Investigation Of The Entropy Change Effect In Ferroelectrics And Multiferroics

Electrocaloric effect (ECE) occurs when an electric field applied under reversible and adiabatic conditions changes the temperature of a polarizable material. The ECE may provide an efficient means to realize solid-state cooling devices for a broad range of applications such as on-chip cooling and temperature regulation for sensors and electronic devices. Besides, refrigeration based on the ECE is more energy-efficient and environment-friendly and hence may offer an alternative to the usual vapor-compression refrigeration technology. On the other hand, Magnetocaloric effect (MCE) is analogous to the ECE. Generally, a large ECE/MCE requires a large entropy variation associated with the polarization/magnetization change, so the ferroelectric materials, which are capable of generating large polarization change, and the multiferroic materials, which exhibit the strong correlation between spin and dipole orderings resulting in remarkable magnetoelectric effects, may be considered as appreciable candidates. Consequently, the purpose of our work is to explore the electrocaloric effect of ferroelectric superlattices and magnetocaloric effect of BiFeO₃ ceramics by the modified phenomenological Landau-Ginzburg-Devonshire(LGD) theory.1. Electrocaloric effect in ferroelectric superlattices.we have investigated the ECE of compositionally modulated PbZr_xTi_1-xO₃/PbTiO₃ (PZT/PT) and Ba_0.7Sr_0.3TiO₃/SrTiO₃ superlattices using the modified LGD thermodynamic theory, in which the misfit strain between the two slabs is taken into account. Caculated results show that the adiabatic temperature change in such superlattices is larger than that of the BaTiO₃ bulk material (△E=100kV/cm,△T=1.6K) and PbZr_xTi_1-xO₃ solid solution (△E=100kV/cm,△T=6.5K). Besides, the effect of interfacial coupling strength and modulated compositional concentration in one period on the adiabatic temperature change is discussed. One can find that the adiabatic temperature change reaches the maximum at an appropriate temperature corresponding to the ferroelectric-paraelectric phase transition temperature. The maximum of adiabatic temperature change not only increases with the increasing coupling factor, but also shifts to the higher temperature region due to the strong interfacial coupling between the two slabs. In addition, for a given thickness of one period, the maximum of adiabatic temperature change can be dramatically improved with the increase in the thickness proportion of slab PZT. Most importantly, we can get a giant adiabatic temperature change, which is larger than that observed experimentally in Pb(Zr_0.95Ti_0.05)O₃ thin film (△T=12K at△E=480kV/cm). As for Ba_0.7Sr_0.3TiO₃/SrTiO₃ superlattice, the adiabatic temperature change can be effectively improved not only with the increase in the thickness proportion of slab SrTiO₃, but also with the increase in the interfacial coupling strength. Therefore, it may provide an effective means to achieve the pronounced ECE by adjusting the ratio of component materials and the strength of interface coupling in the ferroelectric superlattices.2. The magnetocaloric and electrocaloric effect in polycrystalline BiFeO₃ ceramics.The magnetocaloric and electrocaloric effect of multiferroic materials BiFeO₃ ceramics has been investigated, combining the modified LGD theory and the two sublattices model. The dependence of magnetization on the temperature is simulated successfully, which is in good agreement with the experimental results. The results show that magnetocaloric effect of BiFeO₃ ceramics is relatively small, which may be related to the magnetic structure of incommensurate sinusoidal modulation. Besides, it is interesting to see that multiferroic BiFeO₃ exhibits the excellent electrocaloric effect, and the maximum of adiabatic temperature change reaches 4.8K while△E=200kV/cm.