Electrically conductive composites via infiltration of single-walled carbon

nanotube-based aerogels

Marcus A. Worsley*, Joshua D. Kuntz, Sergei O. Kucheyev, Alex V. Hamza, Joe H.

Satcher, Jr. and Theodore F. Baumann

Physical and Life Sciences Directorate, Lawrence Livermore National Laboratory, 7000

East Avenue, Livermore, California 94550

AUTHOR EMAIL ADDRESS: worsley1@llnl.gov

ABSTRACT

Many challenges remain in the effort to realize the exceptional properties of

carbon nanotubes (CNT) in composite materials. Here, we report on electrically

conductive composites fabricated via infiltration of CNT-based aerogels. The ultra low-

density, high conductivity, and extraordinary robustness of the CNT aerogels make them

ideal scaffolds around which to create conductive composites. Infiltrating the aerogels

with various insulating materials (e.g. epoxy and SiO2) resulted in composites with

electrical conductivities over 1 Scm-1

with as little as 1 vol% nanotube content. The

electrical conductivity observed in the composites was remarkably close to that of the

CNT scaffold in all cases.

INTRODUCTION

Carbon nanotubes (CNTs) possess a number of intrinsic properties that have made

them promising candidates for a range of composite materials. CNTs can have electrical

conductivities [1]

as high as 106 Sm

-1, thermal conductivities

[2] as high as 3000 Wm

-1K

-1,

elastic moduli [3]

on the order of 1 TPa, and are extremely flexible [4]

. Unfortunately, the

realization of these properties in macroscopic forms such as composites [5-13] has been

limited. With polymer/CNT composites, though adding as little as 0.007 wt% CNTs can

achieve a measurable increase in electrical conductivity [6], typically to reach

conductivities >1 S cm-1

much larger quantities of CNTs (>10 wt%) are required [7-10].

This makes it an expensive endeavor to create polymer composites with conductivities on

par with highly conductive semiconductors and metals for applications such as

electromagnetic interference shielding [14].

Only recently have loadings of <2 wt% CNTs produced composites with

conductivities >1 S cm-1

[15]. However, this level of conductivity was achieved with

specially-prepared multi-walled CNTs, and attaining high conductivity composites with

commercial single-walled CNTs (SWNTs) remains a serious challenge. An additional

obstacle to fabricating composites is that the method for dispersing CNTs tends to vary

greatly depending on the matrix material [12, 13, 16-19]. With a mechanically robust,

electrically conductive CNT foam, one could imagine simply infiltrating this low-density

CNT scaffold with the matrix material of choice, yielding a conductive composite. This

would generalize the fabrication process for making CNT composites and keep the

amount of CNTs used to a minimum.

Mater. Res. Soc. Symp. Proc. Vol. 1258 © 2010 Materials Research Society 1258-R05-31

We recently reported the synthesis of ultralow-density SWNT-based aerogel

nanofoam (SWNT-CA) monoliths with exceptional electrical and mechanical properties

[20]. These SWNT-CAs simultaneously exhibited increased stiffness, and high electrical

conductivity even at densities approaching 10 mg cm-3

without reinforcement [20]. The

foams are stable to temperatures approaching 1000°C and have been shown to be

unaltered by exposure to extremely low temperatures during immersion in cryogenic

liquids (such as liquid hydrogen) [21]. So, in addition to use in applications such as

catalyst supports, sensors, and electrodes, these ultralight, robust foams could allow the

formation of novel CNT composites. As the conductive network is already established, it

could be impregnated through the wicking process [22] with a matrix of choice, ranging

from inorganic sols to polymer melts to ceramic pastes. Thus, a variety of conductive

CNT composites could be created using the SWNT-CA foam as a pre-made scaffold.

Previously, the synthesis of a highly conductive poly(dimethlysiloxane) (PDMS)

composite was reported using these materials as a scaffold.[23] In this study, we show

that the same methods can be extended to other insulating matrices (e.g. epoxy and silica)

with similar improvements in electrical conductivity.

EXPERIMENT

Materials

All reagents were used without further purification. Resorcinol (99%) and

formaldehyde (37% in water), sodium carbonate (anhydrous), and highly purified

SWNTs were supplied by Aldrich Chemical Co, J.T. Baker Chemical Co., and Carbon

Solutions, Inc., respectively.

SWNT-CA preparation

The SWNT-CAs nanofoams, with a SWNT loading of 55 wt% (1 vol%) and a

monolith density of 30 mg/cc, were prepared as described in details elsewhere [20]. The

volume percent of SWNT in each sample was calculated from the initial mass of SWNTs

added, with an assumption of a CNT density of 1.3 g/cm3, and the final volume of the

sample.

SWNT-CA composite preparation

Composites were prepared by immersing the SWNT-CA (as prepared) in the SiO2

sol or epoxy resin prior to cure. The immersed SWNT-CA was placed under vacuum

until no more air escaped from the scaffold, indicating full penetration of the sol or resin.

The immersed SWNT-CA was then cured at elevated temperature (150°C) to produce the

epoxy/SWNT-CA composite and room temperature to produce the wet gel SiO2/SWNT-

CA composite. In the case of the SiO2/SWNT-CA composites, after curing, the wet gels

required supercritical extraction with liquid CO2 to yield the final SiO2/SWNT-CA

composite.

Characterization

Scanning electron microscopy (SEM) characterization was performed on a JEOL

7401-F at 5-10 keV (20mA) in secondary electron imaging mode with a working distance

of 2-8 mm. Electrical conductivity was measured using the four-probe method with

metal electrodes attached to the ends of cylindrical samples. The amount of current

transmitted through the sample during measurement was 100 mA, and the voltage drop

along the sample was measured over distances of 3 to 6 mm. Seven or more

measurements were taken on each sample, and results were averaged.

DISCUSSION

Figure 1 shows SEM images of epoxy/SWNT-CA (Figures 1a-b) and

SiO2/SWNT-CA (Figure 1c-d) composites. The epoxy/SWNT-CA images were very

similar to those of the PDMS/SWNT-CA, [23] illustrating that the SWNTs are

homogenously distributed throughout a fully dense matrix Furthermore, the ends of the

SWNTs that are visible appear to have a polymer sheath around them suggesting good

bonding between the SWNTs and the matrix. The SiO2/SWNT-CA images also show a

uniform distribution of SWNTs, but in a highly porous structure. The SiO2 aerogel

particles appear to preferentially coat the SWNT bundles, as all the SWNTs are

apparently covered. For both epoxy and SiO2, a well-dispersed network of SWNTs is

present suggesting that the CNT-based foam is intact after the infiltration and curing of

two very different matrices.

Figure 1. SEM images (under different magnifications) of SWNT-CA composites

containing (a-b) epoxy and (c-d) silica.

Table I shows CNT content, density, and electrical conductivity of the

epoxy/SWNT-CA and SiO2/SWNT-CA. Values for SWNT-CA, neat epoxy polymer,

and as-prepared SiO2 aerogel are included for reference. Comparing the conductivity of

the composites and SWNT-CA reveal that the electrical conductivity of the CNT aerogel

is maintained in various insulating matrices. Little to no change in the conductivity

indicates that the conductive scaffold is intact. Furthermore, the observed composite

conductivities are similar to those observed previously for PDMS/SWNT-CA suggesting

that the SWNT-CA is unaffected by the very different matrices in which it finds itself.

The consistent performance of the SWNT-CA over a range of matrices demonstrates its

robustness as a conductive scaffold for infiltration and supports its potential as the basis

for a variety of conductive composites.

Table I. Physical properties of SWNT-CA scaffold, insulating matrices, and conductive

composites.

Material CNT, vol% (wt%) Density, g/cm3

σ, Scm-1

SWNT-CA 1 (55) 0.028 1.12

Epoxy 0 1.44 <0.001

Epoxy/SWNT-CA 1 (1.2) 1.20 1.00

SiO2 0 0.12 <0.001

SiO2/SWNT-CA 1 (16) 0.080 1.00

CONCLUSIONS

In summary, SWNT-CAs were used as scaffolds to fabricate highly conductive

(epoxy and silica) composites via the infiltration method. Little to no degradation of the

conductive network (i.e. CNT-based scaffold) occurred in these composites.

Conductivities as high as 1 Scm-1

were observed for SWNT loadings as low as 1 vol% in

both the epoxy/SWNT-CA and SiO2/SWNT-CA composites. These observations in the

electrical conductivity are in excellent agreement with previous work on PDMS/SWNT-

CA showing the versatility of the infiltration method with the SWNT-CA scaffold. The

exceptional properties of these composites and the general nature of the fabrication

method provide the potential for a whole new class of composites based on the SWNT-

CA scaffold.

ACKNOWLEDGEMENTS

This work was performed under the auspices of the U.S. Department of Energy by

Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344 and

funded by the DOE Office of Energy Efficiency and Renewable Energy.

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