Researchers Predict
Material with
Record-Setting Melting Point
Record-Setting Melting Point
Providence, R.I., Brown University, July 27, 2015 -- Using advanced computers and
a computational technique to simulate physical processes at the atomic level,
researchers at Brown University have predicted that a material made from
hafnium, nitrogen, and carbon would have the highest known melting point, about
two-thirds the temperature at the surface of the sun.
The computations, described in the journal Physical Review B (Rapid Communications),
showed that a material made with just the right amounts of hafnium, nitrogen,
and carbon would have a melting point of more than 4,400 kelvins (7,460 degrees
Fahrenheit). That’s about two-thirds the temperature at the surface of the sun,
and 200 kelvins higher than the highest melting point ever recorded
experimentally.
The experimental record-holder is a substance made from
the elements hafnium, tantalum, and carbon (Hf-Ta-C). But these new
calculations suggest that an optimal composition of hafnium, nitrogen, and
carbon — HfN0.38C0.51 — is a promising candidate to set a
new mark. The next step, which the researchers are undertaking now, is to
synthesize material and corroborate the findings in the lab.
“The advantage of starting with the computational approach is we can try
lots of different combinations very cheaply and find ones that might be worth
experimenting with in the lab,” said Axel van de Walle, associate professor of
engineering and co-author of the study with postdoctoral researcher Qijun Hong.
“Otherwise we’d just be shooting in the dark. Now we know we have something
that’s worth a try.”
The researchers used a computational technique that
infers melting points by simulating physical processes at the atomic level,
following the law of quantum mechanics. The technique looks at the dynamics of
melting as they occur at the nanoscale, in blocks of 100 or so atoms. It's more
efficient than traditional methods, but still computationally demanding due to
the large number of potential compounds to test. The work was done using the
National Science Foundation’s XSEDE computer network and Brown’s “Oscar”
high-performance computer cluster.
Van de Walle and Hong started by analyzing the Hf-Ta-C
material for which the melting point had already been experimentally
determined. The simulation was able to elucidate some of the factors that
contribute to the material’s remarkable heat tolerance.
The work showed that Hf-Ta-C combined a high heat of
fusion (the energy released or absorbed when it transitions from solid to
liquid) with a small difference between the entropies (disorder) of the solid
and liquid phases. “What makes something melt is the entropy gained in the
process of phase transformation,” van de Walle explained. “So if the entropy of
the solid is already very high, that tends to stabilize the solid and increase
the melting point.”
The researchers then used those findings to look for
compounds that might maximize those properties. They found that a compound with
hafnium, nitrogen, and carbon would have a similarly high heat of fusion but a
smaller difference between the entropies of the solid and the liquid. When they
calculated the melting point using their computational approach, it came out
200 kelvins higher than the experimental record.
Van de Walle and Hong are now collaborating with
Alexandra Navrotsky’s lab at the University
of California–Davis
The work could ultimately point toward new
high-performance materials for a variety of uses, from plating for gas turbines
to heat shields on high-speed aircraft. But whether the HfN0.38C0.51
compound itself will be a useful material isn’t clear, van de Walle says.
“Melting point isn’t the only property that’s important
[in material applications],” he said. “You would need to consider things like
mechanical properties and oxidation resistance and all sorts of other
properties. So taking those things into account you may want to mix other
things with this that might lower the melting point. But since you’re already
starting so high, you have more leeway to adjust other properties. So I think
this gives people an idea of what can be done.”
The work also demonstrates the power of this relatively
new computational technique, van de Walle says. In recent years, interest in
using computation to explore the material properties of a large number of
candidate compounds has increased, but much of that work has focused on
properties that are far easier to compute than the melting point.
“Melting point is a really difficult prediction problem
compared to what has been done before,” van de Walle said. “For the modeling
community, I think that’s what is special about this.”
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