Methods and Apparatus for Plasma Processing

Abstract

The present invention relates to a method for treating a sample using glow-discharge plasma, in an apparatus comprising a treatment vessel, an electrode, a counter-electrode, and a power supply comprising one or more transformers and having a first transformer setting and a second transformer setting, the method comprising: (i) a loading step, involving loading the sample into the treatment vessel; (ii) a first treatment step involving treating the sample in a glow-discharge plasma formed within the treatment vessel by applying an electric field between the electrode and counter-electrode at the first transformer setting; (iii) a second treatment step involving treating the sample in a glow-discharge plasma formed within the treatment vessel by applying an electric field between the electrode and counter-electrode at the second transformer setting; and (iv) a removal step, involving removing treated sample from the treatment vessel. The method can be used to functionalize a sample. The present invention also relates to an apparatus for use in such a method.

Claims

1. A method for treating a sample using glow-discharge plasma, in an apparatus comprising a treatment vessel, an electrode, a counter-electrode, and a power supply comprising one or more transformers and having a first transformer setting and a second transformer setting, the method comprising: a loading step, involving loading the sample into the treatment vessel; a first treatment step involving treating the sample in a glow-discharge plasma formed within the treatment vessel by applying an electric field between the electrode and counter-electrode at the first transformer setting; a second treatment step involving treating the sample in a glow-discharge plasma formed within the treatment vessel by applying an electric field between the electrode and counter-electrode at the second transformer setting; and a removal step, involving removing treated sample from the treatment vessel.

2. The method for treating a sample according to claim 1, wherein the first and second transformer settings have voltage ratios of 0.5 or less, 0.45 or less, 0.4 or less, 0.35 or less, 0.3 or less, 0.25 or less, 0.2 or less, 0.15 or less, 0.1 or less, 0.05 or less, 0.025 or less, or 0.01 or less.

3. The method for treating a sample according to claim 1, wherein the first transformer setting has a different voltage ratio to the second transformer setting.

4. The method for treating a sample according to claim 3, wherein the difference between the first and second transformer voltage ratios is at least 0.01, at least 0.025, at least 0.05, at least 0.1, at least 0.15, at least 0.2, at least 0.25, at least 0.3, at least 0.35, at least 0.4, at least 0.45, or at least 0.5.

5. The method for treating a sample according to claim 1, wherein the first transformer setting has a different secondary voltage rating to the second transformer setting.

6. The method for treating a sample according to claim 5, wherein the difference between the secondary voltage ratings of the first and second transformer settings is at least 100 V, at least 200 V, at least 300 V, at least 400 V, at least 500 V, at least 750 V, at least 1 kV, at least 1.5 kV, at least 2.0 kV, at least 2.5 kV, at least 3.0 kV, at least 4.0 kV, at least 5 kV, or at least 10 kV.

7. The method for treating a sample according to claim 1, wherein the power supplied from the power supply during the first and/or second treatment step is modulated periodically between a higher power level and a lower power level.

8. The method for treating a sample according to claim 7, wherein the power is modulated between >0 W and 0 W.

9. The method for treating a sample according to claim 1, wherein the apparatus further comprises an arc detection system.

10. The method for treating a sample according to claim 1, wherein the sample is agitated within the treatment vessel during the first and/or second treatment step.

11. The method for treating a sample according to claim 10, wherein the sample is agitated by rotating the treatment vessel about an axis in a first direction, and then rotating in the opposite direction about the same axis.

12. The method for treating a sample according to claim 10, wherein the treatment vessel is provided with an evacuation port comprising a vessel filter which is protected by a guard element, wherein the guard element blocks the sample from contacting the vessel filter during agitation, while whilst still allowing gas to flow to and through the vessel filter.

13. The method for treating a sample according to claim 12, wherein the treatment vessel is a drum capped by end plates with the vessel filter(s) provided on one or both of the end plate(s), generally spaced from the edges of the end plate so as to be placed above the level of the sample in use, wherein the guard element comprises a wall extending from the end-plate into the interior of the treatment vessel and at least partially surrounding the filter element.

14. The method for treating a sample according to claim 12, wherein the guard element is a tube extending from the end plate into the interior of the treatment vessel and surrounding the vessel filter.

15. The method for treating a sample according to claim 14, wherein the tube has a first end extending into the interior of the treatment vessel, and a second end extending to the exterior of the treatment vessel, the apparatus further comprising a guard filter disposed towards the first end of the tube, and a vessel filter disposed towards the second end of the tube.

16. The method for treating a sample according to claim 1, wherein the treatment vessel is a cylindrical drum capped with two end-plates, wherein the interior surface of the drum is made from an electronically conductive material, and the internal surface of the end plates is made from an electrically insulating material.

17. The method for treating a sample according to claim 1, wherein the apparatus has at least one electrode extending within the interior of the treatment vessel and at least one electrode shield extending within the treatment vessel between the electrode and an interior wall of the treatment vessel, wherein the electrode shield is made from an insulating material and is positioned to block arcing to said interior wall of the treatment vessel in use.

18. The method for treating a sample according to claim 1, wherein the treatment vessel is a temperature controlled treatment vessel and wherein within a given treatment step the temperature is held at a constant temperature of from about −20° C. to about 120° C.

19. The method for treating a sample according to claim 1, wherein the first and second treatment steps are functionalization steps.

20. Apparatus for treating a sample in a method as defined in claim 1, comprising a treatment vessel, an electrode, a counter-electrode, and a power supply comprising one or more transformers.

21. A method for treating a sample using glow-discharge plasma, the method comprising one or more treatment steps, wherein during the one or more treatment steps a glow discharge plasma is formed by supplying power to the treatment apparatus and wherein during the one or more treatment steps the power is modulated periodically between different power levels according to a set pattern.

22-25. (canceled)

26. The method for treating a sample according to claim 7, wherein the power is modulated at a frequency of from 500 Hz to 1000 Hz.

27. The method for treating a sample according to claim 10, wherein the sample is agitated by rotating the treatment vessel through an angle of no more than ±120°.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0198] The present proposals are now explained further with reference to the accompanying figures in which:

[0199] FIG. 1 is a side cross-sectional view of a plasma treatment apparatus used in examples 1 to 3;

[0200] FIG. 2 is a side cross-sectional view of plasma treatment apparatus incorporating an electrode shield according to the present invention;

[0201] FIG. 3 is a front cross sectional view of the plasma treatment apparatus in FIG. 2;

[0202] FIG. 4 is a diagram of an electrode shroud used to mount the electrode shield in FIG. 2;

[0203] FIG. 5A is a partial side cross-sectional view of plasma treatment apparatus showing guard elements according to a first embodiment;

[0204] FIG. 5B is a partial side cross-sectional view of plasma treatment apparatus showing guard elements according to a second embodiment;

[0205] FIG. 6 is a diagram of the fluid delivery system for the plasma treatment apparatus;

[0206] FIG. 7 is a plot showing the stability of dispersions of graphene nanoplatelets subjected to oxygen plasma treatment using different transformer settings;

[0207] FIG. 8 is a section of the plot shown in FIG. 7;

[0208] FIG. 9 is a plot showing the stability of dispersions of GNP-type materials subjected to oxygen plasma treatment;

[0209] FIG. 10 is a plot showing the stability of dispersions of FLG-type materials subjected to oxygen plasma treatment;

[0210] FIG. 11 is a plot showing the number of arcs detected for a number of different carbon materials using plasma treatment apparatus with and without end plates;

[0211] FIG. 12 is a plot showing the pressure and voltage over time for a plasma treatment apparatus without endplates;

[0212] FIG. 13 is a plot showing the pressure and voltage over time for a plasma treatment apparatus with endplates;

[0213] FIG. 14 is a plot showing the number of arcs detected before and after the use of a pulsing generator with the plasma treatment apparatus.

[0214] FIG. 15 is a plot showing the atomic percentage of oxygen, carbon, nitrogen, fluorine, boron and silicon in a sample of boron nitride after plasma treatment with a plasma formed from argon, acrylic, ammonia, oxygen or tetrafluoromethane (CEO.

[0215] FIG. 16 is a plot showing the effect of heating the reaction chamber on the degree of functionalisation of FLG-type materials.

DETAILED DESCRIPTION

[0216] Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which the invention pertains. Although, any methods and materials similar or equivalent to those described herein can be used in practice for testing of the present invention, the preferred materials and methods are described herein. In describing and claiming the present invention, the following terminology will be used in accordance with the definitions set out below. Unless clearly indicated otherwise, use of the terms “a,” “an,” and the like refers to one or more.

[0217] The apparatus shown in FIG. 1 consists of a treatment vessel 1, having a central axial electrode 3 extending therein, loaded into a support container 5 and held in position by locating pads 7. The support container is rotatably mounted in a fixed sealable housing (not shown), so as to allow rotation of the treatment vessel during use. The central axial electrode 3 incorporates multiple gas feed channels, for feeding gas to the vessel interior via a filter at the front end of the electrode.

[0218] To use the equipment, a sample is loaded into the treatment vessel 1 via the removable lid 9, and the pressure in the treatment vessel is reduced by applying a vacuum to an evacuation port on the vessel housing, with the vacuum extending to the treatment vessel through vacuum port 11 and front filter port 13 of the treatment vessel. Next a plasma-forming gas is supplied to the treatment vessel interior via the gas feed channel in electrode 3, and a plasma formed through application of power to the central axial electrode 3. During processing, the treatment vessel 1 is rotated relative to the sealable housing, such that the sample held in the treatment vessel is tumbled through the plasma during processing.

[0219] The power supply includes a power source 15 capable of supplying AC power to the electrode via an array of step-up transformers, T.sub.1, T.sub.2, T.sub.3, having different secondary voltage ratings. The power source is designed to supply up to 400 V at a frequency of between 25 and 35 kHz. In the experiments described below, the apparatus is switched between seven different transformers, having secondary voltage ratings of 0.5, 1.0, 1.5, 2.0, 2.5, 3.0 and 3.5 kV respectively.

[0220] The apparatus includes an arc detection unit, which monitors the power supply to look for changes in the power, voltage, and frequency required to maintain the desired settings which are indicative of arc formation. Upon detection of an abnormality in the power supply, the system is configured to temporarily shut down for several seconds before restarting.

[0221] The power source 15 outputs a modulated power supply, switching between higher and lower power levels during the course of a treatment step. In this particular embodiment, the modulation occurs according to a sine wave.

[0222] FIGS. 2-4 show a modified plasma treatment apparatus, the features of which can be incorporated into the apparatus of FIG. 1. The apparatus consists of a treatment vessel rotatable about a fixed axial electrode 24 extending into the treatment vessel through seal 25. The axial electrode 24 is fixed to an electrode collar 27 (shown in more detail in FIG. 4), which support a number of subsidiary electrodes 29 between the axial electrode and particulate sample 28, separated from the axial electrode 24 by a distance “A”. Electrode shield 21 is also mounted to electrode collar 27, and covers the electrode assembly. The electrode shield 21 is formed from an electrically insulating material, so as to focus plasma formation in the lower half of the treatment vessel, and interrupt formation of arcs to the drum of the treatment vessel (which serves as the counter-electrode in this case). The front of the treatment vessel takes the form of a removable lid 22. The lid is made from an insulating material to prevent the formation of arcs. Gas feed 23 is provided to supply plasma-forming feedstock to the treatment vessel.

[0223] FIGS. 5A and 5B show the front end of a treatment vessel according to the invention comprising a vessel filter and guard element as described above.

[0224] In the embodiment shown in FIG. 5A, the treatment vessel is loaded in and rotatable relative to housing 32. Gas is supplied to the treatment vessel through electrode 31. Gas is removed from the system by a vacuum applied to the housing, operating via housing filter 33, which reduces pressure in the treatment vessel via vessel filters 34. The vessel filter is separated from the material being treated by a guard element, formed from upright wall 35 extending from the drum's interior surface, and a top wall 36 extending from the end-plate of the drum. The top wall 36 is provided with a bleed-through hole “B” to allow air to exit the treatment vessel through the filters.

[0225] In the embodiment of the treatment vessel shown in FIG. 5B, the guard element instead takes the form of a tube 37 extending through the front end-plate of the drum, capped by a guard filter 39 and a vessel filter 38. The tube is positioned above the level of the particulate sample, and thus prevents ingress of the sample through the guard filter.

[0226] FIG. 6 is a diagram of the how gas, liquids or vapours may be delivered to a treatment vessel. Gases, liquids or vapours may be delivered through vents along the length of a central electrode A, through a vent at the end of a central electrode B, through vents in the front wall of the treatment vessel C, through vents in the side walls of the treatment vessel D or through vents in the rear wall of the treatment vessel. An injection unit allows liquid or vapour to be delivered into the treatment vessel. A mix box comprising a mass flow controller allows two or more different gases to be fed into the treatment vessel. The gas lines may also comprise trace heaters, which allow the gas lines to be held at a particular temperature.

EXAMPLES

Examples 1 to 3

[0227] Examples 1 to 3 were conducted to demonstrate the effect of the transformer setting on the performance of apparatus as described in FIG. 1 above.

Example 1

[0228] A series of experiments were conducted to show the effect of selecting different transformers on the power supplied to the electrode during plasma formation.

[0229] An air plasma was formed at a pressure of 70 Pa with 100 W of power supplied via a 0.5 kV transformer. The experiments were then repeated with different transformers in place of the 0.5 kV transformer. The treatment vessel did not include any particles.

[0230] For each transformer, the voltage and frequency required to maintain the 100 W power level was recorded. The voltage was then converted to a voltage rating percentage (“% V”) value by expressing the voltage generated by the transformer (as measured at the electrode) as a percentage of the transformer secondary voltage rating of the transformer.

TABLE-US-00001 TABLE 1 Transformer Run 1 Run 2 Run 3 Average (kV) % V kHz % V kHz % V kHz % V kHz 0.5 86.8 35.5 87.2 37 86.2 36 86.7 36.2 1 45.9 36 46.2 36.7 44.8 35.8 45.6 36.2 1.5 31.2 33.6 31.2 34 30.3 33 30.9 33.5 2 23.8 27.6 23.7 28.4 23.4 28 23.6 28 2.5 19.9 22.2 19.4 22.5 19.4 22.7 19.6 22.5 3 17.3 18.1 16.8 18.2 16.9 18.6 17 18.3 3.5 15.4 15.2 15.2 15 15.2 15.4 15.3 15.2

[0231] These results show that the power source had difficulty in maintaining the required power level as the rating of the transformer increased. For example, when power was supplied via the 0.5 kV transformer, the source was able to supply power at its rated frequency (25-35 kHz) and the transformer operated at ˜86.7% of the voltage rating. In contrast, when power was supplied via the 3.5 kV transformer the system operated inefficiently, with greater output from the source required to maintain the required power level at the electrode. The greater demands placed on the power source led to a drop in frequency below the rating of 25-35 kHz.

Example 2

[0232] A series of experiments were conducted to show the effect of selecting different transformers on the number of arcing events detected by the plasma apparatus.

[0233] Graphene nanoplatelets (260 g) were loaded into the treatment vessel, and subjected to functionalisation with an oxygen plasma treatment at 70 Pa with 100 W of power supplied via a 0.5 kV transformer. The experiments were then repeated with different transformers in place of the 0.5 kV transformer.

[0234] For each transformer, the voltage rating percentage and frequency required to maintain the 100 W power level was recorded, along with the number of arcs detected by the arc detection unit. The detected arcs were observed to be “phantom” arcs, caused through changes in the power supply. In each case, detection of the arc led to shutdown of the apparatus for several seconds before restarting.

TABLE-US-00002 TABLE 2 Run 1 Run 2 Number of Transformer (kV) % V kHz % V kHz arcs detected 0.5 99.9 37.3 93.7 36.9 5 1 48.1 37 47.2 37.2 32 1.5 34.2 36.1 32.9 35.9 75 2 25.1 33.3 24.8 33 75 2.5 20.8 29.9 20.3 29.4 39 3 17.8 25.7 17.3 25 33 3.5 15.6 22 15.3 21.3 33

[0235] These results show that the power source had difficulty in maintaining the required power level as the rating of the transformer increased, in a similar manner to that observed in Example 1. In addition, the data show that the number of phantom arcs detected increased markedly from the 0.5 kV transformer to the 1.5 kV transformer, and then decreased again at transformer rating above 2.5 kV. These “phantom” arcs are indicative of electrical fluctuations in the power source caused by incompatibility of the transformer setting with the particular conditions chosen.

Example 3

[0236] A series of experiments were conducted to show the effect of selecting different transformers on the degree of graphene nanoplatelet functionalisation.

[0237] Graphene nanoplatelets were subjected to oxygen-plasma functionalisation following the procedure described in Example 2, but using a different power setting. The resulting graphene nanoplatelets were then dispersed in water, and the degree of functionalisation due to oxygen-plasma treatment was assessed by monitoring light transmittance through the dispersion over time, following the methods described in the examples of WO 2015/150830. The stability of a dispersion of untreated graphene nanoplatelets was also assessed, to serve as a control experiment. In all cases, the slower the decrease in light transmittance, the more stable the dispersion.

[0238] As shown in FIGS. 7 and 8, dispersions of the oxygen-plasma-treated GNPs were significantly more stable than the untreated GNPs, indicating that functionalisation of the GNPs had occurred.

[0239] In addition, there was a noticeable difference between the degree of functionalisation of the GNPs treated using the different transformers. The results for plasma-treated GNPs can be collected into two groups.

[0240] The first group, consisting of the GNPs functionalised using the 0.5 kV transformer and 3.5 kV transformer, displayed moderate stability. The second group, consisting of the GNPs functionalised using the transformers between 1.0-3.0 kV, displaying relatively higher stability. These results indicate that the GNPs of the second group have a higher degree of surface functionalisation than the first group.

[0241] The lower degree of functionalisation of the first group can be attributed to poorer efficiency of the plasma treating process. In the case of the 0.5 kV transformer, the measured voltage rating percentage was around 100%, which led to a reduction in power output from the transformer and consequently intermittent flickering of the plasma. In the case of the 3.5 kV transformer, the power source struggled to supply sufficient power to the electrode to maintain the plasma, and arcing events were detected, both of which led to the plasma intermittently cutting out. Thus, for both the 0.5 and 3.5 kV transformers, interruption of plasma production led to interruption of the surface functionalisation of the GNPs.

[0242] In contrast, in the higher functionalisation group, the transformers were able to efficiently produce plasma at the required power settings, leading to a more stable plasma, and hence a higher degree of functionalisation.

Examples 4-6

[0243] Examples 4 to 6 were conducted to demonstrate the effect of using a guard element according to FIG. 5A on the performance of the plasma treatment apparatus described in FIG. 1 above.

Example 4

[0244] A series of experiments were conducted to show the effect of using a guard element on the degree of functionalisation of graphitic materials.

[0245] Tests were conducted with two different types of graphitic materials: few layered graphene (FLG) and graphene nanoplatelets (GNP). Samples of each of these materials were loaded into the treatment vessel and subjected to treatment with an oxygen plasma. The conditions used during the treatment of the different materials are given in Table 3 (see below).

TABLE-US-00003 TABLE 3 Amount of material Pressure/ Treatment loaded/g Power/W mbar time/mins GNP 520 70 0.7 60 FLG 130 70 0.7 180

[0246] After treatment the samples were dispersed in water, and the degree of functionalisation was assessed by monitoring light transmittance through the dispersion over time, according to the following method:

[0247] Dispersion stability analysis method [0248] 1. 10 mg of each material was added to 25 ml of deionised water in a surfactant-free vial. [0249] 2. The mixtures were agitated for 30 s to create a colloidal suspension. [0250] 3. The transmission of light through the colloid was measured over a 4-hour period. [0251] 4. Measurements were recorded by a Dispersion Stability Analyser in conjunction with Velleman data logger and PCLab 2000SE software. [0252] 5. Slower increase of light transmission over time is directly related to better dispersion stability.

[0253] Generally, 3 sets of samples were compared each time. The stability of a dispersion of untreated nanomaterials was also assessed, to serve as a control experiment. In addition, the stability of a dispersion of a sample treated using an apparatus without a guard element was also assessed. In all cases, the slower the decrease in light transmittance, the more stable the dispersion.

[0254] GNPs

[0255] FIG. 9 shows dispersions of treated (in treatment vessels with and without a guard element) and untreated GNPs. The GNPs treated in a treatment vessel with a guard element were significantly more stable than the untreated GNPs, indicating that functionalisation of the GNPs had occurred after treatment in a treatment vessel with a guard element.

[0256] The sample treated in a treatment vessel with no guard element demonstrates inferior stability than the sample of untreated GNPs and consequently, also inferior stability than the sample of GNPs treated in a treatment vessel with a guard element. The lower stability of the GNPs treated in a treatment vessel without a guard element may be attributed to the treatment process removing contaminants that prevented close particle interaction and encouraging sedimentation by agglomeration. The treatment in a treatment vessel without a guard element however, has not led to functionalisation of the GNPs due to the system continually arcing.

[0257] For the GNPs treated in a treatment vessel with a guard element it was possible to efficiently functionalize the GNPs. Dispersability was improved to the point where no measurable sedimentation was seen after 12000 s (=3 hrs 20) and the colloid blocked out all light. The dispersion stability index data for each of the GNP materials is given in Table 4 below.

TABLE-US-00004 TABLE 4 Treatment type Stability Index.sup.1 Treatment with guard element 20 (+/−0.55) Treatment without guard element  6 (+/−0.55) None (untreated sample) 13 (+/−0.55) .sup.1The stability index is proportional to the absorption measured through the sample after 3 hrs 20 mins.

[0258] FLG

[0259] FIG. 10 shows dispersions of treated and untreated FLG materials. Both sets of treated FLG materials were more stable than the untreated FLG, indicating that functionalisation had taken place.

[0260] In addition, there was a noticeable difference between the degree of functionalisation of the FLG treated in a treatment vessel with a guard element than the FLG treated in a treatment vessel without a guard element. The sample of the FLG treated in a treatment vessel without a guard element demonstrated inferior dispersion stability than the FLG treated in a treatment vessel with a guard element. These results indicate that the FLG functionalised in a treatment vessel with a guard element had a higher degree of surface functionalisation than the sample treated in a treatment vessel without a guard element.

[0261] For the FLG functionalised in a treatment vessel with a guard element it was possible to efficiently functionalize the FLG and dispersability was improved to the point where no measurable sedimentation was seen after 17000 s (=4 hrs 40).

[0262] The light transmittance at 120 minutes for each of the FLG materials is given in Table 5 below.

TABLE-US-00005 TABLE 5 Light Transmittance Treatment type at 120 minutes None (untreated sample) 55 (+/−15.4) Treatment without guard element 10 (+/−15.4) Treatment with guard element  1 (+/−15.4)

Example 5

[0263] A series of experiments were conducted to show the effect of a guard element on the number of arcing events detected by the arc detection system.

[0264] Tests were conducted with three different types of carbon materials: GNPs, FLG and MWCNT (Multi wall carbon nanotubes). Samples of each of these materials were loaded into the treatment vessel and subjected to treatment with an oxygen-plasma. The conditions used during the treatment of the different materials are given in Table 6 (see below).

TABLE-US-00006 TABLE 6 Amount of material Pressure/ Treatment loaded/g Power/W mbar time/mins GNP 520 70 0.7 60 FLG 130 70 0.7 180 MWCNT 130 70 0.7 180

[0265] FIG. 11 shows the average number of arcs generated for each of the materials, when treated in a treatment apparatus with a guard element according to FIG. 5A and without a guard element according to FIG. 5A. Error bars show the standard error of the mean, calculated according to equation 1:

[00002]$\begin{array}{cc}\mathrm{Standard}\mathrm{Error}\mathrm{of}\mathrm{mean}=\frac{\mathrm{StdDev}}{\sqrt{n}}& \mathrm{Equation}1\end{array}$

[0266] whereby, StdDev is the standard deviation and n is the number of runs conducted.

[0267] The power and treatment times used were the same for the tests carried out in the treatment apparatus with and without a guard element for each of the materials tested.

[0268] The numerical data for all of the runs is given in Table 7 below.

TABLE-US-00007 TABLE 7 Mean number Number of Standard Standard of arcs runs (n) Deviation Error No guard 223.2 1421 657.0 37.9 element Guard 144.3 1413 398.3 18.9 element

[0269] These results show that for all of the materials tested (GNPs, FLG and MWCNT) fewer arcs were detected when a guard element was used.

Example 6

[0270] A series of experiments were conducted to show the effect of using a guard element on the pressure and voltage observed inside the treatment vessel during a given treatment step.

[0271] FLG type material was loaded into the treatment vessel and subjected to treatment with an oxygen plasma.

[0272] The treatment vessel was fitted with two pressure sensors one just prior to the gas inlet (barrel pressure) and one at the gas outlet, after the filters (chamber pressure). If the chamber pressure differs from the barrel pressure this indicates that the filters are becoming blocked.

[0273] FIG. 12 shows the barrel pressure and voltage inside the reaction vessel for a system without guard elements. The chamber pressure registered at around 0.7 mbar throughout and so is omitted for clarity. The process was paused every hour and the filters were back flushed to remove any sample trapped in the filters (back flushing of filters refers to process of removing reactor barrel from chamber and agitating filters to clear built-up material).

[0274] FIG. 12 shows that the voltage increases in response to increases in the barrel pressure (generally, voltage and pressure are expected to be related according to Paschen's Law). However, discontinuity between the barrel pressure and the chamber pressure shows that there must be a partial physical barrier between the barrel and the rest of the chamber (where the chamber pressure is measured) suggesting that the chamber filters have become blocked. This is attributed to FLG clogging the filters. The voltage during the treatment step has a range of approximately 4 kV %.

[0275] Back flushing of the filters is shown to return pressure and voltage to within normal limits, this again demonstrates that the filters are clogging and hence causing the pressure in the barrel to rise. Plasma quality is known to depend on fine control of voltage and pressure during the treatment step and so clogged filters results in inferior quality plasma and hence less even functionalisation of the material being treated.

[0276] FIG. 13 shows the pressure and voltage inside the reaction vessel with a vessel filter and guard element according to FIG. 5A. The experiment was performed in the same way as described above, apart from the filters were not back flushed.

[0277] In this case the voltage is very stable, with the voltage range being within 0.5 kV % after equilibration. The barrel pressure was not measured and only chamber pressure is shown in FIG. 13, but stable voltage is taken as evidence of stable pressure. Therefore, suggesting that better quality plasma is achieved with the endplate comprising a vessel filter and guard element, which is expected to lead to more even functionalisation of the sample being treated.

Example 7

[0278] Example 7 was conducted to demonstrate the effect of modulating the power between a higher power and a lower power on the number of arcing events detected by the arc detection system during oxygen plasma treatment.

[0279] Tests were carried out with MWCNTs in a plasma treatment vessel according to FIG. 2. Samples of MWCNTs (27 g) were loaded into the treatment vessel and subjected to treatment with an oxygen plasma at 0.7 mbar for 180 minutes (for all treatment runs shown).

[0280] Runs 1-16 and 20 were conducted without modulating the power i.e. at a constant power level. The average arc count during these tests was 922.4. During runs 17-19 and 21-24 the power was modulated according to a set pattern corresponding to a square waveform, repeated at a frequency of 500 Hz to 1000 Hz, wherein the lower power level corresponds to no power being supplied during a given treatment step and the ratio of time spent at the higher power level compared to the lower power level is at least one. During runs 17-19 and 21-24 the arc count was reduced to effectively zero. Run 20 was a control run without power modulation, helping to confirm the reduction in arc count is due to the introduction of pulsed power and not a result of any other changes that may have happened to the treatment apparatus.

[0281] The power data shows that modulating the power facilitates power increases to as much as 500 W without arcs and without the associated risk of damage to the treatment apparatus due to thermal arc formation.

Examples 8-9

[0282] Examples 8 and 9 were conducted to demonstrate the types of functionalisation that can be achieved using the apparatus according to FIG. 2 and in particular an apparatus including a vessel filter and filter guard as in FIG. 5A.

Example 8

[0283] Tests were conducted with FLG type materials. A sample of FLG (40 g) was loaded into the treatment vessel and subjected to treatment with a fluorination plasma, formed using CF.sub.4 gas at 0.7 mbar with 500 W of power supplied via a 1/5 kV transformer for 180 minutes. The power was modulated during the treatment step in the same way as in example 7. The weight percentage of carbon, oxygen, nitrogen and fluorine was determined using X-Ray Photoelectron Spectroscopy (XPS). The results are given in Table 8 below.

TABLE-US-00008 TABLE 8 Concentration/at % XPS XPS XPS XPS Sample C (%) O2 (%) N (%) F (%) Unfunctionalised Average 94.58375 5.02875 0.545 0 (n = 8) Standard 0.993579 0.904647 0.22243 0 Deviation Functionalised Average 2.715 68.245 0.285 28.76 (n = 2) Standard 0.275 0.335 0.035 0.64 Deviation

[0284] For all untreated FLG materials (8 repeats in total) fluorine content was confirmed to be zero.

[0285] In contrast, the treated particles demonstrate an increase in atomic percentage of fluorine of 28.76% (based on 2 repeats).

[0286] Addition of high levels of fluorine renders graphitic material hydrophobic and has been likened to ‘Teflonisation’ because the highly fluorinated polymer PTFE/Teflon is known for its intermolecular repulsion and inert nature. This opens up markets for solid lubricants, anti-fouling surfaces and PTFE fillers.

Example 9

[0287] A sample of boron nitride (40 g) was loaded into the treatment vessel and subjected to treatment with argon gas at the conditions given in Table 9. Samples of boron nitride (40 g) were also subjected to treatment with a number of different plasma forming feedstocks, using the conditions given in Table 9. The power was held constant (not modulated) during the course of the treatment steps.

[0288] In this example a temperature controlled treatment vessel was used and the temperature was adjusted to be suitable for the different treatment types (Raw materials). For example for ammonia (NH.sub.3) treatment temperatures of greater than 28° C. were used and for 02 temperatures of below 20° C.

[0289] The transformer setting was also adjusted for the different treatment types (raw materials) for example a lower setting was used for O.sub.2 than for NH.sub.3. This demonstrates that a single machine can be used to carry out a range of different functionalisation steps with a range of different raw materials. The presence of the guard elements also helps to prevent arcing during treatment with a range of different raw materials.

TABLE-US-00009 TABLE 9 Treatment Plasma Material Treatment Type Power/W Loading/g Time/mins Raw material N/A N/A N/A Ar 70 190 180 COOH 70 190 180 NH.sub.3 70 190 180 O.sub.2 70 190 180 F 70 190 180

[0290] The degree of functionalisation for each of the boron nitride samples following treatment with the different plasma forming feedstocks is shown in FIG. 15.

[0291] In summary: [0292] Oxygen (O.sub.2) treatment increased O content by around 3.5%. [0293] Acrylic acid (COOH) treatment increased O by 2.5% [0294] Tetrafluoromethane (F) treatment increased F (0.7%) and C levels (2%). [0295] Neither argon (Ar) nor ammonia (NH.sub.3) treatment had a significant effect on the composition.

[0296] This shows that a treatment apparatus with a temperature controlled treatment vessel, guard elements and a transformer having two or more different settings allows a range of different raw materials to be functionalised.

Example 10

[0297] The plasma treatment apparatus according to FIG. 1 incorporating a system for delivering a liquid into the treatment vessel according to FIG. 6 was used to demonstrate that the plasma treatment apparatus could be used for silane functionalisation.

[0298] Two different graphitic materials were treated under similar conditions to those used in example 8. The results of these tests are given in table 9 below.

TABLE-US-00010 TABLE 9 O1s C1s N1s F1s Si2p Material: Edge Oxidised Graphene Oxide.sup.1 Raw (Ave) 4.86 94.53 0.61 0 0 Treated 8.87 90.24 0.59 0 0.29 Material: Graphene Nanoplatelets.sup.2 Raw (Ave) 4.28 95.35 0 0 0.3 Treated 6.95 90.29 0.96 0.05 1.75 .sup.1The power was modulated during the treatment of the edge oxidised graphene oxide; .sup.2The power was held at a constant level during the treatment of the graphene nanoplatelets.

[0299] Experiments demonstrated that silicon can be incorporated onto the surface of the carbon materials after treatment. This demonstrates that the liquid injection system can be used to provide plasma feedstocks to effectively functionalize the carbon materials.

Example 11

[0300] A series of experiments were conducted to show the effect of heating on the degree of functionalisation of graphitic materials.

[0301] FLG type material was subjected to oxygen-plasma functionalisation in a treatment apparatus described in FIG. 2 above.

[0302] FIG. 16 shows the acid number (approximately proportional to the number of R—COOH groups on the surface of the sample) for samples after treatment against the current hours per gram of sample treated (A.h/g) used for the treatment. The acid number was determined by titration in a Mettler Toledo Autotitrator. EQP 1 corresponds to treating samples for 3 hours at various loadings and at moderate powers (insufficient to lead to significant heating, <500 W). A logarithmic trend line was plotted for EQP 1 with an R2 value of 0.9598.

[0303] For the points corresponding to EQP 1 hot the samples were treated at higher powers (>800 W, corresponding to higher currents), which generated temperatures in the barrel of >100° C. For the points corresponding to EQP 1 cooled, the materials were also treated at higher powers (>800 W), but the treatment was paused intermittently to allow the temperature of the barrel to return to ambient temperature. The results of these tests are also shown in table 10 below.

TABLE-US-00011 TABLE 10 Treatment Acid Number/ Time/h Power/W Loading/g Current/Amps mgKOH/g Amp .Math. h/g EQP 1 3 200 40 4.1 102.2 0.3075 3 300 40 5.5 120.34 0.4125 3 200 40 3.9 98.53 0.2925 3 300 40 5.2 117.7 0.39 3 300 80 5.208333333 89.7 0.195313 3 300 130 4.901960784 58 0.113122 3 500 40 7.3 122.46 0.5475 3 500 40 7.42 123.05 0.5565 3 500 40 7.34 122.46 0.5505 EQP 1 3 804 40 10.89 85.46 0.81675 hot 3 1000 80 12.1 82.2185 0.45375 1 1000 40 12.03 82 0.30075 EQP 1 1 844 40 10.67 92.34 0.26675 cooled 3 1000 40 12.08 127.3 0.906

[0304] The values for EQP 1 hot fall below the trend line for acid number on FIG. 16; whereas the EQP1 cooled values show much better agreement with the trend line. This demonstrates that excess heat is causing the degree of functionalisation of the samples to go down, leading to lower acid numbers. Without wanting to be bound by any theory it is believed that this is a result of decarboxylation occurring due to elevated temperatures during treatment.

[0305] For the avoidance of doubt it is confirmed that in the general description above, in the usual way the proposal of general preferences and options in respect of different features and embodiments of the methods and apparatus constitutes the proposal of general combinations of those general preferences and options for the different features and embodiments, insofar as they are combinable and compatible and are put forward in the same context.

[0306] In respect of numerical ranges disclosed in the present description it will of course be understood that in the normal way the technical criterion for the upper limit is different from the technical criterion for the lower limit, i.e. the upper and lower limits are intrinsically distinct proposals.