Particulate materials, composites comprising them, preparation and uses thereof

Abstract

Methods of processing particulate carbon material, such as graphic particles or agglomerates of carbon nanoparticles such as CNTs are provided. The starting material is agitated in a treatment vessel in the presence of low-pressure (glow) plasma generated between electrodes. The material is agitated in the presence of conductive contact bodies such as metal balls, on the surface of which plasma glow is present and amongst which the material to be treated moves. The methods effectively deagglomerate nanoparticles, and exfoliate graphitic material to produce very thin graphitic sheets showing graphene-type characteristics. The resulting nanomaterials used by dispersal in composite materials, e.g. conductive polymeric composites for electric or electronic articles and devices. The particle surfaces can be functionalized by choosing appropriate gas in which to form the plasma.

Claims

1. A particle treatment method for disaggregating, deagglomerating, exfoliating, cleaning or functionalizing particles, the method comprising: placing particles to be treated in a plasma treatment chamber; placing a plurality of freely-moveable electrically-conductive solid contact bodies in the plasma treatment chamber so as to be in direct contact with the particles to be treated; and agitating the freely-moveable electrically-conductive solid contact bodies together with the particles to be treated in the plasma treatment chamber such that the particles to be treated directly contact the freely-moveable electrically-conductive solid contact bodies and plasma in the treatment chamber, wherein the particles are of graphitic carbon, which is exfoliated by the treatment, and after the treatment the treated particles comprise discrete graphitic or graphene platelets having a platelet thickness less than 100 nm and a major dimension perpendicular to the thickness which is at least 10 times the thickness.

2. Particle treatment method of claim 1 in which the particles to be treated are graphite particles or carbon nanotubes.

3. Particle treatment method of claim 1 in which the treatment chamber is a rotatable drum in which the contact bodies are tumbled with the particles to be treated.

4. Particle treatment method according to claim 1 in which the treatment chamber has a wall which defines an interior space, an electrode extends into the interior space, and the wall of the treatment chamber is conductive and forms a counter-electrode to said electrode.

5. Particle treatment method according to claim 1 in which glow plasma forms on the surfaces of the contact bodies.

6. Particle treatment method according to claim 1 in which the contact bodies are metal balls or metal-coated balls.

7. Particle treatment method according to claim 1 in which the contact bodies have a diameter, and the diameter is at least 1 mm and not more than 60 mm.

8. Particle treatment method according to claim 1 in which a pressure in the treatment chamber is less than 500 Pa.

9. Particle treatment method according to claim 1 in which, during the treatment, gas is fed to the treatment chamber and gas is removed from the treatment chamber through a filter.

10. Particle treatment method of claim 1 in which the treated particles or disaggregated, deagglomerated or exfoliated components thereof resulting from the treatment, are chemically functionalised by components of the plasma-forming gas, forming carboxy, carbonyl, OH, amine, amide or halogen functionalities on their surfaces.

11. Particle treatment method according to claim 1 in which plasma-forming gas in the treatment chamber is or comprises any selected from oxygen, water, hydrogen peroxide, alcohol, nitrogen, ammonia, amino-bearing organic compound, halogen, halogydrocarbon and noble gas.

12. Particle treatment method according to claim 1 in which said treatment is continued for at least 30 minutes.

13. Particle treatment method according to claim 12 in which said treatment is continued until the treated particles comprise by weight at least 80% of platelets less than 30 nm thick, and in which the major dimension is at least 10 times the thickness.

14. Particle treatment method according to claim 12 in which said treatment is continued until the treated particles comprises by weight at least 90% of platelets less than 20 nm thick, and in which the major dimension is at least 10 times the thickness.

15. Particle treatment method according to claim 1 in which said treatment is continued until the treated particles comprises by weight at least 90% of platelets less than 100 nm thick and in which the major dimension is at least 10 times the thickness.

16. A method of preparing a particle dispersion or a composite material, comprising: (a) treating particles by a particle treatment method for disaggregating, deagglomerating, exfoliating, cleaning or functionalizing particles, the particle treatment method comprising: placing particles to be treated in a plasma treatment chamber; placing a plurality of freely-moveable electrically-conductive solid contact bodies in the plasma treatment chamber so as to be in direct contact with the particles to be treated; and agitating the freely-moveable electrically-conductive solid contact bodies together with the particles to be treated in the plasma treatment chamber such that the particles to be treated directly contact the freely-moveable electrically-conductive solid contact bodies and plasma in the treatment chamber, (b) dispersing the treated particles in a matrix material which is polymeric or is a precursor of a polymer; wherein the treated particles comprise carbon nanotubes, or graphitic or graphene platelets having a platelet thickness less than 100 nm and a major dimension perpendicular to the thickness which is at least 10 times the thickness, and are dispersed in a said polymeric matrix material to make an electrically-conductive composite material.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The present proposals are now explained further with reference to the accompanying drawings, in which:

(2) FIG. 1 is a perspective view of a treatment vessel;

(3) FIG. 2 is a schematic view of a central electrode formation in one version;

(4) FIG. 3 is a schematic view of a central electrode formation in another version;

(5) FIG. 4 is a schematic end view of the treatment vessel operating in plasma-generating apparatus;

(6) FIG. 5 is a side view of the same thing;

(7) FIG. 6 is a perspective view of a further embodiment of treatment drum, and

(8) FIG. 7 is an axial cross-section thereof.

(9) FIGS. 8 to 16 show details of actual carbon materials before and after treatment in accordance with the new proposals:—

(10) FIGS. 8 and 9 are SEM images of a MWCNT material before treatment;

(11) FIGS. 10 and 11 are SEM images of the same MWCNT material after treatment;

(12) FIGS. 12(a) and 12(b) are particle size data for the MWCNT material before and after treatment;

(13) FIGS. 13 and 14 are SEM images of a disordered graphitic or graphene material made by arc discharge, before and after treatment;

(14) FIGS. 15 and 16 are SEM images of a natural graphite material before and after treatment.

(15) FIGS. 17 and 18 are SEM images of a disordered graphitic or graphene material made by arc discharge, before and after treatment;

(16) FIGS. 19 and 20 are SEM images of a natural graphite material before and after treatment;

(17) FIGS. 21 and 22 are face views and an edge view of product obtained in Example 6;

(18) FIG. 23 shows a selected nanoplatelet material obtained in Example 7;

(19) FIG. 24 shows a further version of treatment drum (3rd apparatus embodiment);

(20) FIGS. 25 and 26 are ESCA (XPS) results showing the surface elemental analysis of CNTs functionalised by the present treatment methods.

DETAILED DESCRIPTION

(21) With reference to FIG. 1 a generally cylindrical glass vessel or drum 4 has an integral glass rear end wall 43 and a front opening 41. Quartz or borosilicate glass is suitable. Axially-extending rib formations 44 are distributed circumferentially and project inwardly from the interior surfaces of the drum wall 42. They may be formed integrally with the glass of the wall, or be bonded-on plastics components.

(22) The rear wall 43 has a central re-entrant portion or socket 431 forming an insulative locating support for an electrode formation extending forward axially through the drum interior. This formation may be a fixed metal electrode insert, as exemplified in FIG. 2. The embodiment of FIG. 2 is a tubular electrode with a gas feed port via a fine filter disc 32 closing off its front (free) end e.g. clamped by a screw ring cap 33. Its open rear end is sealingly bonded, or more preferably sealingly but removably connected (e.g. by a thread or tapered plug as shown), into a central opening of the glass socket 431.

(23) Alternatively the interior electrode formation may be or comprise a dielectric electrode cover, e.g. an integral tubular forward extension 3′ of the glass wall itself as shown in FIG. 3, having a fine particle filter 32′ e.g. of sintered glass or ceramics at its front end. An alternative has a discrete tubular dielectric electrode cover element fixed or bonded in, like the electrode of FIG. 2.

(24) An advantage of removable electrodes/electrode covers is ease of cleaning, replacement or substitution with different ones e.g. of different size, material, filter type etc.

(25) A plastics sealing lid 5 is provided for the open front end of the glass treatment vessel. This lid has a peripheral sealing skirt 53 to plug tightly into the drum opening 41, a filter port 52 incorporating a HEPA filter element, for pressure equalisation with a vacuum system, and a fluid injection port 51 having a sealing cover, for the introduction of liquid.

(26) In use, a charge of particles is put into the vessel 4. The lid 5 is sealed. The HEPA filter 52 is sufficiently fine that the particles cannot escape, and can in any case be covered with a seal as a precaution against damage. The particle-loaded vessel is sent for plasma treatment using plasma-generating apparatus having a treatment chamber with vacuum generation, plasma-forming gas feed, means for rotating the vessel and system electrode drive for generating a suitable electric field for plasma generation, e.g. RF energy.

(27) In the case as in FIG. 2 where the electrode 3 is integrated, it is necessary to connect this by a suitable connector, e.g. a threaded element 6 with a gas feed conduit 70, to the electrical drive. Of course, this connector could alternatively extend further into or all along inside the tubular electrode 3. However the connector is in any case removably or releasably connected.

(28) In the case as in FIG. 3 where the drum comprises a dielectric electrode cover 3′, an elongate electrode 7 of the plasma-generating apparatus is inserted, fitting closely to avoid intervening space (the slight clearance in the drawing being only to indicate the discrete parts).

(29) A central gas feed channel 70 can be provided inside the connector 6 or electrode 7, for feed of gas to the vessel interior via the filter 32,32′ at the front end of the electrode.

(30) FIGS. 4 and 5 show a plasma treatment apparatus schematically: a support container 8 is mounted rotatably in a fixed sealable housing 9. Either of these or part thereof may comprise the counter-electrode. The counter-electrode should be shaped and positioned in relation to the axial electrode to enable stable glow plasma to form substantially all along the axial electrode inside the treatment chamber. The particle treatment vessel 4 is loaded into the support container 8 through a front hatch 81, and held axially in position by locating pads 82, and by connection of the axial electrode at its rear end. The housing 9 is evacuated via an evacuation port V, and the vacuum applies through the system via container vacuum port 83 and the front filter port 52 of the treatment vessel. Gas is fed in axially via the filter 32,32′ in the electrode formation. Application of RF or other suitable power according to known principles creates plasma in the vessel 4, especially in the region adjacent the axial electrode formation 3. As the drum rotates (FIG. 4) the internal vanes 44 carry the nanoparticles up and cast them down selectively through this plasma-rich zone.

(31) The treatment atmosphere may be chosen freely provided that it will sustain plasma. An oxygen-containing atmosphere is an example, and is effective to produce oxygen-containing functional groups on the particles, thereby activating them.

(32) Thus, the treatment vessel 4 can be plugged into a plasma apparatus and operated to plasma-activate the particles without ever needing to be opened. After treatment, the liquid introduction port 51 can be used for the injection of a suitable liquid to disperse and/or carry the particles. This might be e.g. a solvent vehicle, water or polymer material.

(33) For the injection of process gas the treatment chamber may be provided with more than one gas injection point (e.g. different points in the housing or drum and/or different options for injecting gas at or along the central electrode). The appropriate point can then be selected to produce effective treatment according to the material to be treated.

(34) The rotation speed of the treatment drum is adjustable so that the particles can be made to fall selectively through the glow plasma region.

(35) The drum may be formed in various ways. One possibility is a conductive drum wall itself forming a counter-electrode for plasma formation. Front and back end plates may be dielectric. A further possibility is a fully dielectric drum, with a separate counter-electrode structure or other plasma energising structure. This structure may be an external housing.

(36) Glass is a suitable and readily available dielectric material for forming any of the baffles, drum end plates and drum wall. Plastics or ceramic materials may also be used.

(37) Second Apparatus Embodiment

(38) FIGS. 6 and 7 show a further treatment drum suitable for treatment of particles comprising CNTs, or graphitic granules. It has a cylindrical drum wall 2004 of metal e.g. steel or aluminium to act as counter-electrode. It is to be mounted for rotation in a vacuum chamber, e.g. on support rollers.

(39) The end walls are insulative. A rear end wall is of glass or inert plastics e.g. PTFE and comprises inner and outer layers 2432,2431 between which a filter layer (not shown) is clamped. This end wall filter module has large windows 2111 occupying more than half its area so that gas flow speed through the filter is low. This is found to improve plasma stability i.e. inhibit arcing. The centre of the rear end wall has a holder for the axial electrode, not shown. The electrode is a tubular metal electrode along which process gas is fed in use. It may be housed in a sheath.

(40) A set of eight non-conductive (plastics) lifter vanes 244 is mounted around the inside of the metal drum. The front end wall has a simple insulating sealing wall or lid held on by a tight collar which may optionally—as may the module at the rear end—be screwed onto the metal drum end.

(41) Third Apparatus Embodiment

(42) FIG. 24 shows a third embodiment of the treatment drum, in slightly more detail. This is a larger drum, volume about 60 liters and without interior baffles or lifters i.e. so that the bed of contact bodies e.g. steel balls will reside at the bottom during treatment. The tubular central electrode is used for feeding gas, through a brass sintered plug at the front end (not shown). The front wall is formed into a cone with a limited opening (having a window plug, not shown) to facilitate emptying out of product after treatment. The rear wall is a filter, as before. Elements of the mechanical drive, vacuum communication and gas feed are also shown, to assist the skilled reader. The gas flow through the large volume of the system is relatively slow, and we find there is no tendency for the very fine particulate product to escape through the filter i.e. the product is not “carried out” by gas flow.

EXAMPLES

(43) Apparatus and Conditions

(44) In experimental work we used a steel treatment drum substantially as shown in FIGS. 6 and 7 and also as shown in FIG. 24, without any internal lifter baffles. Internal volume about 12 liters, diameter 400 mm, central electrode diameter 3 mm, steel central electrode and with an observation window in the front wall. As contact bodies we used ordinary steel ball bearings: size 10 mm, weight 12 grams, number about 500. Each charge of starting material (aggregated or initial carbon particles to be treated) weighing about 100 grams was put in the drum with the steel balls and the lid closed. For the treatment, conditions in the drum were e.g. as follows:

(45) TABLE-US-00001 Gas atmosphere fed Oxygen Rate of gas flow 1000 cm.sup.3 per minute Pressure 50 torr Speed of drum rotation 60 rpm Voltage applied (plasma) 100 volts Period of treatment 30 mins

(46) Best results were found at rotation speeds at which a mass of the particles being treated, mixed with the mobile bodies (steel balls), resides at the bottom of the drum as it rotates. At 60 rpm the bed of balls and particles is gently agitated but remains at the bottom of the drum.

(47) Carbon sample materials used in Examples 1 to 3 were as follows.

(48) (1) MWCNT material made by the CVD process, from Bayer;

(49) (2) largely graphitic material produced by an arc discharge process, from Rosseter (Cyprus);

(50) (3) natural graphite powder.

(51) During the treatments we observed plasma-like light haloes around the steel balls, especially those at the top of the bed nearest the central electrode, as they tumbled in the drum with the carbon particles.

(52) Particle sizes were measured in water dispersion (using the standard laser diffraction method) by a MasterSizer 2000 machine (Malvern Instruments, UK). (The skilled person will appreciate that this gives only relative measurements, because of the high aspect ratio of the product.) The SEM images are from a Hitachi S-4800.

Example 1

(53) The MWCNT material as supplied, i.e. as manufactured, is seen in the SEM images FIGS. 8 and 9 and its particle size distribution is in FIG. 12(a). These are large, tightly aggregated granules approaching 1 mm (1000 μm) in size. The treated material is seen in the SEM images of FIGS. 10 and 11 and its particle size distribution is in FIG. 12(b). It can readily be seen that the particle size has been drastically reduced to a range between 1 and 10 μm, i.e. there has been substantial de-aggregation, and also that the treated material has a substantial proportion of discrete, liberated CNTs, visible in the SEM images.

Example 2

(54) The starting material, consisting primarily of disordered, stacked graphite lumps and platelets with a few small fullerenes (FIG. 13), was subjected to the same treatment as described above. Portions of the treated material are seen in FIG. 14. It can readily be seen that there has been substantial thinning of the platelets, exfoliation of some graphene and reduction of size.

(55) BET methods were used to measure the specific surface area, with a 2 hr degas at 300° C.:

(56) treated=92 m.sup.2/g

(57) untreated=62 m.sup.2/g

(58) Increase=48%

Example 3

(59) The starting material was powdered natural graphite. FIG. 15 shows a typical particle: a graphite platelet with multiple layers which will not show the special properties of graphene. FIG. 16 shows the material after treatment. There has been substantial exfoliation, producing a large number of single graphene flakes. These can be functionalised at their edges, as is known.

Example 4

(60) The starting material, consisting primarily of disordered, stacked graphite lumps and platelets with a few small fullerenes (FIG. 17), was subjected to the same treatment as described above. Portions of the treated material are seen in FIG. 18. It can readily be seen that there has been substantial thinning of the platelets, exfoliation of some graphene and reduction of size.

(61) BET methods were used to measure the specific surface area, with a 2 hr degas at 300° C.:

(62) treated=92 m.sup.2/g

(63) untreated=62 m.sup.2/g

(64) Increase=48%

Example 5

(65) The starting material was powdered natural graphite. FIG. 19 shows a typical particle: a graphite platelet with multiple layers which will not show the special properties of graphene. FIG. 20 shows the material after treatment. There has been substantial exfoliation, producing a large number of single graphene flakes. These can be functionalised at their edges, as is known.

Example 6

(66) The starting material was powdered natural graphite of Chinese origin. FIG. 21 is a representative view of the treated product, with fully separated platelets. No measured platelet was thicker than 57 nm. Most were less than 25 nm thick. The thinnest was 2.7 nm.

(67) This material, which carries oxygen-containing functionalities from the plasma treatment, was readily dispersed at 2 wt % in molten polyethylene which was then drawn into a yarn. In a qualitative laboratory comparison the filled yarn had much higher tensile strength than a yarn of the corresponding unfilled material.

Example 7

(68) Exfoliated graphite obtained as in Example 6 was subjected to classification by dispersion in water and ultrasonication, whereupon only the finest particles remained at the top of the jar. These were separated physically and recovered. FIG. 23 shows that they are remarkably small and uniformly very thin platelets; a very high-value material obtained by a simple and economical process.

(69) Functionalisation

(70) FIGS. 25 and 26 show XPS (ESCA) surface analysis for treated carbon nanotubes (Baytubes™). The untreated tubes showed 96% carbon, 4% oxygen.

(71) After thirty minutes of treatment of a 25 g sample in an ammonia-containing plasma (ammonia diluted in Ar), using the steel balls as above, the analysis showed carbon at 97.2%, oxygen 0.9%, nitrogen 1.9%: see FIG. 25. Unwanted O had been reduced and N—H functions introduced.

(72) FIG. 26 shows corresponding results after the same treatment but in a plasma containing CF.sub.4. After treatment, carbon was 83.3%, oxygen 2.6% and fluorine 14.1%. This represents a high level of surface fluorine functionalisation.