Characterization of the thermal response under hydrostatic pressure in organic and ferroelectric compounds with orientational disorder

Artificial refrigeration is considered one of humanity’s greatest technological achievements in recent centuries. It is essential in both industrial and domestic sectors such as for food and medicine preservation, and space cooling. Moreover, global demand for refrigeration is expected to grow significantly in the coming decades due to increasing wealth, the global population, and the intensification of global warming. However, common current technologies are heat pumps that use harmful fluids and have low or moderate efficiency. Additionally, they are increasingly used for heating to replace inefficient and strongly pollutant gas burners. Caloric effects across solid-state first-order phase transitions driven by external fields promise more sustainable cooling and heating methods. Among them, caloric effects induced by hydrostatic pressure, known as barocaloric effects, have recently drawn the attention of the scientific community due to the identification of plastic crystals displaying phase transitions with colossal latent heat. Interestingly, the colossal latent heat in these materials can also be used for thermal energy storage and waste heat management.

This thesis investigates barocaloric effects in two types of plastic crystals. The characterization of their thermodynamic properties and barocaloric response allowed to assess their suitability for applications in both industrial and domestic cooling and heating. In particular, colossal irreversible barocaloric effects have been observed; however, significant reversible effects can be achieved in the plastic under moderate pressure changes, surpassing reported materials away from the transition. Ferroelectric plastic crystals are a novel family of materials that combine a high-temperature plastic phase with a low temperature ferroelectric phase with multiaxial ferroelectricity whose direction can be tuned applying electric fields. This work is the first report of barocaloric effects in this material family. Despite the effects are smaller than those by organic plastic crystals, this study opens the door to possible multicaloric effects under pressure and electric field. For these two particular materials, however, large electric fields appear to be necessary to achieve significant electrocaloric effects.