Hydrocarbon Disordering Transitions for Solid-State Cooling

Abstract

Cooling technologies permeate nearly every aspect of life in the twenty-first century, from maintaining livable standards amidst ever-increasing global temperatures to preserving foods and medicines essential to human health. At the same time, current cooling technologies pose genuine environmental and safety concerns. Not only do the gaseous hydrofluorocarbon (HFC) and hydrofluoroolefin (HFO) refrigerants that power air conditioners and refrigerators enforce significant greenhouse effects, but they also generate highly persistent compounds that pollute land and water sources. To circumvent the emission-related risks associated with gaseous refrigerants, interest has, in recent years, turned to barocaloric materials—solid materials that undergo pressure-induced thermal changes—for use as sustainable, alternative refrigerants. Among barocaloric materials, those undergoing hydrocarbon disordering transitions have emerged as especially promising candidates for solid-state cooling, owing to their significant thermal changes that can be accessed under relatively mild driving pressures. To fully realize the potential of these solid refrigerants, however, it is critical to understand and be able to controllably manipulate the hydrocarbon disordering processes that drive their solid-state thermal behavior. The work presented herein details our efforts to understand and leverage hydrocarbon disordering transitions for solid-state cooling, and, in so doing, to advance meaningful progress towards a more sustainable cooling future. Chapter One introduces caloric materials as solid-state alternatives for conventional gaseous refrigerants, emphasizing barocaloric materials as a particular promising subset of caloric materials. Shortly thereafter, metrics relevant to assessing the practical candidacy of these novel solid refrigerants are detailed. Finally, with an understanding of what metrics ideal barocaloric candidates should achieve, we discuss the utility of solid-state hydrocarbon disordering transitions for designing high-performing barocaloric materials. In Chapter Two, we interrogate through combined experimental and computational efforts how differences in hydrocarbon disordering between two families of barocaloric materials—two-dimensional organic-inorganic perovskites and dialkylammonium salts—inform the thermal changes they display. Specifically, we employ quasielastic neutron scattering (QENS) experiments and machine-learned molecular dynamics (MLMD) simulations to establish molecular-level insights into why a hydrocarbon chain in a dialkylammonium salt undergoes a molar entropy change nearly twice that of the same length-hydrocarbon chain in a two-dimensional perovskite. Equipped with the finding that dialkylammonium salts access particularly large entropy changes among barocaloric materials, we turn, in Chapter Three, to expanding the phase space encompassed by dialkylammonium salts. In this chapter, we report first the synthesis and characterization—both structural and thermal—of a new library of asymmetric dialkylammonium salts. Then, after investigating the barocaloric properties of these new materials, we identify a promising subset of asymmetric dialkylammonium salts that can be driven with substantially lower pressures than the state-of-the-art while maintaining significant entropy changes. Finally, in Chapter Four, we explore the effects of liquid immersion on the barocaloric behavior of dialkylammonium salts. Through calorimetry and X-ray diffraction studies, we monitor how immersion of dialkylammonium salts in different organic solvents affects their hydrocarbon disordering transition behavior. We also investigate the implications of elevated liquid pressures and binary solvent mixtures on liquid-immersed barocaloric systems for gas-free cooling applications.