Researchers taking the first-ever
direct measurement of atom temperature in extremely hot materials inadvertently
disproved a decades-old theory and upended our understanding of superheating.
It's notoriously difficult to take
the temperature of really hot things. Whether it's the roiling plasma in our
sun, the extreme conditions at the core of planets or the crushing forces at
play inside a fusion reactor, what scientists call "warm dense
matter" can reach hundreds of thousands of degrees kelvin.
Knowing precisely how hot these
materials are is crucial for researchers to fully understand such complex
systems, but taking these measurements has been, until now, virtually
impossible.
"We have good techniques for
measuring density and pressure of these systems, but not temperature,"
said Bob Nagler, staff scientist at the Department of Energy's SLAC National
Accelerator Laboratory. "In these studies, the temperatures are always
estimates with huge error bars, which really holds up our theoretical models.
It's been a decades-long problem."
Now, for the first time, a team of
researchers report in the journal Nature that they have directly
measured the temperature of atoms in warm dense matter.
While other methods rely on complex
and hard-to-validate models, this new method directly measures the speed of
atoms, and therefore the temperature of the system. Already, their innovative
method is changing our understanding of the world: In an experimental debut,
the team superheated solid gold far beyond the theoretical limit, unexpectedly
overturning four decades of established theory.
Nagler and researchers at SLAC's
Matter in Extreme Conditions (MEC) instrument co-led this study with Tom White,
associate professor of physics at University of Nevada, Reno. The group
includes researchers from Queen's University Belfast, the European XFEL (X-ray
Free-Electron Laser), Columbia University, Princeton University, University of
Oxford, University of California, Merced, and the University of Warwick,
Coventry.
Taking the temperature
For nearly a decade, this team has
worked to develop a method that circumvents the usual challenges of
measuring extreme temperatures—specifically, the brief duration of the conditions that create those
temperatures in the lab and the difficulty of calibrating how these complex
systems affect other materials.
"Finally, we've directly and
unambiguously taken a direct measurement, demonstrating a method that can be
applied throughout the field," White said.
Under extreme conditions—like those
in the hearts of planets or in exploding stars—materials can enter other exotic
phases with unique characteristics. At SLAC, researchers are studying some of
the most extreme and exotic forms of matter ever created, in detail never
before possible.
At SLAC's MEC instrument, the team
used a laser to superheat a sample of gold. As heat flashed through the
nanometer-thin sample, its atoms began to vibrate at a speed directly related
to their rising temperature. The team then sent a pulse of ultrabright X-rays
from the Linac Coherent Light Source (LCLS) through the superheated sample. As
they scattered off the vibrating atoms, the X-rays' frequency shifted slightly,
revealing the atoms' speed and thus their temperature.
"The novel temperature
measurement technique developed in this study demonstrates that LCLS is at the
frontier of laser-heated matter research," said Siegfried Glenzer,
director of the High Energy Density Science division at SLAC and co-author on
the paper. "LCLS, paired with these innovative techniques, play an
important role in advancing high energy density science and transformative
applications like inertial fusion."
The team was thrilled to have
successfully demonstrated this technique—and as they took a deeper look at the
data, they discovered something even more exciting.
"We were surprised to find a much higher temperature in these superheated solids than we initially expected, which disproves a long-standing theory from the 1980s," White said. "This wasn't our original goal, but that's what science is about—discovering new things you didn't know existed."
Surviving the entropy catastrophe
Every material has specific melting
and boiling points, marking the transition from solid to liquid and liquid to
gas, respectively. However, there are exceptions. For instance, when water is
heated rapidly in very smooth containers—such as a glass of water in a
microwave—it can become "superheated," reaching temperatures above
212 degrees Fahrenheit (100 degrees Celsius) without actually boiling. This
occurs because there are no rough surfaces or impurities to trigger bubble
formation.
But this trick of nature comes with
an increased risk: The further a system strays from its normal melting and
boiling points, the more vulnerable it is to what scientists call a
catastrophe—a sudden onset of melting or boiling triggered by slight environmental
change. For example, water that has been superheated in a microwave will boil
explosively when disturbed, potentially causing serious burns.
While some experiments have shown
it is possible to bypass these intermediary limits by rapidly heating
materials, "the entropy catastrophe was still viewed as the ultimate
boundary," White explained.
In their recent study, the team
discovered that the gold had been superheated to an astonishing 19,000 kelvins
(33,740 degrees Fahrenheit)—more than 14 times its melting point and well
beyond the proposed entropy catastrophe limit—all while maintaining its solid
crystalline structure.
"It's
important to clarify that we did not violate the Second Law of
Thermodynamics," White said with a chuckle. "What we demonstrated is
that these catastrophes can be avoided if materials are heated extremely
quickly—in our case, within trillionths of a second."
The
researchers believe that the rapid heating prevented the gold from expanding,
enabling it to retain its solid state. The findings suggest that there may not
be an upper limit for superheated materials, if heated quickly enough.
Fusion and beyond
SLAC enables
research on materials for fusion power plants and fusion fuel targets, as well
as atomic-level observations of fusion reactions.
Nagler noted
that researchers who study warm
dense matter have likely been surpassing the entropy
catastrophe limit for years without realizing it, due to the absence of a
reliable method for directly measuring temperature.
"If our
first experiment using this technique led to a major challenge to established
science, I can't wait to see what other discoveries lie ahead," Nagler
said.
As just one
example, White and Nagler's teams used this method again this summer to study
the temperature of materials that have been shock-compressed to replicate the
conditions deep inside planets.
Nagler is also
eager to apply the new technique—which can pinpoint atom temperatures from
1,000 to 500,000 kelvins—to ongoing inertial fusion energy research at SLAC.
"When a fusion fuel target implodes in a fusion reactor, the targets are in a warm dense state," Nagler explained. "To design useful targets, we need to know at what temperatures they will undergo important state changes. Now, we finally have a way to make those measurements."
Source: Superheated gold withstands 'entropy catastrophe': New method challenges established physics
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