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Enhanced Electrical Conductivity in Extruded Single-Wall Carbon Nanotube Wires from Modified Coagulation Parameters and Mechanical Processing Andrew R. Bucossi,†,‡ Cory D. Cress,§ Christopher M. Schauerman,‡ Jamie E. Rossi,‡,∥ Ivan Puchades,‡,∥ and Brian J. Landi*,‡,∥ †
Department of Microsystems Engineering, ‡NanoPower Research Laboratory, and ∥Department of Chemical Engineering, Rochester Institute of Technology, Rochester, New York 14623, United States § Electronics Science & Technology Division, U.S. Naval Research Laboratory, Washington, DC, 20375, United States ABSTRACT: Single-wall carbon nanotubes (SWCNTs) synthesized via laser vaporization have been dispersed using chlorosulfonic acid (CSA) and extruded under varying coagulation conditions to fabricate multifunctional wires. The use of high purity SWCNT material based upon established purification methods yields wires with highly aligned nanoscale morphology and an over 4× improvement in electrical conductivity over as-produced SWCNT material. A series of eight liquids have been evaluated for use as a coagulant bath, and each coagulant yielded unique wire morphology based on its interaction with the SWCNT-CSA dispersion. In particular, dimethylacetamide as a coagulant bath is shown to fabricate highly uniform SWCNT wires, and acetone coagulant baths result in the highest specific conductivity and tensile strength. A 2× improvement in specific conductivity has been measured for SWCNT wires following tensioning induced both during extrusion via increased coagulant bath depth and during solvent evaporation via mechanical strain, over that of as-extruded wires from shallower coagulant baths. Overall, combination of the optimized coagulation parameters has yielded acid-doped wires with the highest reported room temperature electrical conductivities to date of 4.1−5.0 MS/m and tensile strengths of 210−250 MPa. Such improvements in bulk electrical conductivity can impact the adoption of metal-free, multifunctional SWCNT materials for advanced cabling architectures. KEYWORDS: single-wall carbon nanotubes, chlorosulfonic acid, dispersion, extrusion, coagulation, electrical conductivity conductivities as high as observed in individual CNTS7 due to CNT-CNT junctions, poor alignment, and lack of free carriers in some CNT types.8−10 Some approaches to increase the conductivity of bulk CNT structures have focused on using chemical treatments to enhance the intrinsic conductivity of individual CNTs11 and to decrease conduction barriers between CNTs in a network.12 Recent studies using iodine doping of CNT bundles have achieved room temperature electrical conductivities of 1 × 107 S/m.11 The conductivity of bulk CNT conductors has also been successfully increased utilizing methods to alter the network morphology regarding CNT alignment and inter-CNT contact through mechanical densification or through mechanical straining techniques.12,13 The highest electrical conductivity from the chemical doping and densification approach in a large diameter CNT wire (>100 μm) has been 1.3 × 106 S/m.12 Alternatively, acid dispersion and extrusion of CNTs have provided a means to influence alignment of CNTs on a nanoscale level,14,15 and
1. INTRODUCTION Macroscopic assemblies of carbon nanotubes (CNTs) in wire format are being considered for a variety of applications including power and data transmission for both aerospace and terrestrial scenarios.1−3 Benefits for adoption of CNT wires include superior flexure tolerance, weight savings, and corrosion resistance over their traditionally used metal counterparts.1 For example, CNT conductors have been successfully used as metal replacements in RG-402 and RG-58 coaxial data transmission cables.1,2,4 Improvements in the attenuation of these cables derived from enhanced bulk CNT electrical conductivity are due to CNT type selection and chemical doping. However, limitations in performance for the center conductor in a CNT coaxial cable and direct current power cables still exist and warrant further advancement in the bulk CNT electrical conductivity.2 The theoretical electrical conductivity for individual metallic CNTs has been reported to be 3.0 × 108 S/m.5 In practice, the electrical conductivity at room temperature for individual CNTs has been measured up to 9.7 × 107 S/m, which is an increase over the conductivity for bulk copper of 5.9 × 107 S/m.6 However, bulk CNT conductors have yet to demonstrate © XXXX American Chemical Society
Received: September 14, 2015 Accepted: November 25, 2015
A
DOI: 10.1021/acsami.5b08668 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
depth of the coagulant bath, and the middle third of each segment was cut and mounted on glass slides for characterization. Electrical resistance was measured using an inline four-point probe connected to a National Instruments NI PXI-5652 source/measure unit and an NI PXI-4071 digital multimeter at room temperature (∼20 °C). The wire mass and length were measured using a MettlerToledo XP2U ultramicrobalance with 0.1 μg resolution and a Fowler caliper with a resolution of 0.01 mm, respectively. The length of samples used for mass and electrical characterization was 4−5 cm. SEM images of SWCNT materials prior to dispersion and extruded wires were taken on a field emission Hitachi S900 with an accelerating voltage of 2 kV and an emission current of 10 μA. Tensile testing of wire segments was conducted on a TA Instruments Q800 dynamic mechanical analyzer using a film tension clamp on samples with 1−2 cm gauge length by applying a constant force ramp rate of 0.01 N/min at 20 °C to determine the ultimate tensile strength at failure.
these techniques can be expanded to use various types of CNT starting materials.16,17 Wires formed by extrusion of CNTchlorosulfonic acid (CSA) dispersions (a wet-spinning technique) have the highest reported bulk conductivity values for manufactured wires (∼10 μm in diameter with acid doping) of 3 × 106 S/m at room temperature, and after subsequent doping with iodine the conductivity increased to 5.5 × 106 S/m (also at room temperature).17 Thus, the combination of chemical doping and extruded wires displays tremendous promise toward enhancing the overall conductivity and motivates optimization of CNT wire extrusion. In the present work, an alternative source of low defect single-wall carbon nanotubes (SWCNTs) from laser vaporization synthesis is utilized for wet spinning SWCNT wires. Typically, the CNT starting material for wet spun wires has been either HiPco SWCNTs or few-walled 5 μm long CNTs.14,16,17 High conductivities have been measured on papers of SWCNTs produced by laser vaporization,1 making them a desirable and unexplored material of choice for wet spinning SWCNT wires. The use of a new starting material motivates examination of the in situ and postprocessing parameters of wire extrusion. In this report, wires are fabricated from CSA dispersions of both as-produced and purified SWCNT starting materials. The in situ parameters of coagulant composition and coagulant bath depth are evaluated for their impact on the resulting wire tensile strength and electrical conductivity. Postprocessing is also explored using mechanical strain during drying to realize SWCNT wires with high room temperature electrical conductivity.
3. RESULTS AND DISCUSSION Laser-synthesized SWCNTs were selected for this study due to the established control over purity,19,20 high reported intrinsic conductivity,1 and for their unexplored potential for wet spun CNT wires. Carbonaceous and metal catalyst particles are two types of impurities commonly found in SWCNT samples which result from their synthesis.20 Two sets of wires were extruded for comparison to determine the effects of impurities in a SWCNT dispersion on the electrical properties of the resulting wires. The first set of wires was extruded from a dispersion containing 38.3 mg AP-SWCNTs per mL CSA (i.e., 2.1 wt % AP-SWCNT material in dispersion). An SEM of the APSWCNTs used to create this dispersion is presented in Figure 1a. AP-SWCNT samples contain 20−30% SWCNTs by carbonaceous mass.19 Small bundles of SWCNTs can be seen in the SEM image as well as a large volume fraction of impurities. The second set of wires was extruded from a dispersion containing 40.0 mg purified SWCNTs per mL CSA (i.e., 2.2 wt % purified SWCNT material in dispersion). An SEM image of the purified SWCNTs used to create the second dispersion is shown in Figure 1b, demonstrating excellent SWCNT quality and a dramatic reduction in the amount of impurities over the AP-SWCNT sample. Purified SWCNT samples contain >99% SWCNTs by carbonaceous mass.19 Wires were extruded from both dispersions into 20 °C acetone coagulant baths which had a depth of 4.5 cm. SEM images of wires extruded from AP-SWCNTs and purified SWCNTs are shown in panels c and d of Figure 1, respectively. The presence of carbonaceous impurities in the wires extruded from the APSWCNT dispersion is evident in the high magnification SEM images (Figure 1e) since bundles of SWCNTs cannot be seen on the wire surface. In comparison, well resolved bundles of SWCNTs are observed on the wires extruded from the purified SWCNT dispersion (Figure 1f). The electrical performance of the wires was quantitatively compared by measuring the resistance per length (R/L) which is related to electrical conductivity by cross sectional area and to specific conductivity by the wire’s mass per length (M/L), as described in eq 1
2. EXPERIMENTAL SECTION SWCNT soot was synthesized using the laser vaporization technique.18,19 Graphite targets pressed at 20,000 psi and containing 3 wt % Ni (submicron diameter) and 3 wt % Co (
