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).

固固相变通常由室温下的有序相转变为处于液固边界的取向无序高温相。固固相变材料（Solid-solid phase-change materials, SSPCMs）可应用于下一代热能存储装置，相较固液相变材料（Phase Change Materials, PCMs）具备多项显著优势：无热液泄漏的安全特性、更低的热膨胀率、更弱的腐蚀性，且无需封装。将固固相变材料投入实际应用，需深入理解其热力学与动力学层面的无序化机制。本研究将针对两种具备高相变潜热的四氟硼酸盐展开研究：氟硼酸钾（KBF₄）与氟硼酸钠（NaBF₄），二者分别在285℃与230℃下发生有序-无序相变，相变潜热ΔH依次为120 J/g与70 J/g。本研究将通过变温粉末X射线衍射（Variable-temperature Powder X-ray Diffraction, PXRD）实验，阐明多次升降温循环过程中的长程结构变化。