Dolores Beasley<br />Headquarters, Washington June 30, 2003<br />(Phone: 202/358-1753)<br /><br /><br />Steve Roy<br />Marshall Space Flight Center, Huntsville, Ala.<br />(Phone: 256/544-0034)<br /><br /><br />Susan Killenberg McGinn<br />Washington University in St. Louis, Mo.<br />(Phone: 314/935-5254)<br /><br /><br />RELEASE: 03-218<br /><br /><br />NASA EXPERIMENTS VALIDATE 50-YEAR-OLD HYPOTHESIS<br /><br /><br /> NASA-funded researchers recently obtained the first <br />complete proof of a 50-year-old hypothesis explaining how <br />liquid metals resist turning into solids.<br /><br /><br />The research is featured on the cover of the July issue of <br />Physics Today. It challenges theories about how crystals <br />form by a process called nucleation, important in everything <br />from materials to biological systems. <br /> <br />"Nucleation is everywhere," said Dr. Kenneth Kelton, the <br />physics professor who leads a research team from Washington <br />University in St. Louis. "It's the major way physical <br />systems change from one phase to another. The better we <br />understand it, the better we can tailor the properties of <br />materials to meet specific needs," he said. <br /><br /><br />Using the Electrostatic Levitator at NASA's Marshall Space <br />Flight Center (MSFC) in Huntsville, Ala., Kelton's team <br />proved the hypothesis by focusing on the "nucleation <br />barrier." German physicist Gabriel D. Fahrenheit, while <br />working on his temperature scale, first observed the barrier <br />in the 1700s. When he cooled water below freezing, it didn't <br />immediately turn into ice but hung around as liquid in a <br />supercooled state. That's because it took a while for all <br />the atoms to do an atomic "shuffle" arranging in patterns to <br />form ice crystals.<br /><br /><br />In 1950, Dr. David Turnbull and Dr. Robert Cech, in <br />Schenectady, N.Y., showed liquid metals also resist turning <br />into solids. In 1952, physicist Dr. Charles Frank, of the <br />University of Bristol in England, explained this <br />"undercooling" behavior as a fundamental mismatch in the way <br />atoms arrange themselves in the liquid and solid phases. <br />Atoms in a liquid metal are put together into the form of an <br />icosahedron, a pattern with 20 triangular faces that can't <br />be arranged to form a regular crystal.<br /><br /><br />"The metal doesn't change to a solid instantly, because it <br />costs energy for the atoms to move from the icosahedral <br />formation in the liquid to a new pattern that results in a <br />regular crystal structure in the solid metal," explained <br />Kelton. "It's like being in a valley and having to climb <br />over a mountain to get to the next valley. You expend energy <br />to get over the barrier to a new place," he said.<br /><br /><br />Frank didn't know about quasicrystals, first discovered in <br />1984, and researchers didn't have tools like NASA's <br />Levitator. Using electrostatic energy to levitate the sample <br />was crucial, because stray contamination from containers <br />cause crystals to form inside liquid metals, which would <br />have ruined Kelton's measurements on pure samples.<br /><br /><br />To measure atom locations inside a drop of titanium-<br />zirconium-nickel alloy, the levitator was moved to the <br />Advanced Photon Source at Argonne National Laboratory in <br />Chicago. There, an energetic beam of X-rays was used to map <br />the average atom locations as the metal turned from liquid <br />to solid. The experiment was repeated several times, and the <br />data definitively verified Frank's hypothesis. <br /><br /><br />As the temperature was decreased to solidify the molten <br />sample, an icosahedral local structure developed in the <br />liquid metal. It cost less energy to form the quasicrystal, <br />because it had an icosahedral structure. This caused the <br />quasicrystal to nucleate first, even though it was less <br />stable than the crystal phase that should have formed. The <br />icosahedral liquid structure was therefore directly linked <br />to the nucleation barrier, as proposed by Frank.<br /><br /><br />To prepare for an International Space Station experiment, <br />the team is continuing levitator experiments. NASA's Office <br />of Biological and Physical Research in Washington and the <br />MSFC Science Directorate fund the research. A peer-reviewed <br />article that discusses this work appeared in the May 16 <br />issue of Physical Review Letters. The research was featured <br />in the May 30 issue of Science.<br /><br /><br />For information and images about NASA and the research on <br />the Internet, visit:<br /><br /><br />http://www.nasa.gov