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LITTLE is known about the chemistry
that produces minerals in the deep regions of Earth or that creates
the ammonia oceans of the outer planets and moons. What is known
is that an element’s fundamental properties—its optical,
structural, electrical, and magnetic characteristics—can completely
change when it is put under extreme conditions. In fact, when a material
is exposed to pressures up to one million times the atmospheric pressure
at Earth’s surface and to temperatures above 6,000°C, its
atoms can completely rearrange themselves, rendering an entirely
new substance.
Similar conditions exist near Earth’s core and in other planets,
both inside and outside the solar system. They also occur in high-explosive
reactions and impacts from meteorites and comets. Understanding chemical
reactions at such extreme conditions is a critical research area
for the National Nuclear Security Administration’s (NNSA’s)
Stockpile Stewardship Program. To maintain the reliability and performance
of the nation’s nuclear weapon stockpile, scientists must be
able to more accurately predict the performance of high explosives
(HE). However, to refine the computer codes that simulate these reactions,
they need more detailed information on the chemical, mechanical,
and energetic properties of the water, carbon, and nitrogen produced
by an HE detonation.
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Livermore’s
Extreme Chemistry Group studies chemical reactions that
occur under high-pressure, high-temperature conditions—up
to one million times the atmospheric pressure at Earth’s
surface and temperatures above 6,000°C. Similar conditions
exist near Earth’s core and in other planets.
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At
Lawrence Livermore, much of this research is conducted by the Extreme
Chemistry Group in the Chemistry and Materials Science (CMS)
Directorate. According to chemist Larry Fried, who leads this group,
the team’s objective is to understand the physical and chemical
processes at extreme conditions as well as scientists now comprehend
those processes at ambient conditions.
The
group’s research applies not only to stockpile stewardship
but also to emerging technologies in nanoscale and energetic materials. “We’re
finding answers to important technical questions regarding reaction
chemistry,” says Fried. “What we’re learning can
be applied to various phenomena, from planetary evolution to new
states of matter.” (See the box below.)
Predicting New Materials
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A
few years ago, using computer simulations, theoretical
chemist Riad Manaa predicted the possibility of a fullerene
made of 60 purely single-bonded nitrogen atoms. (See S&TR,
June 2001, This
Nitrogen Molecule Really Packs Heat.) Although
still strictly theoretical—no naturally occurring
polymeric forms of nitrogen have ever been found—the
calculations have shown that it may be possible to
link together six molecules of N10, or dipentazole,
to create N60, a fullerene expected to have
unprecedented energetic potential. N10 has
yet to be synthesized, but simulations show that linking
two stable pentazole (N5) ions could create
the metastable building block needed to put together
a nitrogen fullerene. (See the figure at right.)
Also
necessary for the complex work would
be extreme chemistry. Tremendous pressures
would be needed to force the dipentazole
molecules into the novel configuration.
The
first fullerene, C60, was
synthesized in the laboratory nearly
20 years ago and named after R. Buckminster
Fuller, whose geodesic dome is brought
to mind by the shape of the fullerene
molecule. The discovery of C60 is
considered by many to be the beginning
of nanotechnology. Says Livermore chemist
Larry Fried, “This work on N60 may
mark the beginning of the field of extreme
nanotechnology.”
The
prediction of new forms of polymeric
nitrogen is not new, however. More than
20 years ago, theoretical physicists
Andy McMahan and Christian Mailhiot predicted
that if squeezed hard enough, molecular
nitrogen could be turned into a solid—a
covalently bonded nonmolecular network
like diamond. In 2002, Alexander Goncharov,
now with Livermore, conducted
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experiments that confirmed
the McMahan and Mailhiot prediction. “The calculations
were not exactly in concert with the original prediction,” says
Fried, “but they did show that a nonmolecular
state of nitrogen could be created with extreme chemistry.”
Manaa
hopes to see this same progression with
his group’s prediction of N60 and
other nitrogen fullerenes. (See S&TR,
December 2003, Predicting
Stability for the High-Energy Buckyball.)
And while scientific discovery rarely
happens overnight, the past decade’s
advances in computing capabilities and
the strides made in extreme chemistry
are continually pushing theory to something
more tangible.
Calculations show
that six molecular units of N10 can be combined to form the nitrogen fullerene, N60 , which is expected to have unprecedented energetic potential.
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Achieving the Extreme
An
essential element in studies of extreme chemistry is a material’s
equation of state—a mathematical expression showing the relationship
of a material’s pressure, temperature, and density. Many of
the Laboratory’s high-temperature, high-pressure experiments
are designed to obtain more accurate data on the equation-of-state
properties for various materials.
Measuring
these properties takes ingenuity and a host of technologies. For
example, two-stage gas guns at Livermore and at the Nevada Test
Site have been instrumental in providing equation-of-state data.
(See S&TR, September 2000, “Shocking” Gas
Gun Experiments; June 2004, Shocking
Plutonium to Reveal Its Secrets.) With gas guns, scientists can
fire hypervelocity projectiles into highly instrumented targets,
shocking matter to extreme conditions
for a millionth of a second or less.
These
experiments create pressures of a million-plus atmospheres, temperatures
up to thousands of degrees, and densities several times
that of a material’s solid state. Shock-compression experiments
have been used to evaluate liquefied gases such as hydrogen, nitrogen,
carbon dioxide, and oxygen as well as solids such as aluminum, copper,
tantalum, and carbon (graphite).
Mixtures
of elements are more difficult to characterize. For those experiments,
scientists use the diamond anvil cell, a device small
enough to fit in the palm of one’s hand. (See S&TR,
March 1996, The Diamond
Anvil Cell: Probing the Behavior of Metals under Ultrahigh Pressures.)
Diamond anvil cells can create the temperature–pressure
conditions required to transform a substance: up to about 3.6 million
atmospheres of pressure at room temperature and 1.7 million atmospheres
at about
3,000°C. Lasers can also be used to heat the samples in a diamond
anvil cell, so that temperatures reach nearly 6,000°C at pressures
below 1 million atmospheres. By comparison, the Sun’s
surface is about 5,500°C.
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Formic
acid reacts to modest pressures and temperatures (about
2,000 atmospheres and 150°C).
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One
advantage of the diamond anvil cell is that is does not necessarily
destroy the sample being tested. “In shock experiments, the
sample is destroyed, and the reaction occurs in less than a microsecond,” says
physicist Bill Nellis, who worked on some of the Laboratory’s
first gas-gun experiments and is now at the Harvard University Lyman
Laboratory of Physics. (See S&TR, September 1996, Jumpin’ Jupiter!
Metallic Hydrogen.) “With
the diamond anvil cell, samples can be held under controlled extreme
conditions long enough for us to observe the reaction.”
Another
option for extreme chemistry research is radiation technology. For
example, Livermore researchers are conducting experiments on
the new high-pressure xray beam line at the Advanced Light Source
at Lawrence Berkeley National Laboratory. In the first experiment
at the new facility, they observed the pressure-induced reactions
of white phosphorous. They have also used
Lawrence Berkeley’s synchrotron infrared beam line to monitor
chemical reactions in experiments with the diamond anvil cell.
To improve
experimental capabilities for high-pressure materials studies, the
Laboratory has joined the High Pressure Collaborative
Access Team (HPCAT). This collaboration is developing new beam lines
at the Advanced Photon Source at Argonne National Laboratory for
such research as characterizing materials at pressures greater than
500,000 atmospheres and improving x-ray diagnostics for shock-wave
experiments.
Chemical
kinetic studies are yet another way to look at reactions under extreme
conditions. With an infrared system, scientists can
monitor spectral features during a reaction, which allows them to
study the process of product formation. From these observations,
they can then determine the activation energy—the energy required
for reactant A to become reaction product B. By better understanding
activation energy, scientists can more accurately predict the pressure
dependence of chemical reaction rates.
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The diamond
anvil cell is small enough to fit in the palm of one’s
hand, but it can compress a sample to extreme pressures—up
to about 3.6 million atmospheres at room temperature
and 1.7 million atmospheres at about 3,000°C.
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Tailored High Explosives
The
Livermore team uses the data generated from experiments to refine
the computer codes that simulate HE performance. Theoretical chemist
Riad Manaa says, “We know how to engineer high explosives,
but we don’t know how they work on an atomic level—how
and why they release energy. By improving our predictive capabilities,
we can design HE materials that are safer and more energetic than
the ones currently used.”
For
example, a new trend in HE formulations is to add nanometer-scale
metal particles to the mix. Metals make an HE reaction hotter but
reduce its pressure. Very small metal particles react much more easily
than large metal particles. In the right proportion, metals can almost
double the energy content of an explosive, but the challenge is finding
the optimal mix. A formulation with too much metal will not burn
completely, so the pressure exerted in the explosion may be lower
than needed. If metal levels are too low, the reaction may not produce
the energy required for a specific application. Heavy metals such
as tungsten, titanium, and zirconium can also be used to alter the
energy delivery rate of a reaction.
“With computer simulations,” says Fried, “we can better
understand how to customize materials for a specific target or situation.” This
capability is of particular interest to the Department of Defense
(DoD) because it will allow scientists to develop weapons systems
that reduce collateral damage, say, when targets are near an enemy’s
weapons stockpiles or a civilian structure.
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Pressure-induced
reactions occur in white phosphorous during the first experiment
on the new beam line at the Advanced Light Source at Lawrence
Berkeley National Laboratory.
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One
code the team uses to model detonations is CHEETAH, which was begun
in 1993 by Fried and his colleagues. (See S&TR, November
1997, Improved Detonation
Modeling with CHEETAH.) CHEETAH uses data
from high-pressure experiments to simulate the performance of different
HE formulations.
Used by about 300 Department of Energy and DoD contractors, it allows
scientists to determine how best to mix materials for various applications.
For example, a shaped charge designed to penetrate armor must deliver
its energy quickly, say, in 10 microseconds. By contrast, high explosives
used in rock blasting are more effective if energy is delivered over
tens of milliseconds. Researchers can use CHEETAH to tailor formulations
for specific purposes and to examine several mix options without
the time and expense involved in conducting small-scale experiments.
Fried
is now linking CHEETAH to Livermore’s hydrodynamic codes,
so it can model the chemistry of entire systems of materials. “Historically,
most chemical studies have examined individual materials in isolation,” says
Fried. “But with the advances in algorithms and computing power,
we’ve moved from looking at elemental hydrogen to HE mixtures
to whole explosives systems. These calculations, which were impossible
to do 20 years ago, allow us to study systems such as warheads, reactive
armor, or target effects.”
Another series
of calculations modeled the rapid decomposition of HMX, a commonly
used propellant and explosive. These simulations showed how and
in what order the molecules
rearrange
themselves as well as what intermediate products are formed.
At about
3,200°C and reaction times of up to 100 picoseconds,
HMX decomposes through an initial step of nitro group elimination.
Stable molecules such as water form rapidly—the first stable
products appearing in less than 1 picosecond. But the transformation
doesn’t stop there. Slower chemical processes continue to change
the chemical composition of HMX throughout the decomposition process.
According
to chemist Joe Zaug, Livermore scientists are also beginning to
examine in more detail the complex chemical systems on board weapon
devices. “Moreover, the ultimate result of a detonated weapon
is chemical species at extreme conditions,” says Zaug. “If
we can fully understand the behavior of a candidate species at relevant
conditions, then we can work backward using the computational codes
to better understand the detonation process.”
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(a)
A simulation of HMX under extreme conditions shows that stable
molecules such as water form in less than 1 picosecond.
(b) Slower chemical processes continue to change the chemical
composition of HMX throughout the decomposition process. |
Beyond Stockpile Stewardship
Understanding
the stepwise changes that occur during these ultrafast reactions
has far broader applications than high explosives. Data
from HE experiments can be used to study the fundamental behavior
of many materials under extreme conditions, from fossil fuels to
water to the elements that form stars and planets.
One
of these studies used laser-based ultrasonic technology to characterize
iron, the main constituent of Earth’s core. With this technique,
the Livermore team recorded the precise sound velocity of iron under
a pressure seven times greater than in previously reported experiments.
Other
characterization techniques, which take less direct measurements,
had led scientists to infer the makeup of Earth’s internal
structure, but those inferences were not borne out by the Livermore
team’s measurements. Instead, results indicate that the density
of Earth’s inner core is consistent with that of pure iron
between 5,000 and 6,000°C, and the liquid outer core is less
dense than pure iron. Such data help scientists better understand
phenomena such as the strength of Earth’s magnetic field and,
in turn, begin to answer related questions—for example, by
determining the role of the magnetic field in protecting Earth from
solar flares.
Other
experiments performed with diamond anvil cells or gas guns can mimic
conditions similar to the atmospheres of giant planets
and the outer envelopes of low-mass stars composed mostly from hydrogen.
Data from these experiments can then be used to characterize the
composition and evolution of the giant planets.
For
example, scientists have learned that Neptune and Uranus are rich
in methane gas, which transforms to hydrogen and various hydrocarbons
under extremely high pressures and temperatures. Indeed, researchers
have suggested that the methane in large planets may conceivably
turn into diamond at shallow depths, about one-tenth of the way to
the center.
Nearly
two decades ago, Nellis led a project that shocked cooled, compressed
liquid methane. “Francis Ree then ran a chemical
equilibrium calculation with our data,” says Nellis. “His
simulation showed that methane at these conditions produced diamond
and molecular hydrogen. So we have reason to believe the cores of
Neptune and Uranus are most likely made of diamond.”
Nellis
adds that the project’s findings were a spin-off result
from the Laboratory’s weapons research, which routinely used
the single-carbon molecule methane in place of real explosives. “Hydrocarbons
such as methane are a simple way to test an HE detonation,” says
Nellis. “Real explosives are complex, and testing with simpler
species helps us detonation.” For example, some chemical reactions
cause complex materials to rapidly decompose into simpler molecules,
such as water, oxygen, nitrogen, carbon monoxide, and methane. The
same reaction happens in giant planets.
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Applying
extreme pressures and temperatures to water creates transient
chains of oxygen–hydrogen–oxygen (O–H–O)
molecules—a new phase of water.
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“Uranus and Neptune are big balls of explosive materials,” says
Nellis. “Hydrogen accounts for 90 percent of all the atoms
in the universe. The combination of hydrogen with oxygen gives us
water, hydrogen with nitrogen gives us ammonia, and with carbon,
we get methane. When all these accrete into one planet—collect
in a snowballing effect—the planet gets bigger, and the materials
deeper down are under even greater pressure. Because these materials
are poor thermal conductors, the heat that’s generated is trapped
for millions of years, which leads to a very hot interior—much
like the center of a detonated high explosive.”
Fried’s group also simulated water under high pressures and
temperatures. In those calculations, the hydrogen atoms moved quickly,
but the oxygen atoms were slower. As a result, short-lived transient
chains of oxygen–hydrogen–oxygen (O–H–O)
were being continually created and broken. Because the O–H
bond is weak under these extreme conditions, the molecules exhibited
a weak covalent character that is mostly ionic.
“We could say that we’re turning water into salt by squeezing
it really hard,” says Fried. “What’s important
about our results is the information we acquire about the behavior
of water under planetary conditions, which helps us better understand
the magnetic properties of planets. We may find that the electrical
conductivity of these molten salts is an important characteristic
of giant planet interiors.”
Extreme
chemistry research has even challenged some of the most fundamental
assumptions about the composition of fossil fuels. In 2003, a team
of scientists from the Carnegie Institute of Washington recreated
the conditions that exist more than 100 kilometers below Earth’s
surface.
The
team compressed marble, iron oxide, and water at pressures about
50,000 times atmospheric pressure and at temperatures of about 1,500°C.
Those experiments produced methane, the main constituent of natural
gas. When the Carnegie data were modeled with the CHEETAH code, the
simulations indicated that near the Earth’s mantle—where
temperatures and pressures are extremely elevated—petroleum
may be forming inorganically.
Is It All So Extreme?
Hot
outer planets, molten iron at the outer core of Earth, detonation
processes—extreme conditions seem to be everywhere. But is
it all really extreme? According to Zaug, the conditions at Earth’s
surface and within its delicate atmosphere are far from common. “The
conditions we live under are unusual,” says Zaug. “The
conditions we create during so-called extreme chemistry experiments
are not extreme—they’re normal.”
And
as scientists learn more about the chemical reactions that can initiate
a detonation sequence, they glimpse the balance of the universe.
—Maurina
S. Sherman
Key Words: CHEETAH code, chemical kinetics, diamond anvil cell,
equation of state, extreme chemistry, gas gun, high explosives
(HE), planetary physics, shock physics, thermochemical code.
For further information contact Larry Fried
(925) 422-7796 (fried1@llnl.gov).
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