Pyroelectric/electrocaloric energy scanvenging and cooling capabilities in ferroelectric materials

Abstract

Current needs for clean energy sources aroused the problem of energy scavenging. Significant advances have been achieved in the framework of energy harvesting on solar energy, wind and vibrations (mechanical quantities). Different systems are already available: solar cells, electromechanical conversion for harvesting from vibrations or from mechanical stress (i.e. produced by a person walking or an object movement) [1]. Wasted heat is rarely harvested, mainly because of the very low efficiency of harvesting devices and their large consumption. Pyroelectric materials may be used for energy scavenging, due to their ability to convert temperature variations into usable electric energy. The reverse effect of pyroelectric activity is the electrocaloric effect. The electrocaloric effect (ECE) is a general property of dielectrics to change their temperature under applied electric field at adiabatic conditions. When having heat exchange with an outer medium, it defines the exchanged heat as a function of the applied electric field under isothermal conditions. In the case of ferroelectric materials ECE is strongly correlated with the pyroelectric effect, which gives the change of electric induction for a given temperature variation. The ECE effect may be used for refrigeration devices whereas the pyroelectric effect may be used for temperature/heat sensors or energy harvesting. Electrocaloric materials were the focus of significant scientific interest in the 1960s and 1970s, but were not commercially exploited as the electrocaloric effects were insufficient for practical applications [2]. (1-x)Pb(Mg1/3Nb2/3)O3xPbTiO3 (PMN-PT) materials show a quite high pyroelectric and electrocaloric activity [3,4]. However the practical use of such materials is their electric field limitations before electric breakdown. It is in fact limited to 3–5 kV/mm on bulk ceramics, and could be largely increased for fields up to 50–100 kV/mm. Such large electric fields associated with outstanding electrocaloric conversion were obtained on PZT thin films and shown by Mischenko et al. [5] with a predicted controllable temperature of 12 K for 25 V input voltage.