Meeting Abstract

11.3  Wednesday, Jan. 4  A Multiscale Structural Design of a Natural Transparent Armor: Placuna placenta LI, L.*; ORTIZ, C.; Massachusetts Institute of Technology, Cambridge; Massachusetts Institute of Technology, Cambridge

A number of species of mollusks possess transparent highly mineralized exoskeletons which combine optical and mechanical functionalities that originate from their intricate and hierarchical structures. In this study, the structure of the highly translucent shell from the mollusk Placuna placenta (Linnaeus 1758) was investigated from nanometer to millimetre length scales using a variety of high resolution experimental techniques. Electron microscopy showed that the entire P. placenta shell (~0.5 mm) has a single primary foliated layer (further stratified with ~1750 individual layers), which is made up of an organic-inorganic nanocomposite (98.93 wt% calcite and 1.07 wt% organic, as measured by thermogravimetric analysis). Each individual foliated layer is composed of fundamental building blocks which are elongated diamond-shaped plates with a characteristic length (141.8 ± 43.4 μm), width (5.54 ± 1.36 μm) and tip angle (10.45 ± 2.95°). Two salient microstructural features of the surface of the individual layers in the foliated structure were observed: 1) micro-ridges (maximum height: 67.0 ± 37.2 nm; inclination angles: 1.4 ± 0.4 and 2.6 ± 0.5°) and 2) nanoasperities (maximum dimension and height: 54.8 ± 17.9 nm and 4.2 ± 2.5 nm), as measured by tapping mode atomic force microscopy. Both electron backscattered diffraction analysis and selected area electron diffraction show that the calcite c-axis was tilted (24.44±6.81°) towards to the end of the diamond-shaped building block plates and the misorientation between adjacent crystalline grains was 13.52 ± 6.63°. The multiscale structural characteristics and the crystallographic arrangement features of this natural nanocomposite contribute to its unique transparent optical behaviour by lowering the scattering and absorption.