Measurement of Thermal Conductivity in a Supercooled Hydrogel-Salt Complex Near Its Phase Transition

Solid-liquid phase transitions, i.e. solidification processes, have applications in data storage, development of novel thermoelectric materials, cooling of microelectronic substrates and air conditioning condensers. Standard analyses of solidification (Stefan problem) assume constant thermal properties of the solid and liquid sides. It is not known how these properties change across the spatial transition interface, though most studies report a discontinuity in the solid and liquid properties through the transition temperature [1]. When the phase transition releases enthalpy, recent research has shown if the phonon or electron transport time is of the same order of magnitude as the time scale of the atomic transformation, this increases the heat capacity of the solid material at temperatures near the phase transition temperature [2]. A fundamental understanding of the transport and thermal properties spatially near the phase transition may help shed light on the molecular origins of supercooling and spontaneous nucleation, which will help with applications involving them. We report studies of supercooling and nucleation of sodium sulfate decahydrate, a salt hydrate which is of recent interest in thermal storage, and of a hydrogel-sodium sulfate complex which shows limited supercooling. We will also report the measurement of their thermal conductivity during the phase transition process. This will involve a transient heated wire technique and setup which produces small temperature changes of the order of ~1 C.<br/><u>References:</u><br/>1. Shamberger, P. J., and Reid, T. "Thermophysical properties of lithium nitrate trihydrate from (253 to 353) K." <i>Journal of Chemical & Engineering Data</i> 57.5 (2012): 1404-1411.<br/>2. Agne, M. T., Voorhees, P.W. and Snyder, G. J. "Phase transformation contributions to heat capacity and impact on thermal diffusivity, thermal conductivity, and thermoelectric performance." <i>Advanced Materials</i> 31.35 (2019): 1902980.