by Staff Writers
Houston TX (SPX) Jan 15, 2015
Even when building big, every atom matters, according to new research on particle-based materials at Rice University. Rice researchers Rouzbeh Shahsavari and Saroosh Jalilvand have published a study showing what happens at the nanoscale when "structurally complex" materials like concrete -- a random jumble of elements rather than an ordered crystal -- rub against each other. The scratches they leave behind can say a lot about their characteristics.
The researchers are the first to run sophisticated calculations that show how atomic-level forces affect the mechanical properties of a complex particle-based material. Their techniques suggest new ways to fine-tune the chemistry of such materials to make them less prone to cracking and more suitable for specific applications.
The research appears in the American Chemical Society journal Applied Materials and Interfaces.
The study used calcium-silicate-hydrate (C-S-H), aka cement, as a model particulate system. Shahsavari became quite familiar with C-S-H while participating in construction of the first atomic-scale models of the material.
C-S-H is the glue that binds the small rocks, gravel and sand in concrete. Though it looks like a paste before hardening, it consists of discrete nanoscale particles. The van der Waals and Coulombic forces that influence the interactions between the C-S-H and the larger particles are the key to the material's overall strength and fracture properties, said Shahsavari. He decided to take a close look at those and other nanoscale mechanisms.
"Classical studies of friction on materials have been around for centuries," he said. "It is known that if you make a surface rough, friction is going to increase. That's a common technique in industry to prevent sliding: Rough surfaces block each other.
"What we discovered is that, besides those common mechanical roughening techniques, modulation of surface chemistry, which is less intuitive, can significantly affect the friction and thus the mechanical properties of the particulate system."
Shahsavari said it's a misconception that the bulk amount of a single element -- for example, calcium in C-S-H -- directly controls the mechanical properties of a particulate system. "We found that what controls properties inside particles could be completely different from what controls their surface interactions," he said. While more calcium content at the surface would improve friction and thus the strength of the assembly, lower calcium content would benefit the strength of individual particles.
"This may seem contradictory, but it suggests that to achieve optimum mechanical properties for a particle system, new synthetic and processing conditions must be devised to place the elements in the right places," he said.
The researchers also found the contribution of natural van der Waals attraction between molecules to be far more significant than Coulombic (electrostatic) forces in C-S-H. That, too, was primarily due to calcium, Shahsavari said.
To test their theories, Shahsavari and Jalilvand built computer models of rough C-S-H and smooth tobermorite. They dragged a virtual tip of the former across the top of the latter, scratching the surface to see how hard they would have to push its atoms to displace them. Their scratch simulations allowed them to decode the key forces and mechanics involved as well as to predict the inherent fracture toughness of tobermorite, numbers borne out by others' experiments.
Shahsavari said atomic-level analysis could help improve a broad range of non-crystalline materials, including ceramics, sands, powders, grains and colloids.
Space Technology News - Applications and Research
|The content herein, unless otherwise known to be public domain, are Copyright 1995-2014 - Space Media Network. All websites are published in Australia and are solely subject to Australian law and governed by Fair Use principals for news reporting and research purposes. AFP, UPI and IANS news wire stories are copyright Agence France-Presse, United Press International and Indo-Asia News Service. ESA news reports are copyright European Space Agency. All NASA sourced material is public domain. Additional copyrights may apply in whole or part to other bona fide parties. Advertising does not imply endorsement, agreement or approval of any opinions, statements or information provided by Space Media Network on any Web page published or hosted by Space Media Network. Privacy Statement All images and articles appearing on Space Media Network have been edited or digitally altered in some way. Any requests to remove copyright material will be acted upon in a timely and appropriate manner. Any attempt to extort money from Space Media Network will be ignored and reported to Australian Law Enforcement Agencies as a potential case of financial fraud involving the use of a telephonic carriage device or postal service.|