Aerogels are ultralight, highly porous materials typically fabricated by subjecting a wet-gel precursor to critical-pointdrying (CPD) or lyophilization (freeze-drying) in order to remove background liquid without collapsing the network. Microscopically, aerogels are composed of tenuous networks of clustered nanoparticles, and the materials often have unique properties, including very high strength-to-weight and surface-area-to-volume ratios. To date most aerogels are fabricated from silica [1] or pyrolized organic polymers. [2,3] Practical interest in the former stems from their potential for ultralight structural media, radiation detectors, and thermal insulators, [1] and in the latter from their potential for battery electrodes and supercapacitors. [2] In this paper we investigate the properties of a new class of aerogels based on carbon nanotubes (CNTs). Small-diameter CNTs, such as single-and few-wall CNTs, are exciting candidates for electrically conducting aerogels. Individually, these nanotubes are extraordinarily stiff [4] and their electrical conductivity can be very large. [4,5] Furthermore, ensembles of such nanotubes are useful aerogel precursors: they form electrically percolating networks at very low volume fractions [6] and elastic gels in concentrated suspensions through van der Waals interaction mediated cross-linking. [7,8] Here we report the creation of CNT aerogels from aqueousgel precursors by CPD and freeze-drying. CNT aerogels have been produced previously as intermediate phases during the process of drawing nanotube fibers [9] from a furnace and during the process of making sheets from multiwall CNT forests.[10] By contrast, our aerogels were derived directly from CNT networks in suspension, and we could readily manipulate the network properties as a result. The flexibility afforded by this process enabled us to control CNT concentration, to utilize optimized CNT dispersion processes, [11] to reinforce the networks with, for example, polyvinyl alcohol (PVA), and to infiltrate or backfill them with polymeric fluids. Here we describe these CNT aerogels and the processing methodologies used to synthesize them, and we characterize their electrical and mechanical properties. The CNT aerogels supported thousands of times their own weight after PVA-reinforcement, and, depending on processing conditions, their electrical conductivity ranged as high as ca. 1 S cm -1. Although our starting chemical vapor deposition (CVD) nanotube material contained single-and few-wall CNTs (the latter being predominantly double-wall CNTs, DWNTs), the dispersion and preparation processes employed here are directly applicable to pure single-wall CNTs (SWNTs).[11] CNT aerogel electrical and structural properties are also expected to be similar to pure SWNT samples because the electrical [12] and tensile [13] properties of bulk SWNTs and DWNTs are comparable. Images of typical critical-point-dried aerogels are seen in Figure 1. Unreinforced aerogels were fragile, but strong enough to permit careful handling. Reinforceme...

(M w = 70 000, Aldrich) and poly(styrene sulfonate) (PSS, M w = 500 000, Polysciences), according to the procedure by Barker et al [20]. The PSS layer was sandwiched between two PAH layers.Tetramethylorthosilicate (TMOS, Aldrich) was hydrolyzed under acidic conditions (molar ratio of TMOS/HCl/H 2 O was 1:1:55.6) for 30 min. The hydrolyzed solution was diluted with H 2 O in a ratio of 1:100. The diluted solution was then introduced into a two-compartment chamber separated by the nanonozzle-array film. The reaction was carried out in the presence of a direct-current electric field (80 V cm ±1 ), with the cathode immersed in the chamber facing the sharp end and the anode facing the large end. In this arrangement, the direction of EOF was from the sharp end to the large end. The reaction proceeded for 15 min, after which the nanonozzle array was taken out, rinsed thoroughly with water and dried. [2] radio-frequency shielding, and field-emission sources. [3,4] Recently, single-walled carbon nanotubes (SWNTs) have emerged as an attractive option for conductive composite materials. Their small size, large aspect ratio, and high conductivity make it possible to create conductive composites at very low filling concentrations and with smaller inhomogeneities than can be achieved with larger particles. Lower filling fractions imply smaller perturbations of bulk physical properties, such as strength and optical transparency, as well as lower cost. We describe here a simple procedure for making conductive SWNT±epoxy composites that result in exceptionally low threshold concentrations with minimal modification of the epoxy matrix material. Thus far, studies on the conductivity of SWNT±polymer composites [5±24] have reported low thresholds at volume fractions ranging from [7]~1 10 ±4 to several percent, [10] in some cases outperforming current technologies. Many aspects of the problem, however, are poorly understood and optimization remains elusive. The starting formulations of dispersed SWNTs frequently contain dense aggregates of nanotubes, as well as amorphous carbon and metallic impurities that can persist throughout processing and affect performance. Stabilizing the nanotubes in suspension can reduce aggregates, but introduces other problems. For example, covalent stabilization modifies the intrinsic SWNT conductivity, while steric stabilization can degrade contacts between nanotubes, and generally introduces additional impurities into the matrix. In addition, while welldispersed SWNTs typically have a higher length-to-diameter ratio than aggregates, which is important for obtaining low thresholds, [25] interactions between particles also contribute to the formation of percolating networks. Network formation through particle chaining, in particular, can be a key factor in COMMUNICATIONS

We report thermal conductivity measurements of purified single-wall carbon nanotube ͑SWNT͒ epoxy composites prepared using suspensions of SWNTs in N-N-Dimethylformamide ͑DMF͒ and surfactant stabilized aqueous SWNT suspensions. Thermal conductivity enhancement is observed in both types of composites. DMF-processed composites show an advantage at SWNT volume fractions between ϳ 0.001 to 0.005. Surfactant processed samples, however, permit greater SWNT loading and exhibit larger overall enhancement ͑64± 9͒% at ϳ 0.1. The enhancement differences are attributed to a tenfold larger SWNT/solid-composite interfacial thermal resistance in the surfactant-processed composites compared to DMF-processed composites. The interfacial resistance is extracted from the volume fraction dependence of the thermal conductivity data using effective medium theory. ͓C.

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built, it has the potential for a full color display, with A new display technology is being developed, which utilizes the electrophoretic properties of reverse emulsions. This display targets the market for highly reflective passive displays. It offers low power OpWatiOII, transmissivity in excess of 70~0, wide viewing angIe, bright colors, low material cost, and an inexpensive production process.