Electrocaloric Effect of Ferroelectrics and Relaxors

Since the discovery of the giant electrocaloric effect in PbZr0.95Ti0.05O3, the electrocaloric effect of ferroelectrics has received increasing research interest and has been demonstrated to have great potential for cooling application. For successful implementation and commercialization of electrocaloric refrigeration, it is necessary to reveal the mechanism and the factors influencing the electrocaloric effect. The general goal of this thesis is to develop computational methods for evaluating and understanding the electrocaloric effect in complex ferroelectric materials and to derive optimization strategies with respect to the caloric cycle and the materials. For evaluating the electrocaloric effect, two methods are developed. One is based on a thermodynamical analytical model with the entropy analysis using the Landau theory and the work loss associated with irreversibility. This allows for a quick calculation of the temperature change on the basis of thermodynamics. The other method is based on the strict enforcement of energy conservation under adiabatic conditions and is implemented in the framework of lattice-based Monte-Carlo microcanonical simulations using a novel Ginzburg-Landau type effective Hamiltonian. Using this method the electrocaloric effect in complex materials can be explicitly interpreted on the domain structure level, and by adjusting the Hamiltonian it can be applied to study complex materials such as relaxors and ferroelectrics with defect dipoles. Based on Monte-Carlo simulations and experimental measurements, an improved thermodynamic cycle is validated, where the cooling effect is enhanced by applying a reversed electric field. In comparison with conventional adiabatic cooling by on-off cycles of the external electric field, applying a reversed field can enhance the cooling efficiency by more than 20% in standard ferroelectrics and also relaxor ferroelectrics, like Pb(Mg1/3Nb2/3)0.71Ti0.29O3. The optimal reversed field corresponds to the shoulder of the P-E loop, which is thermodynamically explained and quantitatively determined by the analytical model based on the entropy calculation. It signifies in general the importance of considering irreversible process in the electrocaloric cycles. By considering oxygen vacancy-acceptor associates by fixed local dipoles, simulation results demonstrate that defect dipoles have a significant influence on the electrocaloric effect in acceptor doped BaTiO3. In particular, defect dipoles anti-parallel to the external field can lead to abnormal electrocaloric features like inverse effect and double peaks, which stem from the delicate interplay of internal and external fields and are systematically explained by the domain structure evolution and related entropy analysis. The results are in good agreement to those from Molecular-Dynamics simulations employing an ab initio based effective Hamiltonian. By making use of the inverse electrocaloric effect in the presence of defect dipoles, improved electrocaloric cycles are proposed with enhanced cooling effect. Generic effective Hamiltonian models are presented for relaxors based on the random field theory, and the corresponding direct electrocaloric calculations reveal that the presence of random fields reduces the entropy variation in an electrocaloric cycle by pinning local polarization. With increasing strength or density of the random fields, the electrocaloric peak shifts to a lower temperature. The effective temperature range becomes wider, but the temperature variation is reduced. The dielectric and electrocaloric properties of the model solid solution BaZrxTi1-xO3 are also simulated by a composition-sensitive effective Hamiltonian, which differentiates the polar Ti-occupied sites from the nonpolar Zr-occupied sites. The model is verified by corresponding experimental measurements. Based on systematic simulations, distinct regimes of ferroelectrics, relaxors, polar clusters, and paraelectric phases are identified sequentially as the Zr-concentration increases. The correlation between the internal random fields induced by the composition fluctuation and the state regimes demonstrates the fundamental role of random fields in relaxors