HIGH-ACCURACY THERMAL-ANALYSIS OF THE SOLID-SOLID PHASE-TRANSITION OF LITHIUM-SULFATE POWDERS

Powder samples of Li2SO4 from three different manufactures were analysed using an accurate high-resolution heat flow calorimeter. It is shown that some of the results presented are endowed with more than ten times smaller uncertainty limits than are usually encountered in the technical literature covering the state of the art of thermal analysis. Details of the measuring system are given, and reasons for the higher accuracy are discussed. The average enthalpy change associated with the transformation between the two high temperature forms of solid lithium sulfate, resulted in DELTAH = 26.1 +/- 1.7 kJ mol-1. The relatively large uncertainty (twice as large as expected from the calorimeter's performance) is due to unexplained differences between the three powder samples investigated. Different transformation temperatures were determined for each one of the samples. The origin of the differences, as well as the shape of the measured heat-flow curves, can be explained semi-quantitatively by means of the Gibbs-Thomson effect, in terms of the size distribution of the crystallites that compose the sample, which should depend on its initial granulometry and the dehydration procedure used. The onset temperatures for the monoclinic to cubic transformation, for sample 1 (Aldrich), sample 2 (Merck), and sample 3 (Alfa), are 577.82 +/- 0.08, 577.98 +/- 0.05 and 578.30 +/- 0.03-degrees-C, respectively. They are believed to represent the transformation temperature of the smallest crystallites present in the sample. Extrapolated temperatures, representative of the transformation temperature corresponding to the average crystallite size in samples 1, 2 and 3, are 578.17 +/- 0.04, 578.28 +/- 0.03 and 578.38 +/- 0.02-degrees-C, respectively. For the cubic to monoclinic transformation, the corresponding onset temperatures for the same three samples, are 577.90 +/- 0.02, 577.84 +/- 0.02 and 577.78 +/- 0.02-degrees-C. These are believed to represent the end temperatures of undercooling, just as nucleation of the new phase begins. Interphase curvature is such that the monoclinic phase is on the concave side of the interphase.