(735o) Exploring the Origin of Hysteresis in Plastic Crystal Compounds for Engineering Pressure Tunable Thermal Energy Storage

Plastic crystals are a class of small, globular molecules that exhibit promising characteristics for use as tunable thermal energy storage (TES) materials. At low temperature or high pressure, plastic crystal molecules adopt regular translational and orientational ordering, while at higher temperatures or lower pressures, the bonds between adjacent molecules begin to break and molecules begin to rotate about their centers of mass. This transition from rotational order to disorder has a large associated isothermal entropy and adiabatic temperature change. The magnitude of this isothermal entropy and adiabatic temperature change makes plastic crystals a promising class of alternatives to environmentally harmful hydrofluorocarbons (HFCs) used in modern day refrigeration and heat pump cycles. This also gives them great potential for use as pressure-tunable TES. However, the relatively large degree of undercooling, on the order of 10-20 °C, even the best plastic crystals exhibit greatly impacts the overall cyclic efficiency. Understanding the rate-dependent phase transition mechanisms of the plastic crystal transition could help to formulate a method for reducing the undercooling. Ultimately, this could propel plastic crystals forward as next generation solid-state refrigerants or TES materials, helping to reduce the effects of man-made climate change and the energy grid demand.

In the present work, we utilize optical microscopy and calorimetric observations and analyses to investigate the kinetics in a model plastic crystal, neopentyl glycol. On heating, we demonstrate a process in which domains of the high symmetry crystal structure preferentially nucleate along grain boundaries. Subsequent growth of the isotropic high symmetry phase limits this heating transformation, which differs from the case on cooling. Observations through both methods suggest a nucleation limited cooling transformation. The metastable plastic crystal state persists well below the expected transition temperature until one domain of the ordered crystal nucleates, upon which a rapid transformation occurs. We reveal the impact of cycling on the microstructure and find that despite its associated increase in grain boundary density, the asymmetric transformation kinetics remain conserved. Lastly, we explore the magnitude of the thermal hysteresis through both experimental techniques, and discuss methods for reducing this hysteresis in future studies.

Building on this understanding, we explore the relationship between lattice correspondence between the ordered and plastic crystal structures and the thermal hysteresis in a variety of plastic crystal systems. We aim to demonstrate that the magnitude of transformation strain associated with the structural transition between the ordered and plastic crystal phases plays a critical role in the magnitude of thermal hysteresis. Through temperature dependent X-ray diffraction, we evaluate the crystal structures of the ordered and plastic crystal phases and utilize the crystal structures to determine lattice disregistry and transformation strain, serving as proxies for the strain energy at the interface between parent and daughter phases. Further, we use calorimetry to quantify the magnitude of undercooling associated with the plastic crystal transition in a variety of compounds. Combining the lattice disregistry and undercooling calculations, we demonstrate that phases which have some degree of lattice correspondence (i.e. small lattice disregistry) between the ordered and plastic crystal phase exhibit smaller degrees of undercooling. In contrast, molecules that share little to no similarity between the crystal structures of their ordered and plastic crystal phases have large degrees of undercooling, contributing to the large thermal hysteresis observed through calorimetry. Lastly, we investigate compounds with relatively low to moderate degrees of undercooling using polarized light microscopy to identify differences in kinetics for the forward and reverse transformations. We discuss the relationship these optical observations have to the crystal structure and lattice correspondence, as well as their implications on the thermal hysteresis of the plastic crystal compounds.

Utilizing the information revealed through the initial kinetics study on the model plastic crystal neopentyl glycol, as well as the study on lattice correspondence in a variety of plastic crystals, we conclude by discussing pathways for materials development. More specifically, we discuss methods for engineering low-hysteresis plastic crystal compounds for use in barocaloric refrigeration systems, or for use as thermal energy storage mediums.