Electronic Entropy Effect Of The Ferroelectric Nanomaterials Research

Recently, giant electrocaloric effect (ECE) in ferroelectric materials has drawn great attentions for its potential applications such as on-chip cooling and temperature regulation for sensors and electronic devices, heat pumping, electrical generator and energy harvesting devices. Refrigeration based on the ECE is more environmentally friendly and hence may also provide an alternative to the existing vapor-compression approach. The study of ECE is not only of theoretical importance but also valuable for the technological application in material physics and condensed matter physics.The thesis has been concentrated on the ECE of nano-ferroelectrics from the following two aspects:1 Influence of thermal strains on the electrocaloric and dielectric properties of ferroelectric nanoshellsThe ECE and dielectric tunability of BaTiO₃ ferroelectric nanoshell on Si and MgO cores are investigated using modified Landau-Ginzburg-Devonshire theory, in which both the surface tension and thermal strain are taken into account. Numerical results show that the nanoshell still keeps its ferroelectricity while shell thickness decreases to 2 nm,and the nanoshell exhibits giant electrocaloric coefficient near the critical size. In addition to the enhanced ECE, the compressive strain also significantly improves the dielectric tunability. More importantly, the ferroelectric nanoshell displays large ECE:ΔT (Tm) = 2.09 K for the nanoshell grown on Si core andΔT (Tm) = 2.33 K on MgO core, respectively. Essentially, the ferroelectric nanoshell provides an effective means to acquire large ECE and high dielectric tunability by adjusting the wall thickness, core radius, annealing temperature and various core materials, which may effectively contribute to the stress level in the ferroelectric nanoshell. 2 Large abnormal electrocaloric effect at field-induced antiferroelectric to ferroelectric transition in PbZr_0.95Ti_0.05O₃ thin filmsThe abnormal electrocaloric effect at the field-induced antiferroelectric to ferroelectric transition in PbZr_0.95Ti_0.05O₃ thin has been investigated by Kittle's sublattices theory. Firstly, the double hysteresis loop of antiferroelectric PbZr_0.95Ti_0.05O₃ thin films can be simulated, which is in good qualitative agreement with the experimental results. Secondly, calculated results show that the entropy of PbZr_0.95Ti_0.05O₃ thin film in the antiferroelectric phase is much lower than the one in the ferroelectric phase, so the entropy increases near the antiferroelectric phase to ferroelectric phase induced by the applied electric field. Correspondingly, a positive electric differenceΔE can result in a positive entropy changeΔS, which is defined as"abnormal electrocaloric effect". This behavior is totally different from the common electrocaloric effect. The PbZr_0.95Ti_0.05O₃ thin films exhibit the excellent abnormal electrocaloric effect. For example, the adiabatic temperature change can reachΔT = - 8.8 K (ΔE = 600 kv / cm) , which is comparable with the experimental resultΔT= -1 0.8 Kestimated from experimental data. Besides, this abnormal electrocaloric effect could appear near room temperature in quite wide temperature range. Therefore, this abnormal electrocaloric effect will provide another effective approach to acquire the giant ECE in ferroelectric thin films, which ould be of great values for potential applications.