FLUIDIC EXFOLIATION

Abstract

The invention provides an apparatus for fluidic exfoliation of a layered material comprising: a housing of circular cross-section defined by a housing wall; a hollow rotor of circular cross-section having a first end and a second end and a wall positioned therebetween arranged concentrically within the housing, wherein the wall of the hollow rotor defines an inner chamber and the space in between the wall of the hollow rotor and the housing wall defines an outer chamber, and wherein a fluid flow path is provided between the inner chamber and the outer chamber; a fluid inlet in fluid communication with the inner chamber or the outer chamber; and a fluid outlet in fluid communication with the other of the inner chamber or the outer chamber; wherein the outer chamber has a width such that on passage of a fluid comprising the layered material from the inlet to the outlet through the outer chamber, a shear rate sufficient to exfoliate the layered material may be applied to the fluid comprising the layered material in the outer chamber by rotation of the hollow rotor.

Claims

1. An apparatus for fluidic exfoliation of a layered material comprising: a housing of circular cross-section defined by a housing wall; a hollow rotor of circular cross-section having a first end and a second end and a wall positioned therebetween arranged concentrically within the housing, wherein the wall of the hollow rotor defines an inner chamber and the-a space between the wall of the hollow rotor and the housing wall defines an outer chamber, and wherein a fluid flow path is provided between the inner chamber and the outer chamber; a fluid inlet in fluid communication with the inner chamber or the outer chamber; and a fluid outlet in fluid communication with the other of the inner chamber or the outer chamber; wherein the outer chamber has a width such that on passage of a fluid comprising the layered material from the inlet to the outlet through the outer chamber, a shear rate sufficient to exfoliate the layered material may be applied to the fluid comprising the layered material in the outer chamber by rotation of the hollow rotor.

2. The apparatus of claim 1, wherein the housing is in a fixed position; and/or wherein the outer chamber has a constant width throughout the apparatus.

3. (canceled)

4. The apparatus of claim 1, wherein the outer chamber has a width not exceeding about 1 cm.

5. The apparatus of claim 1, wherein the rotor is cylindrical; and/or wherein the housing wall is cylindrical.

6. (canceled)

7. The apparatus of claim 1, further comprising a pump arranged to drive the fluid comprising the layered material through the apparatus.

8. The apparatus of claim 1, further comprising a fluid reservoir in fluid communication with the fluid inlet for holding the fluid comprising the layered material.

9. The apparatus of claim 1, further comprising a motor configured to provide a rotational force to rotate the rotor.

10. The apparatus of claim 1, further comprising a source of heat to heat the fluid comprising the layered material passing through the apparatus.

11. The apparatus of claim 1, wherein the layered material is graphite, BN, GaTe, Bi.sub.2Se.sub.3, Bi.sub.2Te.sub.3, Sb.sub.2Te.sub.3, TiNC1, black phosphorus, layered silicate, layered double hydroxide or a transition metal chalcogenide having the formula MX.sub.n, wherein M is a transition metal selected from the group comprising Ti, Zr, Hf, V, Nb, Ta, Cr, Mn, Mo, W, Tc, Re, Ni, Pd, Pt, Fe and Ru, X is a chalcogen selected from the group comprising O, S, Se, and Te; and n is 1 to 3, or a combination thereof.

12. (canceled)

13. (canceled)

14. (canceled)

15. (canceled)

16. A process for fluidic exfoliation of a layered material using an apparatus as claimed in claim 1, comprising: introducing the fluid comprising the layered material through the fluid inlet; and applying a shear rate to the layered material by rotating the rotor at a speed sufficient to exfoliate the layered material.

17. A process for fluidic exfoliation of a layered material using an apparatus as claimed in claim 1, comprising: introducing the fluid comprising the layered material through the fluid inlet; passing the fluid into the inner chamber; passing the fluid through the fluid flow path to the outer chamber; passing the fluid from the outer chamber to the fluid outlet; wherein the rotor is rotating at a speed sufficient to apply a shear rate to exfoliate the layered material.

18. A process for fluidic exfoliation of a layered material using an apparatus as claimed in claim 1, comprising: introducing the fluid comprising the layered material through the fluid inlet; passing the fluid into the outer chamber; passing the fluid from the outer chamber through the fluid flow path to the inner chamber; passing the fluid from the inner chamber to the fluid outlet; wherein the rotor is rotating at a speed sufficient to apply a shear rate to exfoliate the layered material.

19. The process of claim 16, wherein the process is a continuous process.

20. (canceled)

21. (canceled)

22. The process of claim 16, wherein the shear rate applied is greater than about 1000 s.sup.1.

23. The process of claim 16, further comprising heating the fluid comprising the layered material while the fluid is in the apparatus and/or prior to introducing the fluid into the apparatus.

24. The process of claim 16, wherein the fluid comprises particles of the layered material; and/or wherein the fluid comprises about 0.1 to about 15 wt % of the layered material calculated as a total weight of the fluid and layered material.

25. (canceled)

26. The process of claim 16, wherein the layered material is graphite, BN, GaTe, Bi.sub.2Se.sub.3, Bi.sub.2Te.sub.3, Sb.sub.2Te.sub.3, TiNC1, black phosphorus, layered silicate, layered double hydroxide (such as Mg.sub.6Al.sub.2(OH).sub.16) or a transition metal chalcogenide having the formula MX.sub.n, wherein M is a transition metal selected from the group comprising Ti, Zr, Hf, V, Nb, Ta, Cr, Mn, Mo, W, Tc, Re, Ni, Pd, Pt, Fe and Ru, X is a chalcogen selected from the group comprising O, S, Se, and Te and n is 1 to 3, or a combination thereof.

27. (canceled)

28. (canceled)

29. The process of claim 16, wherein the fluid comprises an organic solvent, for example selected from the group consisting of N-methyl pyrrolidone (NMP), cyclohexylpyrrolidone, di-methyl formamide, cyclopentanone (CPO), cyclohexanone, N-formyl piperidine (NFP), vinyl pyrrolidone (NVP), 1,3-dimethyl-2-imidazolidinone (DMEU), bromobenzene, benzonitrile, N-methyl-pyrrolidone (NMP), benzyl benzoate, N,N-dimethylpropylene urea, (DMPU), gamma-butrylactone (GBL), Dimethylformamide (DMF), N-ethyl-pyrrolidone (NEP), dimethylacetamide (DMA), cyclohexylpyrrolidone (CHP), dimethyl sulfoxide (DMSO), dibenzyl ether, chloroform, isopropylalcohol (IPA), cholobenzene, l-octyl-2-pyrrolidone (N8P), 1-3 dioxolane, ethyl acetate, quinoline, benzaldehyde, ethanolamine, diethyl phthalate, N-dodecyl-2-pyrrolidone (N12P), pyridine, dimethyl phthalate, formamide, vinyl acetate and acetone or a combination thereof.

30. The process of claim 16, wherein the fluid further comprises: a polymer selected from polyvinyl alcohol (PVA), polybutadiene (PBD), poly(styrene-co-butadiene) (PBS), polystyrene (PS), polyvinylchloride (PVC), polyvinylacetate (PVAc), polycarbonate (PC), polymethylmethacrylate (PMMA), polyvinylidene chloride (PVDC) and cellulose acetate (CA); and/or a surfactant selected from the group comprising consisting of sodium cholate (NaC), sodium dodecylsulphate (SDS), sodium dodecylbenzenesulphonate (SDBS), lithium dodecyl sulphate (LDS), sodium cholate (SC), sodium deoxycholate (DOC), sodium taurodeoxycholate (TDOC), polyoxyethylene (40) nonylphenyl ether, branched (IGEPAL CO-890 (IGP)), polyethylene glycol p-(1,1,3,3-tetramethylbutyl)-phenyl ether (Triton-X 100 (TX-100)), cetyltrimethyl ammoniumbromide (CTAB), tetradecyltrimethylammonium bromide (TTAB), Tween 20 and Tween 80.

31. The process of claim 16, wherein the fluid is a printable ink composition or a polymer or copolymer selected from a thermoplastic, a thermoset, an elastomer and a biopolymer or a combination thereof.

32. The process of claim 16, wherein the exfoliated layered material is removed from the fluid, optionally by low-speed centrifugation, gravity settling, filtration or flow separation.

33. The process of claim 32, further comprising the step of placing the exfoliated layered material into a matrix to form a composite.

34. (canceled)

35. (canceled)

Description

BRIEF SUMMARY OF FIGURES

[0073] FIG. 1 shows an apparatus for fluidic exfoliation of a layered material.

[0074] FIG. 2 shows Transmission Electron Microscopy images of exfoliated graphene.

[0075] FIG. 3 shows a section of an apparatus illustrating the housing, the rotor and the outer chamber.

[0076] FIG. 4 shows an apparatus for direct fluidic exfoliation of a layered material into a matrix comprising a heat source.

[0077] FIG. 5 shows an apparatus for fluidic exfoliation of a layered material highlighting the inner and outer chambers (numbering corresponds to the numbering of FIG. 1).

[0078] FIG. 6 shows graphene concentration over a fluidic exfoliation processing time of 10 hours.

[0079] FIG. 7 shows the production rate of graphene over a fluidic exfoliation processing time of 10 hours.

[0080] FIG. 8 shows the average number of layers over time for the graphene produced in FIG. 6.

[0081] FIG. 9 shows the Raman shift for a fluidic exfoliated graphene product.

[0082] FIG. 10 shows a Transmission Electron Microscopy image of an exfoliated graphene nanosheet produced from a fluidic exfoliation process.

DETAILED DESCRIPTION

[0083] The invention provides an apparatus for continuous fluidic exfoliation of a layered material comprising: [0084] a housing of circular cross-section defined by a housing wall; [0085] a hollow rotor of circular cross-section having a first end and a second end and a wall positioned therebetween arranged concentrically within the housing, wherein the wall of the hollow rotor defines an inner chamber and the space between the wall of the hollow rotor and the housing wall defines an outer chamber, and wherein a fluid flow path is provided between the inner chamber and the outer chamber; [0086] a fluid inlet in fluid communication with the inner chamber or the outer chamber; and [0087] a fluid outlet in fluid communication with the other of the inner chamber or the outer chamber; [0088] wherein the outer chamber has a width such that on passage of a fluid comprising the layered material from the inlet to the outlet through the outer chamber, a shear rate sufficient to exfoliate the layered material may be applied to the fluid comprising the layered material in the outer chamber by rotation of the hollow rotor.

[0089] The rotation of the hollow rotor relative to the housing simultaneously creates two fluidic zones. The first fluidic zone is in the inner chamber within the hollow rotor. An axially centred vortex provides the initial (stage 1) mixing and shearing of the fluid comprising the layered material. An external pump may be used to drive this fluid through the fluid flow path towards the top of the inner chamber at a user-specified flow rate. The fluid leaves the inner chamber and enters the outer chamber between the wall of the rotor and the housing wall. This annular gap is the second fluidic zone. The motion of the rotating rotor, relative to the housing, generates higher mixing and shearing forces (stage 2). Circumferential vortices, known as Taylor vortices, are generated within this fluid gap when the Taylor number exceeds a certain critical value (Ta>Ta.sub.c) that depends on the width of the outer chamber, the radius of housing and the relative rotational speed of the rotor (Ta and Ta.sub.c may be calculated using the Equation 8, where Ta.sub.c is the Taylor number when the Reynolds number is at the critical value of about 95). Particles of the layered material to be exfoliated are transported along the streamlines of these well-controlled vortices. The result is homogeneous mixing and shearing of the layered material. The flow rate of the pump can be adjusted independently to the exfoliator rotational speed. Hence, the residence time of a particle of the layered material can be set to anything from seconds to infinite time (i.e., pump at zero flow rate).

[0090] Of course, as would be appreciated by a skilled person, the flow may be reversed and the fluid may pass through the outer chamber before the inner chamber during operation of the device.

[0091] The Taylor number of system may exceed a critical value that depends on the width of the outer chamber, the radius of housing and the relative rotational speed of the rotor. For example, the shear rate applied to the layered material during operation of the device may be at a rate greater than about 1000 s.sup.1. Preferably, the shear rate may be greater than about 10000 s.sup.1. The maximum shear rate is preferably applied to the layered material in the outer chamber. For example, where the layered material is graphene, a shear rate of {dot over ()}.sub.tam110.sup.4 s.sup.1 in the outer chamber may be applied.

[0092] Where the rotor and housing are cylindrical, the radius ratio between the outer radius of the inner cylindrical rotor () and the housing radius (r.sub.o) (i.e., the outer chamber) is:

[00001]$\begin{array}{cc}=\frac{r_i}{r_o}& \left(1\right)\end{array}$

[0093] The shear rate in a laminar Taylor-Couette flow is

[00002]${\stackrel{.Math.}{}}_{\mathrm{lam}}=r\left(\frac{d.Math..Math.{}_{\mathrm{lam}}}{\mathrm{dr}}\right),$

where .sub.tam=u.sub.74 /r is the angular laminar azimuthal velocity, r is the radius and u.sub.74 is the laminar azimuthal velocity. When approaches unity, this shear rate can be estimated for a inner cylindrical rotor with stationary outer cylindrical housing by:

[00003]$\begin{array}{cc}{\stackrel{.}{}}_{\mathrm{lam}}\frac{r_i.Math.{}_i}{d}& \left(2\right)\end{array}$

[0094] The shear rate defined above scales with three parameters: r.sub.i, d and .sub.i, the outer radius of the inner cylindrical rotor, the outer chamber width and the cylindrical rotor relative rotational speed respectively. As would be appreciated by a skilled person, it can, therefore, be increased by increasing the rotor radius, rotor rotational speed (where housing is fixed), and/or decreasing the outer chamber width. The apparatus will be a trade-off between all three parameters. For example, using a gap of 2 mm and inner cylindrical rotor radius of 50 mm, a rotational speed of 3820 r.p.m. may be required to achieve a shear rate of at least 1000 s.sup.1. Increasing the gap to 3 mm and a speed of 5730 r.p.m. may be necessary to achieve a shear rate of at least 1000 s.sup.1. Millimeter scale outer chamber widths have been considered and found to work best, as this prevented blockages of the precursor. For example, the outer chamber may have a width of less than about 1 cm.

[0095] The above shear rate calculation is for laminar flow. If the apparatus is operated in a transitional or turbulent flow regime, additional stresses within the fluid may be generated by the formation of fluid structures, such as eddies. This may increase the shearing on the layered material. Thus, the laminar equations described herein may be used to calculate the minimum shear rate that the apparatus may generate. The Reynolds number may be used to determine the flow regime of the apparatus (Equation 7).

[0096] Where the outer chamber has variable width (for example, if the housing is conical shaped and the rotor is cylindrical), the average outer chamber width may be used to determine the average shear rate. The minimum outer chamber width may be used to determine the maximum shear rate and the maximum outer chamber width may be used to determine the minimum shear rate.

[0097] Inner Chamber (Stage 1)Reynolds Numbers

[0098] The cylindrical rotor defines the inner chamber. The outer radius of the rotor has been defined as r.sub.i above. The internal radius of this cylindrical rotor (i.e., the radius of the inner chamber), is defined herein as r.sub.ii. This radius also impacts the initial, Stage 1 mixing/shearing. The level of mixing within the inner chamber depends on the rotating Reynolds number:

[00004]$\begin{array}{cc}R.Math.e_r=\frac{{}_i.Math.D_{i.Math.i}^2}{2.Math.v}& \left(3\right)\end{array}$

[0099] where D.sub.ii=2r.sub.ii and v is the kinematic viscosity of the fluid. Kinematic viscosity may be determined using, for example, a glass capillary kinematic viscometer. Standard methods for determining kinematic viscosity are set out in ASTM D445-17a (Standard Test

[0100] Method for Kinematic Viscosity of Transparent and Opaque Liquids) and ASTM D446-12 (Standard Specifications and Operating Instructions for Glass Capillary Kinematic Viscometers). For example, kinematic viscosity may be determined by measuring the time for a volume of fluid to flow under gravity through a calibrated glass capillary viscometer. The kinematic viscosity is the product of the measured flow time and the calibration constant of the viscometer. Viscometers shall be mounted in the constant temperature bath in the same manner as when calibrated and stated on the certificate of calibration of the viscometer.

[0101] When the pump is set at zero flow rate, the transport phenomena (heat/mass) scales with this Reynolds number to an exponent that depends on the flow regime (laminar/turbulent) such that Re.sub.r.sup.b, where b is the exponent typically 0.5-1.0 (see A. Bejan, Convection Heat Transfer, (2004) 3.sup.rd Ed., Wiley). When the pump continuously delivers fluid into the device, an additional axial Reynolds number is necessary to correlate the influence of a continuous flow on transport phenomena in the inner chamber (Re.sub.a,1.sup.b):

[00005]$\begin{array}{cc}R.Math.e_{a,1}=\frac{4.Math.\stackrel{.}{Q}}{.Math.D_{i.Math.i}.Math.v}& \left(4\right)\end{array}$

[0102] where {dot over (Q)} is the volumetric flow rate delivered by the pump.

[0103] Outer Chamber (Stage 2)Reynolds and Taylor Numbers

[0104] Another consideration is providing a production approach which is inherently repeatable. Although turbulent flows provide additional stresses that enhance exfoliation, the stochastic nature of high levels of turbulence may have an adverse effect on production repeatability to an extent. It is preferable, therefore, to exfoliate layered materials at reasonably low Reynolds numbers, where the fluid motion is inherently repeatable (i.e. laminar). For example, the Reynolds number may less than about 210.sup.4. These flow regimes can be described using the relationship for Reynolds number in a Taylor-Couette flow arrangement:

[00006]$\begin{array}{cc}R.Math.e=\frac{2}{1+}.Math..Math..Math..Math.R_o-R_i.Math.& \left(5\right)\end{array}$

[0105] where R.sub.i and R.sub.o are the inner and outer chamber Reynolds numbers:

[00007]$\begin{array}{cc}{R}_i=\frac{r_i.Math.{}_i.Math.d}{v};.Math.R_o=\frac{r_o.Math.{}_o.Math.d}{v}& \left(6\right)\end{array}$

[0106] The apparatus preferably has a stationary cylindrical housing (.sub.o=0). This reduces the general Reynolds number definition, Re, to:

[00008]$\begin{array}{cc}R.Math.e=\frac{2}{1+}.Math.R_i& \left(7\right)\end{array}$

[0107] Taylor vortices exist due to inertial instabilities that occur beyond a critical condition. These vortices are useful for mixing the fluid, ensuring that particles of the layered material experience a similar shear (integrated over time). The occurrence of these vortices is defined by the Taylor number:

[00009]$\begin{array}{cc}T.Math.a=4.Math.R.Math.e^2\left(\frac{1-}{1+}\right)& \left(8\right)\end{array}$

[0108] Equations (7) and (8) describe the rotational parameters for the outer chamber. When a fluid is continuously passed through the device, the axial Reynolds number is described as:

[00010]$\begin{array}{cc}R.Math.e_{a,2}=\frac{2.Math.\stackrel{.}{Q}}{.Math..Math.r_i\left(1+\frac{1}{}\right).Math.v}& \left(9\right)\end{array}$

[0109] where {dot over (Q)} is the volumetric flow rate delivered by the pump.

[0110] It is worth noting that Stage 1 & Stage 2 mixing/exfoliation regions are coupled, when comparing the equations 2-4 & 7-9. For example, increasing rotational speed will increase shear rates and Reynolds numbers in both the inner hollow cylindrical rotor and fluid gap between inner rotor and outer housing. Conversely, decreasing speed decreases the shearing/mixing intensity.

[0111] Outer Chamber (Stage 2)Dimensionless Torque

[0112] When using the apparatus with a range of different fluids, or entirely new fluids, it can be challenging to predict the flow regime within the device (i.e. laminar/transitional/turbulent). This flow regime, however, can be determined by monitoring the torque characteristics of the device. The dimensionless torque describes this:

[00011]$\begin{array}{cc}G=\frac{T}{.Math.H.Math.v^2}& \left(10\right)\end{array}$

[0113] where T is the torque, and H is the height of the outer chamber between the housing wall and the rotor wall. The dimensionless torque scales with radius ratio (equation 1) and Reynolds number (equation 5). This scaling depends on the flow regime of the device. Three torque regimes have been classified for a system with rotating inner cylindrical rotor and a stationary cylindrical housing, and include: laminar, transitional, soft turbulence and hard turbulence. For laminar flow (Re<Re.sub.c):

[00012]$\begin{array}{cc}{G}_i=G_{\mathrm{lam}}=\frac{2.Math.}{{\left(1-\right)}^2}.Math..Math.R.Math.e& \left(11\right)\end{array}$

[0114] As Reynolds number increases beyond a critical point (Re.sub.c95), where viscosity can no longer dampen instabilities in a supercritical case, Taylor vortices occur. This has been shown to occur at Re95 (see D. P. Lathrop et al., Physical Review A, 46 (1992), 6930).

[0115] There is a change in the torque scaling for this regime (Re.sub.cReRe.sub.T):

[00013]$\begin{array}{cc}{G}_i=\frac{{\left(3+\right)}^{1/4}.Math..Math.R.Math.e^{3/2}}{{\left(1-\right)}^{7/4}.Math.{\left(1+\right)}^{1/2}}& \left(12\right)\end{array}$

[0116] Within this regime, the onset of soft turbulence may occur at Re1.510.sup.4. Finally, another transitional point in torque scaling has been observed at larger Reynolds numbers (Re>Re.sub.T). This hard turbulence transitional point has been associated with a featureless turbulence regime and occurs beyond Re10.sup.5. The apparatus is preferably not intended to operate in this regime, however, the inclusion of inner cylindrical rotor roughness/microscale geometric features could lead to this featureless effect at lower Re than the classical case. Dimensionless torque in a hard turbulence regime and rough walls is:

[00014]$\begin{array}{cc}{G}_i=0.1.Math.0.Math.7.Math.\frac{\sqrt{3+}}{\left(1+\right).Math.{\left(1-\right)}^{3/2}}.Math.{\left(.Math.R.Math.e\right)}^2& \left(13\right)\end{array}$

[0117] The dimensionless torque in a hard turbulence regime and smooth walls is:

[00015]$\begin{array}{cc}{G}_i=0.3.Math.3.Math.\frac{\sqrt{3+}}{\left(1+\right).Math.{\left(1-\right)}^{3/2}}.Math.\frac{{\left(.Math.R.Math.e\right)}^2}{{\left(\ln \left[\left(M\left(\right).Math.{\left(.Math.R.Math.e\right)}^2\right)\right]\right)}^{3/2}}& \left(14\right)\\ \mathrm{where}.Math..Math.M\left(\right)=0.0.Math.0.Math.0.Math.1.Math.\frac{\left(1-\right).Math.\left(3+\right)}{{\left(1+\right)}^2}& \left(15\right)\end{array}$

[0118] Flows transitioning to turbulence can be observed when

[00016]$\frac{G_i}{G_{\mathrm{la}.Math..Math.m}}1.$

[0119] Outer Chamber (Stage 2)Shear Rate for Exfoliation

[0120] When using the apparatus with a range of different fluids, or entirely new fluids, the flow regime (i.e. laminar/transitional/turbulent) influences the shear rate that is generated by the device. This shear rate can be determined using the relationship between wall shear stress and dimensionless torque. The shear rate in the outer chamber is:

[00017]$\begin{array}{cc}\stackrel{.}{}=\frac{G_i.Math.v}{2.Math..Math..Math.r_o^2}& \left(16\right)\end{array}$

[0121] where the selection of G.sub.i (Equations 11-15) is dependent on the operating Reynolds number for the device (Equation 7).

[0122] Properties of Fluids Used in Exfoliation

[0123] The equations that describe the fluid motion within the device have been outlined above. Viscosity and density fluid properties are included in these expressions. By adjusting the parameters in the equations above, the device can be operated to provide the necessary shear rate conditions and flow regimes with any working fluid. This results in a broad/robust approach. Fluids particularly suited to exfoliation and long-term dispersion have been previously defined as having a surface tension which is close to that of the material being exfoliated (see Y. Hernandez et al., Langmuir, 26 (2010), 3208-3213, the contents of which are herein incorporated by reference in their entirety).

[0124] In the apparatus, use or process of the invention, the fluid may comprise particles of the layered material. The fluid may comprise up to about 15 wt % of the layered material calculated as a total weight of the fluid and layered material, preferably about 0.1 to about 15 wt %, preferably about 1 to about 10 wt %, preferably about 5 wt %.

[0125] As used herein, particles may have an average maximum dimension of less than about 500 m, preferably less than about 400 m, preferably less than about 300 m, preferably less than about 200 m, preferably less than about 150 m. It would be appreciated by a skilled person that depending on morphology of the particulate material, the average maximum dimension may be an average diameter or, for example in the case of platelets, an average lateral dimension. It would also be appreciated by a skilled person that, depending on the type of particle, the average diameter may be determined by any of the methods described herein.

[0126] Particles may be provided in the form of platelets or flakes (used interchangeably) of the layered material. The flakes may have an average thickness of for example up to about 10 m, preferably about 100 nm. The flakes may have an average lateral dimension (maximum diameter) of up to about 1000 m, preferably about 500 m. The average lateral dimension and average thickness is the arithmetic mean of the lateral dimension and thickness, respectively. The lateral dimensions of the layered material flakes may be measured using optical and/or scanning electron microscopy. The thickness may be determined using atomic force microscopy or transmission electron microscopy.

[0127] Alternatively, particles may be provided as a powder, for example having an average particle diameter of about 1 to about 500 m. As used herein, average particle diameter refers to the modal value of a particle diameter distribution, for example the modal intensity count value of a distribution of particle diameters measured by dynamic light scattering (DLS) using a light scattering detector, for example that of a Zetasizer pV instrument (Malvern, UK). Intensity counts are the first order output for samples measured by dynamic light scattering (DLS) using a light scattering detector. For example, particle diameters may be determined by diluting a dispersed particle sample in an aqueous solvent sufficiently to allow DLS to be applied, using a Zetasizer pV instrument (Malvern, UK). Other methods such as laser diffraction or sedimentation may alternatively be used.

[0128] The layered material may be graphite, boron nitride (BN), gallium telluride (GaTe), bismuth selenide (Bi.sub.2Se.sub.3), bismuth telluride (Bi.sub.2Te.sub.3), antimony telluride (Sb.sub.2Te.sub.3), titanium nitride chloride (TiNCI), black phosphorus, layered silicates, layered double hydroxides (such as Mg.sub.6Al.sub.2(OH).sub.16) or a transition metal chalcogenide having the formula MX.sub.n, wherein M is a transition metal, X is a chalcogen and n is 1 to 3, or a combination thereof. M may be selected from the group comprising Ti, Zr, Hf, V, Nb, Ta, Cr, Mn, Mo, W, Tc, Re, Ni, Pd, Pt, Fe and Ru; and X may be selected from the group comprising O, S, Se, and Te. Exemplary metal chalcogenides include molybdenum disulfide (MoS.sub.2) and molybdenum trioxide (MoO.sub.3). Further layered materials that may be used in the present invention are disclosed in V. Nicolosi et al., Science, 340 (2013), 1420.

[0129] The fluid may be selected from any suitable solvent or polymer. Solvents with a surface tension which is close to that of the exfoliated material have been determined to be most likely to give the best exfoliation and dispersion performance. For example, for graphene dispersion, the best solvents may have a surface tension from about 30 to about 50 mJ m.sup.2. Surface tension may be determined using a tensiometer as set out in ASTM Standard D1331-14. For example, surface tension may be determined using du Noy ring (platinum wire ring) methods or Wilhelmy plate (flat, thin plate made of glass or platinum) methods

[0130] The fluid may be an organic solvent, for example N-methyl pyrrolidone (NMP), cyclohexylpyrrolidone, di-methyl formamide, cyclopentanone (CPO), cyclohexanone, N-formyl piperidine (NFP), vinyl pyrrolidone (NVP), 1,3-dimethyl-2-imidazolidinone (DMEU), bromobenzene, benzonitrile, N-methyl-pyrrolidone (NMP), benzyl benzoate, N,N-dimethylpropylene urea, (DMPU), gamma-butrylactone (GBL), dimethylformamide (DMF), N-ethyl-pyrrolidone (NEP), dimethylacetamide (DMA), cyclohexylpyrrolidone (CHP), dimethyl sulfoxide (DMSO), dibenzyl ether, chloroform, isopropylalcohol (IPA), cholobenzene, l-octyl-2-pyrrolidone (N8P), 1-3 dioxolane, ethyl acetate, quinoline, benzaldehyde, ethanolamine, diethyl phthalate, N-dodecyl-2-pyrrolidone (N12P), pyridine, dimethyl phthalate, formamide, vinyl acetate or acetone or a combination thereof.

[0131] The fluid may further comprise a polymer, for example selected from polyvinyl alcohol (PVA), polybutadiene (PBD), poly(styrene-co-butadiene) (PBS), polystyrene (PS), polyvinylchloride (PVC), polyvinylacetate (PVAc), polycarbonate (PC), polymethylmethacrylate (PMMA), polyvinylidene chloride (PVDC) and cellulose acetate (CA).

[0132] The fluid may further comprise a surfactant, for example selected from the group comprising sodium cholate (NaC), sodium dodecylsulphate (SDS), sodium dodecylbenzenesulphonate (SDBS), lithium dodecyl sulphate (LDS), sodium cholate (SC), sodium deoxycholate (DOC), sodium taurodeoxycholate (TDOC), polyoxyethylene (40) nonylphenyl ether, branched (IGEPAL CO-890 (IGP)), polyethylene glycol p-(1,1,3,3-tetramethylbutyl)-phenyl ether (Triton-X 100 (TX-100)), cetyltrimethyl ammoniumbromide (CTAB), tetradecyltrimethylammonium bromide (TTAB), Tween 20 and Tween 80. Further surfactants that may be used in the present invention are disclosed in R. J. Smith et al., New Journal of Physics 12 (2010) 125008.

[0133] Exfoliated graphene or exfoliated boron nitride nanosheets produced by a process of the present invention, for example, may be used for the mechanical reinforcement of polymers, to reduce the permeability of polymers, to enhance the conductivity (electrical and thermal) of polymers, and to produce transparent conductors and electrode materials.

[0134] The layered material may therefore be directly exfoliated into a matrix material such as a polymer. Accordingly, the fluid may therefore be a suitable matrix material such as a printable ink composition or a polymer or copolymer, for example selected from a thermoplastic, a thermoset, an elastomer or a biopolymer or a combination thereof.

[0135] As used herein, printable ink compositions are composition suitable for use as a printing ink and include inks suitable for use in 3D printing techniques.

[0136] The term polymer refers to a compound composed of repeating units, or a salt thereof. These units are typically connected by covalent chemical bonds. A polymer preferably comprises at least 10, at least 20, at least 50, at least 100 units or at least 200 units. A polymer may be terminated by any group, for example hydrogen. A polymer may be a homopolymer or a copolymer. Although the term polymer is sometimes taken to refer to plastics, it actually encompasses a large class comprising both natural and synthetic materials with a wide variety of properties. Such polymers may be thermoplastics, elastomers, or biopolymers.

[0137] The term copolymer should be understood to mean a polymer derived from two (or more) monomeric species, for example a combination of any two of the below-mentioned polymers. An example of a copolymer, but not limited to such, is PETG (polyethylene terephthalate glycol), which is a PET modified by copolymerization. PETG is a clear amorphous thermoplastic that can be injection moulded or sheet extruded and has superior barrier performance used in the container industry. The term thermoset should be understood to mean materials that are made by polymers joined together by chemical bonds, acquiring a highly cross-linked polymer structure. The highly cross-linked structure produced by chemical bonds in thermoset materials is directly responsible for the high mechanical and physical strength when compared with thermoplastics or elastomers materials.

[0138] The polymer may be a thermoplastic which may be selected from, but not limited to, the group comprising acrylonitrile butadiene styrene, polypropylene, polyethylene, polyvinylchloride, polyamide, polyester, acrylic, polyacrylic, polyacrylonitrile, polycarbonate, ethylene-vinyl acetate, ethylene vinyl alcohol, polytetrafluoroethylene, ethylene ch lorotrifluoroethylene, ethylene tetrafluoroethylene, liquid crystal polymer, polybutadiene, polychlorotrifluoroethylene, polystyrene, polyurethane, and polyvinyl acetate.

[0139] The polymer may be a thermoset which may be selected from, but not limited to, the group comprising vulcanised rubber, Bakelite (polyoxybenzylmethylenglycolanhydride), urea-formaldehyde foam, melamine resin, polyester resin, epoxy resin, polyimides, cyanate esters or polycyanurates, silicone, and the like known to the skilled person.

[0140] The polymer may be an elastomer which may be selected from, but not limited to, the group comprising polybutadiene, butadiene and acrylonitrile copolymers (NBR), natural and synthetic rubber, polyesteramide, chloropene rubbers, poly(styrene-b-butadiene) copolymers, polysiloxanes (such as Polydimethylsiloxane (PDMS)), polyisoprene, polyurethane, polychloroprene, chlorinated polyethylene, polyester/ether urethane, poly ethylene propylene, chlorosulfanated polyethylene, polyalkylene oxide and mixtures thereof.

[0141] The polymer may be a biopolymer which may be selected from, but not limited to, the group comprising gelatin, lignin, cellulose, polyalkylene esters, polyvinyl alcohol, polyamide esters, polyalkylene esters, polyanhydrides, polylactide (PLA) and its copolymers and polyhydroxyalkanoate (PHA).

[0142] The polymer may be a copolymer selected from, but not limited to, the group comprising copolymers of propylene and ethylene, acetal copolymers (polyoxymethylenes), polymethylpentene copolymer (PMP), amorphous copolyester (PETG), acrylic and acrylate copolymers, polycarbonate (PC) copolymer, styrene block copolymers (SBCs) to include poly(styrene-butadiene-styrene) (SBS) , poly(styrene-isoprene-styrene) (SIS), poly(styrene-ethylene/butylene-styrene) (SEBS), ethylene vinyl acetate (EVA) and ethylene vinyl alcohol copolymer (EVOH) amongst others.

[0143] Apparatus Residence Time

[0144] The shear experienced by the layered material to be exfoliated is primarily governed by the parameters described above. The time a particle stays within the apparatus, under the influence of this shear/mixing, is controlled by the external pump flow rate, {dot over (Q)}. For example, particles of a layered material introduced into the apparatus can be kept there indefinitely by setting the pump flow rate to zero. Conversely, short residence times can be achieved by setting high flow rates.

[0145] The housing of the apparatus of the invention may be cylindrical such that the housing and the rotor may be arranged as concentric cylinders. Alternatively, the housing may have a conical shape.

[0146] As used herein, a cylinder or a cylindrical object, is a 3-dimensional geometric object having two ends and a constant circular cross section (i.e., which is the same from one end to the other) with a curved side wall provided between the two ends.

[0147] As used herein, a cone or conical object, is a 3-dimensional object having a circular cross-section and two ends and a curved side wall, where the radius of the cross-section is largest at one end and decreases to the other end such that, at the other end, the curved wall ends in an apex point. Thus, the other end is an apex. A cone is preferably a right cone, where the apex is aligned directly above the center of the cross-section of the cone. A cone or conical, as used herein includes a frustum of a cone, where the apex has been cut-off to leave a circular other end.

[0148] The housing may comprise a first end and a second end, with the housing wall provided therebetween, arranged in the same orientation as the first and second end of the rotor. The apparatus may further comprise a base at the second end of the housing. As would be appreciated by a skilled person, during operation of the apparatus, the apparatus is sealed at the second end of the housing. The seal at the second end of the housing may form part of the housing.

[0149] In the apparatus of the invention, the fluid flow path between the inner and outer chambers may be provided at the first end of the rotor, which during operation of the apparatus is towards the top of the apparatus. The fluid inlet and outlets to the apparatus into the inner and outer chambers may be located at the second end of the rotor, which during operation of the apparatus is below the fluid flow path. Thus, during operation of the apparatus, both the inflow and outflow of the fluid are positioned towards the bottom of the apparatus. Where, for example, the fluid inlet is in fluid communication with the inner chamber, this configuration delivers the unexfoliated layered material to the inside of the rotor and against gravity. This advantageously eliminates layered material particle build-up that could lead to a flow blockage. Both the inlet and outlet are provided at the second end of the rotor, i.e., below the fluid flow path, thus, the flow directions can be easily reversed so the fluid is introduced into the device through inlet into the outer chamber against gravity. This makes it robust to different mixing/shearing needs of the user.

[0150] Embodiments are now described by way of non-limiting example to illustrate aspects and principles of the disclosure, with reference to the accompanying drawings.

[0151] With reference to FIG. 1, there is provided an apparatus 100 for the continuous exfoliation of a layered material. The apparatus comprises a housing 101 of circular cross-section, a rotor of circular cross-section 102 arranged concentrically within the housing. The rotor defines an inner chamber 103 and the space between the rotor and the housing defines an outer chamber 104. A fluid flow path 105 is provided between the inner chamber and the outer chamber. The apparatus also comprises a fluid inlet 107 in fluid communication with the inner chamber and a fluid outlet 108 in fluid communication with the outer chamber. The outer chamber has a width 106 of about 3 mm. The apparatus further comprises a motor and shaft 109 configured to rotate the rotor.

[0152] The following steps outline an exemplary process for exfoliating a layered material using an apparatus as shown in FIG. 1: [0153] 1. Flakes of a layered material (graphite, Sigma-Aldrich 332461) having average lateral dimension of about 150 m and organic solvent (N-Methyl-2-Pyrrolidone, VWR 26211.425) was placed in a reservoir (80 mL) at a fixed concentration (50 g/L). [0154] 2. A peristaltic pump, located between the reservoir and the apparatus inlet, was initially run at a low pump flow rate (10 mL min.sup.1). This slowly moved the fluid into the apparatus for bleeding of the system. [0155] 3. At this flow rate, air was removed from within the apparatus, to ensure the entire fluid loop was without trapped air during the exfoliation operation. [0156] 4. Once this was completed, the motor connected to the rotor was switched on and rotated at speed that resulted in exfoliation (8000 r.p.m.) [0157] 5. Maintaining a constant rotor rotational speed, the fluid was circulated from the reservoir to the device using the peristaltic pump (20 mins at 50 mL min.sup.1). [0158] 6. The exfoliated fluid was then removed from the reservoir and any remaining layered material was allowed to sediment (sedimentation conditions were 24 hrs at 1 g) [0159] 7. Mono- and few-layer material (graphene) was decanted and tested with Transmission Electron Microscopy to examine the characteristics of the 2d materials produced. FIG. 2 shows Transmission Electron Microscopy images of the graphene product. FIG. 2, top image shows graphene mono-layers with a sheet length of approximately 1-2 m, supported on a holey carbon grid. The bottom image shows graphene multi-layer sheets with a sheet length of approximately 1-2 m, supported on a holey carbon grid. The shear rate was determined to be 110.sup.4 s.sup.1, Re=1.810.sup.4, and Ta=1.4910.sup.8).

[0160] It will be appreciated that the values in the above example will change depending on the scale of the apparatus for production and production requirements.

[0161] With reference to FIG. 3, there is provided apparatus 100, wherein the housing has a conical shape and the rotor has cylindrical shape creating a tapered outer chamber where the width of the outer chamber is smaller at the top of the device near the fluid flow path than at the bottom of the device near the fluid outlet.

[0162] With reference to FIG. 4, there is provided an apparatus 400 for the continuous exfoliation of a layered material. This embodiment demonstrates the homogeneous heat transport capability of the inventive apparatus. Situations can occur where material heating or cooling is necessary during production. For example, this invention can enable the direct exfoliation and dispersion of 2D materials (and other 0D/1D materials) into a matrix material, for example a polymer. This has unique benefits, including the removal of complex processing steps currently involved in composite production. Surface heating/cooling is imposed on the housing. This can be introduced using flexible heater mats 401 (i.e. for heating only), or an outer heating/cooling jacket 301, which circulates hot/cold coolant. The millimetre-scale outer chamber 106 (3mm), in combination with the numerous vortices outside and inside the rotating rotor 102 during operation, provides a low convective-diffusive thermal resistance between the heat source and the product. A basic estimate of the thermal resistance, R.sub.th, is 0.06 K/W by extending heat transport correlations in the literature to this invention (and assuming a Prandtl number of 10) (see S. Seghir-Ouali et al., Int. J. Thermal Sciences, 45 (2006), 1166-1178 and S. Poncet et al., Int. J. Heat Fluid Flow, 32 (2010), 128-144, the contents of which are herein incorporated by reference in their entirety). This low value of thermal resistance demonstrates that near homogeneous heating/cooling of the product will occur. This invention also exfoliates and disperses small volumes (100 mL) in a continuous manner. The heat capacity of the exfoliator at zero mass flow rate is, therefore, also small. Rapid heating and cooling of the product is enabled, intensifying the exfoliation and dispersion characteristics by adjusting the thermophysical properties (i.e. viscosity) and/or processing solid polymer granules/pellets through a change of phase.

[0163] Throughout the description and claims of this specification, the words comprise and contain and variations of the words, for example comprising and comprises, mean including but not limited to, and are not intended to (and do not) exclude other components.

[0164] It will be appreciated that variations to the foregoing embodiments of the invention can be made while still falling within the scope of the invention. Each feature disclosed in this specification, unless stated otherwise, may be replaced by alternative features serving the same, equivalent or similar purpose. Thus, unless stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features.

[0165] All of the features disclosed in this specification may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive. In particular, the preferred features of the invention are applicable to all aspects of the invention and may be used in any combination. Likewise, features described in non-essential combinations may be used separately (not in combination).

[0166] It will be appreciated that many of the features described above, particularly of the preferred embodiments, are inventive in their own right and not just as part of an embodiment of the present invention. Independent protection may be sought for these features in addition to or alternative to any invention presently claimed.

[0167] Reference is now made to the following examples, which illustrate the invention in a non-limiting fashion.

EXAMPLES

[0168] FIG. 6 presents graphene concentration over a processing time of 10 hours for the invention. This exfoliation performance was achieved in a device as illustrated in FIG. 1 at a moderate cylindrical rotor operating speed of 1333 rpm, cylindrical rotor diameter of 101 mm, cylindrical rotor height of 100 mm, outer chamber width of 2 mm, and pump flow rate of 320 ml/min. This resulted in a rotational Reynolds number of 9500. This corresponds to the Taylor vortex regime.

[0169] As a comparison, the performance of the device was compared to that of the Shear Mixing approach presented by Paton et al., Nature Materials, 13 (2014), 624-630, the contents of which are herein incorporated by reference in their entirety. In both cases, the starting graphite (Sigma Aldrich product no. 332461), solvent (NMP), and graphite concentration (10 g/L) were identical. The volume used in both processes was also closely matched at around 1.5 L. The invention is shown to outperform the Shear Mixing approach by a factor of 7. The concentration data is replotted in terms of production rate in FIG. 7, suggesting a processing time of 2 hours may provide the optimum to scale-up material output.

[0170] FIG. 8 presents the average number of layers over time for the graphene produced in FIG. 6. This has been determined through UV-Vis-nIR measurement and the spectroscopic method described by Backes et al., Nanoscale, 8 (2016), 4311-4323, the contents of which are herein incorporated by reference in their entirety. The number of layers decreases with processing time from 11.5 to 8.5. The invention can, therefore, be operated to selectively achieve a required average layer number.

[0171] FIG. 9 provides the Raman shift for the fluid exfoliated graphene product. This has been acquired through vacuum filtering the dispersed graphene nanosheets onto a PTFE membrane with 250nm thick layer. The Raman data for different sampling points demonstrate the characteristics of few-layer graphene (2D band), reinforcing the UV-Vis-nIR findings. The graphite precursor is shown also, indicating that it can also have a D band 0.15, close to that of the graphene produced. This suggests that the product is defect-free (basal-plane defects). The increase to the D band (0.17-0.25) for the exfoliated product is predominantly due to the nanosheet edge contributions (Paton et al., Nature Materials, 13 (2014), 624-630), and the graphene is of high quality.

[0172] Finally, a typical graphene nanosheet produced from the invention is shown in FIG. 10 and obtained using Transmission Electron Microscopy (TEM). From TEM observations, it was found that the graphene produced may range in length between 100 nm and 10 microns.

[0173] While preferred embodiments of the present invention have been shown and described herein, it will be apparent to those skilled in the art that such embodiments are provided by way of example only. Numerous variations, changes, and substitutions will now occur to those skilled in the art without departing from the invention. It should be understood that various alternatives to the embodiments of the invention described herein may be employed in practicing the invention and that methods and structures within the scope of these claims and their equivalents be covered thereby.