The most commonly asked questions about our metallic hydrogen experiments are:
1)Do any metallic systems have electrical conductivities similar to those of metallic hydrogen?
Yes. The electrical conductivities of metallic fluid Cs and Rb at 2000 K are identical to those of fluid hydrogen when all three undergo the same continuous transition from a semiconducting to metallic fluid.
2)What is different about these shock experiments compared to previous ones which tried to metallize hydrogen at static high pressures in a diamond cell?
We shock-heated hydrogen to about 3000 K, which produced a fluid. Previous experiments with static megabar pressures were performed at room temperature or below, which produced solid hydrogen.
3)Why does metallization occur in the fluid at a lower pressure than for the solid?
Metallization in solid hydrogen is inhibited by phase transitions in crystal structure and molecular orientation. Neither exists in the disordered fluid.
4)Does any other element become metallic at a lower pressure in the fluid than in the solid?
Yes. Iodine becomes metallic at 30 kbar in the fluid and 160 kbar in the solid.
5)What do you mean by a metal?
Metallic fluid hydrogen is achieved when high pressures reduce the energy gap Eg between the filled valence-electron band and the unfilled conduction-electron band down to Eg~kBT, where kB is Boltzmann's constant and T is the temperature. When Eg~kBT, thermal smearing in the disordered fluid fills in the energy gap, a metallic density of electronic states is achieved, and the electronic system has a Fermi surface characteristic of a metal.
6)Why not heat hydrogen in a diamond cell to achieve the same high pressures and temperatures as in the shock experiments?
Because of the high mobility of hydrogen at high temperatures, the diamond cell is essentially transparent to hydrogen diffusion. The highest recorded temperature of hydrogen in a diamond cell is 500 K. At higher temperatures the cell is empty because hydrogen diffuses away.
7)How does shock compression heat hydrogen?
Because shock compression is so fast, it is also adiabatic. That is, heat produced by compression has insufficient time to be conducted or radiated away. Also, by reaching final pressure with a shock reverberating in soft hydrogen between stiff sapphire anvils, the final hydrogen temperature is about 1/10 what it would be if final pressure were reached in a single shock. In this way the temperature is about twice the melting temperature and 2% of the Fermi temperature, just the right temperature to produce melted condensed matter.
8)Why doesn't hydrogen diffuse out of a shock-compressed sample?
The experiment lasts only 100 ns, too short a time for the hydrogen to diffuse away.
9)Is metallic hydrogen in thermal equilibrium?
Yes. Within the 3-ns time resolution of the diagnostic system, there are ~105 intermolecular collisions and 4 times as many vibrations. This number of collisions is larger by 3-4 orders of magnitude than required to achieve thermal equilibrium.
10)Is metallic hydrogen in electrical equilibrium?
Yes. The thickness of the hydrogen layer decreases from the initial value of 500 mm down to the compressed value of 50 mm. The calculated flux diffusion time for a layer 50 mm thick with our highest electrical conductivity of 2000 (W-cm)-1 is < 1 ns, which indicates that the electrical current reaches its equilibrium flow pattern in <1 ns.
11)Is the experiment affected by hydrodynamic interfacial instabilities?
No. Rayleigh-Taylor and Richtmyer-Meshkov instabilities did not occur when shock waves traversed the planar interfaces between sapphire and hydrogen because the initial surfaces of the sapphire crystals were optically flat (300 A rms surface roughness) and the time duration of the experiment (100 ns) was too short to allow such small instabilities to grow during the duration of the measurement.
12)Is the temperature lowered by radiative cooling?
No. The temperature lost by radiation in 100 ns is <1 K out of 3000 K, a negligible amount. Thermal radiation is emitted from about 2 optical depths (640 A@) at the surface of the metallic hydrogen. The energy radiated at 3000 K was calculated from the Stefan-Boltzmann radiation law and converted to the radiated temperature using the calculated heat capacity of 2 optical depths of metallic hydrogen.
13)Is the temperature lowered by thermal conduction?
No. An interfacial layer of metallic hydrogen ~0.5 mm thick is cooled about 200 K in 100 ns; both are negligible compared to the 50-mm total thickness at 3000 K. The heat conducted was calculated using values of thermal conductivity and diffusivity of metallic hydrogen calculated with the Wiedemann-Franz law; values for alumina at 1500 K, its calculated shock temperature, were taken as handbook values.
14)Is the measured conductivity caused by a metallic interfacial layer formed by high-temperature chemical reactions?
No. Such a layer, if it were to exist, would be too thin after 100 ns and its electrical conductivity too small to account for the measured signal. For example, there is insufficient time to form a thin metallic layer of pure Al by the reduction of Al2O3 by metallic hydrogen. In 100 ns the conservatively estimated diffusion constant of H2 into Al2 O3is too small for the hydrogen to get sufficiently deep inside the Al2O3 and the diffision constants of O2, H2O, and OH->are far too small for them to get out of the anvil to allow formation of a metallic layer of Al.
15)Is current carried by ions, rather than by electrons.
No. Current is carried by electrons. The Drude conductivity depends inversely on the mass of the carrier. The masses of an electron and of a proton, the lightest possible ion, differ by a factor of 2000, which indicates that electronic conduction dominates. For example, the electrical conductivities of ionic alkali-halide fluids are typically about 1 (W-cm)-1, while conductivities of pure metallic alkali fluids are typically a few 1000 (W-cm)-1. We measured a metallic hydrogen conductivity of 2000 (W-cm)-1.
16)Is current carried by electrons in an impurity band of H monomers in a semiconducting H2 host, as in the degenerate doped semiconductor Si(P)?
No. The electronic structures of the H atom and the H2 molecule are very similar, which means the energy gap we observe is that of the H2-H mixture. Also, fluid H2 at 3000 K does not have a high dielectric constant, which means that H2 and H only interact at short range. In contrast, the electronic structure of crystalline Si and P are very different. Also, crystalline Si has a high dielectic constant, which permits dilute concentrations of P to interact at long range in a semiconducting Si host and form a conducting P impurity band in the bandgap of Si. Metallic fluid hydrogen is nothing like Si(P).
17)Is the value of the electrical resistivity of metallic hydrogen consistent with simple expectations?
Yes. The measured value of 500 mW-cm is bracketed by simple models. The formula for the calculated maximum resistivity of a metal gives a value of 250 mW-cm. A calculation of the electrical resisivity using the Ziman weak-scattering model for a molecular liquid metal gives a value of 100 mW-cm. The electrical resistivity calculated in the strong-scattering free-electron model gives 1700 mW-cm. All these values are within a factor of 5 or less of the measurement, which indicates the measured value is reasonable.
18)Why has the measured resistivity value of metallic hydrogen, 500 mW-cm, not been calculated theoretically?
Because no theory exists for this novel state of condensed matter. For example, tight-binding molecular-dynamics calculations indicate that the energies of translation, vibration, and rotation of H2are all comparable in fluid metallic hydrogen. Present theories of liquid metals assume these energies are significantly different from one another.
19)If metallic hydrogen could be retained metastably on release of pressure, what properties would it have?
Metallic hydrogen is speculated to have a number of interesting properties and important applications, if it could be quenched from high pressures to ambient. It is not known how, nor even whether, metallic hydrogen could be quenched to ambient but the potential benefits are enormous if it could be. For this reason it is worth speculating on possible uses of metastable solid metallic hydrogen.
The ten times higher density of metallic DT fuel pellets, relative to molecular solid ones, would increase substantially the energy produced in laser-driven ICF, giving giant lasers, such as LLNL's NOVA and future NIF, an even larger margin for success than expected previously. The higher starting density of the fuel might produce a sufficiently large increase in ICF energy yield such that it might be possible to use only deterium as the fuel. The absence of radioactive tritium might make the ICF energy source much more attractive to commercial energy producers.
Metallic solid hydrogen has been predicted to be a room-temperature superconductor, which would result in substantial energy conservation nationwide.
Metastable metallic hydrogen would have a very high density of stored energy because it would have a density about ten times that of liquid H2 at 1 bar. Thus, the stored energy released by reversion to the diatomic insulating fluid would also be very large and metastable metallic hydrogen would have widespread applications as fuels. If this energy were released relatively slowly or quickly, metallic hydrogen would be either a clean propellant, as gasoline, or an explosive, respectively. The predicted specific impulse of metallic hydrogen is about 5 times that of liquid H2/O2 fuel now used to launch rockets into space. This large increase in specific impulse would result in smaller cheaper spacecraft, which would greatly facilitate space travel.
If solid metallic hydrogen has sufficient strength, it might be useful as a light-weight structural material. For example, automobiles made of metallic hydrogen would be ~10 times lighter than current ones made of steel, enhancing fuel efficiency and reducing conventional fuel emissions. The ideal would be to synthesize metallic hydrogen to be either extremely metastable, as diamond, for use as a structural material or readily reactive, as gasoline.
Large quantities of metallic hydrogen might be made by shock recovery methods using large systems of chemical explosives, as DuPont now shock-synthesizes diamond.
Reference:
http://www-phys.llnl.gov/Organization/HDiv...n/ComQuest.htmlThe nucleus of the hydrogen atom has it's own magnetic field.
