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                 Table 1. CTE comparisons at temperatures below and above the Tg. All
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                    In summary, the results in this study highlight the enhance-
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CTE of CNT Epoxy Composite

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CTE of CNT Epoxy Composite

  1. 1. www.advmat.de COMMUNICATION A Combined Process of In Situ Functionalization and Microwave Treatment to Achieve Ultrasmall Thermal Expansion of Aligned Carbon Nanotube–Polymer Nanocomposites: Toward Applications as Thermal Interface Materials By Wei Lin, Kyoung-Sik Moon, and C. P. Wong* In the past twenty years, substances with low or negative thermal a novel assembly process of the ACNT–polymer TIMs is needed. expansivities have attracted much interest because of their In this study, we report a strong anisotropy in the CTE of significance in electronic packaging, precision equipment, and ACNT–epoxy composites, and achieve an ultrasmall CTE in the intelligent materials.[1–3] In electronic packaging systems, CNT-aligned direction by enhancing the ACNT–epoxy interface mismatch in the coefficient of thermal expansion (CTE) between with a combined process of in situ CNT functionalization and various materials has become a key issue for developing the next microwave treatment. Furthermore, an assembly process is generation of electronic packaging with higher system reliability. proposed to adapt the ACNT–epoxy TIM structure into real-life CTE values of polymer portions are much higher than those of applications, an issue never previously clarified in the literature. silicon, ceramic, and copper metallization. These large CTE Table 1 shows the CTE measurement results for a thermally mismatches lead to thermal-stress accumulation at contact cured ACNT–epoxy composite (TCOM), a microwave-cured interfaces during both packaging and device performance, which ACNT/epoxy composite (MCOM), a thermally cured epoxy triggers component failure by, for example, warpage and (TEP), and a microwave-cured epoxy (MEP). Both the TEP and rupture.[4] So far, the effective approach to reduce the CTE of MEP samples display the typical CTE–temperature relation for the polymer portion has been to add fillers of low or negative amorphous polymers, with the CTE being much higher above CTEs into polymer matrices.[2,4] The low or negative CTEs of the glass transition points (Tg) than below them. No apparent carbon nanotube (CNTs), together with their low mass density difference in CTE was observed between TEP and MEP, and outstanding mechanical and thermal properties, renders consistent with their similar Tg values and storage moduli (G0 ) them the right filler for advanced polymer nanocomposites in shown in Table 2. This excludes unexpected influences of the microelectronic applications.[5] One of the most important microwave radiation on the pure epoxy itself. Both the TCOM and applications for these materials is CNT–polymer nanocomposites the MCOM samples show strong anisotropy in thermal for thermal interface materials (TIMs) with enhanced thermal expansion. Before their glass transitions, more than 63% conductivity and, equally importantly, reduced CTEs. Thermal reduction in the through-thickness CTE (designated as aN) with properties of polymer composites filled with randomly dispersed regards to the CTE of the epoxy is observed, while their in-plane CNTs have been extensively studied in the past decade.[6] CTE values (aP) are similar to that of the epoxy. Unexpectedly, the Unfortunately, real-life applications of these materials were aN of the MCOM above its Tg is extremely small and close to the inhibited by their low thermal conductivity and relatively high CTE of copper, which is even smaller than that below the Tg, that CTEs, caused mainly by the weak CNT–polymer interface.[3,6] is, a 90% reduction of the CTE of the epoxy above its Tg. In Alternatively, researchers turned to a simple infiltration process to comparison, the TCOM, as usual, shows a large CTE increase at prepare polymer composites filled with aligned carbon nanotubes Tg as the turning point. One point that we would like to emphasize (ACNT), because the CNT alignment ensures much higher here is the special importance of the CTE minimization of TIM thermal conductivities than a random dispersion.[7] However, a composites at temperatures above their Tg, which seems to have CTE study of ACNT–polymer composites has never been been neglected or considered nonfeasible in the past. As is reported. The ACNT/polymer interface is still an issue, as such known, it is common for polymers to have, upon heating, a higher positive CTE at their Tg.[8] Similar phenomena have also been observed in polymer-based composites, including CNT–polymer [*] Prof. C. P. Wong, W. Lin, Dr. K. S. Moon School of Material Science & Engineering composites. For example, Wang et al. and Xu et al. observed that and Packaging Research Center the CTE values of CNT/polymer composites at temperatures Georgia Institute of Technology above their Tg were at least 70 times that for single-crystal silicon, 771 Ferst Drive NW and 13 times that for pure copper.[3] Although semiconductor Atlanta, GA 30332 (USA) devices are expected to operate below 150 8C, the thermal E-mail: cp.wong@mse.gatech.edu nonuniformity usually referred to as hot spots, where the power DOI: 10.1002/adma.200803548 density could be >300 W cm–2, is a factor that eventually Adv. Mater. 2009, 21, 2421–2424 ß 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 2421
  2. 2. www.advmat.de Table 1. CTE comparisons at temperatures below and above the Tg. All COMMUNICATION values were extracted from the slopes in quasi-equilibrium cooling curves. aN and aP indicate the through-thickness and in-plane CTEs, respectively. Samples aN [ppm KÀ1] aP [ppm KÀ1] Below Tg Above Tg Below Tg Above Tg TEP 81 191 80 186 MEP 80 188 82 191 TCOM 30 68 81 310 MCOM 23 18 80 240 determines the device reliability.[9] The hot-spot issue will cause the local temperature to be higher than the Tg of the TIM composite; in this case, the intensively heated part expands much Figure 1. DMA results of TCOM and MCOM. Only cooling curves are more than the nearby components. How can the CTE of polymer included. composites above the Tg be reduced to, for example, a value close to the CTE of copper ($16 ppm KÀ1) or silicon ($3 ppm KÀ1)? In avoiding degradation of intrinsic CNT properties are important other words, how can we realize the principle of ‘1 þ 1 >2’? challenges in this field. Therefore, a novel approach is necessary To the best of our knowledge, this is the first report so far on an to enhance the ACNT–polymer interface. Appropriately, the ultralow CTE above the Tg for CNT/polymer composites. It seems combined processes of ACNT in situ functionalization and that microwave treatment plays a key role in obtaining such an microwave curing helps to improve the ACNT–epoxy interfaces ultralow aN of the MCOM above the Tg. During curing, while maintaining their well-aligned structures. The distinct microwaves selectively heat up the CNTs and the polymer at differences in Tg and G0 between the TCOM and the MCOM, as the interfaces.[10] The fast coupling between the ACNTs, shown in Figure 1 and Table 2, give us a hint on the enhanced functionalized sites on the ACNT surface, and the reactive CNT–epoxy interface status in the MCOM. Notably, a 6 8C functional groups in the epoxy matrix with the oscillating increase in Tg and a great enhancement in G0 with the electromagnetic field is equally important.[11] In fact, microwave variable-frequency microwave (VFM) treatment compared with irradiation has attracted much interest in synthetic organic the pure epoxy and the thermally cured composite samples is chemistry because of its special role in dramatically increasing observed. Consistent with the DMA results, MCOM displays a reaction rates, and its capability of inducing chemical reactions much higher thermal conductivity than TCOM (Fig. 2), which that cannot proceed by thermal heating alone.[11,12] It is further verifies our postulation. postulated that during the curing process in the microwave Why does the better interface lead to the ultralow aN above the field, the interfacial bonding between the ACNTs and the epoxy Tg? The first possibility is the frozen, or at least partially frozen, matrix was dramatically improved. orientation of polymer segments along the thickness direction— As is known, a weak CNT–polymer interface has been a key the CNT-alignment direction. A bonded interface is the right challange for CNT/polymer composites since CNTs were force to freeze the polymer segments close to the interface, even discovered. A weak interface results in inefficient load transfer, above the Tg. At temperatures below the Tg, a sideward expansion and consequently only moderate improvement, in mechanical of the polymer molecules contributes more to the aP, while a properties. Phonon coupling across the weak interface is also lengthwise expansion of the molecules dominates the aN.[8] If it is limited. This has been considered the main reason for the high interfacial thermal resistance, and eventually, why the thermal conductivities of CNT/polymer composites are much lower than expected.[6e,6f,13] Conventionally, people use a wet chemical method to functionalize the CNT surface to increase interfacial bonding between the CNTs and the polymer matrix.[14] However, the CNT alignment is destroyed and the surface of the CNT is damaged. Minimizing unnecessary structural defects and Table 2. DMA result comparisons. Samples Tg [a] [8C] Tg [b] [8C] G0 (100 8C) [MPa] G0 (50 8C) [MPa] TEP 120.72 129.9 1336 1464 MEP 120.62 128.9 1339 1489 TCOM 120.86 131.15 1732 2198 MCOM 126.9 136.19 2332 2662 [a] Extracted from storage modulus curves in the quasi-equilibrium cooling process. [b] Extracted from tan d peaks in the quasi-equilibrium cooling process. Figure 2. Thermal conductivities of TCOM, MCOM, and epoxy. 2422 ß 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Adv. Mater. 2009, 21, 2421–2424
  3. 3. www.advmat.de COMMUNICATION the case, the aN of the MCOM should be smaller than that of the TCOM, while with a larger aP. Although it looks like the aN of the MCOM is slightly smaller than that of the TCOM, their aP are close to each other. It is likely that radial contraction of the ACNTs in the MCOM obliterates this difference based on the better interface. At temperatures above the Tg, the overall volume expansion of the TCOM should roughly equal that of the MCOM. Even if the polymer chains do not lose their orientation, not a big difference is expected between the aN of the MCOM and that of the TCOM. However, at temperatures above the Tg, the aN of the MCOM is only a third of the TCOM. This indicates the existence of a second mechanism that contributes to such an ultralow aN, as discussed below. Molecular dynamics simulations suggest longitudinal con- traction of a single-walled carbon nanotube (SWNT).[5c] Low- dimensional systems gain structural and vibrational entropy by exploring the voids in configurational space at relatively small energy cost, resulting in thermal contraction in the harmonic regime at moderate temperatures. In Ref. [5c], a longitudinal thermal contraction of a (10, 10) single-walled carbon nanotube was found to be shared by bending, twist, and pinch modes of the tube, whereas the volumetric contraction was dominated by the pinch mode. A radial contraction was also expected. A negative radial CTE of À1.5 ppm KÀ1 in SWNTs was experi- Figure 4. Illustration of the ACNT–epoxy TIM assembly process and the mentally estimated.[5b] Although it is not yet clear whether CNT status at the interface. multiwalled carbon nanotubes (the CNTs used in this study are multiwalled) contract longitudinally or not, it is reasonable to expect that they will have a much-lower CTE than epoxy. Upon heating, there is an in-plane stretching coupled with a normal shows a much larger aP than its aN, where aP is even larger than compression imposed on the polymer network because of the the CTE of the epoxy. The even larger aP of the TCOM is probably ACNT anchoring effects at interfaces, where the in-plane a result of an increase in excluded volume,[3a] where the weak stretching comes from the radial contraction of the ACNTs, CNT–polymer interfaces cannot exert the additional influences as while the normal compression is a result of a large CTE mismatch in the MCOM. between the ACNTs and the polymer matrix. Given that Poisson’s One may worry that the large aP will cause a big in-plane CTE ratio of a rubbery polymer network (a crosslinked network above mismatch between the TIM and the mating surfaces. Actually, it its Tg) is close to 0.5, which indicates nearly no volume change, is not the case for the pressure-involved TIM assembly process. the in-plane stretching and the normal compression have Figure 4 shows the development of the CNT–epoxy TIM assembly overlapping effects that reduce the aN. Figure 3 illustrates such process. During the TIM assembly, ACNTs lie down on the a mechanism. Below the Tg, since segmental movements of mating surface with dramatically increased CNT coverage.[16] For polymer molecules are frozen, such stretching/compression example, the thickness of a ACNT 100 mm thick reduces to forces cannot exert much influence on the epoxy network. Above $15 mm upon a 0.4 MPa compression force. The SEM images in the Tg, they do. Although this overall influence may not be as Ref. [16] show clearly the cranked CNT-array structure after strong as the interfacial anchoring effect that acts on a thin compression. In the Supporting Information, we show the top polymer film by a specific substrate, where even an ultranegative view of a slightly compressed ACNTarray at a mating surface after CTE was observed,[15] it is possible to reduce aN to 1/10 of the the mating substrate is removed. Furthermore, the mating CTE or lower for pure epoxy. This also explains why the MCOM surface is rough, the surface roughness being simplified as waviness in Figure 4. Therefore, on the nanometer scale, the ACNTs within the TIM layer are no longer ‘vertically’ oriented, but more likely adopt the orientation shown in Figure 4, where a CTE gradient is expected. This provides an insight into the real status of the ACNT–epoxy TIMs. The epoxy infiltration that accom- panies the pressure will improve the interfacial adhesion and fix the CNT orientation so that 1) tangential stress (sT in Fig. 4) at the mating interface is relatively small, because the mismatch between aN and the CTE of the mating substrate is effectively reduced; 2) normal stress (sN) is beneficial in holding the pressure to ensure close contact between the CNTs and the Figure 3. Illustration of the mechanism for the ultrasmall through- mating surface, so that thermal conductance degradation during thickness CTE of MCOM above its Tg. device performance can be minimized. Adv. Mater. 2009, 21, 2421–2424 ß 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 2423
  4. 4. www.advmat.de In summary, the results in this study highlight the enhance- COMMUNICATION [1] a) T. A. Mary, J. S. O. Evans, T. Vogt, A. W. Sleight, Science 1996, 272, 90. ment of the CNT–polymer interface by the combined process of b) R. Roy, D. K. Agrawal, H. A. Mckinstry, Annu. Rev. Mater. Sci. 1989, 19, 59. in situ functionalization and microwave treatment, which confer c) S. Numata, N. Kinjo, D. Makino, Polym. Eng. Sci. 1988, 28, 906. benefits in CTE reduction, load transfer, and phonon transport [2] a) J. S. O. Evans, J. Chem. Soc. Dalton Trans. 1999, 19, 3317. b) K. Yano, A. across the interface in ACNT–polymer TIM composites. Usuki, A. Okada, T. Kurauchi, O. Kamigaito, J. Polym. Sci. Part A: Polym. Chem. 1993, 31, 2493. [3] a) S. Wang, Z. Liang, P. Gonnet, Y. Liao, B. Wang, C. Zhang, Adv. Funct. Mater. 2007, 17, 87. b) Y. Xu, G. Ray, B. Abdel-Magid, Compos. A 2006, 37, Experimental 114. [4] Z. Q. Zhang, C. P. Wong, IEEE Trans. Adv. Packag. 2004, 27, 515. Vertically aligned CNT arrays 2–3 mm thick were grown on a SiO2/Si [5] a) Y. Yosida, J. Appl. Phys. 2000, 87, 3338. b) Y. Maniwa, R. Fujiwara, H. substrate with 10 nm thick Al2O3 and 2 nm thick Fe films through a thermal Kira, H. Tou, H. Kataura, S. Suzuki, Y. Achiba, Phys. Rev. B 2001, 64, chemical vapor deposition (CVD). The CVD growth was carried out at 750 8C, 241402. c) Y. Kwon, S. Berber, D. Tomanek, Phys. Rev. Lett. 2004, 92, 015 with a gas-flow rate ratio of Ar/H2/C2H4 ¼ 380/150/150 standard cubic cm 901. d) P. M. Ajayan, Chem. Rev. 1999, 99, 1787. e) J. N. Coleman, U. Khan, minÀ1 (sccm) [17]. ACNTs were functionalized in situ during the CVD growth Y. K. Gun’ko, Adv. Mater. 2006, 18, 689. f) R. H. Baughman, A. A. Zakhidov, process, as reported in a recent communication [18]. The ACNT arrays were flipped onto polyimide double-sided tape and then infiltrated with epoxy to W. A. d de Heer, Science 2002, 297, 787. prepare ACNT–epoxy composites, followed by degassing under vacuum for [6] a) C. Wei, D. Srivastava, K. J. Cho, Nano Lett. 2002, 2, 647. b) E. T. 40 min. Samples were thermally cured in a common convection oven at 155 8C Thostenson, Z. F. Ren, T. W. Chou, Compos. Sci. Technol. 2001, 61, 1899. for 40 min, or in a VFM chamber (central frequency: 6.4250 GHz) set at the c) E. S. Choi, J. S. Brooks, D. L. Eaton, M. S. Al-Haik, M. Y. Hussaini, H. same temperature. After furnace cooling, the ACNT–epoxy samples were Garmestani, D. Li, K. Dahmen, J. Appl. Phys. 2003, 94, 6034. d) C. H. Liu, H. peeled off the double-sided tape and polished to be double-sided, parallel, Huang, Y. Wu, S. S. Fan, Appl. Phys. Lett. 2004, 84, 4248. e) M. B. Bryning, flat, and smooth, and were cut into pieces for specific measurements. The D. E. Milkie, M. F. Islam, J. M. Kikkawa, A. G. Yodh, Appl. Phys. Lett. 2005, samples maintain vertical alignment of the CNTs in the epoxy matrix, as 87, 161909. f) S. Shenogin, L. P. Xue, R. Ozisik, P. Keblinski, D. G. Cahill, shown in the Supporting Information, consistent with the SEM image shown J. Appl. Phys. 2004, 95, 8136. in the literature [19]. The epoxy used was bisphenol-F (EPON862), [7] a) H. Huang, C. H. Liu, Y. Wu, S. S. Fan, Adv. Mater. 2005, 17, 1652. b) S. with 4-methylhexahydrophthalic anhydride and 1-cyanoethyl-2-ethyl-4- Sihn, S. Ganguli, A. K. Roy, L. Qu, L. Dai, Compos. Sci. Technol. 2008, 68, methylimidazole as the curing agent and catalyst, respectively. 658. c) T. Borca-Tasciuc, M. Mazumder, Y. Son, S. K. Pal, L. S. Schadler, CTE was measured using a thermal mechanical analyzer (TMA, TA P. M. Ajayan, J. Nanosci. Nanotechnol. 2007, 7, 1581. Instruments Model 2940), at a heating rate of 5 8C minÀ1. When measuring [8] R. F. Boyer, R. S. Spencer, J. Appl. Phys. 1945, 16, 594. the through-thickness CTE (along the CNT-aligned direction), the [9] a) R. S. Prasher, J. Chang, I. Sauciuc, S. Narasimhan, D. Chau, G. Chrysler, bulk-mode fixture and specimens $1 mm thick were used. The quartz A. Myers, S. Prstic, C. Hu, Intel. Technol. J. 2005, 9, 285. b) R. Prasher, Proc. probe was seated normal to the specimen top surface. Two different modes IEEE. 2006, 94, 1571. c) R. Mahajan, C. P. Chiu, G. Chrysler, Proc. IEEE. were used to measure the in-plane CTE. The film-mode fixture was used for 2006, 94, 1476. thin specimens. The bulk-mode fixture was used for thick specimens, [10] a) T. J. Imholt, C. A. Dyke, B. Hasslacher, J. M. Perez, D. W. Price, J. A. where the quartz probe was seated normal to the side of the specimen, that is, normal to the CNT orientation. Storage modulus and tan d were Roberts, J. B. Scott, A. Wadhawan, Z. Ye, J. M. Tour, Chem. Mater. 2003, 15, measured using a dynamical mechanical analyzer (DMA, TA Instruments 3969. b) C. Y. Wang, T. H. Chen, S. C. Chang, S. Y. Cheng, T. S. Chin, Adv. Model 2940) at a heating rate of 5 8C minÀ1. The single-cantilever mode Funct. Mater. 2007, 17, 1979. was used, with the CNTs in the specimen vertically oriented, that is, along [11] D. D. Young, J. Nichols, R. M. Kelly, A. Deiters, J. Am. Chem. Soc. 2008, 130, the vibration direction. A constant frequency of 10 Hz and an amplitude of 10048. 10 mm were adopted. At least five measurements were recorded to obtain [12] C. O. Kappe, Angew. Chem. Int. Ed. 2004, 43, 6250. the average values in Tables 1 and 2. Effective thermal conductivities were [13] a) C. W. Nan, G. Liu, Y. H. Lin, M. Li, Appl. Phys. Lett. 2004, 85, 3549. b) S. T. obtained by measuring thermal diffusivities with a Netzsch laser flash Huxtable, D. G. Cahill, S. Shenogin, L. P. Xue, R. Ozisik, P. Barone, M. apparatus (LFA447); mass densities and specific heat were measured using Usrey, M. S. Strano, G. Siddons, M. Shim, P. Keblinski, Nat. Mater. 2003, 2, a differential scanning calorimeter (DSC, TA Instruments Model 2940). 731. c) S. Shenogin, A. Bodapati, L. Xue, R. Ozisik, P. Keblinski, Appl. Phys. Lett. 2004, 85, 2229. [14] D. Tasis, N. Tagmatarchis, A. Bianco, M. Prato, Chem. Rev. 2006, 106, 1105. Acknowledgements [15] a) M. Mukherjee, M. Bhattacharya, M. K. Sanyal, Phys. Rev. E 2002, 66, 061801. b) T. Kanaya, T. Miyazaki, R. Inoue, K. Nishida, Phys. Status Solidi B The authors acknowledge the NSF (#0621115 and #0422553) for financial 2005, 242, 595. support of this work, and Prof. K. Jacob and Dr. X. He at Georgia Institute of Technology for helpful discussions. Supporting Information is available [16] L. Qu, L. Dai, Adv. Mater. 2007, 19, 3844. online from Wiley InterScience or from the author. [17] L. Zhu, D. Hess, C. P. Wong, Nano Lett. 2005, 5, 2641. [18] W. Lin, Y. Xiu, H. Jiang, R. Zhang, O. Hildreth, K. Moon, C. P. Wong, J. Am. Received: December 1, 2008 Chem. Soc. 2008, 130, 9636. Revised: January 8, 2009 [19] B. L. Wardle, D. S. Saito, E. J. Garcia, A. J. Hart, R. G. de Villoria, E. A. Published online: March 19, 2009 Verploegen, Adv. Mater. 2008, 20, 2707. 2424 ß 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Adv. Mater. 2009, 21, 2421–2424

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