The mechanism of porosity formation during solvent-mediated phase transformations

TL;DR

This study investigates how solvent-mediated solid-solid phase transformations create stable, anisotropic porous structures that facilitate solute transport, with implications for natural and engineered systems.

Contribution

It reveals the formation mechanism of porous structures during phase transformations in the KBr-KCl-H2O system, highlighting the role of pore connectivity and anisotropy.

Findings

Porous structures form during transformation, enabling efficient solute transport.

Discontinuities in compositional profiles are associated with disc-like cavities.

Similar mechanisms may influence phase transformation rates in various systems.

Abstract

Solvent-mediated solid-solid phase transformations often result in the formation of a porous medium, which may be stable on long time scales or undergo ripening and consolidation. We have studied replace- ment processes in the KBr-KCl-H2O system using both in situ and ex situ experiments. The replacement of a KBr crystal by a K(Br,Cl) solid solution in the presence of an aqueous solution is facilitated by the gen- eration of a surprisingly stable, highly anisotropic and connected pore structure that pervades the product phase. This pore structure ensures efficient solute transport from the bulk solution to the reacting KBr and K(Br,Cl) surfaces. The compositional profile of the K(Br,Cl) solid solu- tion exhibits striking discontinuities across disc-like cavities in the product phase. Similar transformation mechanisms are probably important in con- trolling phase transformation processes and rates in a variety of natural and man-made systems.