Phase Transitions at Reduced Dimension Junqiao Wu, University of California-Berkeley , DMR 1055938.
Phase Transitions at Reduced DimensionJunqiao Wu, University of California-Berkeley, DMR 1055938
Outcome: Many materials can change structure and properties under influences, called phase transition. Researchers at U. C. Berkeley have discovered that some phase transitions occur very differently when the materials are small in size.
Impact: Such discoveries and understanding enable methods to make new materials, as well as trick existing materials to offer new properties.
Explanation:Two solid materials may uniformly mix and form a liquid, called an eutectic alloy, at a temperature that is too cold to melt each of the constituents alone.
Metal-semiconductor eutectic alloys are widely used for materials synthesis, assembly, and bonding. The Berkeley team discovered and understood an unusual effect that the formation of gold-silicon eutectic layers is much easier when they are thinner. The thickness-dominated alloying reaction provide new routes for nanoscale materials engineering and processing.
Professor Junqiao Wu, of U. C. Berkeley’s Department of Materials Science and Engineering and recipient of an NSF Faculty Early Career Development (CAREER) award, led the team.
Prof. Wu next to a micro Raman system used to identify different phases during phase transition. (courtesy of J. Wu)
Ultra-Thin Layers Reveal Secret of EutecticsJunqiao Wu, University of California-Berkeley, DMR 1055938
Surprising new physical effects were recently discovered at the surface or reduced sizes of the long-studied gold-silicon eutectic liquids. The majority of these investigations were performed after the eutectics were formed. Thus, it remains unclear how they are formed as this is very difficult to probe due to high temperatures, instabilities, small sizes and fast time scales involved.
Using a high-temperature in situ scanning electron microscope, the researchers discovered that the alloying reaction rate is enhanced by over 20 times when the thickness is reduced from 300 nanometers to 20 nanometers.
The enhancement was attributed to strain and surface effects which becomes dominant at ultra-thin thicknesses.
The discovery may lead to synthesis of new materials, such as monolayer silicon, the silicon equivalent of graphene.
Crop circle-likefeatures develop upon annealing gold layer on top of a silicon wafer, revealing large reaction rate enhancement in formation of ultra-thin gold-silicon eutectic layers.
(courtesy of J. Wu)
“No Materials, No Engineering”Junqiao Wu, University of California-Berkeley, DMR 1055938
An integrated education and outreach effort of this project is to teach pre-college students the importance of materials. Although many of them are interested in engineering, few are aware of the discipline and career option called Materials Science.
The slogan of the effort is “No Materials, No Engineering” or “NMNE”.
The group teams up with middle schools and organizations and offers on-campus lectures, exhibits, and visits centered around materials science and engineering.
One such event was on 3/26/2012, when the PI hosted a one-day visit of Techbridge middle-school girls to the U. C. Berkeley campus. During the visit, the PI's students and the PI demonstrated experimentally to the visitors how materials behave differently at the nanoscale, how SEM works, how shape memory alloy work, how superconductors work, etc.
Prof. Wu demonstrates superconductor levitation to Techbridge middle-school girls. (courtesy of J. Wu)