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Dolores Beasley
Headquarters, Washington                 June 30, 2003
(Phone: 202/358-1753)

Steve Roy
Marshall Space Flight Center, Huntsville, Ala.
(Phone: 256/544-0034)

Susan Killenberg McGinn
Washington University in St. Louis, Mo.
(Phone: 314/935-5254)

RELEASE: 03-218

NASA EXPERIMENTS VALIDATE 50-YEAR-OLD HYPOTHESIS

     NASA-funded researchers recently obtained the first 
complete proof of a 50-year-old hypothesis explaining how 
liquid metals resist turning into solids.

The research is featured on the cover of the July issue of 
Physics Today. It challenges theories about how crystals 
form by a process called nucleation, important in everything 
from materials to biological systems. 
 
"Nucleation is everywhere," said Dr. Kenneth Kelton, the 
physics professor who leads a research team from Washington 
University in St. Louis. "It's the major way physical 
systems change from one phase to another. The better we 
understand it, the better we can tailor the properties of 
materials to meet specific needs," he said. 

Using the Electrostatic Levitator at NASA's Marshall Space 
Flight Center (MSFC) in Huntsville, Ala., Kelton's team 
proved the hypothesis by focusing on the "nucleation 
barrier." German physicist Gabriel D. Fahrenheit, while 
working on his temperature scale, first observed the barrier 
in the 1700s. When he cooled water below freezing, it didn't 
immediately turn into ice but hung around as liquid in a 
supercooled state. That's because it took a while for all 
the atoms to do an atomic "shuffle" arranging in patterns to 
form ice crystals.

In 1950, Dr. David Turnbull and Dr. Robert Cech, in 
Schenectady, N.Y., showed liquid metals also resist turning 
into solids. In 1952, physicist Dr. Charles Frank, of the 
University of Bristol in England, explained this 
"undercooling" behavior as a fundamental mismatch in the way 
atoms arrange themselves in the liquid and solid phases. 
Atoms in a liquid metal are put together into the form of an 
icosahedron, a pattern with 20 triangular faces that can't 
be arranged to form a regular crystal.

"The metal doesn't change to a solid instantly, because it 
costs energy for the atoms to move from the icosahedral 
formation in the liquid to a new pattern that results in a 
regular crystal structure in the solid metal," explained 
Kelton. "It's like being in a valley and having to climb 
over a mountain to get to the next valley. You expend energy 
to get over the barrier to a new place," he said.

Frank didn't know about quasicrystals, first discovered in 
1984, and researchers didn't have tools like NASA's 
Levitator. Using electrostatic energy to levitate the sample 
was crucial, because stray contamination from containers 
cause crystals to form inside liquid metals, which would 
have ruined Kelton's measurements on pure samples.

To measure atom locations inside a drop of titanium-
zirconium-nickel alloy, the levitator was moved to the 
Advanced Photon Source at Argonne National Laboratory in 
Chicago. There, an energetic beam of X-rays was used to map 
the average atom locations as the metal turned from liquid 
to solid. The experiment was repeated several times, and the 
data definitively verified Frank's hypothesis. 

As the temperature was decreased to solidify the molten 
sample, an icosahedral local structure developed in the 
liquid metal. It cost less energy to form the quasicrystal, 
because it had an icosahedral structure. This caused the 
quasicrystal to nucleate first, even though it was less 
stable than the crystal phase that should have formed. The 
icosahedral liquid structure was therefore directly linked 
to the nucleation barrier, as proposed by Frank.

To prepare for an International Space Station experiment, 
the team is continuing levitator experiments. NASA's Office 
of Biological and Physical Research in Washington and the 
MSFC Science Directorate fund the research. A peer-reviewed 
article that discusses this work appeared in the May 16 
issue of Physical Review Letters. The research was featured 
in the May 30 issue of Science.

For information and images about NASA and the research on 
the Internet, visit:

http://www.nasa.gov

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