Mother-of-Pearl on Ice: New ceramics might serve in bones and machines

January 25, 2006 at 11:57 am

Beneath the shimmer of an oyster’s mother-of-pearl, an intricate microstructure bestows both strength and toughness on the natural ceramic. Now, scientists have come up with a way to replicate that structure in humanmade substances.

BRICK-A-STACK. Novel materials made from microplates of ceramic such as alumina (left) joined by metal mortar replicate the microstructure and properties of natural mother-of-pearl of an abalone shell (right). The two micrographs are at different scales; the artificial plates are actually about 10 times as thick as those in the mother-of-pearl. Deville et al./Science

The process exploits one of the most common transformations in nature—the freezing of water—so it’s remarkably simple and potentially inexpensive and environmentally friendly, its developers say.

These researchers, at the Lawrence Berkeley (Calif.) National Laboratory, have used their new approach to create an exceptionally rugged substance that may serve as a scaffold for new bone growth. The method also works well with nonbiological materials, report Sylvain Deville and his colleagues in the Jan. 27 Science. Using it, the team has fabricated novel metal-ceramic composites that benefit from a seashell-like internal architecture.

Mollusks such as abalone and oysters create their iridescent armor, known as nacre, from brittle calcium carbonate microcrystals and pliant proteins arranged like bricks and mortar, respectively (SN: 5/16/92, p. 328). Materials specialists have long envied the composite’s resilience, which is superior to that of human-made ceramics.

Past efforts to artificially replicate the shells’ architecture have typically stalled after a few microlayers or generated cruder laminations than those in the real stuff, says team member Eduardo Saiz (SN: 6/21/03, p. 397: Available to subscribers at Material mimics mother-of-pearl in form and substance). Using the new method, he, Deville, and Antoni P. Tomsia of the Lawrence Berkeley lab and Ravi K. Nalla, now at Intel Corp. in Chandler, Ariz., fabricated centimeters-thick chunks of ceramic with internal layering almost as thin as that of natural nacre.

“This is an exciting paper,” comments Manfred Rühle of the Max Planck Institute for Metals Research in Stuttgart, Germany. The new approach “represents a breakthrough in processing advanced materials,” he adds.

To make a microstructured ceramic, Deville and his colleagues mixed water with finely ground ceramic powder and polymer binders. They then poured the blend into a chamber a few centimeters across. By carefully controlling subfreezing temperatures at the chamber’s bottom and top, the researchers produced a temperature gradient that generated an ice structure sometimes observed in frozen seawater.

In that structure, sheets of microscopic hexagonal ice crystals formed vertically in the chamber. As those crystals grew, they forced the powder and binders to congregate between the pure-ice sheets. Freeze-drying removed the ice, and high-temperature sintering then solidified each ceramic-binder layer into a solid plate. Finally, the researchers selected a substance to play the role of nacre’s protein and introduced it into the spaces between the ceramic plates.

To create bonelike composites, the researchers employed epoxy as the mortar between plates of hydroxyapatite, which is the predominant ceramic in bone and teeth. For nonbiological materials, they bound alumina plates with a mortar containing an alloy of aluminum and silicon and, in some cases, titanium. Such composites may prove useful to many industries, including electronics, machining, and aerospace manufacturing.