Araceli Aznar Luque defends the thesis on barocaloric effects in first-order phase transitions.

Current refrigeration devices, based on vapour-compression cycles, employ refrigerants such as HFCs, which exhibit a global warming potential a thousand times higher than the one produced by CO₂. Furthermore, the increasing middle class, associated with their refrigeration needs, urges for research in new environmentally friendly refrigerant alternatives.

Solid-state caloric effects have been proposed as potential alternatives to replace the vapour-compression refrigeration technologies. They may become giant under the cyclic application and removal of an external field which induces changes in entropy and temperature associated with the occurrence of a first-order phase transition. In this work we specifically focus on caloric effects driven by means of hydrostatic pressure (barocaloric effects, BCEs), which allow us to operate with powder compounds, avoiding fatigue upon cycling. Additionally, a wide variety of materials can be used with BC purposes, due to the possibility of working with powdered samples and to the emergence of BCE associated with any transition volume change.

In this dissertation we carried out the study of the BC performance of a series of compounds belonging to four different material families: Plastic crystals (PC), hybrid organic-inorganic perovskites (HOIPs), magnetic alloys and a superionic conductor. The election of these materials is not arbitrary, but relies on several features which anticipate good BCEs, such as large transition entropy changes, pressure sensitivity of the transition temperature and small thermal hysteresis. The small hysteresis avoids losses related with the refrigeration cycle and ensures smaller pressures under which reversibility is observed (which at the same time enable smaller applied work to the refrigerant). Finally, other properties must also be taken into account when designing a refrigeration device: Density, thermal conductivity and costs of production.

BCEs are determined by means of a combination of quasi-direct and indirect methods. Firstly, we conduct measurements of atmospheric pressure and high-pressure calorimetry (DSC and DTA, respectively), along with experiments of X-ray diffraction and dilatometry. Then, the isobaric entropy curves are constructed, from which by means of curves subtraction the BCEs can be obtained. Additionally, special attention has been put on reversibility, since cyclability is mandatory for a real refrigeration device.