Yang-Bin Ma

Positive and negative electrocaloric effect in BaTiO3 in the presence of defect dipoles

The influence of defect dipoles on the electrocaloric effect (ECE) in acceptor doped BaTiO3 is studied by means of lattice-based Monte-Carlo simulations using a Ginzburg-Landau type effective Hamiltonian. Oxygen vacancy-acceptor associates are described by fixed local dipoles with orientation parallel or antiparallel to the external field. By a combination of canonical and microcanonical simulations the ECE is directly evaluated. Our results reveal that in the case of antiparallel defect dipoles the ECE can be positive or negative depending on the dipole density. Moreover, a transition from a negative to positive ECE can be observed when the external field increases. These transitions are due to the delicate interplay of internal and external fields and are explained by the domain structure evolution and related field-induced entropy changes. The results are in good qualitative agreement to those obtained by molecular dynamics simulations employing an ab initio based effective Hamiltonian. Finally, a modified electrocaloric cycle, which makes use of the negative ECE in the presence of defect dipoles, is proposed to enhance the cooling effect.

State transition and electrocaloric effect of BaZrxTi1−xO3: Simulation and experiment

We present a systematic study on the relation of the electrocaloric effect (ECE) and the relaxor state transition of BaZrxTi1−xO3 (BZT) using a combination of computer simulation and experiment. The results of canonical and microcanonical lattice-based Monte Carlo simulations with a Ginzburg-Landau-type Hamiltonian are compared with measurements of BaZrxTi1−xO3 (x = 0.12 and 0.2) samples. In particular, we study the ECE at various temperatures, domain patterns by piezoresponse force microscopy at room temperature, and the P-E loops at various temperatures. We find three distinct regimes depending on the Zr-concentration. In the compositional range 0≤x≤0.2, ferroelectric domains are visible, but the ECE peak drops considerably with increasing Zr-concentration. In the range 0.3≤x≤0.7, relaxor features become prominent, and the decrease in the ECE with Zr-concentration is moderate. In the range of high concentrations, x≥0.8, the material is almost nonpolar, and there is no ECE peak visible. Our results reveal that BZT with a Zr-concentration around x=0.12∼0.3 exhibits a relatively large ECE in a wide temperature range at rather low temperature.

Optimized electrocaloric effect by field reversal: Analytical model

Application of a negative field on a positively poled ferroelectric sample can enhance the electrocaloric cooling and appears as a promising method to optimize the electrocaloric cycle. Experimental measurements show that the maximal cooling does not appear at the zero-polarization point, but around the shoulder of the P-E loop. This phenomenon cannot be explained by the theory based on the constant total entropy assumption under adiabatic condition. In fact, adiabatic condition does not imply constant total entropy when irreversibility is involved. A direct entropy analysis approach based on work loss is proposed in this work, which takes the entropy contribution of the irreversible process into account. The optimal reversed field determined by this approach agrees with the experimental observations. This study signifies the importance of considering the irreversible process in the electrocaloric cycles.Applying a negative field on a positively poled ferroelectric sample can enhance the electrocaloric cooling and is a promising method to optimize the electrocaloric cycle. Experimental measurements show that the maximal cooling is not obtained, when the electric field is removed, but reversed to a value corresponding to the shoulder of the P-E loop. This phenomenon cannot be explained if a constant total entropy is assumed under adiabatic conditions. Thus, a direct analysis of entropy changes based on work loss is proposed in this work, which takes the entropy contribution of the irreversible process into account. The optimal reversed field determined by this approach agrees with the experimental observations. This study signifies the importance of considering irreversible process in the electrocaloric cycles.

Origins of the Inverse Electrocaloric Effect

Abstract The occurrence of the inverse (or negative) electrocaloric effect, where the isothermal application of an electric field leads to an increase in entropy and the removal of the field decreases the entropy of the system under consideration, is discussed and analyzed. Inverse electrocaloric effects have been reported to occur in several cases, for example, at transitions between ferroelectric phases with different polarization directions, in materials with certain polar defect configurations, and in antiferroelectrics. This counterintuitive relationship between entropy and applied field is intriguing and thus of general scientific interest. The combined application of normal and inverse effects has also been suggested as a means to achieve larger temperature differences between hot and cold reservoirs in future cooling devices. A good general understanding and the possibility to engineer inverse caloric effects in terms of temperature spans, required fields, and operating temperatures are thus of fundamental as well as technological importance. Here, the known cases of inverse electrocaloric effects are reviewed, their physical origins are discussed, and the different cases are compared to identify common aspects as well as potential differences. In all cases the inverse electrocaloric effect is related to the presence of competing phases or states that are close in energy and can easily be transformed with the applied field.

Monte Carlo simulations of the electrocaloric effect in relaxor ferroelectrics

By combination of the canonical and microcanonical Monte Carlo simulations a direct evaluation of the electrocaloric (ECE) effect is carried out in relaxor ferroelectrics. A Hamiltonian is introduced, which includes a thermal energy, a Ginzburg-Landau static ground state term, a dipole-dipole interaction energy, a domain wall energy that arising from the short-range interaction and an electrostatic energy contribution describing the coupling to external and random fields. By incorporating the frozen random dipoles random fields are induced to reproduce the relaxor behavior. The influence of the density of the frozen dipoles on the hysteresis and ECE is investigated. The hysteresis begins to resemble a relaxor-type with 20% density of the frozen dipoles. Upon increasing the density of the frozen random dipoles, the ECE peak position shifts to a lower temperature but the temperature variation is reduced. In relaxor ferroelectrics the ECE is maximum at the freezing temperature where the nonergodic-to-ergodic transition takes place. Our results, especially the evolving domain structures, imply that the entropy variation in an ECE cycle is reduced since the random frozen dipoles destabilize the polarization of their neighbor sites.