Method for producing carbon nanotube structures

Abstract

The invention relates to a method for producing a carbon nanotube structure with substantially aligned carbon nanotubes (CNTs), and a temperature-controlled flow-through reactor.

Description

[0001] The present invention relates to a method for producing a carbon nanotube structure with substantially aligned carbon nanotubes (CNTs), and a temperature controlled flow-through reactor.

[0002] There is an increasing demand for lightweight materials with high strength and stiffness, especially when combined with high electrical and thermal conductivity. Products have been made from aggregates of CNTs, but their properties have not reached their theoretical potential. The main reason for this deficiency is the imperfect alignment of the molecular scale CNTs within the macro scale fibres resulting from their aggregation.

[0003] CNTs are molecular scale structures comprising layers of carbon atoms covalently linked and forming closed tubes. The walls of CNTs can consist of a single layer (single-walled CNTs (SWCNTs)) or multiple layers (multi-walled CNTs (MWCNTs)). Individual CNTs have a diameter typically between 0.4 nm and 40 nm and a length typically more than 100 times their diameter.

[0004] To make CNTs form structures such as fibres or mats for macroscopic applications, it is necessary to produce aggregates of large numbers of CNTs. When in close proximity, CNTs attract each other through van der Waals forces and other atomic and molecular level interactions. A method for forming a plurality of long filaments each comprising a very large number of CNTs is called floating catalyst chemical vapour deposition (FCCVD). In this method, a carbon-rich feedstock (such as methane or acetylene) is introduced into a ceramic tube together with catalytic precursors containing iron and sulphur (such as ferrocene and thiophene, respectively) and raised to extremely high temperatures (typically in excess of 1000°C). After decomposition, carbon atoms provided by the precursors form an aerogel which can be extracted from the ceramic tube to form fibres or mats. FCCVD and the arrangement of the equipment are disclosed in EP-A-3227231. In practice, it was found that the alignment of the CNTs forming the aerogel was poor. This resulted in mechanical, electrical and thermal properties of the fibres far below the values obtainable from well-aligned CNT bundles.

[0005] Figure 1 schematically shows the basic elements of a conventional FCCVD temperature- controlled flow-through reactor. An electrically insulating refractory tube 1 is axially positioned within and surrounded by a furnace comprising a metal outer shell 2, thermal insulation 3 and an elongated electric heating element 4. After heating the furnace to a typical temperature of 1300 °C, feedstock (such as methane) and catalytic precursors (such as ferrocene and thiophene) are fed into the input end 5 of the tube 1 together with a carrier gas (such as hydrogen). An important function of the carrier gas is to exclude oxygen from the inside of the tube 1, which would otherwise cause combustion of the CNTs being formed. The catalytic reaction occurs at high temperature and results in the formation of fibre networks, each fibre network comprising a bundle of CNTs in the form of an aerogel sock 6. The aerogel sock 6 is withdrawn from the output end 7 of the tube 1, where it is drawn into individual fibres by being wound onto a spool 8. Post-processing by, for example, twisting or acid treatment is used to enhance the mechanical properties of the resulting fibres. Examples of such processes are described in Lee et al., “Direct spinning and densification method for high-performance carbon nanotube fibres”, Nature Communications, Volume 10, Article 2962 (2019) and J Bulmer et al., “Extreme stretching of high G:D ratio carbon nanotube fibres using super-acid”, Carbon, 153 725-736. An advantageous aspect of the FCCVD temperature-controlled flow-through reactor is that it can be used for continuous production. Precursor materials are continuously fed into the input end of the temperature-controlled flow-through reactor and aerogel is continuously withdrawn from the output end.

[0006] Methods for producing short well-aligned CNT fibres are known, but are not suitable for the continuous production of long fibres. The use of electric fields has been reported (for example in CN-A-101254914), but in many cases these have been applied to very small scale CNTs. A typical conventional arrangement comprises nanoscale channels or closely spaced plates, each with a few volt potential difference applied across them. For example Chen et al, "Aligning single-wall carbon nanotubes with an alternating-current electric field", Applied Physics Letters, Vol. 78, No. 23, June 2001 describes an arrangement of interdigitated electrodes spaced about 25 μιη apart and with an applied alternating voltage of 10 V peak to peak. Most researchers have described arrangements in which an electric field is applied to CNTs suspended in a liquid medium. While such arrangements cause the CNTs to align, their movement in the liquid is slow and the field strength that can be applied is limited by the properties of the liquid. Some researchers have applied an alternating electric field (see for example Liu et al, "Electric-field oriented carbon nanotubes in different dielectric solvents", Current Applied Physics, Vol. 4 (2004), pages 125-128). C Bower et al, "Plasma-induced alignment of carbon nanotubes", Applied Physics Letters, Vol. 77 No. 6, August 2000 describe a method of aligning CNT bundles grown normal to the surface on which they are formed. Bower reports that nanotubes can be grown on a contoured surface and aligned in a direction always normal to the local substrate surface in the presence of a field generated in the presence of a microwave plasma. This growth is similar to a lawn in which many closely spaced fibres grow about 50 um long normal to the surface on which they are formed. M.T. Cole and W.L. Milne, "Plasma Enhanced Chemical Vapour Deposition of Horizontally Aligned Carbon Nanotubes", Materials, 2013, Vol. 6, pages 2262-2273 also describe an arrangement employing a plasma to produce short aligned CNTs and observe that field strengths of the order of 0.1 to 0.5 μν / η are required.

[0007] Growth of short aligned CNTs between a pair of plates with an applied potential has also been reported (see for example Y. Avigal and R. Kalish, "Growth of aligned carbon nanotubes by biasing during growth", Applied Physics Letters, 78, pages 2291-2293, 2001 and Q Bao and C Pan, "Electric field induced growth of well aligned carbon nanotubes from ethanol flames", Nanotechnology 17 (2006) 1016-1021). Related arrangements are described in W. Merchan-Merchan et al., "Combustion synthesis of carbon nanotubes and related nanostructures", Progress in Energy and Combustion Science, Volume 36 (2010) pages 696-727.

[0008] The use of an alternating field rather than a static (DC) field is described in Chen et al. "Quantitatively Control of Carbon Nanotubes Using Real Time Electrical Detection Dielectrophoresis Assembly", Proceedings of the 15th IEEE International Conference on Nanotechnology, 27-30 July 2015, Rome, Italy, pages 1029-1032.

[0009] A method for forming longer aggregates of CNTs is described in L.R. Bornhoeft et al. ("Teslaphoresis of Carbon Nanotubes", ACS Nano 2016, 10, 4873-4881, American Chemical Society). This method involves "explosive self-assembly" of powder CNTs in air and slow alignment of liquid CNT suspensions.

[0010] US-A-2012 / 0282453 discloses a continuous process for producing aligned CNT ribbons by applying a polymer spray to form a composite material.

[0011] The combination of FCCVD and the application of an electric field is described in Peng et al. (Enrichment of metallic carbon nanotubes by electric field-assisted chemical vapor deposition, Carbon, Vol. 49 (2011), pp. 2555-1560). However, the orientation of the electric field is orthogonal to the direction of the gas flow, so it is not possible to continuously produce long aggregates of oriented CNTs.

[0012] None of the prior art methods are suitable for producing continuous oriented macro-scale fibres for engineering applications.

[0013] The present invention relates to a method and a temperature-controlled flow-through reactor for the continuous production of CNT structures, such as fibres, in which the orientation of the constituent CNTs is improved, which contributes to improved mechanical, electrical or thermal properties. In particular, the present invention relates to a floating catalyst (CVD) method in which there is a direct interaction with the self-assembly of CNT bundles in the gas phase.

[0014] Thus, viewed from a first aspect, the present invention provides a method for producing carbon nanotube structures, the method comprising:

[0015] (a) introducing a metal catalyst precursor into a continuous carrier gas stream in a temperature-controlled flow-through reactor;

[0016] (b) exposing the metal catalyst precursor in the carrier gas stream to a first temperature zone sufficient to generate a particulate metal catalyst;

[0017] (c) releasing a carbon source into the carrier gas stream;

[0018] (d) exposing the particulate metal catalyst and the carbon source to a second temperature zone downstream of the first temperature zone, wherein the second temperature zone is sufficient to produce carbon nanotube aggregates;

[0019] (e) generating an electric field in the temperature-controlled flow-through reactor at or near the second temperature zone;

[0020] (f) discharging the carbon nanotube aggregates as a continuous discharge through a discharge outlet of the temperature-controlled flow-through reactor; and

[0021] (g) collecting the continuous discharge in the form of carbon nanotube structures.

[0022] Typically, the continuous carrier gas stream follows a substantially linear flow path.

[0023] Preferably, the orientation of the electric field is substantially parallel to the flow path of the carrier gas. Particularly preferably, the orientation of the electric field is substantially coaxial with the flow path of the carrier gas.

[0024] Preferably, the temperature-controlled flow-through reactor comprises:

[0025] an elongate refractory housing extending from an upstream end to a downstream end, the metal catalyst precursor being introduced into the elongate refractory housing in step (a) and the carbon source being released into the elongate refractory housing in step (c);

[0026] a thermal enclosure surrounding the elongate refractory housing, the thermal enclosure being adapted to provide an axial temperature variation between temperature zones in the elongate refractory housing, wherein the temperature zones comprise a first temperature zone and a second temperature zone; and

[0027] an electrode positioned inside or outside the elongate refractory housing.

[0028] The electrode can be positioned partially inside the elongate refractory housing. For example, the electrode can extend upstream from the upstream end.

[0029] Preferably, the orientation of the electrode is substantially parallel to the flow path of the carrier gas. Particularly preferably, the orientation of the electrode is substantially coaxial with the flow path of the carrier gas.

[0030] The electric field can be generated by an electric field generator having a first terminal (e.g. a metal housing) electrically grounded and a second terminal electrically connected to the electrode.

[0031] In step (a), the metal catalyst precursor can be introduced axially or radially into the temperature-controlled flow-through reactor. The metal catalyst precursor can be introduced by a probe or a syringe. The metal catalyst precursor can be introduced at multiple locations.

[0032] The metal catalyst precursor can be suspended as solid particles (preferably solid nanoparticles) in the carrier gas.

[0033] The metal catalyst precursor can be a metal compound of at least one of the group consisting of Fe, Ru, Co, W, Cr, Mo, Rh, Ir, Os, Ni, Pd, Pt, Ru, Y, La, Ce, Mn, Pr, Nd, Tb, Dy, Ho, Er, Lu, Hf, Li and Gd.

[0034] The metal catalyst precursor can be a metal complex or an organometallic compound.

[0035] Preferably, the metal catalyst precursor is sulfur-containing.

[0036] The metal catalyst precursor can be introduced in step (a) together with a sulfur-containing additive. The sulfur-containing additive can be thiophene, iron sulfide, a sulfur-containing ferrocenyl derivative (e.g. a ferrocenyl sulfide), hydrogen sulfide or carbon disulfide.

[0037] Typically, the particulate metal catalyst is a nanoparticulate metal catalyst. Preferably, the nanoparticles of the nanoparticulate metal catalyst have an average diameter (e.g. number, volume or surface average diameter) in the range 1 to 50 nm, preferably 1 to 10 nm. Preferably, 80% or more of the particles of the nanoparticulate metal catalyst have a diameter less than 30 nm. Particularly preferably, 80% or more of the particles of the nanoparticulate metal catalyst have a diameter less than 12 nm. The concentration of the particulate metal catalyst can be in the range 10 6 to 10 10 particles cm -3

[0038] In step (c), the carbon source can be released axially or radially into the temperature controlled flow-through reactor. The carbon source can be introduced through a probe or syringe. The carbon source can be introduced at multiple locations.

[0039] The carbon source can be an optionally substituted and / or optionally hydroxylated aromatic or aliphatic, acyclic or cyclic hydrocarbon (e.g. alkyne, alkane or alkene) optionally interrupted with one or more heteroatoms (e.g. oxygen). Preferred are optionally halogenated C 1-6 hydrocarbons (e.g. methane, propane, ethylene, acetylene or tetrachloroethylene), optionally mono-, di- or tri-substituted benzene derivatives (e.g. toluene), C 1-6 -alcohols (e.g. ethanol or butanol) or aromatic hydrocarbons (e.g. benzene or toluene).

[0040] The generation of the particulate metal catalyst can be initiated in step (b) by thermal decomposition or dissociation of the metal catalyst precursor into metal species (e.g. atoms, radicals or ions). The generation of the particulate metal catalyst in step (b) can comprise nucleation of the metal species into nucleated metal species (e.g. clusters). The generation of the particulate metal catalyst can comprise growth of the nucleated metal species into the particulate metal catalyst.

[0041] In a preferred embodiment, the carrier gas comprises dispersed substrate particles. Typically, the substrate particles are fine particles. By forming a substrate supported particulate metal catalyst dispersed in the carrier gas, the substrate particles are used to promote nucleation in the first temperature zone. The substrate particles can be Si or Si02particles.

[0042] Preferably, the method further comprises introducing substrate particles into the continuous carrier gas stream.

[0043] In a preferred embodiment, steps (a) and (c) are concurrent.

[0044] The first and second temperature zones can extend in the range at least 600 to 1300 °C.

[0045] ​The carrier gas is typically one or more of nitrogen, argon, helium or hydrogen. The flow rate of the carrier gas can be in the range 1000 to 50000 seem (e.g. 30000 seem).

[0046] The carbon aggregates can comprise multi-walled carbon nanotubes (e.g. double-walled carbon nanotubes) and / or single-walled carbon nanotubes.

[0047] The carbon aggregates can take the form of a 3D continuous network (e.g. aerogel).

[0048] Preferably, the carbon aggregates are aerogels.

[0049] The carbon nanotube structures can be powders, fibres, wires, films, tapes, strands, sheets, plates, meshes or mats.

[0050] The carbon nanotube aggregates or carbon nanotube structures can comprise carbon nanotube bundles (i.e. an array of substantially parallel CNTs (typically 3-20 CNTs) attracted to each other by van der Waals forces).

[0051] The carbon nanotube aggregates or carbon nanotube structures can comprise carbon nanotube bundles having a median diameter (e.g. as measured by SEM and manual image analysis) of 16 nm or greater, preferably 20 nm or greater, particularly preferably 25 nm or greater, more preferably greater than 50 nm, even more preferably 75 nm or greater. Preferably, the diameters of the carbon nanotube bundles follow a lognormal distribution.

[0052] The carbon nanotube aggregates or carbon nanotube structures can comprise carbon nanotube bundles having a median diameter (e.g. as measured by SEM and manual image analysis) that is axially variable (i.e. along the length). Preferably, the diameters of the carbon nanotube bundles vary axially from a normal distribution to a lognormal distribution.

[0053] Viewed from another aspect, the present application provides a temperature-controlled flow-through reactor for producing carbon nanotube structures, the temperature-controlled flow-through reactor comprising:

[0054] an elongate refractory housing extending from an upstream end to a downstream end;

[0055] an inlet at or near the upstream end of the elongate refractory housing for introducing a continuous carrier gas stream from the upstream end to and past the downstream end;

[0056] first feed means for releasing a carbon source into the continuous carrier gas stream;

[0057] second feed means for introducing a metal catalyst precursor into the continuous carrier gas stream;

[0058] a hot enclosure around the elongate refractory housing, the hot enclosure being adapted to provide an axial temperature variation between temperature zones in the elongate refractory housing, wherein the temperature zones include a first temperature zone sufficient to generate particulate metal catalyst and a second temperature zone sufficient to produce carbon nanotube aggregates;

[0059] a collector for collecting a continuous discharge of carbon nanotube aggregates in the form of carbon nanotube structures from the downstream end;

[0060] a first electrode positioned inside or outside the elongate refractory housing; and

[0061] an electric field generator electrically connected between the ground and the first electrode so as to apply a high potential thereto, the high potential being sufficient to generate an electric field at or adjacent the second temperature zone in the elongate refractory housing.

[0062] The temperature-controlled flow-through reactor can further include a second electrode. The electric field generator can be electrically connected to the second electrode so as to apply a high potential or a low potential thereto. Preferably, the second electrode is electrically grounded.

[0063] The temperature-controlled flow-through reactor can further include a third electrode. The electric field generator can be electrically connected to the third electrode so as to apply a high potential or a low potential thereto. Preferably, the third electrode is electrically grounded. The third electrode can be used to control the form, strength and location of the electric field.

[0064] The temperature-controlled flow-through reactor can further include a plurality of additional electrodes positioned outside the elongate refractory housing. The plurality of additional electrodes can be alternately connected to the electric field generator of high potential and to ground.

[0065] The (or each) electrode can be an elongate electrode (e.g. an elongate solid or elongate hollow electrode). The (or each) electrode can be substantially cuboid, cylindrical or annular. Typically, the (or each) electrode is substantially coaxial with the elongate refractory housing.

[0066] The first electrode can be positioned at least partially inside the elongate refractory housing (e.g. at or adjacent the upstream end of the elongate refractory housing). The first electrode can be positioned at or adjacent the second temperature zone. The first electrode can be positioned upstream of the second temperature zone.

[0067] The (or each) electrode is typically formed of an electrically conductive material capable of withstanding the temperature and chemical environment inside the refractory tube. Suitable materials include molybdenum or glassy carbon. The (or each) electrode can be provided with an inert sleeve (e.g. an alumina sleeve). The sleeve can expose only the downstream end of the electrode.

[0068] Preferably, the electric field is substantially coaxial with the elongate refractory housing.

[0069] Preferably, the collector is electrically grounded. The carbon nanotube aggregates are grounded as they are in operative connection with the collector during use.

[0070] In a first preferred embodiment, the first electrode is positioned at or near the second temperature zone inside the elongated refractory shell, and the collector is electrically grounded. Preferably, a grounded portion (e.g., a terminal) of the electric field generator is electrically connected to the collector.

[0071] In a second preferred embodiment, the temperature-controlled flow-through reactor further comprises a second electrode outside the elongated refractory shell, and the first electrode is positioned at or near the second temperature zone inside the elongated refractory shell. Particularly preferably, a grounded portion (e.g., a terminal) of the electric field generator is electrically connected to the collector.

[0072] In the second preferred embodiment, the second electrode can be electrically connected to the hot enclosure, and the hot enclosure can be grounded. This serves to ground the second electrode. For example, the second electrode can be electrically connected to a metal shell of the hot enclosure.

[0073] In a third preferred embodiment, the first electrode is positioned outside the elongated refractory shell adjacent to the second temperature zone. Particularly preferably, a grounded portion (e.g., a terminal) of the electric field generator is electrically connected to the collector.

[0074] In a fourth preferred embodiment, the temperature-controlled flow-through reactor further comprises a second electrode positioned outside the elongated refractory shell, wherein the first electrode is positioned outside the elongated refractory shell, and the second electrode is electrically grounded.

[0075] In a fifth preferred embodiment, the temperature-controlled flow-through reactor further comprises a second electrode positioned inside the elongated refractory shell, wherein the first electrode is positioned inside the elongated refractory shell, and the second electrode is electrically grounded.

[0076] The first electrode can be positioned adjacent to the second temperature zone. A tip of the first electrode can be positioned upstream of a midpoint of the elongated refractory shell.

[0077] The second electrode can be positioned adjacent to the second temperature zone. A tip of the second electrode can be positioned downstream of a midpoint of the elongated refractory shell.

[0078] Preferably, the electric field generator applies an AC potential (e.g., in the range of 500 V to 5000 V peak to peak).

[0079] Preferably, the electric field generator is an AC source. AC electric fields are advantageously used to continuously orient CNTs in situ before they form a dense network and aerogel. In particular, the AC field creates a CNT hardening effect (z-pinch) induced by Lorentzian forces. As an illustration, in one example the CNT bundle diameter was determined to widen from 16 to 25 nm, and the electrical and tensile properties increased significantly (up to 90 and 380%, respectively) without changing the fundamental nature of the constituent nanomaterials (as verified by Raman spectroscopy). The enhanced properties correlated with the degree of CNT orientation within the textile, as quantified by small-angle X-ray scattering and innovative SEM image analysis. At applied field strengths in the range of 0.5-1 kV cm -1 Significant orientation (T2 = 0.5) was achieved relative to the original material (T2 = 0.2) at applied field strengths in the range of 0.5-1 kV cm

[0080] Preferably, the electric field generator applies the AC potential at a field strength in the range of 0.1 to 2.0 kV cm -1 Particularly preferably, the electric field generator applies the AC potential at a field strength in the range of 0.5 to 1.0 kV cm -1 More preferably, the electric field generator applies the AC potential at a field strength in the range of 0.35 to 0.75 kV cm -1

[0081] Preferably, the electric field generator is operable at radio frequency (RF). Particularly preferably, the electric field generator is operable at high frequency (HF) (e.g., a frequency in the range of 10 to 20 MHz).

[0082] Preferably, the temperature-controlled flow-through reactor further comprises a third feed device for introducing substrate particles into the continuous carrier gas stream.

[0083] The first, second, and third feed devices can be a nozzle, a spray gun, a probe, or a multi-barrel syringe (e.g., a showerhead injector).

[0084] The elongated refractory housing can be substantially cylindrical (e.g., tubular).

[0085] Typically, the thermal enclosure contains a thermally insulating material. The thermal enclosure can be a grounded metal enclosure.

[0086] The axial temperature variation can be non-uniform (e.g., stepped). The temperature of the temperature-controlled flow-through reactor can be controlled by resistive heating, plasma, or laser.

[0087] The temperature-controlled flow-through reactor can be substantially vertical or horizontal.

[0088] The collector is typically electrically conductive (e.g., metallic). The collector can be a rotating mandrel, a spool, or a drum.

[0089] ​The methods and reactors of the present invention advantageously allow control of the size and distribution of CNT bundles (i.e. arrays of substantially parallel CNTs (typically 3-20 CNTs) that are attracted to each other by van der Waals forces) by, for example, adjusting the electric field strength.

[0090] Viewed from yet another aspect, the present invention provides a carbon nanotube aggregate or carbon nanotube structure comprising carbon nanotube bundles having a median diameter (e.g. as measured by SEM and manual image analysis) of 16 nm or greater, preferably 20 nm or greater, particularly preferably 25 nm or greater, more preferably greater than 50 nm, even more preferably 75 nm or greater.

[0091] Preferably, the diameter of the carbon nanotube bundles follows a lognormal distribution.

[0092] Viewed from even yet another aspect, the present invention provides a carbon nanotube aggregate or carbon nanotube structure comprising carbon nanotube bundles having a median diameter (e.g. as measured by SEM and manual image analysis) that is axially variable along the carbon nanotube aggregate or carbon nanotube structure.

[0093] Preferably, the diameter of the carbon nanotube bundles varies axially from a normal distribution to a lognormal distribution.

[0094] The present invention will now be described in a non-limiting sense with reference to the accompanying drawings in which:

[0095] Figure 1 shows a simplified view of a conventional FCCVD furnace for producing carbon nanotubes in the form of aerogels.

[0096] Figure 2 A first embodiment of the temperature-controlled flow-through reactor of the present invention is shown, having a first electrode contained within a refractory tube.

[0097] Figure 3 Computer simulation results of the electric field generated in the first embodiment when a potential difference is applied between the first electrode and a second electrode formed from an aerogel are shown.

[0098] Figure 4 Computer simulation results of the electric field generated in the second embodiment of the temperature-controlled flow-through reactor of the present invention when a potential difference is applied between a first electrode in a refractory tube surrounded by a third hollow cylindrical electrode and a second electrode formed from an aerogel are shown.

[0099] Figure 5 Computer simulation results of the electric field generated in the third embodiment of the temperature-controlled flow-through reactor of the present invention when a potential difference is applied between a hollow cylindrical first electrode outside the refractory tube and a second electrode formed from an aerogel are shown.

[0100] Figure 6Computer simulation of the electric field generated by the fourth embodiment of the temperature-controlled flow-through reactor of the present invention when a potential difference is applied between two hollow cylindrical electrodes outside the refractory tube.

[0101] Figure 7 is an exemplary embodiment of a circuit configured to resonate at a selected operating frequency.

[0102] Figure 8a and 8b show SEM images of CNT aggregates formed with and without the application of an RF electric field.

[0103] Figure 9 and 10 show perspective and cross-sectional views, respectively, of an embodiment of the temperature-controlled flow-through reactor of the present invention with external electrodes.

[0104] Figure 11 shows an AC field directing system. (a) Adapted FCCVD reactor with RF electrodes inserted in its front portion, while the CNT aerogel being formed is collected on a grounded spool acting as a counter electrode. CNTs are aligned along the resulting field lines before the aerogel is formed. (b) Enlarged schematic showing events in the interelectrode gap. (i) AC field induces "Lorentz pinch" hardening the super-long CNTs; (ii) rigid super-long CNTs are under the influence of field-induced aligning torque; (iii) CNTs are aligned according to field lines; the schematic is not drawn to scale, and i-iii occur simultaneously. (c) FEM numerical results depicting the field distribution inside the reactor tube. CNT aerogel ("jacket") approximated as a 28 mm OD (25 mm ID) cylinder. The packing density of the equipotential lines indicates the local field strength. The model shows the presence of field-aligned lines bridging the two electrodes within the interelectrode gap (50 mm wide).

[0105] Figure 12 shows continuous CNT alignment using internal RF electrodes. (a) Image taken looking upstream from the end of the reactor. The image shows CNT aerogel collection on a rotating spool when an AC field is applied. Elongated whiskers "grow" from the end of the graphite RF electrode towards the aerogel being formed. (b) SEM image showing the micro-morphology of the CNT final product. CNT alignment is evident, although it does not appear to be optimal. The inset shows a 15 cm long single CNT jacket produced during AC alignment. The jacket appears more rigid than usual and is able to support its own weight.

[0106] Figure 13Figure 1 shows the physical properties of CNT oriented materials. (a) Graph showing the specific conductivity (black, left axis) and G / D ratio (red, right axis) of CNT materials and reference materials (0W) collected under different applied AC field strengths. Although the G / D ratio did not change significantly, the specific conductivity increased by up to 90%. Error bars represent the standard deviation using at least three different samples. (b) The stress-strain curves of the tensile measurements show a unique change in mechanical properties from ductility (0W) to a more brittle tendency behavior of the oriented samples. The mechanical transition of the properties is well correlated with the applied field strength (∝P 1 / 2).

[0107] Figure 14 shows the WAXS orientation of CNT materials. (a) At 0.7 to 0.8 nm -1 Intensity normalized azimuth scan of the sample at 0W (ref) and 300W under the Q range. (b) corresponds to the x-axis (equator) and is perpendicular to the fiber axis. The inset shows the corresponding 2D SAXS pattern. The reference material does not show any obvious scattering pattern, confirming the anisotropic nature of the textile. The 300W sample shows a unique Lorentzian intensity distribution, confirming the presence of CNT orientation. (b) shows a graph of the Herman parameter (P2) calculated from the azimuthal scan (inset) as a function of the sample's elastic modulus.

[0108] Figure 15 This is the z-pinch mechanism. (a) Illustration of the electromagnetic fields in a CNT associated with the z-pinch stiffening effect. An axial current (orange) is confined to the CNT wall and induces a circumferential magnetic field (blue). (bc) Cross-sectional free-body diagrams of a continuum CNT model with a z-pinch. The internal forces along the upper contours of the two surfaces are shown in red. The pressure (b) and equivalent restoring force (c) acting on the CNT wall are shown in blue.

[0109] Figure 16 Modeling of CNT electric field orientation (ab) surface plots of T2_min versus CNT length (log) and electric field strength (log) for DC (a) and AC (b) electric fields. Contours are plotted in red, black, and blue for different T2_min values. The white dashed line indicates the rigid-elastic transition for the DC field. (c) Log-log plot of electric field strength versus CNT length for the contours taken from (a) (dashed line) and (b) (solid line). The orange dashed line shows the rigid-elastic transition. (de) Log-log plot of the electric field strength required to reach T2_min = 0.5 versus CNT length for different (10,10) SWCNT bundles (d) and MWCNTs with different chair-shaped walls (e). (f) TEM image of the reference sample shows the widespread presence of few-walled MWCNTs with three to five walls (red line).

[0110] Figure 17 A two electrode configuration is shown. (a) RF electrode (graphite; 6 mm) inserted at the front and ground electrode (Mo; 6 mm) through the back. Both electrodes are free to pass through the central axis, enabling control of the depth (Δχ) and width (ΔL) of the inter-electrode gap. CNTs orient along the resulting field lines. (b) Photographs of the inter-electrode gap; (i) undergoing hydrogen breakdown due to high field strength (on the order of at least a few kV cm -1 (ii) Vapor grown carbon fiber (VGCF) whiskers growing in the inter-electrode gap according to the bridging field lines between the electrodes. (c) FEM numerical results depicting the internal field distribution of the furnace cavity, with equipotential lines (blue) and orthogonal field lines (red). The packing density of the equipotential lines indicates the local strength of the field. The model shows a directed induction field in the 50 mm inter-electrode gap similar to that exhibited in bii. (d) Low magnification SEM images showing the highly oriented CNT micro-morphology seen in the material produced under the influence of applied field strengths of ~0.75 kV cm -1 -1 -1 -

[0111] Figure 18 shows image analysis of CNT materials. (a) Plot of the Chebyshev orientation order parameter (T2) calculated by the Fibre COP software depicting the orientation (with accompanying typical SEM images) for the applied field strengths generated in the inter-electrode gap. While field strengths less than 0.23 kV cm -1 did not appear to affect orientation, field strengths reaching 0.3 kV cm -1 and above showed a significant increase in orientation. The Y value variance is based on the standard deviation of the calculated T2 values derived from at least three images from two different samples; the X value variance is based on the voltages generated in the two limit set points of the system. (b) Beam diameter distribution (lognormal fit) shows the median beam thickness shifts from 16.44 ± 0.10 to 18.87 ± 0.87 and 25.40 ± 0.46 for materials produced at field strengths of 0.23, 0.35 and 0.75 kV cm -

[0112] Figure 19VGCF formation in the FCCVD reactor is shown. (a) VGCF whiskers grow radially from the RF electrode surface towards the reactor wall, causing shorting. (b) SEM image of the whiskers reveals an isotropic network of VGCF. Inset shows individual VGCF at higher magnification. (c) SEM image of whiskers produced under the influence of HV shows more orientation in the VGCF network. Inset shows finer "dendritic" whiskers produced when HV is applied during whisker synthesis.

[0113] Figure 20 VGCF "extension" whisker growth in the FCCVD reactor is shown. (a) VGCF whiskers grow axially downstream from the RF electrode, producing extensions towards the RF electrode. (b) Some "extension" whisker growths are 150 mm long. (c) SEM image of VGCF "extension" whiskers, showing that they are made of long and oriented VGCF. Inset shows high magnification image revealing that the VGCF are very thin (-100 nm diameter) with a CNT core (arrow).

[0114] Figure 21 Raman spectra of various CNT samples produced by the internal RF electrode setup. There is no significant difference between the reference sample spectrum and the other spectra of materials produced under the influence of the electric field.

[0115] Figure 22 SEM image of CNT material produced at a field strength of 0.75 kV cm -1 The arrow traces the trajectory of an ultra-long CNT bundle with a length greater than 100 μm.

[0116] Figure 2 A first embodiment of the temperature-controlled flow-through reactor of the present invention is shown. An elongated first electrode 9 is provided at the input end 5 of an electrically insulating refractory tube 1 which is axially positioned within and surrounded by a furnace. The furnace comprises a metal housing 2 which is grounded through a connector 16, an insulating material 3 and an elongated electric heating element 4. The first electrode 9 is formed of an electrically conductive material, such as molybdenum, which is capable of withstanding the temperature and chemical environment within the refractory tube 1. A second electrode is formed by the trailing end of a conductive aerogel sleeve 6 which is produced during processing and discharged from the output end 7 of the refractory tube 1 onto a conductive spool 8. A first conductor 10 connects the first electrode 9 to the live terminal of a high voltage power supply 13 and a second conductor 11 connects the conductive spool 8 to the terminal of the high voltage power supply 13 which has a ground connection 14. The high voltage power supply 13 delivers a radio frequency voltage. The effect of establishing a high potential difference (voltage) between the leading end of the first electrode 9 and the trailing end of the aerogel sleeve 6 is to produce an electric field in the region represented by the dashed outline 15 which is substantially axial and coincident with the axis of the refractory tube 1.

[0117] Figure 3Computer simulation results are shown for the electric field lines 13 and equipotential lines 14 generated between the first electrode 9 and the second electrode formed by the tail end of the aerogel sleeve 6 in the first embodiment. It has been found that effective CNT orientation is observed by applying an AC voltage at a frequency of 13.64 MHz. This is a frequency specified internationally for industrial and scientific use. The source of the applied potential is configured such that the applied potential is sufficient to provide maximum CNT orientation while avoiding arcing or corona discharge.

[0118] Figure 4 The simulated electric field generated by a second embodiment of a temperature-controlled flow-through reactor of the present invention between a first electrode 9, a second electrode formed of aerogel 6 (as described for the first embodiment), and an elongated hollow cylindrical third electrode 20 on the exterior of the refractory tube 1 is shown. The third electrode 20 is maintained at ground potential by a conductive connection to the metal (grounded) shell 2 of the furnace. Compared to the first embodiment, the second embodiment generates a more uniform axial field in the region between the first electrode 9 and the second electrode formed of aerogel 6.

[0119] Figure 5 The simulated electric field generated by a third embodiment of a temperature-controlled flow-through reactor of the present invention is shown between a first electrode 21, which takes the form of a ring or hollow cylinder surrounding the exterior of a refractory tube 1. In this embodiment, the first electrode 21 is at a high potential, and the second electrode formed of aerogel 6 is grounded via a conductive spool 8 (as described for the first embodiment).

[0120] Figure 6 The simulated electric field generated by a fourth embodiment of a temperature-controlled flow-through reactor of the present invention is shown between a first electrode 22, which takes the form of an elongated hollow cylinder, and a second electrode 23, which similarly takes the form of an elongated hollow cylinder. The first electrode 22 is at a high potential, while the second electrode 23 is grounded. The first and second electrodes 22, 23 surround the exterior of a refractory tube 1. The fourth embodiment generates a substantially axial electric field over a longer axial distance than the first, second, and third embodiments.

[0121] exist Figures 2 to 6 In the embodiment of the present invention, the metal housing 2 forms a grounded electromagnetic shield, which shields the environment from radiation caused by the alternating fields in the furnace. This ensures the safety of personnel and prevents interference with electrical or electronic equipment.

[0122] For a refractory tube 1 having a diameter of 55 mm, the voltage required to provide CNT alignment by an alternating electric field is found to be typically between 500 V and 5000 V peak to peak. The maximum field strength that can be used is lower than the field strength that can cause a corona discharge or formation of a plasma within the refractory tube 1. The optimum axial position of the electrodes and the field strength between the electrodes are functions of the diameter of the refractory tube 1, the properties and flow rate of the reactive and transport gases within the refractory tube 1, the temperature profile along the axis of the refractory tube 1 and the configuration of the one or more high voltage electrodes and the one or more grounded electrodes. Although the frequency of the applied field can be in the range 13.553 - 13.567 MHz, other frequencies can also be used.

[0123] It is convenient to generate the electric field using a radio frequency generator, the output of which is applied to a circuit arrangement configured to resonate at the selected operating frequency. Figure 7 An exemplary embodiment of such a circuit arrangement is shown in Figure 1, in which a radio frequency power generator (provided with a circuit arrangement comprising an oscillator and power amplifier, and control and monitoring facilities) is connected to an input port 30 by means of a radio frequency transmission line. An inductor 31 and a variable capacitor 33 constitute a series resonant circuit, the function of which is to increase the voltage applied to it from the input port 30. A connection is provided between an output port 35 and the junction of the inductor 31 and the variable capacitor 33, in which the voltage applied at the input port 30 is multiplied by the voltage amplification factor ("Q factor") of the resonant circuit 31, 33. A variable capacitor 32 is connected in parallel with the inductor 31 to allow variation of the effective inductive reactance of the resonant circuit 31, 33. The values of the variable capacitors 32, 33 are selected to allow control of the Q factor of the resonant circuit 31, 33, enabling control of the relationship between the voltages at the input port 30 and the output port 35. The inductor 31 can be provided with a variable tap to allow direct adjustment of its inductance. A variable capacitor 34 is provided to enable the input impedance of the circuit to be adjusted to match the 50 ohm impedance typically required by a connected radio frequency generator. To allow high voltages to be generated (for example between 500 V and 10000 V), the variable capacitors 32, 33, 34 are vacuum variable capacitors.

[0124] A stray (parasitic) capacitance 36 exists between the metal housing 2 of the furnace and the high voltage electrode and the associated conductive connections. The effect of this parasitic capacitance is to load the resonant circuit 31, 33, resulting in a reduction in the output voltage at the output port 35. The effect of the parasitic capacitance 36 can be reduced by connecting an inductor 37 in parallel with it. The effective value of the inductor 37 is selected to produce parallel resonance with the parasitic capacitance 36 at the operating frequency.

[0125] Port 38 is connected by means of a radio frequency transmission line to a resistive termination typically having a value of 50 ohms. The monitor port 39 is provided with a current connected conductive loop 48 and capacitor 40 together enabling the measurement of the output voltage after a one time calibration procedure to relate the output voltage at the monitor port 39 to the much lower voltage at the output port 35. After calibration, the voltage at the output port 35 can be estimated by measuring the low voltage at the output port 35 using, for example, a standard oscilloscope. This arrangement eliminates any requirement to frequently measure high radio frequency voltages which can be hazardous to personnel operating the apparatus.

[0126] A radio frequency generator connected to the input port 30 provides a selectable output power level and contains an arrangement to reduce the output power in the event that the reflected power increases to a level which can cause damage. Monitoring the reflected power provides an indication of changes within the reactor such as the initial formation of a corona or other electrical discharge, or contact between the grounded CNT aerogel and the high voltage electrode. The monitoring information can be provided to the radio frequency generator by a digital interface and can be used to control the rate at which the aerogel is withdrawn from the refractory tube or the flow rate of the reagent.

[0127] Figure 8a is a scanning electron microscope image showing a sample of CNT aggregates produced using the temperature controlled flow through reactor of Figure 1. The fibres formed from the assembled CNTs show little degree of orientation. Figure 8b is a scanning electron microscope image of a sample of fibres produced in the first embodiment of the application with an applied axial electric field at a frequency of 13.56 MHz. The temperature profile and other operating parameters were essentially unchanged. Figure 8b The fibres shown exhibit a significant degree of orientation.

[0128] Figure 9 and 10 show respectively a perspective view and a cross sectional view of an embodiment of the temperature controlled flow through reactor 94 of the application having Kanthal loop electrodes 97 and Kanthal RF electrodes 98 outside the refractory tube into which methane / thiophene / hydrogen 91 and ferrocene 99 are fed. The outer loop electrodes 97 produce electric field lines 92 to orient the CNTs 93 which form an aerogel 95 which is wound onto a spool 96. This facilitates a continuous process and eliminates unwanted growth of VGCF. Example

[0129] Method

[0130] High voltage system and final element modelling

[0131] Custom-made cabinets were manufactured to act as RF shielded chambers for the HV components, thus ensuring personnel and equipment safety. The system housed a 300W RF generator (Dressler Cesar 1312) operating in the license-exempt 13.56-MHz frequency band. The output of the generator was connected to a 50-ohm load through a series L-C circuit tuned to 13.56 MHz. This arrangement results in the generation of high voltages at the connection between the inductor and the capacitor. A second variable capacitor (C1) was connected in parallel to the inductor, thus allowing its effective reactance to be varied. To project the HV generated by the system into the reactor, the L-C junction was connected to the RF electrode. According to equation (1), the voltage is tuned by modifying the reactance of the main capacitor and the parallel combination of the inductor and its capacitor:

[0132]

[0133] where Q is the voltage amplification factor, L is the inductance, C is the capacitance, and V is the output voltage.

[0134] The RF output voltage was measured by connecting a resistive voltage divider (985 kQ + 1 kQ) across the high voltage output of the network and measuring the voltage across the 1 kQ resistor using an oscilloscope (72-8705A Tenma) and a 1:1 probe (30 W input power applied). A correction was applied to account for the stated input impedance of the probe. Since the output voltage is proportional to the square of the output power, the measured value at 30 W was scaled appropriately.

[0135] The field distribution inside the furnace was modeled using the AC / DC module of COMSOL Multiphysics. The small size of the furnace interior (total length 500 mm) compared to the free-space wavelength (22 m) allowed the field to be modeled on a quasi-DC basis. In this model, the form of the electric field is independent of the applied voltage. The reactor assembly dimensions and material properties were faithful to the real system. The electric field was modeled in the absence of the CNT aerogel, and the presence of the CNT aerogel was modeled as a cylinder with OD 28 mm (25 mm ID). Figure 11 The CNT aerogel seen in c was modeled as a cylinder with OD 28 mm (25 mm ID).

[0136] Continuous CNT alignment by a single RF electrode

[0137] The FCCVD reactor was equipped with a single RF graphite electrode oriented along the central axis of the tube. Conceptually, the electrically conductive CNT aerogel formed at the end of the reactor acts as a ground electrode (see Figure 11a). Grounding of the CNT aerogel was ensured by collecting it on a ground wire spool, which was grounded by a dedicated copper stake through the ground. The RF electrode was connected to the HV system and inserted into the reactor through a custom-made syringe flange. The RF electrode tip was fixed and positioned 95 mm upstream of the reactor midpoint. The power supply of the HV unit was set to 0, 200, 250, and 300 W. The reflected power during collection was minimal (<10 W). Each power configuration was run at least three times. After the end of the collection, the CNT material was hand-rolled perpendicular to the collection axis to produce a "cigar" thin wire on the circumference of the spool. The thin wire was cut at random points to produce ~160 mm long CNT fibrous material. All runs employed the same process parameters as described for CNT orientation by double electrode, and the collection speed was 30 revolutions per minute (linear speed of 0.157 m s -1 ).

[0138] CNT fibrous material characterization

[0139] Fibers were weighed using a microbalance (Sartorius SE2-F) and their length was measured to calculate the linear density (in g km -1 (tex) for each sample. The linear resistance of each sample was determined by measuring the resistance of a 100 mm section using a custom four-point probe jig connected to a milliohmmeter (Aim-TTi Bs407). The specific conductivity was calculated by normalizing the linear conductance (inverse of linear resistance) to the linear density of each sample. The specific conductivity values (Sm 2 kg -1 ) were averaged according to a set of at least three samples.

[0140] Fibers tenacity (ultimate tensile stress normalized by linear density) and breaking strain were determined using an Instron mechanical tester (5500R) equipped with a 10 N load cell. The initial gauge length was 20 mm, and the sample displacement rate was 1 mm min -1 . The sample pre-stress was fixed at 0.1 N. To prevent slippage, the ends of the CNT fiber samples were clamped between aluminum foils and glued before clamping to the grips. The fiber tenacity and breaking strain values were averaged according to a set of at least three samples.

[0141] Raman analysis was performed in a Horiba XploRA PLUS confocal microscope system using a 638 nm laser, a 50x objective, a 1200 grating, 25% laser power, and a 30-second triple integration. The spectra were presented with baseline correction applied. The G / D ratio was averaged according to a set of at least three replicates on three different samples.

[0142] 2D SAXS patterns of CNT materials were collected at the BL11-NCD-SWEET amorphous beamline at the ALBA synchrotron light facility (Barcelona, Spain), which is equipped with a Dectris (Pilatus 1M) photon counting and Rayonix LX255-HSCDD detector. A microfocus spot of ~10-μm diameter was used and the samples were collected at a radiation wavelength of The scattering of the samples was collected using a silver behenate (AgBh) calibration of the sample holder position before the collection of the patterns. The collected patterns were first corrected for background scattering and then analysed using the DAWN software (v.2.20) which obtains azimuthal angular distribution after radial integration in the Q range of 0.7 to 0.8 nm-1. The intensity was normalised by the scattering invariant Q obtained from the Kratky plot (q2• I(q) vs q).

[0143] For HRTEM imaging, a sample was prepared by ultrasonication of ~10 mg of CNT material in 200 ml of 1 -methyl-2-pyrrolidone (NMP 99% purity; Merck) in an ultrasonicator (Hielscher, UP400ST) for 60 minutes. 1 ml of the dispersion was pipetted onto a Formvar / Carbon TEM grid (Ted Pella) and left for 1 minute before being blotted dry. The residual NMP was dried by baking the grid in a vacuum oven at 70 °C overnight. Imaging was performed in high resolution mode using a monochromatic FEI Titan 80-300 TEM operated at 300 KV.

[0144] CNT orientation by a two-electrode setup

[0145] The FCCVD reactor was equipped with a 50 mm (OD) alumina tube (Almath Crucibles; see Figure 17two electrodes. A 6 mm graphite electrode (Beijing Great Wall Co) referred to as the RF electrode was connected to the HV system and inserted into the reactor through an injector flange. The injector flange allowed the RF electrode to be freely moved laterally while using two side ports to introduce ferrocene from one port and other precursors from the other. A 6 mm molybdenum electrode (Goodfellow) referred to as the ground electrode was inserted from the far end of the reactor. To facilitate orienting the electrodes and holding them in place, a grounded z-axis displacement stage (Optics Focus Instruments Co) was used. To maximize the electric field uniformity, both electrode tips were polished to produce hemispherical smooth ends. Experiments were run by discretely varying the inter-electrode gap (AL) between 200, 150, 130, and 50 mm. This was facilitated by changing the RF electrode tip position while the ground electrode end was fixed (140 mm downstream of the reactor midpoint). The power supply for the HV unit was set to 300 W (maximum output) except for AL = 50 mm, where 0 W (reference) and 180 W power settings were also used. Each setting was run at least twice. All experimental runs were short periods (<5 s) as the process was immediately turned off when the reflected power indicated on the HV power console reached its maximum capacity (100 W). In all runs (unless otherwise noted), the method was run as follows: the furnace was set to 1300 °C, and the precursors included hydrogen (1400 standard cubic centimeters per minute; sccm, BOC); methane (160 sccm, BOC); ferrocene (200 sccm of hydrogen through a canister heated to 110 °C, 98% purity, Merck); and thiophene (60 sccm of hydrogen through an ice slurry cooled reservoir at ~0 °C, >99% purity, Merck).

[0146] SEM imaging and image analysis

[0147] CNT samples were imaged using a MIRA3 field emission gun-SEM (Tescan). An In-Beam SE detector with a working distance of 3-5 mm was used to image at an acceleration voltage of 5 kV. The samples were not sputter coated. For directional quantification, images were acquired at 50 kX magnification using a 4096 X 3072 raster. In cases where the orientation was visually apparent, images were taken manually at an angle where most CNTs were parallel to the long axis of the rectangular frame. At these imaging parameters, the resolution was calculated to be 2.9-4.7 pixels / CNT bundle (based on the finding of a CNT bundle median diameter of 16-26 nm as shown in the results section), and as such the number of CNTs per frame should be higher than 500. The resolution and number of CNTs per frame met the requirements for successful image analysis. SEM image analysis was performed to acquire the orientation distribution function (ODF) and further extract the orientation order parameter (i.e. the second moment, which is the average of the Chebyshev polynomial T2). The analysis was performed by using the open-access Fibre COP program. The program parameters were set to 5 scans, a bin size of 0.25, and a filter interval of 5. The number of peaks was set to 3, while a Lorentzian fit was performed for each peak. The average T2 orientation parameter for each bi-polar setup was acquired based on the analysis of at least 3 SEM images (more than 1500 CNTs in total). SEM images for CNT bundle diameter analysis were taken using the same configuration as above but at a 200 kX magnification. The diameter of 200 CNT bundles was measured manually using Fiji, and the histogram was fitted by a log-normal distribution using OriginPro 2021.

[0148] Modeling of CNT orientation under the influence of RF fields

[0149] CNT orientation under alternating electric fields can be described using a worm-like chain model with energy contributions from bending, electric polarization, and additional electromagnetic interactions resulting from z-pinch hardening effects.

[0150] Current, pressure, and force

[0151] As a first approximation, any variation of the current amplitude in the CNT along its profile and with time is neglected. Thus, a constant current J in the CNT is assumed. In the derivation of the Lorentz pressure, a continuous CNT with a finite wall thickness is assumed. Using Ampere’s law, the magnetic field strength inside the CNT wall can be calculated. The axial current and the circumferential magnetic field are shown in Figure 14a The magnetic field and current in the CNT interact, resulting in a uniform compressive Lorentz force on the CNT wall. By integrating over the width of the CNT and taking the limit of vanishing wall thickness, the Lorentz pressure acting on the CNT wall is equal to:

[0152]

[0153] By further integrating over the surface along the points of the profile, from s parametrization, one can derive the following restoring line force density:

[0154]

[0155] A = πR 2 is the cross-sectional area of the CNT, and is the tangent vector along the CNT. Thus, the pressure generated by the current always acts on the curvature of the chain. In Figure 15 The pressure and restoring force of the 2D continuum model of the CNT are shown in b-c.

[0156] Energy contribution

[0157] Using the variational method, the energy contribution of the restoring force density due to z pinching hardening is calculated:

[0158]

[0159] This energy naturally explains that the two halves of the chain are pulled in the direction of their respective nearest end, while the midpoint of the chain is fixed in place.

[0160] The current and the resulting pressure need to be externally induced in the CNT. This can be done by applying an electric field E on the CNT. Assuming a simple model where the charge can only move tangentially within the CNT, the following energy contribution of the electric field itself has been proposed:

[0161]

[0162] where A is again the cross-sectional area of the CNT.

[0163] Combining the energy terms discussed above with the regular curvature term of the WLC, one obtains the full free energy functional of the model:

[0164]

[0165] where a simply denotes the bending stiffness of the CNT.

[0166] Harmonic approximation

[0167] For the purposes of the present invention, it is sufficient to assume that the CNT is already strongly aligned with the electric field. Without loss of generality, allow the electric field point to lie along the z-axis and to have magnitude e. Then, one can expand the tangent vector and its second-order derivatives in the x and y components of the tangent vector θ(s) to obtain the following harmonic approximation of the free energy, up to additive constant:

[0168]

[0169] This approximate model is the basis for the current results and can be solved exactly using methods from the theory of Gaussian statistical fields.

[0170] Results and discussion

[0171] Continuous CNT alignment using internal RF electrode

[0172] The field alignment adapted FCCVD rig uses a graphite electrode (RF electrode) connected to the HV unit and inserted through the reactor top. The conductive CNT aerogel (synthesized continuously in the reactor) is collected on a grounded spool to act as a grounded electrode (see Figure 11 a). To minimize artifacts related to the mechanical spooling speed, the linear speed of the spool was set to ~0.16 m s -1 , which is considered to be the inefficient speed of CNT alignment. Figure 11 b depicts the mechanism of the inter-electrode alignment process. The alignment process is based on the internal AC current, the z-pinch hardening effect, and the induced dipole back torque. From initial experiments it can be seen that after the injection of the process precursor, there is a whisker-like material growing radially outwards from the electrode surface (see Figure 19 a). These whiskers grow in a section upstream of the reactor's midpoint by 70-90 mm (equivalent to a temperature range of 1100-1200 °C). SEM analysis revealed that those whiskers are made of an isotropic network of sub-micron vapor grown carbon fibers (VGCF, see Figure 19 b). These whiskers grow without the application of HV, but in the presence of an electric field, an instantaneous surge of whisker growth is evident once the precursor is injected. Under the influence of the electric field, it is clear that the VGCF do not spontaneously self-assemble, but rather align themselves according to the electric field lines (see Figure 19 c). In this configuration, some individual VGCF show lengths of more than 100 pm. This preferential growth of the aligned VGCF whiskers can be explained by a finite element field distribution model for the internal furnace cavity (see Figure 11 c). This model indicates that a strong radial field (represented by the dense accumulation of blue equipotential lines) is located between the RF electrode and the alumina tube. The model also ensures that there is a well-defined field line between the RF electrode and the CNT aerogel "jacket", enabling the CNT to align accordingly.

[0173] Due to the fast radial growth of the VGCF whiskers on the RF electrode and the inevitable electrical contact with the ceramic tube, a low resistance between the RF electrode and ground is created within seconds. This detunes the HV setup and leads to a strong decrease of the voltage and field strength. Due to this, the RF electrode is retracted 100 mm upstream of the midpoint. At this position, the RF electrode is at least 10 mm further upstream of the position where the growth of VGCF whiskers was detected and thus avoids RF short circuits during continuous operation. In such a setup, it is visually evident that no radial growth of VGCF whiskers occurs during operation, but some axial whisker growth can be detected (see Figure 12 a). The CNT material created when applying HV appears to be more rigid than their reference, as single CNT sheaths can self-support without collapsing (see Figure 12 b). SEM imaging shows a CNT orientation pattern. However, it is not predominant. This is expected, as the interelectrode gap in this configuration is huge, which in turn reduces the applied field strength. However, as Figure 12 a and 20a show, it is assumed that due to the presence of the electric field, the whiskers self-assemble and grow laterally from the RF electrode tip side, thus artificially narrowing the interelectrode gap. In fact, some of those "extended" whiskers grow up to 150 mm long and are composed of well-oriented, extremely thin VGCF (see Figure 20 c).

[0174] Experiments with this setup were run with the RF power source set to 0 (reference), 200, 250 and 300 W (maximum power output). In this setup, the applied field strength cannot be evaluated, as the interelectrode gap is unknown. However, as V ∝ P 1 / 2 , the increment of the field strength should be proportional to the square root of the RF generator power. Electrical measurements on different samples revealed a significant increase of the specific conductivity of 75-90% compared to the reference sample (0 W), while the G / D ratio retrieved from the Raman spectra did not change significantly (see Figure 13 a). Furthermore, the Raman distribution profiles remained similar between all samples (see Figure 21 ). These findings indicate that the improvement of the electrical properties is not due to a shift in the synthesis process that creates less defective CNTs, but to a transformation of the microstructure that leads to smaller resistive CNT-CNT junctions. Further mechanical analysis revealed a unique transformation of the tensile behavior of the samples, as observed by their stress / strain curves (see Figure 13b). The CNT material produced by the original method (0W) shows a ductile behavior with a high breaking strain ratio and a fuzzy breaking point. In contrast, all CNT materials produced under AC fields have a more brittle-like behavior with a lower strain to break ratio and a clear breaking point. Interestingly, the in-situ oriented material exhibits a significant increase in the elastic modulus (δ) of up to 375% and a specific tensile breaking (UTS) of up to 358%. This significant shift in mechanical behavior is a strong evidence that the load-bearing microstructure of the CNT network has changed due to the CNT orientation.

[0175] To directly evaluate the degree of orientation, additional SAXS analysis was performed on the samples. Figure 14a Overlaid azimuthal scans of the reference (0W) and 300W samples normalized by the invariant (scattered power) are shown, accompanied by the associated 2D SAXS patterns. It can be clearly seen that while the reference sample does not show any orientation pattern (as it is intrinsically isotropic), the 300W sample shows a distinct Lorentzian-type profile associated with a more deeply oriented pattern. Additional analysis based on the raw data integration to calculate the Herman parameter (P2) reveals a trend between the applied voltage (which is related to the square root of the RF power) and the degree of orientation (see Figure 14b ). An apparent correlation between P2 and δ can also be seen, which supports the concept that the stiffness of the CNT network is mainly controlled by the internal orientation of its CNT bundles. Interestingly, in addition to the P2 values, the profile of the azimuthal scans (as seen in the inset) from the non-existent (0W), Gaussian-type (200, 250W) to Lorentzian-type (300W) supports the idea that the orientation is related to the field strength.

[0176] CNT orientation using RF fields - theoretical model

[0177] Z-pinch hardening

[0178] The CNTs are modeled as continuous shells with vanishing thickness. As an average field approximation, it is assumed that the current density within the CNT wall is constant along the entire CNT profile. The current in the CNT is limited by the scattering of the electrons by optical phonons. Modeling of the current carrying mode within the SWCNTs indicates that the current of the RF electric field should exceed the maximum saturation current of the CNT wall of J0~ 25 μΑ. Therefore, when assuming current saturation, it is assumed that the SWCNTs carry a saturation current J0when an RF AC field is applied. This is in contrast to a simple DC field where no current flows after the initial polarization of the CNT. Furthermore, experiments show that each CNT wall carries its own saturation current in SWCNT and MWCNT bundles. Therefore, the total current is proportional to the number of walls present in the CNT fiber.

[0179] The axial current in the CNT then induces a circumferential magnetic field within the CNT wall, asFigure 15 a. Ampere's law can be used to calculate the magnitude of the field. Due to the presence of the magnetic field, the axial current then experiences a Lorentz force. Effectively, this can be modeled as a pressure acting on the CNT wall. The name z-pinch refers to this "pinching" of the CNT around its vertical z-axis, and is derived from a similar effect used to compress plasmas sufficiently to undergo nuclear fusion. While the effect in CNTs is less dramatic, it can harden the CNT to facilitate alignment.

[0180] If a curved CNT segment is considered, it is clear that the side facing (away from) the center of curvature is compressed (stretched). Therefore, the Lorentz pressure acts on a larger surface area on the side facing away from the center of curvature, resulting in an effective restoring force. Since this force counteracts any curvature, the CNT is hardened by the z-pinch effect. A pictorial illustration of the pressure and restoring force is shown in Figure 15 b-c.

[0181] Model results

[0182] The main measure used to quantify alignment is the two-dimensional alignment order parameter T2, defined as follows:

[0183] T2= 2<cosθ 2D -1

[0184] where θ 2D represents the two-dimensional alignment angle of the CNTs with the electric field. This quantity can be easily measured in two-dimensional SEM images of the CNT material, thus allowing direct comparison of the theoretical model with experimental data. The average value of T2varies along the CNT, being lowest at the CNT ends and highest at the CNT midpoint.

[0185] As a conservative measure of CNT alignment, the minimum value T 2,min found at the CNT ends is chosen.

[0186] Rigid-elastic transition

[0187] Intuitively, CNT alignment improves with increasing electric field strength and CNT length up to a certain point. For DC, the behavior changes significantly (see Fig. 2,min a) at T Figure 16 a threshold length. Below the threshold length, the CNT can be considered rigid. Above the threshold length, elastic bending dominates the system, limiting the CNT's coupling with the electric field. For AC, the rigid state still exists, but for lower values of T 2,min and long CNTs, the behavior deviates from the elastic state and returns to the rigid state (see Fig. Figure 16b). This indicates that the z-pinch effect hardens the CNT (i.e. effectively makes them rigid). For essentially aligned SWCNTs, this effect only sets in on the millimeter length scale (see Figure 16 c), and is limited by the relatively low value of the saturation current. However, this result demonstrates that Z-pinch hardening can in principle favor alignment even for SWCNTs.

[0188] SWCNT bundles and MWCNTs

[0189] The strength of the Z-pinch hardening is limited by the current saturation in SWCNTs. However, the saturation current is proportional to the number of CNT walls in a SWCNT bundle or a single MWCNT. Therefore, in both cases, the Z-pinch hardening should be significantly more pronounced. Figure 16 d-e show the electric field strength E plotted against the CNT length L for different (10,10) SWCNT bundles and MWCNTs. For the curves, T 2,min = 0.5 was chosen to represent essentially aligned material. Both curves contain a single (10,10) SWCNT for reference, where the z-pinch hardening only becomes dominant on the millimeter length scale. As soon as about three CNT walls are present, either as a single SWCNT in a bundle or as a wall of a MWCNT (which is the most dominant nanostructure in aerogels) (see Figure 16 f), the z-pinch hardening is already significant at the rigid-elastic transition threshold length. Therefore, the z-pinch effect can effectively harden CNT structures containing more than three CNT walls, thereby favoring their electric field alignment. The required electric field strength for alignment then drops below the typical dielectric breakdown field strength of FCCVD process gases, making the alignment of single MWCNTs and small diameter SWCNT bundles technically feasible.

[0190] Dual electrode configuration

[0191] As a means of controlling and increasing the applied field strength and its lateral occurrence in the reactor volume, an additional two electrode setup was developed (see Figure 17 a). The same graphite RF electrode and molybdenum electrode (ground electrode) as in the original setup were inserted through the back. Both electrodes are oriented along the central axis of the reaction tube and can be moved freely laterally. This configuration allows for the individual setting of each electrode position and enables the adjustment of the inter-electrode position and gap width (Δχ and ΔL, respectively, as seen in Figure 17 a). Such means enable the control of the applied field strength (in combination with the input RF power) and the lateral position of the inter-electrode gap. Obviously, this system can achieve HV by creating a hydrogen breakdown between the two electrodes, which is a phenomenon that requires at least a few kV cm -1 order field strength for a hydrogen atmosphere ( Figure 17When the inter-electrode gap is positioned as a VGCF growth regime, a visual indication of good agreement with the field distribution model is evident. In this configuration, when the precursor is injected and the HV is turned on, whiskers are clearly observed to grow between the electrodes ( Figure 17 bii). The field lines are most prominently observed in the center and at the periphery tangential to the electrode tips.

[0192] Due to the rapid radial growth of VGCF whiskers on the RF electrodes and the unavoidable detuning of the HV setup, which leads to a large drop in voltage and field strength even within the short period of applied HV (~5 seconds), it is clear that this setup has a significant impact on the formation and orientation of the CNT aerogel. This can be easily seen from Figure 17 d, which compares the reference sample (top) revealing its isotropic nature with a sample at ∼0.75 kV cm that exhibits significant orientation. -1 Microscopic morphology of the material synthesized under the influence of an in situ electric field (bottom). Since the CNT bundles are well oriented, some individual CNT bundles can be tracked along the entire frame, making it possible to discern that some bundles are at least 50 μm long, and in other cases even more than 100 μm (see Figure 22 After running a large number of experimental setups with various ΔX and ΔL configurations, it became clear that CNT alignment was only achieved when the ground electrode was positioned at least 140 mm downstream from the furnace midpoint. This result is consistent with the knowledge that the majority of CNT aerogel is synthesized in the last third of the reactor.

[0193] Because the amount of material produced in the two-electrode setup is negligible, the degree of orientation as a function of the applied field strength can only be quantified by SEM image analysis. An open-access program (Fibre COP) specifically designed to quantify uniaxial orientation order based on 2D images is used to adapt to this need. For the orientation distribution derived from the 2D images, the software calculates the average of the second moments of the Chebyshev (as opposed to Legendre) polynomials based on the Lorentzian fiber orientation distribution. Therefore, the calculated orientation order parameter is referred to as T2 in this section, rather than the more common Herman parameter (P2), which is applicable to data obtained from, for example, 3D bulk samples (e.g. obtained from x-ray diffraction). It is also noted that on the same data set, the T2 values ​​based on the Lorentzian distribution always show lower values ​​than P2. Therefore, the current values ​​of T2 should not be directly compared with the Herman parameters published elsewhere, but rather used as an internal scale for orientation. As Figure 18a As shown, the reference sample (0 kV cm -1) is visually isotropic. However, it does exhibit a T2 of 0.19 (a perfect isotropic should result in a zero value). This can be related to some inherent orientation in the material due to the associated gas flow in the reactor. The system was set to 0.23 kV cm -1 -1, the applied field strength does not seem to change the basic isotropic nature of the CNT aerogel, with the orientation parameter not changing practically at 0.20. Only when the field strength is changed to 0.30-0.35 kV cm 1 -1, a clear CNT orientation pattern is revealed. Although a part of the CNT bundles does not follow a horizontal pattern, most do, thus the T2 value increases to 0.41-0.42. When the field strength is increased to 0.75-0.95 kV cm -1 -1, a very unique orientation pattern is noticed. Image analysis reveals that the orientation parameter jumps to 0.46-0.51, which is found to be very similar to the value calculated from SEM images taken from a commercial CNT fibre (5 tex; Tortech Nano Fibres Ltd.). Although the orientation parameter follows a non-linear trend, the increase of T2 from ~0.2 to ~0.5 corresponds to a decrease of the full width at half maximum (FWHM) from ~100 to ~43.5°, and should thus be considered significant. There seems to be an order of magnitude difference between the experimental applied field strength and the field strength at which T2 reaches ~0.5 Figure 16 d), but in the case of 1D nanomaterials such as CNTs, there is a dominant field enhancement effect. As a first order approximation, this enhancement factor is proportional to the aspect ratio of the 1D material, and in the current case, according to a higher order approximation, it should be > 500. Another change observed in the micro-morphology of the material is related to the CNT bundle diameter. As shown in Figure 18b , the higher the field strength employed in the inter-electrode gap, the thicker the CNT bundles become. For field strengths of 0.23, 0.35 and 0.75 kV cm -1 -1, the CNT median diameter is analysed to be 16.44, 18.87 and 25.40 nm, respectively. It is expected that the CNT bundles would become thinner due to the orientation forces, as there should be fewer collisions between adjacent CNTs. This counter-intuitive phenomenon can be explained by the presence of a compressive Lorentz pinch induced on the CNTs due to the AC field.

[0194] CONCLUSIONS

[0195] This novel approach takes advantage of an external electric field (e.g. up to ~1 kV cm -1The strength of the applied field has a significant effect on the self-assembly mechanism of CNTs in the gas phase, as embodied by a clear thickening of the CNT bundles from ~16 to ~25 nm. This system enables continuous in-situ manipulation of nanomaterials while simultaneously collecting the nanomaterials to form macroscopic textiles. This method has been shown to generate a unique alignment pattern compared to the isotropic nature of the original bulk material, as determined by SAXS. The microstructural reorganization is closely related to the transition in the mechanical behavior of the textile from ductile to brittle-like, increasing the elastic modulus by up to 375%. The higher portion of the load-bearing nanotubes resist the tensile load due to the alignment, increasing the failure stress by up to 358%. This also results in fewer resistive CNT-CNT junctions, and an associated electrical enhancement of up to 90%. Interestingly, the electric field does not affect the CNT synthesis, as no significant changes can be detected using Raman spectroscopy. A well-developed model recognizes the feasibility of MWCNT bundle alignment below the carrier gas breakdown threshold, and reveals the benefits of applying an AC field (rather than a DC field).

[0196] It is confidently believed that this novel use of external fields to manipulate and control the assembly process of CNT networks in the gas phase will unlock the full potential of high aspect ratio (~10 4 ) CNT-based textiles without sacrificing the cost-effectiveness of the underlying process.

Claims

1. A method for producing a carbon nanotube structure, comprising: (a) introducing a metal catalyst precursor into a continuous carrier gas flow in a temperature-controlled flow-through reactor; (b) exposing the metal catalyst precursor in the carrier gas stream to a first temperature zone sufficient to produce a particulate metal catalyst; (c) releasing a carbon source into the carrier gas stream; (d) exposing the particulate metal catalyst and the carbon source to a second temperature zone downstream from the first temperature zone, wherein the second temperature zone is sufficient to produce carbon nanotube aggregates; (e) generating an electric field in the temperature-controlled flow-through reactor at or near the second temperature zone; (f) discharging the carbon nanotube aggregates as a continuous discharge material through a discharge outlet of the temperature-controlled flow-through reactor; as well as (g) collecting the continuous discharge in the form of carbon nanotube structures, The electric field is oriented substantially parallel to the flow path of the carrier gas. 2 . The method of claim 1 , wherein the electric field is oriented substantially coaxially with the flow path of the carrier gas.

3. A method according to any one of the preceding claims, wherein the electric field is generated by an AC source.

4. The method according to any one of the preceding claims, wherein the electric field is 0.35 to 1.0 kV cm -1 Field strength generated within the range.

5. The process according to any one of the preceding claims, wherein the temperature-controlled flow-through reactor comprises: an elongated refractory shell extending from an upstream end to a downstream end, wherein the metal catalyst precursor is introduced into the elongated refractory shell in step (a) and the carbon source is released into the elongated refractory shell in step (c); a thermal enclosure surrounding the elongated refractory shell, the thermal enclosure adapted to provide axial temperature variation between temperature zones in the elongated refractory shell, wherein the temperature zones include the first temperature zone and the second temperature zone; as well as Electrodes are positioned inside or outside the elongated refractory housing. The method of claim 5 , wherein the electrodes are oriented substantially parallel to the flow path of the carrier gas.

7. A method according to claim 5 or 6, wherein the electrode is oriented substantially coaxially with the flow path of the carrier gas.

8. The method according to any one of the preceding claims, wherein the carbon nanotube aggregates are aerogels.

9. A temperature-controlled flow-through reactor for producing carbon nanotube structures, comprising: an elongated refractory shell extending from an upstream end to a downstream end; an inlet at or near the upstream end of the elongated refractory housing for introducing a continuous flow of carrier gas from the upstream end to and across the downstream end; a first feed device for releasing a carbon source into the continuous carrier gas flow; a second feed means for introducing a metal catalyst precursor into the continuous carrier gas flow; a thermal enclosure surrounding the elongated refractory shell, the thermal enclosure adapted to provide an axial temperature variation between temperature zones in the elongated refractory shell, wherein the temperature zones include a first temperature zone sufficient to generate a particulate metal catalyst and a second temperature zone sufficient to produce carbon nanotube aggregates; a collector for collecting the continuous discharge of the carbon nanotube aggregates in the form of carbon nanotube structures from the downstream end; a first electrode positioned inside or outside the elongated refractory housing; as well as an electric field generator electrically connected to the first electrode so as to apply a high electric potential thereto, the high electric potential being sufficient to generate an electric field at or near the second temperature zone in the elongated refractory housing; wherein the electric field is substantially coaxial with the elongated refractory housing.

10. The temperature-controlled flow-through reactor of claim 9, further comprising a second electrode.

11. The temperature-controlled flow-through reactor according to any one of claims 9 to 10, wherein the collector is electrically grounded.

12. The temperature-controlled flow-through reactor of any one of claims 9 to 10, wherein the first electrode is positioned inside the elongated refractory shell at or near the second temperature zone, and the collector is electrically grounded.

13. The temperature-controlled flow-through reactor of any one of claims 9 to 11, further comprising a second electrode outside the elongated refractory shell, wherein the first electrode is positioned inside the elongated refractory shell at or near the second temperature zone.

14. The temperature-controlled flow-through reactor of claim 13, wherein the second electrode is electrically connected to the thermal housing, and the thermal housing is grounded.

15. The temperature-controlled flow-through reactor of any one of claims 9 to 11, wherein the first electrode is positioned outside the elongated refractory housing adjacent the second temperature zone.

16. The temperature-controlled flow-through reactor of any one of claims 9 to 11, further comprising a second electrode positioned outside the elongated refractory shell, wherein the first electrode is positioned outside the elongated refractory shell and the second electrode is electrically grounded.

17. The temperature-controlled flow-through reactor of any one of claims 9 to 16, wherein the electric field generator is an AC source.

18. The temperature-controlled flow-through reactor of claim 17, wherein the electric field generator is operable at high radio frequency (HF).

Images