Hydrogenated carbon nanotubes
Stretched for storage

Carbon nanotube is an excellent material for hydrogen storage and molecular sensory applications thanks to its quasi-one dimensional geometry and unique structure-property relationship. First-principles calculations show that the atomic structures, mechanical and electronic properties of carbon nanotubes can be significantly modified by hydrogenation. More impressively, applying strain can further control the hydrogenation process. These findings offer a way not only to engineer the properties of carbon nanotubes, but also to tune the binding strength of hydrogen in a controllable and reversible manner. See Xue and Xu, J. Comp. Theo. Nanosci., 2011 for details.
Spider silk
Secondary structures of graphene unveiled

Graphene features a two-dimensional structure, where applications from electronic building blocks to reinforced composites are emerging, enabled through the utilization of its intriguing electrical, mechanical and thermal properties. These properties are controlled by the structural makeup of graphene, which is known to display multiple morphologies that change under thermal fluctuations and variations of its geometry. A conformational phase diagram is recently presented for rectangular graphene sheets, defined by their geometry (size and aspect ratio), boundary conditions, and the environmental conditions such as supporting substrates and chemical modifications, as well as changes in temperature. See Keten, Xu and Buehler, ACS Nano, 2010 for details.
Biophysical Journal 2010
More dimensions for better dissipation

Upon the impact from trapped prey, spider's capture silk has the capacity to dissipate kinetic energy into the heat as fast as possible. We investigate this process, i.e. mechanical energy transfer and dissipation, in the crystalline domain of spider silk. Comparison is also made with other fibrous protein such as Alzheimer's amyloid fibril and beta solenoids. These biological materials share a common beta-sheet structure, which is the molecular basis for their outstanding mechanical properties. Our research shows that, althrough sharing similarily large stiffness and strength, silk nanocrystals feature much stronger energy dissipation capacity. The underlying two-dimensional hydrogen bonds network is the key. See Xu et al., 2010 for details.
Spider silk
Unraveling silks' secrets

A new analysis of the structure of silks explains the paradox at the heart of their super-strength, and may lead to even stronger synthetic materials.
Spiders and silkworms are masters of materials science, but scientists are finally catching up. Silks are among the toughest materials known, stronger and less brittle, pound for pound, than steel. Now scientists at MIT have unraveled some of their deepest secrets in research that could lead the way to the creation of synthetic materials that duplicate, or even exceed, the extraordinary properties of natural silk. See Keten, Xu, Britini and Buehler, Nature Materials, 2010 for details.
Mind the helical crack

Catastrophic breakage of brittle materials such as ceramics is usually triggered by the rapid spreading of cracks. Computer simulations have now cracked the three-dimensional details of this process.
Earthquakes, the damage to biological tissues caused by disease, and the wear of materials in aeroplanes share an underlying feature — all eventually reach a point at which the breakdown of material constituents leads to the failure of a functional system. In brittle materials such as ceramics, rocks and glass, a fundamental mechanism of failure is the spreading of cracks, a phenomenon also seen in the shearing of tectonic plates in earthquakes. Even though the phenomenon of fracture is seen throughout our world, the mechanisms by which cracks actually propagate remain largely unknown. On page 85 of this issue, Pons and Karma describe a computational study that reveals the origin of segmented fracture surfaces, which are found widely in fracture phenomena in engineering and geology.
The cracking process is complex, and predicting the path a crack will take remains a challenge. Part of the difficulty is that cracking is a multiscale phenomenon — that is, it depends on mechanisms that operate across multiple length scales. The spreading of cracks concurrently involves the overall material breakdown at the macroscopic scale, the evolution of crack fronts at intermediate micrometre scales, and the breaking of molecular or chemical bonds on nanometre or angstrom scales... See Buehler and Xu, Nature, 2010 for details.
Graphene Oxide
Nanoengineering graphene with oxygen 

Researchers from MIT and Beijing Institute of Technology have investigated graphene epoxide as an example of engineered graphene materials by functional groups. Their work focuses on the mechanical and electronic properties at various oxidation conditions. For regularly patterned epoxy structures, two phases are revealed to have considerable binding strength consistent with previous experimental observations: the clamped structure where the oxygen adsorbed on the sp2 bond and the unzipped structure where the epoxy binding breaks the sp2 bond. The clamped phase presents at high oxidation density and forms a regular lattice, while the unzipped phase is more stable and results in line defects in graphene. See Xu and Xue, Nanotechnology, 2010 for details.
Hydrogen bonded graphene nanoribbons
Scaling up graphene nanoribbons – a bioinspired solution

Graphene nanoribbons present intriguing electronic properties due to their characteristic size and edge shape, and have been suggested for a wide range of uses from electronics to electromechanical systems. However, there are hurdles to achieving larger-scale applications of graphene nano-ribbons. Specifically, the need for nanoscale precision, the material’s structural instability at elevated temperature, and its chemical activity along open edges. To be successful, a material approach must 1) be able to assemble graphene nanoribbons into macroscopic materials, while 2) preserving their physical significance and novel properties at larger scales. See Xu and Buehler, Nanotechnology, 2009 for details.
Hierarchical network
The new heat order

The extreme miniaturization that is reached in micro- and nano-electromechanical devices also generates a lot of heat at specific points. This heat has to be dissipated to stop the device from deteriorating, but normal heat-management techniques, such as metal wiring or fluid cooling, do not work well for sources of this size.
Zhiping Xu and Markus Buehler at the Massachusetts Institute of Technology in Cambridge used a theoretical model to evaluate the heat-dissipation performance of hierarchical networks composed of one-dimensional filaments — for example, linked carbon nanotubes. The authors discovered that, even with the same number of dissipating nodes, these structures are much more effective than non-hierarchical configurations ... See Xu and Buehler, Nano Letters, 2009 for details.
Carbon nanotubes: from stress to strength

Prestressed multiwalled carbon nanotubes have enhanced mechanical properties that are ideal for building space elevators. Scientists have envisioned a space elevator that could deliver people or cargos from Earth to a geostationary satellite ever since the discovery of carbon nanotubes. Extremely strong multiwalled carbon nanotubes (MWNTs) have been produced in the laboratory, but their specific strengths are still well below the minimum space-cable requirement of 48.5 GPa. Numerical simulations by Quanshui Zheng and co-workers at Tsinghua University in Beijing1 show that prestressed MWNTs — MWNTs with smaller interwall spacings than conventional MWNTs — have enhanced properties that might meet this requirement ... See Xu et al., Small, 2008 for details.
Nanotubes go transphononic

Researchers in China have discovered a new phenomenon in carbon nanotube-based devices that they have dubbed the transphonon effect. This resembles the transonic effect in aerodynamics. "The effect could be important for device design and may inspire research into nanoscale dynamics and energy-transfer problems," said team member Zhiping Xu of Tsinghua University in Beijing ...  See Xu et al., Nanotechnology, 2008 for details.

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