Three parallel algorithms for classical molecular dynamics are presented. The first assigns each processor a fixed subset of atoms; the second assigns each a fixed subset of inter-atomic forces to compute; the third assigns each a fixed spatial region. The algorithms are suitable for molecular dynamics models which can be difficult to parallelize efficiently—those with short-range forces where the neighbors of each atom change rapidly. They can be implemented on any distributed-memory parallel machine which allows for message-passing of data between independently executing processors. The algorithms are tested on a standard Lennard-Jones benchmark problem for system sizes ranging from 500 to 100,000,000 atoms on several parallel supercomputers--the nCUBE 2, Intel iPSC/860 and Paragon, and Cray T3D. Comparing the results to the fastest reported vectorized Cray Y-MP and C90 algorithm shows that the current generation of parallel machines is competitive with conventional vector supercomputers even for small problems. For large problems, the spatial algorithm achieves parallel efficiencies of 90% and a 1840-node Intel Paragon performs up to 165 faster than a single Cray C9O processor. Trade-offs between the three algorithms and guidelines for adapting them to more complex molecular dynamics simulations are also discussed.

Fast parallel algorithms for short-range molecular dynamics

Worldwide commercial interest in carbon nanotubes (CNTs) is reflected in a production capacity that presently exceeds several thousand tons per year. Currently, bulk CNT powders are incorporated in diverse commercial products ranging from rechargeable batteries, automotive parts, and sporting goods to boat hulls and water filters. Advances in CNT synthesis, purification, and chemical modification are enabling integration of CNTs in thin-film electronics and large-area coatings. Although not yet providing compelling mechanical strength or electrical or thermal conductivities for many applications, CNT yarns and sheets already have promising performance for applications including supercapacitors, actuators, and lightweight electromagnetic shields.

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Carbon Nanotubes: Present and Future Commercial Applications

Human skin is a remarkable organ. It consists of an integrated, stretchable network of sensors that relay information about tactile and thermal stimuli to the brain, allowing us to maneuver within our environment safely and effectively. Interest in large-area networks of electronic devices inspired by human skin is motivated by the promise of creating autonomous intelligent robots and biomimetic prosthetics, among other applications. The development of electronic networks comprised of flexible, stretchable, and robust devices that are compatible with large-area implementation and integrated with multiple functionalities is a testament to the progress in developing an electronic skin (e-skin) akin to human skin. E-skins are already capable of providing augmented performance over their organic counterpart, both in superior spatial resolution and thermal sensitivity. They could be further improved through the incorporation of additional functionalities (e.g., chemical and biological sensing) and desired properties (e.g., biodegradability and self-powering). Continued rapid progress in this area is promising for the development of a fully integrated e-skin in the near future.

25th Anniversary Article: The Evolution of Electronic Skin (E-Skin): A Brief History, Design Considerations, and Recent Progress

All carbon aerogels (up to 1000 cm(3)) with ultralow density (down to 0.16 mg cm(-3)) and temperature-invariant (-190-900 °C) super-elasticity are fabricated by facile assembling of commercial carbon nanotubes (CNTs) and chemically-converted giant graphene sheets, on the basis of the synergistic effect between elastic CNTs ribs and giant graphene cell walls.

Multifunctional, ultra-flyweight, synergistically assembled carbon aerogels.

Chemically converted graphene aerogels with ultralight density and high compressibility are prepared by diamine-mediated functionalization and assembly, followed by microwave irradiation. The resulting graphene aerogels with density as low as 3 mg cm(-3) show excellent resilience and can completely recover after more than 90% compression. The ultralight graphene aerogels possessing high elasticity are promising as compliant and energy-absorbing materials.

Ultralight and Highly Compressible Graphene Aerogels

A mechanically fragile aerogel made of single-walled carbon nanotubes can be transformed into a superelastic material by coating it with graphene.

Graphene coating makes carbon nanotube aerogels superelastic and resistant to fatigue

IO N Materials that can remain electrically conducting under large elastic stretching and bending are needed for fabrication of fl exible displays, stretchable electronic implants, artifi cial mechanoreceptors, electrically actuated elastomers for artifi cial muscles, and loudspeakers. [ 1–8 ] One approach to fabricate stretchable or elastic conductors has been to imprint patches of inorganic thin fi lms with conductive interconnects on a prestrained elastomeric substrate. [ 9–12 ] However, the performance and diversity of such elastic conductors are limited by the degree of initial prestrain, the brittleness of the inorganic fi lms, and the compatibility between the inorganic materials and polymeric substrates. [ 1 ] Another approach developed in recent years is to embed networks of conductive nanoparticles such as graphene or carbon nanotubes (CNTs), primarily by direct dispersion using sonication or high shear mixing, into an elastomeric polymer matrix. [ 13–21 ] Graphene-based elastic conductors are transparent and electrically conductive, but electrical conductivity decreases rapidly with tensile strain larger than ≈ 10%. [ 13 , 14 ] Stretchable conductors have also been fabricated using tens of micrometerto millimeter-long bundles and ropes of single-walled (SWCNTs) and multiwalled (MWCNTs) CNTs. However, with a few exceptions, [ 15–19 ] CNT-based elastic conductors offer modest electrical conductivity, require high concentrations of CNTs, are opaque, and their electrical conductivity decreases signifi cantly when stretched. [ 15–17 , 19 ] Furthermore, graphene and CNTs phase-segregate or agglomerate within elastomers during dispersion, which hinders practical-scale usage of grapheneor CNT-based elastic conductors. To avoid such segregation or agglomeration of CNTs, we report a novel stretchable conductor fabrication method utilizing preformed, highly porous 3D networks of SWCNTs, called SWCNT aerogels, and then integrating elastomeric polymers. SWCNT aerogels are ultralight and electrically conducting created by critical-point drying of aqueous elastic gel of individually dispersed SWCNTs. [ 22–26 ] The SWCNTs are connected to each other within the network via van der Waals interactions at the SWCNT–SWCNT junctions. [ 24 , 25 ] The concentration of SWCNTs within the aerogel can be as low as 6 mg mL − 1 and the rest is void space. [ 26 ] The aerogel shapes and sizes are readily

Single-walled carbon nanotube aerogel-based elastic conductors.

Lightweight aerogels with large specific surface area (SSA) have numerous applications. Free-standing aerogels are created from single-walled carbon nanotubes (SWCNTs), and their SSA and pore characteristics, electrical conductivity, mechanical properties, and thermal management attributes are determined. The SSA of the aerogels is extraordinarily high and approaches 1291 m2 g−1 at a density of 7.3 mg mL−1, which is close to the theoretical limit (≈1315 m2 g−1). Mechanical characterization shows that these aerogels have open-cell structures and their Young's moduli are higher than other aerogels at comparable density. The aerogels also enhance heat transfer in a forced convective process by ≈85%, presumably due to their large porosity and surface area.

Mechanical and Thermal Management Characteristics of Ultrahigh Surface Area Single-Walled Carbon Nanotube Aerogels

The thermal conductivity of gas-permeated single-walled carbon nanotube (SWCNT) aerogel (8 kg m − 3 density, 0.0061 volume fraction) is measured experimentally and modeled using mesoscale and atomistic simulations. Despite the high thermal conductivity of isolated SWCNTs, the thermal conductivity of the evacuated aerogel is 0.025 ± 0.010 W m − 1 K − 1 at a temperature of 300 K. This very low value is a result of the high porosity and the low interface thermal conductance at the tube‐tube junctions (estimated as 12 pW K − 1 ). Thermal conductivity measurements and analysis of the gas-permeated aerogel (H 2 , He, Ne, N 2 , and Ar) show that gas molecules transport energy over length scales hundreds of times larger than the diameters of the pores in the aerogel. It is hypothesized that ineffi cient energy exchange between gas molecules and SWCNTs gives the permeating molecules a memory of their prior collisions. Low gas-SWCNT accommodation coeffi cients predicted by molecular dynamics simulations support this hypothesis. Amplifi ed energy transport length scales resulting from low gas accommodation are a general feature of CNT-based nanoporous materials.

Gas Diffusion, Energy Transport, and Thermal Accommodation in Single‐Walled Carbon Nanotube Aerogels

Lightweight, superelastic foams that resist creep and fatigue over a broad temperature range are being developed as structural and functional materials for use in numerous diverse applications. Unfortunately, conventional foams display superelasticity degradation, undergo considerable creep, show fatigue under repeated usage, or fracture over large strains, particularly under significant temperature variations. We report that graphene-coated single-walled carbon nanotube (SWCNT) aerogels remain superelastic, and resist fatigue and creep over a broad temperature range of −100–500 °C. The microstructure of these ultralow density (≈14 mg/mL; corresponding volume fraction ≈9 × 10–3) aerogels is composed of a three-dimensional network of randomly oriented SWCNTs with junctions between SWCNTs coated with 2–5 layers of ≈3 nm long graphene nanoplatelets. Compressive stress (σ) versus compressive strain (e) curves show that the aerogels fully recover their shapes even when strained by at least 80% over −100–300 °C...

Kyu Hun Kim

Papers

Graphene coating makes carbon nanotube aerogels superelastic and resistant to fatigue

Single-walled carbon nanotube aerogel-based elastic conductors.

Mechanical and Thermal Management Characteristics of Ultrahigh Surface Area Single-Walled Carbon Nanotube Aerogels

Gas Diffusion, Energy Transport, and Thermal Accommodation in Single‐Walled Carbon Nanotube Aerogels

Graphene-Coated Carbon Nanotube Aerogels Remain Superelastic while Resisting Fatigue and Creep over −100 to +500 °C