Treating Particles

Abstract

A method of treating particles by disaggregating, deagglomerating, exfoliating, cleaning, functionalising, doping, decorating and/or repairing said particles, in which the particles are subjected to plasma treatment in a treatment chamber containing a plurality of electrodes which project therein and wherein plasma is generated by said electrodes which are moved during the plasma treatment to agitate the particles.

Claims

1. A method of treating particles by disaggregating, deagglomerating, exfoliating, cleaning, functionalising, doping, decorating and/or repairing said particles, in which the particles are subjected to plasma treatment in a treatment chamber containing a plurality of electrodes which project therein and wherein plasma is generated by said electrodes which are moved during the plasma treatment to agitate the particles.

2. A method according to claim 1 in which the particles treated are graphite particles.

3. A method according to claim 2 in which the graphite particles are exfoliated to produce graphene containing particles, preferably graphene nanoplatelets (GNPs).

4. A method according to claim 1 in which the particles treated are nano-particles.

5. A method according to claim 4 in which the nanoparticles are carbon nano-particles, preferably fullerenes such as carbon nanotubes (CNTs), or graphene containing nanoparticles such as GNPs.

6. A method according to claim 1 in which glow discharge plasma is generated by the electrodes.

7. A method according to claim 1 in which plasma is formed in a localised region around each electrode.

8. A method according to claim 1 in which a gas or gaseous mixture is introduced into the chamber to sustain the plasma, in which said gas or gaseous mixture is introduced from each electrode.

9. A method according to claim 1 in which the electrodes are moved at a speed which is varied during the course of the plasma treatment so as to control the interaction between the plasma generated by the electrodes and the particles.

10. A method according to claim 1 in which at least a portion of the treatment chamber is rotated thereby causing the electrodes to move during the plasma treatment.

11. Apparatus for treating particles in a method according to claim 1 including a treatment chamber containing a plurality of electrodes which project therein, each electrode for generating a plasma, in which said electrodes are moveable to agitate particles disposed in the treatment chamber.

12. Apparatus according to claim 11 in which at least a portion of the treatment chamber is rotatable thereby to cause the electrodes to move thereby agitating particles disposed in the treatment chamber.

13. Apparatus according to claim 12 in which the treatment chamber is rotatable around a longitudinal axis, and the electrodes are radially disposed around the longitudinal axis.

14. Apparatus according to claim 11 in which three or more, and preferably between 3 and 20, electrodes project into the treatment chamber.

15. A method of repairing defects in fullerene or graphene containing nanoparticles in which said nanoparticles are subjected to a plasma treatment wherein plasma is formed in a carbon containing gas or gaseous mixture including a carbon containing gas, the carbon containing gas being present in an amount sufficient to repair defects in said nanoparticles.

16. A method according to claim 15 in which the plasma treatment is a plasma enhanced chemical vapour deposition (PECVD) process.

17. A method according to claim 15 in which the carbon containing gas is a hydrocarbon, an alcohol or a halocarbon.

18. A method according to claim 17 in which the carbon containing gas is methane, methanol, or carbon tetrachloride.

19. A method according to claim 18 in which the plasma is formed in pure methane or in a gaseous mixture which consists essentially of methane.

20. A method according to claim 15 in which the nanoparticles are CNTs or GNPs.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0063] Embodiments of apparatus and methods in accordance with the invention will now be described with reference to the accompanying drawings, in which:

[0064] FIG. 1 is a perspective view of apparatus of the invention;

[0065] FIG. 2 is a partially cutaway perspective view of a rotating drum of the invention;

[0066] FIG. 3 is an exploded view of the interior of the rotating drum;

[0067] FIG. 4 shows (a) an exploded perspective view and (b) a side view of an end plate having a plurality of electrodes disposed therein;

[0068] FIG. 5 is a perspective front view of an end plate having electrodes disposed therein; and

[0069] FIG. 6 shows (a) an electron microscope image and (b) an EDX spectrum of GNP treated using the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0070] FIG. 1 shows a treatment chamber 10 of the invention positioned on a bed 12 through coupling portions 14, 16. The coupling portion 16 is in operative connection with a suitable motor or actuator disposed in a housing 18. The motor or actuator is coupled to the treatment chamber 10 so that, in operation, the treatment chamber 10 may be rotated at a desired rotational speed.

[0071] The treatment chamber 10 is a three-part modular arrangement comprising a central drum 20 and first and second frusto-conical sections 22, 24. The first frusto-conical section 22 is in contact with the receiving portion 16, and second frusto-conical section 24 is in contact with the receiving portion 14 so as to allow the treatment chamber 10 to be rotated. The drum 20 and first and second frusto-conical sections 22, 24 can be formed from any suitable material, such as stainless steel.

[0072] FIGS. 2 and 3 show the internal arrangement of the drum 20 in more detail. In particular, the drum 20 comprises a cylindrical portion 26 and first and second circular end plates 28, 30. The first end plate 28 is in communication with the first frusto-conical section 22 and the second end plate 30 is in communication with the second frusto-conical section 24. A plurality of electrodes 32 project out of the first end plate 28 into the interior of the drum 20. The electrodes 28 are radially disposed around the longitudinal axis of the drum 20 in a circular pattern. As shown in FIG. 3, the electrodes are equally spaced, although it is not critical that this is so. The electrodes 28 are arranged towards the circumferential edge of the first end plate 28. As explained in more detail below, this arrangement is preferred in order to provide agitation of particles which are disposed in the treatment chamber in order to undergo plasma treatment. In the embodiment shown in the figures, an arrangement of twelve electrodes project into the treatment chamber. However, a greater or a lesser number of electrodes might be used. In general, it is envisaged that between three and twenty electrodes will be suitable for most applications. However, a smaller number of electrodes or a larger number of electrodes might be used. The skilled reader will be able to readily determine the number of electrodes and the shape of the electrodes which are suitable for any given application. In the embodiment shown in the figures, the electrodes 32 are formed from an electrically conductive material such as stainless steel. Isolator sleeves 34 formed from an electrically insulating material such as ceramic are disposed at both ends of each electrode 32. The sleeves may be coatings on the electrodes. The electrodes should generally be arranged to project a significant way into the chamber. It is possible in other embodiments for them to extend between the end plates. The active plasma producing areas of the electrodes may be coated with a conductive ceramic coating such as boron silicate glass. This can act to reduce unwanted sputtering.

[0073] The supply and removal of gases to and from the treatment chamber 10 will now be described with particular reference to FIGS. 4 and 5 which show a gas inlet module 38 (also shown in FIG. 3) which is in gas conducting connection with a plurality of gas inlet lines 40. Each gas inlet line 40 is connected to an electrode 32. Each electrode 32 is hollow, having an internal gas conducting conduit (not shown) leading to a gas outlet aperture at the distal end of each electrode 32 (not shown). The gas inlet module 38 is housed in the first frusto-conical section 22, and is supplied with the gases to be used during plasma processing from one or more external gas supply sources (not shown). The supply of gas can be controlled using known means such as mass flow controllers.

[0074] The first end plate 28 has an exhaust port 42 formed therein. As shown in FIGS. 4(b) and 5, a filter 44 is disposed in the exhaust port 42. The exhaust port 42 is in connection with a pumping arrangement (not shown) which is used to create a vacuum in the treatment chamber and to pump away process gases in a manner which is well-known to the skilled reader.

[0075] Rotatable drum chambers are available commercially and may be adapted in order to produce apparatus of the invention. For example, a rotating drum plasma reactor is produced commercially by Diener Electronic GmbH & Co. KG, D-72224 Ebhausen, Germany having the product name Tetra 500. This apparatus may be adapted in accordance with the invention, for example by providing the plurality of electrodes described above.

[0076] In use, a charge of the particles to be processed is disposed on the floor of the drum 20. The chamber is evacuated to a desired baseline pressure, and the process gas or gases are introduced into the treatment chamber 20 through the electrodes 32. The treatment chamber 10 is rotated at a desired rotational speed. A plasma is generated at each electrode 32 in order to initiate processing of the particles. During the processing of the particles, the electrodes 32 are rotating, and this acts to continuously agitate the charge of particles in the treatment chamber. The particles may be physically transported through this agitation, for example through sideways displacement of the particles, or by way of the particles being thrown upwards the interior of the chamber. The scoops 36 can significantly assist in the process.

[0077] In the embodiment shown in FIGS. 1 to 5, glow discharge plasmas are formed at each electrode 32. RF power is applied to electrode 32. A convenient RF frequency such as 13.56 MHz may be used. The electrodes 32 thereby act as working electrodes in a glow discharge system. A counter-electrode is provided, and conveniently this can be the inner surface of the drum 20 which might be coated with a conductive ceramic such as boron silicate glass. The RF power establishes a negative DC bias voltage on the electrodes 32 which thereby act as cathodes in the flow discharge system. It is also possible to use other methods to obtain glow discharge plasmas, such as through the application of a DC voltage through electrodes 32. Other forms of plasma might be utilised instead.

[0078] Localised plasmas are generated around each electrode 32, but the process conditions are selected so that these plasmas are discrete and separated from one another. In this way, each electrode is surrounded with a plasma halo which contains clouds of energetic electrons, UV photons, ions and, typically, reactive neutral species. This rich plasma can be used to perform one or a number of useful particle treatment steps. The use of multiple electrodes increases the number of electron clouds and other useful species associated with the plasmas, and this has beneficial effect on processing efficiency. Additionally, the use of the electrodes to agitate the particles to be treated can also have a beneficial effect on processing efficiency as well as improving the results achieved.

[0079] The apparatus may be used to exfoliate particles of graphite to produce a graphene containing material. GNP may be produced in this way. This is achieved through exfoliation, and typically a high plasma power is utilised, at least in the initial stages of the process, to facilitate ion bombardment and ion intercalation of the target material. Effective bombardment and intercalation causes layers of the target material such as graphite to exfoliate. Without wishing to be limited by a particular theory or conjecture, it is believed that a result of this is that a nett negative charge is imparted onto the exfoliated layers so that they can push off from each other. This charge overcomes attractive van der Waals' forces, thereby retarding the usual inclination of the particles produced by the exfoliation to reagglomerate. A plasma formed in oxygen is effective in producing exfoliation to provide graphene containing material such as GNP. The process can be used more generally to exfoliate and/or deagglomerate fibres and sheet material.

[0080] An oxygen plasma is also an advantageous means of cleaning particles. Nanoparticles such as CNTs and GNPs are conveniently cleaned by an oxygen plasma. Representative but non-limiting process conditions are a temperature of less than 100 C. with a plasma power of 120 W for around thirty minutes at a pressure of 1.5 Torr (200 Pa). The present inventors have recognised that the particles to be treated may contain defects, and also that processes such as cleaning can introduce further defects. Very advantageously, it has been realised that effective repair of defects can be achieved through plasma processing using apparatus of the invention. Effective repair can be achieved using a PECVD process involving a plasma in pure methane. Representative conditions are a plasma power of 100 W and a chamber gas pressure of 0.2 Torr (27 Pa). GNPs of excellent crystallinity have been obtained following repair treatment in a methane plasma. The plasma process can be terminated with nitrogen prior to vacuum packing to preserve the cleanliness of the processed particles.

[0081] Experiments were performed on GNP. Mined graphite from Sri Lanka was size reduced in a mechanical mill and introduced to a treatment chamber of the type shown in FIG. 1. An oxygen plasma was used to exfoliate layers from the graphite to produce GNP. The oxygen plasma also acted as a cleaning plasma and as a functionalising plasma. Following the oxygen plasma treatment, the GNP was subjected to a methane treatment to repair defects in the GNP. FIG. 6(a) shows an electron microscope image of the GNP after cleaning and repair. FIG. 6(b) is an EDX spectrum. Table 1 shows the associated elemental analysis.

TABLE-US-00001 TABLE 1 Elemental Analysis of GNP Element Weight % Atomic % C 95.01 96.20 O 4.99 3.80

[0082] The results indicate that graphene nanoplatelets of extremely high purity and crystallinity have been produced using the invention. The functionalisation with oxygen mainly producesCOOH functionalities, which are extremely dense on the GNP. Although the GNP has been produced from mined graphite, there are no signs of mined contaminants such as metals, sand or chalk. This indicates that the cleaning part of the treatment cycle has worked extremely well.

[0083] A further possibility when treating particles in accordance with the invention is to remove treated particles from the multi-electrode treatment chamber to a final stage treatment chamber using vacuum transfer or other appropriate means. The final treatment chamber may be used to facilitate high temperature processing which will provide additional treatment options, for example for decorating the particles with a desired material. The final stage treatment may be a microwave induced plasma treatment. In these embodiments, the final treatment chamber may have glass windows and an inner surface coated with a ceramic or glass. Appropriate wave guides would be used to couple microwave energy into the chamber through the windows. The final treatment chamber can be configured to rotate in order to agitate the particles. In another alternative, a final treatment step such as this can be performed in the original treatment chamber. In these embodiments, the original treatment chamber is provided with microwave means to generate the microwave induced plasma. The multi-electrode array can be used for this purpose if the electrodes are made from suitable materials such as a conductive glass, for example boron silicates.

[0084] A potential problem during processing is electrical shorting of the electrodes. This can be at least ameliorated by reducing the plasma power as the processing continues. The likelihood of shorting occurring increases if the material volume increases during processing which is likely to occur if deagglomeration and/or disaggregation occurs. One approach which may be adopted is to reduce the plasma power as the volume of the treated particles increases. For example, an inverse relationship between the volume of the treated particles and the applied plasma power, or another relationship determined by routine investigation may be followed.