Prospects for Metallic Hydrogen
The theoretical predictions of the transition of hydrogen from an insulating molecular crystal to an electrical conducting alkali metal are reviewed in the light of Hugoniot experiments carried out to 0.9 Mbars and isentropic experiments carried out to a claimed 8 Mbars. This isentropic experiment provides no evidence for the existence of a first order transition contrary to earlier claims.
If a pair potential model is accurate and if available isentropic and Hugoniot experiments are accurate, the transition to an alkali metal phase is quite high (several megabars). However, there are theoretical predictions (which appear to have a sounder basis than the pair potential model) which predict that molecular hydrogen itself becomes metallic at modest pressures; this result by itself suggests that the pair potential model is inadequate. New results of Ashcroft based on the behavior of metallic molecular hydrogen suggest that a first order transition may be possible at modest pressures (near one megabar).
The state of static experimental research is reviewed. In general the pressure in such experiments has been overestimated. The evidence of production of metallic hydrogen is, at this time, very weak, and methods are suggested to test these claims.
Finally, the strength of perfect diamond crystals is considered and shown to be very high. Experiments by Ruoff and Wanagel, in which static contact pressures of 1.4 Mbars have been reached in a contact circle having a radius of one micrometer, validate these predictions. It is concluded that pressures as high as 3 – 5 Mbars may be achieved in extremely small contact regions without yielding and that, moreover, scientific observations will be possible in those tiny regions.
KeywordsOrder Transition Perfect Crystal Free Energy Curve Cement Tungsten Carbide Metallic Hydrogen
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