Benji Maruyama

Strong, Light, Multifunctional Fibers of Carbon Nanotubes with Ultrahigh Conductivity

Optimizing Carbon Nanotubes Shorter carbon nanotubes are easier to make, but, when assembled into fibers, the resulting fiber properties are much poorer than might be predicted by theory. Conversely, longer carbon nanotubes have much better properties but are harder to process. Behabtu et al. (p. 182) combined the best of both worlds through scalable wet spinning method, in which they dissolved longer carbon nanotubes and then spun them into fibers that showed excellent strength, stiffness, and thermal conductivity. Exceptional carbon nanotube fibers are produced by a wet spinning process using longer nanotubes as feedstock. Broader applications of carbon nanotubes to real-world problems have largely gone unfulfilled because of difficult material synthesis and laborious processing. We report high-performance multifunctional carbon nanotube (CNT) fibers that combine the specific strength, stiffness, and thermal conductivity of carbon fibers with the specific electrical conductivity of metals. These fibers consist of bulk-grown CNTs and are produced by high-throughput wet spinning, the same process used to produce high-performance industrial fibers. These scalable CNT fibers are positioned for high-value applications, such as aerospace electronics and field emission, and can evolve into engineered materials with broad long-term impact, from consumer electronics to long-range power transmission.

Role of water in super growth of single-walled carbon nanotube carpets.

The Ostwald ripening behavior of Fe catalyst films deposited on thin alumina supporting layers is demonstrated as a function of thermal annealing in H2 and H2/H2O. The addition of H2O in super growth of single-walled carbon nanotube carpets is observed to inhibit Ostwald ripening due to the ability of oxygen and hydroxyl species to reduce diffusion rates of catalyst atoms. This work shows the impact of typical carpet growth environments on catalyst film evolution and the role Ostwald ripening may play in the termination of carpet growth.

In situ evidence for chirality-dependent growth rates of individual carbon nanotubes

Chiral-selective growth of single-walled carbon nanotubes (SWNTs) remains a great challenge that hinders their use in applications such as electronics and medicine. Recent experimental and theoretical reports have begun to address this problem by suggesting that selectivity may be achieved during nucleation by changing the catalyst composition or structure. Nevertheless, to establish a rational basis for chiral-selective synthesis, the underlying mechanisms governing nucleation, growth, and termination of SWNTs must be better understood. To this end, we report the first measurements of growth rates of individual SWNTs through in situ Raman spectroscopy and correlate them with their chiral angles. Our results show that the growth rates are directly proportional to the chiral angles, in agreement with recent theoretical predictions. Importantly, the evidence singles out the growth stage as responsible for the chiral distribution-distinct from nucleation and termination which might also affect the final product distribution. Our results suggest a route to chiral-selective synthesis of SWNTs through rational synthetic design strategies based on kinetic control.

Multi-scale characterization of spatially heterogeneous systems: implications for discontinuously reinforced metal–matrix composite microstructures

Abstract A new multi-scale technique is presented for characterizing the spatial distribution of second-phase particles in two-dimensional distributed multi-phase systems. The implications for the characterization of reinforcement distributions in discontinuously reinforced metallic matrix composite microstructures are discussed, along with results of the analysis both for simulated and experimental discontinuously reinforced aluminum (DRA) materials. A systematic variation in the degree of spatial heterogeneity is observed with increasing length scale. This result leads to the definition of the parameter LH or homogeneous length scale. The relevance of LH measured for a real DRA microstructure is then discussed in the context of statistical variations in mechanical properties such as tensile strength, ductility, and fracture toughness.

Double-Walled Boron Nitride Nanotubes Grown by Floating Catalyst Chemical Vapor Deposition

One-dimensional nanostructures exhibit quantum confinement which leads to unique electronic properties, making them attractive as the active elements for nanoscale electronic devices. Boron nitride nanotubes are of particular interest since, unlike carbon nanotubes, all chiralities are semiconducting. Here, we report a synthesis based on the use of low pressures of the molecular precursor borazine in conjunction with a floating nickelocene catalyst that resulted in the formation of double-walled boron nitride nanotubes. As has been shown for carbon nanotube production, the floating catalyst chemical vapor deposition method has the potential for creating high quality boron nitride nanostructures with high production volumes.

Revealing the impact of catalyst phase transition on carbon nanotube growth by in situ Raman spectroscopy.

The physical state of the catalyst and its impact on the growth of single-walled carbon nanotubes (SWNTs) is the subject of a long-standing debate. We addressed it here using in situ Raman spectroscopy to measure Fe and Ni catalyst lifetimes during the growth of individual SWNTs across a wide range of temperatures (500-1400 °C). The temperature dependence of the Fe catalyst lifetimes underwent a sharp increase around 1100 °C due to a solid-to-liquid phase transition. By comparing experimental results with the metal-carbon phase diagrams, we prove that SWNTs can grow from solid and liquid phase-catalysts, depending on the temperature.

Harvesting electrical energy from carbon nanotube yarn twist

Making the most of twists and turns The rise of small-scale, portable electronics and wearable devices has boosted the desire for ways to harvest energy from mechanical motion. Such approaches could be used to provide battery-free power with a small footprint. Kim et al. present an energy harvester made from carbon nanotube yarn that converts mechanical energy into electrical energy from both torsional and tensile motion. Their findings reveal how the extent of yarn twisting and the combination of homochiral and heterochiral coiled yarns can maximize energy generation. Science, this issue p. 773 Twisted and coiled carbon nanotubes can harvest electrical energy from mechanical motion. Mechanical energy harvesters are needed for diverse applications, including self-powered wireless sensors, structural and human health monitoring systems, and the extraction of energy from ocean waves. We report carbon nanotube yarn harvesters that electrochemically convert tensile or torsional mechanical energy into electrical energy without requiring an external bias voltage. Stretching coiled yarns generated 250 watts per kilogram of peak electrical power when cycled up to 30 hertz, as well as up to 41.2 joules per kilogram of electrical energy per mechanical cycle, when normalized to harvester yarn weight. These energy harvesters were used in the ocean to harvest wave energy, combined with thermally driven artificial muscles to convert temperature fluctuations to electrical energy, sewn into textiles for use as self-powered respiration sensors, and used to power a light-emitting diode and to charge a storage capacitor.

Millimeter-Long Carbon Nanotubes: Outstanding Electron-Emitting Sources

We are reporting the fabrication of a very efficient electron source using millimeter-long and highly crystalline carbon nanotubes. These devices start to emit electrons at fields as low as 0.17 V/μm and reach threshold emission at 0.24 V/μm. In addition, these electron sources are very stable and can achieve a peak current density of 750 mA cm(-2) at only 0.45 V/μm. In order to demonstrate intense electron beam generation, these devices were used to produce visible light by cathodoluminescence. Finally, density functional theory calculations were used to rationalize the measured electronic field emission properties in open carbon nanotubes of different lengths. The modeling establishes a clear correlation between length and field enhancement factor.

Growth, new growth, and amplification of carbon nanotubes as a function of catalyst composition.

Carbon nanotubes (CNTs) have been grown using Fe, Co, Ni, and Co/Fe spin-on-catalyst (SOC) systems, involving the metal salt dispersed with a spin-on-glass precursor. During initial growth runs (CH4/H2/900 degrees C), the CNT yield followed the order Co-SOC > Fe-SOC >> Ni-SOC. The Fe catalysts produced the longest nanotubes at the expense of a larger average CNT diameter and broader diameter distribution than the Co-SOC system. A series of Co/Fe-SOCs were prepared where as the atomic percentage of Co is increased nucleation of CNT increases but the CNT length decreases. The linear relationship between the diameter and length of CNTs grown from the Co/Fe-SOC suggests that slow growth is beneficial with respect to control over CNT diameter. After initial CNT growth, the original samples were subjected to additional growth runs. Four individual reactions were observed in the Fe-SOC and binary Co/Fe-SOC: regrowth (amplification), double growth (a second CNT growing from a previously active catalyst), CNT etching, and nucleation from initially inactive catalysts (new growth). CNT etching was observed for the mixed catalyst systems (Co/Fe-SOC) but not for either Fe-SOC or Co-SOC. During the regrowth experiments, CNTs were observed that were not present after the initial growth run (and were not as a result of amplification or double growth). Thus, catalysts, which were initially inactive toward nucleation of CNTs in the original growth run, are capable of becoming activated when placed back into the furnace and submitted to regrowth under identical conditions.

Catalyst and catalyst support morphology evolution in single-walled carbon nanotube supergrowth: Growth deceleration and termination

Detailed understanding of growth termination in vertically aligned single-walled carbon nanotubes (SWNTs) made via supergrowth, or water-assisted growth, remains critical to achieving the ultralong SWNTs necessary for next-generation applications. We describe the irreversible catalyst morphology evolution that occurs during growth, and which limits the lifetime of surface supported catalysts. Growth termination is strongly dependent on growth temperature, but not sensitive to C 2 H 2 :H 2 O ratio. In addition to both planar Ostwald ripening of small (sub-5 nm) Fe catalyst particles and diffusion of metal atoms into the alumina support, other features that contribute to growth termination or deceleration are described, including center-of-mass particle motions and coalescence of smaller species of surface supported Fe nanoparticles. Additionally, a temperature-induced structural transition in the alumina catalyst support is found to be coincident with abrupt growth termination at temperatures of 800 °C and higher. In situ electron microscopy observations are used to directly support these observations.