On electrocaloric effects in ferroic nano-thin films

Ferroelectrics are insulating materials with spontaneous polarization that can be reversed through the application of electric fields. Their coupled electrothermal and electromechanical properties have granted them as promising candidates in a vast number of applications. Among them, the potential of ferroelectrics as the working materials of solid state coolers has attracted tremendous research interests in the recent years due to the discovery of giant electrocaloric effects (ECEs), which are characterized by adiabatic temperature changes of dipolar materials in response to applied electric fields. In the present thesis, the ECEs of PbZr1-xTixO3 (PZT) thin films are calculated and their trends are linked to the electric field driven phase transitions through the temperature-electric field (T-E) phase diagrams. Moreover, the influence of domain structures on the ECEs of PbTiO3 (PTO) thin films is investigated to explore the tunability of ECEs through epitaxial strains. In chapter 2, the magnitude of ECE in PZT thin films is characterized for a wide range of Ti composition, misfit strain, temperature and electric field by performing thermodynamic calculations. At relatively high misfit strains, the electric field is found to have no out of the ordinary effect on the ECEs of PZT thin films, with only the typical enhancement of ECE with increasing electric field observed. Whereas at intermediate misfit strains, several interesting trends in ECE are observed, i.e. the sharp increase in the ECE, the existence of negative ECE as well as the existence of double and triple peaks in ECE, all of which are electric field dependent. Giant ECEs are found to exist in PZT thin films with high Ti composition, in which the highest magnitude of ECE occurs in PTO thin film whereas the weakest ECE occurs in PbZr0.5Ti0.5O3 thin film. The robustness of the T-E phase diagrams has been demonstrated. They are useful in the study of ECE as they provide guidance on the selection of the range of operating electric field and it is also possible to predict the number of ECE peaks from the diagrams. In chapter 3, the influence of epitaxial strains and domain structures on the ECEs in PTO thin films is investigated through phase field modeling. The simulated domain switching dynamics obtained from the proposed model is compared with the existing experimental results. In the case of single-domain PTO thin films, the ECE gradually increases with increasing epitaxial strain. Whereas, in the case of poly-domain PTO thin films the influence of epitaxial strain on ECE can be divided into three regions of stable domain structures, i.e., c-domains, a/c-domains and a-domains regions. It is worth noting that the maximum ECE occurs under a tensile epitaxial strain in the a/c-domains region, which is the consequence of the competition between the enhancement of ECE and the reduction of c-domain volume fraction, both caused by increasing tensile epitaxial strain. This study illustrates the important influence of domain structures on the ECEs of thin films, which has not gained much attention, since most researchers still focus on single- domain thin films.