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Alamos Scientific Laboratory of the University of California 10S ALAMOS, NEW MEXICO A7545 LA-61 72-MS Informal Report UC-34a Reporting Date: November 1975 Issued: January 1976 CIC-14 REPORT COLLECTION REPRODUCTION COPY Metallic Hydrogen: A Brief Technical Assessment by L. A. Gritzo N. H. Krikorian —L Contributors D. T. Vier A. T. Peaslee, Jr. r ) } alamos scientific laboratory of the university of California 10s ALAMOS, NEW MEXICO a7545 \ An Affirmative Action/Equal Opportunity Employer uNITEO STATES ENERGY RESEARCH AND DEVELOPMENT ADMINISTRATION CONTRACT W.7405.ENG. 36 In the interest of prompt distribution, this report was not edited by the Technical Information staff. This work was supported by the DoD Defense Advanced Research Projects Agency. The views and conclusions contained in this report are those of the authors and should not be interpreted as necessarily representing the official policies, either expressed or implied, of the Defense Advanced Research Pro- jects Agency or of the United States Government. Printed in the United States of America. Available from Nationsl Technical Information Service U.S. Department of Commerce 6286 Port Royal Road Springfield. VA 22161 Price: Printed CopY $4.50 Microfiche $2.26 7%1, ,ePort w., Prep. red ,, . ●ccount ofwork ,p.a.,.,ed h k UnitedSW- Government. Neither the UqiwJSm- .or the U“itrd Stat” l?nersy Scwarch ..d Development Ad- ministration, nor any or their ●mpl.yccs. “or any of their con. tractors. ..bco.tr.clors, or their empl.>~, !n.kes .ny wmr..tv. ●xpress or implied, . ..s. nms .ny Irr.1 Ii.btlity or rcspywibility for the ●ccuracy, campleten”., or .sef.lnts. of an? mformattom apparatus, LWXIuct, w D,,JWSJ di~lo,~, or rrr.ramt. that its use would not irifrlnce pri..tely owned rights. METALLIC HYDROGEN: A BRIEF TECHNICAL ASSESSMENT by L. A. Gritzo and N. H, Krikorian Contributors D. T. Vier and A, T. Peaslee, Jr. ABSTRACT A brief technical assessment of the status of inter- national research on metallic hydrogen is presented. Research methods are outlined; published work in the USSR, the US, and several other countries is reviewed. A prognosis is given for metallic hydrogen. Potential military applications and future research requirements are discussed, A bibliography is in- cluded. 1. INTRODUCTION Chemical classification of elements ity; it could be expected to have some un- places those with a single outer (valence) usual properties. electron, i.e., a single electron in the Molecular hydrogen exists as a gas un- first unfilled shell, into a category call- der normal conditions. It can be liquefied ed alkali metals. This category includes at about 20”K through cooling at atmospher- lithium, potassium, sodium, etc., in which ic pressure. It will form a solid at 14°K the bonding of the valence electron to the with a complex crystal structure. Solid atom is weakened by the presence of addi- molecular hydrogen is a low-density (about tional electrons in interior shells. Hy- 0.09 g/cm3) dielectric (electrical non- drogen (H) is placed in the alkali metals conductor) material . category by default, since the valence elec- Since about 1935, numerous theorists tron is the only electron present in the have postulated the existence of an addi- neutral atom. The first electronic shell tional solid state for hydrogen, the metal- of an atom is filled only when exactly two lic state. In the metallic state, the atoms electrons are present. There is thus a would no longer exist as diatomic molecules. strong tendency for hydrogen atoms to exist Instead, they would be considered as a reg- paired as diatomic molecules (H2). Although ular array of protons embedded in a sea of the hydrogen atoms form diatomic molecules free-moving electrons. Many calculations with strong internal bonding forces, the have been made that indicate a degree of molecules experience only very weak forces stability for this type of atomic hydrogen in interactions with each other. Hydrogen structure. However, the computed interatom- is unique among the elements in its simplic- ic distances required for stability in the 1 atomic form are considerably less than those necessary to resolve existing questions will observed for the molecular form. In order be discussed. to achieve the compact atomic form, it is believed that a large external pressure is II. THE WORLD-WIDE METALLIC HYDROGEN EFFORT required to be exerted on the molecular Both theoretical and experimental form. The present literature indicates research has been conducted on molecular significant disagreement on the magnitude and atomic hydrogen. In the following text, of the pressure required to obtain the several research methods that have been used molecular-atomic solid transition. The d“ s- or proposed will be reviewed, The review is agreement appears to arise from both the followed by a discussion of work that has current lack of knowledge regarding the been done in the USSR, in the US, and in response of hydrogen molecules to such other countries. external forces, and the insufficiencies in A. Research Methods the mathematical models used to describe Nearly all experimental work has been systems of atoms and molecules. oriented toward the production of very high Two important properties have been pressures and the application of these pres- postulated for the metallic form. The first sures to molecular hydrogen and deuterium. of these is metastability, i.e., retention The work generally falls into two major of the metallic form (produced at low tem- categories, dynamic and static. perature and under high pressure) after the The dynamic technique is exemplified by pressure is reduced and the temperature is the work of Fowler at the Los Alamos Scien- raised. The second is superconductivity at tific Laboratory (LASL) and of Hawke at the temperatures far above that of ordinary Lawrence Livermore Laboratory (LLL). Hawke’s superconductors (about 15”K), Most re- description of the technique is:l searchers today expect that a molecular- Figure 1 shows a cross section of the experimental apparatus. An initial mag- atomic transition will occur in hydrogen. netic field is generated by use of a pair Such transitions have been observed in the of coils and a capacitor bank. The magne- tic field diffuses through a stainless- USSR for other crystalline materials (e.g., steel liner surrounded by high explosive. salt, sulfur, silica and alumina). Many When the diffused magnetic field is close to its peak value, the high explosive is researchers question the postulated meta- detonated and the liner is cylindrically stability of the metallic form, especially imploded. The implosion compresses the magnetic flux and increases the magnetic at temperatures higher than the melting field intenkity, During the flux cumula- point of solid molecular hydrogen. Corres- tion, eddy currents generated in the sample tube and liner interact with the pondingly, most researchers will admit the magnetic field and exert an outward pres- possibility of the metallic form being sure on the liner and an inward pressure on the sample tube. superconducting, but many question the postulated high critical temperature (the Pressures up to about 4 Mb have been ob- temperature at which it would cease being tained with this method; the duration of a superconductor). the high-pressure pulse is very short The potential value of metallic hy- (submicro-second) . Diagnostic equipment drogen hinges on a favorable outcome for can include high-speed framing cameras its postulated metastability and super- with high-explosive-driven argon flash conductivity. In the following sections, lamps, inductive pickup loops, and pulsed the known international research in this x-ray radiographic machines. The pressure area will be reviewed, the prognosis for obtained is not measured directly, but is metallic hydrogen will be discussed, poten- deduced from radiographic measurements and tial military applications for the material indirect computations. Vereshchagin of the will be listed, and additional research Soviet Union says of the dynamic technique: 2 material onto an anvil of Carbonado, and then loading the sample by means of a Carbonado plunger, as shown schematically in Fig. 3. Pressures of several megabars W-Al’e’ona’orsare attainable over very tiny nonuniform areas. An alternate approach to the formation of metallic hydrogen has been proposed by Hess of Germany. This method involves the formation of atomic hydrogen by dissocia- tion via an electric discharge in a cryo- statted apparatus. The atomic form is - N-J{ maintained (without recombination to the /’4-\- \ -. molecular form) by means of high magnetic ‘Copper tube \\ fields. The magnetic fields would be used Stainless steelliner Vacuum to maintain spin alignment of the electron magnetic moments. Condensation of the atom- ic form at temperatures below 1.2°K might Fig. 1. Cutaway vielw of experimental apparatus. lead to metallic hydrogen. Further inves- tigation of this method is being done at NASA’s Lewis Research Center. The disadvantages of pulsed methods include the inability of saving the phases Reviews of the Soviet and US literature produced; moreover, the investigated phase on metallic hydrogen, to be presented in the transitions in many cases cannot take place within the time of existence of high pres- remainder of this section, show that most of sures. the reported work is of a theoretical The static technique is exemplified by nature. Despite the thorough theoretical the work of Vereshchagin et al. His understanding of the detailed behavior of a description of the technique is: single hydrogen atom, the ab initio deter- “Reaching such pressures by static mination of the characi eristics of a many- methods requires not only the forces of colossal presses, but especially new bodied substance (such as solid hydrogen) construction materials with outstanding from quantum theory is intractable. How- properties. Work is being conducted. .. on a press for investigating the problem ever> theoretical techn ques have success- of metallic hydrogen. The press is design- ed to produce a maximum force of 50 000 tons,” (See Fig.
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