Biomimicry, technological innovation inspired by nature is one of the hottest ideas in science but has yet to yield many practical advances.
Scientists with the U.S. Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) have mimicked the structure of mother of pearl to create what may well be the toughest ceramic ever produced.
Through the controlled freezing of suspensions in water of an aluminum oxide and the addition of a well known polymer, polymethylmethacrylate (PMMA), a team of researchers has produced ceramics that are 300 times tougher than their constituent components.
The team was led by Robert Ritchie, who holds joint appointments with Berkeley Lab’s Materials Sciences Division and the Materials Science and Engineering Department at the University of California, Berkeley.
Mother of pearl, or nacre, the inner lining of the shells of abalone, mussels and certain other mollusks, is renowned for both its iridescent beauty and its amazing toughness.
Nacre is 95-percent aragonite, a hard but brittle calcium carbonate mineral, with the rest of it made up of soft organic molecules. Yet nacre can be 3,000 times (in energy terms) more resistant to fracture than aragonite.
No human-synthesized composite outperforms its constituent materials by such a wide margin. The problem has been that nacre’s remarkable strength is derived from a structural architecture that varies over lengths of scale ranging from nanometers to micrometers.
Two years ago, however, Berkeley Lab researchers Tomsia and Saiz found a way to improve the strength of bone substitutes through a processing technique that involved the freezing of seawater.
This process yielded a ceramic that was four times stronger than artificial bone. When seawater freezes, ice crystals form a scaffolding of thin layers.
These layers are pure ice because during their formation impurities, such as salt and microorganisms, are expelled and entrapped in the space between the layers. The resulting architecture roughly resembles that of nacre.
In this latest research, Ritchie, working with Tomsia and Saiz, refined the freeze-casting technique and applied it to alumina/PMMA hybrid materials to create large porous ceramic scaffolds that much more closely mirrored the complex hierarchical microstructure of nacre.
To do this, they first employed directional freezing to promote the formation of thin layers (lamellae) of ice that served as templates for the creation of the layered alumina scaffolds.
After the ice was removed, spaces between the alumina lamellae were filled with polymer.
For ceramic materials that are even tougher in the future, Ritchie says he and his colleagues need to improve the proportion of ceramic to polymer in their composites.
The alumina/PMMA hybrid was only 85-percent alumina. They want to boost ceramic content and thin the layers even further. They also want to replace the PMMA with a better polymer and eventually replace the polymer content altogether with metal.
Such future composite materials would be lightweight and strong as well as tough, he says, and could find important applications in energy and transportation.
This research was supported by DOE’s Office of Science, through the Division of Materials Sciences and Engineering in the Basic Energy Sciences office.
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