Multicaloric Effect Of Multiferroic And Ferroelectric Materials

In recent years,the solid-state refrigeration technology based on the caloric effect in multiferroic materials has attracted people's attention due to its clean and efficient refrigeration method,which is expected to replace the traditional air compression refrigeration technology.Caloric effect refers to the isothermal entropy change or adiabatic temperature change that occurs when a material is stimulated by an external force such as electric field,magnetic field or mechanical stress.According to the external stimulus,the corresponding caloric effect is called the electrocaloric effect(electric field),magnetocaloric effect(magnetic field),elastocaloric effect(mechanical stress),and barocaloric effect(hydrostatic pressure).In the thesis,the multicaloric effect of multiferroic EuTiO3 thin films and ferroelectric Ba1-xSrxTiO3 thin films has been investigated based on the modified Landau-Ginzburg Devonshire(LGD)theory and thermodynamic theory.1.Multicaloric effect of multiferroic EuTiO3 thin filmsA modified Landau-Ginzburg Devonshire phenomenological thermodynamic theory is used to investigate both the influence of biaxial compressive misfit strain between film and the substrate and the mechanical stress on the multicaloric response in epitaxial EuTiO3 thin films.The complicated compressive misfit strain-temperature phase diagram in the low temperature region is developed.The excellent electrocaloric and elastocaloric effect can be achieved:?T(TC)=3.66K while ?E=100kV/cm,um=-0.04;?T=5.31K at 200MPa.Besides,the maximal adiabatic temperature change ?T(TM)=17.3K while?H=5T,um=-0.04,in the low temperature region,which is comparable with the experimental result.Both the misfit strain and the external mechanical stress play a crucial role in the multicaloric response in EuTiO3 films.It may open more opportunities for practical application in refrigeration devices,especially for low temperature solid state multicaloric cooling.2.Multicaloric effect of ferroelectric Ba1-xSrxTiO3 thin filmsBaTiO3,as a traditional ferroelectric material,has been received much attention by the researchers because of its excellent electrocaloric effect.However,its high Curie temperature makes it difficult to have a good caloric effect at room temperature.In the thesis,the impact of mismatch strain,uniaxial stress and external electric field on the caloric behavior in the doped Ba1-xSrxTiO3 thin film has been investigated.It is shown that the doped Ba1-xSrxTiO3 may have a higher adiabatic temperature change near the room temperature,and exhibit the high adiabatic temperature change in the wide temperature region by applying the uniaxial stress.While um=-0.2%,?E=100kV/cm,?=600MPa,the maximum of adiabatic temperature difference?Tmax=2.12 and ?Tmax=2.35 for the doped Ba1-xSrxTiO3(x=0.4)and Ba1-xSrxTiO3(x=0.3),respectively.It may also open the more opportunities for the traditional ferroelectric materials in the practical application of refrigeration devices,especially in the room temperature range.