desirably, the process according to the present invention operates without the requirement for a liquid phase carrier.SUMMARY OF INVENTION

[0006] In accordance with the present invention there is provided apparatus for processing a bulk material, such as graphite, to separate layers of the bulk material, whereby to convert the bulk material into a laminar form, such as graphene. The apparatus comprises a containment vessel for containing the bulk material during processing, at least one movable member located within the containment vessel and having a first surface, and a second surface within the containment vessel opposed to the first surface to form a gap between the first surface and the second surface. The movable member is arranged to move the first surface across the second surface and thereby capture bulk material in thegap, whereby a shear force is generated across the bulk material in the gap by the relative movement of the first and second surfaces, which shear force acts to separate layers of the bulk material.

[0007] Embodiments of the present invention do not require a liquid carrier for the bulk material to achieve effective processing. Consequently, the apparatus of the invention is simpler and has a generally lower environmental impact than known apparatus for converting bulk material to laminar material.

[0008] In embodiments of the invention, the first surface or the second surface may be substantially smooth. Preferably, both the first surface and the second surface are substantially smooth.

[0009] In embodiments of the invention, the second surface may be associated with a further movable member. In the presently preferred embodiment, however, the second surface is substantially static.

[0010] In one embodiment, the second surface is an internal surface of the containment vessel. The second surface may be flat, but in the presently preferred embodiment the second surface is curved.

[0011] In one embodiment, the containment vessel comprises a cylindrical drum. The second surface may be the internal surface of the cylindrical drum.

[0012] The movable member may be arranged to move the first surface repeatedly across the second surface during processing of the bulk material. In this way, particles of the bulk material may be repeatedly captured in the gap. For example, the movable member may be arranged for reciprocal motion. In the presently preferred embodiment, however, the movable member is arranged for rotation about a rotational axis. The rotational axis may be substantially horizontal in use of the apparatus. The rotational axis may be substantially colinear with the axis of the cylindrical drum. The speed of rotation of the shaft may be less than 10 revolutions per second, feasibly less than 5 revolutions per second.

[0013] The apparatus may comprise a plurality of movable members equally distributed about the rotational axis. The distribution of the movable members in this way provides rotational balance to the apparatus. The plurality of movable members may form a group of movable members located at substantially the same position along the rotational axis. For example, the apparatus may comprise a pair of diametrically opposed movable members. The apparatus may comprise a plurality of groups of movable members, the groups being mutually spaced along the rotational axis. The movable members of adjacent groups of movable members may be mutually offset in the circumferential direction of the rotational axis. For example, adjacent pairs of movable members may be arranged mutually perpendicularly. The movable members of adjacent groups may be interdigitated so that the axial extent of the first surfaces of the movable members of adjacent groups overlap. This allows the entire second surface to be covered by the movable members.

[0014] The movable member(s) may comprise a resilient member arranged to urge the first surface towards the second surface. The resilient member may be a spring, in particular a compression spring. Alternatively, the resilient member may be an elastomer or a pneumatic component.

[0015] The first surface may comprise a high friction coating. The high friction coating may be an elastomer, for example silicone or rubber. Other suitable materials may include PTFE or other polymers.

[0016] The first surface may deviate away from the second surface towards a leading edge of the first surface whereby to guide bulk material into the gap.

[0017] The gap may be less than 20 microns, preferably less than 10 microns, more preferably less than 5 microns, ideally less than 1 micron.

[0018] The containment vessel may be fixedly mounted to a chassis. The movable member(s) may be arranged for movement relative to the fixed containment vessel.

[0019] The containment vessel may comprise a closable loading port and / or a closable discharge port. The containment vessel may be arranged on the chassis to be tiltable whereby to discharge the processed laminar material through the opened discharge port.

[0020] The present invention extends to a method of operating the apparatus described herein. The method may comprise the steps of loading the containment vessel with a bulk material, processing the bulk material using the apparatus to separate layers of the bulk material, whereby to convert the bulk material into a laminar form, and discharging the laminar material from the containment vessel to provide a product.

[0021] According to the method, the bulk material may be processed within the containment vessel substantially dry. This is particularly advantageous because it allows processing in the absence of a chemical carrier for the bulk material, which has environmental and cost advantages.

[0022] The containment vessel may be loaded with the bulk material and the first surface may be moved across the second surface repeatedly until the bulk material has been converted to the laminar form. The processed laminar material may then be discharged from the containment vessel. Thus, the method may be a batch process.

[0023] In embodiments of the invention, the volume of bulk material loaded into the containment vessel is less than 40%, preferably less than 30%, of the internal volume of the containment vessel. In this way, there is sufficient space within the containment vessel for the processed laminar material and unprocessed or partially processed bulk material to fly around the containment vessel during processing and for the heavier bulk material to fall to the bottom of the containment vessel for capture in the gap.

[0024] The bulk material may have a particle size less than 500 microns. In embodiments of the invention the bulk material has a particle size less than 200 microns. The laminar material may comprise fewer than 20 atomic layers, preferably fewer than 15 atomic layers, more preferably fewer than 10 atomic layers, most preferably fewer than 5 atomic layers.BRIEF DESCRIPTION OF DRAWINGS

[0025] Embodiments of the invention are further described hereinafter by way of example only and with reference to the accompanying drawings, in which:Figure 1 is a schematic representation of apparatus according to an embodiment of the present invention;Figure 2 is a magnified view of details of the apparatus of Figure 1;Figure 3 is a schematic representation of the crystal structure of graphite;Figure 4 is a schematic representation of the crystal structure of graphite under a lateral shear force;Figure 5 is a diagrammatic representation of the effect of a lateral shear force on the crystal structure of graphite;Figure 6 is a schematic representation of a component of the apparatus of Figure 1 according to an alternative embodiment; andFigure 7 shows four scanning electron microscope images of graphene platelets produced by the apparatus of Figure 1.DESCRIPTION OF EMBODIMENTS

[0026] Figure 1 is a schematic representation of apparatus according to an embodiment of the present invention. The apparatus converts bulk material, such as graphite, into laminar material, such as graphene. As shown in Figure 1, the apparatus comprises a cylindrical drum 1 which contains a plurality of movable members 2 mounted for rotation on a shaft 3. The drum 1 is formed of stainless steel (304), and has an internal diameter of 800mm, an outer diameter of 820mm and a length of 800mm. A thick polymer-based material may be used as an alternative to stainless steel. The rotational axis of the shaft 3 is colinear with the axis of the cylindrical drum. The shaft 3 is driven by an electric motor 4 to cause the movable members 2 to rotate about the axis of the shaft 3. The drum 1 is provided with a closable loading port 5 which allows bulk material to be added to the drum 1 from a loading hopper 6. The drum 1 is further provided with a closable discharge port 7 which allows processed laminar material to be discharged from the drum 1 into a de-loading system 8.

[0027] The drum 1 and the electric motor 4 are fixedly mounted to a chassis 9. The chassis 9 is pivotally mounted to a stand 10 by means of a pivot 11, which allows the chassis 9 and drum 1 to tilt relative to the stand 10 as indicated by the dashed arrows in Figure 1. The tilting of the drum 1 assists in discharging the processed laminar material through the discharge port 7 after successful processing. The tilting of the chassis 9 and drum 1 is effected by a double action hydraulic jack 12, which can control the angular orientation of the chassis 9 and thereby the drum 1. The chassis 9 and drum 1 can be tilted from -40° to 40°.

[0028] The movable members 2 each comprise a pad 13 and an arm 14 which connects the pad 13 to the shaft 3. The pads 13 and arms 14 are formed of stainless steel. In the embodiment shown there are ten movable members arranged in five groups (pairs) of two spaced along the axis of the shaft 3. Other embodiments may have, for example, eight pairs of movable members 2. As shown in Figure 2, which is a view along the axis of the shaft 3, the two movable members 2 of each group are distributed equally about the shaft 3, i.e. the pads 13 are diametrically opposed. The even circumferential distribution of the movable members 2 about the shaft 3 balances the pads 13 and thereby minimises vibration of the apparatus as the shaft 3 and movable members 2 rotate. The present embodiment uses the minimum number of movable members 2 in each group that will achieve balanced rotation while maximising the space within the drum 1 for the material being processed. This also minimises the number of pads 13 that must be driven across the internal surface 15 of the drum 1 by the electric motor 4.

[0029] As shown in Figures 1 and 2, adjacent groups of movable members 2 are mutually offset by 90°, so that their respective arms 14 are perpendicular. This allows the width of the pads 13 in the axial direction of the shaft 3 to be greater than the spacing between the pads 13 of adjacent groups, i.e. the movable members 2 are interdigitated. In this way coverage of the pads 13 in the axial direction of the shaft 3 overlap so that it is ensured that the entire internal surface 15 of the drum 1 is covered by the pads 13. This prevents accumulation of material on the internal surface 15 of the drum 1.

[0030] Referring to Figure 2, it will be seen that the outer surface 16 of the pads 13 is shaped to closely follow the internal surface 15 of the drum 1 to form a small gap 17. The dashed lines in Figure 2 indicate progressive magnifications of the gap size. In the embodiment shown, the gap 17 is at most 10 microns. The gap 17 is desirably as small as possible to maximise the delamination of the bulk material as will be explained below. However, a maximum gap size of 10 microns has been found to be an acceptable compromise between effective processing and practical manufacturing tolerances. The effective surface area of the outer surface 16 of the pads 13 that follows the internal surface 15 of the drum 1 is 300 by 110 mm. As shown in Figure 2, the internal surface 15 of the drum 1 and the outer surface 16 of the pads 13 are both substantially smooth.

[0031] As shown in Figure 2, the outer surface 16 of each pad 13 deviates away from the internal surface 15 of the drum 1 at its outer edge. In this way, the outer surface 16 of the pad 13 forms a funnel for the bulk material to guide the material into the gap 17 as the shaft 3 rotates the movable members 2. Note that is only necessary for the pads 13 to have this deviating surface on their leading edge in the direction of rotation, as is the case in the embodiment of Figure 6.

[0032] Referring now to Figure 3, the structure of graphite is shown schematically. As shown in Figure 3, layers of carbon atoms 18 are held together by pi bonds 19 to form the graphite structure. The distance between the layers is approximately 0.34nm. As shown in Figure 4, in the presence of a lateralshearing force 20, the pi bonds 19 are stretched as the layers of carbon atoms 18 are mutually displaced. With a sufficient shearing force 20, the pi bonds will be stretched beyond their effective length and will break, allowing the layers of carbon atoms 18 to separate from each other. Repeated breaking of the pi bonds in this way will eventually convert the graphite to graphene (or graphene platelets, which are carbon particles comprising a small number of layers of carbon atoms, for example 10 layers or fewer). In this way, by applying a shearing force 20 to the bulk graphite, it can be converted into graphene.

[0033] Figure 5 indicates diagrammatically the required magnitude of the shearing force 20 to achieve separation. The graphite crystal lattice can tolerate some lattice deformation, but when the shear force 20 is higher than the lattice tolerance, the distance between bonded atoms 18 reaches a critical distance in which atoms cannot be bonded anymore and the lattice will fail from these two layers. With increasing shear force 20, the angle 0 increases and the distance between the bonded atoms 18 reaches the critical distance for de-bonding.

[0034] According to the present embodiment, the required shearing force 20 to achieve separation of the graphene layers is achieved by the frictional engagement of the internal surface 15 of the drum 1 and the outer surface 16 of the pads 13 with the graphite in the gap 17 as the pad 13 moves across the internal surface 15 of the drum 1. As shown in Figure 6, the frictional engagement of the outer surface 16 of the pad 13 is enhanced by the provision of a silicone coating 21. The silicone coating is not essential and the stainless-steel surface of the pad 13 may be used. Alternatively, other covering materials such as rubber, PTFE, or other polymers may be used. It is important to seal the gap 17 between the outer surface 16 of the pad 13 and the internal surface 15 of the drum 1 to trap the graphite particles and maximise the shear force 20.

[0035] The pad 13 is connected to the arm 14 by means of two springs 22, which act to urge the pad 13 towards the internal surface 15 of the drum 1, also enhancing frictional engagement. The end 23 of the arm 14 is slidably received within a complementary recess in the pad 13 and the degree of compression of the springs 22 can be adjusted by means of a plate 24 which compresses the springs 22 under the action of two adjustment bolts 25 received in threaded holes formed in respective lateral projections 26 from the arm 14. Also to be noted in Figure 6 is the shape of the leading end 27 of the pad 13 which deviates from the internal surface 15 of the drum 1 and acts to funnel bulk material into the gap 17 as explained above.

[0036] The apparatus of the present embodiment breaks the layered crystal structure of the bulk material through the application of a shearing force 20 in a selective direction to decrease the number of layers rather than smashing the crystals to smaller pieces in random directions as in a mill or similar device. The apparatus applies a high shear force 20 to the crystal structure to break it from its weakest point which is somewhere in the middle of the crystal. As the apparatus continues to operate with aload of bulk material it will continue to break the crystals to lower numbers of layers until the required thickness is achieved, typically between 2 and 5 sheets. The graphene platelets produced are in the form of sheets and comprise the minimum of defects and broken particles. The lower layered structures have a larger distance between their layers as there is no lattice pressure on the layers. The present process produces graphene platelets having mostly 10 or fewer than 10 layers with a resultant thickness under 3nm.

[0037] Figure 7 shows scanning electron microscopy (SEM) images of outputted few-layered graphene sheets.

[0038] In operation, the drum 1 is loaded with bulk material, such as graphite, by filling the loading hopper 6 and opening the loading port 5 so that the bulk material enters the interior of the drum 1 via a flexible tube 28. The loading port 5 is then closed. The discharge port 7 is already closed at this point. A typical load from 10 to 15 kg bags of graphite may be 15 to 20 kg. A mass controller may be used to close the valve of the loading port 5 when the appropriate loading of the drum 1 has been achieved. A 20 kg load is around 25% of the volume of the drum 1. After successful processing, the volume of material approximately doubles to roughly half the free internal space of the drum 1.

[0039] The apparatus of the present embodiment is not limited to graphite; any layered structure crystal may be treated, but the present embodiment is focused on graphite. The process is not limited by the purity of the graphite, but pure raw materials result in a higher quality of product. Most of the common impurities such as Fe, Ca and Si are unacceptable in many applications. The particle size of the raw materials is important as breaking the bigger crystal particles needs more shear force energy and the apparatus naturally has a maximum power. The apparatus of the present embodiment is able to treat raw materials from 200 microns to less than 1 micron.

[0040] With the drum 1 loaded, the electric motor 4 is activated to rotate the shaft 3 and hence the arms 14 and pads 13. In this embodiment, the rotational speed is one revolution per second, but a speed of five revolutions per second is feasible with a faster electric motor 4. Rotation of the shaft 3 is in the same direction; there is no reversal of the direction of rotation. Rotation of the shaft continues for 30 to 90 minutes, typically 60 minutes, until all the graphite has been successfully converted into graphene platelets by passing through the gap 17 between the pad 13 and internal surface 15 of the drum. During processing, particles of graphite and graphene will tend to fly around the interior of the drum 1 as a result of their interactions with the arms 14 and pads 13. The lighter graphene platelets will tend to be suspended in the air inside the drum 1, while the heavier unprocessed or partially processed graphite particles will tend to fall to the bottom of the drum 1 to be sheared by the pads 13.Consequently, graphite particles may pass through the gap 17 multiple times before they are successfully converted to graphene platelets. The amount of time required to convert a given mass of graphite tographene platelets is determined empirically by analysing the results of test processing. It is then assumed that the same consistency of the product will be achieved after the same processing time for the same loading. A larger load will require a longer processing time.

[0041] It is important to understand that the process according to the present embodiment does not require a liquid carrier for the graphite or any other chemicals or hazardous materials, which results in a very low environmental impact and is cost effective. The graphite is processed in air and is substantially dry. A vacuum is not required. Another advantage over other techniques is that the x-y dimensions of the graphite particles are not changed. The processing is focused on a selective direction which targets intralayer bonding (n-bond), whereas in grinding or ball milling techniques particles are smashed by grinder blades or stainless-steel ball surfaces resulting in broken particles rather than thinner ones.

[0042] Once the material has been processed for the required amount of time, the electric motor 4 is stopped and the material in the drum 1 is allowed to settle for a few minutes. Once the material has settled the hydraulic jack 12 is activated to tilt the chassis 9 and drum 1 approximately 30° so that the material inside the drum 1 slides towards the discharge port 7. A valve at the discharge port 7 controls the rate of discharge of the processed material. The discharge port 7 is opened to allow the processed material to exit the drum 1 into a collecting bag 29 of the de-loading system 8 for packaging and transport.

[0043] In summary, apparatus for processing a bulk material, such as graphite, to convert the bulk material into a laminar form, such as graphene, comprises a containment vessel, in the form of a cylindrical drum 1, for containing the bulk material during processing. Pairs of arms 14 and pads 13 are located within the drum 1 and are mounted for rotation with a shaft 3. An outer surface 16 of the pads 13 opposes the internal surface 15 of the drum 1 to form a gap 17 between the outer surface 16 and the internal surface 15. The arms 14 move the pads 13 across the internal surface 15 of the drum 1 and thereby capture bulk material in the gap 17. A shear force 20 is generated across the bulk material in the gap 17 by the relative movement of the outer and internal surfaces, which acts to separate layers of the bulk material. The apparatus can process the bulk material in air without a carrier liquid, which provides for simpler and more environmentally friendly processing than known techniques.

[0044] Throughout the description and claims of this specification, the words "comprise" and "contain" and variations of them mean "including but not limited to", and they are not intended to (and do not) exclude other components, integers or steps. Throughout the description and claims of this specification, the singular encompasses the plural unless the context otherwise requires. Where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise.

[0045] Features, integers, characteristics or groups described in conjunction with a particular aspect, embodiment or example of the invention are to be understood to be applicable to any other aspect, embodiment or example described herein unless incompatible therewith. All features disclosed in this specification (including any accompanying claims, abstract and drawings), and / or all steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and / or steps are mutually exclusive. The invention is not restricted to the details of any foregoing embodiments. The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed. Modifications and improvements can be incorporated herein without departing from the scope of the invention as defined in the appended claims and their equivalents.

Claims

CLAIMS1. Apparatus for processing a bulk material, such as graphite, to separate layers of the bulk material, whereby to convert the bulk material into a laminar form, such as graphene, the apparatus comprising: a containment vessel for containing the bulk material during processing; at least one movable member located within the containment vessel and having a first surface; and a second surface within the containment vessel opposed to the first surface to form a gap between the first surface and the second surface, wherein the movable member is arranged to move the first surface across the second surface and thereby capture bulk material in the gap, whereby a shear force is generated across the bulk material in the gap by the relative movement of the first and second surfaces, which shear force acts to separate layers of the bulk material.

2. Apparatus as claimed in claim 1, wherein the first surface and / or the second surface is substantially smooth.

3. Apparatus as claimed in any preceding claim, wherein the second surface is an internal surface of the containment vessel.

4. Apparatus as claimed in any preceding claim, wherein the containment vessel comprises a cylindrical drum.

5. Apparatus as claimed in any preceding claim, wherein the movable member is arranged to move the first surface repeatedly across the second surface during processing of the bulk material.

6. Apparatus as claimed in claim 5, wherein the movable member is arranged for rotation about a rotational axis.

7. Apparatus as claimed in claim 6, wherein the rotational axis is substantially horizontal in use of the apparatus.

8. Apparatus as claimed in claim 4 in combination with claim 6 or 7, wherein the rotational axis is substantially colinear with the axis of the cylindrical drum.

9. Apparatus as claimed in any of claims 6 to 8 comprising a plurality of movable members equally distributed about the rotational axis.

10. Apparatus as claimed in claim 9, wherein the plurality of movable members forms a group of movable members located at substantially the same position along the rotational axis.

11. Apparatus as claimed in claim 10 comprising a plurality of groups of movable members, the groups being mutually spaced along the rotational axis.

12. Apparatus as claimed in claim 11, wherein the movable members of adjacent groups of movable members are mutually offset in the circumferential direction of the rotational axis and the movable members of adjacent groups are interdigitated so that the axial extent of the first surfaces of the movable members of adjacent groups overlap.

13. Apparatus as claimed in any preceding claim, wherein the movable member(s) comprise a resilient member arranged to urge the first surface towards the second surface.

14. Apparatus as claimed in any preceding claim, wherein the first surface comprises a high friction coating, such as an elastomer, for example silicone.

15. Apparatus as claimed in any preceding claim, wherein the first surface deviates away from the second surface towards a leading edge of the first surface whereby to guide bulk material into the gap.

16. Apparatus as claimed in any preceding claim, wherein the gap is less than 20 microns, preferably less than 10 microns.

17. Apparatus as claimed in any preceding claim, wherein the containment vessel is fixedly mounted to a chassis and the movable member(s) is / are arranged for movement relative to the fixed containment vessel.

18. Apparatus as claimed in claim 17, wherein the containment vessel comprises a closable loading port and a closable discharge port and the containment vessel is arranged on the chassis to be tiltable whereby to discharge the processed laminar material through the opened discharge port.

19. A method of operating the apparatus of any preceding claim comprising the steps of: loading the containment vessel with a bulk material; processing the bulk material using the apparatus to separate layers of the bulk material, whereby to convert the bulk material into a laminar form; and discharging the laminar material from the containment vessel to provide a product.

20. A method as claimed in claim 19, wherein the bulk material is processed within the containment vessel substantially dry.

21. A method as claimed in claim 19 or 20, wherein the containment vessel is loaded with the bulk material and the first surface is moved across the second surface repeatedly until the bulk material has been converted to the laminar form and the processed laminar material is then discharged from the containment vessel.

22. A method as claimed in any of claims 19 to 21, wherein the volume of bulk material loaded into the containment vessel is less than 30% of the internal volume of the containment vessel.

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