Metallic Hydrogen

Hydrogen:
the most abundant element in the universe.

Normally it has been considered to remain a
non-metal at any range of temperatures and
pressures. That is, until now. Recently this year,
hydrogen was changed into a metallic substance,
which could conduct electricity. An experiment
conducted by William J. Nellis et al. at the
Lawrence Livermore National Laboratory
accomplished this feat. Hydrogen was converted
from a non-metallic liquid, into a liquid metal. The
likelihood that the most abundant element in the
universe could be converted into metallic form at
sufficient pressures was first theorized in 19351,
but tangible evidence has eluded scientists in the
intervening decades. “Metallization of hydrogen
has been the elusive Holy Grail in high-pressure
physics for many years,” said Bill Nellis, one of
three Livermore researchers involved in the
project. “This is a significant contribution to
condensed matter physics because a pressure and
temperature that actually produce metallization
have finally been discovered.”2 Livermore
researchers Sam Weir, Art Mitchell, and Bill
Nellis used a two-stage gas gun at Livermore to
create enormous shock pressure on a target
containing liquid hydrogen cooled to 200 K (-
4200 F). Sam Weir, Arthur Mitchell (a Lab
associate), and Bill Nellis published the results of
their experiments in the March 11 issue of Physical
Review Letters under the title “Metallization of
Fluid Molecular Hydrogen at 140 GPa (1.4
Mbar).” When asked about the significance of the
work, Nellis had this to say: “Hydrogen makes up
90 percent of the universe. Jupiter is 90 percent
hydrogen and contains most of the mass in our
planetary system. Hydrogen is very important to a
lot of work done at the Lab. Hydrogen in the form
of deuterium and tritium isotopes is the fuel in
laser-fusion targets and how it behaves at high
temperatures and pressures is very important to
Nova and the National Ignition Facility.”3 By
measuring the electrical conductivity, they found
that metallization occurs at pressure equivalent to
1.4 million times Earth’s atmospheric pressure,
nine times the initial density of hydrogen, and at a
temperature of 30000 K (50000 F). Because of
the high temperature, the hydrogen was a liquid.

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The intense pressure lasted less than a
microsecond. Optical evidence of a new phase of
hydrogen has been previously reported using an
experimental approach that involves crushing
microscopic-sized samples of crystalline hydrogen
between diamond anvils.4 However, metallic
character has not been established. Metallic
character is most directly established by electrical
conductivity measurements which are not yet
possible in diamond anvil cells at these pressures.

The Livermore team’s results were surprising
because of their methods, the form of hydrogen
used and the pressure needed to achieve the result
(which was much lower than previously believed).

Virtually all predictions surrounding metallic
hydrogen have been made for solid hydrogen at
low temperatures (around absolute zero). The
Livermore team tried a different approach. They
looked at hydrogen in liquid form at relatively high
temperature, for which no predictions have been
made. Some of the theorists who proposed the
existence of metallic hydrogen also believed the
substance would remain metallic after the
enormous pressures required to produce it were
removed, and that it might also be a
superconducter.5 Additionally, solid metallic
hydrogen is predicted to contain a large amount of
energy that might be released quickly as an
explosive or relatively slowly as a lightweight
rocket fuel. Metallic hydrogen’s light weight might
also have implications for material science. The
metallization events at Livermore occurred for
such a brief period of time, and in such a manner,
that questions about its superconducting properties
and retention of metallic form following pressure
removal could not be answered. “The potential
uses of metallic hydrogen are fascinating to
contemplate, but they are far down the road, and
we’ve only reached the first mile post on that
road,” said Nellis.6 Future experiments will be
aimed at learning more about the dependence of
metallization pressure on temperatures achieved in
liquid hydrogen. This understanding is vital for
Laboratory applications, according to Nellis, as
well as furthering collective knowledge about the
interiors of giant planets, such as Jupiter and those
recently discovered around nearby stars.7
Because hydrogen is the lightest and simplest off
all elements and composes about 90% of the
atoms in the visible universe, scientists have a
broad spectrum of interest in its properties and
phases. In the case of astrophysics, metallic
hydrogen is thought to exist in the interior of
Jupiter and Saturn. Its presence in large planets
both within and outside our solar system has a
significant effect on their behavior. Laser fusion,
which uses isotopes of hydrogen as targets,
exerting enormous pressure on them with laser
beams, may also be influenced by research on
metallic hydrogen. A better understanding of the
temperature/pressure relationship in hydrogen
could lead to higher fusion energy yields. The
experiments at Livermore were accomplished with
a two-stage gas gun. In the first stage, gunpowder
is used to drive a piston down the pump tube,
compressing hydrogen gas ahead of it. Squeezed
to sufficient pressure, the hydrogen breaks a
rupture valve and accelerates a projectile down
the second stage barrel at velocities up to 7km/s
(16,000 mph). The projectile generates a strong
shock-wave on impact with an aluminum sample
container, which is cooled to 20 degrees Kelvin
(-4200 F). Entering the liquid hydrogen, the shock
pressure first drops, then reverberates many times
between parallel sapphire anvils until the final
pressure, density and temperature are reached.

This reverberation produces 1/10 the temperature
that would be created by a single shock to the
same pressure. The temperatures achieved keep
hydrogen in the form of molecules, rather than
letting molecules break into atoms. Because the
experiments were done at higher temperatures
than originally predicted, the results suggest that
the metallization pressure of hydrogen is
temperature- dependent. A trigger pin in the target
produces an electrical signal when it is struck by
the initial shock wave; this signal is used to turn on
the data recording system at the proper moment.

The electrical conductivity of the hydrogen shock
is then measured to determine if metallization has
occurred. The Livermore team credited the
national laboratory’s unique multidisciplinary
capabilities for making possible their success. “A
lot of technology was brought to bear on the
experiment,” said Weir. “We couldn’t have done it
without the cryogenic and computational capability
that exists – along with the gas gun – only at
Livermore.”8 With more extensive research, the
full potential of metallic hydrogen can be reached.

The development of a metallic hydrogen is only in
its primary stages. This metal can have several
important properties which would make it a
valuable asset. Its formation was something that
many scientistists believed they would never see in
their lifetimes. After many failed attempts, it has
finally been achieved. But as Bill Nellis has said,
“Liquid metallic hydrogen turns out to be a rather
ordinary metal.”9 1 Coontz, Robert J. “Out Of
Thin Air,” The Sciences v. 36 (July/August 1996),
p12. 2 Lipkin, Richard. “The Lightest Metal in the
Universe,” Science News v. 149 (April 20 1996),
p250. 3 Johnston, Don. “Lab Team Hits Success
With Metallized Hydrogen,” Science v. 271
(March 22 1996), p1624. 4 Coontz, Robert J.

“Out Of Thin Air,” The Sciences v. 36
(July/August 1996), p12. 5 Hensel, Friedrich and
Edwards, Peter. “Hydrogen: The First Metallic
Element,” Science v. 271 (March 22 1996),
p1692. 6 Nellis, W. et al. “Neutralization and
Electrical Conductivity of Hydrogen,” Science v.

273 (August 16 1996), p937. 7 Nellis, W. et al.

“Neutralization and Electrical Conductivity of
Hydrogen,” Science v. 273 (August 16 1996),
p937. 8 Hemley, Russell and Ashcroft, Neil.

“Shocking States of Matter,” Nature v. 380 (April
25 1996), p671. 9 Geller, M.J. “Just Gas,”
Discover v. 17 (October 1996), p21.

Bibliography Coontz, Robert J. “Out Of Thin Air,”
The Sciences v. 36 (July/August 1996), p11-12.

Geller, M.J. “Just Gas,” Discover v. 17 (October
1996), p20-21. Hemley, Russell and Ashcroft,
Neil. “Shocking States of Matter,” Nature v. 380
(April 25 1996), p671- 672. Hensel, Friedrich
and Edwards, Peter. “Hydrogen: The First
Metallic Element,” Science v. 271 (March 22
1996), p1692. Johnston, Don. “Lab Team Hits
Success With Metallized Hydrogen,” Science v.

271 (March 22 1996), p1624-1625. Lipkin,
Richard. “The Lightest Metal in the Universe,”
Science News v. 149 (April 20 1996), p250-251.

Nellis, W. et al. “Neutralization and Electrical
Conductivity of Hydrogen,” Science v. 273
(August 16 1996), p937-940. Rao, C. N. and
Edwards, Peter. “Livermore’s Big Guns Produce
Liquid Metallic Hydrogen,” Physics Today v. 49
(May 1996), p17-18.
Science