Control Of Electrocaloric Effect In Ferroelectrics

Ferroelectrics has the spontaneous polarization below its Curie temperature, which has two or more directions. The direction of spontaneous polarization can be changed by the applied electric field. Therefore, ferroelectrics has more applications based on polarization switching. In the past decades, ferroelectrics had attracted great attentions and had applications in dynamic random access memory, sensor, capacitor, and transducer due to its high dielectric, piezoelectric and pyroelectric properties. In fact, as the inverse effect of pyroelectric effect, ferroelectrics has the giant electrocaloric effect. Electrocaloric effect is a reversible temperature change of insulating polarizable dielectrics upon the application or withdrawal of an electric field under adiabatic conditions. Therefore, electrocaloric effect has great potential for solid state cooling applications. However, giant electrocaloric effect takes place near the Curie temperature which is much higher than room temperature and the application of electrocaloric effect is limited. In order to achieve a giant electrocaloric effect over a broad range of temperature and near room temperature, we focus on control of electrocaloric effect in ferroelectrics. The main results have been presented as follow.1. The thermodynamic theory of PbTiO3/SrTiO3 bilayer thin film and Pb1-xSrxTiO3 thin film was constructed by using the phenomenological Landau-Devonshire theory. Effect of orientation on the electrocaloric effect of PbTiO3/SrTiO3 bilayer thin film was investigated. The relationship between Sr content and electrocaloric effect of Pb1-xSrxTiO3 thin film was revealed.Conventional ferroelectrics, such as PbTiO3, has high pyroelectric property whose Curie temperature is much higher than room temperature. In order to decrease the Curie temperature and utilize its high pyroelectric property, the electrocaloric effect of PbTiO3SrTiO3 bilayer thin film was investigated, because the ferroelectricity of bilayer thin film is higher than that of PbTiO3 and SrTiO3 thin film. The PbTiO3/SrTiO3 bilayer thin film had the giant electrocaloric effect at the critical volume fraction of SrTiO3 layer. Moreover, the electrocaloric effect of (111) oriented bilayer thin film was larger than that of (001) and (110) oriented. However, the adiabatic temperature change at room temperature had just few degrees. On the other hand, the electrocaloric effect of Pb1-xSrxTiO3 thin film was investigated. Theoretical results indicated that the electrocaloric effect largely depends on the Sr content. Moreover, control of the misfit strain between film and substrate can be used to adjust the electrocaloric effect of Pb1-xSrxTiO3 thin film.2. A thermodynamic model of ferroelectric polymer was constructed based on the Landau-Dovenshire theory. The effects of misfit strain and stress on the electrocaloric effect were investigated to understand the mechanism of electrocaloric effect in polymers.Because its Curie temperature is near room temperature, ferroelectric polymer thin film was chosen and the effect of misfit strain on the electrocaloric effect was investigated for P(VDF-TrFE) (70/30) and (65/35) copolymer thin films. Under the constant electric field, the compressive misfit strain suppressed the polarization and moved the Curie temperature to the lower temperature, meanwhile the tensile misfit strain enhanced the polarization and made the Curie temperature move to higher temperature. They were different from the results of the conventional ferroelectric thin films that the compressive misfit strain increased the polarization and the tensile misfit strain resulted in the opposite effect, which may attribute to the intrinsic structure of the copolymers. The compressive misfit strain decreased the maximum of the adiabatic temperature change, and reduced the working temperature to a great extent, while the tensile misfit strain increased it and shifted the working temperature to higher temperature. Therefore, control of the misfit strain was an effective method to adjust the electrocaloric effect. However, the giant electrocaloric effect can not be achieved at room temperature only by controlling of misfit strain. Consequently, the effect of misfit strain and external stress on electrocaloric effect was investigated. The tensile external stress or the compressive misfit strain decreased both the maximum of the adiabatic temperature change and the working temperature. The largest room temperature adiabatic temperature change of 22.6 K can be taken place at the critical misfit strain and the external stress.3. Taken into account the phase transition properties, the ferroelectric, dielectric, piezoelectric, electrooptic, and electrocaloric properties of (1-x) Pb(Mg1/3Nb2/3)O3-x PbTiO3 single crystals were explained by an eighth-order phenomenological theory.Recently, the giant electrocaloric effect of (1-x) Pb(Mg1/3Nb2/3)O3-x PbTiO3 single crystal was found near its Curie temperature and over a broad range of temperature. It provides great potentials for solid state cooling applications. Moreover, the properties of (1-x) Pb(Mg1/3Nb2/3)O3-x PbTiO3 single crystal largely depended on the composition of PbTiO3. However, the phase transitions, polarization, dielectric properties and piezoelectric properties of (1-x) Pb(Mg1/3Nb2/3)O3-x PbTiO3 single crystal can not be explained by existing theory. Therefore, an eighth-order Landau-Devonshire theory was constructed to explain these properties. Theoretical results were in good agreement with the experimental measurements. However, the eighth-order thermodynamic potential had the difficulty in analyzing the large piezoelectric property of (1-x) Pb(Mg1/3Nb2/3)O3-x PbTiO3 single crystals which is around the morphotropic phase boundary. It may attribute to the polarization rotation induced by an electric field, relaxor ferroelectric state, or polydomain crystal structure of (1-x) Pb(Mg1/3Nb2/3)O3-x PbTiO3 single crystals. Based on this theory, the electrocaloric effect of (1-x) Pb(Mg1/3Nb2/3)O3-x PbTiO3 single crystals was investigated. A negative electrocaloric effect was observed near the Mc-T phase transition in 0.69 Pb(Mg1/3Nb2/3)O3-0.31 PbTiO3 single crystal. A similar behavior was obtained in experimental measurements and theoretical calculations. The application of a strong enough electric field change preserved a high electrocaloric effect over a broad range of temperature which is in agreement with the observations. It is most likely due to the formation and alignment of field-induced polar nanodomains, which are maintained above the temperature of depolarization.4. Using the mechanical and electric boundary conditions, a phenomenological theory for polydomain BaTiO3 thin films was presented. The effect of domain structures on the electrocaloric effect was investigated.The effect of misfit strain on the electrocaloric effect in polydomain epitaxial BaTiO3 thin films at room temperature was investigated using the Ginzburg-Landau-Devonshire thermodynamic theory. Numerical calculations indicated that the misfit strain has a large impact on the ferroelectric polarization states and the electrocaloric effect. Most importantly, the electrocaloric effect in the polydomain ca1/ca2/ca1/ca2 phase was much larger than that of monodomain c phase and the other polydomain phases. Consequently, a large electrocaloric effect can be obtained by carefully controlling the misfit strain, which may provide potential applications in refrigeration devices.