Single-wall carbon nanotubes (SWNTs) were used to augment the thermal transport properties of industrial epoxy. Samples loaded with 1 wt% unpurified SWNT material show a 70% increase in thermal conductivity at 40K, rising to 125% at room temperature; the enhancement due to 1 wt% loading of vapor grown carbon fibers is three times smaller. Electrical conductivity data show a percolation threshold between 0.1 and 0.2 wt% SWNT loading. The Vickers hardness rises monotonically with SWNT loading up to a factor of 3.5 at 2 wt%. These results suggest that the thermal and mechanical properties of SWNT-epoxy composites are improved, without the need to chemically functionalize the nanotubes

Biercuk, M. J

Fischer, John E

Hyun, J. K

Johnson, Alan T

Llaguno, Mark C

Radosavljevic, M.

English

ScholarlyCommons@Penn

University of PennsylvaniaScholarlyCommonsDepartmental Papers (MSE) Department of Materials Science & EngineeringApril 2002Carbon nanotube composites for thermalmanagementM. J. BiercukUniversity of PennsylvaniaMark C. LlagunoUniversity of PennsylvaniaM. RadosavljevicUniversity of PennsylvaniaJ. K. HyunUniversity of PennsylvaniaAlan T. JohnsonUniversity of Pennsylvania, cjohnson@sas.upenn.eduSee next page for additional authorsFollow this and additional works at: http://repository.upenn.edu/mse_papersPostprint version. Published in Applied Physics Letters, Volume 80, Issue 15, April 15, 2002, pages 2767-2769.Publisher URL: http://dx.doi.org/10.1063/1.1469696This paper is posted at ScholarlyCommons. http://repository.upenn.edu/mse_papers/51For more information, please contact libraryrepository@pobox.upenn.edu.Recommended CitationBiercuk, M. J., Llaguno, M. C., Radosavljevic, M., Hyun, J. K., Johnson, A. T., & Fischer, J. E. (2002). Carbon nanotube composites forthermal management. Retrieved from http://repository.upenn.edu/mse_papers/51Carbon nanotube composites for thermal managementAbstractSingle-wall carbon nanotubes (SWNTs) were used to augment the thermal transport properties of industrialepoxy. Samples loaded with 1 wt% unpurified SWNT material show a 70% increase in thermal conductivity at40K, rising to 125% at room temperature; the enhancement due to 1 wt% loading of vapor grown carbonfibers is three times smaller. Electrical conductivity data show a percolation threshold between 0.1 and 0.2wt% SWNT loading. The Vickers hardness rises monotonically with SWNT loading up to a factor of 3.5 at 2wt%. These results suggest that the thermal and mechanical properties of SWNT-epoxy composites areimproved, without the need to chemically functionalize the nanotubes.CommentsPostprint version. Published in Applied Physics Letters, Volume 80, Issue 15, April 15, 2002, pages 2767-2769.Publisher URL: http://dx.doi.org/10.1063/1.1469696Author(s)M. J. Biercuk, Mark C. Llaguno, M. Radosavljevic, J. K. Hyun, Alan T. Johnson, and John E. FischerThis journal article is available at ScholarlyCommons: http://repository.upenn.edu/mse_papers/515/13/02 Biercuk et al. 1Carbon nanotube composites for thermal managementPublished in Applied Physics Letters, volume 80, page 2767 (2002).M.J. Biercuka), M.C. Llaguno, M. Radosavljevicb), J.K. Hyunc), and A.T. Johnsond)Department of Physics & Astronomy and Laboratory for Research on the Structure ofMatter, University of Pennsylvania, Philadelphia, Pennsylvania   19104J.E. FischerDepartment of Materials Science & Engineering and Laboratory for Research on theStructure of Matter, University of Pennsylvania, Philadelphia, Pennsylvania   19104Single-wall carbon nanotubes (SWNTs) were used to augment the thermal transportproperties of industrial epoxy. Samples loaded with 1 wt% unpurified SWNT materialshow a 70% increase in thermal conductivity at 40K, rising to 125% at room temperature;the enhancement due to 1 wt% loading of vapor grown carbon fibers is three timessmaller. Electrical conductivity data show a percolation threshold between 0.1 and 0.2wt% SWNT loading. The Vickers hardness rises monotonically with SWNT loading upto a factor of 3.5 at 2 wt%. These results suggest that the thermal and mechanicalproperties of SWNT-epoxy composites are improved, without the need to chemicallyfunctionalize the nanotubes.a) present address: Department of Physics, Harvard University, Cambridge,Massachusetts   02138.b) present address: IBM T.J. Watson Research Center, Yorktown Heights, New York10598c) present address: Department of Applied Physics, Columbia University, New York,New York   10027d) electronic mail: cjohnson@physics.upenn.edu5/13/02 Biercuk et al. 2Rapid advance in the large-scale synthesis of single wall carbon nanotubes (SWNTs),coupled with their remarkable mechanical properties, are positive signs for application ina host of composite materials. First, the high Young’s modulus and strength-to-weightratio of SWNTs are extremely promising for enhanced strength in mechanicalcomposites. Second, and only more recently appreciated, because phonons dominatethermal transport at all temperatures in carbon materials [1], and since the phononthermal conductivity (κ) can be estimated by κ = CPvsl (where CP is the specific heat, vsthe speed of sound, and l the mean free path), SWNTs should be ideal for high-performance thermal management [2]. Indeed, theory predicts an extremely high value(6000 W/m-K) for the room temperature thermal conductivity of an isolated SWNT [3].Measurements of κ(T) for bulk SWNT samples agree with predictions for one-dimensional phonon systems [4, 5], while a recent measurement of 3000 W/m-K for theroom temperature thermal conductivity of an individual multiwall nanotube (MWNT) [6]shows directly that the superior thermal properties of the graphene plane are realized inits related nanomaterials.Despite this great promise, progress in mechanical composites has proved difficult todate. Although enhanced strength was observed in SWNT-polymethylmethacrylate(PMMA) composites [7], SWNT-epoxy and MWNT-epoxy composites are typicallyweaker or only slightly stronger than the pristine epoxy [8, 9, 10]. This has beenattributed to a weak nanotube-matrix interaction but may also reflect processingdifficulties that result in poor SWNT dispersion [11]. We report here our results on thethermal and mechanical properties of SWNT-epoxy composites.5/13/02 Biercuk et al. 3We fabricated nanotube-epoxy thermal composites and measured a thermalconductivity enhancement of more than 125% at 1.0 wt% nanotube loading. Compositesloaded with vapor grown carbon fibers (VGCF) at the same mass fraction showconsiderably weaker enhancement (45%). As is typical for composite materials, both ofthese measured enhancements are significantly lower than the “law of mixtures”prediction, even taking a conservative estimate of 1000 W/m-K for the SWNT thermalconductivity, and the published value of 1900 W/m-K for the VGCFs. Measurements ofthe electrical conductivity show a sharp increase by a factor of nearly 105 between 0.1and 0.2 wt% SWNT loading but not until between 1 and 2 wt% for VGCF composites.This likely indicates easier formation of a percolation network of SWNTs because oftheir larger aspect ratio. We used micro-indentation testing (Vickers hardness or VH) toevaluate the mechanical properties because of limited material quantity. There is amonotonic increase in VH with SWNT loading that reaches a factor of 3.5 at 2 wt%.Samples were based on the Shell Chemicals Epon 862 epoxy resin and Air ProductsAncamine 2435 dimethane-amine curing agent. Composite samples were loaded witheither raw SWNT soot grown by the HiPCO method [12] or vapor grown carbon fibers(VGCF) from Applied Sciences, Inc. The SWNT material contained approximately 15 –25 wt% Fe catalyst in the form of isolated nanoparticles. Quoted loading values are basedon the mass of as-grown SWNT material added to epoxy and have not been reduced toaccount for the Fe impurities. Transmission electron microscopy revealed that the SWNTmaterial had a broad distribution in tube diameter peaked at 1.1 nm and tube lengths fromhundreds of nanometers to several microns. Nanotube bundles were small (3 – 30 nm),and neutron diffraction indicated poor bundle crystallinity [13]. “Pyrograf-III” vapor5/13/02 Biercuk et al. 4grown carbon fibers (VGCF) have an average diameter of 200 nm and lengths fromseveral microns to over 10 mm. During sample production (mixing, sonication; seebelow) these fibers tend to break to lengths below 100 µm. The thermal conductivity ofthe fibers is 1900 W/m–K at room temperature [14], close to that predicted for SWNTs.Control samples of pristine epoxy were also measured.Care was taken to disperse carbon material uniformly through the composite. Carbonmaterials were dispersed ultrasonically for as long as 48 hours in an organic solvent(dichloroethane or N-N dimethylformamide) to promote the formation of a stablesuspension.  The epoxy resin was subsequently dissolved in the carbon/solvent mixture;high quality dispersal was indicated by the formation of a smooth emulsion. This solutionwas placed under vacuum to remove trapped air.  After degassing, samples were placedon a hotplate at 130 C for an hour to completely evaporate the solvent [15].  The curingagent was added, and the samples cured at room temperature for two to four daysfollowed by a 2 h post-cure bake at 120 C.  Loading was studied up to 5 wt%; Fig. 1shows that the SWNTs were well dispersed in the material on the micrometer scale, withrandom tube/rope orientation. Samples were black, however, indicating that the dispersalwas still not ideal.From the bulk product we cut samples (typically 1 mm x 1 mm x 2 mm), taking careto exclude regions with macroscopic air inclusions. We verified that different samples cutfrom the same epoxy composite gave identical results for all measurements. Thermalconductivity was measured from 20 – 300K with a comparative technique [16]. Briefly, asample is mounted between two constantan rods of known thermal conductivity. A heatcurrent is passed through this thermal “circuit” and then to thermal ground (the sample5/13/02 Biercuk et al. 5stage). Temperature drops are measured across each rod and the sample using differentialthermocouples, yielding the relative thermal conductivity of the sample. The secondstandard is used to monitor heat current loss due to radiation and other sources, with caretaken to minimize these negative effects. In calibration runs, this method accuratelyreproduced the known thermal conductivity of numerous different samples of metals andcarbon fibers.For the room temperature data as a function of carbon loading, results from 5 – 10measurements were averaged for each sample. The thermal conductivity enhancement inSWNT-epoxy samples rises much more rapidly than in samples loaded with VGCF. Weobserve a 125 % enhancement in κ at 1 wt% SWNT loading while equal loading ofVGCF produces but a 45% increase. Data for κ(T) were collected in a closed-cyclehelium-cooled system; typical raw data and the enhancement in κ(T) are presented in Fig.3. From this data we see that SWNT-epoxy composites show a markedly higher κenhancement at all temperatures than VGCF-loaded samples.Room temperature electrical conductivity of the composites was measured tocharacterize the extent of the carbon material network. For both carbon materials, low-loading samples display only a factor of 2 or 3 change in conductivity followed by asudden jump by more than 104, consistent with the formation of a percolating networkthrough the sample. The percolation threshold, where the sharp onset in conductivity isobserved, is between 0.1 and 0.2 wt% for SWNT-epoxy samples, while VGCF requiresloading between 1 and 2 wt%. These values are reasonable in light of experimental,17numreical,18 and theoretical19 work indicating that the percolation threshold in dilute,random rod systems is approximately equal to the inverse of the aspect ration, roughly5/13/02 Biercuk et al. 61000 for SWNTs and 100 for VGCF. Our measured critical loading for SWNT is fourtimes smaller than that found by others for SWNT-PMMA composites [20]. Thisdifference reflects the fact that their samples were formed by spin coating whichpreferentially aligns nanotubes in the plane of the sample, normal to the current directionin their experiment. An even more extreme case is presented by SWNT-PMMA fibers ofRef. 7. They were made with a very large draw ratio, leading to high SWNT alignment ,[7] and are electrical insulators even at loading as  high as 5 wt %. [21] In sharp contrastto these cases, the SWNTs in our samples are oriented randomly, and the percolationnetwork forms at lower loading.Resistance to plastic flow for HiPCO-epoxy samples was measured using a TukonMicrohardness Tester with a Vickers indenter. Epoxy samples were mechanicallypolished to ensure a smooth sample surface, and VH measured using a 100 gram load.The VH increased almost linearly from 0.4 for the pristine epoxy to 1.4 at 2 wt% loading.These experiments demonstrate convincingly that a small fraction of SWNT materialcan dramatically enhance the thermal properties of an epoxy matrix, and that they aremore effective than larger diameter carbon fibers for this purpose. It is likely that SWNTsare superior to VGCFs because their nanoscale diameter and larger aspect ratio enable amore extensive network to form at the same weight loading, as indicated by the lowercritical loading for percolation seen in the electrical conductivity measurements. Theobserved increase in VH is intriguing. First, it implies that SWNT-based thermalcomposites do not suffer any degradation in mechanical strength, a fact that augurs wellfor applications. Second, the increase in VH suggests that high-quality SWNT dispersionmay be an important factor in enhancing the strength of SWNT-epoxy composites.5/13/02 Biercuk et al. 7Ongoing TEM imaging to characterize the SWNT dispersion on the nanometer scale, andmechanical tests that diretly measure the elastic moduli of the composites will furtherilluminate this point.In summary, we demonstrated that SWNT-epoxy composites have significantlyenhanced thermal conductivity, and SWNTs are much more effective for this than larger-diameter carbon fibers. We observed an increase in Vickers hardness with increasingSWNT loading that will be the subject of additional investigation. Future improvementsmight result from chemical functionalization of the SWNTs to enhance the SWNT-matrixinteraction and the creation of anisotropic thermal management composites via nanotubealignment.This work was supported by the US Department of Energy, DEFG02-98ER45701(MCL and JEF), and the Laboratory for Research on the Structure of Matter (LRSM), anNSF-supported MRSEC, DMR00-79909 (ATJ, MR). MJB and JKH acknowledge thesupport of the LRSM Research Experience for Undergraduates program.5/13/02 Biercuk et al. 8FIGURE CAPTIONSFigure 1: Scanning electron micrograph of 1 wt% SWNT-epoxy composite. Randomlyoriented nanotube bundles are clearly visible throughout the matrix.Figure 2: Enhancement in thermal conductivity relative to pristine epoxy as a function ofSWNT and VGCF loading.Figure 3: (a) Thermal conductivity vs. temperature for the pristine epoxy and epoxy with1 wt% SWNT loading. (b) Enhancement in the thermal conductivity vs. temperature forcomposites loaded with 1 wt% SWNT and VGCF. The SWNT-epoxy has a largerenhancement at all temperatures.5/13/02 Biercuk et al. 9Figure 1, Biercuk et al.Figure 2, Biercuk et al.5/13/02 Biercuk et al. 10Figure 3, Biercuk et al.5/13/02 Biercuk et al. 11                                                 [1] L.X. Benedict, S.G. Louie, M.L. Cohen, Solid State Commun. 100, 177 (1996).[2] J. Hone, B. Batlogg, Z. Benes, A.T. Johnson, J.E. Fischer, Science 289, 1730 (2000).[3] S. Berber, Y.K. Kwon, D. Tomanek, Phys. Rev. Lett. 84, 4613 (2000).[4] J. Hone,  M. Whitney, C. Piskoti and A. Zettl, Phys. Rev. B Rapid Comm. 59, R2514(1999).[5]J. Hone, M.C. Llaguno, N.M. Nemes, A.T. Johnson, J.E. Fischer, D.A. Walters, M.J.Casavant, J. Schmidt, R.E. Smalley, Appl. Phys. Lett. 77, 666 (2000).[6] P. Kim, L. Shi, A. Majumdar, and P. L. McEuen, Phys. Rev. Lett. 87, 215502 (2001).[7] R. Haggenmueller, H.H. Gommans, A.G. Rinzler, J.E. Fischer, and K.I. Winey,Chem. Phys. Lett. 330, 219 (2000).[8] L. Vaccarini, G. Desarmot, R. Almairac, S. Tahir, C. Goze, and P. Bernier, p. 521,Proceedings of the XIV International Winterschool, Kirchberg, Tyrol, 2000, edited by H.Kuzmany, J. Fink, M. Mehring, and S. Roth (AIP Conference Proceedings 544,Woodbury, New York).[9] P. Ajayan, L. Schadler, C. Giannaris and A. Rubio, Advanced Materials 12, 750(2000).[10] L.S. Schadler, S.C. Giannaris and P.M. Ajayan, Appl. Phys. Lett. 73, 3842 (1998).[11] H.Z. Geng, R. Rosen, B. Zheng, H. Shimoda, L. Fleming, J. Liu, and O. Zhou,unpublished.[12]M. J. Bronikowski, P. A. Willis, D. T. Colbert, K. A. Smith, and R. E. Smalley, J.Vac. Sci. Technol. A, 19(4), 1800-1805 (2001).[13] W. Zhou, Y.H. Ooi, R. Russo, P. Papanek, D.E. Luzzi, J.E. Fischer, M.J.Bronikowski, P.A. Willis, and R.E. Smalley, Chem. Phys. Lett. 350, 6 (2001).[14] Physical properties of Pyrograf-III were found on the Applied Sciences website:www.apsci.com.[15] Solvent evaporation was considered complete when the remaining sample mass waswithin 0.1 % of the original mass of resin plus carbon material.[16] M.C. Llaguno, J. Hone, A.T. Johnson, and J.E. Fischer, AIP Conference Proceedings591, 384 (2001).5/13/02 Biercuk et al. 12                                                                                                                                                  [17] D.M. Bigg, in Metal-Filled Polymers, edited by S.K. Bhattacharya (Marcel Dekker,New York, 1986).[18] A.P. Philipse, Langmuir 12, 1127 (1995).[19] I. Balberg, N. Binenbaum, and N. Wagner, Phys. Rev. Lett. 52, 1465 (1984).[20] C. Stephan, T.P. Nguyen, S. Lefrant, L. Vaccarini, P. Bernier, p. 363, AIPConference Proceedings 544, 363 (2000).[21] M. Llaguno, J. Vavro, R. Haggenmueller, A.T. Johnson, J.E. Fischer, and K.I.Winey (unpublished).

Carbon nanotube composites for thermal management

https://repository.upenn.edu/cgi/viewcontent.cgi?article=1058&amp;context=mse_papers

Carbon nanotube composites for thermal management

Abstract

Similar works

Full text

Available Versions

ScholarlyCommons@Penn