This material is based on work supported by the Air Force Office of Scientific Research under award number FA9550-14-1 -0070 P0002.

BACKGROUND TO THE INVENTION

Field of the invention

The present invention relates to a carbon nanotube-based material, a method for the production of a carbon nanotube-based material and a method for the treatment of a carbon nanotube-based material.

Related art

Processes are known for the production of high quality carbon nanotube-based materials. For example, US 2013/0228830 builds on a process for the production of an aerogel of carbon nanotubes and associated impurities via a floating catalyst CVD method, the aerogel then being consolidated into a fibre or a film. US 2013/0228830 discloses further densification of the fibre by applying an aerosol of acetone to the fibre, the acetone subsequently being removed by evaporation and thereby causing further densification of the fibre. Additionally, US 2013/0228830 proposes treatment of the fibre by laser illumination. An infrared (wavelength 15000nm) 600 W C02 pulsed laser was used to illuminate the entire fibre sample for 10, 20, 30, 50, 100 or 300 ms. This has the effect of ablation of impurities in the fibre by melting, vaporizing or exploding them. From this explanation can be understood that the laser illumination is conducted in vacuum or inert atmosphere. The effect of illumination for 30 ms is explained in US 2013/0228830 as being an improvement in densification and alignment of the carbon nanotubes.

US 7,973,295 discloses a process of making a CNT film, irradiating the CNT film with a laser with a power density of greater than 0.1 x 104 W/m2, thereby converting the CNT film to a transparent CNT film. In this process, the CNT film is made by forming a super-aligned CNT array on a substrate and removing these by pulling with adhesive tape. The CNT film is therefore supported on a substrate during the irradiation process, which is carried out in an oxidative atmosphere. US 8,889,217 provides a similar disclosure.

US 7,659,139 discloses a process of irradiating a mixture of semiconducting and metallic CNTs formed as a film on a substrate using a laser in order selectively to destroy semiconducting or metallic CNTs by virtue of resonant absorption of the laser energy.

US 7,880,376 discloses the formation of mats of CNTs by electrophoresis, for example, onto a substrate. The CNT mats are then subjected to laser treatment in order to promote their utility in field emission devices. US 7,341 ,498 provides a similar disclosure.

In the academic literature, various work is reported relating to the effect of laser irradiation of carbon nanotubes. Some of this literature is discussed below.

Ajayan et al. (2002) disclose the effect of a conventional photographic flash on single wall carbon nanotubes (SWCNTs). Their testing was carried out on a sample containing SWCNTs, multiwall carbon nanotubes (MWCNTs), graphite powder, fluffy soot, C6o and metal catalyst particles. Their work showed that SWCNTs ignite and oxidize, leaving the multiwall carbon nanotubes (MWCNTs), graphite powder, fluffy soot, C6o and oxidized metal catalyst particles. Braidy et al. (2002) provides similar disclosure.

Yudasaka et al. (2003) disclose a process for light-assisted oxidation of SWCNTs.

SWCNTs were treated with H2O2 and irradiated with light. The SWCNTs were formed using the HiPco (high pressure carbon monoxide) process and were purified by O2 treatment and HCI treatment to remove Fe particles. The CNTs were mixed with an

aqueous solution of H2O2 and were subjected to laser irradiation during this time. The temperature of the mixture was up to 70°C. This work appears to show that the oxidation of SWCNTs was enhanced due to the laser irradiation, and furthermore that this process was diameter-selective.

Kichambare et al. (2001 ) disclose the laser irradiation of CNTs in air using laser pulses with different energy fluences. The CNTs were grown by microwave CVD as films on Fe-coated Si substrates. CNTs were transformed into sub-micron sized plates and cauliflower type aggregation of carbon deposits. Raman analysis suggests that a peak at 2700cm"1 in the pure CNTs, attributed to disorder induced by nanotube curvature, is reduced by the laser irradiation treatment.

Corio et al. (2002) disclose work on the evolution of the molecular structure of metallic and semiconducting carbon nanotubes under laser irradiation. The CNTs were produced by the electric arc discharge method. The effect of the laser treatment was to burn off the smaller diameter CNTs, leading to an increase in the mean diameter of the CNTs. Fig. 4 of Corio et al. (2012) shows resonant Raman spectra of SWCNTs before and after laser treatment in air.

Huang et al. (2006) disclose the preferential destruction of metallic single-walled carbon nanotubes by laser irradiation in air, whereas the semiconducting single-walled carbon nanotubes could be retained. Figs. 2 and 4 of Huang et al. (2006) shows an example of how a laser process in air after many minutes preferentially removes metallic single wall CNTs. This is shown with the modification of the radial breathing modes. Mahjouri-Samani et al. (2009) also disclose the laser induced selective removal of metallic carbon nanotubes.

Souza et al. (2015) investigated defect healing and purification of single-wall carbon nanotubes with laser radiation by time-resolved Raman spectroscopy. The SWCNTs were formed by pulsed laser deposition into freestanding mats.

Markovic et al. (2012) studied the effect of laser irradiation on thin films of SWCNTs in air, with different types of SWCNTs (from different sources) responding differently to the laser irradiation treatment. CNTs supported on a substrate experienced a crystallinity enhancement and decrease in amorphous carbon after laser treatment in air.

Mialichi et al. (2013) disclose the effect of laser irradiation of carbon nanotube films in vacuum and in air. Films of MWCNTs irradiated in air showed an enhancement in thermal conductance but an increase in defects.

Wei et al. (1997) showed that laser irradiation can result in the transformation of CNTs to diamond. Ramadurai at al. (2009) disclosed that MWCNTs exposed to high laser power densities could transform into structurally different forms of carbon, although SWCNTs did not show the same effect.

Liu et al. (2012) disclose a process in which CNTs yarns are fabricated and treated by a laser sweep in vacuum in order to recover defects. The authors also speculate that the laser sweep acts to weld carbon nanotube joints.

SUMMARY OF THE INVENTION

It is of particular interest in the present disclosure to consider how the practical performance of carbon nanotube based materials can be improved. As an example of a material type, CNT-based textiles have emerging applications in field emission, flexible touch screens and electrical wire. In each of these exemplary applications, electrical conductivity is important. To date, the highest reported electrical conductivity of such CNT cables is 6 MS/m [Behabtu et al. (2013)]. For an individual CNT however, the typical measured electrical conductivity is about 280 MS/m. This is about five times the electrical conductivity of copper, at about 60 MS/m. It is therefore apparent that there is still room for improvement in the electrical conductivity of CNT cables and, more

generally, for CNT-based materials which are capable of being self-supporting. Such materials are sometimes referred to as "self supporting CNT materials". They are self supporting in the sense that a piece of the material can be suspended, e.g. from two opposing ends of the piece of material, and the piece of material can support at least its own weight without breakage of the piece of material. It is also of interest in the present disclosure to promote thermal conductivity of the CNT-based materials.

The present inventors consider that increasing the internal CNT alignment, enhancing the graphitic crystallinity, preserving single wall CNTs and/or preserving double wall CNTs, and/or removing impurities are of importance for improving the conductivity of self-supporting CNT-based materials.

For practical self-supporting CNT-based materials, there is a spectrum of CNT quality, length, and chirality. In turn, this leads to a large envelope of bulk material properties. Research at Rice University has led to a multistep wet chemistry process that aligns CNTs to achieve a high conductivity fiber. However, this process limits the length of these CNTs individually to less than 20 μηη. An alternative, floating catalyst CVD production process developed at the University of Cambridge creates CNT textiles in which the individual CNTs have lengths of the order of 100 μηη and longer.

It is considered that one disadvantage with the University of Cambridge process over the Rice University process however is a greater degree of residual catalyst, amorphous carbon and/or partly ordered non-tubular carbon in the material following the University of Cambridge process, as well as more defective CNTs. However, the University of Cambridge process provides a fundamental advantage in terms of the length of the

CNTs. It is therefore of significant interest to capitalize on this length advantage, to seek to improve alignment, crystallinity, and/or purity of the self-supporting CNT-based material.

The present invention has been devised in order to address at least one of the above problems. Preferably, the present invention reduces, ameliorates, avoids or overcomes at least one of the above problems.

Accordingly, in a first preferred aspect, the present invention provides a method for treating carbon nanotube-based material including the steps:

providing a carbon nanotube-based material;

suspending the carbon nanotube-based material in an oxidative atmosphere;

illuminating an illumination portion of the carbon nanotube-based material with electromagnetic radiation to heat the illumination portion, the illumination portion being out of direct contact with any supporting surface, heat being continuously conducted away from the illumination portion to a non-illumination portion of the carbon nanotube-based material, said heating in the oxidative atmosphere causing at least partial oxidation and at least partial removal of amorphous carbon, partly ordered non-tubular carbon and/or defective nanotubes in the carbon nanotube-based material, leaving a treated material comprising an arrangement of remaining carbon nanotubes.

In a second preferred aspect, the present invention provides a method for manufacturing and treating a carbon nanotube-based material including the steps:

forming an aerogel comprising at least carbon nanotubes, amorphous carbon, partly ordered non-tubular carbon and catalyst particles by nucleation and growth of carbon nanotubes from a carbon material feedstock and floating catalyst particles in a reactor; extracting and consolidating the aerogel into a carbon nanotube-based material;

suspending the carbon nanotube-based material in an oxidative atmosphere;

illuminating an illumination portion of the carbon nanotube-based material with electromagnetic radiation to heat the illumination portion, the illumination portion being out of direct contact with any supporting surface, heat being continuously conducted away from the illumination portion to a non-illumination portion of the carbon nanotube-based material, said heating in the oxidative atmosphere causing at least partial oxidation and at least partial removal of amorphous carbon, partly ordered non-tubular carbon and/or defective nanotubes in the carbon nanotube-based material, leaving a treated material comprising an arrangement of remaining carbon nanotubes.

In a third preferred aspect, the present invention provides a carbon nanotube-based material comprising carbon nanotubes of average length at least Ι ΟΟμηη, the carbon nanotubes of the material being aligned to the extent that: the material has a Herman orientation parameter of at least 0.5 for morphologies in which micro-structure misalignment relative to the intended axis of micro-structure alignment is not constrained to one direction over another; and a Chebyshev's polynomial factor of at least 0.5 for morphologies in which micro-structure misalignment relative to the intended axis of micro-structure alignment is predominantly constrained to one plane

In a fourth preferred aspect, the present invention provides a carbon nanotube-based material comprising carbon nanotubes of average length at least Ι ΟΟμηη, the carbon nanotubes of the material having graphitic crystallinity to the extent that when the material is subjected to non-polarized Raman spectroscopy to measure the ratio D:G of the magnitude of the D peak to the magnitude of the G peak, with magnitudes calculated by performing a baseline subtraction and integrating under the peaks, with Raman laser intensity sufficiently low to keep the calculated D:G ratio independent of Raman laser intensity within 10%, using light of wavelength 523 nm and 785 nm, the D:G ratio is at most 0.025 for 523 nm light and at most 0.1 for 785 nm light.

In a fifth preferred aspect, the present invention provides a carbon nanotube-based material comprising carbon nanotubes of average length at least Ι ΟΟμηη, the carbon nanotubes of the material having graphitic crystallinity to the extent that when the material is subjected to non-polarized Raman spectroscopy to measure the ratio D:G of the magnitude of the D peak to the magnitude of the G peak, with magnitudes calculated by performing a baseline subtraction and integrating under the peaks, with Raman laser intensity sufficiently low to keep the calculated D:G ratio independent of Raman laser intensity within 10%, using light of different wavelengths, when the D:G ratio is plotted against the fourth power of the wavelength and fitted to a straight line, with the straight line numerically constrained to the origin , the adjusted R2 is at least 0.7.

The first, second, third, fourth and/or fifth aspect of the invention may have any one or, to the extent that they are compatible, any combination of the following optional features.

It is considered at the time of writing, without wishing to be bound by theory, that heating in the oxidative atmosphere causes at least partial oxidation and at least partial removal of nanotubes not part of a sufficient thermally conductive pathway in the material. Those nanotubes, being unable to transport heat away suitably quickly, are consequently heated to a degree that permits their oxidation. Preferably, the carbon nanotube-based material has a footprint area of at least 0.1 cm2. Here it is intended that the "footprint" area is the plan view area or silhouette area of the material. Although the invention can be carried out on relatively small samples of material such as a footprint area of at least 0.1 cm2, in some embodiments the invention is carried out on substantially larger material samples, for example having a footprint area of at least 1 cm2, more preferably a footprint area of at least 5cm2, more preferably a footprint area of at least 10cm2, more preferably a footprint area of at least 50cm2. In some embodiments, the method of the invention may be carried out substantially continuously. As will be understood, the illuminated portion typically takes up only a minor proportion of the entire footprint area of the carbon nanotube-based material at any one time.

Preferably, the carbon nanotube-based material comprises at least 50 wt% carbon nanotubes. This may be assessed by thermogravimetric analysis (TGA). Furthermore, preferably the carbon nanotube-based material comprises at least 5 wt% carbon nanotubes selected from one or more of: single wall carbon nanotubes, double wall carbon nanotubes, and triple walled carbon nanotubes. Again, this may be assessed by TGA.

Preferably, single, double and triple wall carbon nanotubes in the carbon nanotube-based material have an average length of at least 100 μηη. This is a substantial average length (measured as explained below). Suitable carbon nanotube materials may be made via a floating catalyst chemical vapour deposition (CVD) method.

The density of the carbon nanotube-based material may be at least 0.05 gem-3. More preferably, the density of the carbon nanotube-based material may be at least 0.1 gem-3. In some embodiments, the density of the carbon nanotube-based material may be up to about 1 gem-3. More preferably, the density of the carbon nanotube-based material may be up to 0.8 gem"3 or up to 0.7 gem"3 or up to 0.64 gem"3.

Preferably, the non-illumination portion of the carbon nanotube-based material has an area of at least 5 times the area of the illumination portion at a given instant in time during treatment. This is intended to ensure that there is sufficient non-illuminated material at any one time which is available as a heat sink from the illumination portion for those CNTs in the illumination portion forming part of a sufficiently thermally conductive pathway.

Preferably, the electromagnetic radiation is moved relative to the carbon nanotube-based material so as to move the illumination portion progressively along the carbon nanotube-based material. Preferably, such progressive movement is a substantially continuous movement, without stopping (except optionally at the limits of movement of the illumination portion with respect to the material). It has been found that such a scanning type approach can provide the treated material with satisfactory uniform properties, compared with a stop-start approach. Preferably, the carbon nanotube-based material (the 'as-is' material) has a direction of preferential alignment of the carbon nanotubes. The direction of relative movement of the illumination portion is preferably substantially parallel to the direction of preferential alignment of the carbon nanotubes.

Preferably, the illumination of the illumination portion by the electromagnetic radiation takes place over a relatively short time scale. As explained elsewhere in this disclosure, it is considered that the illumination portion undergoes an oxidation chemical reaction. Preferably, the illumination takes place over a time scale not longer than the duration of the oxidation chemical reaction itself. More preferably, this time scale is shorter (more preferably substantially shorter) than the duration of the oxidation chemical reaction.

Preferably, the electromagnetic radiation is pulsed. This is a convenient way to ensure that the duration of the illumination, corresponding to the pulse length, is of the time scale explained above.

Still further, for a region of the material being illuminated, preferably, the total time of illumination by the electromagnetic radiation (corresponding to the sum of the duration of pulses received by the region illumination during a single pass) is not longer than the oxidation chemical reaction itself. The duration of the oxidation chemical reaction may be assessed based on the duration of the white oxidative flash. More preferably, the total time of illumination by the electromagnetic radiation is substantially shorter than the oxidation chemical reaction.

Taking the steps above is found to provide advantages in the sense that the material being treated is then less likely to be burned away completely.

The temperature of the illumination portion may be at least 300°C. This temperature may be achieved as a result of the absorption of the electromagnetic radiation by the carbon nanotube-based material, and any external additional sources of heat, such as a hot plate or furnace. Additional contribution to the temperature of the illumination portion is also provided by resultant oxidation reactions taking place at the illumination portion. The illumination portion may be heated to a temperature of at most 2500°C. In some embodiments, the illumination portion may be heated to a temperature of at most 1600°C. A pyrometer can be used to measure the temperature of the area of interest.

The pyrometer should be aimed immediately adjacent to the oxidation flash in space or immediately after the oxidation event in time. This measurement approach yields a lower bound value in the temperature of the area of interest. If the light from the oxidation chemical reaction itself, beyond black body radiation, is measured by the pyrometer then this reading yields an upper bound value for temperature of the area of interest. Note that the temperature can be measured with Raman spectroscopy by considering the Stokes and anti Stokes modes of the G peak according to [Tristant et al, Nanoscale (2016)].

Preferably, the fluence and/or intensity of the electromagnetic radiation at the illumination portion is sufficient to heat the carbon nanotube-based material to reach at least the lowest ignition temperature of all present carbon species at the illumination portion.

Preferably, the oxidative environment is simply air from the ambient atmosphere, but could be any gas causing an oxidation reaction with the carbon species in the material. It is also possible oxidizing agents could be added to the CNT material to supply and/ or assist in the oxidation reaction, such as hydrogen peroxide. These other additional sources of oxidation are also included in the scope of the patent.

The ratio of the mass of the illumination portion after the process to the mass of the illumination portion before the process may be at most 0.9. The ratio of the mass of the illumination portion after the process to the mass of the illumination portion before the process may be at least 0.01 . In this way, it is clear that the treatment applied to the material results in some mass loss, which is attributed to oxidation of carbon.

The treated material may be further treated to remove at least some residual catalyst particles, as well as any remaining amorphous carbon from the primary process. This may be carried out by acid treatment, preferably non-oxidative acid treatment, in a known manner.

In the treated material, preferably the carbon nanotubes are aligned to the extent that the material has a Herman orientation parameter of at least 0.5 for morphologies such that micro-structure misalignment relative to the intended axis of micro-structure alignment is not constrained to one direction over another. More preferably, the Herman orientation parameter is at least 0.6 or at least 0.7 for these said morphologies. For morphologies such that micro-structure misalignment relative to the intended axis of micro-structure alignment is predominantly constrained to one plane, preferably the Chebyshev's polynomial factor is at least 0.5. More preferably for these morphologies, the

Chebyshev's polynomial factor is at least 0.6 or at least 0.7.

In the treated material, preferably the carbon nanotubes have a graphitic crystallinity to the extent that when the material is subjected to non-polarized Raman spectroscopy to measure the ratio D:G of the magnitude of the D peak to the magnitude of the G peak, with magnitudes calculated by performing a baseline subtraction and integrating under the peaks, with Raman laser intensity sufficiently low to keep the calculated D:G ratio independent of Raman laser intensity within 10%, using light of wavelength 523 nm and 785 nm, the D:G ratio is at most 0.025 for 523 nm light and at most 0.1 for 785 nm light.

In the treated material, preferably the carbon nanotubes have a graphitic crystallinity to the extent that when the material is subjected to non-polarized Raman spectroscopy to measure the ratio D:G of the magnitude of the D peak to the magnitude of the G peak, using light of different wavelengths, when the D:G ratio is plotted against the fourth power of the Raman laser excitation wavelength and fitted to a straight line, with the straight line numerically constrained to the origin, the adjusted R2 is at least 0.7. More preferably, the reduced R2 is at least 0.8.

In some embodiments, the material is in the form of a fibre, textile, sheet or film.

Preferably, the material is provided in a free-standing format, without the need for a substrate for support. The material may be light-transmissive. For example, the material may be substantially transparent, or fully transparent.

The inventors have additionally noted that, in some embodiments, applying the method of the invention to the carbon nanotube-based material does not change the radial breathing modes of the CNTs in the Raman spectrum after treatment according to the preferred methods of the invention.

Further optional features of the invention are set out below.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described by way of example with reference to the accompanying drawings in which:

Fig. 1 A shows an optical image of a carbon nanotube sheet before treatment, suspended between copper terminals using silver paste.

Fig. 1 B shows the carbon nanotube sheet of Fig. 1 A after illumination according to an embodiment of the invention.

Figs. 2A-2D show optical images of treated materials according to embodiments of the invention.

Fig. 3 shows an SEM image of the self-supporting CNT material before the laser treatment.

Fig. 4 shows an SEM image of the self-supporting CNT material after the laser treatment according to an embodiment of the invention.

Fig. 5 shows an SEM image of the self-supporting CNT material after the laser treatment and subsequent acid treatment in order to removes the exposed catalyst.

Fig. 6 shows a Raman spectrum on the self-supporting CNT material before the laser treatment.

Fig. 7 shows a Raman spectrum on the self-supporting CNT material after the laser treatment.

Fig. 8 shows the effect of a preferred embodiment of the invention on the microstructural alignment of a CNT-based material. Images shown as a, b, c and d are described below.

Fig. 9 shows the 'before' and 'after' effects of the Raman spectra from the atmospheric photonic process for different laser wavelengths.

Fig. 10 shows the effect of a preferred embodiment of the invention on the nanostructural sorting and alignment of a CNT-based material. Images shown as a, b, c and d are described below.

Fig. 1 1 shows electrical resistance behaviour with temperature, with electrical resistance normalized to room temperature electrical resistance, for the CNT material before and after laser treatment according to an embodiment of the invention.

Fig. 12 shows a schematic perspective view of photonic treatment of a CNT material sample in air, indicating translational movement of the laser beam relative to the CNT material.

Fig. 13 shows the effect of illumination of a single static area on the morphology and crystallinity of a CNT textile. The images around the circumference are high speed camera images of the oxidation flash from a single point illumination.

Figs. 14a and 14b show TGA analysis results of CNT materials produced using butanol and toluene feedstock in a floating catalyst CVD process.

Fig. 15 shows an X-ray diffraction azimuthal scan for an embodiment of the invention, for use in determining the Herman orientation parameter.

Fig. 16A shows Raman spectra of a treated material according to an embodiment of the invention, in which the microstructure of the product is oriented parallel to the Raman laser polarization (black) and perpendicular (red). This provides an indication of the effect of alignment in the treated material on the polarized light Raman spectrum.

Fig. 16B shows Raman spectra as for Fig. 16A but for the carbon nanotube material before illumination treatment according to an embodiment of the invention.

Fig. 17 shows D:G values plotted against the fourth power of Raman excitation wavelength. As-is material (i.e. carbon nanotube material before treatment according to a method of an embodiment of the invention) does not yield a good fit and has a significant non-zero intercept. The materials according to embodiments of the invention have suitable linear relationships of D:G with the fourth power of the wavelength.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS, AND FURTHER OPTIONAL FEATURES OF THE INVENTION

Overview

Floating catalyst chemical vapor deposition is an easily industrialized, one-step production process that uniquely generates aligned single-walled carbon nanotube (SWCNT) or double-walled carbon nanotubes (DWCNT) textiles with individual CNT lengths magnitudes longer than competing processes. Even after extrinsic bulk imperfections are addressed, atomic scale defects inherent to the growth process could still limit prospects for competitive electrical transport. The preferred embodiments of the present invention seek to address this. The methodology presented here is particularly suited to these textiles, selectively removing amorphous carbon, and/or partly ordered non-tubular carbon, defective CNTs, and CNTs not forming a sufficient thermal pathway. In the preferred embodiments, what endures is an optically transparent SWCNT or DWCNT material (typically in the form of a film) with profound improvement in the microstructure alignment and, in regards to Raman spectroscopy, a D peak disappearing under the noise floor of the spectrometer while preserving the radial breathing modes. Furthermore, residual catalyst particles can be removed with a tailored non-oxidizing acid wash.

The basic procedure of irradiation of the material in air, followed by an acid wash, is shown to increase conductivity (e.g. up to tenfold) and then enables a simple acid treatment to increase conductivity several factors more. Cryogenic transport

measurements show the effect of the new microstructure alignment, crystallinity, purity, and chemical treatment on the electrical transport.

Carbon nanotube (CNT) manufactured electrical cables are incrementally materializing as a disruptive technology in power transmission. Twenty-five years ago, what started as soot on a transmission electron microscopy grid evolved into bulk CNT cables exceeding copper and aluminium in terms of conductivity, current carrying capacity, and strength— if normalized by weight. These results are exciting but must be put into historical context. Over thirty years ago, other sp2 carbon forms, iodine doped polyacetelene and graphitic intercalation compounds, approached and, in the best cases, exceeded the conductivity of copper on its own accord without weight considered. Indeed, in 1984 intercalated graphitized carbon fiber was considered as a replacement for overhead power transmission lines on the grounds of its multifunctional strength and near-to-copper conductivity. In all these carbon materials, including the CNTs now, purity, internal alignment, and graphitic crystallinity are important in achieving highest virgin conductivity, as well as the highest conductivities after chemical treatment.

Single wall CNTs (SWCNTs) and double wall CNTs (DWCNTs) could be superior to the other bulk sp2 carbon forms, including large multiwall CNTs, in that transport may be uniquely both 1 D (inherently suppressing phonon interaction, leading to substantially μηη mean free paths) and intrinsically metallic (metallic resistance temperature dependence approaching absolute zero, without doping complications). Significant for electrical power transfer, researchers have demonstrated that quasi-one dimensional transport persists when combined together in a macroscopic assembly forming a textile. This attribute may yield superior bulk conductivity provided extrinsic factors such as purity, internal alignment, and graphitic crystallinity sufficiently evolve.

In the view of the inventors, floating catalyst chemical vapour deposition is the most scalable route for producing aligned, long length SWCNT and DWCNT textiles developed to date. It generates SWCNT and/or DWCNT textiles in sheet and fiber form where the individual CNTs are hundreds of times longer than CNTs in competing manufacturing processes. The CNT fiber conductivity, however, does not substantially outshine the competition. Crystal defects, as many as one every 10 nm, limits the room temperature mobility.

In the preferred embodiments of the present invention, a multi-step, photonic based post-process is presented which is particularly well suited to floating catalyst derived SWCNT and DWCNT textiles, substantially improving purity, internal alignment, and graphitic crystallinity. It is found that not all SWCNT and DWCNT materials may be successfully laser treated. The inventors speculate, without wishing to be bound by theory, that a high degree of pre-existing order may be required.

In the preferred embodiments of the invention, an incident laser beam continually passes over a stretched SWCNT (or double wall CNT) textile suspended by its ends so as not to be in contact with a substrate (supporting surface) at the treatment region. With each successive laser pass in air, material not forming a thermal conduit is incrementally removed. It is considered that the removed material is typically one or more of:

amorphous carbon, partly ordered non-tubular carbons, defective CNTs, and CNTs not forming a sufficient thermal pathway. This treatment process may be summed up as natural selection - what survives is a transparent SWCNT (or DWCNT) film with substantially greater internal microstructure alignment, specific conductivity (tenfold increase), and a crystallinity which approaches the limits of instrument resolution (near elimination of the Raman spectra's D peak). Residual catalyst emerges to the surface and is easily removed subsequently with an acid bath. The significance of the work presented here is that: 1 ) it demonstrates the true potential of floating catalyst derived SWCNT textiles after substantial improvement of purity, alignment, and crystallinity; 2) it establishes a multi-step, scalable manufacturing process that may be integrated in a straightforward manner after production, or inline.

There has been some progress, reported in the literature, in the graphitization of multiwall CNTs. However, typically, graphitization has failed for SWCNTs. This includes previous attempts at laser annealing of CNTs. This is discussed in the following section of this disclosure. Proof of principle work is then presented, along with characterization techniques. Scale-up to arbitrarily long SWCNT textiles is then discussed, with continuous laser scanning. Without wishing to be bound by theory, the mechanics of the process are then discussed in terms of the differences from other SWCNT annealing and purification techniques.

Further background

Graphitization is the high temperature, inert annealing (2500 to 3500°C) that graphite and carbon fiber requires for particularly high mobility and electrical conductivity. It reduces impurities, heals crystalline point defects, as well as enhances internal microstructure order. Crystal grains grow and stacked graphene planes align with regular ABAB stacking, leading to shrinking graphene plane separation and an increase in bulk density. At first glance, graphitization of CNTs is an obvious course of action and indeed has been successfully applied to the multiwall variety. Transmission electron microscopy shows that the initially wavy and disordered walls of an as-produced multiwall CNT straighten after graphitization. Thermo-gravimetric analysis reveals graphitization increases oxidation temperature a couple of hundred degrees centigrade, indicating removal of defects that are the first points of oxidation. Multiwall graphitization has been shown to improve room temperature conductivity from 10 to 200 kSnr1, to increase thermal conductivity 2.5 to 22.3 W K"1nr1, and to improve a charge carrier's mean free path from about 0.3 μηη to about 2 μηη. Raman spectroscopy on graphitized multiwall CNTs shows a narrowing of the G peak and a shift to higher energy. D:G, the ratio between the Raman spectra's D peak and G peak and a prevalent indicator of graphitic crystallinity, improved from 0.769 to 0.270 (Kajiura et al. (2005)).

SWCNT graphitization is, however, another story. Not even approaching typical graphitization temperatures, there are multiple reports revealing SWCNTs coalescing into larger SWCNTs beginning at about 1400°C in inert backgrounds. By about 1800°C these larger SWCNTs start transforming into multiwall CNTs. By 2400°C it was found all CNTs transformed into multiwall CNTs, and in some cases even graphitic carbon ribbons. Double-wall CNTs performed better and were structurally stable up to 2000°C.

Researchers verified SWCNT coalescence with transmission electron microscopy and

Raman spectroscopy, where shifts of the Raman radial breathing modes to lower energy indicate conversion to wider diameter tubes. Upon conversion to multiwall tubes the radial breathing modes disappear. SWCNTs, and to a lesser degree DWCNTs, are peculiar to other sp2 carbons considering their small cylindrical diameter and curvature induced internal stress. This makes them notoriously vulnerable to oxidation, chemical treatment, and, unfortunately, also includes typical graphitization annealing.

The internal stresses that prevent typical graphitization treatment, however, potentially render defects easier to heal. Defects in CNT crystal structure are not stationary in a fixed location and are in fact highly mobile. First principle modelling shows that single vacancy defects in SWCNTs become mobile at about 100- 200°C and transmission electron microscopy found multiwall CNT defects are perturbed by thermal fluctuations and will travel up heat gradients, at a speed 80 nm s"1. Beyond simply moving defects, another microscopy study directly witnessed healing of double-wall CNT defects. The defect healing rate increases strongly with temperature with the healing rate saturating at about 225°C. Thus, a SWCNT equivalent of graphitization quite possibly requires much lower temperatures then more planar graphite structures. Inert annealing of SWCNTs well below typical graphitization temperatures has been attempted at 1000°C and lead to an improvement in the Raman spectra's D to G ratio, at the best Raman excitation wavelength, from 0.18 to 0.059.

Instead of using typical furnaces for heat treatment, annealing with laser illumination is an alternate heat source with inherently faster heating/cooling rates and selective heat zones allowing a degree of control not found with furnaces. In itself, laser annealing of CNTs is not a new concept. The most successful laser processes involved illuminating SWCNTs in air, where often the annealing laser was also a probe for Raman

spectroscopy - see Corio et al. (2002), Huang et al. (2006) Mahjouri-Samani et al.

(2009), Souza et al. (2015), Markovic et al. (2012), Maehashi et al. (2004) and Mialichi et al. (2013). Experimental parameters between these Raman in-air studies varied significantly. Laser wavelengths spanned from ultraviolet to infrared, and the most

successful average intensities ranged from 1 to 100 kWcnr2. Total treatment time lasted tens of seconds to hours. Despite the parameter spread, the outcome of often was the same - that is, a modification of the Raman spectra's radial breathing modes. An early study alluded this effect to selectively oxidizing smaller diameter CNTs due to their greater chemical activity (Corio et al. (2002)). Other studies determined that this is not exactly the case, and that the laser treatment selectively oxidizes away metallic SWCNTs from the interaction of free charge carriers with the laser light (Huang et al. (2006), Mahjouri-Samani et al. (2009) and Souza et al. (2015)).

Beyond the changes of radial breathing modes, air laser treatment of SWCNTs generally leads to some improvement in D:G - indicating a crystallinity enhancement and/or removal of amorphous carbon. Sometimes D:G improved substantially; in a case of unaligned SWCNTs it was beyond an order of magnitude from 0.67 to 0.04 (Souza et al. (2015)). In another case for unaligned SWCNTs, there was the removal of the D peak (Zhang et al. (2002)). In both of these examples, before laser treatment, the SWCNTs were grown with either the laser ablation or arc discharge methods. These growth processes over a very brief time expose the SWCNTs to higher temperatures (above 1700°C) than floating catalyst derived textiles. The D:G improvement from their laser annealing could be explained by removal of amorphous carbon, leaving behind SWCNTs that are already very crystalline.

Moving away from treating SWCNTs in air, laser annealing SWCNTs in an inert atmosphere such as vacuum, nitrogen, or argon has only led to marginal improvement of the crystallinity (Mialichi et al. (2013)). Researchers noticed that significant heat is lost with convection to the inert gas background compared to the case with vacuum. A

SWCNT sample laser heated to 1000°C in vacuum, for example, would under the same illumination conditions in nitrogen experience only a temperature of 250°C. Laser treating multiwall CNTs, in either air or inert background, has mostly led to only marginal improvement or to deterioration. An exception is aligned multiwall CNT yarn suspended in vacuum and heated by a sweeping CO2 laser (3.8 kW cm-2 over about 20 ms per laser

pass) (Liu et al. (2012)). Conductivity increased about 50% from 42.5 to 65 kSnr1 and D:G ratio improved from 0.45 to 0.08. Note that there was not a clear change in the microstructure or fiber diameter and the yarn toughness decreased appreciably.

A thoroughly discussed parameter in CNT laser annealing is laser wavelength. CNTs in general have four physically distinct electromagnetic absorption mechanisms belonging in the THz, infrared, visible, and ultraviolet regions of the spectrum. Starting with mechanisms in the THz to infrared regime, the plasma frequency of CNT materials ranges from approximately 55.6 μηη (22.3 meV/ 180 cm"1) to 12.4 μηη (100 meV/806 cm" 1). Also in this regime, a broad absorption peak exists for both SWCNTs and multiwall CNTs near 100 μηη (12.4 meV/ 100 cm"1). The basis of this absorption peak has been a source of controversy - attributed to either the small bandgap formed by the curvature of the graphene plane into a CNT or plasmon oscillations along the length of a CNT.

Recent results indicate the latter. While this absorption peak is centred at a wavelength too large for most practical lasers, the peak is broad enough to be a factor for infrared lasers. In regards to laser annealing CNTs in the infrared, a study (Markovic et al.

(2012)) evaluated CNT annealing with multiple wavelengths from visible to infrared. It was found that small wavelengths probed the surface of unaligned SWCNT materials (168 nm penetration for 532 nm laser line) and longer wavelengths penetrated deeper into the bulk (331 nm penetration for 780 nm laser line). This finding supports that longer wavelengths are a perhaps better choice to fully impact the material in a homogeneous manner.

For the higher energy regions of the spectrum, SWCNTs display well-defined visible absorption peaks from the electronic transitions between von Hove singularities. The particular locations of these peaks are chirality dependent and are not present for multiwall CNTs generally. Due to a distribution of chiralities and the effects of SWCNT aggregation/bundling, the absorption peaks will broaden and merge. In regards to laser annealing, at least one study claimed their laser struck a resonance with a van Hove singularity (Maehashi et al. (2004)). The radial breathing modes in their Raman spectra indeed changed after laser illumination. This effect, however, is also explained by selective oxidation of small or metallic tubes, which has been observed previously (Corio et al. (2002), Huang et al. (2006), Mahjouri-Samani et al. (2009) and Souza et al. (2015)) and was not discussed in their paper. Both multiwall CNTs, SWCNTs as well as graphite and graphene, have a prominent absorption band in the ultraviolet regime centred at 248 nm (5 eV) due to resonance of the ττ-plasmon. Researchers showed laser annealing at this wavelength had a particular purification effect where amorphous carbon was selectively oxidized away, sparing the SWCNTs (Hurst et al. (2010) and Gspann et al. (2014)).

US 20130028830 discloses some aspects of work carried out on laser annealing of CNTs in an inert argon environment. This approach was shown to lead to densification of the material. Additionally, the treatment disclosed in US 20130028830 forces residual catalyst to the surface. The process of US 20130028830 does not remove significant amount of material from the sample treated.

In the academic literature which discloses the laser treatment of single wall CNTs in air, there emerges a picture of improvement in graphitic crystallinity but another effect seems to be the removal of metallic SWNTs or small diameter SWNTs. In all of the studies mentioned, the SWNT film is supported on a substrate and is illuminated by the laser at very high laser power and dwell times (i.e. high laser fluence). The literature seems to suggest that longer wavelengths penetrate deeper into the material.

Materials under Test and Set-Up

In the preferred embodiment of the present invention, the treatment process selectively removes non-conductive CNTs, partly ordered non-tubular carbons, and amorphous carbon. Where the self-supporting material at the start of the process is an opaque film, the treatment process renders it transparent, where the CNT microstructure is significantly more aligned. To the knowledge of the inventors, there is no other

disclosure of a similar effect. In particular, the radial breathing modes of the Raman spectroscopy do not change after treatment. This indicates that the SWCNT/ double-wall CNTs distribution has not changed despite being well above their oxidation temperature. This too is a new result whereas other, more primitive oxidative laser annealing altered if not destroyed this distribution. The inventors have found that the effect is accompanied by a profound increase in conductivity, purity and graphitic crystallinity. It is found that the technique has particular applicability to CNT-based materials manufactured by a floating catalyst CVD method.

The primary material under test was somewhat aligned SWCNT/ DWCNT textiles generated from various floating catalyst chemical vapour deposition recipes. The CNT generation process is described in Koziol et al. (2007) and Gspann et al. (2014). Briefly, a liquid carbon source, such as toluene or n-butanol, is evaporated and mixed with sublimed ferrocene, the catalyst precursor, and thiophene, the reaction promoter - all within a hydrogen gas background. The gas mixture is passed through a tube furnace at about 1300°C, forming an elastic CNT cloud. The CNT cloud is directly extracted out of the furnace by mechanical means on to a spool where its winding rate dictates the degree of microstructure alignment. Unaligned CNT buckypaper commercially obtained from Nanolntegris was also investigated.

Aligned CNT textiles were stretched between two scaffolds such that the film was elevated and supported only at its ends with tape. The treatment region of the textile was not in contact with any underlying substrate. As-is film thickness ranged from approximately 5 μηη to 15 μηη and the microstructure alignment was typically in the long direction of the cut film.

A collimated, linearly polarized, 10 μηη wavelength pulsed laser beam illuminated the suspended film directly overhead with the following typical settings: 40 W average power, 5 kHz pulsed repetition rate, 20% duty cycle. The beam profile was Gaussian with a 1/e2 diameter of 10 mm. This yielded an average intensity of 50 W cm-2. Per pulse, the peak

intensity and fluence were 250 W cm"2 and 0.25 J cm"2 respectively. These are the general, not necessarily optimized "sweet spot" parameters that should be assumed if not explicitly stated otherwise.

After atmospheric photonic processing, the primary characterization tool was a Bruker Senterra Raman microscope with 532 nm, 633 nm, and 785 nm laser lines. Incoming laser light was randomly polarized and the 4x objective was used to mitigate signal distortion from heating. The laser accumulation time and intensity also were kept as small as practical to minimize heating; we verified that the accepted spectrum was largely independent of these laser heating parameters. The spectra depicted are averages over at least five different film locations with standard deviation well below the measured values. Every spectra is normalized by the G peak and has been baseline corrected. D:G was calculated by integrating peak areas, which is a more useful metric accounting for peak width changes, rather than simply considering peak height. In cases where the D peak was very small, we found plotting the intensity logarithmically helped with peak boundary identification. The G peak, Raman spectroscopy's well-established prominent peak found with graphitic materials, is typically centred at approximately 1582 cm"1 independent of Raman laser excitation wavelength for undoped CNT materials. The width at full width half maximum can vary considerably although a width of 500 cm"1 is common. The integration of the peak areas is carried out between peak limits established by where the peak meets the base line. The exact position of the D peak depends on the CNT material and the excitation wavelength, although peaks centred at approximately 1350 cm"1 (for 532 nm excitation) and 1300 cm"1 (for 785 nm excitation) are typical.

Scanning electron microscopy was accomplished with a FEI Nova NanoSEM. Evolution of the oxidation flash from the laser CNT material interaction was recorded with a high speed camera (36,000 frames per second) and the CNT textile temperature was measured with a pyrometer. Thermo-gravimetric analysis was accomplished with a TA instruments Q500 in bottled air with a dynamic heating rate. To determine conduction

mechanisms, cryogenic resistance versus temperature was measured in a standard four probe configuration and gradual submersion into a liquid helium Dewar. Probe current was 10 μΑ.

Next is a discussion of the effects of the laser/ CNT/ air interaction at a material point followed by a consideration of continuous scanning, demonstrating scale-up.

The Photonic Procedure

Fig. 12 shows a schematic perspective view of photonic treatment of a CNT material sample in air, indicating translational movement (see arrow) of the laser beam relative to the CNT material. A treatment region of the CNT textile is elevated off the substrate by suspending the textile from its ends. The laser sweeps across the surface leading to selective oxidation. Surviving CNTs have substantially improved chirality, micro-structure alignment, and residual catalyst migration to the surface.

As an initial experiment, the CNT textile was illuminated without translational movement of the laser beam. It is found that such single point illumination does not yield the best results, although its relative simplicity makes the fundamental photonic effect easier to study.

Fig. 13 shows the effect of static illumination for a 150 ms duration shot, which is a train of 750 individual laser pulses. The sample here is considerably larger than the beam diameter so that thermal edge effects are not in play. The optical microscope image (left hand side of the central part of Fig. 13) shows a transparent annulus region where it is apparent that most of the material has vaporized. The Raman map overlay of relative D:G reduction factor (right hand side of the central part of Fig. 13) shows a three to four fold crystallinity improvement in the annulus region and a two to three fold improvement in the inner region. This is the first indication of a general theme that transparency equates to, among several parameters, superior crystallinity.

In more detail, the right hand side of the inner part of Fig. 13 shows a Raman map of the annulus oxidation region produced by a 150 ms application of the laser, comprised of a 5 kH pulse train. Here, the map shows relative reduction factor in D:G, and in this particular example the best improvement is only a factor of four. An optical microscope photograph (left hand side of the central part of Fig. 13) shows the improved annulus region is optically transparent, indicating most of the SWCNTs in the improved region burned away. In the original image, false colour is used, and so selected regions of the image are mapped onto the scale, to guide the eye. The perimeter of Fig. 13 shows a sequence of images captured via high speed camera showing the evolution of the laser heat zone combined with the oxidation reaction flash. Note the camera is at an angle that tilts the perspective. These displayed images are at 277.5 s intervals, an image for approximately every individual laser pulse. The horizontal bar indicates 10 mm.

This annulus form shown in Fig. 13 is unexpected because the laser beam intensity has a Gaussian distribution. The high speed camera images shown around the perimeter of Fig. 13 show the high intensity flash from the laser interaction growing from the inside outward (verifying the Gaussian profile) and reaching the beam diameter size in approximately 3 ms (or 12 laser pulses). Also by this point, the annulus region (and hence the critical CNT oxidation) is also apparent. When watching the motion video, the expanding flash is composed of rhythmic heating of the 5 kHz laser pulses, as well as a constant non-cyclic component that is assumed to be self-sustained oxidation.

Pyrometer measurements indicate sustained temperatures at 1400°C, almost three times the temperature required to initiate oxidation. Note that pyrometers measure black body radiation and light caused from the electronic transitions in exothermic reactions would alter the temperature measurement. Regardless, the visual intensity of the white flash qualitatively indicates temperature almost certainly above the SWCNT oxidation threshold and this is confirmed by the transparency of the annulus region. There is more than sufficient temperature and fuel supply available for oxidation over the entire illuminated region however, so the striking differences between the transparent annulus and opaque inner zones are perhaps best explained by oxygen availability. Also particularly noteworthy, the oxidation and resultant vaporization process terminates in the first 3 ms (approximately 12 laser pulses) of a 150 ms duration shot. This important observation drove development of the scale-up approach.

Based on this initial work, it is found that when there is insufficient laser fluence, there is no substantial effect on the visually appearance of the microstructure of the material or on the properties of the material as determined by Raman spectroscopy. On the other hand, too high a laser fluence simply burns holes in the material. The laser treatment can be carried out at intermediate operating conditions such that the initially opaque CNT textile becomes transparent and it is found that this usually indicates superior properties.

The inventors investigated variables such as film thickness and laser polarization. These changed the precise preferred operating parameters to some degree, but did not result in a fundamental, dramatic consequence.

The inventors also tested a 1 μηη laser, an order of magnitude lower wavelength, and this too yielded similar results in terms of microstructure and Raman spectra to those discussed above. This wavelength independence supports the view that the atmospheric photonic process is thermally driven oxidation without reliance on a particular absorption mechanism or electronic transition.

It was found that the CNT film should not be in thermal contact with a substrate at the treatment region. In this embodiment, this was achieved by elevating the sample from the substrate by suspension from its ends. Highlighting the relevance of heat transport, it was found that regions in thermal contact with a substrate, such as a CNT film supported by a glass slide, will not experience the intense white oxidation flash or any substantial material enhancement.

The photonic process was carried out on unaligned SWCNT buckypaper commercially obtained from Nanolntegris. It was found that this material did not respond in the same way to the atmospheric photonic process. Such buckypaper is a highly purified SWCNT material with residual catalyst and amorphous carbon less than 3% and 2% respectively, as stated by the supplier. They however lack any internal alignment and are composed of SWCNT lengths no longer than about 1 μηη.

In the experiments carried out by the inventors, successful outcomes were obtained with textiles composed of partly aligned, long length CNTs made using floating catalyst chemical vapour deposition. In one such process, a recipe based on a n-butanol carbon feedstock produced CNT textile which did respond well to the laser treatment in terms of improvement in Raman crystallinity and microstructure alignment. However, another recipe using a toluene feedstock did not experience any Raman crystallinity

improvement, although still had microstructure alignment. Thermo-gravimetric analysis (see Fig. 14) reveals the toluene-derived material's greater carbon species diversity. The temperature derivative of weight, for example (Fig. 14b), shows the oxidation

temperature for toluene derived CNTs as two broad peaks at about 550°C, contrasting with n-butanol's single sharp oxidation peak.

In more detail, Figs. 14a and 14b show the results of thermo-gravimetric analysis on as-is material spun from n-butanol and material spun from toluene. Fig. 14a shows the mass in percentage and Fig. 14b shows the normalized mass derivative with respect to temperature showing species oxidation temperatures.

The gradual weight reduction up to CNT oxidation indicates the amount of amorphous and oligomeric carbon present. This is 20% in terms of the total weight for toluene, compared to 6% for n-butanol. The toluene material has a small oxidation peak at about 325°C that point to oligomeric carbon, which coats and cross-links the CNTs. Without being bound by theory, the inventors speculate that the n-butanol derived material has a greater underlying graphitic crystallinity then the toluene derived material, as indicated by Raman spectroscopy after laser treatment. Additionally, the residual Fe content is somewhat higher in the n-butanol derived sample which will also have an effect in triggering vaporization events.

With this better understanding of the basic effects and requirements of photonic processing in an oxidative atmosphere such as air, we now consider a more complex process beyond point illumination that demonstrates uniform treatment of an arbitrarily long CNT textile, as well as superior improvement in crystallinity and microstructure alignment. The high speed camera images in Fig. 13 showed that the critical oxidation process concludes after approximately 3 ms, or 12 laser pulses, and is relatively quick compared to the full duration of the point illumination shot. Rather than discretely starting and stopping the laser to treat a long sample, the inventors found that continuously sweeping the laser quickly across a suspended CNT textile in air leads to a better and more uniform outcome. Approximately 350 mm s"1 was the fastest practical scan speed available in this set-up. Typically, initial transparent regions appear after several laser sweeps and then the next laser pass typically renders the entire sample uniformly transparent. The actual number of required passes is sample dependent and particularly thin CNT films may require only one pass. Additional laser passes beyond uniform transparency incrementally vaporises more material with little or no gains in quality. The width of the SWCNT textile film did not have a major impact on the outcome, except wider films suffered greater macroscopic tears from internal stain after treatment. The initial, as-is microstructure alignment should be substantially parallel to the direction of the laser scanning. Rastering the laser over a film cut against the microstructure grain leads to a mechanically weak and inhomogeneous outcome.

CLAIMS

A method for treating carbon nanotube-based material including the steps:

providing a carbon nanotube-based material;

suspending the carbon nanotube-based material in an oxidative atmosphere;

illuminating an illumination portion of the carbon nanotube-based material with electromagnetic radiation to heat the illumination portion, the illumination portion being out of direct contact with any supporting surface, heat being continuously conducted away from the illumination portion to a non-illumination portion of the carbon nanotube-based material, said heating in the oxidative atmosphere causing at least partial oxidation and at least partial removal of amorphous carbon, partly ordered non-tubular carbon, and/or defective nanotubes in the carbon nanotube-based material, leaving a treated material comprising an arrangement of remaining carbon nanotubes.

2. A method according to claim 1 wherein the heating in the oxidative atmosphere causes at least partial oxidation and at least partial removal of nanotubes not part of a sufficient thermally conductive pathway to allow transport of heat away before oxidation of those nanotubes.

3. A method according to claim 1 or claim 2 wherein the carbon nanotube-based material has a footprint area of at least 0.1 cm2.

4. A method according to any one of claims 1 to 3 wherein the carbon nanotube-based material comprises at least 50 wt% carbon nanotubes.

5. A method according to any one of claims 1 to 4 wherein the carbon nanotube-based material comprises at least 5 wt% carbon nanotubes selected from one or more of: single wall carbon nanotubes, double wall carbon nanotubes, and triple walled carbon nanotubes.

6. A method according to any one of claims 1 to 5 wherein single, double and triple wall carbon nanotubes in the carbon nanotube-based have an average length of at least 100 m.

7. A method according to any one of claims 1 to 6 wherein the density of the carbon nanotube-based material is at least 0.05 gem-3.

8. A method according to any one of claims 1 to 7 wherein the carbon nanotube-based material is manufactured by chemical vapour deposition on floating catalyst particles.

9. A method according to any one of claims 1 to 8 wherein the non-illumination portion of the carbon nanotube-based material has an area of at least 5 times the area of the illumination portion at a given instant in time during treatment.

10. A method according to any one of claims 1 to 9 wherein the electromagnetic radiation is moved relative to the carbon nanotube-based material so as to move the illumination portion progressively along the carbon nanotube-based material.

1 1 . A method according to claim 10 wherein the carbon nanotube-based material has a direction of preferential alignment of the carbon nanotubes, and the direction of relative movement of the illumination portion is substantially parallel to the direction of preferential alignment of the carbon nanotubes.

12. A method according to any one of claims 1 to 1 1 wherein the illumination of the illumination portion by the electromagnetic radiation takes place over a time scale not longer than the duration of an oxidation chemical reaction corresponding to said least partial oxidation.

13. A method according to any one of claims 1 to 1 1 wherein the electromagnetic radiation is pulsed in time so that the duration of each pulse of the electromagnetic radiation is not longer than the duration of an oxidation chemical reaction corresponding to said least partial oxidation.

14. A method according to any one of claims 1 to 1 1 wherein, for a region of the material being illuminated, the electromagnetic radiation is pulsed in time so that the cumulative duration of the pulses of the electromagnetic radiation is not longer than the duration of an oxidation chemical reaction corresponding to said least partial oxidation.

15. A method according to any one of claims 1 to 14 wherein the temperature of the illumination portion is at least 300°C.

16. A method according to any one of claims 1 to 15 wherein the temperature of the illumination portion is at most 2500°C.

17. A method according to any one of claims 1 to 16 wherein the fluence and/or intensity of the electromagnetic radiation at the illumination portion is sufficient to heat the carbon nanotube-based material to reach at least the lowest ignition temperature of all present carbon species at the illumination portion.

18. A method according to any one of claims 1 to 17 wherein the ratio of the mass of the illumination portion after the process to the mass of the illumination portion before the process is at most 0.9 and at least 0.01 .

19. A method according to any one of claims 1 to 18 wherein the treated material is further treated to remove at least some residual catalyst particles and/or some amorphous carbon that remained after the primary treatment

20. A method according to any one of claims 1 to 19 wherein, in the treated material, the carbon nanotubes are aligned to the extent that:

(i) where the treated material has a morphology such that micro-structure misalignment relative to the intended axis of micro-structure alignment is not constrained to one direction over another, the treated material has a Herman orientation parameter of at least 0.5; or

(ii) where the treated material has a morphology such that micro-structure misalignment relative to the intended axis of micro-structure alignment is predominantly constrained to one plane, the treated material has a Chebyshev's polynomial of at least 0.5.

21 . A method according to any one of claims 1 to 20 wherein, in the treated material, the carbon nanotubes have a graphitic crystallinity to the extent that when the material is subjected to non-polarized Raman spectroscopy to measure the ratio D:G of the magnitude of the D peak to the magnitude of the G peak, with magnitudes calculated by performing a baseline subtraction and integrating under the peaks of the non-polarized Raman spectrum, with Raman laser intensity sufficiently low to keep the calculated D:G ratio independent of Raman laser intensity within 10%, using light of wavelength 523 nm and 785 nm, the D:G ratio is at most 0.025 for 523 nm light and at most 0.1 for 785 nm light.

22. A method according to any one of claims 1 to 21 wherein, in the treated material, the carbon nanotubes have a graphitic crystallinity to the extent that when the material is subjected to non-polarized Raman spectroscopy to measure the ratio D:G of the magnitude of the D peak to the magnitude of the G peak, with magnitudes calculated by performing a baseline subtraction and integrating under the peaks of the non-polarized Raman spectrum, with Raman laser intensity sufficiently low to keep the calculated D:G ratio independent of Raman laser intensity within 10%, using light of different wavelengths, when the D:G ratio is plotted against the fourth power of the Raman laser excitation wavelength and fitted to a straight line, with the straight line numerically constrained to the origin, the adjusted R2 is at least 0.7.

23. A method for manufacturing and treating a carbon nanotube-based material including the steps:

forming an aerogel comprising at least carbon nanotubes, amorphous carbon, partly ordered non-tubular carbon, and catalyst particles by nucleation and growth of carbon nanotubes from a carbon material feedstock and floating catalyst particles in a reactor; extracting and consolidating the aerogel into a carbon nanotube-based material;

suspending the carbon nanotube-based material in an oxidative atmosphere;

illuminating an illumination portion of the carbon nanotube-based material with electromagnetic radiation to heat the illumination portion, the illumination portion being out of direct contact with any supporting surface, heat being continuously conducted away from the illumination portion to a non-illumination portion of the carbon nanotube-based material, said heating in the oxidative atmosphere causing at least partial oxidation and at least partial removal of amorphous carbon, partly ordered non-tubular carbon, and/or defective nanotubes in the carbon nanotube-based material, leaving a treated material comprising an arrangement of remaining carbon nanotubes.

24. A carbon nanotube-based material comprising carbon nanotubes of average length at least Ι ΟΟμηη, the carbon nanotubes of the material being aligned to the extent that:

(i) where the treated material has a morphology such that micro-structure misalignment relative to the intended axis of micro-structure alignment is not constrained to one direction over another, the material has a Herman orientation parameter of at least 0.5; or

(ii) where the treated material has a morphology such that micro-structure misalignment relative to the intended axis of micro-structure alignment is predominantly constrained to one plane, the material has a Chebyshev's polynomial of at least 0.5,

and the carbon nanotubes of the material have graphitic crystallinity to the extent that when the material is subjected to non-polarized Raman spectroscopy to measure the ratio D:G of the magnitude of the D peak to the magnitude of the G peak, with

magnitudes calculated by performing a baseline subtraction and integrating under the peaks of the non-polarized Raman spectrum, with Raman laser intensity sufficiently low to keep the calculated D:G ratio independent of Raman laser intensity within 10%, using light of wavelength 523 nm and 785 nm, the D:G ratio is at most 0.025 for 523 nm light and at most 0.1 for 785 nm light.

25. A carbon nanotube-based material according to claim 24 wherein:

(i) where the treated material has a morphology such that micro-structure misalignment relative to the intended axis of micro-structure alignment is not constrained to one direction over another, the material has a Herman orientation parameter of at least 0.6; or

(ii) where the treated material has a morphology such that micro-structure misalignment relative to the intended axis of micro-structure alignment is predominantly constrained to one plane, the material has a Chebyshev's polynomial of at least 0.6.

26. A carbon nanotube-based material according to claim 24 wherein:

(i) where the treated material has a morphology such that micro-structure misalignment relative to the intended axis of micro-structure alignment is not constrained to one direction over another, the material has a Herman orientation parameter of at least 0.7; or

(ii) where the treated material has a morphology such that micro-structure misalignment relative to the intended axis of micro-structure alignment is predominantly constrained to one plane, the material has a Chebyshev's polynomial of at least 0.7.

27. A carbon nanotube-based material comprising carbon nanotubes of average length at least Ι ΟΟμηη, the carbon nanotubes of the material being aligned to the extent that:

(i) where the treated material has a morphology such that micro-structure misalignment relative to the intended axis of micro-structure alignment is not constrained to one direction over another, the material has a Herman orientation parameter of at least 0.5; or

(ii) where the treated material has a morphology such that micro-structure misalignment relative to the intended axis of micro-structure alignment is predominantly constrained to one plane, the material has a Chebyshev's polynomial of at least 0.5,

and the carbon nanotubes of the material have graphitic crystallinity to the extent that when the material is subjected to non-polarized Raman spectroscopy to measure the ratio D:G of the magnitude of the D peak to the magnitude of the G peak, with magnitudes calculated by performing a baseline subtraction and integrating under the peaks of the non-polarized Raman spectrum, with Raman laser intensity sufficiently low to keep the calculated D:G ratio independent of Raman laser intensity within 10%, using light of different wavelengths, when the D:G ratio is plotted against the fourth power of the wavelength and fitted to a straight line, with the straight line numerically constrained to the origin, the adjusted R2 is at least 0.7.

28. A carbon nanotube-based material according to claim 27, the carbon nanotubes of the material being aligned to the extent that:

(i) where the treated material has a morphology such that micro-structure misalignment relative to the intended axis of micro-structure alignment is not constrained to one direction over another, the material has a Herman orientation parameter of at least 0.6; or

(ii) where the treated material has a morphology such that micro-structure misalignment relative to the intended axis of micro-structure alignment is predominantly constrained to one plane, the material has a Chebyshev's polynomial of at least 0.6.

29. A carbon nanotube-based material according to claim 27 wherein:

(i) where the treated material has a morphology such that micro-structure misalignment relative to the intended axis of micro-structure alignment is not constrained to one direction over another, the material has a Herman orientation parameter of at least 0.7; or

(ii) where the treated material has a morphology such that micro-structure misalignment relative to the intended axis of micro-structure alignment is predominantly constrained to one plane, the material has a Chebyshev's polynomial of at least 0.7.

30. A carbon nanotube-based material according to any one of claims 27 to 29 wherein, when the D:G ratio is plotted against the fourth power of the wavelength and fitted to a straight line, with the straight line numerically constrained to the origin, the reduced R2 is at least 0.8.

31 . A carbon nanotube-based material according to any one of claims 24 to 30 wherein the material is in the form of a fibre, textile, sheet or film.

32. A carbon nanotube-based material according to any one of claims 24 to 31 wherein the material is light-transmissive.

33. A carbon nanotube-based material according to claim 31 or claim 32 wherein the material is provided in a free-standing format, without the need for a substrate for support.