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Cross-section of composite specimen in polarized light. Barium
titanate inclusions appear as black spots. The polycrystalline
structure of the tin matrix is shown in gray-scale. (Scale bar,
500 µm.)
(Larger image)
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Hidden gems: New composites
are stiffer than diamond
sing
a unique combination of barium titanate and tin, University of Wisconsin-Madison
researchers have made the first known material that’s stiffer
than diamond.The group published its results in the Feb.
2 issue of Science.
Aside from its value as a
gemstone, diamond has the highest thermal conductivity and is the stiffest,
hardest material around. Yet despite its benefits, diamond is too expensive
to consider in such structural applications as bridges, buildings, airplanes
or golf clubs.
While diamond achieves its
rock-solid stability via dense, directional, extremely tight atomic
bonds, the UW-Madison researchers created their stiff composite from
ordinary materials held together in an extraordinary way, says Wisconsin
Distinguished Professor
Roderic Lakes. “We’re using a material now that’s
chosen for having the ability to change volume during phase transformation,”
he says. “The material we chose—barium titanate—goes
from one solid to another solid.”
Barium titanate is a well
researched crystalline material previously used in such applications
as microphones or cell phone speakers. Embed bits of it in a tin matrix,
and the phase transformation, or shift in the arrangement of atoms,
is held back, creating stored energy. “Imagine water getting into
cracks in the road and freezing,” says Lakes. “It can’t
expand because it’s held in place.”
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| Landau
energy function of strain e and normalized
temperature TN = (αγ/β2)(T
- T1) - 0.25, with unit cells
of BaTiO3 in cubic and tetragonal phases. α, γ,
and β are constants that depend on
the material. (Larger
image)
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The blocked phase transformation
creates negative stiffness, or instability, within the barium titanate,
while the tin has positive stiffness, or stability. “We’ve
finally showed that in the lab, you can make a composite that’s
stiffer than either constituent, which nobody thought was possible before,
because in all of the previous composites both constituents are in a
minimum energy state,” he says. “There’s no stored
energy, and both stiffness values are positive.”
In laboratory experiments,
Lakes and his collaborators showed that if they embed the barium titanate
within the tin, the resulting composite material achieves stiffness
approaching 10 times that of diamond. “You’d think that
if you’d add positive and negative, you’d get zero,”
says Lakes. “Actually, that’s exactly how you get the extreme
stiffness, because you’re adding compliances.”
For example, he says, steel
is very stiff; rubber is very compliant. A positive compliance is the
inverse of a stiffness and a negative compliance is the inverse of a
stiffness. Add positive compliance and negative compliance and the sum
is close to zero—which corresponds to very high stiffness.
Like the phase transformation
of water to ice at 0 degrees Celsius, the barium titanate phase transformation
also is governed by temperature, so the current composite exhibits extreme
stiffness within a temperature range of less than 10 degrees. “The
temperature at which this material works is like a hot day in Libya,”
says Lakes. “So it’s like 65 degrees Celsius, and a hot
day in New York is 40 Celsius. It’s a higher temperature than
is convenient. We think we can tune that, but that’s the future.”
Other contributors to the
research include former UW-Madison PhD student Timothy Jaglinski,
now a research associate with the Washington State University Institute
for Shock Physics; former master’s student Dennis Kochmann,
now of Ruhr-University Bochum, Germany; and Materials Science and Engineering
Associate Professor Donald Stone.
