Order-disorder Transitions in Tetrafluoroborate Salts - Investigation into Structural Stabilities over Many Temperature Cycling

Solid-solid transitions occur from room temperature ordered phases to orientationally disordered high-temperature phases that lie at the boundary between liquids and solids. Solid-solid phase-change materials (SSPCMs) can be potentially used for next generation thermal energy storage devices, as they present several advantages over solid-liquid PCMs including safety (no spillage of hot liquid), lower thermal expansion, lower corrosiveness, and no need for encapsulation. Adoption of SSPCMs for practical applications require detailed understanding of both thermodynamic and kinetic disordering. The proposal will investigate two tetrafluoroborate salts with high latent heat of phase transitions. KBF4 and NaBF4 undergo order-disorder transitions at 285 ℃ and 230 ℃ with ΔH =120 J/g and 70 J/g, respectively. Variable-temperature powder x-ray diffraction (PXRD) will be conducted to elucidate long-range structural changes over many temperature cycling (heating/cooling).