On the Plastic Deformation of Pure Solid Lithium Sulfate and Lithium-Potassium Sulfate Mixtures

A simple rotational viscometer has been constructed for measurements at elevated temperatures in the region 109 — 2 x 1012 poise, and has been used to study the deformation of pure fee lithium sulfate (temperature range 740 —860 °C) and mixtures of this salt with potassium sulfate. These substances resemble highly-viscous non-NEwroidan liquids, and for a given torque the apparent viscosity can be represented by the conventional relation rj = r]0exp(Q/R T). For pure lithium sulfate, measurements with a torque of 19 x 107 dyne-cm gave the values ? / 0 =4 .24 x 10~ 4 poise and £> = 7.4 x 104 cal/mole for the two constants. Two mixtures containing 1.0 and 2.2 mole-% K 2 S 0 4 , respectively, were studied. In the homogeneous solid solution phase the viscosity showed a dependence on the thermal prehistory of the salt during the first few days at a given temperature. This effect was not detected for the pure lithium sulfate or for the two-phase region of the mixtures. The temperature dependence of the viscosity was less pronounced for the homogeneous phase than it was for the pure salt, while it was very great in the two-phase region. It is suggested that viscosity (or electrical conductivity) measurements should be a possible method to determine the position of the solidus curve for mixtures of this type.