Mother-of-Pearl Nanoscale Structures

Mother-of-Pearl Nanoscale Structures

Mother-of-pearl, also called nacre, makes up the inner shell lining of pearl mussels and some other mollusks. Pearls themselves are made of nacre, which is a composite nanomaterial constructed by the biomachinery of the shellfish. Tiny crystal grains of calcium carbonate are arranged in a regular, intricate pattern and bound together by biopolymers in nacre's structure, which adds a tremendous amount of stability to the material: it is some 1000 times more resistant to cracking from impact than the crystalline form of calcium carbonate (the mineral aragonite) that makes up the bulk of nacre.

Indeed, calcium carbonate by itself is perhaps best known as blackboard chalk; its tendency to crumble undermines any notion that it would serve as an effective means of stopping a bullet. And yet nature organizes a complex brick-and-mortar-like structure--with the bricks of calcium carbonate measuring in the range of nanometers--to create an incredibly tough material, much stronger than the sum its parts. Mother-of-pearl's shimmering quality is a by-product of this structure, because the visible light that it reflects has wavelengths that are similar in size to the nanoscale bricks therein.

Nacre's strength under pressure, Li explained, is unusual and somewhat counter to intuition. When squeezed quickly (dynamic loading), it withstands far more pressure than when squeezed slowly (static loading). "This is a feature of natural materials with nanoparticle architectures," said Li, "Hardly any man-made ceramics have this property, which would be invaluable in applications like body armor, so understanding how it works is very important."

The increased strength of nacre in the face of rapid pressure has been known for 10 years, but the reasons underlying it have remained unclear. So Li's team set out to understand the mechanism by focusing on the structure of nacre at the nanoscale. They precisely cut mother-of-pearl samples from California red abalone and subjected them to both dynamic and static loading. The nacre that was squeezed rapidly--the ballistic test, in a sense--put up more than twice as much resistance before fracturing than that squeezed slowly. Then Li and co-workers, which included USC researchers as well as contributors from the University of North Carolina at Charlotte, used transmission electron microscopy to address the details of the fracturing at the nanoscale level.

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