Here we quantify the electron transport properties of aligned carbon nanotube (CNT) networks as a function of the CNT length, where the electrical conductivities may be tuned by up to 10× with anisotropies exceeding 40%. Testing at elevated temperatures demonstrates that the aligned CNT networks have a negative temperature coefficient of resistance, and application of the fluctuation induced tunneling model leads to an activation energy of ≈ 14 meV for electron tunneling at the CNT-CNT junctions. Since the tunneling activation energy is shown to be independent of both CNT length and orientation, the variation in electron transport is attributed to the number of CNT-CNT junctions an electron must tunnel through during its percolated path, which is proportional to the morphology of the aligned CNT network.The quantum confinement mediated landmark properties of one dimensional materials, such as nanowires, nanofibers, and nanotubes, makes them attractive to a number of high value applications. Recently, carbon nanotubes (CNTs) were extensively studied in scalable aligned architectures, commonly known as forests, which promise the design and facile manufacture of multifunctional material architectures with tunable properties.

Polymer derived carbon ceramics are highly desirable for lightweight, high strength, extreme environment material architectures, but their mechanical performance as a function of structure and processing is not currently understood and cannot be predicted. In this study, the mechanical behavior for bulk-scale pyrolytic carbons (PyCs) made via polymer pyrolysis of phenolformaldehyde precursors are established as a function of heat treatment temperature and the resulting average nano-and meso-scale order and disorder via X-ray diffraction, Raman spectroscopy, and Fourier transform infrared spectroscopy. The PyCs exhibit crystallite evolution on both the atomic-and meso-scale for pyrolysis temperatures of 600 • C to 1000 • C, whereas only atomic-scale crystallite evolution is observed for pyrolysis temperatures of 1000 • C to 1400 • C. The measured Vickers hardness of the PyCs is observed to scale non-monotonically as a function of the pyrolysis temperature reaching a peak at ∼ 4 GPa for samples prepared at 1000 • C. New modeling results, based on the elastic constants of disordered graphite, indicate that this counter-intuitive Vickers hardness scaling, which is a decades-old open question, originates from the PyC inter-layer shear elastic constant and the crystallite aspect ratio evolution with processing temperature. PyCs studied here are shown to be the lightest super-hard materials, having Vickers hardness-to-density ratios that are comparable to super-hard carbides, oxides, nitrides, and phosphides.

Here we present a study on the presence of physisorbed water on the surface of aligned carbon nanotubes (CNTs) in ambient conditions, where the wet CNT array mass can be more than 200% larger than that of dry CNTs, and modeling indicates that a water layer > 5 nm thick can be present on the outer CNT surface. The experimentally observed non-linear and non-monotonic dependence of the mass of adsorbed water on the CNT packing (volume fraction) originates from two competing modes. Physisorbed water cannot be neglected in the design and fabrication of materials and devices using nanowires/nanofibers, especially CNTs, and further experimental and ab initio studies on the influence of defects on the surface energies of CNTs, and nanowires/nanofibers in general, are necessary to understand the underlying physics and chemistry that govern this system.One dimensional nanoscale systems, such as nanowires, nanofibers, and nanotubes, are well known for their phenomenal electrical, [1][2][3][4] thermal, [5][6][7][8] and mechanical properties, [9][10][11] which could enable the design and manufacture of next-generation materials with unprecedented properties. [12][13][14][15][16][17][18] However, while many studies have previously explored the synthesis of new architectures and devices using one dimensional nanomaterials, specifically carbon nanotubes (CNTs), the properties they reported were far lower than the properties of predicted using current theory.[12] Some of the main reasons why existing models cannot accurately predict the behavior of CNTs in scalable architectures, such as aligned CNT arrays, are the various CNT morphology and proximity effects, [13][14][15]19] which can strongly impact properties, but are not well understood and cannot be properly integrated into theoretical frameworks. Here we report the presence of a commonly neglected morphological effect, an unexpectedly large (compared to the CNT mass) amount of moisture located on the surface of CNTs in aligned CNT (A-CNT) arrays at ambient conditions; show the non-linear and non-monotonic dependence of this effect on array porosity, which suggests two competing mechanisms; and discuss the strong impact such an effect can have on the structure and properties of nanocomposite architectures composed of A-CNTs.Previous studies on how water interacts with the outer surface of a CNT illustrated that the water molecules form a layer-like shell surrounding the CNTs, [20][21][22][23] and that the water layer density varies greatly and non- * wardle@mit.edu.monotonically with its thickness. [21,22] A recent study on the physisorption of water onto the external surface of a suspended ∼ 1.1 − 1.2 nm diameter single walled CNT showed that more than one layer of water is present on the CNT surface in water vapor, and that water molecules more easily adsorb onto larger diameter CNTs.[24] However, this study was limited to isolated and defect-free CNTs, [24,25] meaning that the interaction of moisture present in ambient air with multiwalled CNTs, which normally have nativ...