The theoretical maximum tensile strain--that is, elongation--of a single-walled carbon nanotube is almost 20%, but in practice only 6% is achieved. Here we show that, at high temperatures, individual single-walled carbon nanotubes can undergo superplastic deformation, becoming nearly 280% longer and 15 times narrower before breaking. This superplastic deformation is the result of the nucleation and motion of kinks in the structure, and could prove useful in helping to strengthen and toughen ceramics and other nanocomposites at high temperatures.
We report on atomic layer deposition of an ∼ 2-nm-thick ZnO layer on the inner surface of ultralow-density (∼ 0.5% of the full density) nanoporous silica aerogel monoliths with an extremely large effective aspect ratio of ∼ 10 5 (defined as the ratio of the monolith thickness to the average pore size). The resultant monoliths are formed by amorphous-SiO2/wurtzite-ZnO nanoparticles which are randomly oriented and interconnected into an open-cell network with an apparent density of ∼ 3% and a surface area of ∼ 100 m 2 g −1. Secondary ion mass spectrometry and high-resolution transmission electron microscopy imaging reveal excellent uniformity and crystallinity of ZnO coating. Oxygen K-edge and Zn L3-edge soft x-ray absorption near-edge structure spectroscopy shows broadened O 2p-as well as Zn 4s-, 5s-, and 3d-projected densities of states in the conduction band.
We report the analysis of repeated transients to monitor the coupled evolution of dislocation velocity and mobile dislocation density in plastically deforming nanocrystalline Ni. The stress relaxation series allowed the determination of the physical activation volume, indicating a rate-controlling mechanism different from that in coarse-grained Ni. The mobile dislocation exhaustion observed is correlated with the unusually high apparent work-hardening rate during the early stage of straining.
The deformation mechanism of body-centered cubic (bcc) nanocrystalline tantalum with grain sizes of 10–30 nm is investigated by nanoindentation, scanning electron microscopy and high-resolution transmission electron microscopy. In a deviation from molecular dynamics simulations and existing experimental observations on other bcc nanocrystalline metals, the plastic deformation of nanocrystalline Ta during nanoindentation is controlled by deformation twinning. The observation of multiple twin intersections suggests that the physical mechanism of deformation twinning in bcc nanocrystalline materials is different from that in face-centered cubic (fcc) nanocrystalline metals.
At liquid nitrogen temperature, the yield strength of nanocrystalline Ni and Co increases by as much as 50%–80% over the already-impressive (∼1GPa) room-temperature values. This unusual strength ratio as well as the remarkable magnitude of flow stress reached (as high as 2.5GPa) are unexpected for conventional close-packed pure metals. The strong temperature dependence is attributed to the unusually small activation volume measured in strain rate change tests. Grain boundary dislocation nucleation is proposed as the thermally activated deformation mechanism in nanocrystalline grains.
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