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Xiaodong Li and colleagues at the
University of South Carolina, Chung
Yuan Christian University, Taiwan, and
the University of British Columbia,
Canada have characterized the
structure and mechanical properties of
seashells in order to understand their
extraordinary toughness [Li et al.,
Nano Lett.(2004) 4 (4), 613].
Nature has evolved complex bottom-up methods for fabricating ordered nanostructured materials ideally suited to their functions. For example, motherof- pearl or nacre is a remarkably robust nanocomposite despite the brittle nature of its components. It is made up of ~95% aragonite, a mineral form of CaCO3?Create, and a few percent organic biopolymers, but the composite has a toughness 1000 times greater than its constituents. Nacre is known to have a ‘bricks and mortar’ structure with aragonite platelets held together by biopolymer glue. The platelets were thought to be single crystals, based on their electron diffraction pattern. Li and colleagues used atomic force and scanning electron microscopy to show, for the first time, that individual platelets consist of ‘cobble-like’, polygonal nanosized grains. During biomineralization, nanoparticles aggregate into platelets with the same crystal orientation.
The researchers also show the platelets are ductile, not brittle – a property that could result from the large number of nanograins. Indentations made with a microindenter produce short, radial cracks that propagate along the biopolymer mortar layers. Around the crack tip, the platelets are plastically deformed. “This will change our conventional concept of nacre aragonite platelets,” says Li.
“The discovery of deformability of platelets is of critical importance to understanding the secret of ultrahigh fracture toughness for seashells.” Understanding how nacre’s structure and toughness are related could enable the development of synthetic nanocomposites that reproduce some of these properties.
Jonathan Wood
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