Highly conducting states in metal–ammonia solutions

Abstract

A peculiar phenomenon of unusually high electrical conduction in metal–ammonia solutions (MAS) has been described in the literature; the conduction of the solutions with metal concentrations from 1 to 5 MPM (mol% metal) increases by several powers of ten when the solutions are in contact with air and the dissolved metal slowly reacts to form solid reaction products. This conduction increase has been explained by the Fröhlich mechanism of one-dimensional superconductivity in the capillaries of the solid. In our experiments we could reproduce this high conductivity, but we have demonstrated that the phenomenon can also be observed in pure MAS without any decomposition, when voluminous or porous material, such as glass-wool, is housed between the current electrodes. Under these conditions the effect is easily observable and fairly reproducible. The reversible transitions from normal to high and from high to normal conduction are relatively slow processes requiring up to 1 h. We explain the conduction increase on the basis of the “pinch effect” as known in plasma physics: the current through a conductor gives rise to a magnetic field which forces, by the Lorentz force, the moving charge carriers (both cations and electrons) into the inner region of the current path and thus gives rise to an increase in their local concentration. Because the specific conductivity of MAS varies rapidly in the range of the non-metal–metal transition, from 1 to 5 MPM by a factor of more than 1000, this concentration increase yields an over-proportional increase in the electrical conductivity of the system.