July 24, 1998 ALBUQUERQUE, N.M. -- Humanity has valued seashells for their beauty as ornaments and utility as tools for thousands of years. But even alchemists never tried transmuting base materials into the very fine interlayering necessary to create a shell's strength, hardness, and toughness.
Now, in a paper published in the July 16 Nature, researchers at the Department of Energy's (DOE) Sandia National Laboratories and the University of New Mexico (UNM) disclose a rapid and efficient method to self-assemble diverse materials into coatings that mimic seashell structures.
The process permits rapid formation of tough, strong, optically transparent coatings suitable for applications such as automotive finishes, as well as coatings for implements and optical lenses.
A preliminary analysis shows the coating to be more than twice as hard as the same materials mixed randomly.
The secret of easily accreting these very thin laminations had eluded scientists for years. Laminations based upon the seashell model are important because of the improved properties achieved by alternating layers of flexible, cushioning biopolymers with hard layers of calcium carbonate. Abalone shell, for example, composed of approximately one percent polymer and 99 percent aragonite (CaCO3) by volume, is two times harder and 1,000 times stronger than its constituent materials.
Cracks that begin in any calcium carbonate layer are immediately intercepted by a polymer layer. The crack thus requires still more energy before it can propagate through succeeding calcium carbonate layers, thus improving their resistance to fractures.
The process, developed by Sandia researchers Alan Sellinger and Jeff Brinker, working with UNM students Pilar Weiss and Yungfeng Lu, is based on the scientifically well-known tendency of two-sided detergent molecules, composed of hydrophilic (water-loving) and hydrophobic (water-hating) portions, to spontaneously form spherical molecular assemblies called micelles.
In water, micelles arrange themselves so that the water-loving part of the detergent is in contact with water, while the water-hating, hydrophobic part is shielded in the micellar interior. This arrangement is useful when washing dishes because oils are quickly adsorbed in the hydrophobic interiors, allowing them to be rinsed away. Similarly, in the composite process, micelles separate and organize inorganic molecules around the micelles' hydrophilic exterior and organic molecules within the hydrophobic interiors.
During the coating process, the intentional evaporation of water further arranges the micelles into alternating layers of organic and inorganic molecules -- called precursors -- in a single step. A low- temperature heat treatment polymerizes the organic and inorganic layers and bonds their interfaces. Previous nanocomposite assembly processes involved tedious, sequential deposition of individual organic and inorganic layers. This approach produces, after much time, only layered constructions. The Sandia process takes only a few seconds and can easily be modified to achieve 1-, 2-, or 3-dimensional connectivity of the reinforcing polymer phase.
The work was funded in part by the UNM/National Science Foundation's Center for Micro-Engineered Materials, DOE's Basic Energy Sciences, and Sandia's Laboratory-Directed Research and Development Program.
Sandia is a multiprogram DOE laboratory, operated by a subsidiary of Lockheed Martin Corp. With main facilities in Albuquerque, N.M., and Livermore, Calif., Sandia has major research and development responsibilities in national security, energy, and environmental technologies.
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