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.
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