Self-assembled nanostructures function better than bone as porosity increases

July 10, 2007
Self-assembled nanostructures function better than bone as porosity increases
On the left is a TEM micrograph of a porous, cube-like nanostructure. On the right is a blow-up of the silica framework (the dark <2-nm thick regions on the left side figure) based on modeling. The highlighted structures represent the small rings referred to in the story. Credit: Sandia National Laboratory

Naturally occurring structures like birds’ bones or tree trunks are thought to have evolved over eons to reach the best possible balance between stiffness and density.

But in a June paper in Nature Materials, researchers at Sandia National Laboratories and the University of New Mexico (UNM), in conjunction with researchers at Case Western Reserve and Princeton Universities, show that nanoscale materials self-assembled in artificially determined patterns can improve upon nature’s designs.

“Using self-assembly we can construct silica materials at a finer scale than those found in nature,” says principal investigator Jeff Brinker. “Because, at very small dimensions, the structure and mechanical properties of the materials change, facile fabrication of stiff, porous materials needed for microelectronics and membrane applications may be possible.”

Nuclear magnetic resonance and Raman spectroscopic studies performed by Sandia researchers Roger Assink (ret.) and Dave Tallant, along with molecular modeling studies performed by Dan Lacks at Case Western Reserve University, showed that, as the ordered porous films became more porous, the silica pore walls thinned below 2 nm, re-arranging the silica framework to become denser and stiffer.

Whereas the stiffness of evolved optimized bone declines proportional to the square of its density, mechanical studies performed by Sandia researcher Thomas Buchheit working with UNM student Christopher Hartshorn showed that the stiffness/modulus of self-assembled materials was much less sensitive to increasing porosity: For a material synthesized with a cubic arrangement of pores, the modulus declined only as the square root of its density.

The silica nanostructures — basically a synthetic analogue of bone-like cellular structures, replicated at the nanoscale using silica compounds — thus may improve performance where increased pore volume is important. These include modern thin-film applications such as membrane barriers, molecular recognition sensors, and low-dielectric-constant insulators needed for future generation of microelectronic devices.

“Bone, closely examined, is a structured cellular material,” says Brinker, a Sandia Fellow and chemical engineering professor at UNM. “Because, using self-assembly, we had demonstrated the fabrication of a variety of ordered cellular materials at the nanoscale with worm-like (curving cylinders), hexagonal (soda straw packing) and cubic sphere arrangements of pores, we wondered whether the modulus-density scaling relationships of these nanoscale materials would be similar to the optimized evolved materials [like bone]. We found that both material structure and pore sizes matter. At all densities we observed that the cubic arrangement was stiffer than the hexagonal arrangement, which was stiffer than the worm-like. For each of these structures, increasing porosity caused a reduction in modulus, but the reduction was less than for theoretically optimized or naturally evolved materials due to the attendant stiffening of the thinning nanoscale silica walls resulting from the formation of small stiff silica rings.

“This change in ring structure only happens at the nanoscale,” says Brinker.

Sandia researcher Hongyou Fan created cubic, cylindrical, and worm-like (or disordered) pores to evaluate differences in stiffness resulting from these differently shaped internal spaces.

Other paper authors include Dave Kissel of UNM, Regina Simpson at Sandia, and Salvatore Torquato of Princeton.

Source: Sandia National Laboratory

Explore further: Reducing power plants' freshwater consumption with new silica filter

Related Stories

Researchers solve mystery of attractive surfaces

August 2, 2006

When smooth surfaces that hate water approach each other underwater, scientists have observed that they snap into contact. This is apparently due to attractive forces that extend for tens to hundreds of nanometers.

FBI unveils science of anthrax investigation

August 22, 2008

They have worked for almost seven years in secret. Most people did not know that the work in Ray Goehner's materials characterization department at Sandia National Laboratories was contributing important information to the ...

Recommended for you

Clothing fabric keeps you cool in the heat

November 16, 2017

(—Researchers have designed a thermal regulation textile that has a 55% greater cooling effect than cotton, which translates to cooler skin temperatures when wearing clothes made of the new fabric. The material ...

Graphene water filter turns whisky clear

November 14, 2017

Previously graphene-oxide membranes were shown to be completely impermeable to all solvents except for water. However, a study published in Nature Materials, now shows that we can tailor the molecules that pass through these ...


Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.