The influence of defect dipoles on the electrocaloric effect (ECE) in acceptor doped BaTiO3 is studied by means of lattice-based Monte-Carlo simulations. A Ginzburg-Landau type effective Hamiltonian is used. Oxygen vacancy-acceptor associates are described by fixed defect dipoles with orientation parallel or anti-parallel to the external field. By a combination of canonical and microcanoncial simulations the ECE is directly evaluated. Our results show that in the case of anti-parallel defect dipoles the ECE can be positive or negative depending on the density of defect dipoles. Moreover, a transition from a negative to positive ECE can be observed from a certain density of anti-parallel dipoles on 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 compared to those obtained by MD simulations employing an ab initio based effective Hamiltonian, and a good qualitative agreement is found. In addition, a novel electrocaloric cycle, which makes use of the negative ECE and defect dipoles, is proposed to enhance the cooling effect.

An improved thermodynamic cycle is proposed, where the cooling effect of an electrocaloric refrigerant is enhanced by applying a reversed electric field. In contrast to conventional adiabatic heating or cooling by on-off cycles of the external electric field, applying a reversed field is significantly improving the cooling efficiency, since the variation in configurational entropy is increased. By comparing results from computer simulations using Monte-Carlo algorithms and experiments using direct electrocaloric measurements, we show that the electrocaloric cooling efficiency can be enhanced by more than 20% in standard ferroelectrics and also relaxor ferroelectrics, like Pb(Mg 1/3 /Nb 2/3 )0.71Ti0.29O3.

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.

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.The electrocaloric (EC) effect shows great application potential in the technology of solid state refrigeration. 1-3Even though much effort has been made to explore material candidates with large EC effect and device concepts, there are few work concerning the optimization of electrocaloric cycle. In a conventional EC cycle of a solid refrigerant, the cooling effect is obtained simply by removing the previously applied electric field. Using direct heat flux calorimetry on poly(vinylidene fuoridetride-trifuoroethylene) films, Basso et al. 4 demonstrated that the electrocaloric cooling can be doubled if a negative electric field to a positively poled sample is applied. The EC hysteresis of ferroelectric ceramics measured by Thacher et al.5 indicated also that a reversed electric field can increase the cooling effect of ferroelectric ceramics. In the authors' previous work, 6 experimental and numerical studies were carried in PMN-29PT and BaTiO 3 , which demonstrated that there exists an optimal reversed electric field, corresponding to a position around the shoulder of the dielectric hysteresis. At this point, the EC cooling effect reaches its maximum (also see Fig. 1). This phenomenon was also observed in Ref. 4. It is of scientific and engineering importance to deter- mine and understand this optimal reversed electric field. In the literature, as reviewed in Ref. 7, the EC cycle is considered to be reversible, with constant total entropy under the adiabatic condition. This assumption leads to the conclusion that the maximal cooling takes place at the zero polarization point, since at this point the dipolar entropy takes maximum (see Model I). This conclusion deviates obviously from the experimental observations. In fact, the total entropy S total should satisfy:where S dip and S vib are the dipolar and the vibrational entropy, respectively. In the case of applying a reversed electric field, the irreversible contribution becomes considerable. In other words, to correctly determine the optimal reversed electric field, the change of the total entropy induced by the work loss W loss due to the irreversible process s...

Canonical and microcanonical Monte Carlo simulations are carried out to study the electrocaloric effect (ECE) in ferroelectrics and relaxor ferroelectrics (RFEs) by direct computation of field-induced temperature variations at the ferroelectric-to-paraelectric phase transition and the nonergodic-to-ergodic state transformation. A lattice-based Hamiltonian is introduced, which includes a thermal energy, a Landau-type term, a dipole-dipole interaction energy, a gradient term representing the domain-wall energy, and an electrostatic energy contribution describing the coupling to external and random fields. The model is first parametrized and studied for the case of BaTiO 3 . Then, the ECE in RFEs is investigated, with particular focus on the influence of random fields and domain-wall energies. If the strength or the density of the random fields increases, the ECE peak shifts to a lower temperature but the temperature variation is reduced. On the contrary, if the domain-wall energy increases, the peak shifts to a higher temperature and the ECE becomes stronger. In RFEs, the ECE is maximum at the freezing temperature where the nonergodic-to-ergodic transition takes place. Our results imply that the presence of random fields reduces the entropy variation in an ECE cycle by pinning local polarization.