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 1935, 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.”
Two 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.”
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 30,000 K (50,000 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. 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 superconductor. 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 milepost on that road,” said Nellis. 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. Because hydrogen is the lightest and simplest of 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 7 km/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.” With more extensive research, the full potential of metallic hydrogen can be reached. The development of metallic hydrogen is only in its primary stages.
This metal can have several important properties that would make it a valuable asset. Its formation was something that many scientists 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.”
References:
- Coontz, Robert J. “Out Of Thin Air,” The Sciences v. 36 (July/August 1996), p12.
Lipkin, Richard. “The Lightest Metal in the Universe,” Science News v. 149 (April 20, 1996), p250. - Johnston, Don. “Lab Team Hits Success With Metallized Hydrogen,” Science v. 271 (March 22, 1996), p1624.
- Coontz, Robert J. “Out Of Thin Air,” The Sciences v. 36 (July/August 1996), p12.
- Hensel, Friedrich and Edwards, Peter. “Hydrogen: The First Metallic Element,” Science v. 271 (March 22, 1996), p1692.
- Nellis, W. et al. “Neutralization and Electrical Conductivity of Hydrogen,” Science v. 273 (August 16, 1996), p937.
- Nellis, W. et al. “Neutralization and Electrical Conductivity of Hydrogen,” Science v. 273 (August 16, 1996), p937.Hemley, Russell and Ashcroft, Neil. “Shocking States of Matter,” Nature v. 380 (April 25, 1996), p671.
- 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”
