Manipulating Phase Transitions in Metal–Organic Materials for Thermal Applications

Abstract

The thermal properties of phase transitions are critical for materials used in thermal energy storage, heating and cooling, memory, and materials processing. Despite their high degree of modularity and tunability, metal–organic materials have received little attention as phase-change materials. In my thesis, I synthesize and establish structure-thermodynamic property relationships for phase changes in metal–organic materials. Importantly, as phase-change thermodynamics depend on the changes in a material across the transition, we not only characterize the ordered, solid-state phases but also investigate disordered, liquid phases. The work presented here provides new fundamental insights into metal–organic materials for thermal energy storage, meltable metal–organic network materials, and metal–organic nanomaterials. I begin by discussing the use of extended X-ray absorption fine structure (EXAFS), which can provide structural information in the absence of long-range order, to characterize the melts of metal–organic materials. Measuring and fitting EXAFS to determine coordination numbers is challenging, so in Chapter 2 I discuss a measurement and fitting strategy that allows us to accurately determine melt coordination numbers. In Chapter 3, I discuss the synthesis and structural and thermal characterization of a library of metal–amide-based coordination complexes for thermal energy storage, which afford materials with energy densities comparable to conventional metal-salt hydrates. Though achieving high energy storage densities is one challenge for phase-change energy storage, another is in realizing tunable transition behavior. In Chapter 4, I discuss a series of metal–imidazole complexes that show thermal history dependent switching between phases with two distinct melting temperatures and use EXAFS to implicate anion binding and unbinding to the metal center in the transition. Having studied meltable zero-dimensional complexes for thermal energy storage, I turn to the structural characterization of meltable metal-bis(acetamide) network materials in Chapter 5, using EXAFS to study the network-forming nature of the molten and glassy phases of these materials. Finally, in Chapter 6, I apply X-ray crystallographic analysis to the structural characterization of an atomically precise metal–organic nanocluster, which affords critical insight into its catalytic properties.