Hydrogen:the most abundant element in the universe.
Normally it has been considered to remain anon-metal at any range of temperatures andpressures. That is, until now. Recently this year,hydrogen was changed into a metallic substance,which could conduct electricity. An experimentconducted by William J.
Nellis et al. at theLawrence Livermore National Laboratoryaccomplished this feat. Hydrogen was convertedfrom a non-metallic liquid, into a liquid metal. Thelikelihood that the most abundant element in theuniverse could be converted into metallic form atsufficient pressures was first theorized in 19351,but tangible evidence has eluded scientists in theintervening decades. “Metallization of hydrogenhas been the elusive Holy Grail in high-pressurephysics for many years,” said Bill Nellis, one ofthree Livermore researchers involved in theproject. “This is a significant contribution tocondensed matter physics because a pressure andtemperature that actually produce metallizationhave finally been discovered.Order now
“2 Livermoreresearchers Sam Weir, Art Mitchell, and BillNellis used a two-stage gas gun at Livermore tocreate enormous shock pressure on a targetcontaining liquid hydrogen cooled to 200 K (-4200 F). Sam Weir, Arthur Mitchell (a Labassociate), and Bill Nellis published the results oftheir experiments in the March 11 issue of PhysicalReview Letters under the title “Metallization ofFluid Molecular Hydrogen at 140 GPa (1. 4Mbar). ” When asked about the significance of thework, Nellis had this to say: “Hydrogen makes up90 percent of the universe.
Jupiter is 90 percenthydrogen and contains most of the mass in ourplanetary system. Hydrogen is very important to alot of work done at the Lab. Hydrogen in the formof deuterium and tritium isotopes is the fuel inlaser-fusion targets and how it behaves at hightemperatures and pressures is very important toNova and the National Ignition Facility. “3 Bymeasuring the electrical conductivity, they foundthat metallization occurs at pressure equivalent to1.
4 million times Earth’s atmospheric pressure,nine times the initial density of hydrogen, and at atemperature of 30000 K (50000 F). Because ofthe high temperature, the hydrogen was a liquid. The intense pressure lasted less than amicrosecond. Optical evidence of a new phase ofhydrogen has been previously reported using anexperimental approach that involves crushingmicroscopic-sized samples of crystalline hydrogenbetween diamond anvils.
4 However, metalliccharacter has not been established. Metalliccharacter is most directly established by electricalconductivity measurements which are not yetpossible in diamond anvil cells at these pressures. The Livermore team’s results were surprisingbecause of their methods, the form of hydrogenused and the pressure needed to achieve the result(which was much lower than previously believed). Virtually all predictions surrounding metallichydrogen have been made for solid hydrogen atlow temperatures (around absolute zero).
TheLivermore team tried a different approach. Theylooked at hydrogen in liquid form at relatively hightemperature, for which no predictions have beenmade. Some of the theorists who proposed theexistence of metallic hydrogen also believed thesubstance would remain metallic after theenormous pressures required to produce it wereremoved, and that it might also be asuperconducter. 5 Additionally, solid metallichydrogen is predicted to contain a large amount ofenergy that might be released quickly as anexplosive or relatively slowly as a lightweightrocket fuel. Metallic hydrogen’s light weight mightalso have implications for material science.
Themetallization events at Livermore occurred forsuch a brief period of time, and in such a manner,that questions about its superconducting propertiesand retention of metallic form following pressureremoval could not be answered. “The potentialuses of metallic hydrogen are fascinating tocontemplate, but they are far down the road, andwe’ve only reached the first mile post on thatroad,” said Nellis. 6 Future experiments will beaimed at learning more about the dependence ofmetallization pressure on temperatures achieved inliquid hydrogen. This understanding is vital forLaboratory applications, according to Nellis, aswell as furthering collective knowledge about theinteriors of giant planets, such as Jupiter and thoserecently discovered around nearby stars. 7Because hydrogen is the lightest and simplest offall elements and composes about 90% of theatoms in the visible universe, scientists have abroad spectrum of interest in its properties andphases.
In the case of astrophysics, metallichydrogen is thought to exist in the interior ofJupiter and Saturn. Its presence in large planetsboth within and outside our solar system has asignificant effect on their behavior. Laser fusion,which uses isotopes of hydrogen as targets,exerting enormous pressure on them with laserbeams, may also be influenced by research onmetallic hydrogen. A better understanding of thetemperature/pressure relationship in hydrogencould lead to higher fusion energy yields. Theexperiments at Livermore were accomplished witha two-stage gas gun. In the first stage, gunpowderis used to drive a piston down the pump tube,compressing hydrogen gas ahead of it.
Squeezedto sufficient pressure, the hydrogen breaks arupture valve and accelerates a projectile downthe second stage barrel at velocities up to 7km/s(16,000 mph). The projectile generates a strongshock-wave on impact with an aluminum samplecontainer, which is cooled to 20 degrees Kelvin(-4200 F). Entering the liquid hydrogen, the shockpressure first drops, then reverberates many timesbetween parallel sapphire anvils until the finalpressure, density and temperature are reached. This reverberation produces 1/10 the temperaturethat would be created by a single shock to thesame pressure.
The temperatures achieved keephydrogen in the form of molecules, rather thanletting molecules break into atoms. Because theexperiments were done at higher temperaturesthan originally predicted, the results suggest thatthe metallization pressure of hydrogen istemperature- dependent. A trigger pin in the targetproduces an electrical signal when it is struck bythe initial shock wave; this signal is used to turn onthe data recording system at the proper moment. The electrical conductivity of the hydrogen shockis then measured to determine if metallization hasoccurred.
The Livermore team credited thenational laboratory’s unique multidisciplinarycapabilities for making possible their success. “Alot of technology was brought to bear on theexperiment,” said Weir. “We couldn’t have done itwithout the cryogenic and computational capabilitythat exists – along with the gas gun – only atLivermore. “8 With more extensive research, thefull potential of metallic hydrogen can be reached. The development of a metallic hydrogen is only inits primary stages.
This metal can have severalimportant properties which would make it avaluable asset. Its formation was something thatmany scientistists believed they would never see intheir lifetimes. After many failed attempts, it hasfinally been achieved. But as Bill Nellis has said,”Liquid metallic hydrogen turns out to be a ratherordinary metal. “9 1 Coontz, Robert J.
“Out OfThin Air,” The Sciences v. 36 (July/August 1996),p12. 2 Lipkin, Richard. “The Lightest Metal in theUniverse,” Science News v.
149 (April 20 1996),p250. 3 Johnston, Don. “Lab Team Hits SuccessWith 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 andEdwards, Peter. “Hydrogen: The First MetallicElement,” Science v.
271 (March 22 1996),p1692. 6 Nellis, W. et al. “Neutralization andElectrical Conductivity of Hydrogen,” Science v.
273 (August 16 1996), p937. 7 Nellis, W. et al. “Neutralization and Electrical Conductivity ofHydrogen,” Science v. 273 (August 16 1996),p937.
8 Hemley, Russell and Ashcroft, Neil. “Shocking States of Matter,” Nature v. 380 (April25 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 (October1996), p20-21. Hemley, Russell and Ashcroft,Neil.
“Shocking States of Matter,” Nature v. 380(April 25 1996), p671- 672. Hensel, Friedrichand Edwards, Peter. “Hydrogen: The FirstMetallic Element,” Science v. 271 (March 221996), p1692.
Johnston, Don. “Lab Team HitsSuccess 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 ElectricalConductivity of Hydrogen,” Science v.
273(August 16 1996), p937-940. Rao, C. N. andEdwards, Peter. “Livermore’s Big Guns ProduceLiquid Metallic Hydrogen,” Physics Today v.
49(May 1996), p17-18. Science