Claims:WE CLAIM:
1. A method for purifying carbon nano materials comprising the steps of:
a. heating a carbon nano material in a gaseous atmosphere at a temperature between about 400 °C and about 600 °C, wherein the gaseous atmosphere comprises a gas selected from the group consisting of argon, nitrogen, hydrogen and mixtures thereof;
b. subsequently treating the carbon nano materials with chlorine gas at a temperature between about 400 °C and about 600 °C;
c. cooling treated carbon nano materials; and
d. recovering a purified product having a higher concentration of carbon nanotubes than the carbon nano material of step (a).
2. The method as claimed in claim 1, wherein the carbon nano material is a carbon nanotube, or a fiber, yarn or fabric comprising of carbon nanotubes.
3. The method as claimed in claim 1, wherein the heating of the carbon nano material in the gaseous atmosphere being performed preferably at temperature between about 450 °C and about 550 °C.
4. The method as claimed in claim 1, wherein the gaseous atmosphere comprises a mixture of hydrogen and argon or nitrogen.
5. The method as claimed in claim 4, wherein the gaseous atmosphere comprises about 5% to about 20% hydrogen.
6. The method as claimed in claim 1, wherein step (b) is repeated.
7. The method as claimed in claim 6, wherein the duration of step (b) is about 15 to about 30 minutes.
8. The method as claimed in claim 1, wherein the concentration of carbon nanotubes is greater than 88% as determined by thermogravimetric analysis.
9. The method as claimed in claim 8, wherein the concentration of carbon nanotubes is greater than 98% as determined by thermogravimetric analysis.
10. A method for purifying carbon nano materials comprising the steps of:
a. heating a carbon nano material in a gaseous atmosphere at a temperature between about 400 °C and about 600 °C, wherein the gaseous atmosphere comprises a gas selected from the group consisting of argon, nitrogen, hydrogen and mixtures thereof;
b. subsequently treating the carbon nano materials with chlorine gas at a temperature between about 400 °C and about 600 °C;
c. heating the carbon nano material in presence of air at a temperature about 350 °C; and
d. recovering a purified product having a higher concentration of carbon nanotubes than the carbon nano material of step (a).
11. The method as claimed in claim 10, wherein the carbon nano material is a carbon nanotube, or a fiber, yarn or fabric comprising of carbon nanotubes.
12. The method as claimed in claim 10, wherein the heating of the carbon nano material in the gaseous atmosphere being performed preferably at temperature between about 450 °C and about 550 °C.
13. The method as claimed in claim 10, wherein the gaseous atmosphere comprises a mixture of hydrogen and argon or nitrogen.
14. The method as claimed in claim 13, wherein the gaseous atmosphere comprises about 5% to about 20% hydrogen.
15. The method as claimed in claim 10, wherein step (b) is repeated.
16. The method as claimed in claim 15, wherein the duration of step (b) is about 15 to about 30 minutes.
17. The method as claimed in claim 10, wherein the concentration of carbon nanotubes is greater than 88% as determined by thermogravimetric analysis.
18. The method as claimed in claim 17, wherein the concentration of carbon nanotubes is greater than 98% as determined by thermogravimetric analysis.
19. A method for increasing the conductivity of carbon nanotube materials comprising the steps of:
a. heating a carbon nanotube material at a predefined pressure and at a temperature between about 100 °C and about 800 °C; and
b. recovering a product having an electrical conductivity higher than the carbon nanotube material of step (a).
20. The method as claimed in claim 19, wherein the carbon nano material is a carbon nanotube, or a fiber, yarn or fabric comprising of carbon nanotubes.
21. The method as claimed in claim 19, wherein the predefined pressure is between about 1 × 10-9 mbar and about 9 × 10-3 mbar.
22. The method as claimed in claim 19, wherein the temperature is preferably between about 350 °C and about 550 °C.
23. The method as claimed in claim 19, wherein the electrical conductivity is increased between about 100 and about 1000 of the electrical conductivity of the original sample as measured from I/V measurements using a 2-point probe.
24. A method for purifying carbon nanotube materials in order to increase conductivity of the carbon nanotube materials comprising the steps of:
a. passing a direct current through a carbon nanotube material at a voltage between about 1 V and 7 V;
b. reversing the flow of the a current through a carbon nanotube material at a voltage the same as in step (a); and
c. recovering a product having an electrical conductivity higher than the carbon nanotube material of step (a).
25. The method as claimed in claim 24, wherein the carbon nano material is a carbon nanotube, or a fiber, yarn or fabric comprising of carbon nanotubes.
26. The method as claimed in claim 24, wherein the voltage is preferably between about 3 V and about 5 V.
27. The method as claimed in claim 24, wherein the direct current is passed through the carbon nanotube material in a reduced, inert or chemically reducing atmosphere.
28. The method as claimed in claim 24, wherein the electrical conductivity is increased between about 100 and about 1000 of the electrical conductivity of the original sample as measured from I/V measurements using a 2-point probe.

Dated this 16th day of July 2020
Signature

Vidya Bhaskar Singh Nandiyal
Patent Agent (IN/PA-2912)
Agent for the Applicant

, Description:FIELD OF THE INVENTION
[0001] Embodiment of the present invention relates to purification of carbon nanotubes (CNTs) by removal of multiple categories of impurity, including, but not limited to, amorphous carbon and residues from catalyst used to grow the CNTs, with an aim to improve the electrical conductivity of individual CNTs and CNT fibers.

BACKGROUND

[0002] The various allotropes of carbon nanomaterials include buckminsterfullerene, graphene, and CNTs. Single walled carbon nanotubes (SWCNTs) and multi walled carbon nanotubes (MWCNTs) are both cylindrical entities in which the crystal lattice remains unbroken along the length of the tubes.

[0003] Carbon nanotubes are a class of nano material that has the potential to provide a variety of new, and previously unattainable, combinations of properties to materials. The global carbon nanotube market from 2018-2023 is forecasted to grow at a CAGR of 16.7%. This projected growth arises from technological advancement and decreased production cost. The market is expected to grow from 4.55 billion (USD) in 2018 to 9.84 billion (USD) by 2023. The investment into R&D has results in new applications of carbon nanomaterials. As the cost of production of carbon materials is decreasing and investment into carbon nanomaterial is increasing, there is an obvious need for purification procedures that reduce contamination in the materials there by minimizing unwanted effects of carbon nanomaterials in their application evaluation. The global market is primarily governed by the increasing demand for lithium-ion batteries. The market is categorized into aerospace and defence, electrical and electronics, energy, automotive, and others.

[0004] Because of their high electrical conductivity, nanometre diameters, high aspect ratios, and high degree of flexibility, CNTs are ideal materials for the preparation of transparent conductive films and coatings. CNTs network films can be prepared on flexible or rigid substrates by various solution processing methods. Flexible transparent conductors prepared from single walled (SWCNTs) and multi-walled (MWCNTs) carbon nanotubes have been reported. SWCNTs have smaller diameters than MWNTs (approx. 1 nm versus 30 nm).

[0005] One emerging area for the use of CNTs is solution-processed, flexible and durable, conductive coatings, another use of CNTs is as a replacement for metallic conductors such as copper and aluminum. Another emerging area for using carbon nanotubes is in the fabrication of conductive polymer composites. Carbon nanotubes can be dispersed in a polymer matrix to yield a conductive polymer composite that retains the mechanical and processing properties of the polymer.

[0006] Most of power distribution today is via the electrical grid, which relies on copper (or aluminum) cables within an iron sheath. Unfortunately, over 10% of the power transmitted is lost due in the main to resistive heating effects within the cables. To compensate for each 200 MW of line loss, another coal plant must be on-line. In 2011, summertime electrical generation in the US was 1,026 GW. Therefore, a 10% loss would be equivalent to ca. 200 average-sized coal power plants. Furthermore, because of limits in grid capacity, wind turbines are routinely turned off, because the excess electricity cannot be transported as needed. In addition to issues associated with power loss, the weight of any conductor has a significant impact on energy consumption, which is particularly true in the automotive and aerospace industries. Proposed long term solutions to low transmission losses of electricity involve CNTs), in particular metallic single walled carbon nanotubes (SWCNTs), or a near term solution involving the improvement in conductivity and ampacity of copper by the addition of CNTs, resulting in a Cu-CNT composite material termed ultra-conductive copper.

[0007] Currently, conductive coating of CNTs have been produced and used as antistatic coatings and for electromagnetic shielding. However, additional potential uses for conductive coatings using carbon nanotubes include touch screens for computers and other video terminals, flat panel displays, and as a substitute for expensive indium tin oxide coatings. There remains a need in the art to provide carbon nanotube coatings having both higher degrees of optical transmission in combination with higher electrical conductivities.

[0008] Among the different methods of growing carbon materials, those using an iron catalyst and a hydrocarbon source have to date been amongst the most successful and widely used. Chemical vapor deposition (CVD) is the current standard route to high-volume carbon nanotubes (CNTs) production. This synthesis method generally leads to the additional presence of particles of carbonaceous materials (amorphous carbon particles, fullerenes and nanocrystalline polyaromatic shells) and high content of metal catalyst residues. Purification steps are necessary for further modification of the CNTs and also for many of their applications such as photovoltaics and drug delivery where an even higher degree of purity is needed.

[0009] CNT fibers may be produced by a range of methods that include spinning a CNT dispersion liquid containing a plurality of CNTs including one or more CNT having at least partially collapsed structures, a dispersant, and a solvent by extruding the CNT dispersion liquid into a coagulant liquid.

[0010] One approach that shows promise in improving both the electrical conductivity and optical transmission of CNT coatings is by adding chemical reactants that p-dope or n-dope the carbon nanotubes, A variety of p-type and n-type dopants have been explored, including Br2, I2, and O2 as p-type and K, Cs, and Na as n-type. Thionyl chloride has been shown to yield substantial improvements in the conductivity of SWCNTs. Thionyl chloride has benefits compared to prior art doping agents such as Br2, Cs, and K, in that it is substantially less reactive and easier to handle. Thionyl chloride is used commercially in lithium-thionyl chloride batteries. SWCNT powders or bucky papers treated with liquid thionyl chloride for 24 h at 45 °C. showed conductivity increase by up to a factor of 5. However, their results indicate that the treatment is less effective for larger diameter SWCNT, such as those obtained by arc discharge and laser ablation processes. In addition, the reaction time is prohibitively long for use in-line during processing of carbon nanotube coatings.

[0011] Hence, there is a need for a method for the improved purification and increased electrical conductivity of carbon nanomaterials.

SUMMARY

[0012] The present invention is directed toward the purification and improved electrical conductivity of carbon nano materials, in particular carbon nanotubes (CNTs) and fibers made therefrom, through removal of impurities by reductive thermal treatment followed by chlorination or voltage annealing.

[0013] As prepared carbon nanotubes contain significant impurities. Amorphous carbon and catalyst residue are normally considered the main impurities; however, other species may be present as a consequence of process steps.

[0014] The optimized purification process achieves substantial removal of oxidized compounds of iron catalyst. The reductive annealing step enables removal of more non-graphitic carbon. The combination of the hydrogen thermal annealing and subsequent chlorination achieves removal of unwanted contaminants achieving an ultra-pure carbon nanomaterial. Unlike other previously described purification processes, this methodology can be expanded to single-walled carbon nanotubes, multi-walled carbon nanotubes and carbon nanofibers, which are notoriously challenging materials to purify.

[0015] Hydrogen-based reduction annealing was used to remove amorphous carbon, considered an impurity when dealing with graphitic carbon in carbonaceous nanomaterials, as well as cleave off functional groups from the CNT sidewalls including hydroxyl and carboxyl groups. Knowing that reduction of iron is a must, the improved purification process uses a combination of H2-annealing (for removing amorphous carbon and reducing oxidized impurities) and chlorination (for removing iron and other metallic impurities).

[0016] In accordance with an embodiment of the invention, a method for purifying carbon nano materials is provided. The method includes heating a carbon nano material in a gaseous atmosphere at a temperature between about 400 °C and about 600 °C, wherein the gaseous atmosphere comprises a gas selected from the group consisting of argon, nitrogen, hydrogen and mixtures thereof. The method also includes subsequently treating the carbon nano materials with chlorine gas a temperature between about 400 °C and about 600 °C. The method further includes cooling treated carbon nano materials. The method includes recovering a purified product having a higher concentration of carbon nanotubes.

[0017] In accordance with alternate embodiment of the invention, a method for purifying carbon nano materials includes heating a carbon nano material in a gaseous atmosphere at a temperature between about 400 °C and about 600 °C. The method also includes subsequently treating the carbon nano materials with chlorine gas a temperature between about 400 °C and about 600 °C. The method further includes heating the carbon nano material in presence of air at a temperature about 350 °C. The method includes recovering a purified product having a higher concentration of carbon nanotubes.

[0018] Another object of the present invention is to purifying carbon nano materials in order to increase the electrical conductivity of CNT fibers through the selective removal of amorphous carbon without damaging or oxidatively functionalizing the CNTs. Furthermore, the improvement on electrical conduction is achieved without the use of dopants, which is required by the prior art.

[0019] In an alternative approach, the selective removal of amorphous carbon without damaging or oxidatively functionalizing the CNTs is achieved by applying a direct current at a determined voltage in a non-oxidative atmosphere.

[0020] In accordance with another embodiment of the invention, a method for increasing the conductivity of carbon nanotube materials is provided. The method includes heating a carbon nanotube material at a predefined pressure and at a temperature between about 100 °C and about 800 °C. The method also includes recovering a product having a higher electrical conductivity.

[0021] In accordance with yet another embodiment, a method for purifying carbon nano materials in order to increase the electrical conductivity of carbon nanotube materials is provided. The method includes passing a direct current through a carbon nanotube material. The method also includes reversing the flow of the a current through a carbon nanotube material. The method further includes recovering a product having a higher electrical conductivity.

[0022] The advantage over prior methods is that all types of impurities may be removed under simple process conditions, and that increased electrical conduction of CNT fibers can be achieved without densification or chemical doping.

[0023] To further clarify the advantages and features of the present disclosure, a more particular description of the disclosure will follow by reference to specific embodiments thereof, which are illustrated in the appended figures. It is to be appreciated that these figures depict only typical embodiments of the disclosure and are therefore not to be considered limiting in scope. The disclosure will be described and explained with additional specificity and detail with the appended figures.

DESCRIPTION OF RELATED ART

[0024] It is understood that as-prepared carbon nanotubes (CNTs) are contaminated with a number of impurities. Chemical vapor deposition (CVD) is the current standard route to high-volume CNT production. This method generally leads to the presence of particles of carbonaceous materials (amorphous carbon particles, fullerenes and nanocrystalline polyaromatic shells) and high content of metal catalyst residues. Purification steps are necessary for further modification of the CNTs and also for many of their applications such as photovoltaics and drug delivery where an even higher degree of purity is needed.

[0025] Carbon nanotube purification methods can be divided in two main groups: physical and chemical. Generally physical purification complex methods involve processes like size exclusion chromatography, microfiltration, centrifugation and high temperature annealing among others. These methods preserve the structure of the carbon nanotubes but are not 100% effective in removing the impurities. Chemical purification methods commonly use gas-phase and wet methods. A summary of the prior art methods and the goal is shown in Table 1.

Method Intended Purpose Detrimental effects
Microwave heating Rapid removal of amorphous carbon Causes oxidation of Fe0 to iron oxides
Air thermal annealing Removal of amorphous carbon Causes oxidation of Fe0 to iron oxides as well as oxidation & functionalization of CNTs
Oxidative annealing Removal of amorphous carbon Causes oxidation of Fe0 to iron oxides as well as oxidation & functionalization of CNTs
H2 thermal annealing Removal of amorphous carbon Causes unzipping of CNTs
Chlorination Removal of residual catalyst (e.g., iron) Does not remove oxidized metals (e.g., iron oxides)
Fluorination Removal of residual catalyst (e.g., iron) Hazardous results in formation of C2.1F
Acid treatment Removal of residual catalyst (e.g., iron) Causes oxidation & functionalization of CNTs
Table 1. Summary of purification methods for CNTs and CNT fibers.

[0026] Liquid-phase oxidation for instance is effective in removing both, amorphous carbon and metallic catalyst particles but often require the use of strong oxidants like HNO3, a mixture of H2SO4:HNO3 and KMNO4. In one example of an oxidative route CNTs are heated in an aqueous solution of an inorganic oxidant, such as nitric acid, a mixture of hydrogen peroxide and sulfuric acid, or a potassium permanganate (Colbert et al. US Patent 7,115,864). One drawback of this prior art is that the CNTs must be refluxed in an aqueous solution of an oxidizing acid at a concentration high enough to etch away amorphous carbon deposits, but not so high that the CNT material will be etched to a significant degree. In an alternative solution based route method in the prior art the addition of reagent, such as a surfactant, is used in addition to the oxidizing agent (Hiura et al. US Patent 5,698,175).

[0027] As an alternative oxidative treatments have been reported that involve heating in air or oxygen at a temperature in the range from 600 °C to 1000 °C until the impurity carbon materials are oxidized and dissipated into gas phase (Ebbesen et al. US Patent 5,641,466).

[0028] A side effect of the oxidation routes is that they result in the functionalization of the graphitic surface of the carbon nanotubes with oxygen-containing groups leading to application issues further down the line. Another problem is that transition metal catalysts can remain encapsulated in the CNTs affecting the performance in many practical applications. Alternatively, the catalyst residue can be oxidized. In order facilitate removal of the catalyst post oxidation it has been reported that treating the CNT material with a solution comprising a halogen-containing acid (Smalley, et al. US Patent 6,752,977; Smalley, et al. US Patent 6,936,233) or other halogen containing species (Smalley, et al. US Patent 7,090,819) is beneficial. Unfortunately, these processes are still accomplished in an oxidizing atmosphere.

[0029] Non-oxidative acid treatments have been employed to purify CNTs. It has been reported that catalyst nanoparticles can be efficiently removed from carbon nanotubes using a high temperature chlorination process. In this treatment, metal catalyst residues are treated with Cl2 gas to create halides that vaporizes at high temperature (Andreoli, et al., 2014). This has the advantages of both the physical and chemical methods, eliminating the carbon and a greater quantity of catalytic residues simultaneously. It is however unable to eliminate the catalytic particles that remain fully encased by either graphitic or amorphous carbon. To overcome this issue, microwave irradiation has been employed to remove the encapsulating carbon and allow the catalyst residue to react with the chlorine (Gomez, et al., 2016). However, this route is only able to remove metal species in the metallic state since Cl2 does not react with the oxide derivatives of the catalyst metal. Reductive processes have been reported but the aim of enables nanotube opening to create carbon ribbons (Talyzin et al., 2011).

[0030] The prior art indicates that that a number of methods can be used to remove impurities from as synthesized CNTs; however, each method has disadvantages and does not remove all impurity types, or as in the reports of hydrogen reduction results in the undesired unzipping of the CNTs. None of these methods are aimed at the removal of sulfide species formed by the reaction of the catalyst (i.e., iron) with sulfur compounds used as catalyst moderators.

One of the reasons for purifying CNTs is that the impurities have a lower conductivity than the CNTs and as such purified samples are generally more conductive. In this regard doping is commonly employed. For example, oxidizers have been reported to increase the conductivity (Yoon et al. US Patent 8,586,458; Heintz, et al. US Patent 9,365,728; Yoon, et al. US Patent 8,501,529). Suitable oxidizers include: thionyl chloride, phosphoryl chloride, selenium oxychloride, iodine monobromide, and aurous chloride.

BRIEF DESCRIPTION OF THE DRAWINGS
[0031] The disclosure will be described and explained with additional specificity and detail with the accompanying figures in which:

[0032] FIG. 1 illustrates a flow diagram (10) representing a method for purifying carbon nano materials in accordance with an embodiment of the present disclosure;

[0033] FIG. 2 illustrates a flow diagram (20) representing a method for increasing the conductivity of carbon nanotube materials in accordance with an embodiment of the present disclosure;

[0034] FIG. 3 illustrates a flow diagram (30) representing an alternative method for increasing the conductivity of carbon nanotube materials in accordance with an embodiment of the present disclosure;

[0035] FIG. 4 illustrates a graphical representation (40) of performance of the current state-of-the-art dry purification process using a combination of inert gas thermal annealing and chlorination in accordance with an embodiment of the present disclosure.
[0036]

[0037] FIG. 5 illustrates a graphical representation (50) of performance of the improved purification method using a combination of hydrogen reductive annealing and chlorination in accordance with an embodiment of the present disclosure;

[0038] FIG. 6 illustrates method steps for performing thermogravimetric analysis of an example carbon nanomaterial (CNT fiber) in accordance with an embodiment of the present disclosure;

[0039] FIG. 7 illustrates method steps for performing thermogravimetric analysis of a CNT fiber after chlorination (involving argon annealing) in accordance with an embodiment of the present disclosure;

[0040] FIG. 8 illustrates a graph plot depicting thermogravimetric analysis of a CNT fiber after the improved purification process showing a reduction in Fe impurity content to ~1.2 wt% in the material after purification in accordance with an embodiment of the present disclosure.

[0041] FIG. 9 illustrates a graph plot depicting SKa WDS spectrum of a CNT fiber showing the presence of FeS2 as an impurity in accordance with an embodiment of the present disclosure.

[0042] FIG. 10 illustrates a graph plot depicting relative atomic percentage of Fe and S in a sample of CNT fiber in accordance with an embodiment of the present disclosure.

[0043] FIG. 11 illustrates a graph plot depicting resonant Raman spectrum, at 785 nm, for a CNT fiber as-is. The IG/ID number is from a minimum of 10 measurements in accordance with an embodiment of the present disclosure.

[0044] FIG. 12 illustrates a graph plot depicting resonant Raman spectrum, at 785 nm, for a CNT fiber after the chlorination purification protocol in accordance with an embodiment of the present disclosure.

[0045] FIG. 13 illustrates a graph plot depicting resonant Raman spectrum, at 785 nm, for a CNT fiber after the combined hydrogen and chlorination purification protocol in accordance with an embodiment of the present disclosure.

[0046] FIG. 14 illustrates a plot of resistance (O) versus annealing temperature (°C) for CNT fiber at a base pressure of 9×10-3 mbar in accordance with an embodiment of the present disclosure;

[0047] FIG. 15 illustrates a plot of resistance (O) versus annealing temperature (°C) for CNT fiber at a base pressure of 1×10-9 mbar in accordance with an embodiment of the present disclosure;

[0048] FIG. 16 illustrates a plot of resistance (O) versus voltage sweep range (V) for CNT fiber in accordance with an embodiment of the present disclosure;

[0049] FIG. 17 illustrates a plot of Raman IG/ID ratio (at 785 nm) as a function of fiber annealing temperature for thermally annealed in-situ in the nanoprobe in accordance with an embodiment of the present disclosure;

[0050] FIG. 18 illustrates a plot of resistance (O) as a function of Raman IG/ID ratio (at 785 nm) for thermally annealed in-situ in the nanoprobe in accordance with an embodiment of the present disclosure;

[0051] FIG. 19 illustrates a plot of Raman IG/ID ratio (at 785 nm) as a function of fiber annealing temperature for thermally annealed under H2 (5%) in a tube furnace in accordance with an embodiment of the present disclosure;

[0052] FIG. 20 illustrates a plot of resistance (O) as a function of Raman IG/ID ratio (at 785 nm) for thermally annealed under H2 (5%) in a tube furnace in accordance with an embodiment of the present disclosure;

[0053] FIG. 21 illustrates a graph plot depicting decrease in linear mass density of the CNT fiber after purification in accordance with an embodiment of the present disclosure.

[0054] FIG. 22 illustrate a plot of resistance (O) as a function treatment of CNT fibers in accordance with an embodiment of the present disclosure;

[0055] FIG. 23 illustrates a graph plot depicting thermogravimetric analysis (TGA, Weight loss) and differential thermal analysis (DTA, heat flow) of the CNT fibers before treatment with four distinct thermal events associated with mass loss: (1) Oxidation of amorphous carbon; (2) Oxidation of Fe to FeO and also oxidation of SWCNTs; (3) Oxidation of MWCNTs; (4) Oxidation of FeO to Fe2O2 and remaining MWCNTs as well as bundles of MWCNTs, in accordance with an embodiment of the present disclosure.

[0056] FIG. 24 illustrates a graph plot depicting the differential thermal analysis (DTA, heat flow) of the CNT fibers after heating to about 300 °C at a base pressure of 9×10-3 mbar in accordance with an embodiment of the present disclosure.

[0057] FIG. 25. illustrates a graph plot depicting the differential thermal analysis (DTA, heat flow) of the CNT fibers after heating to about 400 °C at a base pressure of 9×10-3 mbar in accordance with an embodiment of the present disclosure;

[0058] FIG. 26 illustrates a graph plot depicting the differential thermal analysis (DTA, heat flow) of the CNT fibers after heating to about 700 °C at a base pressure of 9×10-3 mbar in accordance with an embodiment of the present disclosure.

[0059] FIG. 27 illustrates a plot of resistance (O) versus temperature (K) for unpurified CNT fiber that shows metallic behavior due to metallic iron residue from the catalyst used to grow the CNTs in accordance with an embodiment of the present disclosure;

[0060] FIG. 28 illustrates a plot of resistance (O) versus temperature (K) for purified CNT fiber (Example 17) that shows the removal of metallic iron residue from the catalyst used to grow the CNTs in accordance with an embodiment of the present disclosure;

[0061] FIG. 29 illustrates an image of a suitable apparatus used for purification of the CNTs in accordance with an embodiment of the present disclosure;

[0062] FIG. 30 is an image depicting presence of FeCl3 formed from the reaction of the catalyst residue with chlorine, deposited on the outlet for the tube in accordance with an embodiment of the present disclosure.

[0063] FIG. 31 illustrates a SEM image of the commercial Nanocomp roving CNT fiber in accordance with an embodiment of the present disclosure.

[0064] FIG. 32 illustrates a SEM image of the commercial Nanocomp YE-A10 CNT fiber in accordance with an embodiment of the present disclosure.

[0065] FIG. 33 illustrates a graph plot of WDS analysis of HiPco (High Pressure carbon monoxide) SWCNTs for iron content in accordance with an embodiment of the present disclosure.

[0066] FIG. 34 illustrates a graph plot of WDS analysis of HiPco (High Pressure carbon monoxide) SWCNTs for sulfur content in accordance with an embodiment of the present disclosure.

[0067] FIG. 35 illustrates a graph plot of WDS analysis of HiPco (High Pressure carbon monoxide) SWCNTs for oxygen content in accordance with an embodiment of the present disclosure; and

[0068] FIG. 36 illustrates a graph plot depicting the iron-to-oxygen ratio as determined from the individual iron and oxygen WDS scans before in accordance with an embodiment of the present disclosure.

[0069] Further, those skilled in the art will appreciate that elements in the figures are illustrated for simplicity and may not have necessarily been drawn to scale. Furthermore, in terms of the method steps, chemical compounds, and parameters used herein may have been represented in the figures by conventional symbols, and the figures may show only those specific details that are pertinent to understanding the embodiments of the present disclosure so as not to obscure the figures with details that will be readily apparent to those skilled in the art having the benefit of the description herein.

DETAILED DESCRIPTION
[0070] Embodiments of the present invention relates to a method for the improved purification and increased electrical conductivity of carbon nanomaterials. The method mainly focuses on removal of impurities by reductive thermal treatment followed by chlorination or voltage annealing.

[0071] FIG. 1 illustrate a flow diagram (10) representing the method for purifying carbon nano materials in accordance with an embodiment of the present disclosure. The present invention is differentiated from the prior art in that it requires a reductive thermal treatment in combination with chlorine treatment in order to ensure the removal of amorphous carbon impurities as well as catalyst residue and other impurities obtained during fiber processing.

[0072] The method for purifying carbon nano materials begins with heating a carbon nano material in a gaseous atmosphere at a temperature between about 400 °C and about 600 °C at step 12. The gaseous atmosphere comprises a gas selected from the group consisting of argon, nitrogen, hydrogen and mixtures thereof. In an embodiment, the gaseous atmosphere comprises about 5% to about 20% hydrogen. The carbon nano material is a carbon nanotube, or a fiber, yarn or fabric comprising of carbon nanotubes. The heating of the carbon nano material in the gaseous atmosphere being performed preferably at temperature between about 450 °C and about 550 °C.

[0073] The reductive thermal treatment is achieved through either thermal annealing in vacuum or under a reducing atmosphere. In an alternative method the reductive thermal treatment is achieved by passing a direct current through the sample resulting in selective thermal annealing. A selective reductive thermal annealing also results in the reduction of any catalyst oxide residues, such as Fe2O3 and Fe3O4 to elemental FeO that allows for its subsequent reaction with chlorine to form volatile FeCl3 (boiling point = 316 °C) that is sublimed out of the sample. Furthermore, the selective reductive thermal annealing also reduces FeS2 impurities that are formed due to the presence of sulfur containing catalyst moderators. Furthermore, the presence of calcium containing impurities, due to washing of CNT fibers, can also be removed through the selective reductive thermal annealing followed by reaction of the residue with chlorine.

[0074] The prior art provides for performing hydrogen reduction between 400 °C and 500 °C using hydrogen gas in order to remove the amorphous carbon (Talyzin et al., 2011). The present invention unexpectedly demonstrates that diluted hydrogen is sufficient to remove the amorphous hydrogen as well as reduce the metal oxides without harming the CNTs.

[0075] The method includes subsequently treating the carbon nano materials with chlorine gas at temperature between about 400 °C and about 600 °C at step 14. The duration of subsequently treating the carbon nano materials with chlorine gas at temperature between about 400 °C and about 600 °C is about 15 to about 30 minutes. The step 14 may be repeated as per the requirement.

[0076] The method includes cooling treated carbon nano materials at step 16A before recovering a purified product.

[0077] In an alternative embodiment, the method includes heating the carbon nano material in presence of air at a temperature about 350 °C at step 16B.

[0078] The method includes recovering a purified product having a higher concentration of carbon nanotubes than the carbon nano material of step 12 at step 18. The concentration of carbon nanotubes recovered is greater than 88% as determined by thermogravimetric analysis. In another embodiment, the concentration of carbon nanotubes recovered is greater than 98% as determined by thermogravimetric analysis.

[0079] FIG. 4 illustrates a graphical representation (40) of performance of the current state-of-the-art dry purification process using a combination of inert gas thermal annealing and chlorination in accordance with an embodiment of the present disclosure.

[0080] In an exemplary embodiment, the method for purifying carbon nano materials includes heating a fiber under vacuum and argon atmosphere at 500°C. The method also includes evacuating a chamber and filling the chamber with chlorine gas. The method includes evacuating the chamber after 10 minutes. The method includes thermal annealing under argon atmosphere. The method also includes repetition of evacuating the chamber, filling chlorine gas and again evacuating the chamber after 10 minutes. The method further includes repetition of thermal annealing under argon atmosphere and evacuating the chamber and filling chlorine gas. The method includes cooling treated fiber to 25°C under vacuum and argon atmosphere. The method further includes heating the fiber to 350°C in presence of air and cooling it to 25°C.

[0081] FIG. 5 illustrates a graphical representation (50) of performance of the improved purification method using a combination of hydrogen reductive annealing and chlorination in accordance with an embodiment of the present disclosure.

[0082] In another exemplary embodiment, the method for purifying carbon nano materials includes heating a fiber under vacuum and hydrogen atmosphere at 500°C. The method also includes evacuating a chamber and filling the chamber with chlorine gas. The method includes evacuating the chamber after 10 minutes. The method includes thermal annealing under hydrogen atmosphere. The method also includes repetition of evacuating the chamber, filling chlorine gas and again evacuating the chamber after 10 minutes. The method further includes repetition of thermal annealing under hydrogen atmosphere and evacuating the chamber and filling chlorine gas. The method includes cooling treated fiber to 25°C under vacuum and hydrogen atmosphere. The method further includes heating the fiber to 350°C in presence of air and cooling it to 25°C.

[0083] In another embodiment of the present invention, a method for increasing the conductivity of carbon nanotube materials is provided.

[0084] The selective removal of the amorphous carbon without damaging the CNTs results in an increase in the electrical conductivity of CNT fibers. This is unexpected since the resulting fibers will be less dense and such improved conductivity is associated with either densification of the fiber or chemical doping.

[0085] The removal of amorphous carbon has been previously demonstrated through oxidative treatments, including heating under an oxidizing atmosphere, e.g., air or oxygen. Unfortunately, the oxidative process introduces impurities and functional groups. Furthermore, oxidative treatments result in the oxidation of catalyst residue making them more difficult to remove. For example, the majority of commercial CNTs use an iron-based catalyst, which results in nanoparticles of elemental Fe0 to be retained within the CNT sample. Some of this elemental Fe0 is oxidized under ambient air exposure to oxides including Fe2O3 and Fe3O4.

[0086] Although the elemental Fe0 is removed through the reaction with chlorine by the reaction forming volatile iron chloride that is sublimed out of the CNT sample (V. Gomez, et al., 2016):
2 Fe0 + 3 Cl2 ? 2 FeCl3 (1)

[0087] This process is inhibited if the Fe0 is oxidized since the oxides do not react with chlorine and hence the oxides are not removed from the CNT sample. Thus, oxidative treatments are counterproductive, and a reductive process must be employed. Unfortunately, prior art suggests that while a reductive process can remove amorphous carbon it also teaches that the CNTs are opened and unzipped (Talyzin et al., 2011), which is detrimental to the electrical conductivity.

[0088] FIG. 2 illustrates a flow diagram (20) representing the method for increasing the conductivity of carbon nanotube materials in accordance with an embodiment of the present disclosure.

[0089] The method for increasing the conductivity of carbon nanotube begins with heating a carbon nanotube material at a predefined pressure and at a temperature between about 100 °C and about 800 °C at step 22. The carbon nano material is a carbon nanotube, or a fiber, yarn or fabric comprising of carbon nanotubes. The predefined pressure is between about 1 × 10-9 mbar and about 9 × 10-3 mbar. The temperature is preferably between about 350 °C and about 550 °C.

[0090] The method also includes recovering a product having an electrical conductivity higher than the carbon nanotube material of step 22 at step 24. The electrical conductivity is increased between about 100 and about 1000 of the electrical conductivity of the original sample as measured from I/V measurements using a 2-point probe.

[0091] In yet another embodiment of the present invention, a method for purifying carbon nanotube materials in order to increase conductivity of the carbon nanotube materials is provided. FIG. 3 illustrate a flow diagram (30) of method steps for the same.

[0092] The method for increasing the conductivity of carbon nanotube materials begins with passing a direct current through a carbon nanotube material at a voltage between about 1 V and 7 V at step 32. The carbon nano material is a carbon nanotube, or a fiber, yarn or fabric comprising of carbon nanotubes. The voltage is preferably between about 3 V and about 5 V. The direct current is passed through the carbon nanotube material in a reduced, inert or chemically reducing atmosphere.

[0093] The method includes reversing the flow of the a current through a carbon nanotube material at a voltage the same as in step 32 at step 34. The reversing the flow of the current is carried by providing potential bias and measuring current.

[0094] The present invention also shows that this selective removal of amorphous carbon can be achieved through thermal treatment under a reducing atmosphere or using a direct current from an applied voltage. To ensure that unzipping of the fibers does not occur it is important that the temperature and/or voltage be controlled within the desired range.

[0095] In such embodiment, the method also includes recovering a product having an electrical conductivity higher than the carbon nanotube material of step 32 at step 36. The electrical conductivity is increased between about 100 and about 1000 of the electrical conductivity of the original sample as measured from I/V measurements using a 2-point probe.

[0096] In the present invention characteristics of purified carbon nanotube materials with increased electrical conductivity were analysed.

[0097] FIG. 6 illustrates method steps for performing thermogravimetric analysis of an example carbon nanomaterial (CNT fiber), where the test result shows ~13.5 wt% Fe impurity content in the material tested as-is in accordance with an embodiment of the present disclosure.

[0098] FIG. 7 illustrates method steps for performing thermogravimetric analysis of a CNT fiber after chlorination (involving argon annealing). The result shows a reduction in Fe impurity content to ~4.5 wt% in the material after purification in accordance with an embodiment of the present disclosure.

[0099] FIG. 8 illustrates a graph plot depicting thermogravimetric analysis of a CNT fiber after the improved purification process showing a reduction in Fe impurity content to ~1.2 wt% in the material after purification in accordance with an embodiment of the present disclosure. The new process has helped achieve sub-1 wt% impurity super pure samples.

[0100] FIG. 9 illustrates a graph plot depicting SKa WDS spectrum of a CNT fiber showing the presence of FeS2 as an impurity in accordance with an embodiment of the present disclosure.

[0101] FIG. 10 illustrates a graph plot depicting relative atomic percentage of Fe and S in a sample of CNT fiber in accordance with an embodiment of the present disclosure.

[0102] FIG. 11 illustrates a graph plot depicting resonant Raman spectrum, at 785 nm, for a CNT fiber as-is. The IG/ID number is from a minimum of 10 measurements in accordance with an embodiment of the present disclosure.

[0103] FIG. 12 illustrates a graph plot depicting resonant Raman spectrum, at 785 nm, for a CNT fiber after the chlorination purification protocol in accordance with an embodiment of the present disclosure. The IG/ID number is from a minimum of 10 measurements.

[0104] FIG. 13 illustrates a graph plot depicting resonant Raman spectrum, at 785 nm, for a CNT fiber after the combined hydrogen and chlorination purification protocol in accordance with an embodiment of the present disclosure. The IG/ID number is from a minimum of 10 measurements.

[0105] FIG. 14 illustrates a plot of resistance (O) versus annealing temperature (°C) for CNT fiber at a base pressure of 9×10-3 mbar in accordance with an embodiment of the present disclosure.

[0106] FIG. 15 illustrates a plot of resistance (O) versus annealing temperature (°C) for CNT fiber at a base pressure of 1×10-9 mbar in accordance with an embodiment of the present disclosure.

[0107] FIG. 16 illustrates a plot of resistance (O) versus voltage sweep range (V) for CNT fiber in accordance with an embodiment of the present disclosure.

[0108] FIG. 17 illustrates a plot of Raman IG/ID ratio (at 785 nm) as a function of fiber annealing temperature for thermally annealed in-situ in the nanoprobe in accordance with an embodiment of the present disclosure.

[0109] FIG. 18 illustrates a plot of resistance (O) as a function of Raman IG/ID ratio (at 785 nm) for thermally annealed in-situ in the nanoprobe in accordance with an embodiment of the present disclosure.

[0110] FIG. 19 illustrates a plot of Raman IG/ID ratio (at 785 nm) as a function of fiber annealing temperature for thermally annealed under H2 (5%) in a tube furnace in accordance with an embodiment of the present disclosure.

[0111] FIG. 20 illustrates a plot of resistance (O) as a function of Raman IG/ID ratio (at 785 nm) for thermally annealed under H2 (5%) in a tube furnace in accordance with an embodiment of the present disclosure.

[0112] FIG. 21 illustrates a graph plot depicting decrease in linear mass density of the CNT fiber after purification in accordance with an embodiment of the present disclosure. The current state-of-art process prior to our invention brings it down from a linear mass density of 10.8 µg/mm to ~7.8 µg/mm with the removal of Fe0 and amorphous carbon. Our invention reduces oxidized iron to Fe0, and helps with a further mass reduction to an average of 6.75 µg/mm. Removal of physically adsorbed FeCl3 then results in a final value of 6.35 µg/mm- a ~41% reduction from the initial value and ~18.7% reduction from the current best process (before this invention).

[0113] FIG. 22 illustrates a plot of resistance (O) as a function treatment of CNT fibers in accordance with an embodiment of the present disclosure.

[0114] FIG. 23 illustrates a graph plot depicting thermogravimetric analysis (TGA, Weight loss) and differential thermal analysis (DTA, heat flow) of the CNT fibers before treatment with four distinct thermal events associated with mass loss: (1) Oxidation of amorphous carbon; (2) Oxidation of Fe to FeO and also oxidation of SWCNTs; (3) Oxidation of MWCNTs; (4) Oxidation of FeO to Fe2O2 and remaining MWCNTs as well as bundles of MWCNTs, in accordance with an embodiment of the present disclosure.

[0115] FIG. 24 illustrates a graph plot depicting the differential thermal analysis (DTA, heat flow) of the CNT fibers after heating to about 300 °C at a base pressure of 9×10-3 mbar in accordance with an embodiment of the present disclosure.

[0116] FIG. 25 illustrates a graph plot depicting the differential thermal analysis (DTA, heat flow) of the CNT fibers after heating to about 400 °C at a base pressure of 9×10-3 mbar in accordance with an embodiment of the present disclosure.

[0117] FIG. 26 illustrates a graph plot depicting the differential thermal analysis (DTA, heat flow) of the CNT fibers after heating to about 700 °C at a base pressure of 9×10-3 mbar in accordance with an embodiment of the present disclosure.

[0118] FIG. 27 illustrates a plot of resistance (O) versus temperature (K) for unpurified CNT fiber that shows metallic behavior due to metallic iron residue from the catalyst used to grow the CNTs in accordance with an embodiment of the present disclosure.

[0119] FIG. 28 illustrates a plot of resistance (O) versus temperature (K) for purified CNT fiber (Example 17) that shows the removal of metallic iron residue from the catalyst used to grow the CNTs in accordance with an embodiment of the present disclosure.

[0120] FIG. 29 illustrates an image of a suitable apparatus used for purification of the CNTs in accordance with an embodiment of the present disclosure.

[0121] FIG. 30 is an image depicting presence of FeCl3 formed from the reaction of the catalyst residue with chlorine, deposited on the outlet for the tube in accordance with an embodiment of the present disclosure.

[0122] FIG. 31 illustrates a SEM image of the commercial Nanocomp roving CNT fiber in accordance with an embodiment of the present disclosure.

[0123] FIG. 32 illustrates a SEM image of the commercial Nanocomp YE-A10 CNT fiber in accordance with an embodiment of the present disclosure.

[0124] FIG. 33 illustrates a graph plot of WDS analysis of HiPco (High Pressure carbon monoxide) SWCNTs for iron content in accordance with an embodiment of the present disclosure. The typical iron content ranges from 10-20 wt% with some outliers on either side.

[0125] FIG. 34 illustrates a graph plot of WDS analysis of HiPco (High Pressure carbon monoxide) SWCNTs for sulfur content in accordance with an embodiment of the present disclosure. HiPco SWCNTs are seen to have next to no sulfur, in contrast to the CNT fibers prepared using floating catalyst CVD, and shows that iron is not present as iron sulfide here.

[0126] FIG. 35 illustrate a graph plot of WDS analysis of HiPco (High Pressure carbon monoxide) SWCNTs for oxygen content in accordance with an embodiment of the present disclosure. HiPco SWCNTs have a small fraction of oxygen associated with carbon, with the rest associated with the residual iron catalyst.

[0127] FIG. 36 illustrates a graph plot depicting the iron-to-oxygen ratio as determined from the individual iron and oxygen WDS scans before in accordance with an embodiment of the present disclosure. There is a strong and direct correlation between the iron and oxygen contents in HiPco SWCNTs, which shows that iron in individual carbon nanotubes is present as native iron, and also as oxides of iron, with the elemental ratio indicating the presence of Fe2O3. The iron oxide would thus have to be reduced in order to be removed by chlorination similar to CNT fibers with oxidized iron as iron sulfide and/or iron oxide.

[0128] The ability of the present invention to selectively remove of amorphous carbon is demonstrated by comparing the thermogravimetric analysis (TGA) and differential thermal analysis (DTA) of the CNT fibers before treatment and after thermal annealing to different temperatures resulting in improved electrical conductivity (FIG. 11). As shown in FIG. 20, the TGA of the CNT fibers shows four distinct thermal events associated with mass loss. Based upon prior art these can be assigned to the following chemical processes:
Step 1: Oxidation of amorphous carbon
Step 2: Oxidation of Fe to FeO and also oxidation of SWCNTs
Step 3: Oxidation of MWCNTs
Step 4: Oxidation of FeO to Fe2O2 and remaining MWCNTs as well as bundles of MWCNTs.
[0129] As may be seen from the differential thermal analysis (DTA) of the CNT fibers after heating to FIG. 21, as the CNT fiber is heated to between about 200 °C and about 300 °C, there is a decrease in the peak associated with the oxidation of amorphous carbon. This is consistent with the desired removal of the amorphous carbon and a concomitant reduction of the electrical resistance (increased electrical conductivity) of the CNT fiber (FIG. 11).

[0130] Heating the CNT fiber to between about 400 °C and about 500 °C, there is a further dramatic decrease in the peak associated with the oxidation of amorphous carbon (FIG. 22). This is consistent with the desired removal of the amorphous carbon and a greater reduction of the electrical resistance (increase in electrical conductivity) of the CNT fiber (FIG. 11).

[0131] If the CNT fiber is heated further to above about 600 °C the DTA shows a decrease in the peaks associated with the oxidation of both SWCNTs and MWCNTs (FIG. 23), i.e., they have been consumed during the thermolysis process (FIG.11). The resultant material shows an increase in the electrical resistance (decrease in electrical conductivity) of the CNT fiber (FIG.11).

[0132] The present invention allows for the selective removal of amorphous carbon without the damage of the CNTs associated with the prior art processes.

[0133] Furthermore, sulfur is used as common catalyst moderator for the growth of CNTs (Hirai et al. US Patent 10,266,411). It was found that after multiple chlorine reactions with CNT fibers there was still significant iron impurity. WDS analysis (FIG.6 and FIG.7) shows that the iron is associated with sulfur.

[0134] It would be expected that this FeS2 is not removed upon exposure to chlorine or via other prior art methods; however, the present invention allows for its removal. When the CNT the sample is reduced under hydrogen, the FeS2 impurity is reduced to Fe0 (equation 2), which does react with chlorine in the subsequent step and hence allows the residual iron to be removed.
FeS2 + 2 H2 ? Fe0 + 2 H2S (2)

[0135] Furthermore, it has been found that commercial CNT fibers contain traces of calcium compounds associated with water washing of the CNT fiber during processing. The prior art has not addressed removal of trace calcium from CNT materials. However, the advantage of the present invention is that it allows for the removal of calcium species as well as those of elements associated with the catalyst used to grow the CNTs.

[0136] During chlorine treatment the FeCl3 that is formed is flushed from the system (FIG.26); however, if any is retained within the CNT fibers an additional process step of heating under a flow of air or an inert gas such as argon or nitrogen may be added.

[0137] One advantage of the present invention over the prior art is that the CNTs in the sample are not damaged or functionalized. This means that all impurities are removed except for the CNTs.
[0138] The present invention is explained further in the following specific examples which are only by way of illustration and are not to be construed as limiting the scope of the invention.

EXAMPLES

[0139] The carbon nanotube fiber yarns (Roving, and single-ply YE-A10) were purchased from Nanocomp Technologies, Inc. Chlorine gas (>99.9% pure), argon gas (>99.998% pure), hydrogen gas (>99.8% pure), and two composite gas mixtures (5 vol% hydrogen/95 vol% nitrogen; 10 vol% hydrogen/90 vol% nitrogen) were purchased from Matheson Tri-Gas, Inc.

[0140] The fibers, and Roving in particular, were characterized using energy-dispersive X-ray spectroscopy (EDS), wavelength-dispersive X-ray spectroscopy (WDS), thermogravimetric analysis (TGA) in air, and resonant Raman spectroscopy. Careful analysis revealed that the fibers had an average residual iron content of ~15 wt% (in the form of ~21 wt% iron oxide). In addition, the fibers had a significant amount of amorphous (non-graphitic) carbon and YE-A10 also had ~1 wt% calcium introduced presumably via internal processing using hard water at Nanocomp Technologies, Inc. Approximately 25% of the residual iron was present as iron sulfide or iron oxide, with the oxidized Fe2+ form not easily removed relative to Fe0. The only difference in YE-A10 and Roving, as it pertains to this technology, is the additional calcium impurity in the former.

[0141] Example 1
A 1’ long section of Nanocomp YE-A10 was wrapped around a quartz spool holder and placed inside the hot zone of a Lindberg Blue 1” tube furnace. The system was evacuated under vacuum using a mechanical pump to -30” Hg equivalent pressure, and then argon gas was flown through such that the operating pressure was increased to -15” Hg equivalent. The furnace was switched on and set to 500 °C and, once steady state was achieved, the argon gas flow was turned off followed and chlorine gas was allowed to pass through for 30 minutes of continuous flow directed to a dry pump that is halogen-resistant. After 30 minutes, the chlorine flow was turned off and the sample allowed to cool to room temperature under vacuum and argon gas as during the heating stage. Once cooled, the sample was exposed to air and annealed to 350°C to remove any physically adsorbed FeCl3 on the surface of the fiber, then cooled again to room temperature before being removed from the reactor for characterization. The final mass residual by TGA was 10 wt%, compared to ~21 wt% for the control fiber.
[0142] Example 2
Same as above, except a new 1’ long sample of YE-A10 fiber was now isolated in the reactor and chlorine pumped to a positive pressure of 5 PSI. A single batch operation cycle of 30 minutes was used to test the effect of batch versus continuous flow reactions, and the mass residual by TGA was 12 wt%, showing that a single long batch process was not as efficient.
[0143] Example 3
Same as above, except with the use of two 15 min chlorination batch cycles instead of a single 30 min cycle. In between the cycles, chlorine was evacuated via vacuum. The fiber was then exposed to argon again for 5 minutes before the next cycle commenced. The TGA mass residual was 10 wt%, on par with the continuous flow setup but using a lot less chlorine gas.
[0144] Example 4
Same as Example 3, except with the use of three 10 min chlorination batch cycles instead of a single 30 min cycle. The TGA mass residual was 7 wt%, even better than the continuous flow setup but using a lot less chlorine gas. This shows that multiple batch cycles is the way to go, with the argon annealing between cycles helping expose more iron previously trapped under an amorphous carbon shell.
[0145] Example 5
Same as Example 4, except with the use of four 10 min chlorination batch cycles instead of a single 30 min cycle. The TGA mass residual was 6.8 wt%, not showing any improvement over three cycles of chlorination and argon annealing.
[0146] Example 6
A repeat experiment of Example 5, with the TGA mass residual being 6.5 wt%. This shows that three batch chlorination cycles is the optimum way to go at 500 °C.
[0147] Example 7
Same as Example 5, except at 600 °C instead of 500 °C. The efficacy of the purification process was found to be worse here, with the mass residual by TGA showing to be 9 wt%.
[0148] Example 8
Same as Example 4, except at 600 °C instead of 500 °C. The efficacy of the purification process was found to be worse here, with the mass residual by TGA showing to be 10 wt%. This shows that operation at 500 °C is better than at 600 °C, for three or four cycles.
[0149] Example 9
Same as Example 4, except at 400 °C instead of 500 °C. The efficacy of the purification process was found to be worse here, with the mass residual by TGA showing to be 12 wt%. This shows that operation at 500 °C is better than at 400 °C for the 3-cycle operation.
[0150] Example 10
Same as Example 4, except at 450 °C instead of 500 °C. The efficacy of the purification process was found to be worse here, with the mass residual by TGA showing to be 11 wt%. This shows that operation at 500 °C is better than at 450 °C for the 3-cycle operation.
[0151] Example 11
Same as Example 4, except at 550 °C instead of 500 °C. The efficacy of the purification process was found to be worse here, with the mass residual by TGA showing to be 9 wt%. This shows that operation at 500 °C is better than at 550 °C for the 3-cycle operation, and is the optimum temperature for the process.
[0152] Example 12
Same as Example 4, except the initial heating stage took place in a reducing environment of 100% hydrogen. The oxidized iron was reduced to Fe0, which helped remove the ~25% iron that was not being removed by chlorination alone, however the carbon nanotubes were also getting affected and unwrapped.
[0153] Example 13
Same as Example 12, except with a 5% H2/95% N2 gas mixture. The CNTs remained intact, and the final mass residual by TGA was 4.5 wt%. This shows that a reducing environment is needed to fully reduce all iron to Fe0, which is then removed by chlorination. The concentration of hydrogen is also key here to retain the CNTs as-is.
[0154] Example 14
Same as Example 13, except with a 10% H2/90% N2 gas mixture. The CNTs remained intact, and the final mass residual by TGA was 5 wt%. Prior studies in the lab show that an H2 concentration ranging from 5-20% by volume in an inert gas carrier is the optimal range of operation.
[0155] Example 15
Same as Example 13, except with the use of the 5% H2/95% N2 gas mixture even in-between chlorination cycles instead of argon. The TGA mass residual was 2.5 wt%.
[0156] Example 16
A repeat of Example 15, with the TGA mass residual being 2.2 wt% for another fiber sample. Further analysis revealed that iron and calcium were both removed now.
Table 2 enlists calcium content from the samples.
Sample Ca content (wt%)
CNT fiber (control) 0.95 ±0.1 wt%
TGA residue of CNT fiber (control) 24.2 ±4.5 wt%
TGA residue of purified CNT fiber 1.1 ±0.22 wt%
Table 2
[0157] Example 17
Same as Example 15, except with Roving instead of YE-A10. Roving has less calcium to begin with, and the TGA mass residual was 1.6 wt%.
Table 3 enlists composition of the purified CNT fiber with respect to control sample of CNT fiber.
Sample CNT fiber (control) Purified CNT fiber
TGA residue (wt%) 24.2 1.6
Fe from EDS (wt%) 14.4 1.1
S from EDS (wt%) 0.9 0.04
Ca from EDS (wt%) ~0.1 ~0
Table 3
[0158] Example 18
A repeat of example 17, the TGA mass residual was 1.9 wt%.
[0159] Example 19
A repeat of example 17, the TGA mass residual was 1.7 wt%.
[0160] Example 20
A 1 cm piece was cut of YE-A10 fiber was placed in an Omicron LT nanoprobe and two tungsten probes approached to contact fiber with a separation of 20 µm. A voltage was swept from -1 V to 1 V and the current measurement and 4 separate locations were measured over the 1 cm fiber. The resistance was calculated at 0.5 V for each of the 4 locations and the average resistance at 0.5 V was calculated to be 1.28×106 ±3.07×105 ?. Uncertainty calculated using standard error.
[0161] Example 21
YE-A10 fiber was cut to 30 cm length and annealed a tube furnace (base pressure 9×10-3 mbar) for one hour at 100 °C. A 1 cm piece was cut from the annealed fiber was placed in an Omicron LT Nanoprobe (base pressure 1×10-9 mbar) and two tungsten probes approached to contact fiber with a separation of 20 µm. A voltage was swept from -1 V to 1 V and the current measured and 4 separate locations over the 1 cm fiber. The resistance was calculated at 0.5 V for each of the 4 locations and the average resistance at 0.5 V was calculated to be 7.72×105 ±1.61×105 ?. Uncertainty calculated using standard error.
[0162] Example 22
YE-A10 fiber was cut to 30 cm length and annealed a tube furnace (base pressure 9×10-3 mbar) for one hour at 200 °C. A 1 cm piece was cut from the annealed fiber was placed in an Omicron LT Nanoprobe (base pressure 1×10-9 mbar) and two tungsten probes approached to contact fiber with a separation of 20 µm. A voltage was swept from -1 V to 1 V and the current measured and 4 separate locations over the 1 cm fiber. The resistance was calculated at 0.5 V for each of the 4 locations and the average resistance at 0.5 V was calculated to be 2.04×104 ±1.43×104 ?. Uncertainty calculated using standard error.
[0163] Example 23
YE-A10 fiber was cut to 30cm length and annealed a tube furnace (base pressure 9×10-3 mbar) for one hour at 300 °C. A 1 cm piece was cut from the annealed fiber was placed in an Omicron LT Nanoprobe (base pressure 1×10-9 mbar) and two tungsten probes approached to contact fiber with a separation of 20 µm. A voltage was swept from -1 V to 1 V and the current measured and 4 separate locations over the 1 cm fiber. The resistance was calculated at 0.5 V for each of the 4 locations and the average resistance at 0.5 V was calculated to be 7.24×103 ± 4.1×103 ?. Uncertainty calculated using standard error.
[0164] Example 24
YE-A10 fiber was cut to 30cm length and annealed a tube furnace (base pressure 9×10-3 mbar) for one hour at 400 °C. A 1 cm piece was cut from the annealed fiber was placed in an Omicron LT Nanoprobe (base pressure 1×10-9 mbar) and two tungsten probes approached to contact fiber with a separation of 20 µm. A voltage was swept from -1 V to 1 V and the current measured and 4 separate locations over the 1 cm fiber. The resistance was calculated at 0.5 V for each of the 4 locations and the average resistance at 0.5 V was calculated to be 1.63×103 ± 7.73×102 ?. Uncertainty calculated using standard error.
[0165] Example 25
YE-A10 fiber was cut to 30cm length and annealed a tube furnace (base pressure 9×10-3 mbar) for one hour at 500 °C. A 1 cm piece was cut from the annealed fiber was placed in an Omicron LT Nanoprobe (base pressure 1×10-9 mbar) and two tungsten probes approached to contact fiber with a separation of 20 µm. A voltage was swept from -1 V to 1 V and the current measured and 4 separate locations over the 1 cm fiber. The resistance was calculated at 0.5 V for each of the 4 locations and the average resistance at 0.5 V was calculated to be 3.63×103 ±1.37×103 ?. Uncertainty calculated using standard error.
[0166] Example 26
YE-A10 fiber was cut to 30cm length and annealed a tube furnace (base pressure 9×10-3 mbar) for one hour at 600 °C. A 1 cm piece was cut from the annealed fiber was placed in an Omicron LT Nanoprobe (base pressure 1×10-9 mbar) and two tungsten probes approached to contact fiber with a separation of 20 µm. A voltage was swept from -1 V to 1 V and the current measured and 4 separate locations over the 1 cm fiber. The resistance was calculated at 0.5 V for each of the 4 locations and the average resistance at 0.5 V was calculated to be 2.22×105 ±9.71×104 ?. Uncertainty calculated using standard error.
[0167] Example 27
YE-A10 fiber was cut to 30cm length and annealed a tube furnace (base pressure 9×10-3 mbar) for one hour at 700 °C. A 1 cm piece was cut from the annealed fiber was placed in an Omicron LT Nanoprobe(base pressure 1×10-9 mbar) and two tungsten probes approached to contact fiber with a separation of 20 µm. A voltage was swept from -1 V to 1 V and the current measured and 4 separate locations over the 1 cm fiber. The resistance was calculated at 0.5 V for each of the 4 locations and the average resistance at 0.5 V was calculated to be 6.53×105 ±3.72×105 ?. Uncertainty calculated using standard error.
[0168] Example 28
YE-A10 fiber was cut to 30cm length and annealed a tube furnace (base pressure 9×10-3 mbar) for one hour at 800 °C. A 1 cm piece was cut from the annealed fiber was placed in an Omicron LT Nanoprobe (base pressure 1×10-9 mbar) and two tungsten probes approached to contact fiber with a separation of 20 µm. A voltage was swept from -1 V to 1 V and the current measured and 4 separate locations over the 1 cm fiber. The resistance was calculated at 0.5 V for each of the 4 locations and the average resistance at 0.5 V was calculated to be 2.34×106 ±1.76×106 ?. Uncertainty calculated using standard error.
[0169] Example 29
YE-A10 fiber was cut to 1 cm length placed in an Omicron LT Nanoprobe (base pressure 1×10-9 mbar), annealed using a stage heat for 1 hour to 100 °C, allowed to cool and two tungsten probes approached to contact fiber with a separation of 20 µm. A voltage was swept from -1 V to 1 V and the current measured and 8 separate locations over the 1 cm fiber. The resistance was calculated at 0.5 V for each of the 8 locations and the average resistance at 0.5 V was calculated to be 1.26×106 ±2.49×105 ?. Uncertainty calculated using standard error.
[0170] Example 30
YE-A10 fiber was cut to 1 cm length placed in an Omicron LT Nanoprobe (base pressure 1×10-9 mbar), annealed using a stage heat for 1 hour to 200 °C, allowed to cool and two tungsten probes approached to contact fiber with a separation of 20 µm. A voltage was swept from -1 V to 1 V and the current measured and 8 separate locations over the 1 cm fiber. The resistance was calculated at 0.5 V for each of the 8 locations and the average resistance at 0.5 V was calculated to be 1.42×106 ±5.37×105 ?. Uncertainty calculated using standard error.
[0171] Example 31
YE-A10 fiber was cut to 1 cm length placed in an Omicron LT Nanoprobe (base pressure 1×10-9 mbar), annealed using a stage heat for 1 hour to 300 °C, allowed to cool and two tungsten probes approached to contact fiber with a separation of 20 µm. A voltage was swept from -1 V to 1 V and the current measured and 8 separate locations over the 1 cm fiber. The resistance was calculated at 0.5 V for each of the 8 locations and the average resistance at 0.5 V was calculated to be 5.06×105 ±1.93×105 ?. Uncertainty calculated using standard error.
[0172] Example 32
YE-A10 fiber was cut to 1 cm length placed in an Omicron LT Nanoprobe (base pressure 1×10-9 mbar), annealed using a stage heat for 1 hour to 400 °C, allowed to cool and two tungsten probes approached to contact fiber with a separation of 20 µm. A voltage was swept from -1 V to 1 V and the current measured and 8 separate locations over the 1 cm fiber. The resistance was calculated at 0.5 V for each of the 8 locations and the average resistance at 0.5 V was calculated to be 1.25×105 ±5.57×104 ?. Uncertainty calculated using standard error.
[0173] Example 33
YE-A10 fiber was cut to 1 cm length placed in an Omicron LT Nanoprobe (base pressure 1×10-9 mbar), annealed using a stage heat for 1 hour to 500 °C, allowed to cool and two tungsten probes approached to contact fiber with a separation of 20 µm. A voltage was swept from -1 V to 1 V and the current measured and 4 separate locations over the 1 cm fiber. The resistance was calculated at 0.5 V for each of the 4 locations and the average resistance at 0.5 V was calculated to be 2.98×103 ±4.05×103 ?. Uncertainty calculated using standard error.
[0174] Example 34
YE-A10 fiber was cut to 1 cm length placed in an Omicron LT Nanoprobe (base pressure 1×10-9 mbar), annealed using a stage heat for 1 hour to 600 °C, allowed to cool and two tungsten probes approached to contact fiber with a separation of 20 µm. A voltage was swept from -1 V to 1 V and the current measured and 8 separate locations over the 1 cm fiber. The resistance was calculated at 0.5 V for each of the 8 locations and the average resistance at 0.5 V was calculated to be 2.51×105 ±6.88×104 ?. Uncertainty calculated using standard error.
[0175] Example 35
YE-A10 fiber was cut to 1 cm length placed in an Omicron LT Nanoprobe (base pressure 1×10-9 mbar), annealed using a stage heat for 1 hour to 700 °C, allowed to cool and two tungsten probes approached to contact fiber with a separation of 20 µm. A voltage was swept from -1 V to 1 V and the current measured and 8 separate locations over the 1 cm fiber. The resistance was calculated at 0.5 V for each of the 8 locations and the average resistance at 0.5 V was calculated to be 1.01×106 ±5.11×105 ?. Uncertainty calculated using standard error.
[0176] Example 36
YE-A10 fiber was cut to 1 cm length placed in an Omicron LT Nanoprobe (base pressure 1×10-9 mbar), annealed using a stage heat for 1 hour to 800 °C, allowed to cool and two tungsten probes approached to contact fiber with a separation of 20 µm. A voltage was swept from -1 V to 1 V and the current measured and 8 separate locations over the 1 cm fiber. The resistance was calculated at 0.5 V for each of the 8 locations and the average resistance at 0.5 V was calculated to be 3.17×106 ±4.21×105 ?. Uncertainty calculated using standard error.
[0177] Example 37
A 1 cm piece was cut of YE-A10 fiber was placed in an Omicron LT nanoprobe and two tungsten probes approached to contact fiber with a separation of 20 µm. A voltage was swept from -2 V to 2 V and the current measurement and 4 separate locations were measured over the 1 cm fiber. The resistance was calculated at 0.5 V for each of the 4 locations and the average resistance at 0.5 V was calculated to be 1.39×106 ±3.34×105 ?. Uncertainty calculated using standard error.
[0178] Example 38
A 1 cm piece was cut of YE-A10 fiber was placed in an Omicron LT nanoprobe and two tungsten probes approached to contact fiber with a separation of 20 µm. A voltage was swept from -3 V to 3 V and the current measurement and 4 separate locations were measured over the 1 cm fiber. The resistance was calculated at 0.5 V for each of the 4 locations and the average resistance at 0.5 V was calculated to be 2.63×103 ±1.41×103 ?. Uncertainty calculated using standard error.
[0179] Example 39
A 1 cm piece was cut of YE-A10 fiber was placed in an Omicron LT nanoprobe and two tungsten probes approached to contact fiber with a separation of 20 µm. A voltage was swept from -4 V to 4 V and the current measurement and 4 separate locations were measured over the 1 cm fiber. The resistance was calculated at 0.5 V for each of the 4 locations and the average resistance at 0.5 V was calculated to be 1.69×103 ±3.43×102 ?. Uncertainty calculated using standard error.
[0180] Example 40
A 1 cm piece was cut of YE-A10 fiber was placed in an Omicron LT nanoprobe and two tungsten probes approached to contact fiber with a separation of 20 µm. A voltage was swept from -5 V to 5 V and the current measurement and 4 separate locations were measured over the 1 cm fiber. The resistance was calculated at 0.5 V for each of the 4 locations and the average resistance at 0.5 V was calculated to be 1.24×103 ±7.38×101 ?. Uncertainty calculated using standard error.
[0181] Example 41
A 1 cm piece was cut of YE-A10 fiber was placed in an Omicron LT nanoprobe and two tungsten probes approached to make contact with fiber with a separation of 20 µm. A voltage was swept from -6 V to 6 V and the current measurement and 4 separate locations were measured over the 1 cm fiber. The resistance was calculated at 0.5 V for each of the 4 locations and the average resistance at 0.5 V was calculated to be 6.43×109 ±2.42×1010 ?. Uncertainty calculated using standard error.
[0182] Example 42
A 1 cm piece was cut of YE-A10 fiber was placed in an Omicron LT nanoprobe and two tungsten probes approached to contact fiber with a separation of 20 µm. A voltage was swept from -7 V to 7 V and the current measurement and 4 separate locations were measured over the 1 cm fiber. The resistance was calculated at 0.5 V for each of the 4 locations and the average resistance at 0.5 V was calculated to be 7.44×108 ±6.56×108 ?. Uncertainty calculated using standard error.
[0183] While specific language has been used to describe the disclosure, any limitations arising on account of the same are not intended. As would be apparent to a person skilled in the art, various working modifications may be made to the method in order to implement the inventive concept as taught herein.

[0184] The figures and the foregoing description give examples of embodiments. Those skilled in the art will appreciate that one or more of the described elements may well be combined into a single functional element. Alternatively, certain elements may be split into multiple functional elements. Elements from one embodiment may be added to another embodiment. Moreover, the actions of any flow diagram need not be implemented in the order shown; nor do all of the acts need to be necessarily performed. Also, those acts that are not dependent on other acts may be performed in parallel with the other acts. The scope of embodiments is by no means limited by these specific examples.