Alexander P. Moravsky

A Multiscale Study of High Performance Double-Walled Nanotube−Polymer Fibers

The superior mechanical behavior of carbon nanotubes (CNT) and their electrical and thermal functionalities has motivated researchers to exploit them as building blocks to develop advanced materials. Here, we demonstrate high performance double-walled nanotube (DWNT)-polymer composite yarns formed by twisting and stretching of ribbons of randomly oriented bundles of DWNTs thinly coated with polymeric organic compounds. A multiscale in situ scanning electron microscopy experimental approach was implemented to investigate the mechanical performance of yarns and isolated DWNT bundles with and without polymer coatings. DWNT-polymer yarns exhibited significant ductility of ∼20%, with energy-to-failure of as high as ∼100 J g(-1), superior to previously reported CNT-based yarns. The enhanced ductility is not at the expense of strength, as yarns exhibited strength as high as ∼1.4 GPa. In addition, the significance of twisting on the densification of yarns and corresponding enhancement in the lateral interactions between bundles is identified. Experiments at nanometer and macroscopic length scales on DWNT-polymer yarns and bundles further enabled quantification of energy dissipation/storage mechanisms in the yarns during axial deformations. We demonstrate that while isolated DWNT bundles are capable of storing/dissipating up to ∼500 J g(-1) at failure, unoptimal load transfer between individual bundles prevents the stress build up in the yarns required for considerable energy storage at the bundle level. By contrast, through polymer lateral interactions, a much better performance is obtained with the majority of energy dissipated at failure being contributed by the interactions between the polymer coating and the DWNTs as compared to the direct van der Waals interactions between bundles.

Dynamics of water confined in single- and double-wall carbon nanotubes.

Using high-resolution quasielastic neutron scattering, we investigated the temperature dependence of single-particle dynamics of water confined in single- and double-wall carbon nanotubes with the inner diameters of 14+/-1 and 16+/-3 A, respectively. The temperature dependence of the alpha relaxation time for water in the 14 A nanotubes measured on cooling down from 260 to 190 K exhibits a crossover at 218 K from a Vogel-Fulcher-Tammann law behavior to an Arrhenius law behavior, indicating a fragile-to-strong dynamic transition in the confined water. This transition may be associated with a structural transition from a high-temperature, low-density (<1.02 gcm(3)) liquid to a low-temperature, high-density (>1.14 gcm(3)) liquid found in molecular dynamics simulation at about 200 K. However, no such dynamic transition in the investigated temperature range of 240-195 K was detected for water in the 16 A nanotubes. In the latter case, the dynamics of water simply follows a Vogel-Fulcher-Tammann law. This suggests that the fragile-to-strong crossover for water in the 16 A nanotubes may be shifted to a lower temperature.

Bio-Inspired Carbon Nanotube–Polymer Composite Yarns with Hydrogen Bond-Mediated Lateral Interactions

Polymer composite yarns containing a high loading of double-walled carbon nanotubes (DWNTs) have been developed in which the inherent acrylate-based organic coating on the surface of the DWNT bundles interacts strongly with poly(vinyl alcohol) (PVA) through an extensive hydrogen-bond network. This design takes advantage of a toughening mechanism seen in spider silk and collagen, which contain an abundance of hydrogen bonds that can break and reform, allowing for large deformation while maintaining structural stability. Similar to that observed in natural materials, unfolding of the polymeric matrix at large deformations increases ductility without sacrificing stiffness. As the PVA content in the composite increases, the stiffness and energy to failure of the composite also increases up to an optimal point, beyond which mechanical performance in tension decreases. Molecular dynamics (MD) simulations confirm this trend, showing the dominance of nonproductive hydrogen bonding between PVA molecules at high PVA contents, which lubricates the interface between DWNTs.

Extraordinary Improvement of the Graphitic Structure of Continuous Carbon Nanofibers Templated with Double Wall Carbon Nanotubes

Carbon nanotubes are being widely studied as a reinforcing element in high-performance composites and fibers at high volume fractions. However, problems with nanotube processing, alignment, and non-optimal stress transfer between the nanotubes and surrounding matrix have so far prevented full utilization of their superb mechanical properties in composites. Here, we present an alternative use of carbon nanotubes, at a very small concentration, as a templating agent for the formation of graphitic structure in fibers. Continuous carbon nanofibers (CNF) were manufactured by electrospinning from polyacrylonitrile (PAN) with 1.2% of double wall nanotubes (DWNT). Nanofibers were oxidized and carbonized at temperatures from 600 °C to 1850 °C. Structural analyses revealed significant improvements in graphitic structure and crystal orientation in the templated CNFs, with the largest improvements observed at lower carbonization temperatures. In situ pull-out experiments showed good interfacial bonding between the DWNT bundles and the surrounding templated carbon matrix. Molecular Dynamics (MD) simulations of templated carbonization confirmed oriented graphitic growth and provided insight into mechanisms of carbonization initiation. The obtained results indicate that global templating of the graphitic structure in fine CNFs can be achieved at very small concentrations of well-dispersed DWNTs. The outcomes reveal a simple and inexpensive route to manufacture continuous CNFs with improved structure and properties for a variety of mechanical and functional applications. The demonstrated improvement of graphitic order at low carbonization temperatures in the absence of stretch shows potential as a promising new manufacturing technology for next generation carbon fibers.

Key Factors Limiting Carbon Nanotube Yarn Strength: Exploring Processing-Structure-Property Relationships

Studies of carbon nanotube (CNT) based composites have been unable to translate the extraordinary load-bearing capabilities of individual CNTs to macroscale composites such as yarns. A key challenge lies in the lack of understanding of how properties of filaments and interfaces across yarn hierarchical levels govern the properties of macroscale yarns. To provide insight required to enable the development of superior CNT yarns, we investigate the fabrication-structure-mechanical property relationships among CNT yarns prepared by different techniques and employ a Monte Carlo based model to predict upper bounds on their mechanical properties. We study the correlations between different levels of alignment and porosity and yarn strengths up to 2.4 GPa. The uniqueness of this experimentally informed modeling approach is the models ability to predict when filament rupture or interface sliding dominates yarn failure based on constituent mechanical properties and structural organization observed experimentally. By capturing this transition and predicting the yarn strengths that could be obtained under ideal fabrication conditions, the model provides critical insights to guide future efforts to improve the mechanical performance of CNT yarn systems. This multifaceted study provides a new perspective on CNT yarn design that can serve as a foundation for the development of future composites that effectively exploit the superior mechanical performance of CNTs.

Neutron spectroscopy of fullerite hydrogenated under high pressure; evidence for interstitial molecular hydrogen

Inelastic neutron scattering spectra of a hydrofullerite quenched after synthesis at 620 K under a hydrogen pressure of 0.6 GPa, and of the same sample after annealing at 300 K for 35 h, which reduced the hydrogen content by molecules per unit, were measured at 85 K. The quenched sample is shown to consist of molecules with and of interstitial molecular hydrogen. The interstitial molecular hydrogen left the sample during annealing at room temperature, whereas the molecules were stable at this temperature. The intramolecular and intermolecular vibrations of and in the fullerite are discussed in view of the measured spectra.

Commercial Production of Fullerenes and Carbon Nanotubes

It has been slightly over ten years since the development of a way to produce macroscopic quantities of fullerene, and the related discovery of fullerene nanotubes. As a result, over 1500 worldwide patents have been filed for the production and applications of these new materials. These applications are so wide ranging that they extend across different industries with products from additives to polymers, photoconductors, photo-resists, and bio-active agents to cosmetics. MER Corporation in Tucson, Arizona joined the ranks of fullerene enthusiasts at the beginning of its discovery by immediately licensing the Huffman-Kratschmer patents. While we are widely recognized as a producer of fullerene and nanotubes, MER has also been active in developing applications for fullerenes and nanotubes. The different applications investigated by MER will be reviewed in subsequent chapters. The overriding factor for the success of any of these applications, however, is the price of fullerenes. However, the price cannot come down markedly until large-scale applications are found. To introduce the first large-scale application an organization had to take a leap of faith and initiate the large-scale low-cost production. Mitsubishi/FIC Corporations has been a leader and pioneer in recognizing the need to support large-scale production effort to realize the fullerene and nano-technology commercialization dream. It is now our opportunity to realize the commercial applications. The present status of the scale-up production effort of fullerenes and the different nanotubes will be presented in this chapter.

Neutron spectroscopy of fullerite hydrogenated under high pressures

A new hydrofullerite was synthesized when the C 60 fullerite was treated under a hydrogen gas pressure of 30 kbar at 620 K for 24 h and quenched. The inelastic neutron scattering spectra of the as-prepared sample and that after annealing at 300 K showed that the as-prepared hydrofullerite consisted of C 60 H x molecules and interstitially dissolved molecular hydrogen. Both spectra exhibited peaks at 13 meV characteristic of polymeric fullerites. ( 1999 Elsevier Science B.V. All rights reserved.

Physical Hydrogen Storage on Nanotubes and Nanocarbon Materials

The recent developments of carbonaceous material synthesis have resulted in several new forms of carbon such as carbon nanotubes and carbon nanofibers, and super-high surface area activated carbons nano-materials. There are speculations that these materials may have extraordinarily high hydrogen storage capacities. In this work, we examined the hydrogen gas adsorption capacities of these carbonaceous materials at room temperature, as well as at liquid nitrogen temperature, to elucidate the hydrogen storage potential of these materials. Experimental results indicated that none of these materials showed significant hydrogen storage capacities at room temperatures, and only super-high surface area activated carbon showed attractive gravimetric hydrogen storage at cryogenic temperature, of over 5.4% by weight at 77 K and at 300 psi hydrogen gas pressure. However, the nanocarbon materials produced from the activation of fullerene (AC-C60), and vacuum soot (AC-VAS) showed increased hydrogen adsorption capacity compared to the best commercial super-high surface area activated carbons. In addition, nanotubes showed enhanced storage capacity for their surface area. The challenge is to further modify nanotubes materials to achieve high surface area and consequently high hydrogen storage capacity.

Neutron spectroscopy of C60Hx quenched under hydrogen pressure

Abstract Inelastic neutron scattering spectra were measured at 85 K for a hydrofullerite quenched to this temperature after a synthesis at 620 K under a hydrogen pressure of 0.6 GPa, and for the same sample annealed at 300 K. The quenched sample consisted of C 60 H 24 molecules and of interstitial molecular hydrogen (1.4 H 2 molecules per unit C 60 ). The interstitial H 2 left the sample during the anneling at 300 K. The intramolecular and intermolecular C 60 H 24 vibrations and H 2 rotations are discussed based on the measured spectra.