Scientists reveal the way nanotubes grow
In work funded by the Laboratory Directed Research and Development (LDRD) Program, Livermore scientists have clarified the growth process for nanotubes—molecules that are typically 10,000 times smaller than the diameter of a human hair. Because nanotubes are extremely strong and have good thermal conductivity, researchers want to use them in nanoscale electronic and mechanical applications. By understanding their growth structure, researchers will be better able to manipulate the use of nanotubes in technological applications.
The Livermore team performed a series of quantum molecular dynamics simulations using different initial conditions for carbon coverage on iron nanoparticles. The team chose iron as a catalyst because experimentally it has the highest rate of carbon nanotube production. In the simulations, carbon atoms bonded to the iron, forming chains that eventually interconnected to form a sheet of pentagons and hexagons.
“We were surprised that carbon and iron do not mix at the nanoscale during growth and that the tubes grew capped,” says Giulia Galli, a physicist in Livermore’s Physics and Advanced Technologies Directorate. “Our simulations clarified the growth of nanotubes on small metal nanoparticles and thus will help researchers design experiments aimed at controlled growth of nanotubes.
The team’s research appeared in the August 26, 2005, issue of Physical Review Letters and was featured as a highlight in the September 8 issue of Nature Materials.
Contact: Giulia A. Galli (925) 683-4452 (firstname.lastname@example.org).
A new method for making superhard materials
A research collaboration led by scientists at Lawrence Livermore found that high-pressure shock waves applied to nanocrystals may increase a material’s hardness. The research, which was funded by the LDRD Program, combined data from the first shock experiments on nanocrystalline metals with numerical simulations run on the Laboratory’s supercomputers. Results indicate that, with further development, high-pressure shock waves can be used to make superhard materials that are stronger than any materials made with current manufacturing processes.
In general, metallic materials used in everyday applications are made of small “grains” joined by grain boundaries. When the grain size is reduced to less than 100 nanometers, the material is considered nanocrystalline. Nanocrystalline materials have extraordinary properties, such as enhanced hardness. However, extremely small grains tend to slide over each other as a material deforms, which reduces the material’s hardness. “It’s like stepping into sand,” says Livermore laser physicist Bruce Remington, who participated in the research. “The material is solid, but you still sink into it.”
The team used Livermore’s Janus laser to shock samples of nanocrystalline nickel and copper. The shock waves produced by the high-intensity laser move faster than the speed of sound and generate pressures nearly 1 million times greater than atmospheric pressure. “The high pressure increased the friction among grains and decreased the sliding,” says team leader Eduardo Bringa, a materials scientist at the Laboratory. “By turning off the mechanism that softens the grains, we create a material that, being hard to begin with, is even harder during and following the shock-wave application.”
Team members caution that the research is in an early phase. If successful, it could be used to develop superhard materials for shielding military vehicles, protecting spacecraft from damage caused by interplanetary dust particles, or building safer automobile bumpers and frames. The materials also could have applications in inertial confinement fusion experiments. The team’s findings were published in the September 16, 2005, issue of Science.
Contact: Eduardo M. Bringa (925) 423-5724 (email@example.com).
Enhanced adaptive optics for Gemini Telescope
Lawrence Livermore has been selected as the lead institution to build the Extreme Adaptive Optics Coronagraphy (ExAOC) for the Gemini Telescope. Researchers from the University of California at Berkeley and Los Angeles, the Jet Propulsion Laboratory, the American Museum of Natural History, the Hertzberg Institute in Canada, and several smaller institutes will also collaborate on this project.
An enhanced adaptive optics system will allow astronomers to minimize the blurring effects of Earth’s atmosphere, so they can detect planets about 30 to 150 light years from the solar system. Adaptive optics systems use light from a relatively bright star to measure and then correct the atmospheric distortions by bouncing light off a deformable mirror. The ExAOC system will have 3,000 to 4,000 actuators to control the deformable mirror. These actuators will be made of etched silicon microelectromechanical systems instead of reflective glass, which is used on the adaptive optics system at the W. M. Keck Observatory. With the new actuators, researchers will be able to adjust the shape of the mirror by several micrometers with a precision of less than 1 nanometer and, thus, will be able to correct for atmospheric distortions at a rate 10 times greater than the Keck system. Such a high resolution will allow them to detect distant planets and learn more about how solar systems formed.
The ExAOC system is funded primarily by the National Science Foundation through the Association of Universities for Research for Astronomy. First light is predicted near the end of 2009.
Contact: Bruce Macintosh (925) 432-8129 (firstname.lastname@example.org).