Thermodynamic Study Of The Effects Of Misfit Strain On The Electrocaloric Effect Of Ferroelectric Thin Films

The change of the external electric field can affect the order of dipole inside ferroelectrics and cause adiabatic temperature change,which can be used for refrigeration.Compared with bulk materials,ferroelectric thin films have larger breakdown electric field,and thus obtain greater adiabatic temperature change.Excellent electrocaloric response usually occurs near the phase boundary,and the lattice misfit strain between ferroelectric thin film and substrate would affect the phase structure and phase boundary of ferroelectric films,which provides opportunities to improve electrocaloric response of ferroelectric thin films through misfit strain.However,the value of misfit strain in theoretical studies of electrocaloric effect is usually arbitrarily assumed,and the origins of the misfit strain is not considered.Therefore,based on thermodynamic theory and entropy analysis method,this thesis takes Pb(Zr_1-xTi_x)O₃（PZT）thin film as the material system,by considering the origins of misfit strain,the effect of misfit strain caused by substrate and film thickness on electrocaloric effect of ferroelectric films is studied.The main contents and results are as follows:（1）Based on the thermodynamic phenomenological theory,the influence of directions of the applied electric field,in-plane misfit normal strain and in-plane misfit shear strain on the electrocaloric effect of PZT thin film is studied.The results show that misfit strain makes ferroelectrics exhibit more phase structures,the in-plane shear strain affects the combination of in-plane polarization directions,and large electrocaloric effect generally exists near the phase boundary.When[001]-oriented electric field is applied,the Ti-rich PZT thin films under in-plane misfit normal strain have a large positive electrocaloric response near the orthogonal-monoclinic phase boundary,while the PZT thin films with low Ti concentration exhibits negative electrocaloric effect near the tetragonal-monoclinic phase boundary.When a larger[100]oriented electric field is applied,the Ti-rich PZT thin films have a negative electrocaloric effect near the two phase boundaries,while PZT thin films with low Ti concentration produces large positive electrocaloric effect near the monoclinic-tetragonal phase boundary.When the[110]or[111]oriented electric field is applied,a significant change in response to electrocaloric effect occurs under in-plane misfit shear strain.This is due to the presence of both[100]and[010]in-plane electric field components,and the in-plane misfit shear strain determining the in-plane polarization combination.（2）By considering the origin of misfit strain,the influence of substrate and film thickness on the electrocaloric effect of Pb Ti O₃（PTO）films is studied.Taking Sr Ti O₃and Mg O as substrates,the misfit strain of PTO film is calculated.It is found that the PTO film grown on Sr Ti O₃ substrate is mainly subjected to compressive misfit strain,while the PTO film grown on Mg O substrate is mainly subjected to tensile misfit strain.When the substrate is Sr Ti O₃,the large electrocaloric effect response appears near the tetragonal-paraelectric phase boundary,and the peak value of electrocaloric effect temperature shifts towards low temperatures as the PTO film thickness increasing.When the substrate is Mg O,the PTO film exhibits a rich phase structures,and its tetragonal-orthogonal,tetragonal-monoclinic,tetragonal-paraelectric,and orthogonal-monoclinic phase boundaries all exhibit excellent electrocaloric effect response.And with the increase of electric field change,the electrocaloric effect response of the PTO film is also improved.