Relaxation processes in super-cooled nitrate melts

Abstract

Binary mixtures of calcium nitrate with potassium nitrate can be supercooled extensively below their normal solidus temperatures. Comparable binary mixtures have now been studied with sodium, rubidium or caesium as the monovalent cation. Viscosities are in the sequence Na>Cs>Rb>K at high temperatures.

On progressive cooling of these binary melts, the viscosity and electrical resistivity as measured by conventional methods show very marked increases above normal “Arrhenius” values. Viscosities up to 10⁴ poise are observed, with electrical resistivities of 10⁴–10⁵ ohm cm. If crystallization can be avoided, on still further cooling the mass sets to a glass; if crystallization of the viscous melts occurs, an increase of resistivity is observed. Additions of alkali halides as ternary components to the calcium+potassium mixture have opposing effects on viscosity and resistivity. The viscosity is increased by the presence of the spherical halide anions (Cl, Br) but is little affected on replacing a potassium halide by a sodium halide. Addition of a halide with a samller cation (Na) decreases the resistivity; other halides have little effect.

With super-cooled melts of sufficiently high resistivity, measurements of dielectric constants became practicable in the frequency range 10³–10⁷ c/sec; these showed frequency-dependent relaxation effects. Analysis of the data indicates a dominant relaxation process of characteristic time between 10^–6 and 10^–7 sec in the binary nitrate melts at around 90°C. Probable structures and relaxation mechanisms in supercooled nitrate melts are discussed.