- Email: [email protected]
Thermal and electrical conductivity of array-spun multi-walled carbon nanotube yarns
CARBON
5 0 ( 2 0 1 2 ) 2 4 4 –2 4 8
Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/carbon
Thermal and electrical conductivity of array-spun multi-walled carbon nanotube yarns Michael B. Jakubinek a,b,c, Michel B. Johnson b, Mary Anne White b,c,d,*, Chaminda Jayasinghe e, Ge Li e, Wondong Cho e, Mark J. Schulz f, Vesselin Shanov
e
a
Steacie Institute for Molecular Sciences, National Research Council Canada, Ottawa, ON, Canada K1A 0R6 Institute for Research in Materials, Dalhousie University, Halifax, NS, Canada B3H 4R2 c Department of Physics, Dalhousie University, Halifax, NS, Canada B3H 4R2 d Department of Chemistry, Dalhousie University, Halifax, NS, Canada B3H 4R2 e Department of Chemical and Materials Engineering, University of Cincinnati, Cincinnati, OH 45221-0012, USA f Department of Mechanical Engineering, University of Cincinnati, Cincinnati, OH 45221-0072, USA b
A R T I C L E I N F O
A B S T R A C T
Article history:
The electrical resistivity of CNT yarns of diameters 10–34 lm, spun from multi-walled car-
Received 2 February 2011
bon nanotube arrays, have been determined from 2 to 300 K in magnetic fields up to 9 T.
Accepted 17 August 2011
The magnetoresistance is large and negative at low temperatures. The thermal conductiv-
Available online 25 August 2011
ity also has been determined, by parallel thermal conductance, from 5 to 300 K. The roomtemperature thermal conductivity of the 10 lm yarn is (60 ± 20) W m1 K1, the highest measured result for a CNT yarn to date. The thermal and electrical conductivities both decrease with increasing yarn diameter, which is attributed to structural differences that vary with the yarn diameter. 2011 Elsevier Ltd. All rights reserved.
1.
Introduction
Individual carbon nanotubes (CNTs) exhibit excellent mechanical, electrical and thermal properties, but the realization of many potential applications (e.g., responsive materials [1]) requires that these properties be translated to macroscopic materials. CNT fibers, also called yarns, which are produced by liquid- or solid-state spinning processes, are particularly promising for lightweight, strong, and thermally and electrically conductive materials as the CNTs can be highly aligned along the fiber axis. Recent reviews describe advances in CNT fiber production and properties [1–3]. Strengths of CNT fibers are typically one to two orders of magnitude lower than individual nanotubes and increase with CNT length [3]. Electrical conductivities, r, vary from <10 to >1000 S cm1, and the effectiveness of the coalescence, as well as doping, appear to be critical factors [3]. In comparison,
thermal conductivity (j) has been the subject of few studies. A thermal conductivity of 26 W m1 K1 was reported for yarn drawn from a 300 lm tall MWCNT array [4], slightly larger than reported for SWCNT fibers extruded from a super-acid suspension of much shorter CNTs [5]. We produce multi-walled carbon nanotube (MWCNT, diameter ca. 15 nm) yarns by spinning from the sides of vertically aligned MWCNT arrays, 500 lm high (Fig. 1a, see supplementary material online for a video of the spinning) [1]. The arrays are grown by water-assisted CVD [6,7]. Recently, we reported the thermal and electrical conductivities of these arrays: j 1 W m1 K1 and r 10 S cm1, low in part due to the low density of the as-grown arrays [8]. The spun yarns have a density of about 0.9 g cm3 and a twist of about 1 · 104 m1 (slightly lower twist for larger diameter, see Figs. 1b and c). CNT yarns have been reported to have much higher electrical conductivity than arrays [1], and are
* Corresponding author. E-mail address: [email protected] (M.A. White). 0008-6223/$ - see front matter 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2011.08.041
CARBON
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245
Fig. 1 – (a) Spinning CNT yarn from array. (b) SEM image of 10 lm diameter CNT yarn. (c) SEM image of 34 lm diameter CNT yarn. The twist of the 10 lm yarn is 1.9 ± 0.5 · 104 m1 and the outer twist angle is 20. The twist of the 34 lm yarn is 1.2 ± 0.5 · 104 m1 and the outer twist angle is 30.
expected to have improved electrical and thermal conductivities due in part to their higher density. Aliev et al. [9] recently described the importance of CNT length and overlap in optimizing thermal conductivity of CNT bundles. The long length of the MWCNTs used in the present investigation should reduce the effect of interfacial thermal resistance between CNTs in comparison to earlier reports. Here we report characterization of the thermal and electrical conductivity of MWCNT yarns. Magnetoresistance and the effect of yarn diameter also are explored.
2.
Electrical conductivity
2.1.
Measurement methods
Electrical resistivity (q), magnetoresistance (Dq/q), and iV curves were measured using a Physical Property Measurement System (PPMS; Quantum Design, San Diego, CA) for T = 2–300 K and magnetic field (parallel to the yarn axis) 0– 9 T. A 2 cm section of yarn was suspended across four leads (Fig. 2b, inset), held with DuPont 4929 N silver paint, and the excess was cut off. Temperature and field dependences of the resistivity were measured with i = 0.1 mA (Ohmic region of iV curve) and f = 93 Hz, iV curve parameters ±3 mA and f = 60 Hz. Measurements at 300 K before and after cooling showed stable resistivity.
2.2.
Electrical conductivity results and discussion
Fig. 2 shows the electrical and magnetoresistance results. The room temperature r compares favorably to other reports. Although higher r is reported for CNT fibers from super-acid dispersions, they are acid-doped and r drops by an order of magnitude after annealing [10]. Interestingly, the smaller diameter yarn also has r (at 300 K) exceeding the scaled MWCNT value from arrays similar to those used to produce this MWCNT yarn [8]. The iV curves show Ohmic behavior at 300 and 100 K and non-linearity at 10 and 2 K, which also is observed in individual CNTs and is consistent with a fluctuation-assisted tunneling description of conduction in CNT materials [11]. The magnetoresistance at room temperature is small with peaks evident around 2 T and 4.5 T for the d = 34 lm and 10 lm yarns, respectively. At low temperature (10 K and 2 K) Dq/q is negative, similar to that previously reported for MWCNT yarns oriented perpendicular to the field [12]. The 34 lm fiber might be approaching a minimum in Dq/q, observed for SWCNT fibers spun from a superacid suspension [13]. The difference in r with yarn diameter was explored in more detail using many yarns produced from a single MWCNT array. These yarns show the same trend of decreasing conductivity with increasing diameter (Fig. 3). This trend is attributed to structural differences between small and large
246
CARBON
5 0 ( 2 0 1 2 ) 2 4 4 –2 4 8
Fig. 3 – Effect of yarn diameter on electrical conductivity at 300 K.
properties of the yarn. Stress and conduction properties are more uniform across the smaller diameter yarn.
Fig. 2 – (a) Electrical conductivity for 10 lm and 34 lm diameter MWCNT yarns. (b) Selected iV curves for the 10 lm yarn (inset shows the measurement platform). (c) Magnetoresistance of the MWCNT yarns at 2 K, 10 K, and 300 K.
diameter yarns, primarily decreased CNT migration length (i.e., the longitudinal distance over which a CNT shifts from the yarn surface to the interior and back [14]). We also observe some decrease in twist angle, leading to reduced alignment, in the larger diameter yarn (Fig. 1). Our results indicate that a smaller diameter yarn has better electrical (and thermal) conductivity than larger diameter yarn. The smaller diameter yarn has more turns per unit length for a given helix angle as compared to a larger diameter yarn. More turns increases the radial grip, which makes the yarn tighter, thereby improving the physical properties. Furthermore, SEM (Fig. 1) shows that the outer fibers on a larger diameter yarn are more loosely bound, and therefore less effective in contributing to the
3.
Thermal conductivity
3.1.
Measurement method
Thermal conductivity measurements on CNT fibers are complicated by the small sample size and the fact that the sample cannot support heaters or thermometers for 1D steady-state measurements. Others have addressed this by using comparative [5,15] or self-heating [4] methods. We employed the parallel thermal conductance (PTC) method [16,17], essentially a 1D steady-state measurement except that the background conductance, usually minimal in a steady-state measurement, now is the major component that is precisely determined and subtracted. PTC measurement requires three measurement runs: (1) background conductance (Kb) without a sample in place; (2) total conductance (Kt) with the sample in place; and (3) with the sample connected to the hot side only (K3). Measurement of K3 is used to correct for radiation loss [16]. The sample conductance (Ks) is then calculated as 1 Ks ¼ Kt Kb ðK3 Kb Þ: 2
ð1Þ
Here we used a custom sample holder built onto a PPMS resistivity puck (Fig. 4a). The sample holder consisted of two brass islands, one sinked to the base and the other supported by an acetate sheet. A heater (120 X strain gage with most packaging removed) was attached to the latter island and wired using 0.00300 alumel wire (4 cm, bare); a thermocouple (constantan–chromel–constantan, 0.00100 diameter, bare) was used to measure the temperature difference between the islands. This configuration reduces Kb. The MWCNT yarn was placed across the gap between the two islands and attached with DuPont 4929 N. Ks in all cases is approximately 10% of Kb or greater as recommended [16]. To obtain sufficient Ks it was necessary to put 13–20 segments of yarn in parallel across the gap. An additional advantage is that the measurement effectively probes 60–80 mm of yarn rather than a single short piece. The heater current was applied by a variable DC current source. Current was measured using an external meter (HP34401A) and controlled to produce desired
CARBON
5 0 (2 0 1 2) 2 4 4–24 8
247
Fig. 4 – (a) PTC measurement platform along with images showing 34 lm diameter MWCNT yarn segments in parallel. (b) Example data showing a single conductance measurement at 300 K. (c) Thermal conductances from measurement of thirteen 34 lm diameter yarns in parallel. (d) Thermal conductivity of MWCNT yarns (see Supplementary information for data tables). Preliminary data for one set of yarns shown by solid symbols (j for 34 lm and for 10 lm) and more extensive data for different samples are shown with open symbols ( for 34 lm and for 10 lm), indicate the reproducibility of the data.
temperature gradients across the sample. The heater and thermocouple voltages were measured using external meters (HP3456A). These quantities were logged using LabView, which also calculates the temperature difference across the sample using NIST thermocouple tables. At each temperature at least three heater currents were applied and the measured conductance was determined as the slope of P vs. DT graphs. All thermal conductance measurements were conducted under vacuum (<104 Torr) to minimize the effects of convection.
3.2.
Thermal conductivity results and discussion
The thermal conductivity measurement and results are shown in Fig. 4. The thermal conductivity of MWCNT yarn is much higher than for the as-produced arrays, but comparable to the scaled MWCNT value [8]. The error bars include uncertainties in the measured conductances (from fits to DT vs. P) and yarn dimensions. For the larger yarn (Fig. 4c), Ks adds a sufficient thermal pathway above Kb that it can be extracted
through the PTC method except for T < 5 K (<50 K for the smaller yarn). For the 10 lm diameter yarn the room temperature j, (60 ± 20) W m1 K1, appears to be the highest measured result for a CNT yarn to date. However, we note that it remains low in comparison to individual MWCNTs (3000 W m1 K1 [18]) and that higher scaled values, where the measured result has been adjusted to a theoretically densified sample, have been reported for bulk CNT materials [19]. As observed for electrical conductivity, j(300 K) also decreased with increasing yarn diameter and, considering the larger uncertainty in j, the ratio j(10 lm)/j (34 lm) at 300 K (2.5) is in reasonable agreement with the ratio of their electrical conductivities (1.9). This similarity is not related to the mechanisms of electron and heat transport. We know that heat transport in MWCNT yarns is phonon-dominated because the effective Lorentz number, L = j/(rT), is 100· larger than for electronic heat conduction in metals, which is consistent with previous descriptions for individual CNTs [18] and assemblies, e.g., [8]. The similar effect of diameter on electrical and thermal conductivity supports the hypothesis
248
CARBON
5 0 ( 2 0 1 2 ) 2 4 4 –2 4 8
that structural differences between larger and smaller diameter yarns result in higher conductivities (both thermal and electrical) at smaller diameters, as described above in relation to electrical conductivity.
4.
Conclusion
In summary, the electrical conductivity of MWCNTyarns spun from tall MWCNT arrays is competitive with other high conductivity CNT fibers/yarns, and the room temperature thermal conductivity result for the 10 lm diameter MWCNT yarn is the highest measured value reported to our knowledge. While the properties of CNT yarn remain far below those of individual nanotubes and in some cases also below those of traditional high performance carbon fibers [1], this is a new kind of material that can offer multifunctional capabilities. It is very light, not brittle, and does not fatigue when performing. Furthermore, there is considerable potential for continued improvement. In this study the yarns were spun from relatively long (500 lm) MWCNTs; however, longer CNTs also typically have higher defect densities. Improved CNT quality, as well as further increase in length can be expected to lead to continued improvements. Simulations have suggested that optimizing CNT overlap and length, along with improved quality, could provide for MWCNT cables with j greater than copper and competitive with high-performance carbon fibers [9]. The present study identifies differences between larger and smaller diameter yarns, with higher electrical and thermal conductivities achieved for smaller diameter due to structural differences such as variation in the CNT migration length. This points to an additional strategy to improve CNT yarns, wherein the use of multi-ply yarns with smaller diameter strands could provide superior properties.
Acknowledgements The authors thank Prof. T. Tritt (Clemson U.) for helpful information on the PTC method, as well as R. Chen and P. Scallion (Dalhousie) for SEM. The work at Dalhousie was supported by NSERC, the Killam Trusts, and the Sumner Foundation, as well as CFI and other partners that fund the Facilities for Materials Characterization managed by the Institute for Research in Materials. The UC researchers acknowledge the financial support from NSF through Grant CMMI-07272500 with program officer Dr. Haris Doumanidis and from NCA&T through DURIP-ONP.
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.carbon.2011.08.041.
R E F E R E N C E S
[1] Jayasinghe C, Li W, Song Y, Abot JL, Shanov VN, Fialkova S, et al. Nanotube responsive materials. MRS Bulletin 2010;35(9):682–92.
[2] Joeslevich E, Dai H, Liu J, Hata K, Windle AH. Carbon nanotube synthesis and organization. Topics in Applied Physics 2008;111:101–64. [3] Behabtu N, Green MJ, Pasquali. Carbon nanotube-based neat fibers. Nano Today 2008;3(5-6):24–34. [4] Aliev AE, Guthy C, Zhang M, Fang S, Zakhidov AA, Fischer JE, et al. Thermal transport in MWCNT sheets and yarns. Carbon 2007;45(15):2880–8. [5] Zhou W, Vavro J, Guthy C, Winey KI, Fischer JE, Ericson LM, et al. Singe wall carbon nanotube fibers extruded from super-acid suspensions: preferred orientation, electrical, and thermal transport. J Appl Phys 2004;95(2):649–55. [6] Shanov VN, Gorton A, Yun Y-H, Schulz MJ. Composite catalyst and method for manufacturing carbon nanostructured materials. US Patent US2008/0095695 A1; 2008. [7] Yun YH, Shanov V, Tu Y, Subramaniam S, Schulz MJ. Growth mechanism of long aligned multiwall carbon nanotube arrays by water-assisted chemical vapor deposition. J Phys Chem B 2006;110:23920–5. [8] Jakubinek MB, White MA, Li G, Jayasinghe C, Cho W, Schulz MJ, et al. Thermal and electrical conductivity of tall, vertically aligned carbon nanotube arrays. Carbon 2010;48(13):3947–52. [9] Aliev AE, Lima MH, Silverman EM, Baughman RH. Thermal conductivity of multi-walled carbon nanotube sheets: radiation losses and quenching of phonon modes. Nanotechnol 2010;21:035709 (11 pages). [10] Ericson LM, Fan H, Peng H, Davis VA, Zhour W, Sulpizio J, et al. Macroscopic, neat, single-walled carbon nanotube fibers. Science 2004;305:1447–50. [11] Kaiser AB, Park YW. Current-voltage characteristics of conducting polymers and carbon nanotubes. Synth Met 2005;152:181–4. [12] Sheng L, Gao W, Ma X, Zhao X, Cao S, Zhang J. Electrical properties in magnetic field of macroscopic carbon nanotube objects. J Nanosci Nanotechnol 2010;10(6):4049–53. [13] Ksenevich VK, Odzaev VB, Martunas Z, Seliuta D, Valusis G, Galibert J, et al. Localization and nonlinear transport in single walled carbon nanotube fibers. J Appl Phys 2008;104(7):073724 (7 pages). [14] Zhang M, Atkinson KR, Baughman RH. Multifunctional carbon nanotube yarns by downsizing an ancient technology. Science 2004;306:1358–61. [15] Badaire S, Pichot V, Zakri C, Poulin P, Launois P, Vavro J, et al. Correlation of properties with preferred orientation in coagulated and stretch-aligned single-wall carbon nanotubes. J Appl Phys 2004;96(12):7509–13. [16] Zawilski BM, Littleton RT, Tritt TM. Description of the parallel thermal conductance technique for the measurement of the thermal conductivity of small diameter samples. Rev Sci Instrum 2001;72(3):1770–4. [17] Aaron K, Measurement of the in-plane thermal conductivity of single crystals by the modified parallel thermal conductance technique. Masters thesis in Physics. Clemson University; 2005. [18] Kim P, Shi L, Majumdar A, McEuen PL. Thermal transport measurements of individual multiwalled nanotubes. Phys Rev Lett 2001;87(21):215502 (4 pages). [19] Hone J, Llaguno MC, Nemes NM, Johnson AT, Fischer JE, Walters DA, et al. Electrical and thermal transport properties of magnetically aligned single wall carbon nanotube films. Appl Phys Lett 2000;77(5):666–8.
5 0 ( 2 0 1 2 ) 2 4 4 –2 4 8
Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/carbon
Thermal and electrical conductivity of array-spun multi-walled carbon nanotube yarns Michael B. Jakubinek a,b,c, Michel B. Johnson b, Mary Anne White b,c,d,*, Chaminda Jayasinghe e, Ge Li e, Wondong Cho e, Mark J. Schulz f, Vesselin Shanov
e
a
Steacie Institute for Molecular Sciences, National Research Council Canada, Ottawa, ON, Canada K1A 0R6 Institute for Research in Materials, Dalhousie University, Halifax, NS, Canada B3H 4R2 c Department of Physics, Dalhousie University, Halifax, NS, Canada B3H 4R2 d Department of Chemistry, Dalhousie University, Halifax, NS, Canada B3H 4R2 e Department of Chemical and Materials Engineering, University of Cincinnati, Cincinnati, OH 45221-0012, USA f Department of Mechanical Engineering, University of Cincinnati, Cincinnati, OH 45221-0072, USA b
A R T I C L E I N F O
A B S T R A C T
Article history:
The electrical resistivity of CNT yarns of diameters 10–34 lm, spun from multi-walled car-
Received 2 February 2011
bon nanotube arrays, have been determined from 2 to 300 K in magnetic fields up to 9 T.
Accepted 17 August 2011
The magnetoresistance is large and negative at low temperatures. The thermal conductiv-
Available online 25 August 2011
ity also has been determined, by parallel thermal conductance, from 5 to 300 K. The roomtemperature thermal conductivity of the 10 lm yarn is (60 ± 20) W m1 K1, the highest measured result for a CNT yarn to date. The thermal and electrical conductivities both decrease with increasing yarn diameter, which is attributed to structural differences that vary with the yarn diameter. 2011 Elsevier Ltd. All rights reserved.
1.
Introduction
Individual carbon nanotubes (CNTs) exhibit excellent mechanical, electrical and thermal properties, but the realization of many potential applications (e.g., responsive materials [1]) requires that these properties be translated to macroscopic materials. CNT fibers, also called yarns, which are produced by liquid- or solid-state spinning processes, are particularly promising for lightweight, strong, and thermally and electrically conductive materials as the CNTs can be highly aligned along the fiber axis. Recent reviews describe advances in CNT fiber production and properties [1–3]. Strengths of CNT fibers are typically one to two orders of magnitude lower than individual nanotubes and increase with CNT length [3]. Electrical conductivities, r, vary from <10 to >1000 S cm1, and the effectiveness of the coalescence, as well as doping, appear to be critical factors [3]. In comparison,
thermal conductivity (j) has been the subject of few studies. A thermal conductivity of 26 W m1 K1 was reported for yarn drawn from a 300 lm tall MWCNT array [4], slightly larger than reported for SWCNT fibers extruded from a super-acid suspension of much shorter CNTs [5]. We produce multi-walled carbon nanotube (MWCNT, diameter ca. 15 nm) yarns by spinning from the sides of vertically aligned MWCNT arrays, 500 lm high (Fig. 1a, see supplementary material online for a video of the spinning) [1]. The arrays are grown by water-assisted CVD [6,7]. Recently, we reported the thermal and electrical conductivities of these arrays: j 1 W m1 K1 and r 10 S cm1, low in part due to the low density of the as-grown arrays [8]. The spun yarns have a density of about 0.9 g cm3 and a twist of about 1 · 104 m1 (slightly lower twist for larger diameter, see Figs. 1b and c). CNT yarns have been reported to have much higher electrical conductivity than arrays [1], and are
* Corresponding author. E-mail address: [email protected] (M.A. White). 0008-6223/$ - see front matter 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2011.08.041
CARBON
5 0 (2 0 1 2) 2 4 4–24 8
245
Fig. 1 – (a) Spinning CNT yarn from array. (b) SEM image of 10 lm diameter CNT yarn. (c) SEM image of 34 lm diameter CNT yarn. The twist of the 10 lm yarn is 1.9 ± 0.5 · 104 m1 and the outer twist angle is 20. The twist of the 34 lm yarn is 1.2 ± 0.5 · 104 m1 and the outer twist angle is 30.
expected to have improved electrical and thermal conductivities due in part to their higher density. Aliev et al. [9] recently described the importance of CNT length and overlap in optimizing thermal conductivity of CNT bundles. The long length of the MWCNTs used in the present investigation should reduce the effect of interfacial thermal resistance between CNTs in comparison to earlier reports. Here we report characterization of the thermal and electrical conductivity of MWCNT yarns. Magnetoresistance and the effect of yarn diameter also are explored.
2.
Electrical conductivity
2.1.
Measurement methods
Electrical resistivity (q), magnetoresistance (Dq/q), and iV curves were measured using a Physical Property Measurement System (PPMS; Quantum Design, San Diego, CA) for T = 2–300 K and magnetic field (parallel to the yarn axis) 0– 9 T. A 2 cm section of yarn was suspended across four leads (Fig. 2b, inset), held with DuPont 4929 N silver paint, and the excess was cut off. Temperature and field dependences of the resistivity were measured with i = 0.1 mA (Ohmic region of iV curve) and f = 93 Hz, iV curve parameters ±3 mA and f = 60 Hz. Measurements at 300 K before and after cooling showed stable resistivity.
2.2.
Electrical conductivity results and discussion
Fig. 2 shows the electrical and magnetoresistance results. The room temperature r compares favorably to other reports. Although higher r is reported for CNT fibers from super-acid dispersions, they are acid-doped and r drops by an order of magnitude after annealing [10]. Interestingly, the smaller diameter yarn also has r (at 300 K) exceeding the scaled MWCNT value from arrays similar to those used to produce this MWCNT yarn [8]. The iV curves show Ohmic behavior at 300 and 100 K and non-linearity at 10 and 2 K, which also is observed in individual CNTs and is consistent with a fluctuation-assisted tunneling description of conduction in CNT materials [11]. The magnetoresistance at room temperature is small with peaks evident around 2 T and 4.5 T for the d = 34 lm and 10 lm yarns, respectively. At low temperature (10 K and 2 K) Dq/q is negative, similar to that previously reported for MWCNT yarns oriented perpendicular to the field [12]. The 34 lm fiber might be approaching a minimum in Dq/q, observed for SWCNT fibers spun from a superacid suspension [13]. The difference in r with yarn diameter was explored in more detail using many yarns produced from a single MWCNT array. These yarns show the same trend of decreasing conductivity with increasing diameter (Fig. 3). This trend is attributed to structural differences between small and large
246
CARBON
5 0 ( 2 0 1 2 ) 2 4 4 –2 4 8
Fig. 3 – Effect of yarn diameter on electrical conductivity at 300 K.
properties of the yarn. Stress and conduction properties are more uniform across the smaller diameter yarn.
Fig. 2 – (a) Electrical conductivity for 10 lm and 34 lm diameter MWCNT yarns. (b) Selected iV curves for the 10 lm yarn (inset shows the measurement platform). (c) Magnetoresistance of the MWCNT yarns at 2 K, 10 K, and 300 K.
diameter yarns, primarily decreased CNT migration length (i.e., the longitudinal distance over which a CNT shifts from the yarn surface to the interior and back [14]). We also observe some decrease in twist angle, leading to reduced alignment, in the larger diameter yarn (Fig. 1). Our results indicate that a smaller diameter yarn has better electrical (and thermal) conductivity than larger diameter yarn. The smaller diameter yarn has more turns per unit length for a given helix angle as compared to a larger diameter yarn. More turns increases the radial grip, which makes the yarn tighter, thereby improving the physical properties. Furthermore, SEM (Fig. 1) shows that the outer fibers on a larger diameter yarn are more loosely bound, and therefore less effective in contributing to the
3.
Thermal conductivity
3.1.
Measurement method
Thermal conductivity measurements on CNT fibers are complicated by the small sample size and the fact that the sample cannot support heaters or thermometers for 1D steady-state measurements. Others have addressed this by using comparative [5,15] or self-heating [4] methods. We employed the parallel thermal conductance (PTC) method [16,17], essentially a 1D steady-state measurement except that the background conductance, usually minimal in a steady-state measurement, now is the major component that is precisely determined and subtracted. PTC measurement requires three measurement runs: (1) background conductance (Kb) without a sample in place; (2) total conductance (Kt) with the sample in place; and (3) with the sample connected to the hot side only (K3). Measurement of K3 is used to correct for radiation loss [16]. The sample conductance (Ks) is then calculated as 1 Ks ¼ Kt Kb ðK3 Kb Þ: 2
ð1Þ
Here we used a custom sample holder built onto a PPMS resistivity puck (Fig. 4a). The sample holder consisted of two brass islands, one sinked to the base and the other supported by an acetate sheet. A heater (120 X strain gage with most packaging removed) was attached to the latter island and wired using 0.00300 alumel wire (4 cm, bare); a thermocouple (constantan–chromel–constantan, 0.00100 diameter, bare) was used to measure the temperature difference between the islands. This configuration reduces Kb. The MWCNT yarn was placed across the gap between the two islands and attached with DuPont 4929 N. Ks in all cases is approximately 10% of Kb or greater as recommended [16]. To obtain sufficient Ks it was necessary to put 13–20 segments of yarn in parallel across the gap. An additional advantage is that the measurement effectively probes 60–80 mm of yarn rather than a single short piece. The heater current was applied by a variable DC current source. Current was measured using an external meter (HP34401A) and controlled to produce desired
CARBON
5 0 (2 0 1 2) 2 4 4–24 8
247
Fig. 4 – (a) PTC measurement platform along with images showing 34 lm diameter MWCNT yarn segments in parallel. (b) Example data showing a single conductance measurement at 300 K. (c) Thermal conductances from measurement of thirteen 34 lm diameter yarns in parallel. (d) Thermal conductivity of MWCNT yarns (see Supplementary information for data tables). Preliminary data for one set of yarns shown by solid symbols (j for 34 lm and for 10 lm) and more extensive data for different samples are shown with open symbols ( for 34 lm and for 10 lm), indicate the reproducibility of the data.
temperature gradients across the sample. The heater and thermocouple voltages were measured using external meters (HP3456A). These quantities were logged using LabView, which also calculates the temperature difference across the sample using NIST thermocouple tables. At each temperature at least three heater currents were applied and the measured conductance was determined as the slope of P vs. DT graphs. All thermal conductance measurements were conducted under vacuum (<104 Torr) to minimize the effects of convection.
3.2.
Thermal conductivity results and discussion
The thermal conductivity measurement and results are shown in Fig. 4. The thermal conductivity of MWCNT yarn is much higher than for the as-produced arrays, but comparable to the scaled MWCNT value [8]. The error bars include uncertainties in the measured conductances (from fits to DT vs. P) and yarn dimensions. For the larger yarn (Fig. 4c), Ks adds a sufficient thermal pathway above Kb that it can be extracted
through the PTC method except for T < 5 K (<50 K for the smaller yarn). For the 10 lm diameter yarn the room temperature j, (60 ± 20) W m1 K1, appears to be the highest measured result for a CNT yarn to date. However, we note that it remains low in comparison to individual MWCNTs (3000 W m1 K1 [18]) and that higher scaled values, where the measured result has been adjusted to a theoretically densified sample, have been reported for bulk CNT materials [19]. As observed for electrical conductivity, j(300 K) also decreased with increasing yarn diameter and, considering the larger uncertainty in j, the ratio j(10 lm)/j (34 lm) at 300 K (2.5) is in reasonable agreement with the ratio of their electrical conductivities (1.9). This similarity is not related to the mechanisms of electron and heat transport. We know that heat transport in MWCNT yarns is phonon-dominated because the effective Lorentz number, L = j/(rT), is 100· larger than for electronic heat conduction in metals, which is consistent with previous descriptions for individual CNTs [18] and assemblies, e.g., [8]. The similar effect of diameter on electrical and thermal conductivity supports the hypothesis
248
CARBON
5 0 ( 2 0 1 2 ) 2 4 4 –2 4 8
that structural differences between larger and smaller diameter yarns result in higher conductivities (both thermal and electrical) at smaller diameters, as described above in relation to electrical conductivity.
4.
Conclusion
In summary, the electrical conductivity of MWCNTyarns spun from tall MWCNT arrays is competitive with other high conductivity CNT fibers/yarns, and the room temperature thermal conductivity result for the 10 lm diameter MWCNT yarn is the highest measured value reported to our knowledge. While the properties of CNT yarn remain far below those of individual nanotubes and in some cases also below those of traditional high performance carbon fibers [1], this is a new kind of material that can offer multifunctional capabilities. It is very light, not brittle, and does not fatigue when performing. Furthermore, there is considerable potential for continued improvement. In this study the yarns were spun from relatively long (500 lm) MWCNTs; however, longer CNTs also typically have higher defect densities. Improved CNT quality, as well as further increase in length can be expected to lead to continued improvements. Simulations have suggested that optimizing CNT overlap and length, along with improved quality, could provide for MWCNT cables with j greater than copper and competitive with high-performance carbon fibers [9]. The present study identifies differences between larger and smaller diameter yarns, with higher electrical and thermal conductivities achieved for smaller diameter due to structural differences such as variation in the CNT migration length. This points to an additional strategy to improve CNT yarns, wherein the use of multi-ply yarns with smaller diameter strands could provide superior properties.
Acknowledgements The authors thank Prof. T. Tritt (Clemson U.) for helpful information on the PTC method, as well as R. Chen and P. Scallion (Dalhousie) for SEM. The work at Dalhousie was supported by NSERC, the Killam Trusts, and the Sumner Foundation, as well as CFI and other partners that fund the Facilities for Materials Characterization managed by the Institute for Research in Materials. The UC researchers acknowledge the financial support from NSF through Grant CMMI-07272500 with program officer Dr. Haris Doumanidis and from NCA&T through DURIP-ONP.
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.carbon.2011.08.041.
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