On the Constant Volume Viscous Properties of Molten Alkali Nitrates

The viscosities of molten NaNO3 and KNO3 have been determined by a rolling ball technique over a pressure range from 1 to 414 bar and over temperature ranges from 328 to 444°C for NaNO3 and from 355 to 467°C for KNO3. Constant pressure activation energies, Ep = − R[d ln φ/d (1/T)]p, are fairly constant for a given salt at different pressures and are about 16.3 to 17.5 kJ for NaNO3 and about 18.2 to 19.0 kJ for KNO3, with the larger values occurring at higher pressures. There appears to be a tendency towards temperature dependent Ep values at the highest pressures employed, which could be traceable to compression forcing the melts towards a glassylike state, especially at lower temperatures. Activation volumes, Δ V* = − RT (d ln φ/d P)T, are typically from about 14 to 17 cm3 mole−1 for each melt, which are quite large values in contrast to those obtained by other workers for equivalent electrical conductance. These large ΔV* values lead to very small constant volume activation energies, Ev = − R[d ln φ/d (1/T)]v = Ep − (α/β) T Δ V*, of only about 2% to 6% of Ep. These are much smaller Ev values than those obtained in conductance work, and even in comparison to those obtained for the viscosities of ordinary nonpolar liquids. A modified Batchinski free volume model: φ = V sp, 0 (1−β P) / B − b/B, even with crude approximations for isothermal compressibility, β, and with assumed pressure independence of excluded volume, b, and the flow unit size parameter, B, gives fairly good agreement with experiment. It may be that viscous properties of fused NaNO3 and KNO3 are, then, more exactly describable by a hard-sphere free volume type of model and that the average size of a neutral flow unit is greater than that of the average charged unit involved in conductance.