EFFECTS OF HIGH-PRESSURE HELIUM ARGON AND NITROGEN ON VISCOSITY OF FUSED SODIUM NITRATE

The viscosity of molten NaNO3 has been measured by a capillary flow technique, from 328 to 444°, while pressurized with He, Ar, and N2 at pressures from 1 to 414 bars. Increasing the pressure of He resulted in diminishing the fluidity, φ, to nearly the same extent at each temperature. Use of Ar, however, actually increased φ with increasing pressure at each temperature, although the effect became less and less pronounced with increasing temperature, being almost nonexistent at 444°. A similar trend was observed with N2, although this gas increased φ only slightly at 328° and decreased φ to an ever-increasing extent with increasing temperature. The results are interpreted in terms of the Ar and N2 molecules diluting the melt, tending to make it less viscous in competition with the effect of pressure alone. The different extents of the effects of Ar and N2 are attributed to the degrees of the ion-gas molecule interactions, as reflected by the heats and entropies of solution of these gases in fused NaNO3. If the dilution effect by the very small, virtually non-polarizable He atoms is negligible, an activation volume for viscous flow at 400° is calculated as ΔVη* = -RT(d ln φ/dP)T = 8.79 cm3 mol-1, and the activation energies at constant pressure, EP, and at constant volume, EV, evaluated at 400° (1 atm) result as 3.93 and 1.50 kcal, respectively.