Microstructural Control and Heat Transport Enhancement in Lanthanum Sulfate for Thermochemical Heat Storage

Abstract

Enhancing heat transport within thermochemical heat storage (TCHS) materials is essential for improving their heat output. A common strategy is to combine salts with highly thermally conductive additives, such as carbon or metallic materials. However, such composites often exhibit interfacial instability and reduced gas permeability. In this work, we propose an alternative approach based on microstructural orientation control, aiming to create efficient heat-transport pathways without relying on conductive additives. $β$-La$_2$(SO$_4$)$_3$, which undergoes reversible hydration and dehydration below 250 $^\circ$C, was selected as a model TCHS material. Highly oriented rod-like La$_2$(SO$_4$)$_3\cdot$9H$_2$O crystals with centimeter-scale lengths were grown from solution, cut into plate-shaped specimens, and then dehydrated to $β$-La$_2$(SO$_4$)$_3$. Two types of specimens with different microstructural orientations, which form spontaneously during the dehydration of La$_2$(SO$_4$)$_3\cdot$9H$_2$O to $β$-La$_2$(SO$_4$)$_3$, were prepared. In the "cross-plane-gb" specimen, the aligned grain boundaries were predominantly oriented parallel to the through-thickness direction of the plate, whereas in the "in-plane-gb" specimen, they were predominantly oriented perpendicular to this direction. Laser flash analysis (LFA) of $β$-La$_2$(SO$_4$)$_3$ revealed a clear orientation dependence of heat transport: the apparent thermal diffusivity was approximately 0.24 mm$^2$/s for the cross-plane-gb specimens, in which the grain boundaries are aligned along the heat-flow direction during LFA, and it was approximately 0.15 mm$^2$/s for the in-plane-gb specimens. These findings demonstrate that controlling the microstructural orientation is a viable route for enhancing heat transport in TCHS materials, offering an additive-free design strategy.