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.
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