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Honolulu, Hawaii, United States, 2006/10/02 - New research findings may lead to the development of ultra-tough nanocomposites, for instance for armor material, by realizing the rotation mechanism of the nanograins in seashells.
Seashells are natural armor materials. The need for toughness arises because aquatic organisms are subject to fluctuating forces and impacts during motion or through interaction with a moving environment. Nacre (mother-of-pearl), the pearly internal layer of many mollusc shells, is the best example of a natural armor material that exhibits structural robustness, despite the brittle nature of their ceramic constituents. This material is composed of about 95% inorganic aragonite with only a few percent of organic biopolymer by volume. New research at the university of South Carolina reveals the toughening secrets in nacre: rotation and deformation of aragonite nanograins absorb energy in the deformation of nacre. The aragonite nanograins in nacre are not brittle but deformable. The new findings may lead to the development of ultra-tough nanocomposites, for instance for armor material, by realizing the rotation mechanism.
Super-tough and ultra-high temperature resistant materials are in critical need for applications under extreme conditions such as jet engines, power turbines, catalytic heat exchangers, military armors, aircrafts, and spacecrafts. Structural ceramics have largely failed to fulfill their promise of revolutionizing engines with strong materials that withstand very high temperature. The major problem with the use of ceramics as structural materials is their brittleness. Although many attempts have been made to increase their toughness, including incorporation of fibers, whiskers, or particles, and ZrO2 phase transformation toughening, currently available ceramics and their composites are still not as tough as metals and polymers. The brittleness of ceramic materials has not yet been overcome. It has proven difficult to solve this problem by conventional approaches.
On the other hand, Nature has evolved complex bottom-up methods for fabricating ordered nanostructured materials that often have extraordinary mechanical strength and toughness. One of the best examples is nacre. It has evolved through millions of years to a level of optimization not currently achieved in engineered composites.
This material has a brick-and-mortar-like structure with highly organized polygonal aragonite platelets of a thickness ranging from 200 to 500 nm and an edge length about 5 µm sandwiched with a 5-20 nm thick organic biopolymer interlayer, which assembles the aragonite platelets together. The combination of the soft organic biopolymer and the hard inorganic calcium carbonate produces a lamellar composite with a 2-fold increase in strength and a 1000-fold increase in toughness over its constituent materials.
Such remarkable properties have motivated many researchers to synthesize biomimetic nanocomposites that attempt to reproduce nature’s achievements and to understand the toughening and deformation mechanisms of natural nanocomposite materials.
Dr. Xiaodong Li, who heads the Nanostructures and Reliability Laboratory at the University of South Carolina, and his team have published two papers that examine the role of nanostructures in the amazing properties of nacre. In a first paper (" Nanoscale Structural and Mechanical Characterization of a Natural Nanocomposite Material: The Shell of Red Abalone"), the group reported the discovery of nanosized grains (particles) in nacre. However, the functionality of these aragonite nanograins was entirely unknown. Subsequently, many research groups asked: What roles do the nanoscale structures play in the inelasticity and toughening of nacre? Can we learn from this to produce nacre-like nanocomposites?
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By Michael Berger, Copyright 2006 Nanowerk, LLC