method

Abstract

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

Description

[Technical Field]

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

[0002] There is a growing demand for lightweight materials with high strength and stiffness, especially when combined with high electrical and thermal conductivity. While products formed from CNT aggregates have been manufactured, their properties fall short of their theoretical potential. The primary reason for this shortfall is the imperfect alignment of molecular-scale CNTs within the macroscale fibers produced by the aggregation.

[0003] CNTs are molecular-scale structures comprising sheets of carbon atoms bound by covalent bonds to form closed tubes. CNT walls can be single-walled (single-walled CNTs (SWCNTs)) or multi-walled (multi-walled CNTs (MWCNTs)). Individual CNTs typically have diameters between 0.4 nm and 40 nm, and lengths typically greater than 100 times their diameters.

[0004] Forming CNTs into structures such as fibers or mats for macroscale applications requires the creation of large numbers of CNT aggregates. When CNTs are brought into close proximity, they are attracted to each other by van der Waals forces and other atomic and molecular interactions. A method for forming multiple long, thin strands, each containing a very large number of CNTs, is known as floating catalyst chemical vapor deposition (FCCVD). In this method, a carbon-rich feedstock (e.g., methane or acetylene) is introduced into a ceramic tube along with iron- and sulfur-containing catalyst precursors (such as ferrocene and thiophene, respectively) and heated to very high temperatures (typically above 1000°C). Following decomposition, the carbon atoms provided by the precursor form an aerogel, which can be extracted from the ceramic tube to form fibers or mats. The FCCVD and equipment configuration are disclosed in EP 3227231. In practice, the CNTs that form the aerogel are found to be poorly aligned. This results in fibers with mechanical, electrical, and thermal properties that are far lower than those obtained from aligned CNT bundles.

[0005] The essential elements of a conventional FCCVD temperature-controlled flow-through reactor are shown schematically in Figure 1. An electrically insulating refractory tube 1 is positioned axially within and surrounded by a furnace comprising a metallic outer case 2, thermal insulation 3, and an elongated electric heating element 4. After heating the furnace to a typical temperature of 1300 °C, feedstocks such as methane and catalyst precursors such as ferrocene and thiophene are fed into the input end 5 of the tube 1 along with a carrier gas such as hydrogen. The key function of the carrier gas is to exclude oxygen from the interior of the tube 1, which would otherwise cause combustion of the forming CNTs. A catalytic reaction occurs at high temperature, resulting in the formation of a network of fibers, each comprising a bundle of CNTs in the form of an aerogel sock 6. The aerogel sock 6 is drawn from the output end 7 of the tube 1, where it can be stretched into a single fiber by winding it on a reel 8. Post-treatment (for example, by twisting or treatment with acid) can help enhance the mechanical properties of the resulting fiber. Examples of such processes are described in Lee et al., "Direct spinning and densification method for high-performance carbon nanotube fibers," Nature Communications, Vol. 10, Article 2962 (2019), and J. Bulmer et al., "Extreme stretching of high G:D ratio carbon nanotube fibers using super-acid," Carbon, 153, 725-736. An advantage of FCCVD temperature-controlled flow-through reactors is that they 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 discharged from the output end.

[0006] Although methods for producing well-aligned, short CNT fibers are known, they are not suitable for the continuous production of long fibers. While the use of electric fields has been reported (e.g., in Chinese Patent Application Publication No. 101254914), these methods are often applied to very small CNTs. Typical conventional configurations include nanoscale channels or closely spaced plates, each of which is subjected to a potential difference of several volts. 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 interdigitated electrode configuration with electrodes spaced approximately 25 μm apart and an alternating voltage of 10 V peak-to-peak applied. Most researchers have described configurations in which an electric field is applied to CNTs suspended in a liquid medium. While such configurations can align CNTs, CNT movement in liquids is slow, and the strength of the electric field 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), pp. 125-128). A method for aligning CNT bundles that grow perpendicular to the surface on which they are formed is described by C. Bower et al., "Plasma-induced alignment of carbon nanotubes," Applied Physics Letters, Vol. 77, No. 6, August 2000. Bower reported that nanotubes can be grown on uneven surfaces in the presence of microwave plasma-generated electric fields and can be consistently oriented perpendicular to the local substrate surface. This growth resembles a lawn with many closely packed fibers about 50 μm long growing perpendicular to the surface on which they are formed.MTCole and WLMilne, "Plasma Enhanced Chemical Vapor Deposition of Horizontally Aligned Carbon Nanotubes," Materials, 2013, Vol. 6, pp. 2262-2273, also describes a configuration for producing short, aligned CNTs using plasma, and indicates that a field strength on the order of 0.1-0.5 μV / m is required.

[0007] The growth of short, aligned CNTs between a pair of plates to which an electric potential is applied 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, pp. 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). A related configuration is described in W. Merchan-Merchan et al., "Ombustion synthesis of carbon nanotubes and related nanostructures," Progress in Energy and Combustion Science, Vol. 36 (2010) pp. 696-727.

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

[0009] A method for producing longer aggregates of CNTs was described by L.R. Bornhoeft et al. ("Teslaphoresis of Carbon Nanotubes," ACS Nano 2016, 10, 4873-4881, American Chemical Society). The method involves the "explosive self-assembly" of powdered CNTs in air and the slow alignment of a liquid CNT suspension.

[0010] US Patent Application Publication No. 2012 / 0282453 discloses a continuous method for producing ribbons of aligned CNTs by applying a polymer spray to form a composite.

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

[0012] None of the prior art methods are suitable for producing continuously oriented macroscale fibers for engineering applications. Summary of the Invention

[0013] The present invention relates to a method and temperature-controlled flow-through reactor that can continuously produce CNT structures (e.g., fibers) with improved orientation of the constituent CNTs, which contributes to improved mechanical, electrical, or thermal properties. In particular, the present invention relates to a floating catalyst (CVD) method that directly interacts with the self-assembly of CNT bundles in the gas phase.

[0014] Viewed from a first aspect, the present invention therefore provides a method for producing a medicament for the treatment of a malaria parathyroidectomy, comprising: 1. A method for producing a carbon nanotube structure, comprising: (a) introducing a metal catalyst precursor into a continuous flow of a carrier gas in a temperature-controlled flow-through reactor; (b) exposing the metal catalyst precursor in a stream of carrier gas to a first temperature zone sufficient to produce a particulate metal catalyst; (c) releasing the carbon source into a stream of carrier gas; (d) exposing the particulate metal catalyst and the carbon source to a second temperature zone downstream of the first temperature zone, the second temperature zone being sufficient to produce carbon nanotube aggregates; (e) generating an electric field within the temperature-controlled flow-through reactor at or near the second temperature zone; (f) discharging the carbon nanotube aggregates as a continuous discharge from an outlet of a temperature-controlled flow-through reactor; (g) collecting the continuous emission in the form of a carbon nanotube structure; The present invention provides a method for producing a carbon nanotube structure, comprising:

[0015] Typically, the continuous flow of carrier gas follows a substantially linear flow path.

[0016] Preferably, the electric field is oriented substantially parallel to the flow path of the carrier gas, and particularly preferably, the electric field is oriented substantially coaxially with the flow path of the carrier gas.

[0017] Preferably, the temperature controlled flow-through reactor comprises: an elongated refractory housing extending from an upstream end to a downstream end into which a metal catalyst precursor is introduced in step (a) and into which a carbon source is discharged in step (c); a thermal enclosure surrounding the elongated, fire-resistant housing, the thermal enclosure adapted to provide axial temperature variation between temperature zones within the elongated, fire-resistant housing, the temperature zones including a first temperature zone and a second temperature zone; an electrode located inside or outside the elongated refractory housing; Equipped with.

[0018] The electrode may be located partially inside the elongated refractory housing, for example, the electrode may extend upstream from the upstream end.

[0019] Preferably, the electrodes are oriented substantially parallel to the flow path of the carrier gas, and particularly preferably, the electrodes are oriented substantially coaxially with the flow path of the carrier gas.

[0020] The electric field may be generated by an electric field generator having a first terminal electrically connected to ground (eg, a metal case) and a second terminal electrically connected to an electrode.

[0021] 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 through a probe or injector. The metal catalyst precursor can be introduced at multiple locations.

[0022] The metal catalyst precursor may be suspended as solid particles (preferably solid nanoparticles) in a carrier gas.

[0023] The metal catalyst precursor can be a compound of at least one metal from 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.

[0024] The metal catalyst precursor can be a metal complex or an organometallic compound.

[0025] Preferably, the metal catalyst precursor is sulfur-containing.

[0026] The metal catalyst precursor may be introduced in step (a) together with a sulfur-containing additive, which may be thiophene, iron sulfide, a sulfur-containing ferrocenyl derivative (e.g., ferrocenyl sulfide), hydrogen sulfide, or carbon disulfide.

[0027] Typically, the particulate metal catalyst is a nanoparticle metal catalyst. Preferably, the nanoparticles of the nanoparticle metal catalyst have an average diameter (e.g., number, volume, or surface average diameter) in the range of 1 to 50 nm (preferably 1 to 10 nm). Preferably, 80% or more of the particles of the nanoparticle metal catalyst have a diameter of less than 30 nm. Particularly preferably, 80% or more of the particles of the nanoparticle metal catalyst have a diameter of less than 12 nm. The concentration of the particulate metal catalyst is 10 6 ~10 10 particle cm -3 The range may be:

[0028] 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 via a probe or injector. The carbon source can be introduced at multiple locations.

[0029] The carbon source may be an optionally substituted and / or optionally hydroxylated aromatic or aliphatic, acyclic or cyclic hydrocarbon (e.g., alkyne, alkane, or alkene), which may optionally be interrupted by one or more heteroatoms (e.g., oxygen). Preferably, an optionally halogenated C 1-6 hydrocarbons (e.g. methane, propane, ethylene, acetylene or tetrachloroethylene), optionally mono-, di- or trisubstituted benzene derivatives (e.g. toluene), C 1-6 alcohols (for example ethanol or butanol) or aromatic hydrocarbons (for example benzene or toluene).

[0030] The production of the particulate metal catalyst in step (b) may be initiated by thermal decomposition or dissociation of the metal catalyst precursor into metal species (e.g., atoms, radicals, or ions). The production of the particulate metal catalyst in step (b) may include nucleation of the metal species into nucleated metal species (e.g., clusters). The production of the particulate metal catalyst may include growth of the nucleated metal species into the particulate metal catalyst.

[0031] In a preferred embodiment, the carrier gas contains dispersed substrate particles, typically finely divided. The substrate particles serve to promote nucleation in the first temperature zone by forming a substrate-supported granular metal catalyst dispersed in the carrier gas. The substrate particles can be Si or SiO2 particles.

[0032] Preferably, the method further comprises introducing the substrate particles into a continuous flow of carrier gas.

[0033] In a preferred embodiment, steps (a) and (c) are simultaneous.

[0034] The first and second temperature zones may range from at least 600 to 1300°C.

[0035] The carrier gas is typically one or more of nitrogen, argon, helium, or hydrogen. The flow rate of the carrier gas can range from 1,000 to 50,000 sccm (e.g., 30,000 sccm).

[0036] The carbon aggregates can comprise multi-walled carbon nanotubes (eg, double-walled carbon nanotubes) and / or single-walled carbon nanotubes.

[0037] The carbon aggregates may take the form of a 3D continuous network (eg, an aerogel).

[0038] Preferably, the carbon aggregate is an aerogel.

[0039] The carbon nanotube structure can be a powder, fiber, wire, film, ribbon, strand, sheet, plate, mesh, or mat.

[0040] The carbon nanotube aggregates or carbon nanotube structures may comprise carbon nanotube bundles (ie, arrays of substantially parallel CNTs (typically 3-20 CNTs) that are attracted to each other by van der Waals forces).

[0041] The carbon nanotube aggregates or carbon nanotube structures may comprise carbon nanotube bundles having a median diameter (e.g., as measured by SEM and manual image analysis) of 16 nm or more, preferably 20 nm or more, particularly preferably 25 nm or more, more preferably more than 50 nm, and even more preferably 75 nm or more. Preferably, the diameters of the carbon nanotube bundles follow a log-normal distribution.

[0042] The carbon nanotube aggregates or carbon nanotube structures may comprise carbon nanotube bundles having a median diameter (e.g., as measured by SEM and manual image analysis) that is variable axially (i.e., along its length). Preferably, the diameters of the carbon nanotube bundles vary axially from a normal to a log-normal distribution.

[0043] Viewed from a further aspect, the present invention provides a temperature controlled flow-through reactor for producing carbon nanotube structures, comprising: an elongated refractory housing 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 the downstream end and beyond; a first feed for discharging a carbon source into a continuous flow of a carrier gas; a second feed for introducing a metal catalyst precursor into the continuous flow of carrier gas; a thermal enclosure surrounding the elongated refractory housing adapted to provide axial temperature variation between temperature zones within the elongated refractory housing, the temperature zones including a first temperature zone sufficient to produce a particulate metal catalyst and a second temperature zone sufficient to produce carbon nanotube aggregates; a collector for collecting the continuous release of carbon nanotube aggregates in the form of carbon nanotube structures from the downstream end; a first electrode located inside or outside the elongated refractory housing; an electric field generator electrically connected between ground and the first electrode to apply a high potential sufficient to generate an electric field inside the elongated refractory housing at or near the second temperature zone; A temperature-controlled flow-through reactor for producing carbon nanotube structures is provided, comprising:

[0044] The temperature-controlled flow-through reactor may further include a second electrode. The electric field generator may be electrically connected to the second electrode to apply a high or low potential. Preferably, the second electrode is electrically connected to ground.

[0045] The temperature-controlled flow-through reactor may further include a third electrode. The electric field generator may be electrically connected to the third electrode to apply a high or low potential. Preferably, the third electrode is electrically connected to ground. The third electrode may be used to control the shape, strength, and position of the electric field.

[0046] The temperature-controlled flow-through reactor may further comprise a plurality of additional electrodes located outside the elongated refractory housing, the plurality of additional electrodes being alternately connected to a high potential electric field generator and to ground.

[0047] The (or each) electrode may be an elongated electrode (e.g., an elongated solid or elongated hollow electrode). The (or each) electrode may be substantially cubic, cylindrical, or annular. Typically, the (or each) electrode is substantially coaxial with the elongated refractory housing.

[0048] The first electrode may be located at least partially inside the elongated refractory housing (e.g., at or near the upstream end of the elongated refractory housing). The first electrode may be located at or near (e.g., adjacent to) the second temperature zone. The first electrode may be located upstream of the second temperature zone.

[0049] The (or each) electrode is typically formed from an electrically conductive material capable of withstanding the temperature and chemical environment inside the refractory tube. Suitable materials include molybdenum or vitreous carbon. The (or each) electrode may include an inert sleeve (e.g., an alumina sleeve). The sleeve may leave only the downstream tip of the electrode exposed.

[0050] Preferably, the electric field is substantially coaxial with the elongated refractory housing.

[0051] Preferably, the collector is electrically connected to earth ground, and by effective connection to the collector in use the carbon nanotube aggregates are grounded.

[0052] In a first preferred embodiment, the first electrode is located inside the elongated refractory housing at or adjacent to the second temperature zone, and the collector is electrically connected to ground. Preferably, a grounded portion (e.g., a terminal) of the electric field generator is electrically connected to the collector.

[0053] In a second preferred embodiment, the temperature-controlled flow-through reactor further comprises a second electrode on the exterior of the elongated refractory housing, and the first electrode is located at or adjacent to a second temperature zone inside the elongated refractory housing. Particularly preferably, a grounded portion (e.g., a terminal) of the electric field generator is electrically connected to the collector.

[0054] In a second preferred embodiment, the second electrode may be electrically connected to the thermal enclosure, and the thermal enclosure may be grounded. This serves to ground the second electrode. For example, the second electrode may be electrically connected to the metal case of the thermal enclosure.

[0055] In a third preferred embodiment, the first electrode is located outside the elongated refractory housing 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.

[0056] In a fourth preferred embodiment, the temperature-controlled flow-through reactor further comprises a second electrode located outside the elongated refractory housing, wherein the first electrode is located outside the elongated refractory housing and the second electrode is electrically connected to ground.

[0057] In a fifth preferred embodiment, the temperature-controlled flow-through reactor further comprises a second electrode located inside the elongated refractory housing, wherein the first electrode is located inside the elongated refractory housing and the second electrode is electrically connected to ground.

[0058] The first electrode may be located adjacent to the second temperature zone, and a tip of the first electrode may be located upstream of a midpoint of the elongated refractory housing.

[0059] The second electrode may be located adjacent to the second temperature zone, and the tip of the second electrode may be located downstream of the midpoint of the elongated refractory housing.

[0060] Preferably, the electric field generator applies an AC potential (eg, in the range of 500V to 5000V peak to peak).

[0061] The electric field generator is preferably an AC power source. The AC field advantageously serves to continuously orient the CNTs in situ before they form dense networks and aerogels. Specifically, the AC field generates a Lorentz force-induced CNT stiffening effect (z-pinch). For example, we observed that the diameter of CNT bundles expanded from 16 to 25 nm, resulting in dramatic increases in electrical and tensile properties (up to 90% and 380%, respectively) without altering the fundamental properties of the nanobuilding block components (verified by Raman spectroscopy). The enhanced properties correlated with the degree of CNT orientation within the textile, quantified by small-angle X-ray scattering and innovative SEM image analysis. The AC field was 0.5 to 1 kV cm. -1 A clear orientation (T2 = 0.5) was achieved relative to the original material (T2 = 0.2) for applied field strengths in the range of

[0062] Preferably, the electric field generator has a voltage of 0.1 to 2.0 kVcm -1 , and particularly preferably 0.5 to 1.0 kVcm -1 , more preferably 0.35 to 0.75 kVcm -1 An AC potential is applied with a field strength in the range of

[0063] Preferably, the electric field generator is operable at radio frequency (RF), and particularly preferably at high radio frequency (HF) (for example at a frequency in the range of 10-20 MHz).

[0064] Preferably, the temperature-controlled flow-through reactor further comprises a third feed for introducing substrate particles into the continuous flow of carrier gas.

[0065] The first, second, and third feeds can be injection nozzles, lances, probes, or multi-orifice injectors (eg, showerhead injectors).

[0066] The elongated refractory housing may be substantially cylindrical (eg, tubular).

[0067] The thermal enclosure typically contains thermal insulation and can be a grounded metal case.

[0068] 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.

[0069] The temperature-controlled flow-through reactor can be substantially vertical or horizontal.

[0070] The collector is typically electrically conductive (e.g., metallic). The collector can be a rotating spindle, a reel, or a drum.

[0071] The methods and reactors of the present invention facilitate control of the size and distribution of CNT bundles (i.e., arrays of substantially parallel CNTs (typically 3-20 CNTs) attracted to each other by van der Waals forces) by (for example) adjusting the electric field strength.

[0072] Viewed from yet another aspect, the present invention provides carbon nanotube aggregates or carbon nanotube structures comprising carbon nanotube bundles having a median diameter (e.g., as measured by SEM and manual image analysis) of 16 nm or more, preferably 20 nm or more, particularly preferably 25 nm or more, more preferably more than 50 nm, and even more preferably 75 nm or more.

[0073] Preferably, the diameters of the carbon nanotube bundles follow a log-normal distribution.

[0074] Viewed from yet another aspect, the present invention provides carbon nanotube aggregates or carbon nanotube structures comprising carbon nanotube bundles having a median diameter (e.g., as measured by SEM and manual image analysis) that is variable axially along the carbon nanotube aggregate or carbon nanotube structure.

[0075] Preferably, the diameters of the carbon nanotube bundles vary axially from a normal distribution to a log-normal distribution.

[0076] The invention will now be described in a non-limiting sense with reference to the accompanying drawings, in which: [Brief explanation of the drawings]

[0077] [Figure 1] 1 is a simplified diagram of a conventional FCCVD furnace for producing carbon nanotubes in the form of aerogel. [Figure 2] FIG. 1 shows a first embodiment of a temperature-controlled flow-through reactor of the present invention having a first electrode contained within a refractory tube. [Figure 3]FIG. 10 shows the results of a computer simulation of the electric field generated in the first embodiment when a potential difference is applied between a first electrode and a second electrode formed by aerogel. [Figure 4] FIG. 10 shows the results of a computer simulation of the electric field generated by the second embodiment of the temperature-controlled flow-through reactor of the present invention when a potential difference is applied between a first electrode and a second electrode formed by an aerogel within a refractory tube surrounded by a third hollow cylindrical electrode. [Figure 5] FIG. 10 shows the results of a computer simulation of the electric field generated by 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 external to the refractory tube and a second electrode formed by an aerogel. [Figure 6] FIG. 10 shows the results of a computer simulation of the electric field generated by a fourth embodiment of the temperature-controlled flow-through reactor of the present invention when a potential difference is applied between two hollow cylindrical electrodes external to the refractory tube. [Figure 7] 1 is an exemplary embodiment of a circuit configured to resonate at a selected operating frequency. [Figure 8] 8a and 8b are SEM images of CNT aggregates formed by applying an RF field. [Figure 9] FIG. 1 is a perspective view of an embodiment of a temperature-controlled flow-through reactor of the present invention having external electrodes. [Figure 10] FIG. 1 is a cross-sectional view of an embodiment of a temperature-controlled flow-through reactor of the present invention having an external electrode. [Figure 11]The AC electric field orientation system is shown. (a) An FCCVD reactor with an RF electrode inserted in front, while the forming CNT aerogel is collected on a grounded bobbin, which acts as a counter electrode. The CNTs are oriented along the resulting field lines before forming the aerogel. (b) A close-up view showing what happens in the inter-electrode gap. (i) The AC electric field induces a "Lorentz pinch" that stiffens the ultralong CNTs. (ii) The stiffened ultralong CNTs are under the influence of an electric field-induced orientation torque. (iii) The CNTs are oriented along the field lines. The schematic is not to scale; steps (i) to (iii) occur simultaneously. (c) FEM numerical results of the electric field distribution inside the reactor tube, showing equipotential lines (blue) and orthogonal field lines (red). The CNT aerogel ("Sock") was approximated as a cylinder with an outer diameter of 28 mm (inner diameter of 25 mm). The packing density of the equipotential lines indicates the local electric field strength. The model shows the presence of orientation-induced field lines bridging the two electrodes within the interelectrode gap (50 mm wide). [Figure 12] Continuous CNT alignment using an internal RF electrode is shown. (a) Image looking upstream from the end of the reactor. The image shows CNT aerogel collecting on a rotating bobbin while an AC field is applied. Extended whiskers "grow" from the end of the graphite RF electrode toward the forming aerogel. (b) SEM image showing the micromorphology of the final CNT product. CNT alignment is evident, but appears suboptimal. The inset shows a single 15 cm long CNT sock produced during AC alignment. The sock appears to be stiffer than normal and is able to support its own weight. [Figure 13] Physical properties of CNT-aligned materials are shown. (a) Plot showing the specific conductivity (black, left axis) and G / D ratio (red, right axis) of CNT materials collected under different applied AC electric field strengths and a reference material (0 W). The G / D ratio does not change significantly, while the specific conductivity increases by up to 90%. Error bars indicate the standard deviation using at least three different samples. (b) Stress-strain curves from tensile measurements showing a clear change in mechanical performance from ductile (0 W) to more brittle behavior for the aligned samples. The mechanical shift in properties correlates well with the applied electric field strength (∝P 1 / 2). [Figure 14]WAXS analysis of the CNT material demonstrates its orientation. (a) Intensity-normalized azimuthal scans of the 0W (reference) and 300W samples over the Q range of 0.7–0.8 nm. The azimuthal angle φ = 0° corresponds to the x-axis (equator) and is perpendicular to the fiber axis. The inset shows the corresponding 2D SAXS patterns. The reference material exhibits no obvious scattering pattern, confirming the anisotropy of the textile. The 300W sample exhibits a characteristic Lorentzian intensity distribution, confirming the presence of CNT orientation. (b) Plot of the Harman's parameter (P2) calculated from the azimuthal scan (inset) as a function of the sample's elastic modulus. [Figure 15] The z-pinch mechanism. (a) Diagram of the electromagnetic field in a CNT related to the z-pinch hardening effect. Axial current (orange) is confined to the CNT wall and induces a circumferential magnetic field (blue). (b) Cross-sectional free-body diagram of a continuum CNT model of the z-pinch. Internal forces on both sides along the contour are shown in red. The pressure acting on the CNT wall (b) and the equivalent restoring force (c) are shown in blue. [Figure 16] Modeling of CNT field orientation is shown. (a-b) Surface plots of T2_min versus CNT length (log) and field strength (log) for DC (a) and AC (b) electric fields. Contours at different values ​​of T2_min are depicted in red, black, and blue. The white dashed line indicates the rigid-elastic transition for DC electric fields. (c) Log-log plots of field strength versus CNT length for contours obtained from (a) (dashed lines) and (b) (solid lines). The orange dashed line indicates the rigid-elastic transition. (d-e) Log-log plots of the field strength required to reach T2_min = 0.5 versus CNT length for different (10,10) SWCNT bundles (d) and MWCNTs with different armchair walls (e). (f) TEM image of the reference sample shows the widespread presence of several-walled MWCNTs with 3-5 walls (red lines). [Figure 17]The twin-electrode configuration is shown. (a) An RF electrode (graphite, 6 mm) is inserted at the front and a ground electrode (Mo, 6 mm) is threaded from the back. Both electrodes are free to run along their central axes, allowing for control of the depth (ΔX) and width (ΔL) of the inter-electrode gap. CNTs align along the resulting field lines. (b) Photograph of the inter-electrode gap. (i) Hydrogen decomposition is observed due to the high electric field strength (on the order of at least several kV cm-1). (ii) Vapor-grown carbon fiber (VGCF) whiskers grow in the inter-electrode gap, following the bridging field lines between the electrodes. (c) FEM numerical results of the electric field distribution inside the furnace cavity, showing equipotential lines (blue) and orthogonal field lines (red). The packing density of the equipotential lines indicates the local strength of the electric field. The model shows the orientation-induced electric field at a 50 mm inter-electrode gap similar to that revealed in (b)(ii). (d) Low-magnification SEM image showing the isotropic nature of the CNT network, comparing the reference material (no voltage, top) with the highly aligned CNT micromorphology seen in the material produced under the influence of an applied electric field strength of approximately 0.75 kV cm-1 (bottom). [Figure 18] Image analysis of CNT materials is shown. (a) Plot comparing the alignment depicted by the Chebyshev orientational order parameter (T2) calculated by the FiberCOP software (accompanied by a typical SEM image) with the applied electric field strength generated across the interelectrode gap. While electric field strengths below 0.23 kV cm-1 did not appear to affect the alignment, alignment significantly increased when electric field strengths reached 0.3 kV cm-1 or higher. The variance in Y values ​​is based on the standard deviation of T2 values ​​calculated from at least three images of two different samples. The variation in X values ​​is based on the voltage generated at the two extreme set points of the system. (b) Bundle diameter distribution (log-normal fitting) shows that the median bundle thickness of material generated at electric field strengths of 0.23, 0.35, and 0.75 kV cm-1 changed from 16.44 ± 0.10 to 18.87 ± 0.87 and 25.40 ± 0.46 nm, respectively. For each sample, the diameters of 200 bundles were manually measured. [Figure 19]Formation of VGCF in an FCCVD reactor. (a) VGCF whiskers grow radially from the RF electrode surface toward the reactor wall, causing a short circuit. (b) SEM image of the whiskers revealing an isotropic network of VGCF. The inset shows a single VGCF at higher magnification. (c) SEM image of whiskers produced under the influence of HV shows more orientation in the VGCF network. The inset shows that application of HV during whisker synthesis produces much finer, "dendrite"-like whiskers. [Figure 20] Growth of VGCF "extended" whiskers in an FCCVD reactor. (a) VGCF whiskers grow axially from downstream of the RF electrode, creating an extension to the RF electrode. (b) Some of the "extended" whiskers grew to a length of 150 mm. (c) SEM image of VGCF "extended" whiskers, showing that they are made of long, aligned VGCF. The inset shows a higher magnification image revealing that the VGCF has a CNT core (arrow) and is very thin (approximately 100 nm in diameter). [Figure 21] Raman spectra of various CNT samples produced with the internal RF electrode setup. There is no significant difference between the spectrum of the reference sample and the other spectra of materials produced under the influence of an electric field. [Figure 22] SEM image of CNT material produced at an electric field strength of 0.75 kV cm-1. Arrows trace the trajectories of ultralong CNT bundles with lengths exceeding 100 μm. DETAILED DESCRIPTION OF THE INVENTION

[0078] Figure 2 shows a first embodiment of the temperature-controlled flow-through reactor of the present invention. An elongated first electrode 9 is attached to the input end 5 of an electrically insulated refractory tube 1, which is positioned axially within and surrounded by the furnace. The furnace includes a metal outer case 2, thermal insulation 3, and an elongated electric heating element 4, which are grounded by a connector 16. The first electrode 9 is formed from a conductive material, such as molybdenum, that can withstand the temperature and chemical environment inside the refractory tube 1. The second electrode is formed by the rear end of a conductive aerogel sock 6, which is produced during processing and ejected from the output end 7 of the refractory tube 1 onto a conductive reel 8. A first conductor 10 connects the first electrode 9 to a live terminal of a high-voltage source 13, and a second conductor 11 connects the conductive reel 8 to a terminal of the high-voltage source 13, which has a connection 14 to ground. The high-voltage source 13 supplies radio-frequency voltage. The effect of establishing a high potential difference (voltage) between the front end of the first electrode 9 and the rear end of the aerogel sock 6 creates a substantially axial electric field in the region indicated by the dotted outline 15 that is substantially aligned with the axis of the refractory tube 1.

[0079] Figure 3 shows the results of a computer simulation of the electric field lines 13 and equipotential lines 14 generated in the first embodiment between the first electrode 9 and the second electrode formed by the rear end of the aerogel sock 6. It was found that effective alignment of the CNTs was observed by applying an AC voltage at a frequency of 13.64 MHz, an internationally allocated frequency for industrial and scientific use. The applied potential source is configured so that the applied potential is sufficient to provide the maximum degree of CNT alignment while avoiding arcing or corona discharge.

[0080] 4 shows a simulated electric field generated by a second embodiment of the temperature-controlled flow-through reactor of the present invention between a first electrode 9, a second electrode formed from aerogel 6 (as described for the first embodiment), and an elongated, hollow, cylindrical third electrode 20 external to the refractory tube 1. The third electrode 20 is maintained at ground potential by a conductive connection to the furnace's metallic (grounded) outer casing 2. The second embodiment generates a more uniform axial electric field in the region between the first electrode 9 and the second electrode formed from aerogel 6 than the first embodiment.

[0081] 5 shows a simulated electric field generated by a third embodiment of the temperature-controlled flow-through reactor of the present invention between a first electrode 21 in 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 from aerogel 6, is grounded by a conductive reel 8 (as described for the first embodiment).

[0082] 6 shows a simulated electric field generated by a fourth embodiment of the temperature-controlled flow-through reactor of the present invention between a first electrode 22 in the form of an elongated hollow cylinder and a second electrode 23 in the form of a similar elongated hollow cylinder. The first electrode 22 is at a high potential, and the second electrode 23 is grounded. The first and second electrodes 22, 23 surround the outside of the heat-resistant tube 1. The fourth embodiment generates a substantially axial electric field over a longer axial distance than the first, second, and third embodiments.

[0083] In the embodiment of Figures 2 to 6, the metal outer case 2 forms a grounded electromagnetic screen that shields the environment from radiation caused by the alternating electric fields inside the furnace, not only ensuring personnel safety but also blocking interference with electrical or electronic equipment.

[0084] For a refractory tube 1 having a diameter of 55 mm, the voltage required to provide CNT alignment by alternating the electric field was found to typically be between 500 V and 5,000 V peak-to-peak. The maximum electric field strength that can be used is below that which would cause corona discharge or plasma formation within the refractory tube 1. The optimal axial position of the electrodes and the electric field strength between them are a function of the diameter of the refractory tube 1, the type and flow rate of the reactant and transport gases within the refractory tube 1, the temperature profile along the axis of the refractory tube 1, and the configuration of the high-voltage electrode(s) and ground electrode(s). The frequency of the applied electric field can range from 13.553 to 13.567 MHz, although other frequencies can also be used.

[0085] It is convenient to generate the electric field using a radio frequency generator whose output is applied to circuitry configured to resonate at a selected operating frequency. An exemplary embodiment of such circuitry is shown in FIG. 7, in which a radio frequency power generator (provided with circuitry including an oscillator and a power amplifier, along with control and monitoring functions) is connected to an input port 30 by a radio frequency transmission line. An inductor 31 and a variable capacitor 33 form a series resonant circuit whose function is to increase the voltage applied 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, so that the voltage applied to the input port 30 is multiplied by the voltage multiplication factor ("Q factor") of the resonant circuit 31, 33. A variable capacitor 32 is connected in parallel with the inductor 31, allowing for variation of the effective inductive reactance of the resonant circuit 31, 33. Selection of the values ​​of the variable capacitors 32, 33 allows for control of the Q factor of the resonant circuit 31, 33, and therefore the relationship between the voltages at the input port 30 and the output port 35. Inductor 31 may be provided with a variable tap, allowing its inductance to be adjusted directly. Variable capacitor 34 is provided, allowing the input impedance of the circuit to be adjusted to match the 50 ohm impedance typically required by a connected radio frequency generator. To enable the generation of high voltages (for example, between 500V and 10,000V), variable capacitors 32, 33, 34 are vacuum variable capacitors.

[0086] A stray (parasitic) capacitance 36 exists between the furnace's metal outer casing 2 and the high voltage electrode and associated conductive connections. The effect of this stray capacitance is to load the resonant circuit 31, 33, resulting in a reduction in the output voltage at output port 35. The effect of the stray capacitance 36 can be reduced by connecting an inductor 37 in parallel with it. The effective value of the inductor 37 is selected to create a parallel resonance with the stray capacitance 36 at the operating frequency.

[0087] Port 38 is connected by a radio frequency transmission line to a resistive termination, typically having a value of 50 ohms. A monitoring port 39 is provided with a galvanically connected conductive loop 48 and capacitor 40, allowing the output voltage to be measured after a one-time calibration process to relate the output voltage at monitoring port 39 to the much lower voltage at output port 35. After calibration, the voltage at output port 35 can be estimated by measuring the lower voltage at output port 35 using (for example) a standard oscilloscope. This configuration eliminates the need to frequently measure high radio frequency voltages, which could pose a hazard to personnel operating the equipment.

[0088] A radio frequency generator connected to input port 30 provides selectable levels of output power and includes configuration for reducing the output power if reflected power increases beyond a level that could cause damage. Monitoring the reflected power provides an indication of changes within the reactor, such as the initial formation of a corona or other discharge, or contact between the grounded CNT aerogel and the high-voltage electrode. Monitoring information can be provided by a digital interface to the radio frequency generator and used to control the rate of aerogel withdrawal from the refractory tube or the flow rate of reagents.

[0089] Figure 8a is a scanning electron microscope image of a sample of CNT aggregates produced using the temperature-controlled flow-through reactor of Figure 1. The fibers formed from the assembled CNTs exhibit little degree of orientation. Figure 8b is a scanning electron microscope image of a sample of fibers produced in the first embodiment of the present invention by applying an axial electric field at a frequency of 13.56 MHz. The temperature profile and other operating parameters were not substantially changed. The fibers shown in Figure 8b exhibit a significant degree of orientation.

[0090] 9 and 10 show perspective and cross-sectional views, respectively, of an embodiment of a temperature-controlled flow-through reactor 94 of the present invention having a Kanthal ring electrode 97 and a Kanthal RF electrode 98 on the exterior of a refractory tube into which methane / thiophene / hydrogen 91 and ferrocene 99 are fed. The exterior ring electrode 97 generates electric field lines 92 to orient CNTs 93 forming an aerogel 95 wound on a bobbin 96. This promotes a continuous process and eliminates unwanted growth of VGCF. [Example]

[0091] (method) (High Voltage System and End Element Modeling) A custom-built cabinet was fabricated to serve as an RF-shielded compartment for the HV components, thereby ensuring the safety of personnel and equipment. The system housed a 300-W RF generator (Dressler Cesar 1312) operating in the license-free 13.56 MHz band. The generator output was connected to a 50-ohm load through a series-connected LC circuit tuned to 13.56 MHz. This configuration generated a high voltage at the junction between the inductor and the capacitor. A second variable capacitor (C1) was connected in parallel with the inductor to allow its effective reactance to be varied. The LC junction was connected to the RF electrode to project the HV generated by the system into the reactor. The voltage was adjusted by changing the reactance of the main capacitor and the parallel combination of the inductor and the capacitor according to equation (1).

[0092]

number

[0093] where Q is known as the voltage multiplication factor, L is the inductance, C is the capacitance and V is the output voltage.

[0094] The RF output voltage was measured by connecting a resistive voltage divider (985kΩ + 1kΩ) to the high-voltage output of the network and measuring the voltage across the 1kΩ resistor at 30W of input power using an oscilloscope (72-8705A Tenma) and a 1:1 probe. A correction was applied to take into account the nominal input impedance of the probe. Since the output voltage is proportional to the square of the output power, the measurement at 30W was appropriately scaled.

[0095] The electric field distribution inside the furnace was modeled using the AC / DC module in COMSOL Multiphysics. The small dimensions of the furnace (total length 500 mm) compared to the wavelength in free space (22 m) allowed the electric field to be modeled on a quasi-DC basis. In such a model, the shape of the electric field is independent of the applied voltage. The dimensions and material properties of the reactor components were faithful to the actual system. The CNT aerogel, seen in Figure 11c, was modeled as a cylinder with an outer diameter of 28 mm (inner diameter of 25 mm).

[0096] (Continuous alignment of CNTs using a single RF electrode) The FCCVD reactor was equipped with a single RF graphite electrode oriented along the central axis of the tube. Conceptually, the conductive CNT aerogel formed at the end of the reactor served as the ground electrode (see Figure 11a). Grounding of the CNT aerogel was ensured by collecting it on a grounded bobbin, which was grounded by a dedicated copper stake penetrating the ground. The RF electrode was connected to the HV system and inserted into the reactor through a custom-made injector flange. The tip of the RF electrode was stationary and located 95 mm upstream from the midpoint of the reactor. The power of the HV unit was set to 0, 200, 250, and 300 W. Reflected power during collection was minimal (less than 10 W). Each power configuration run was repeated at least three times. After collection was completed, the CNT material was manually wound perpendicular to the collection axis to generate a thin, "cigar-wrapped" thread around the circumference of the bobbin. The thread was cut at random points to produce a CNT fiber-like material approximately 160 mm long. In all runs, the same process parameters were used as described for CNT alignment with twin electrodes, with a collection speed of 30 rounds / min (linear velocity 0.157 ms). -1 ) was.

[0097] (Characteristics evaluation of CNT fibers) Weigh the fibers using a microbalance (Sartorius SE2-F) and measure the fiber length to determine the linear density of each sample in gkm. -1 The linear resistance of the fibers was determined by measuring the resistance of a 100 mm section of each sample using a custom-made four-point probe fixture connected to a milliohmmeter (Aim-TTi BS407). The specific conductivity was calculated by normalizing the linear conductance (which is inversely proportional to the linear resistance) according to the linear density of each sample. The specific conductivity values ​​(Sm 2 kg -1 ) were averaged according to sets of at least three samples.

[0098] Fiber strength (ultimate tensile stress normalized to linear density) and strain at failure were measured using an Instron mechanical testing machine (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 pretension of the sample was fixed at 0.1 N. To prevent slippage, the ends of the CNT fiber samples were sandwiched and glued with aluminum foil before being fixed in the grips. The fiber strength and strain at break values ​​were averaged for at least three sets of samples.

[0099] Raman analysis was performed on a Horiba XploRA PLUS confocal microscope system using a 638 nm laser, a 50x objective, a 1200 grating, 25% laser power, and three 30-second accumulations. Spectra are shown with baseline correction applied. G / D ratios were averaged over a set of at least three replicates for three different samples.

[0100] 2D SAXS patterns of CNT materials were collected at the BL11-NCD-SWEET amorphous beamline at the ALBA Synchrotron Light Facility (Barcelona, ​​Spain) equipped with Dectris (Pilatus 1M) photon counting and a Rayonix LX255-HS CDD detector. Sample scattering was collected at an emission wavelength of λ = 1.0 Å using a microfocus spot approximately 10 μm in diameter. Prior to collecting the patterns, the sample holder was aligned using silver behenate (AgBh). The collected patterns were first corrected for background scattering and then analyzed using DAWN software (v.2.20) to obtain a 0.7-0.8 nm chromatographic image. -1 The azimuthal profiles were obtained after radial integration over a range of Q. The intensity was normalized by the scattering invariant Q obtained from the Kratky plot q2·I(q) vs q.

[0101] For HRTEM imaging, specimens were prepared by sonicating approximately 10 mg of CNT material in 200 ml of 1-methyl-2-pyrrolidinone (NMP, 99% purity, Merck) for 60 min in an ultrasonicator (Hielscher, UP400ST). 1 ml of the dispersion was pipetted onto a Lacey Formvar / Carbon TEM grid (Ted Pella), allowed to stand for 1 min, and then blotted. 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 monochrome FEI Titan 80-300 TEM operating at 300 KV.

[0102] (CNT alignment using twin electrode setup) The FCCVD reactor contained two electrodes oriented along the central axis of a 50 mm (OD) alumina tube (Almath Rucibles, see Figure 17a). 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 the injector flange. The injector flange allowed free lateral movement of the RF electrode, and two side ports were used to introduce ferrocene through one port and other precursors through the other. A 6 mm molybdenum electrode (Goodfellow), referred to as the ground electrode, was inserted from the far end of the reactor. A grounded z-axis translation stage (Optics Focus Instruments Co.) was used to facilitate electrode orientation and fix its position. To maximize electric field uniformity, both electrode tips were polished to produce smooth, hemispherical edges. Experiments were performed by discretely varying the interelectrode gap (ΔL) between 200, 150, 130, and 50 mm. This was facilitated by changing the position of the RF electrode tip while the grounded electrode end was stationary (140 mm downstream from the reactor midpoint). The power supply for the HV unit was set to 300 W (maximum power) except at ΔL = 50 mm, at which point power settings of 0 W (reference) and 180 W were used. Each setup was run at least twice. All experiments were run for a short time (less than 5 seconds), as the process was immediately shut down when the reflected power, indicated by the HV power console on the HV power supply, reached its maximum capacity (100 W). In all runs (unless otherwise noted), the process was carried out as follows: The furnace was set to 1300 °C. The precursors included hydrogen (1400 standard cubic centimeters per minute, sccm, BOC), methane (160 sccm, BOC), ferrocene (200 sccm hydrogen passed through a tank heated to 110°C, 98% purity, Merck), and thiophene (60 sccm hydrogen passed through an ice-cooled reservoir at approximately 0°C, ≥99% purity, Merck).

[0103] (SEM imaging and image analysis) CNT specimens were imaged using a MIRA3 field emission gun-SEM (Tescan). Imaging was performed at an accelerating voltage of 5 kV using an In-Beam SE detector at a working distance of 3-5 mm. The specimens were not sputter-coated. For orientation quantification, images were acquired at 50 kX magnification using a 4096x3072 raster. If orientation was visually evident, images were manually captured at an angle where most CNTs were parallel to the long axis of the rectangular frame. With these imaging parameters, the resolution was calculated to be 2.9-4.7 pixels per CNT bundle (based on the discovery that the median diameter of CNT bundles is 16-26 nm, as shown in the Results section). Therefore, the number of CNTs per frame should exceed 500. The resolution and number of CNTs per frame met the requirements for successful image analysis. SEM image analysis was performed to obtain the image orientation distribution function (ODF) and further extract the orientational order parameter (i.e., the second moment, which is the average of the Chebyshev polynomial T2). The analysis was performed using the open-access FiberCOP program. The program parameters were set to 5 scans, a bin size of 0.25, and a filter spacing of 5. The number of peaks was set to 3, while each peak was Lorentzian-fitted. The acquisition of the average T2 orientation parameter for each twin-electrode setup was based on the analysis of at least three SEM images (totaling more than 1500 CNTs). SEM images for CNT bundle diameter analysis were taken using the same configuration as above, but at a magnification of 200 kX. The diameters of 200 CNT bundles were manually measured using Fiji, and the histograms were fitted with a log-normal distribution using OriginPro 2021.

[0104] (Modeling CNT alignment under the influence of an RF electric field) The alignment of CNTs by an alternating electric field can be explained using a worm-like chain model with energy contributions from bending, electric polarization, and additional electromagnetic interactions due to the z-pinch stiffening effect.

[0105] (Current, Pressure, and Force) As a first approximation, the variations in magnitude and time of the current in the CNT along its contour were neglected. Therefore, 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, it is possible to calculate the magnetic field strength inside the CNT wall. The axial current and the circumferential magnetic field are shown in Figure 14a. The magnetic field and the current in the CNT interact to generate a uniform compressive Lorentz force on the CNT wall. By integrating over the width of the CNT and taking the limit where the wall thickness vanishes, the Lorentz pressure acting on the CNT wall is:

[0106]

number

[0107] By further integrating the surface at each point along the contour and parameterizing it by s, we can derive the restoring line force density:

[0108]

number

[0109] A=πR 2 is the cross-sectional area of ​​the CNT, JPEG0007783264000004.jpg913 is the tangent vector along the CNT. Therefore, the pressure resulting from the current always acts against the curvature of the chain. The pressure and restoring forces are shown for a 2D continuum model of a CNT in Figure 15b and c.

[0110] (Energy contribution) Using the calculus of variations, we calculate the energy contribution of the restoring force density due to z-pinch stiffening as follows:

[0111]

number

[0112] This energy has a natural interpretation: both halves of the chain are pulled towards their nearest ends, with the midpoint of the chain fixed in place.

[0113] The current and resulting pressure need to be induced externally in the CNT, which can be done by applying an electric field, E, to the CNT. Assuming a simple model in which charges can only move tangentially within the CNT, the energy contribution of the electric field itself is proposed as follows:

[0114]

number

[0115] Here again, A is the cross-sectional area of ​​the CNT.

[0116] Combining the above energy terms with the usual curvature terms of the WLC, we obtain the complete free energy functional for the model as follows:

number

[0117] Here, a simply represents the bending rigidity of the CNT.

[0118] (harmonic approximation) For this purpose, it is sufficient to assume that the CNT is already strongly oriented in the electric field. Without loss of generality, the electric field point is allowed along the z-axis and has magnitude E. Then, the tangent vector and its second derivatives in the x and y components of the tangent vector θ(s) can be expanded to arrive at a harmonic approximation of the free energy up to an additive constant as follows:

[0119]

number

[0120] This approximate model is the basis for the present results and can be solved exactly using methods of Gaussian statistical field theory.

[0121] (Results and Discussion) (Continuous alignment of CNTs using internal RF electrodes) The FCCVD rig, adapted for electric field orientation, used a graphite electrode (RF electrode) connected to the HV unit and inserted through the reactor head. Conductive CNT aerogel (continuously synthesized in the reactor) was collected on a grounded bobbin, which served as the grounded electrode (see Figure 11a). To minimize artifacts related to the mechanical winding speed, the linear speed of the bobbin, considered an inefficient speed for CNT orientation, was set to approximately 0.16 ms. -1Figure 11b shows the mechanism of the alignment process between the electrodes. The alignment process is based on the internal AC current, the z-pinch stiffening effect, and the induced dipole alignment torque. Initial trials revealed that whisker-like material grew radially outward from the electrode surface after the injection of the process precursor (see Figure 19a). These whiskers grew in a section of the reactor 70–90 mm upstream from the reactor's midpoint (corresponding to the temperature range of 1100–1200 °C). SEM analysis revealed that these whiskers were made of an isotropic network of submicron vapor-grown carbon fibers (VGCFs; see Figure 19b). These whiskers grew even without the application of a high voltage, but in the presence of an electric field, whisker growth became significant as soon as the precursor was injected. Under the influence of an electric field, it was clear that the VGCFs did not spontaneously self-assemble but instead aligned themselves along the electric field lines (see Figure 19c). In this configuration, some of the individual VGCFs exhibited lengths exceeding 100 μm. This preferential growth of aligned VGCF whiskers can be explained by a finite element electric field distribution model of the furnace cavity (see Figure 11c). This model shows that there is a strong radial electric field (represented by the dense blue equipotential lines) between the RF electrode and the alumina tube. This model also ensures that well-defined electric field lines exist between the RF electrode and the CNT aerogel "sock," allowing the CNTs to be aligned accordingly.

[0122] Due to the rapid radial growth of VGCF whiskers on the RF electrode and the inevitable electrical contact with the ceramic tube, a low resistance was created between the RF electrode and ground within a few seconds. This caused the HV setup to go off-tune, significantly reducing the voltage and field strength. For this reason, the RF electrode was retracted 100 mm upstream of the midpoint. At this position, the RF electrode was at least 10 mm upstream of the location where VGCF whisker growth was detected, thus avoiding RF short circuits during successive runs. With such a setup, it was visually evident that no radial growth of VGCF whiskers occurred during the run, although axial whisker growth could be detected (see Figure 12a). The CNT material produced when the HV was applied appeared more rigid than the reference, as single CNT sockes could self-support without collapse (see Figure 12b). SEM imaging showed a pattern of CNT alignment, but it was not dominant. This was expected due to the large inter-electrode gap in such a configuration, which in turn reduced the applied field strength. Nevertheless, it is hypothesized that the presence of an electric field caused the whiskers to self-assemble and grow laterally from the tips of the RF electrodes, thereby artificially narrowing the inter-electrode gap, as shown in Figures 12a and 20a. Indeed, some of these "extended" whiskers grew up to 150 mm in length and were made of well-oriented, very thin VGCF (see Figure 20c).

[0123] Trials with this setup were performed with the RF power set to 0 (reference), 200, 250, and 300 W (maximum power). The applied field strength could not be evaluated in this setup because the inter-electrode gap was unknown. However, V ∝ P 1 / 2In the case of , the increment in electric field strength is proportional to the square root of the RF generator power. Electrical measurements of the different samples revealed a clear increase in specific conductivity by 75–90% compared to the reference sample (0 W), while there was no obvious change in the G / D ratio obtained from Raman spectroscopy (see Figure 13a). Furthermore, the Raman profile patterns remained similar among all samples (see Figure 21). These findings indicate that the increase in electrical properties was due to a change in the microstructure leading to less resistive CNT-CNT junctions, rather than a change in the synthesis process that produces CNTs with fewer defects. Further mechanical analysis revealed characteristic changes in the tensile behavior of the samples, as can be seen from the stress / strain curves (see Figure 13b). CNT materials produced by the original process (0 W) exhibit ductile behavior with a high strain ratio to fracture and a vague fracture point. In comparison, all CNT materials produced under an AC electric field exhibited much more brittle behavior, with a low strain ratio to fracture and a distinct fracture point. Interestingly, the in situ oriented materials showed a dramatic increase in elastic modulus (δ) of up to 375% and specific tensile failure (UTS) of up to 358%. This dramatic change in mechanical behavior is encouraging evidence of a change in the load-bearing microstructure of the CNT network due to CNT orientation.

[0124] To directly assess the degree of orientation, additional SAXS analysis was performed on the samples. Figure 14a shows overlaid azimuthal scans of the reference (0 W) and 300 W samples normalized by an invariant (scattering power) along with the associated 2D SAXS patterns. It can be clearly seen that the reference sample exhibits no orientation pattern (since it is essentially isotropic), whereas the 300 W sample exhibits a distinctive Lorentzian distribution associated with a more profound orientation pattern. Additional analysis based on integration of the raw data to calculate Herman's parameter (P2) revealed a trend between the applied voltage (which correlates with the square root of the RF power) and the degree of orientation (see Figure 14b). A clear correlation between P2 and δ was also observed, supporting the notion that the stiffness of the CNT network is primarily governed by the internal orientation of its CNT bundles. Interestingly, apart from the P2 values, the distribution of the azimuthal scans (as seen in the inset) evolves from non-existent (0 W) to Gaussian (200, 250 W) to Lorentzian (300 W), supporting the idea that orientation is correlated to field strength.

[0125] (Alignment of CNTs using RF electric fields - theoretical model) (Z-pinch hardening) CNTs are modeled as continuous shells with vanishing thickness. As a mean-field approximation, the current density within the CNT walls was assumed to be constant along the entire CNT contour. Current flow within the CNT is limited by electron scattering by optical phonons. Modeling of current-carrying modes within SWCNTs suggests that the RF field current must exceed the maximum saturation current of the CNT walls, J0 ≈ 25 μA. Therefore, current saturation is assumed, and therefore SWCNTs are assumed to carry a saturation current J0 when an RF AC field is applied. This contrasts with a simple DC field, where no current flows after the initial polarization of the CNT. Further experiments suggest that in SWCNT and MWCNT bundles, each CNT wall carries its own saturation current. Therefore, the total current scales proportionally with the number of walls present in the CNT fiber.

[0126] Axial current flow in a CNT induces a circumferential magnetic field within the CNT wall, as shown in Figure 15a. The magnitude of the electric field can be calculated using Ampere's law. The axial current then experiences a Lorentz force due to the presence of the magnetic field. 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 the vertical z-axis and comes from a similar effect used to compress plasmas strongly enough to undergo nuclear fusion. While this effect is not as dramatic in CNTs, it may stiffen the CNTs and facilitate alignment.

[0127] Considering a curved CNT segment, it is clear that the side facing (away from) the center of curvature is compressed (stretched). Therefore, the surface area over which the Lorentz pressure acts on the side away from the center of curvature is larger, providing an effective restoring force. As this force reacts against the curvature, the CNT stiffens due to the z-pinch effect. A diagram of the pressure versus restoring force is shown in Figures 15b-c.

[0128] (Model results) The primary measure used to quantify orientation is the two-dimensional orientational order parameter, T2, defined as:

[0129]

number

[0130] where θ 2D indicates the two-dimensional orientation angle of the CNT with respect to the electric field. This quantity can be easily measured in two-dimensional SEM images of CNT material, allowing direct comparison of current theoretical models with experimental data. The average value of T varies along the CNT, being lowest at the CNT ends and highest at the midpoint of the CNT. As a conservative measure of CNT orientation, the minimum value T found at the CNT ends 2,min was selected.

[0131] (rigid-elastic transition) Intuitively, increasing the electric field strength and CNT length up to a certain point improves the alignment of the CNTs. For DC, T 2,min There is a clear change in behavior where T no longer depends on the length of the CNT beyond a threshold length (see Figure 16a). This threshold length can be derived analytically and is proportional to the persistent length of the CNT. Below the threshold length, the CNT can be treated as a rigid body. Above the threshold length, elastic bending dominates the system, limiting the coupling of the CNT to the electric field. For AC, a strict regime still exists, but T 2,min For low values ​​of and long CNTs, the behavior deviates from the elastic regime and returns to the rigid regime (see Figure 16b). This indicates that the z-pinch effect can stiffen (i.e., effectively rigidify) CNTs. For substantially aligned SWCNTs, this effect only occurs on the millimeter length scale (see Figure 16c) and is limited by the relatively low values ​​of the saturation current. However, this result indicates that z-pinch stiffening, in principle, can facilitate alignment of SWCNTs.

[0132] (SWCNT bundles and MWCNTs) The strength of the z-pinch stiffening is limited by the current saturation of the SWCNTs. However, the saturation current scales linearly with the number of CNT walls in the SWCNT bundle or a single MWCNT. Therefore, the z-pinch stiffening should be significantly more pronounced in both cases. Figure 16d-e shows the electric field intensity E plotted against the CNT length L for different bundles of (10,10) SWCNTs and MWCNTs. In the plots, T 2,min= 0.5 was chosen to represent substantially aligned material. Both plots include a single (10,10) SWCNT for reference; z-pinch stiffening only becomes dominant at millimeter length scales. With approximately three CNT walls present, either as individual SWCNTs within a bundle or as MWCNT walls, the most dominant nanostructure in aerogels (see Figure 16f), z-pinch stiffening is already noticeable at the threshold length for the rigid-elastic transition. Thus, the z-pinch effect effectively stiffens CNT structures containing three or more CNT walls, promoting field alignment. The field strength required for alignment then falls below the typical dielectric breakdown field strength of FCCVD process gases, making alignment of single MWCNTs and small-diameter SWCNT bundles technically feasible.

[0133] (Twin electrode configuration) As a means to control and increase the applied electric field strength and its lateral development within the reactor volume, an additional two-electrode setup (see Figure 17a) was developed. The same graphite RF electrode as in the original setup and a molybdenum electrode (ground electrode) were inserted from the rear. Both electrodes were oriented along the central axis of the reactor tube and could move freely laterally. This configuration allowed for the position of each electrode to be set independently, allowing for adjustment of the inter-electrode position and gap width (ΔX and ΔL, respectively, as shown in Figure 17a). Such a means allowed for control of the applied electric field strength (coupled with the input power of the RF power supply) and the lateral position of the inter-electrode gap. It was clear that the system could achieve HV by generating hydrogen decomposition between the two electrodes. This was achieved by generating at least several kVcm for a hydrogen atmosphere. -1 This phenomenon requires an electric field strength on the order of 1000 s (Fig. 17b(i)). When the inter-electrode gap was located in the VGCF growth region, a visual indication was evident that agreed well with the electric field distribution model. In that configuration, when precursors were injected and the HV was on, whiskers were clearly seen to grow between the electrodes (Fig. 17b(ii)). Most notably were the central field lines and the peripheral lines bordering the electrode edges.

[0134] The rapid radial growth of VGCF whiskers on the RF electrodes and the consequent off-tune of the HV setup led to a significant drop in voltage and field strength even for short periods of HV application (approximately 5 seconds), clearly demonstrating the significant impact of the setup on the formation and orientation of the CNT aerogel. This can be easily inferred from Figure 17d (top) comparing the micromorphology of the reference sample, whose isotropy was observed at approximately 0.75 kV cm. -1 The results show that the material synthesized under the influence of an in situ electric field (bottom) exhibited significant orientation. The CNT bundles were so well aligned that it was possible to track several individual bundles running along the entire frame, some of which were at least 50 μm long and in other cases even exceeding 100 μm (see Figure 22). After running a significant number of experimental setups with various ΔX and ΔL configurations, it became clear that CNT alignment was only achieved when the grounded electrode was positioned at least 140 mm downstream from the furnace midpoint. This result is in good agreement with the knowledge that most CNT aerogels are synthesized in the last third of the reactor.

[0135] Because the amount of material produced with the twin-electrode setup was minute, quantifying the degree of orientation as a function of applied electric field strength could only be done by SEM image analysis. To address this demand, an open-access program (Fibre COP) specialized in quantifying uniaxial orientation order based on 2D images was used. For orientation distributions obtained from 2D images, the software calculated the average of the second moments of the Chebyshev polynomials (rather than Legendre) according to the Lorentzian fiber orientation distribution. Therefore, the calculated orientation order parameter is referred to in this section as T2 rather than the more common Herman's parameter (P2), which is more appropriate for data obtained from 3D bulk samples, e.g., obtained from X-ray diffraction. It should also be noted that T2 values ​​based on the Lorentzian distribution always exhibit lower values ​​than P2 for the same data set. Therefore, rather than directly comparing the present value of T2 with Herman's parameters published elsewhere, it should be used as an internal scale of orientation. As shown in Figure 18a, the T2 value for the reference sample (0 kV cm) is 0.01%. -1 ) is visually isotropic. However, it exhibits a T of 0.19 (perfect isotropy would lead to a value of zero). This may be related to the inherent orientation of the material due to the associated gas flows in the reactor. The system was operated at 0.23 kV cm -1 Even when the electric field strength is set to 0.30-0.35 kV cm, the basic isotropy of the CNT aerogel appears unchanged, and the orientation parameter remains practically unchanged at 0.20. -1 A significant CNT alignment pattern was revealed only when the electric field strength was changed from 0.75 to 0.95 kV cm. Although some of the CNT bundles did not follow a horizontal pattern, most did, resulting in an increase in the T2 value from 0.41 to 0.42. -1As T2 increased, a very distinctive orientation pattern was observed. Image analysis revealed a sharp increase in the orientation parameter from 0.46 to 0.51. This was found to be very similar to the value calculated from SEM images acquired from commercially available CNT fibers (5tex, Tortech Nano Fibers Ltd.). Although the orientation parameter follows a nonlinear trend, an increase in T2 from approximately 0.2 to approximately 0.5 is considered significant because it corresponds to a decrease in the full width at half maximum (FWHM) from approximately 100 to approximately 43.5°. Although there appears to be an order of magnitude discrepancy between the experimental applied field strength and the field strength required to reach a T2 of approximately 0.5 (Figure 16d), there is a dominant field enhancement effect in the case of 1D nanomaterials such as CNTs. To a first approximation, this enhancement factor is proportional to the aspect ratio of the 1D material, which in the present case should be greater than 500 according to a higher-order approximation. Another change observed in the material's micromorphology was related to the diameter of the CNT bundles. As shown in Figure 18b, the higher the electric field strength used in the interelectrode gap, the thicker the CNT bundles became. The median diameter of the CNTs was 0.23, 0.35, and 0.75 kVcm. -1 The thicknesses of the CNT bundles were analyzed to be 16.44, 18.87, and 25.40 nm for electric field strengths of 1000 Å and 1000 Å, respectively. The CNT bundles were expected to become thinner due to the orientation forces, as fewer collisions occurred between neighboring CNTs. This counterintuitive phenomenon can be explained by the existence of a compressive Lorentz pinch induced in the CNTs as a result of the AC electric field.

[0136] (Conclusion) This new approach allows for the application of external electric fields (e.g., up to 1 kV cm -1Utilizing an electric field (up to 1000 Hz) substantially impacts the self-organization mechanism of CNTs in the gas phase, as revealed by apparent CNT bundle thicknesses of approximately 16 to 25 nm. This system allows for continuous in situ manipulation while collecting nanomaterials to form macroscopic textiles. As determined by SAXS, this method proved to produce unique orientation patterns compared to the isotropy of the original bulk material. The microstructural reorganization correlated well with the transition of the textile's mechanical behavior from ductile to brittle, increasing the elastic modulus by up to 375%. The specific stress leading to fracture increased by up to 358%, as the orientation resulted in a higher proportion of load-bearing nanotubes resisting tensile loads. This reduced resistive CNT-CNT junctions and resulted in an associated electrical enhancement of up to 90%. Interestingly, the electric field did not affect CNT synthesis, as no obvious changes could be detected using Raman spectroscopy. A well-developed model acknowledged that MWCNT bundle orientation could occur below the decomposition threshold of the carrier gas, highlighting the advantages of applying an AC electric field rather than a DC electric field.

[0137] This novel use of external electric fields to manipulate and control the assembly process of CNT networks in the gas phase allows for the fabrication of high aspect ratios (approximately 10 s) without sacrificing the cost-effectiveness of the underlying process. 4 )We believe that we can maximize the potential of CNT-based textiles.

Claims

1. 1. A method for producing a carbon nanotube structure, comprising: (a) introducing a metal catalyst precursor into a continuous flow of a carrier gas in a temperature-controlled flow-through reactor; (b) exposing the metal catalyst precursor in the flow of the carrier gas to a first temperature zone sufficient to produce a particulate metal catalyst; (c) releasing a carbon source into the flow of the carrier gas; (d) exposing the particulate metal catalyst and the carbon source to a second temperature zone downstream of the first temperature zone, the second temperature zone being sufficient to produce carbon nanotube aggregates; (e) generating an electric field within the temperature-controlled flow-through reactor in the second temperature zone; (f) discharging the carbon nanotube aggregates as a continuous discharge from an outlet of the temperature-controlled flow-through reactor; (g) collecting said continuous emission in the form of a carbon nanotube structure; Including, the electric field is generated by an AC power source; The method for producing a carbon nanotube structure, wherein the electric field is oriented substantially coaxially with a flow path of a carrier gas.

2. The electric field is 0.35 to 1.0 kVcm －１ 10. The method of claim 1, wherein the electric field strength is generated in the range of .

3. The temperature-controlled flow-through reactor comprises: an elongated refractory housing extending from an upstream end to a downstream end into which the metal catalyst precursor is introduced in step (a) and into which the carbon source is discharged in step (c); a thermal enclosure surrounding the elongated refractory housing adapted to provide axial temperature variation between temperature zones within the elongated refractory housing, the temperature zones including the first temperature zone and the second temperature zone; and an electrode located inside or outside the elongated refractory housing; The method of claim 1 or 2, comprising:

4. The method of claim 3 , wherein the electrodes are oriented substantially parallel to the flow path of the carrier gas.

5. The method of claim 3 or 4, wherein the electrode is oriented substantially coaxially with the flow path of the carrier gas.

6. The method of claim 1 , wherein the carbon nanotube aggregate is an aerogel.

7. 1. A temperature-controlled flow-through reactor for producing carbon nanotube structures, comprising: an elongated refractory housing 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 the downstream end and beyond; a first feed for releasing a carbon source into said continuous flow of said carrier gas; a second feed for introducing a metal catalyst precursor into said continuous flow of said carrier gas; a thermal enclosure surrounding the elongated refractory housing adapted to provide axial temperature variation between temperature zones within the elongated refractory housing, the temperature zones including a first temperature zone sufficient to produce a particulate metal catalyst and a second temperature zone sufficient to produce carbon nanotube aggregates; a collector for collecting a continuous discharge of the carbon nanotube aggregates in the form of carbon nanotube structures from the downstream end; a first electrode located inside or outside the elongated refractory housing; an electric field generator electrically connected to the first electrode to apply a high potential sufficient to generate an electric field within the elongated refractory housing at the second temperature zone; Equipped with A temperature-controlled flow-through reactor for producing carbon nanotube structures, wherein the electric field is generated by an AC power source, and the electric field is oriented substantially coaxially with a flow path of a carrier gas.

8. A temperature-controlled flow-through reactor as described in claim 7, further comprising a second electrode located inside or outside the elongated refractory housing.

9. 9. The temperature-controlled flow-through reactor of claim 7 or 8, wherein the electric field is substantially coaxial with the elongated refractory housing.

10. 10. The temperature-controlled flow-through reactor of claim 7, wherein the collector is electrically connected to ground.

11. 10. The temperature-controlled flow-through reactor of claim 7, wherein the first electrode is located inside the elongated refractory housing at or adjacent to the second temperature zone, and the collector is electrically connected to ground.

12. 11. The temperature-controlled flow-through reactor of claim 7, further comprising a second electrode on the exterior of the elongated refractory housing, the first electrode being located inside the elongated refractory housing at or adjacent to the second temperature zone.

13. 13. The temperature-controlled flow-through reactor of claim 12, wherein the second electrode is electrically connected to the thermal enclosure, and the thermal enclosure is grounded.

14. 11. The temperature-controlled flow-through reactor of claim 7, wherein the first electrode is located outside the elongated refractory housing adjacent the second temperature zone.

15. 11. The temperature-controlled flow-through reactor of claim 7, further comprising a second electrode located outside the elongated refractory housing, wherein the first electrode is located outside the elongated refractory housing and the second electrode is electrically connected to ground.

16. The temperature-controlled flow-through reactor of any one of claims 7 to 15, wherein the electric field generator is operable at high radio frequency (HF).

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