Theory and simulations of caloric effects in ferroelectric materials

In this thesis, I present the outcomes of two projects -both theoretical- on the study of the temperature changes that are triggered through the application of mechanical stress -first project- and electric fields -second project-, i.e. mechano- and electrocaloric effects.
These effects are studied for cooling applications in the hope of being able to replace the current polluting gas-based cooling devices. Up to now, most of the temperature changes obtained from mechanocaloric effects are below 5 K -only a few effects are above 35 K-, for which metals, ceramics, or polymers are used. While in electrocaloric effects, most of the temperature changes found in the literature lie below 5 K, and only some are of the order of 40 K. For these effects, only non-conducting materials are used, e.g., ceramics, polymers, and their combination (polymeric composites with ceramic fillers).
These temperature changes triggered by an electric field are large in ferroelectric materials, particularly at their Curie temperature TC, where the material changes from being ferroelectric to paraelectric. It is in the vicinity of TC where the materials are highly responsive or susceptible to external electric fields, and this is translated, as I will explain in subsequent sections in this chapter, in a maximum response in the electrocaloric effect.
The majority of studies on the optimization of these (electrocaloric) effects have a “system perspective” character, id est, they focus on the optimization in the synthesis of these materials, their deposition over the electrodes and substrates, on improving the contact with these ones, and the heating transmitting medium. Or also, they just focus on altering the composition of the material, i.e., doing solid solutions or adding dopants to improve the response. However, very little is done in the engineering of the intrinsic electrocaloric response of the material.
Here I discuss the intrinsic mechanocaloric and electrocaloric responses with a focus on the latter. I explore the possibility of engineering the intrinsic electrocaloric responses from a more fundamental perspective, considering the influence of electrical and mechanical boundary conditions. And study the microscopic aspects that control a particular behavior in these electrocaloric effects.