Production of Nanotube Forests

Field of the invention

[1] This invention relates generally to the production of carbon nanotube (CNT) forests but has a particularly advantageous application to the production of CNT forests that are drawable laterally of the nanotube orientation as a longitudinal nanotube assembly in the manner of a yarn, ribbon, sheet or web. Twist can be inserted into such assemblies to create a twisted yarn or they can be used directly in an untwisted state as a yarn, ribbon, sheet or web.

Background of the invention [2] A carbon nanotube (CNT) is comprised of one or more concentric cylinders of' graphene, that is sheets of planar sp2 bonded carbon that are one atom thick and rolled into a cylinder with the edges joined. If only one cylinder is present, the CNT is classed as a single walled nanotube (SWNT), if two concentric cylinders, then as a double wall nanotube (DWNT) and if more then as a multi-wall nanotube (MWNT). The term 'CNT' or 'nanotube' is generically used to imply SWNT, DWNT or MWNT and is in the plural as well as the singular. The diameters of SWNTs are typically about 1 nm, whereas MWNTs, which may comprise many tens of concentric tubes, can have outside diameters of up to 100 nm. Lengths are commonly in the order tens of microns for SWNTs up to several millimetres for MWNTs. CNTs are also termed 'Buckytubes' or 'Fullerenes'.

[3] Carbon nanotubes, particularly of the single-walled variety, have a range of spectacular properties such as high elastic modulus (~1 TPa) and high mechanical strength (~30 GPa) that are of great technological interest. A low volumetric density (~1330 kg/m ³ ) means that the specific properties are even more exceptional when compared with most other materials commonly available, e.g., the modulus and strength of SWNTs are ~20 and ~50 times that of high tensile steel. SWNTs also display excellent properties such as high electrical conductivity (10-30 kS/cm) and high thermal conductivity (-2000 W nV ¹ K ^'1 ).

[4] Carbon nanotubes are synthesised as randomly oriented tangles or bundles, or as highly aligned arrays in which the CNTs are substantially of the same length and diameter, and stand closely packed and in parallel on a substrate as trees in a forest. In order to capture the properties of CNTs in macroscopic structures, it is necessary to disperse them in and combine them with other materials such as polymers to create composite structures, or, following dispersion, to remove the polymer or other dispersion medium such as a solvent to create pure CNT structures. This dispersion operation is termed 'wet processing'. Either as composites or as pure materials, the CNTs can be randomly oriented or can be aligned in one, two or (pseudo) three dimensions. Two dimensional alignment, wherein the CNTs are arranged in parallel within a plane, and pseudo-three dimensional alignment, wherein the CNTs are aligned in a yarn, are most desirable as these alignments enable the properties of CNTs to be most effectively realised.

[5] CNT composites have the advantage that the CNTs are dispersed and handled with relatively little difficulty. The CNTs are either randomly oriented, or can be aligned by drawing the composites into films and fibres. One disadvantage of composites is that the maximum loading of CNTs to polymer that can be achieved is rarely above 10% and often less than 5% as the CNTs greatly influence the melt or solution properties of the composite, such as by increasing viscosity. As a consequence of the CNTs having relatively little interaction with each other and being present in small proportion, the composite exhibits mainly the properties of the polymer rather than the nanotubes. Nevertheless, impressive mechanical benefits have been obtained for polymer-solution-spun SWNTs, which in large part can be attributed to the mechanical properties of the nanotubes.

[6] An alternative to composite formation is to disperse the CNTs in a continuous medium such as a solvent or polymer, which is then partially or substantially removed during or after the process of structure formation to leave a mat or paper of CNTs.

[7] In general, there are several problems associated with "wet" processing techniques. Firstly, dispersing CNTs into the fluids causes significant breakage of the CNTs and so inhibits the properties of the composites. Another problem is that the

viscosity of the fluid increases rapidly with the concentration of the CNTs, which limits ultimate concentrations to less than 10%. Finally, if the CNTs are filtered from the dispersion to produce a CNT paper, it is found that residual traces of the fluids may remain on the nanotubes that significantly reduce transport of electrons or phonons and that physical properties of the material are poor.

[8] A method that avoids the problems of wet processing utilises forests of close packed parallel aligned nanotubes on a solid substrate to fabricate nanotube yarn. As described in K. Jiang et al., Nature 419, 801 (2002); and in U.S. patent application US20040053780 "Method for fabricating carbon nanotube yarn", this method entails the capturing by forceps and drawing away from the face of a forest of a bundle of nanotubes, whereby nanotubes behind the bundle are sufficiently attached to also be drawn out, and whereby contiguously attached nanotubes are also drawn out. The result is that a continuous chain of nanotubes in the nature of a yarn is drawn away from the forest. However, the yarn so produced is of very poor strength and deficient in other physical properties.

[9] In a surprising development, it was shown (Mei Zhang, Ken R. Atkinson, Ray

H. Baughman, Science, 306, 1358 (2004) and International patent application PCT/US2005/41031) that twist could be inserted into the CNT assembly that is drawn from a forest, thereby producing a yarn of reasonable strength and tenacity in much the same way as twist is used for conventional fibres. The CNT yarns required about 100 times the twist to achieve reasonable tensile properties compared with conventional yarns, because of their much smaller diameters. The authors of the cited paper reported that yarns with diameters of about 1 μm, which is about 150 times smaller than a typical single-ply worsted wool yarn, had quite good tenacity and high electrical conductivity for twists of about 50,000 m ^"1 . This level of twist gives about the same 'helix angle' or orientation of the fibres relative to the yarn axis as seen in conventional yarns.

[10] This method utilises MWNTs grown in forests with the important property that they are "drawable" laterally of the nanotube orientation as a longitudinal nanotube assembly. If the forest is drawable, when the row of nanotubes of an outer face of the

forest is withdrawn generally laterally of the nanotube orientation, the nanotubes in the next row are sufficiently connected to the first row to then be pulled out after it, followed by each successive row or rank. This process continues indefinitely through the ranks of nanotubes in the forest, ultimately creating a longitudual nanotube assembly in the form of a continuous yarn, ribbon, sheet or web of nanotubes. The individual nanotubes are largely aligned in two dimensions in that they are parallel and within a plane, having been rotated into the plane of the yarn or web by the action of drawing. This web of nanotubes has sufficient integrity to be used by itself or to be twisted into a yarn. By drawing "laterally" in this context is meant at an angle to the nanotube orientation. The angle may be, and indeed is preferably, about 90 °.

[11] The webs and yarns have excellent mechanical strength and electrical conductivity and can be used in many applications. As no dispersion or wet processing is required, this operation is termed 'dry processing' or 'dry spinning' or 'dry drawing' of CNTs, and the CNT forests from which they are drawn are termed 'drawable' or 'spinnable' nanotube forests. A difficulty for the methods that rely on the use of drawable nanotube forests is that existing processes for forming such nanotube forests are very sensitive to conditions of reaction and hence are susceptible to failure. Known such processes include those described by Jiang et al in US patent application 2004053053 "Carbon nanotube array and method for forming same".

[12] In order to obtain nanotube forests that are drawable, the various process parameters need to be carefully controlled within specific ranges of values. For example, in the most common process the forest is grown from an iron catalyst layer vapour-deposited on a silicon wafer as substrate placed in a reactor having an inert gas atmosphere to which acetylene is admitted as a reactive gas. The critical parameters include the purity and method of deposition and subsequent treatment of the catalyst layer, the choice and handling of the silicon substrate, the quality of the inert and reactive gas supplies, and the exact flow rates, temperature profiles and gas compositions and running times for the reaction, etc. Precise control of the process parameters within the narrow acceptable ranges is difficult and this sensitivity has to- date resulted in poor yield, reliability and quality in the production of drawable nanotube

forests. The processes have also been found to be very limited in their suitability for increased scale of production.

[13] It is an object of the invention to at least in part alleviate one or more of the difficulties noted above and in general to provide a method, at least in one or more embodiments, of producing a nanotube forest that is drawable laterally of the nanotube orientation as a longitudinal nanotube assembly having satisfactory strength and structural integrity. It is a further object of the invention to provide a method capable of adaptation to viable commercial scale production of nanotube forests.

[14] It is known that a single pair of silicon substrates each coated with titanium nitride, then an overlying AI ₂ O ₃ iron-based catalyst, separated by a sacrificial layer or air gap will grow a single nanotube forest between them. The forest exhibits tip growth to both substrates from catalyst nano particles intermediate the substrates. (Z. Chen et al., Diam. ReI. Mat. (2006) 15 104-108 and Z. Chen et al., Carbon (2006) 44 225-230)

It is further known that a single silicon substrate coated with aluminium oxide and a catalyst layer of iron and bearing a cap of silicon that has been coated with silicon nitride and where the catalyst is pretreated at high temperature with hydrogen, will grow a single nanotube forest under the cap, in which forest growth is predominantly at the periphery and tangled nanotubes grow at the centre. If the cap is a weight, the growing forest exhibits a mechanical force, and the forest develops defects if the weight applies pressure to the forest. (A.Hart and A. Slocum, J Phys Chem B, (2006) 110 8250-8257, and A.Hart and A. Slocum, Nano Lett, (2006) 6 6 1254-1260).

[15] There is no suggestion in or expectation from any of the above-referenced Chen et al and Hart et al papers that the reported forests were drawable, especially drawable laterally of the nanotube orientation as a longitudinal nanotube assembly.

Summary of the invention

[16] The concept of the present invention is essentially to grow nanotube forests on substrates in such a fashion that the outer tips of the nanotubes bear upon a closure that, in certain embodiments, may be a further substrate or an oppositely growing forest of similar nanotubes. The substrates upon which the nanotubes are grown may be

silicon dioxide or quartz or any other compatible refractory material and are preferably coated with a simple single layer of catalyst that does not require pretreatment with hydrogen, and the closure may be of an identical material to the substrate, whether or not coated with catalyst, or may be any other compatible refractory material. The forest thereby exhibits base growth as the seed catalyst nano particles remain at the substrate surface(s). It is preferred, at least in some aspects of the invention, that there is no additional layer of catalyst or catalyst sandwich material such as titanium nitride of aluminium or aluminium oxide or silicon nitride, nor an air gap or sacrificial layer.

[17] Accordingly, in one aspect, the invention provides a method of producing a nanotube forest, including causing a nanotube forest to grow, in an atmosphere that includes one or more suitable reactive carbon gases, on and outwardly from a first substrate having a coating of a material that is a catalyst for growth of the forest, the substrate and coating being configured so that the nanotube forest exhibits base growth, and, as the forest grows, maintaining across the tips of the nanotubes of the forest a closure that restricts access to the coated substrate to traversal of the growing forest by said reactive carbon gas(es) which therefore must pass laterally through the forest among the nanotubes thereof, rather than being able to enter at the tips of the nanotube forest and pass down between the nanotubes to their base at the substrate, wherein the closure is dimensioned and reaction parameters set whereby the mean nanotube height across the forest is substantially uniform.

[18] In the first aspect of the invention, there is further provided apparatus for producing a nanotube forest, including:

[18.1] means for defining a first substrate having a coating of a material that is a catalyst for growth of a nanotube forest thereon, the substrate and coating being configured so that the nanotube forest exhibits base growth;

[18.2] means for defining a chamber about said substrate for an atmosphere that includes one or more suitable reactive carbon

gases, whereby a nanotube forest can be caused to grow on and outwardly from said coated first substrate; and

[18.3] means for maintaining across the tips of the nanotubes of the forest as the forest grows, a closure that restricts access to the substrate to traversal of the forest by the reactive carbon gas(es) which pass laterally through the forest among the nanotubes thereof;

[18.4] wherein the closure is dimensioned and reaction parameters set whereby the mean nanotube height across the forest is substantially uniform.

[19] In an embodiment of the first aspect of the invention, the closure is a second substrate for a further nanotube forest on its obverse face, arranged so that the reverse face of the second substrate contacts said tips of the nanotubes of the first mentioned forest.

[20] In another embodiment, the aforesaid closure is an assembly of a second substrate and a further nanotube forest growing thereon, arranged so that the respective nanotube forests grow by base growth towards each other.

[21] Preferably, the nanotube forest is a forest that is drawable laterally of the nanotube orientation as a longitudinal nanotube assembly. By "laterally" in this specification is meant at an angle to the nanotube orientation. The angle may be about 90° and indeed is preferably about 90°.

[22] In a second aspect, the invention provides a method of producing a nanotube forest, including:

[22.1] providing a first substrate having a surface with a coating of a material that is a catalyst for growth of a nanotube forest thereon, and further having a second substrate substantially in contact with said coating;

[22.2] causing a nanotube forest to grow by base growth on and outwardly from the coated surface, in an atmosphere that includes one or more suitable reactive carbon gases; and

[22.3] allowing said substrates to move apart as said forest grows, to accommodate the growth of the forest between them, wherein the closure is dimensioned and reaction parameters set whereby the mean nanotube height across the forest is substantially uniform.

[23] In the second aspect of the invention, the second substrate may have an obverse face coated for growing a further nanotube forest thereon, such forest being itself covered by a third substrate, and a reverse face that contacts the tips of the nanotubes of the first substrate. In this arrangement, the second substrate may be coated on both its obverse and reverse sides and grow nanotube forests on both sides, being in contact respectively with the first and third substrates. In an alternative arrangement, the first and second substrates are arranged so that the respective nanotube forests that grow thereon grow towards each other.

[24] Preferably, the nanotube forest is a forest that is drawable laterally of the nanotube orientation as a longitudinal nanotube assembly that grow thereon grow towards each other.

[25] In a third aspect, the invention provides a carbon nanotube structure or stack comprising multiple carbon nanotube forests interspersed with substantially parallel substrates supporting the forests. There may be a single nanotube forest layer between adjacent substrates, or alternatively there may be pairs of forests between adjacent substrates in which the tips of the respective forests are substantially coplanar or overlapping.

[26] In the third aspect, the invention also provides a carbon nanotube structure comprising a pair of carbon nanotube forests projecting towards each other from respective substrates supporting the forests, the respective forests being distinct and separate and their carbon nanotubes substantially not bonded at their adjacent tips.

[27] In third aspect, each forest is preferably drawable laterally of the nanotube orientation as a longitudinal nanotube assembly

[28] In each aspect of the invention, the atmosphere about the substrate(s) is preferably primarily an inert gas or mix of such gases. The reactive carbon gas(es) preferably include(s) acetylene. The catalyst is preferably iron, cobalt, nickel or combinations or alloys thereof. The catalyst is conveniently vapour-deposited on the substrate.

[29] It is thought that by restricting access to the substrate to traversal of the catalyst and the forest by the reactive gas(es), the gases are favourably conditioned by the catalyst and by the initially formed nanotubes nearest the edges of the substrates and so beneficiated for better performance in ongoing growth of the forest further inward from the edge. This conditioning may arise from the carbon nanotubes behaving as a form of activated carbon that absorbs or adsorbs gaseous impurities and by-products entrained with the reactive carbon gas. For example, where the reactive carbon gas is acetylene, these gaseous impurities may typically include phosphine, arsine, acetone, hydrogen sulphide and water, benzene, styrene and other thermal degradation products of acetylene, and also hydrogen as a by-product of the reaction of acetylene with the catalyst. Other mechanisms of action are possible.

[30] The invention therefore also provides, in a fourth aspect, a method of producing a nanotube forest, including causing a nanotube forest to grow, in an atmosphere that includes one or more suitable reactive carbon gases, on and outwardly from a first substrate having a coating of a material that is a catalyst for growth of the forest, the substrate and coating being configured so that the nanotube forest exhibits base growth and, as the forest grows, causing the reactive carbon gas(es) to flow into the forest primarily by flowing through the forest among the nanotubes thereof, rather than into the forest past the tips of the nanotubes, thereby resulting in enhanced conditioning of the reactive carbon gas(es) by the nanotubes of the forest, wherein the closure is dimensioned and reaction parameters set whereby the mean nanotube height across the forest is substantially uniform. Such conditioning of the reactive carbon gas(es) may include absorption or adsorption of gaseous impurities and by-products

entrained with the reactive carbon gas. Again, the forest is preferably a forest that is drawable laterally of the nanotube orientation as a longitudinal nanotube assembly.

[31] In its fourth aspect, the invention also provides apparatus for producing a nanotube forest, including:

[31.1] means for defining a first substrate having a coating of a material that is a catalyst for growth of a nanotube forest thereon, the substrate and coating being configured so that the nanotube forest exhibits base growth;

[31.2] means for defining a chamber about said substrate for an atmosphere that includes one or more suitable reactive carbon gases, whereby a nanotube forest can be caused to grow on and outwardly from said coated first substrate; and

[31.3] means for restricting access to the coated substrate to traversal of the growing forest by said reactive carbon gas(es) which therefore must pass laterally through the forest among the nanotubes thereof and thereby be filtered, conditioned or otherwise beneficiated, rather than being able to enter at the tips of the nanotube forest and pass down between the nanotubes to their base at the substrate;

[31.4] wherein the closure is dimensioned and reaction parameters set whereby the mean nanotube height across the forest is substantially uniform.

[32] In a fifth aspect, the invention provides a method of improving any gas comprising passing that gas among the nanotubes of a nanotube forest whereby the gas is filtered, conditioned or otherwise beneficiated by the nanotubes of the forest, for example, by absorption or adsorption of gaseous impurities and by-products entrained with the gas.

[33] In each aspect of the invention, the catalyst-coated substrate(s) may conveniently be a regenerated substrate obtained by heating and thereby oxidising and regenerating a catalyst-coated substrate on which a nanotube forest has previously been grown.

Brief description of the drawings

[34] The invention will be further described, by way of example only, with reference to the accompanying drawings, in which:-

[35] Figure 1 is a diagrammatic representation of the side elevation of a starting pair of substrates catalyst-coated with a single simple layer of catalyst on both of the opposed sides that are in substantial contact to be treated in accordance with a first embodiment of the invention;

[36] Figure 2 is a similar diagram of the same pair of catalyst-coated substrates, after nanotube forests have been grown on the substrates;

[37] Figure 2a is a diagram of the pair of substrates, as depicted in Figure 2, in a reactor in which the nanotube forests have grown;

[38] Figures 3 and 4 are isometric views of alternate suitable substrates for carrying out embodiments of the invention;

[39] Figures 5 and 6 are views corresponding to Figures 1 and 2 for a second embodiment of the invention, in which a single forest is grown between two substrates;

[40] Figures 7 and 8 are views corresponding to Figures 1 and 2 for a third embodiment of the invention, in which there is a small stack of alternate substrates and forests;

[41] Figures 9 and 10 are views corresponding to Figures 1 and 2 for a fourth embodiment of the invention, in which the substrates are of different dimensions;

[42] Figure 11 is a photograph showing a nanotube ribbon being drawn from a drawable forest grown on a catalyst-coated quartz substrate that was overlain by a catalyst-coated silicon dioxide substrate, illustrating that substrates need not be the same size or material;

[43] Figures 12 and 13 are views similar to Figures 1 and 2 for a fifth embodiment of the invention, in which the structure of Figures 1 and 2 is repeated in a stack;

[44] Figure 14a is a photograph showing a stack of 26 pairs of substrates about to be loaded into a reactor. Figure 14b is a photograph of stacks of 30 pairs of substrates before (right) and after (left) growth of nanotube forests, illustrating the increase in stack height due to the accumulated length of 60 nanotube forests.

[45] Figure 15 depicts a sixth embodiment of the invention comprising a stack containing substrates coated with catalyst on both sides; and

[46] Figure 16 is a diagram of a structure for conditioning a gas or liquid stream.

Detailed Description of Embodiments of the Invention [47] In a first embodiment, (Figures 1 and 2) of the invention, drawable nanotube forests are grown on each of a pair of facing substrates 101 , 104 each coated with a catalyst 102, 105. The substrate can be silicon dioxide of any thickness commensurate with structural and mechanical integrity. It is to be understood that this invention is applicable to any substrates that are or can be made suitable for the base growth of preferably drawable carbon nanotube forests. The following description will use the term 'substrate' for convenience but it is understood that this term implies substrates in the form of silicon dioxide, silicon dioxide coated wafers, quartz slides, and all other substrates of the suitable materials in any shape or size.

[48] Each substrate 101 , 104 should have at least one surface that is smooth by virtue of having been polished, fused, crystallised, planed, coated, or by any other natural or artificial means, and may be of any size or thickness that may be conveniently handled. In one preferred embodiment, the substrate is silicon dioxide of 100 nm

thickness supported by a P type silicon wafer of 550 μm thickness and 100 mm diameter, and in another embodiment, substrates are quartz glass slides 126 mm square and 2 mm thick.

[49] The smooth surface of each substrate 101 , 104 is coated with catalyst 102, 105 by any means including electron beam, plasma, sputter, thermal filament or boat, or chemical vapour deposition, or solution deposition. The catalyst may be of the metals iron, cobalt or nickel or combinations or alloys thereof, with iron being the preferred metal at a thickness of from 1 to 10 nm but preferably 5 nm. The catalyst is applied directly to the substrate surface. This ensures the aforementioned preferred base growth of the nanotubes rather than tip growth. While not essential in all aspects of the invention, it is generally preferred that there is no additional layer of catalyst or catalyst sandwich material such as titanium nitride of aluminium or aluminium oxide or silicon nitride, nor an air gap or sacrificial layer. It is also preferred that the catalyst not be pretreated with hydrogen prior to nanotube growth.

[50] Substrate 101 , 104 with their catalyst layer 102, 105 are paired and matched for material, size and shape, and arranged such that the coated sides are placed face to face and such that they are aligned and substantially in contact with one another, having substantially no space or gap between them. The pair of substrates is placed in a reactor being, in one embodiment (Figure 2a), a horizontal quartz glass tube 200 of 45 mm internal diameter and of 1000 mm in length with a surrounding furnace 205 for heating that section holding the substrates, and a means of supplying inert and reactive carbon and other gases at one end 202. The pair of substrates 101 ,104 may be placed at any angle within the reactor but most conveniently horizontally, and be supported at its edges by the wall of the tube or be placed on or in a support 210 of inert material.

[51] The reactor is closed and flushed thoroughly with an inert gas of high purity such as nitrogen or a noble gas, but preferably helium. The substrates are heated in the reactor within the atmosphere of the inert gas to a temperature in the range 500 ⁰ C to 1200 ⁰ C but preferably around 680 ⁰ C. The inert gas is adjusted to a flow rate of from 100 to 2000 seem, preferably 700 seem, and a reactive gas that is a source of carbon is added to the inert gas flow in the amount of up to 100% but preferably 5% by volume.

The reactive gas may be, for example, methane or a higher alkane, ethylene or a higher alkene, acetylene or a higher alkyne, benzene or its derivatives, or any derivatives and combinations of these, but is preferably acetylene. The reactive gas flow may optionally include additional gases such as hydrogen and ammonia.

[52] A controlled flow of inert and reactive gases, and a controlled temperature, is maintained for 1 to 60 minutes, preferably 20 minutes, after which the reactive gas is stopped. During the reaction time, inert and reactive gas or gases diffuse or flow or insinuate between the substrates and there interact with the catalyst coating or grow carbon nanotubes in respective forests 103, 106 on substrates 101 , 104. If it is desired to remove the CNTs and substrates from the reactor, heating is continued for a short further time with the inert gas flowing to remove residual reactive material before the reactor and contents are cooled and unloaded. Those skilled in the art will recognise that the times, temperatures, flow rates and many other variables can be adjusted over a broad range to produce nanotubes of various characteristics and that use of such conditions does not preclude them from application within the spirit of the present invention.

[53] In Figure 2, and correspondingly in several other figures, the nanotube forest

103, 106 is represented by a repeating pattern of lines. It will be understood that this is not intended to indicate the appearance of the nanotubes: it is simply a drawing device to deal with objects that cannot practically be illustrated at their real size or in their precise geometry. Nanotubes in fact are often not vertically straight, often have wavy- style forms, and regularly touch each other at locations remote from their base. There may be of the order of 10 ¹⁰ to 10 ¹² nanotubes per cm ² of substrate.

[54] The nanotube forests 103, 106 grow (Figure 2) perpendicular to the respective substrate surfaces 101 , 104 coated with catalyst 102, The nanotube forests

103,106 forests exhibit base growth in that the seed catalyst nano particles remain at the substrate surface: the result is separate and distinct forests growing from the respective substrates. : The nanofibre tips of the forests, being the points farthest from their respective substrates, impinge and do not substantially interpenetrate. The adjacent tips of the opposing forests substantially do not bond together: the forests

remain distinct and separable. As a result, the nanotubes push each other and the respective substrates apart as the growth proceeds.

[55] As the reactive and inert gases insinuate between and diffuse from the edges into the interior regions of the substrates 101 , 104 across the catalyst layers 102, 105, and among nanotubes 107 of the growing forests 103, 106, the most reactive components and impurities react at the edges 109 of the substrates and hence are effectively removed. The reactive and inert gases are thus filtered, purified, or otherwise beneficiated and flow to the central area of the substrates at which they reliably and reproducibly react to produce high quality drawable forests. Mean nanotube height is substantially uniform across each forest, including at the periphery and in the central area.

[56] The size of the substrates 101 , 104 is thought to influence the structure of the. forest grown, with distance from the edge being the key variable. With reference to Figure 3, the preferred maximum distance of any point on a substrate 101 to an edge, in particular to ensure the aforesaid feature of substantially uniform mean nanotube height, can be from 0.1 mm up to 150 mm or more with the optimum distance dependent upon the reactor conditions. The desired distance, for example 10 mm, is achieved either by cutting the substrate into pieces of suitable dimension, such as 20 mm wide and of greater length than 20 mm as illustrated, or by taking a large substrate 101 (Figure 4) and perforating it 115 such that no point is greater than the required distance from a perforation or edge 108.

[57] Although the embodiment just outlined with reference to Figures 1 and 2 describes the use of a pair of substrates 101 , 104 that are placed face to face and both coated with a catalyst 102, 105, it is also to be understood that this method for growing drawable forests may be carried out with a pair of substrates 501 , 504 (Figures 5 and 6) where only one of the pair, eg 501 , is coated with a catalyst 502. Treatment of this pair results (Figure 6) in nanotubes 503 growing on the coated surface 502 of the substrate 501 only, whereby the uncoated substrate 504 is pushed apart from the coated substrate 501. Again, because of the base growth of the forest, the nanotubes 503 of

the forest do not bond to, and are distinct and separable from, the overlying substrate 504.

[58] In a further embodiment of the invention, (Figures 7 and 8) one substrate 704 is smooth and coated with catalyst 702, 705 on both sides, and each coated side is placed substantially in contact with respective other substrates 701 , 707 that may or may not themselves be coated (the latter is illustrated). Growth of nanotubes on such an assembly (Figure 8) results in the coated substrate 704 growing nanotube forests 703, 706 on the respective coatings 702, 705 on both sides and thereby pushing the uncoated substrates 701 , 707 apart.

[59] Although the preferred embodiment describes pairs of substrates that are matched in size and shape, it is recognised (Figures 9 and 10) that this is for convenience and efficiency only and that mismatched substrates 901 , 904, where both are coated 902, 905, will also grow high quality drawable nanotube forests. Treatment of such mismatched pairs results (Figure 10) in the substrates 901 , 904 growing high quality nanotubes 903, 906 on the catalyst layers 902, 905 that were in substantial contact. Areas that were not in substantial contact 902a, 902b may grow nanotube forests 903a, 903b however these will be of inferior length and quality as illustrated.

[60] Figure 11 shows nanotube ribbon being drawn from a drawable forest grown on a quartz slide - the second substrate in this case was of smaller dimensions than the slide, and the substrates were also of different materials.

[61] Although the preferred embodiment describes pairs of substrates of like material such as silicon dioxide, it is also to be understood that pairs of substrates of unlike material may also be used. Such combinations are not limited to silicon dioxide and quartz or quartz and magnesia but encompass any dissimilar materials that are compatible with the growth of CNTs, where the word 'substrate' is understood to stand for any material of the types described in any size or shape, and that the word 'pair' is understood to stand for any two pieces of material whether or not substantially similar in size, shape or substance.

[62] Although the preferred embodiment describes the use of one pair of substrates, it is also to be understood that pairs of substrates may be stacked one upon another (Figure 12), in which each substrate 1201 has a catalyst coating 1202. Stacks can be placed side by side or in line to the maximum extent of the reactor capacity or to an extent limited by the capacity to maintain the appropriate reactor environment with respect to reactant and waste gases, gas flows, temperatures etc. Treatment of the stacks results (Figure 13) in each catalyst layer eg 1202 generating a base-growth nanotube forest eg 1203 and thereby pushing the substrates eg 1201 apart; a portion of such a stack is illustrated. Also shown (Figure 14a) is a photograph of a stack of 26 pairs of substrates, being loaded into a reactor and (Fig 14 b), stacks of 30 pairs before (right) and after (left) treatment, illustrating the increase in stack height due to the accumulated length of 60 drawable nanotube forests produced during a single reaction cycle. It is also understood that other compatible materials that may or may not grow nanotubes can be placed on or under or around the pairs or stacks of substrates to support or constrain them or to direct the flow of gases.

[63] It is also understood that stacks (Figure 15) can be formed by placing multiple single substrates eg 1501 rather than pairs one atop another, such that the back of one substrate is substantially in contact with the front of the next and wherein either one or both of each pair of surfaces that are in substantial contact is coated with catalyst and hence can grow a drawable forest. This is illustrated for a portion of a stack wherein both sides of each substrate eg 1501 are coated 1502, 1505 and grow nanotube forests 1503, 1506, thereby mutually pushing the substrates apart.

[64] Catalyst coating can be accomplished prior to or following division of the substrate into sizes for insertion into the reactor. The coating can be uniform over the substrate area or can be patterned to produce growth in specific areas and of specific character as desired.

[65] Although the preferred embodiment describes the use of a substrate newly coated with iron, it has been found that substrates that were previously coated with iron and used to grow drawable forests can be oxidised in air at 400 ⁰ C to 1000 ⁰ C, preferably 700 ⁰ C for 10 minutes to 12 hours preferably one hour, with or without the

careful prior removal of the drawable forest. Such substrates, when placed in the reactor face to face with another substrate, whether or not coated, and processed as described, again grow drawable forests (not illustrated). This process can be repeated at least 10 times with no diminution in quality.

[66] Remarkably, the iron, which, following heating in air is present as the oxide, is directly reduced to the catalytic form by the reactive carbon gas rather than requiring an alternative reductant such as hydrogen or ammonia. Remarkably also, substrates that in their first exposure failed to grow drawable forests, and are oxidised and treated as described will not grow drawable forests in subsequent exposures, whereas substrates that succeeded on the first cycle will succeed on subsequent cycles, indicating that the first cycle permanently templates the iron structure.

[67] Those skilled in the art will recognise that the reactor size, geometry and operation are able to be widely varied and still achieve the objective of CNT synthesis. For example the reactor may be horizontal or vertical or any angle, the diameter and length may be varied as may be the gas flows, temperatures, catalysts, running times, and substrate treatments.

[68] Although the preferred embodiments and illustrations are of a vertical stack of wafers in a horizontal reactor, it is recognised that the reactor and the stack can be oriented at any convenient angle.

[69] The process described to produce forests, including drawable forests, on paired or stacked substrates in part entails a mechanism of purification wherein the inert and reactive carbon gases pass over a catalyst and between the fibres of a carbon nanotube forest, thereby being filtered, purified, modified or conditioned. It is understood that rather than the purified or conditioned gases then being utilised to grow high quality drawable forests directly, they can be passed out of the substrate structure to be utilised elsewhere either to grow drawable forests or for any other purpose, the essential function of the stack structure now being one of purification or conditioning rather than drawable forests growth. Figure 16 depicts such a structure. The substrates eg 1601 can be of a size and number and constrained within a framework eg

1600 that obliges gases (arrows 1620) to enter at one location, being the upstream face of a stack for example, and having passed over the catalyst coatings eg 1602 and through the forests eg 1605 and been filtered, purified, modified or conditioned, to leave at another location, being the downstream face of the stack. It is also recognised that the gases so processed (arrows 1625) and the conditions used can be other than those required to grow the forest once the forest structure has been generated.

[70] It is further recognised that the fluid to be filtered or purified can be a liquid of any type compatible with carbon nanotubes and with the substrates upon which they are grown.

[71] It is further recognised that the capacity for the substrates to be heated in air to 700°C, whereby any carbon species including carbon nanotube forests, whether drawable or not, and other volatile material is destroyed to leave the original catalyst coating as the oxide, thence for the substrates to have the forest regrown as before, enables the formation of a filter which, when blocked or exhausted, can be burned off and regrown in or ex situ.

[72] The present invention provides for reliable growth of drawable carbon nanotube forests, even if gases of less than optimal purity are in use. In addition, previous methods have described only the use of single substrates or small assemblages where the catalyst coated growth surface has been open, uncovered and exposed to the reactor environment. This drastically limits the quantity of material that can be processed in each reaction cycle. The present invention enables a reactor space to be packed with substrates to an extent limited only by the size and shape of the reactor and the maintenance of the gaseous environment with respect to reactants and products, increasing the productivity by around two orders of magnitude. In addition, the present invention enables a broader range of flow rates and temperatures and running times than have been found practical with previously reported methods, and enables the recycling of growth substrates. In addition the present invention enables a broader range of catalyst thickness and purity to be effectively utilised.