Electrical Conductivity of Noble Gases at High Pressures

Abstract

We present experimental and theoretical results for the electrical conductivity of noble gases (He, Ne, Ar, Kr, Xe) up to high pressures where a transition from nonmetallic to metallic-like conductivities occurs. In addition, we show the behavior of the thermal conductivity and thermopower for xenon as an example. The experiments were performed using explosively driven shock waves. Different geometries allow to probe various parameter regions up to several megabars. Besides single-shock experiments along the principal Hugoniot curve, also multiple-shock experiments were performed which follow almost an isentrope. The theoretical calculations were performed within a partially ionized plasma model. The composition is determined by solving a system of mass action laws. The transport coefficients are calculated within linear response theory taking into account the relevant scattering mechanisms of electrons at different ion species, atoms, and other electrons. The general trends of the experimental results can be explained within this theoretical approach.

Opacity and conductivity measurements in noble gases at conditions of planetary and stellar interiors

“…High absorption (α > 0.1 μm −1 ) is observed above 6,700 K (0.6 eV). At this temperature the DC conductivity is ∼1 S/cm (12,13,17,30,31) (SI Text), but the corresponding free electron absorption (∼0.01 μm −1 ) is an order of magnitude lower than the observed absorption.…”

“…Significant theoretical and observational evidence for phase separation of helium and neon in the hydrogen-rich outer envelopes of giant planets has been reported (1-8), yet questions over the nature and even existence of this phase separation remain (2,42,43). This is in part due to limited direct experimental data on the hydrogen-helium-neon system at the conditions of phase separation: although hydrogen is observed in a metallic state at the interior conditions of giant planets (44), measurements on the electronic character of helium and neon at relevant densities and temperatures have been limited (12,13,17,45). As phase separation may be controlled by the electronic properties of the constituents (1, 2, 4), the present measurements have a direct bearing on this problem.…”

“…Xe is observed to metallize near room temperature under pressures similar to those at Earth's coremantle boundary (18,19). Ar and He are observed to conduct only at combined high pressure and temperature (12,13,17). Ne is predicted to have the highest metallization pressure of all known materials-10 3 times that of Xe and 10 times that of He (14,18,20,21)-and has never been documented outside of its insulating state.…”

“…Dense noble gases are expected to show systematic similarities in their properties at extreme conditions (13)(14)(15)(16)(17); however, a general understanding of their insulator-conductor transformation remains to be established. Xe is observed to metallize near room temperature under pressures similar to those at Earth's coremantle boundary (18,19).…”

“…Ne is predicted to have the highest metallization pressure of all known materials-10 3 times that of Xe and 10 times that of He (14,18,20,21)-and has never been documented outside of its insulating state. Experimental probes of extreme densities and temperatures in noble gases have previously relied on dynamic compression by shock waves (12,13,17,(22)(23)(24). However, in such adiabatic experiments, light and compressible noble gases heat up significantly and can ultimately reach density maxima (12,13,17,21,24,25), so that conditions created often lie far from those deep within planets (7,8) and stars (9).…”