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A Combined Process of In Situ Functionalization andMicrowave Treatment to Achieve Ultrasmall ThermalExpansion of Aligned Carbon Nanotube–PolymerNanocomposites: Toward Applications as ThermalInterface Materials

By Wei Lin, Kyoung-Sik Moon, and C. P. Wong*

[*] Prof. C. P. Wong, W. Lin, Dr. K. S. MoonSchool of Material Science & Engineeringand Packaging Research CenterGeorgia Institute of Technology771 Ferst Drive NWAtlanta, GA 30332 (USA)E-mail: cp.wong@mse.gatech.edu

DOI: 10.1002/adma.200803548

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In the past twenty years, substances with low or negative thermalexpansivities have attracted much interest because of theirsignificance in electronic packaging, precision equipment, andintelligent materials.[1–3] In electronic packaging systems,mismatch in the coefficient of thermal expansion (CTE) betweenvarious materials has become a key issue for developing the nextgeneration of electronic packaging with higher system reliability.CTE values of polymer portions are much higher than those ofsilicon, ceramic, and copper metallization. These large CTEmismatches lead to thermal-stress accumulation at contactinterfaces during both packaging and device performance, whichtriggers component failure by, for example, warpage andrupture.[4] So far, the effective approach to reduce the CTE ofthe polymer portion has been to add fillers of low or negativeCTEs into polymer matrices.[2,4] The low or negative CTEs ofcarbon nanotube (CNTs), together with their low mass densityand outstanding mechanical and thermal properties, rendersthem the right filler for advanced polymer nanocomposites inmicroelectronic applications.[5] One of the most importantapplications for these materials is CNT–polymer nanocompositesfor thermal interface materials (TIMs) with enhanced thermalconductivity and, equally importantly, reduced CTEs. Thermalproperties of polymer composites filled with randomly dispersedCNTs have been extensively studied in the past decade.[6]

Unfortunately, real-life applications of these materials wereinhibited by their low thermal conductivity and relatively highCTEs, caused mainly by the weak CNT–polymer interface.[3,6]

Alternatively, researchers turned to a simple infiltration process toprepare polymer composites filled with aligned carbon nanotubes(ACNT), because the CNT alignment ensures much higherthermal conductivities than a random dispersion.[7] However, aCTE study of ACNT–polymer composites has never beenreported. The ACNT/polymer interface is still an issue, as such

a novel assembly process of the ACNT–polymer TIMs is needed.In this study, we report a strong anisotropy in the CTE ofACNT–epoxy composites, and achieve an ultrasmall CTE in theCNT-aligned direction by enhancing the ACNT–epoxy interfacewith a combined process of in situ CNT functionalization andmicrowave treatment. Furthermore, an assembly process isproposed to adapt the ACNT–epoxy TIM structure into real-lifeapplications, an issue never previously clarified in the literature.

Table 1 shows the CTE measurement results for a thermallycured ACNT–epoxy composite (TCOM), a microwave-curedACNT/epoxy composite (MCOM), a thermally cured epoxy(TEP), and a microwave-cured epoxy (MEP). Both the TEP andMEP samples display the typical CTE–temperature relation foramorphous polymers, with the CTE being much higher abovethe glass transition points (Tg) than below them. No apparentdifference in CTE was observed between TEP and MEP,consistent with their similar Tg values and storage moduli (G0)shown in Table 2. This excludes unexpected influences of themicrowave radiation on the pure epoxy itself. Both the TCOMandthe MCOM samples show strong anisotropy in thermalexpansion. Before their glass transitions, more than 63%reduction in the through-thickness CTE (designated as aN) withregards to the CTE of the epoxy is observed, while their in-planeCTE values (aP) are similar to that of the epoxy. Unexpectedly, theaN of the MCOM above its Tg is extremely small and close to theCTE of copper, which is even smaller than that below the Tg, thatis, a 90% reduction of the CTE of the epoxy above its Tg. Incomparison, the TCOM, as usual, shows a large CTE increase atTg as the turning point. One point that we would like to emphasizehere is the special importance of the CTE minimization of TIMcomposites at temperatures above their Tg, which seems to havebeen neglected or considered nonfeasible in the past. As isknown, it is common for polymers to have, upon heating, a higherpositive CTE at their Tg.

[8] Similar phenomena have also beenobserved in polymer-based composites, including CNT–polymercomposites. For example, Wang et al. and Xu et al. observed thatthe CTE values of CNT/polymer composites at temperaturesabove their Tg were at least 70 times that for single-crystal silicon,and 13 times that for pure copper.[3] Although semiconductordevices are expected to operate below 150 8C, the thermalnonuniformity usually referred to as hot spots, where the powerdensity could be >300W cm–2, is a factor that eventually

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Table 1. CTE comparisons at temperatures below and above the Tg. Allvalues 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 K�1] aP [ppm K�1]

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

Figure 1. DMA results of TCOM and MCOM. Only cooling curves areincluded.

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determines the device reliability.[9] The hot-spot issue will causethe local temperature to be higher than the Tg of the TIMcomposite; in this case, the intensively heated part expands muchmore than the nearby components. How can the CTE of polymercomposites above the Tg be reduced to, for example, a value closeto the CTE of copper (�16 ppm K�1) or silicon (�3 ppm K�1)? Inother words, how can we realize the principle of ‘1þ 1 >2’?

To the best of our knowledge, this is the first report so far on anultralow CTE above the Tg for CNT/polymer composites. It seemsthat microwave treatment plays a key role in obtaining such anultralow aN of the MCOM above the Tg. During curing,microwaves selectively heat up the CNTs and the polymer atthe interfaces.[10] The fast coupling between the ACNTs,functionalized sites on the ACNT surface, and the reactivefunctional groups in the epoxy matrix with the oscillatingelectromagnetic field is equally important.[11] In fact, microwaveirradiation has attracted much interest in synthetic organicchemistry because of its special role in dramatically increasingreaction rates, and its capability of inducing chemical reactionsthat cannot proceed by thermal heating alone.[11,12] It ispostulated that during the curing process in the microwavefield, the interfacial bonding between the ACNTs and the epoxymatrix was dramatically improved.

As is known, a weak CNT–polymer interface has been a keychallange for CNT/polymer composites since CNTs werediscovered. A weak interface results in inefficient load transfer,and consequently only moderate improvement, in mechanicalproperties. Phonon coupling across the weak interface is alsolimited. This has been considered the main reason for the highinterfacial thermal resistance, and eventually, why the thermalconductivities of CNT/polymer composites are much lower thanexpected.[6e,6f,13] Conventionally, people use a wet chemicalmethod to functionalize the CNT surface to increase interfacialbonding between the CNTs and the polymer matrix.[14] However,the CNT alignment is destroyed and the surface of the CNT isdamaged. Minimizing unnecessary structural defects and

able 2. DMA result comparisons.

amples Tg [a] [8C] Tg [b] [8C] G0 (100 8C) [MPa] G0 (50 8C) [MPa]

EP 120.72 129.9 1336 1464

EP 120.62 128.9 1339 1489

COM 120.86 131.15 1732 2198

COM 126.9 136.19 2332 2662

] Extracted from storage modulus curves in the quasi-equilibrium cooling process.

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[b] Extracted from tan d peaks in the quasi-equilibrium cooling process.

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avoiding degradation of intrinsic CNT properties are importantchallenges in this field. Therefore, a novel approach is necessaryto enhance the ACNT–polymer interface. Appropriately, thecombined processes of ACNT in situ functionalization andmicrowave curing helps to improve the ACNT–epoxy interfaceswhile maintaining their well-aligned structures. The distinctdifferences in Tg and G0 between the TCOM and the MCOM, asshown in Figure 1 and Table 2, give us a hint on the enhancedCNT–epoxy interface status in the MCOM. Notably, a 6 8Cincrease in Tg and a great enhancement in G0 with thevariable-frequency microwave (VFM) treatment compared withthe pure epoxy and the thermally cured composite samples isobserved. Consistent with the DMA results, MCOM displays amuch higher thermal conductivity than TCOM (Fig. 2), whichfurther verifies our postulation.

Why does the better interface lead to the ultralow aN above theTg? The first possibility is the frozen, or at least partially frozen,orientation of polymer segments along the thickness direction—the CNT-alignment direction. A bonded interface is the rightforce to freeze the polymer segments close to the interface, evenabove the Tg. At temperatures below the Tg, a sideward expansionof the polymer molecules contributes more to the aP, while alengthwise expansion of the molecules dominates the aN.

[8] If it is

Figure 2. Thermal conductivities of TCOM, MCOM, and epoxy.

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Figure 4. Illustration of the ACNT–epoxy TIM assembly process and theCNT status at the interface.

the case, the aN of the MCOM should be smaller than that of theTCOM, while with a larger aP. Although it looks like the aN of theMCOM is slightly smaller than that of the TCOM, their aP areclose to each other. It is likely that radial contraction of the ACNTsin the MCOM obliterates this difference based on the betterinterface. At temperatures above the Tg, the overall volumeexpansion of the TCOM should roughly equal that of the MCOM.Even if the polymer chains do not lose their orientation, not a bigdifference is expected between the aN of the MCOM and that ofthe TCOM. However, at temperatures above the Tg, the aN of theMCOM is only a third of the TCOM. This indicates the existenceof a secondmechanism that contributes to such an ultralow aN, asdiscussed below.

Molecular dynamics simulations suggest longitudinal con-traction of a single-walled carbon nanotube (SWNT).[5c] Low-dimensional systems gain structural and vibrational entropy byexploring the voids in configurational space at relatively smallenergy cost, resulting in thermal contraction in the harmonicregime at moderate temperatures. In Ref. [5c], a longitudinalthermal contraction of a (10, 10) single-walled carbon nanotubewas found to be shared by bending, twist, and pinch modes ofthe tube, whereas the volumetric contraction was dominated bythe pinch mode. A radial contraction was also expected. Anegative radial CTE of �1.5 ppm K�1 in SWNTs was experi-mentally estimated.[5b] Although it is not yet clear whethermultiwalled carbon nanotubes (the CNTs used in this study aremultiwalled) contract longitudinally or not, it is reasonable toexpect that they will have a much-lower CTE than epoxy. Uponheating, there is an in-plane stretching coupled with a normalcompression imposed on the polymer network because of theACNT anchoring effects at interfaces, where the in-planestretching comes from the radial contraction of the ACNTs,while the normal compression is a result of a large CTEmismatchbetween the ACNTs and the polymer matrix. Given that Poisson’sratio of a rubbery polymer network (a crosslinked network aboveits Tg) is close to 0.5, which indicates nearly no volume change,the in-plane stretching and the normal compression haveoverlapping effects that reduce the aN. Figure 3 illustrates sucha mechanism. Below the Tg, since segmental movements ofpolymer molecules are frozen, such stretching/compressionforces cannot exert much influence on the epoxy network. Abovethe Tg, they do. Although this overall influence may not be asstrong as the interfacial anchoring effect that acts on a thinpolymer film by a specific substrate, where even an ultranegativeCTE was observed,[15] it is possible to reduce aN to 1/10 of theCTE or lower for pure epoxy. This also explains why the MCOM

Figure 3. Illustration of the mechanism for the ultrasmall through-thickness CTE of MCOM above its Tg.

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shows a much larger aP than its aN, where aP is even larger thanthe CTE of the epoxy. The even larger aP of the TCOM is probablya result of an increase in excluded volume,[3a] where the weakCNT–polymer interfaces cannot exert the additional influences asin the MCOM.

One may worry that the large aP will cause a big in-plane CTEmismatch between the TIM and the mating surfaces. Actually, itis not the case for the pressure-involved TIM assembly process.Figure 4 shows the development of the CNT–epoxy TIM assemblyprocess. During the TIM assembly, ACNTs lie down on themating surface with dramatically increased CNTcoverage.[16] Forexample, the thickness of a ACNT 100mm thick reduces to�15mm upon a 0.4MPa compression force. The SEM images inRef. [16] show clearly the cranked CNT-array structure aftercompression. In the Supporting Information, we show the topview of a slightly compressed ACNTarray at amating surface afterthe mating substrate is removed. Furthermore, the matingsurface is rough, the surface roughness being simplified aswaviness in Figure 4. Therefore, on the nanometer scale, theACNTs within the TIM layer are no longer ‘vertically’ oriented, butmore likely adopt the orientation shown in Figure 4, where a CTEgradient is expected. This provides an insight into the real statusof the ACNT–epoxy TIMs. The epoxy infiltration that accom-panies the pressure will improve the interfacial adhesion and fixthe CNTorientation so that 1) tangential stress (sT in Fig. 4) at themating interface is relatively small, because the mismatchbetween aN and the CTE of the mating substrate is effectivelyreduced; 2) normal stress (sN) is beneficial in holding thepressure to ensure close contact between the CNTs and themating surface, so that thermal conductance degradation duringdevice performance can be minimized.

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In summary, the results in this study highlight the enhance-ment of the CNT–polymer interface by the combined process ofin situ functionalization and microwave treatment, which conferbenefits in CTE reduction, load transfer, and phonon transportacross the interface in ACNT–polymer TIM composites.

Experimental

Vertically aligned CNT arrays 2–3mm thick were grown on a SiO2/Sisubstrate with 10nm thick Al2O3 and 2nm thick Fe films through a thermalchemical vapor deposition (CVD). The CVD growthwas carried out at 750 8C,with a gas-flow rate ratio of Ar/H2/C2H4¼ 380/150/150 standard cubic cmmin�1 (sccm) [17]. ACNTswere functionalized in situ during the CVD growthprocess, as reported in a recent communication [18]. The ACNT arrays wereflipped onto polyimide double-sided tape and then infiltrated with epoxy toprepare ACNT–epoxy composites, followed by degassing under vacuum for40min.Sampleswere thermallycured inacommonconvectionovenat155 8Cfor 40min, or in a VFM chamber (central frequency: 6.4250GHz) set at thesame temperature. After furnace cooling, the ACNT–epoxy samples werepeeled off the double-sided tape and polished to be double-sided, parallel,flat, and smooth, and were cut into pieces for specific measurements. Thesamples maintain vertical alignment of the CNTs in the epoxy matrix, asshown in the Supporting Information, consistent with the SEM image shownin the literature [19]. The epoxy used was bisphenol-F (EPON862),with 4-methylhexahydrophthalic anhydride and 1-cyanoethyl-2-ethyl-4-methylimidazole as the curing agent and catalyst, respectively.

CTE was measured using a thermal mechanical analyzer (TMA, TAInstrumentsModel 2940), at a heating rate of 5 8Cmin�1. Whenmeasuringthe through-thickness CTE (along the CNT-aligned direction), thebulk-mode fixture and specimens �1mm thick were used. The quartzprobe was seated normal to the specimen top surface. Two differentmodeswere used to measure the in-plane CTE. The film-mode fixture was used forthin specimens. The bulk-mode fixture was used for thick specimens,where the quartz probe was seated normal to the side of the specimen, thatis, normal to the CNT orientation. Storage modulus and tan d weremeasured using a dynamical mechanical analyzer (DMA, TA InstrumentsModel 2940) at a heating rate of 5 8C min�1. The single-cantilever modewas used, with the CNTs in the specimen vertically oriented, that is, alongthe vibration direction. A constant frequency of 10Hz and an amplitude of10mm were adopted. At least five measurements were recorded to obtainthe average values in Tables 1 and 2. Effective thermal conductivities wereobtained by measuring thermal diffusivities with a Netzsch laser flashapparatus (LFA447); mass densities and specific heat weremeasured usinga differential scanning calorimeter (DSC, TA Instruments Model 2940).

Acknowledgements

The authors acknowledge the NSF (#0621115 and #0422553) for financialsupport of this work, and Prof. K. Jacob and Dr. X. He at Georgia Institute ofTechnology for helpful discussions. Supporting Information is availableonline from Wiley InterScience or from the author.

Received: December 1, 2008

Revised: January 8, 2009

Published online: March 19, 2009

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