GB2458776A - Chemical vapour deposition process - Google Patents

Abstract

A thermal CVD process for growing carbon structures on a continuous substrate 1 that is moved into and out of a deposition chamber 5 through one or more non-airtight openings 13, 15 located at or near one end or portion thereof, with a reactive gas mixture being fed in a sealed manner from an opposing end 11. The substrate may reverse its direction inside the chamber and at least one reversing device 17 may be located in the chamber. The continuous substrate may be a wire, ribbon, yarn, filament or fabric. Preferably, the substrate comprises a continuous tow of reinforcing fibres for use in a composite. The carbon structures may be carbon nanotubes. In another aspect, the thermal CVD process is preceded by two pre-treatment stages to facilitate carbon structure deposition.

Description

I

Chemical Vapour Deposition Process This invention relates to a process for growing carbon structures on the surface of a substrate, and especially to such a process for growing carbon structures using chemical vapour deposition (CVD) on continuous substrates, especially moving, continuous fibres. In particular the invention relates to a process for growing carbon nanostructures on the surface of continuous, moving substrates using thermal CVD. The carbon-bearing substrates, especially when fibres, find particular application as reinforcing members in composites.

CVD processes for depositing carbon onto continuous fibres are known. For example, the article uEvidence for the benefit of adding a carbon interphase in an all-carbon composite uby Hariom Dwivedi et al, Carbon 44 (2006) 699 -709, available online at www. science direct.com, www.elsevier.com/locate/carbon, describes a process for depositing pyrolytic carbon on continuous tows of carbon fibres using a chemical vapour deposition (CVD) device in which a carbon fibre tow comprising 6000 filaments is delivered into a vertical, alumina reactor tube from a drum located on top of the tube, travels along the reactor tube across a furnace, then is rewound on a motorised gathering drum located at the bottom of the tube. The fibres are then incorporated into a coal tar matrix to form an all-carbon composite. The mechanical behaviour of the carbon fibre/carbon matrix is improved by the presence of the pyrolytic carbon coating, which acts as an interphase between fibre and matrix.

US 4859503 describes continuous coating of a strand of carbon filaments with a refractory carbide in a hermetic enclosure, by a reactive CVD deposition technique. US 5547512 describes deposition of a barrier coating of an oxide, carbide, boride, nitride, silicide, carbon or metal onto a continuous fibre tow, such as, for example, aluminium borosilicate and mullite fibres, alumina-zirconia fibres or carbon fibres, by CVD, in a horizontal cylindrical furnace tube having one end through which the reactive gas and an inert carrier gas are fed and the fibre tow is also introduced. The total flux, when subjected to the high temperature of the furnace, deposits a continuous coating by CVD onto the filaments of the moving tow. The other end of the furnace tube is unconstricted, and the flux exiting through the unconstricted outlet is sufficient to minimize the back diffusion of air such than non-oxide coatings can be attained with acceptable levels of oxygen contamination. A reactive gas flow of 1 90cc/mm and a high inert argon gas flow rate of 2100cc/mm are exemplified. The barrier coating is required so that when the fibres are incorporated into a matrix and used at high operating temperatures, the matrix does not chemically react with or dissolve the fibres.

Similar methods for fabricating carbon nanostructures by chemical vapour deposition are also known. Carbon nanostructures are discrete, ordered structures grown from specific catalytic sites on a substrate and include carbon nanotubes, nanostacks, nanofibres and the like, which group of structures grow axially along their respective lengths away from the substrate surface. The carbon nanotubes (CNT's), which may be single wall or multi-wall carbon nanotubes, and carbon nanostacks or graphite nanofibres, may have dimensions, for example, of from I nm to 500nm wide and 1 pm to several mm long. In a typical carbon nanostructure, each carbon atom is bound to three neighbouring carbon atoms to form a flat plane of repeating hexagonal rings resembling a honeycomb. In a carbon nanotube, this honeycomb is rolled to form a cylinder or tube.

CVD methods for growing nanocarbons usually rely on the presence of a specific metal powder catalyst, such as iron, cobalt or nickel, to initiate growth of the carbon nanostructures and to control their subsequent morphology. For example, Applicant's earlier application, PCT/GBO3/00674, describes a process for growing carbon nanostructures, in which the nanostructures are grown on a continuous, elongate, heated, catalytic filament, wire, ribbon or tape passing through a sealed CVD chamber, the carbon nanostructures being scraped off upon their exit from the chamber.

The review article "Carbon Nanotube Synthesis via the Catalytic CVD Method: A Review on the Effect of Reaction Parameters", Fullerenes, Nanotubes, and Carbon Nanostructures, 14:17-37, 2006 explains that, in principle, thermal chemical vapour deposition to achieve carbon nanostructures is achieved by the catalytic decomposition of a gaseous carbon precursor, such as a hydrocarbon or carbon monoxide feedstock, with the aid of a supported transition metal catalyst in a furnace. Typical catalysts include cobalt, iron, titanium, nickel, some zeolites and combinations of these metals and or their oxides. On page 25, it states: uTo avoid oxidation of the carbon, the chamber is kept free of oxygen during the production process. Generally continuous inert gas flow is supplied to the reaction chamber. Nitrogen and argon are the most extensively used inert gases." The flow rate of the carbon source gas is described as being typically 10-30 ml per minute, with the flow rate of the inert gas being typically 1 OOml per minute.

The following prior art processes grow CNT's under a variety of conditions, using different catalytic methods. None, however, are concerned with growing CNT's on a continuous substrate that is moved into and out of the deposition chamber; rather they involve static substrates.

The article 1Hierarchical Composites Reinforced with Carbon Nanotube Grafted Fibres: The Potential Assessed at the Single Fiber Level", Chem. Mater, 2008, 20,1862- 1869, published 9 February 2008 describes grafting CNT's on stationary carbon fibres using a static CVD method, wherein iron was selected as the catalyst and was predeposfted using the incipient wetness technique before the growth reaction.

W02007/026213 describes a static process for the continuous production of CNT's using a fluidized mixture of a floating catalyst (comprising ferrocene in a carrier gas, xylene) and a hydrocarbon feed gas, in a CVD chamber having stainless steel or glass lined walls. The article "Production of Controlled Architectures of Aligned Carbon Nanotubes by an Injection Chemical Vapour Deposition Method", Carbon 41(2003) 359- 368 also describes a static CNT growth process using an injection CVD method in which ferrocene as catalyst is introduced by injection into a CVD chamber and CNT growth on a quartz substrate or chamber walls is achieved. The article "Long Bundles of Aligned Carbon Nanofibers Obtained by Vertical Floating Catalyst Method", Materials Chemistry and Physics 87 (2004) 241-245 is a further example of a static CNT growth process and uses a vertical quartz reactor.

The present invention provides a process for growing carbon structures on a continuous substrate by thermal CVD, wherein the continuous substrate is moved along its length into and out of a deposition chamber through one or more non-airtight openings located in one end or portion of an otherwise sealed deposition chamber, into which a reactive gas mixture for generating the crbon structures is also introduced in a sealed manner.

Such a set-up permits the substrate to be movable along its length without needing to seal its entry and exit points yet still retaining a relatively oxygen-free i.e. substantially inert atmosphere.

By ensuring both the entry and exit of the substrate is confined to one end or portion of the chamber, for example, through reversing or redirecting of the moving substrate inside the chamber, it is possible to grow carbon structures, such as pyrolytic (amorphous) carbon or nanocarbons, at the high temperatures required for CVD (chemical vapour deposition) using non-airtight inlets/outlets, that is with those openings open to the atmosphere. This is highly convenient for a substrate that needs to move, where it is difficult to seal such openings, and enables carbon structures to be deposited on continuous substrates such as fibres in large quantities and at reasonable costs. This is in contrast to prior art arrangements, which usually need completely sealed chambers to prevent oxidation of the carbon structures, or use very high reactant/inert/reductant gas flow rates to prevent air ingress. Indeed, the present arrangement has been found suitable for use with substrates of combustible material, for example, carbon fibre tow, which would otherwise be burnt up at the elevated temperatures used in the thermal CVD process, if such air seepage had occurred.

Preferably, the reactive gas mixture for generating the carbon structures is introduced at a location remote from the one end or portion, although it could be introduced from a plurality of well spaced positions. Ideally, the reactive gas mixture is introduced from the bottom or a lower part of the chamber so that it flows upwards. The mixture will usually include an inert gas component and the mixture will be continuously supplied for the entire duration of the process, the intention being to prevent air or moisture ingress.

There is further provided a process for growing carbon nanostructures on a continuous substrate by thermal CVD, wherein the continuous substrate is moved through a deposition chamber, entering and exiting it through one or more non-airtight openings located at or near one end of the deposition chamber, and a reactive gas mixture is fed in from an opposing end of the deposition chamber.

Usually the one or more substrate inlet/outlet openings are substantially the only non-airtight openings in the chamber, so that the other end or portion of the chamber is sealed. All the gases in the chamber are therefore forced to exit through those opening(s), and given they are introduced at the opposite end or portion of the chamber, generally leads to a one-way flow of gases within the chamber. Those openings will usually only be slightly larger in cross-section than the substrate and may even be constricted so as to minimise air ingress.

Most end use applications will require the carbon to be deposited in the form of carbon nanostructures.

There is further provided a process for growing carbon nanostructures on a continuous substrate by thermal CVD, preferably by atmospheric pressure CVD (APCVD), wherein the continuous substrate is moved through a deposition chamber, entering and exiting it through one or more (e.g. a pair of) non-airtight openings located at or near one end of the deposition chamber, and a reactant gas mixture is fed in from an opposing end of the deposition chamber.

The reactive gas mixture will comprise one or more reactant (carbon source) gases and one or more inert gases in a desired ratio, as well as any required reductant gases or catalytic gases. Usually the rest of the chamber is sealed and any reactant/reductant/inert or catalytic gases are all introduced in an airtight manner and forced to exit through the non-airtight openings. While the substrate may enter and exit through a single non-airtight opening, separate respective unsealed inlet and outlet openings, but in the same portion of the chamber, are usually needed to facilitate transfer to adjacent spooling mechanisms.

The substrate needs to stay in the chamber for a sufficient duration to enable growth of carbon structures to occur. The substrate may follow a path whereby it reverses its directton inside the chamber, so as to allow it to enter and exit the chamber from that one end or portion. While the substrate could follow a path where it is redirected by passing around the inner periphery of a reaction chamber of a particular shape (e.g. a drum or ring-shaped, i.e. a solid or hollow doughnut shaped chamber), conveniently, the substrate follows a path whereby it reverses its direction of movement inside the chamber.

At least one reversing device may be provided in the chamber. The device is preferably located at or near the remote location or the opposing end or portion of the chamber.

Normally, the chamber is an elongate tubular chamber. In such a chamber, where the substrate is set up to follow a path where it reverses its direction, then this may be achieved using at least one reversing device located at or near the opposing end or portion of the chamber. Such reversing devices may be static devices including loops, channels or guides or the like, but to minimise damage to the deposited carbon coating are preferably mobile devices including rotating wheels, guides, pulleys, or the like, or may include resilient mechanisms such as springs that attach guides or pulleys, etc. to the chamber walls.

If one device is provided, the substrate will completely reverse its path so as to follow the same or a closely aligned path. However, the substrate may be guided around two or more devices so that the return path is spaced from the initial path. In any event, the substrate reverses such that it exits and enters the chamber in the same vicinity, through which all gases must exit, thereby ensuring a clear gas flow direction in the chamber and minimising oxygen ingress.

To ensure a uniform residence time, the reversing device is normally either fixedly or flexibly attached to the interior of the chamber. However, it may hang freely (e.g. a weighted pulley); the latter arrangement would suffice, for example, where the substrate reverses its direction at its lowermost end, as in a vertically arranged deposition chamber.

Preferably, the non-airtight opening(s) of the chamber are located in the uppermost part of the chamber, so as to take advantage of upward convection.

The CVD chamber may be any suitable shape. An especially preferred shape is an elongate chamber, for example an elongate cylindrical chamber.

In a highly preferred arrangement, the chamber is an elongate, vertically aligned (i.e. upright) chamber with the openings located uppermost through which the hot gases exit.

The substrate moves in a direction along its length through the chamber, usually with a continuous motion at a calculated spool rate that gives the correct residence time in the chamber. Continuous movement provides for a more uniform coverage of carbon structures.

However, similar advantages may be achieved where the substrate is moved semi-continuously i.e. batchwise through the chamber, for example, equal lengths are successively moved on into the chamber at selected intervals. Preferably, such lengths will be short enough for each section to reside in the chamber for multiple residences (e.g. four or more) for improved uniformity of coverage. However, where an elongate tubular chamber is used and the substrate is reversed at the furthermost end thereof, a length of substrate approximately equal to the length of the chamber may be introduced into the chamber and held stationary for a first residence period, and then advanced around the reversing device for a second stationary residence period, before exiting the chamber. Alternatively, a length double the length of the chamber may be introduced into the chamber for a single stationary residence period before exiting the chamber.

The continuous substrate may take any suitable form, but may be, for example, a yarn, ribbon, wire, filament or fibre, a tow of filaments or fibres, a fabric or a narrow fabric strip. Similarly a number of different materials are suitable for the continuous substrate for example, ceramic such as alumina or silane, carbon, glass or iron wire. The substrate material must be sufficiently heat resistant to resist the high temperatures (typically 600-800°C) typically used in the thermal CVD process. In a highly preferred application, the continuous substrate comprises a continuous tow of reinforcing fibres for use in a composite.

There is also provided a process for preparing a continuous substrate having carbon nanostructures deposited thereon, which process includes moving the substrate continuously or semi-continuously along its length through successive processing stages including:-i) an optional pre-treatment stage; ii) an optional catalytic pre-treatment stage to facilitate carbon nanostructure deposition; and, iii) a thermal CVD deposition stage comprising a process as described above.

A pre-treatment stage will usually be required. For example, carbon fibres are generally unreactive, non-absorbent materials and usually require some surface treatment to improve the fibre surface wettability or to increase the quantity of surface functional groups. The pre-treatment stage may comprise an oxidation stage.

Stage ii) may comprise passing the substrate through at least one bath containing a catalytic agent, for example, where the incipient wetness impregnation technique is used. Alternatively an optional catalytic treatment may form part of stage iii), such as, for example, the injection technique which is used to introduce catalytic gas-borne particles into the chamber during carbon growth.

After stage iii), a further downstream process may involve removing the carbon structures from the substrate.

The present invention further provides a process for depositing carbon structures on a continuous substrate by thermal CVD, wherein the continuous substrate is moved into and out of a deposition chamber through one or more non-airtight openings located at or near one end or portion of the deposition chamber, and a reactive gas mixture is fed in from an Opposing end or portion of the deposition chamber.

The present invention will now be described, by way of example only, with reference to the accompanying drawings in which:-Figs. IA and I B are respective schematic front views of two alternative CVD rigs in accordance with the present invention; Fig. 2 is a schematic front view of a third alternative CVD rig in accordance with the present invention; Fig. 3 is a pair of scanning electron micrographs (SEMs), at two different magnifications, of CNT's growth on Continuous carbon fibres; Figs. 4A and 48 are respective schematic front views of two alternative, comparative CVD rigs (i.e. not according to the present invention); and, Fig. 5 is a schematic side view of a Continuous production process for generating CNT coated carbon fibre tows.

Example I -Carbon Fibre Tow The following example describes a process for growing carbon nanotubes on continuous carbon fibres using a CVD process according to the invention. In this example, polyacrylonitrile (PAN) based carbon fibres were used in a tow comprising about 12,000 continuous fibres of average diameter 7pm.

The tow fibres were surface treated to improve their absorption and interaction properties. This surface treatment process Comprised oxidation by refluxing in nitric acid, washing of the fibres, and then soaking of the fibres in sodium hydroxide solution at ambient temperature, and finally refluxing in the same solution.

Carbon nanotubes (CNT's) synthesis usually requires the introduction of a catalyst often in the form of gas particulates or as a solid support. (Otherwise, amorphous or pyrolytic carbon coatings will result.) A variety of catalytic methods may be used with thermal CVD, and these include the incipient wetness impregnation technique and the injection technique. In the incipient wetness technique, the substrate is uniformly impregnated with a catalyst containing solution prior to CVD, while the injection technique involves injecting a metal containing solution into a preheated chamber where it is vaporiseci and the catalyst carried into the CVD chamber. In the present example, the incipient wetness technique was used on the carbon fibres. The choice of metallic catalyst may affect the morphology and growth of the CNTs. The use of transition metals iron, cobalt and nickel (or titanium) alone or in bimetallic combinations is well known for catalysing nanotubes growth by thermal CVD. The incipient wetness technique was used to incorporate an iron catalyst onto the surface treated PAN-based fibres. Iron nitrate nanohydrate (Fe(N03)3.9H20), as a catalyst precursor, was dissolved in propan-2-ol, and the surface treated PAN-based fibres were impregnated with this solution, and then washed in fresh propan-2-ol to remove the excess nitrate solution, and then dried at 200- 300°C. The final stage in the incipient wetness technique, calcination, involves treating the samples at a high temperature of about 750°C to form the Fe catalyst particles.

Since this calcination step needs to be carried out at the same temperature as is required for CNT growth, it was decided to attempt to carry out this final stage simultaneously with growing the carbon nanostructures on the iron catalyst coated fibres.

This would result in less physical damage to the lengthy fibre tow and lead to a shorter and more cost effective process.

The fibres 1 were subjected to thermal CVD in the CVD rig 3 shown schematically in Figure lB. The rig 3 comprises a CVD quartz tube 5 housed within a furnace 7.

Heating to a temperature in the range 600-800°C,preferably about 750°C, is provided by a heating element 9 surrounding the tube 5. The tube 5 is arranged vertically within the furnace 7, and has an upper end that is only open to the atmosphere through lateral openings 13 and 15, as well as a sealed lower end that is contiguous with a gas entry pipe 11, so that no air from the atmosphere can enter the tube at its lower end.

A seed carbon tow is pre-threaded through the CVD tube 5. It enters through opening 13 at the upper end of the tube 5, extends down the tube and then passes around reversing device in the form of a freely rotating yam guide 17, and passes back upwards through the tube 5 to pass out of opening 15 at the same upper end level with opening 13. (Separate openings were preferred to accommodate the respective spooling arrangements and because entry and exit vertically out of a single opening would have been difficult in practice.) The tow is then wound via a tow guide 19 onto a take-up roller 21. The yarn guide 17 is fixed to the inner walls of the work tube 5 by fixing strips so as to maintain a constant effective length and dwell time for the tow in the tube 5.

The dried, catalyst pre-coated carbon tow was wound up on a delivery spool (not shown) and the end tied to the seed carbon tow. The rate of passage of the tow through the tube can be anything from 1 to 6 cm/minute, but the take-up roller 21 was usually set to move at a very slow rate of 1 -3 cm/mm. The tube 5 was 40cm long (effective process length 80cm), so the dwell time for most runs was between 30 mm to 1.5 h. In Figure 1 B, the reversing device is fixedly attached to the tube walls to ensure uniform runs. However, in Figure 1A, an alternative rig is shown with a freely hanging reversing device 17'. This may be more convenient and entirely acceptable for some types of substrates or for experimental trials. It does, however, have the disadvantages that it may drop and break the tube when tension is removed from the substrate, and that variation in tow tension can cause it to move up and down in the tube, leading to variable effective dwell times. Referring to Figure 2, this shows a further reversal device where a yarn guide 17" is connected by two or more flexible links to the tube 5, so as to give some play and minimise damage to the nanocarbons.

In operation, a gas system consisting of argon acting as a carrier gas, hydrogen, acting as a reductantfmorphology control gas and ethylene or carbon dioxide, acting as a carbon source gas, were fed into the tube 5 via the gas entry pipe 11. Average flow rates for the ethylene, argon and hydrogen were 5, 50 and 5 mIs/mm respectively (i.e. 1:10:1).

It will be noted that these are very moderate flushing rates, despite the presence of unsealed openings 13 and 15. (This is in contrast to the excessive flushing rates needed in some prior art set-ups that are open to the atmosphere.) The actual pressure may be close to or of the order of atmospheric pressure but it is the inertness of the atmosphere rather than the actual pressure which is important. The reactant gas: inert gas ratio will usually be between 1: 6 to 1: 15. Inert gas flow rates usually will not need to exceed 0.51/mm, more preferably not more than 0.21/mm.

For each run (about 2h), the furnace temperature was raised to and then maintained at 750 °C, with continuous argon flushing (until the run had finished and the apparatus had cooled back down to room temperature). Hydrogen was introduced after the furnace temperature had stabilised, so as to remove oxygen, and ethylene was also introduced as spooling commenced. After nearly all the carbon tow had passed through the apparatus, a further seed tow was attached to the far end of the carbon tow, and the ethylene and then the hydrogen and eventually the argon were switched off, in that order.

The fibres wound well and appeared fully open. Figure 3 shows a pair of SEMs at different magnifications of the tow removed from the take-up reel after one continuous trial run and shows a good growth of C NT's on the surface of the tow. The C NT's exhibited random orientation and coiling. While growth is not as dense as might be achieved in a static environment, it is still encouraging that despite the substrate moving around various guides, reasonable coverage is achieved. Such CNT coated carbon fibres, otherwise known as "hairy fibres", inherently give rise to improved properties when incorporated as reinforcing fibres in composites, due to the enhanced fibre-matrix interactions caused by the numerous tiny CNT "hairs"

In summary, this example has shown that:-

* it is possible to grow CNT's on a continuously moving carbon fibre tow at around atmospheric pressure without the tow being burnt off despite the unsealed openings.

* it is not necessary to have excessively high flushing rates where unsealed openings are used * it is possible to combine the calcination of the catalyst stage and the CVD stage into a one step process (i.e. one high heating step only) by putting the dried pre-coated tow into CVD * the process has the potential to be a single overall continuous process, including the oxidation treatment, catalytic impregnation and drying treatment and ca!cination/CVD treatment.

By way of comparison, the two experimental rigs depicted in Figs. 4a and 4b were tried out, each involving the tow passing in one end of the CVD tube and out of the opposite end. In both cases, it was extremely difficult to seal the tow inlet and outlet adaptor at the CVD tube bottom stopper to prevent entry of environmental air and moisture. Moreover the flow rate of argon was not high enough to prevent the influx. In fact, it was probable that some of the combined gases (Ar/H2/C2H4) escaped through the adaptor. Consequently, in every run attempt the tow burnt off and disintegrated.

Exam�le 2-Glass Fibre Tow In order to grow CNT's on glass fibre tow, the apparatus can be adapted to the CVD rig shown in Fig. 2, which allows the floating catalyst technique, as mentioned in the above acknowledged prior art, to be used instead of the incipient wetness technique.

Thus, following a suitable surface pre-treatment, a glass fibre tow 25 can be exposed in the reaction chamber simultaneously to reactant gases and catalytic gases.

Referring to Fig. 2, this rig differs from those in Figs. 1A and 1 B in that it has an additional pre-heated chamber 23 for introducing heated catalytic gases into the CVD tube, which pre-heated chamber is interposed between the feed gas supply and the gas inlet to the CVD tube. During a run, a ferrocene Fe(C5H5)2 solution in toluene would be injected using controlled delivery means 24 into the preheated chamber 23 (heater jacket not shown) maintained at around 200°C, and would be turned into vapour. As the feed gases pass through the pre-heated chamber 23 and into the CVD tube 5 they would carry that vapour into the CVD tube, to be deposited on the glass fibre substrate 25.

In this case, depending on the operation of the controlled delivery means (i.e. whether it injects solution continuously or periodically), it might be appropriate to move the glass fibre tow batch-wise into and out of the chamber in prescribed lengths, either for a single occupancy stage, or double occupancy or even multiple occupancy, possibly each movement being linked to a simultaneous further injection of catalyst.

As mentioned previously and as shown in Fig. 2, a flexibly mounted yarn guide 17" could be used.

Examole 3-Continuous Production Process for aeneratinQ CNT Coated Carbon Fibre Tows Figure 5 shows a possible scaled up continuous (or semi-continuous) production process for coating carbon fibre tows with CNT's. In essence, since the surface pre-treatment stage and the incipient wetness impregnation treatment and the intervening washing and rinsing steps all involve solution baths, it is possible for the entire process to be automated and carried out on continuously or semi-continuously moving continuous fibre tow. If desired, this could incorporate the CVD process at the end, although the fibre may instead be transferred manually to the CVD rig for separate processing at a preferred spooling rate.

Referring to Figure 5, the tow 29 would travel at a speed of 1-4 cm/mm starting from the left hand side of the diagram in the direction of arrow C. Section A would be the pre-treatment stage (about 3m in length), while section B (about 5m) would be the catalytic treatment stage, before automated or manual transfer to the CVD rig. Section A commences with the carbon fibre tow travelling over and around bobbins 31 successively through a nitric acid bath 33, then a de-ionised water wash bath 37, a sodium hydroxide bath 39 and a de-ionised water wash bath 41, with water sprays 35 being located above transfer bobbins 43 located between each of the baths, and the water spray being collected by a drain bench (not shown). An extraction hood 45 would be provided above the water sprays 35.

The tow 29 would then transfer into Section B and through an oven 47 maintained at 100°C to be dried, variable speed air blowers 49 being provided overhead upon entry and exit from the oven. The tow 29 would then pass successively through a bath 51 of transition metal salt catalyst solution or similar catalytic agent solution (e.g. iron nitrate nanohydrate) and a solvent rinse bath 53, before passing under another variable speed blower 49 and through a further oven 55 maintained at 100-300°C, where the catalytically treated fibres would be dried, before being collected on a wind-up bobbin 57.

The tow 29 could then be manually or automatically transferred to a CVD rig 59 set up and operating in accordance with the invention (as described, for example, in Example I above) underneath another extraction hood 45, before finally being collected on winder 61.

A continuous production scheme with appropriate modifications for glass fibre substrates could similarly be designed based on Example 2.

It will be apparent to the skilled person that numerous modifications could be made to the arrangements described above, but still in accordance with the present invention. The present invention further contemplates any novel feature or novel combination of features described above.

Claims (15)

1. CLAIMS1. A process for growing carbon structures on a continuous substrate by thermal CVD, wherein the continuous substrate is moved along its length into and out of a deposition chamber through one or more non-airtight openings located in one end or portion of an otherwise sealed deposition chamber, into which a reactive gas mixture for generating the carbon structures is also introduced in a sealed manner.
2. 2. A process as claimed in claim 1, wherein the reactive gas mixture for generating the carbon structures is introduced at a location remote from the one end or portion.
3. 3. A process as claimed in claim I or claim 2, wherein the reactive gas mixture is introduced from the bottom or a lower part of the chamber so that it flows upwards.
4. 4. A process for growing carbon nanostructures on a continuous substrate by thermal CVD, wherein the continuous substrate is moved through a deposition chamber, entering and exiting it through one or more non-airtight openings located at or near one end of the deposition chamber, and a reactive gas mixture is fed in from an opposing end of the deposition chamber.
5. 5. A process according to any preceding claim, wherein the substrate follows a path whereby it reverses its direction inside the chamber so as to allow it to enter and exit the chamber from that one end or portion.
6. 6. A process according to claim 5, wherein at least one reversing device is provided in the chamber.
7. 7. A process according to claim 6, wherein the reversing device is either fixedly or flexibly attached to the interior of the chamber.
8. 8. A process according to any preceding claim, wherein the chamber is an elongate tubular chamber.
9. 9. A process according to any preceding claim, wherein the non-airtight opening(s) of the chamber are located in the uppermost part of the chamber.
10. 10. A process according to any preceding claim, wherein the substrate is moved continuously through the chamber.
11. 11. A process according to any preceding claim, wherein the continuous substrate is a wire, ribbon, yam, filament or fibre, or a tow of filaments or fibres, or a fabric or fabric strip.
12. 12. A process according to claim 11, wherein the continuous substrate comprises a continuous tow of reinforcing fibres for use in a composite.
13. 13. A process for preparing a continuous substrate having carbon nanostructures deposited thereon, which process includes moving the substrate continuously or semi-continuously along its length through successive processing stages including:-i) an optional pre-treatment stage; ii) an optional catalytic pre-treatment stage to facilitate carbon nanostructure deposition; and, iii) a thermal CVD deposition stage comprising a process as claimed in any one of claims ito 12.
14. 14. A process substantially as described with reference to any one of Figures 1 to 3 or 5 of the drawings.
15. 15. Any novel feature or combination of novel features hereinbefore mentioned.