 |
|
|||
However, making a material porous is not without cost: Porosity decreases the stiffness of a material, which presents a problem when the material is used in a structural application (such as bone). Thus there is an optimal porosity, which combines high stiffness with low density. It would be expected that the stiffness-density characteristics for bone would be near optimal, since it is a result of hundreds of millions of years of evolution. But in a recent paper in Nature Materials, Case chemical engineering Professor Dan Lacks and collaborators at Sandia National Lab, University of New Mexico, and Princeton University show that nature’s attempts to maximize the stiffness for a given density can be topped with nanotechnology.
This relationship between porosity and stiffness in the nanoporous material cannot be explained by ordinary theories, but was revealed through a combination of spectroscopic experiments by the Sandia-New Mexico team and molecular simulations carried out at Case. These studies show that the nanoscale structure changes the way that the atoms pack together, so that the atomic packing becomes stiffer. As the porosity of the material increases, the increased stiffness of the atomic packing counteracts the usual decrease of stiffness with porosity, making the overall stiffness of the material insensitive to changes in porosity. The paper can be accessed from the publisher's web site here (subscription required). H. Fan, C. Hartshorn, T. Buchheit, T. Tallant, R. Assink, R. Simpson, D. J. Kissel D. J., Lacks, S. Torquato and C. J. Brinker, “Modulus-density scaling behavior and framework architecture of nanoporous, self-assembled silicas”, Nature Materials 6, 418 (2007).
|
||||
It takes less energy to move a light object than a heavy object. An energy saving strategy, exploited in both nature and technology, is to make objects lighter by making them porous. For example, bone has evolved to be porous in order to reduce the weight of the skeleton and therefore minimize the energy required for animals to move 
The team from Sandia and New Mexico, led by Jeff Brinker, developed a novel porous silica material where the porosity is ~50%, but with pore diameters that are only ~2 nanometers. When they measured the stiffness of these materials, they found that the porosity could be increased with only minor changes in the stiffness.