Study of barocaloric and multicaloric effects under hydrostatic pressure and electric field in disordered materials and ferroelectric compounds

Feb 04, 2025

Ming Zeng defended his thesis co-directed by Pol Lloveras and Josep Lluís Tamarit, on December 4, 2024 at the Besòs Campus. Entitled "Barocaloric and multicaloric effects under hydrostatic pressure and electric field", the thesis presents the caloric effects in first-order phase transitions in solids of different types. These effects are of interest for efficient and sustainable refrigeration applications.

Study of barocaloric and multicaloric effects under hydrostatic pressure and electric field in disordered materials and ferroelectric compounds — Barocaloric cycle for refrigeration: A change in pressure causes an increase in temperature in the barocaloric material, which recovers the initial temperature by releasing heat to the environment. During this process the material undergoes an exothermic phase transition to the ordered phase. Then, the reverse change in pressure causes cooling below the environment, being able to absorb heat from what is to be cooled. During this process the material undergoes the reverse endothermic transition to the disordered phase

At present, HFC refrigerants with a global warming potential of thousands of times that of CO₂, are widely used in air conditioners and refrigerators. Due to lack of maintenance and poor management, and low or moderate efficiencies, cooling devices contribute to approximately 8% of total greenhouse gas emissions. Given the increasing global warming, it is urgent to find new cooling technologies with low carbon emissions.

Cooling methods based on solid-state caloric effects (adiabatic temperature changes ∆T and isothermal entropy changes ∆S) driven by external fields have been proposed as an environmentally friendly alternative to today's gas compression equipment. This thesis focus on caloric effects driven by hydrostatic pressure (barocaloric, BC) and/or electric field (electrocaloric, EC) near first-order phase transitions. In particular, BC effects have been investigated in different material families: Inorganic salts, Mn-based antiperovskites, superionic plastic crystals, and melting of stearic acid encapsulated in metal–organic frameworks (MOFs). Multicaloric effects were also investigated under simultaneous application of pressure and electric field on Lead Scandium Tantalate (PbSc0.5Ta0.5O3, PST).

For this purpose, standard and modulated differential scanning calorimetry, and differential thermal analysis under different applied pressure and/or electric field were performed on the mentioned materials. The obtained data were combined with temperature-dependent volume data to construct the isofield entropy curves from which the caloric effects were calculated using the quasi-direct method. The results suggest that MOFs can be used as suitable solid framework for encapsulation of non-solid high-performance BC materials.

Finally, the multicaloric effects in the prototypical ferroelectric PST have also been studied. The so far unexplored 3-dimensional phase diagram T(p,E) was obtained and analyzed. It was demonstrated that multicaloric effects may provide opportunities that cannot be achieved by monocaloric effects, such as tuning or expansion of temperature ranges and efficiency improvement.

This research has demonstrated the feasibility, novelty, and impact of multicaloric effects under p and E, thus opening up a new area of caloric effects that should provide new physical insights on the wide family of ferroelectrics.

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