Carbon nanotube aerogels

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 or pyrolized organic polymers. Practical interest in the former stems from their potential for ultralight structural media, radiation detectors, and thermal insulators, and in the latter from their potential for battery electrodes and supercapacitors. In this paper we investigate the properties of a new class of aerogels based on carbon nanotubes (CNTs). Small-diameter CNTs, such as singleand few-wall CNTs, are exciting candidates for electrically conducting aerogels. Individually, these nanotubes are extraordinarily stiff and their electrical conductivity can be very large. Furthermore, ensembles of such nanotubes are useful aerogel precursors: they form electrically percolating networks at very low volume fractions and elastic gels in concentrated suspensions through van der Waals interaction mediated cross-linking. 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 from a furnace and during the process of making sheets from multiwall CNT forests. 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, 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. Although our starting chemical vapor deposition (CVD) nanotube material contained singleand 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). CNT aerogel electrical and structural properties are also expected to be similar to pure SWNT samples because the electrical and tensile 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. Reinforcement of aerogels with PVA polymer improved the strength and stability of the aerogel. In this case, the aerogels could support at least 8000 times their own weight, as shown in Figure 1b. All of the aerogels (with or without PVA reinforcement) were highly porous, with pore sizes ranging from tens of nanometers up to one micrometer. Transmission electron microscopy (TEM) images like the one in Figure 1d, prepared by shredding (pulling apart) unreinforced CNT aerogels above a TEM sample grid, suggest nanotubes in the aerogels form a random filamentous network with little bundling. In the absence of bundling, the nanotube surface area is very large, a unique feature potentially useful for applications that require materials with large surface-area-to-volume ratios, including chemical sensors, reaction catalysts, and novel electrodes. The freeze-dried samples sometimes had a second porosity length scale of the order of tens to hundreds of micrometers as a result of ice crystals that formed during the freezing process and displaced nanotubes within the network. Densities of the CNT aerogels studied ranged from 10–30 mg mL with no PVA reinforcement, to 40–60 mg mL for aerogels reinforced in a 1 wt % PVA bath. The PVA conC O M M U N IC A IO N