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From:
Physicsworld.com, Sep 16, 2014
http://physicsworld.com/cws/article/news/2014/sep/16/tiny-scaffolds-toughen-ceramics
The author of the blog, Anna Demming, online editor of nanotechweb.org, writes:
"A nanostructured ceramic material that does not break when deformed has been developed by researchers at the California Institute of Technology. The new material incorporates a scaffolding of nanotubes, which gives it ultra-low density and high strength but none of the brittleness that is seen in artificially nanostructured ceramics.
"Julia Greer and colleagues created the material by arranging alumina nanotubes with diameters of about one micron into a trussed lattice structure. This familiar arrangement of criss-crossed struts is widely used in buildings and other large structures. 'It turns out that there is a critical thickness-to-diameter ratio of the [nanotube] struts, below which it is possible for the nanolattice to deform via shell buckling rather than via fracture,' explains Greer.
"Eight triangles bear the load
"Trusses are structures comprising five or more triangular units, and Greer and Caltech graduate students Lucas Meza and Satyajit Das chose a structure that exploits eight individual triangles. Greer explains that an octet-truss is one of very few lattice geometries with mechanical behaviour dominated by stretching, rather than bending, in response to applied loads.
"Were the researchers surprised to find this behaviour in the thinner structures? 'Of course,' says Greer. 'These structures don't fail in a brittle fashion like any macro material of the sort would, or even like the 50 nm-thick walled octet in our study.'
....
"Eager to understand their observations, the team developed a model for the critical transition point between fracture and elastic failure. This was able to predict that the thicker alumina tubes would fail because of brittle fracture, while the thinner tubes were more prone to buckling through an elastic failure mode.
"Like macro-scale engineering structures, the stress was largely localized at the nodes of the truss structures. 'There are several ways to improve nodal strength,' says Greer. 'But that's not really what we are working on. Improving strength is something that engineers do; we are scientists, so we do scientific discovery and leave improvements for engineers.'"
See:
Science. 2014 Sep 12;345(6202):1322-6. doi: 10.1126/science.1255908.
Strong, lightweight, and recoverable three-dimensional ceramic nanolattices.
Meza LR1, Das S1, Greer JR2.
Author information
Abstract
Ceramics have some of the highest strength- and stiffness-to-weight ratios of any material but are suboptimal for use as structural materials because of their brittleness and sensitivity to flaws. We demonstrate the creation of structural metamaterials composed of nanoscale ceramics that are simultaneously ultralight, strong, and energy-absorbing and can recover their original shape after compressions in excess of 50% strain. Hollow-tube alumina nanolattices were fabricated using two-photon lithography, atomic layer deposition, and oxygen plasma etching. Structures were made with wall thicknesses of 5 to 60 nanometers and densities of 6.3 to 258 kilograms per cubic meter. Compression experiments revealed that optimizing the wall thickness-to-radius ratio of the tubes can suppress brittle fracture in the constituent solid in favor of elastic shell buckling, resulting in ductile-like deformation and recoverability.
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