Crystal growth via spiral motion in abalone shell nacre
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We present a structural feature of nacre in the red abalone shell: micrometer-scale screw dislocations in the aragonite layers and resultant growth via spiral motion. Compared to typical ionic or covalent crystals, nacre contains 106 screw dislocations per square centimeter, a difference of three orders of magnitude. Using electron microscopy, ion microscopy, and an in situ nano-manipulator, we studied the structure of screw dislocation cores in detail. We considered that these screw dislocations contribute significantly to the strengthening mechanisms that lead to nacre’s extraordinary work of fracture, which is three orders of magnitude greater than that of aragonite and other monolithic crystals. This work suggests that the lamellar layers of aragonite propagate via a large number of continuous spiral growth domains as the “stacks of coins” become confluent. This model may provide a basis for creating new comparable micro/nanocomposites through synthetic or biomineralization means.
I. INTRODUCTION
Biomineralized materials often exhibit structural robustness in spite of the brittleness of their constituents. Bone, teeth, and mollusk shells are the most common and well-known examples.1–4 Nacre (mother of pearl), the internal iridescent layer of mollusk shells, is a natural ceramic composite comprised of 95% calcium carbonate in the aragonite polymorph with 5% or less organic macromolecules sandwiched in between. Yet its work of fracture is about 3000 times greater than monolithic ceramics, and its strength is among the highest in shell structures.5–7 In addition, its capability for substantial inelastic strain, in contrast to most ceramics, renders it notch insensitive and can eliminate stress concentrations and catastrophic failure.8 Because of nacre’s extraordinary physical properties as a self-assembled organic/ inorganic composite, it has attracted much interest in many disciplines, such as surface chemistry, colloidal physics, molecular biology, and, of course, materials science. A detailed understanding of the mechanisms underlying its robustness could inspire advances in materials design and synthesis, producing synthetic composite materials with optimized desirable properties. The wellorganized structure of nacre might also prove a model system for characterizing biogenic composites in general.9,10 Today’s synthetic versions made of interlocking ceramic tablets bound by a few weight percent of ordinary
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2006.0252 J. Mater. Res., Vol. 21, No. 8, Aug 2006
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adhesives have a fracture toughness higher than ceramics without such microarchitecture, but they still do not have a toughness comparable to that of nacre.11 Mature nacre consists of pseudo-hexagonal or polygonal aragonite platelets about 5–8 m in diameter and 0.5 m in height that generally are both stacked vertically and form horizontal sheets of lamellae similar to a brick wall,7 as shown in Fig. 1. The g
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