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Understanding the Hydration Process of Salts: The Impact of a Nucleation Barrier
journal contribution
posted on 2019-02-14, 00:00 authored by Leyla-Cann Sögütoglu, Michael Steiger, Jelle Houben, Daan Biemans, Hartmut R. Fischer, Pim Donkers, Henk Huinink, Olaf C. G. AdanThe
solid-state hydration of salts has gained particular interest
within the frame of thermochemical energy storage. In this work, the
water vapor pressure–temperature (p–T) phase diagram of the following thermochemical salts was
constructed by combining equilibrium and nonequilibrium hydration
experiments: CuCl2, K2CO3, MgCl2·4H2O, and LiCl. The hydration of CuCl2 and K2CO3 involves a metastable zone
of ca. 10 K, and the induction times preceding hydration are well-described
by classical homogeneous nucleation theory. It is further shown for
K2CO3 (metastable) and MgCl2·4H2O (not metastable) through solubility calculations that the
phase transition is not mediated by bulk dissolution. We conclude
that the hydration proceeds as a solid–solid phase transition,
mobilized by a wetting layer, where the mobility of the wetting layer
increases with increasing vapor pressure. In view of heat storage
application, the finding of metastability in thermochemical salts
reveals the impact of nucleation and growth processes on the thermochemical
performance and demonstrates that practical aspects like the output
temperature of a thermochemical salt are defined by its metastable
zone width (MZW) rather than its equilibrium phase diagram. Manipulation
of the MZW by e.g. prenucleation or heterogeneous nucleation is a
potential way to raise the output temperature and power on material
level in thermochemical applications.