New electrocaloric oxides for sustainable and efficient refrigeration

Abstract

As refrigeration is essential for health and comfort, food, medicine or electronics, it represents a large and growing fraction of the worldwide electricity consumption. The current cooling technology, based on the compression of harmful gases, has a limited energy efficiency and contributes significantly to global warming. Caloric effects are among the most promising climatefriendly alternatives because they lead to higher energy efficiencies and use solid refrigerants that can be prepared without toxic elements. Electrocaloric (EC) materials show reversible thermal changes when subjected to variations of an applied electric (E) field, known as the electrocaloric effect (ECE), which maximizes near a ferroelectric phase transition. While the research in their magnetocaloric (MC) counterparts is rather mature, EC materials have been in the spotlight only in the last ten years, being advantageous over MC because of the ease of application of E fields. Up to now, the research in EC materials has been dominated by lead-based oxides. Sufficiently large ECE values for cooling applications have only been observed in thin-film samples. We have investigated new EC oxides (bulk and thin film) by making use of in-house laboratory methods for the “direct” and “indirect” measurements of the ECE, combined with complementary synchrotron-based X-ray absorption spectroscopy. In particular, layer-structured ferroelectric Aurivillius oxides Srn-3Bi4TinO3n+3 (n = 4, 5), where the A- and B-site of the “n” perovskite-like layers are occupied by Sr/Bi and Ti, respectively. The Curie temperature TC of their ferroelectric transitions is about 800 K and 560 K for n = 4 and 5, respectively. Aiming at enhancing their EC response at near room temperature (RT), chemical substitutions at the A- (La3+) and B-site (Nb5+) have been applied to tune TC close to RT. For both series of compounds (n = 4, 5), we found that La-doping is effective in decreasing TC and in promoting a relaxor ferroelectric behavior but at the expense of weakening the ferroelectricity and EC properties. Preliminary results (Fig. 1) for Sr2Bi4Ti5O18 (n=5) show a promising direction towards enhancement of the EC properties through a combination of La- and Nb-doping featuring an ECE maximum close to RT.