Study On The Preparation And Electric Properties Of Ferroelcetic Thin Film Near The Morphotropic Phase Boundary

Ferroelectric thin films refer to a ferroelectric material with the thickness of tens of nanometers to several micrometers and have a series of functional properties such as dielectricity,piezoelectricity,pyroelectricity,and ferroelectricity,which have a potential broad application prospect in aviation,aerospace,electronics,communications,medical and military fields.With the development of 5G/6G technology,its MEMS strong field electrical applications have higher and higher quality requirements for the ferroelectric thin films.This thesis mainly focuses on the current research status of inorganic ferroelectric thin films,such as low cooling/tunability/energy storage effect and poor thermal stability in the application of strong field electricity in electrocaloric effect（ECE）,dielectric tunability and energy storage.In this thesis,the components of near the morphotropic phase boundary（MPB）of lead-based films were chosen as the research object by the aging treatment,preferred orientation and heterostructure construction strategies or method.The research results are as follows:（1）Pb(Ni_1/3Nb_2/3)_0.5Zr_0.15Ti_0.35O₃（PNNZT）thin films were prepared on SrTiO₃（STO）and Pt（111）/Ti O_x/Si O₂/Si（100）（Pt）substrates by pulse laser deposition and sol-gel techniques,respectively.The results show that coexistence of positive and negative ECE in the（100）and（111）-oriented films deposited on STO single crystal substrates,while only positive ECE occurs in the polycrystalline films deposited on Pt substrates.Under the electric field of 763k V/cm,the absolute value ofΔT increases from 4.7 K to 10.2 K,and its extreme temperature moves to room temperature.The results of XPS and EPR analysis show that the defect dipoles formed by nickel/oxygen vacancies follow the principle of point defect symmetry during aging.In addition,it was found that the saturation polarization intensity of polycrystalline films remained unchanged after aging treatment,but the remanent polarization significantly decreased,and the breakdown electric field significantly decreaseed,from 2167 k V/cm to 1834k V/cm.However,the energy storage density was increased from 14.26 J/cm³ to22.12 J/cm³,and the energy storage efficiency was increased from 23.9%to59.5%.（2）The 0.85Pb(Mg_1/3Nb_2/3)-0.15Pb Ti O₃（PMN-15PT）highly epitaxial thin films were prepared by sol-gel method on Nb doped SrTiO₃（STO）single crystal substrate.The PMN-15PT films deposited on Nb-doped STO（100）,（110）and（111）substrates have high dielectric tunability（η）and frequency stability（in the range of 500 Hz to 10 k Hz）with the values of～72%（E～1500 k V/cm）,～74%（E～1304 k V/cm）,～80%（E～1500 k V/cm）,respectively.The high dielectric tunability of these thin films was associated to the domain-wall motions,the motion of the cluster interphase boundaries and the reorientation of dipolar moments of the polar nanoregions.The（111）-oriented thin film exhibits the highest thermal stability（η=67.5%）with a broad temperature range～150 K,and the maximum fluctuation of the tunability to the middle value is～±2%.This phenomenon may be ascribed to the large dielectric diffuseness degree of the film（γ～1.81）.In addition,the（100）,（110）and（111）-oriented films have obtained high energy storage density and energy storage efficiency.Under the electric field of 3260 k V/cm,their energy storage densities are 45.1 J/cm³,58.5 J/cm³ and 40.3J/cm³,and the corresponding energy storage efficiencies are 60.7%,55.9%and59.1%,respectively.（3）The Ca_0.2Zr_0.8O_1.8/0.85Pb(Mg_1/3Nb_2/3)-0.15Pb Ti O₃（CZ/PMN-15PT）heterojunction films were prepared by sol-gel method.The results show that the heterostructure film exhibits large negative ECE in a board temperature range（298 K-453 K）,and the absolute value of the maximum temperature variations（?T）is 9.35 K.The negative ECE with a board temperature range is mainly contributed by two aspects:on the one hand,the build-in electric field from the CZ layer to the PMN-15PT film is formed in the interface of the heterojunctures,which leads the domain walls to be pinned of and makes the polarization reversal to be difficult in the PMN-15PT film.However,as the temperature increases,the pinning of the domain wall weakens,the polarization reversal becomes easy and the polarization increases.The film shows a positive pyroelectric coefficient and exhibits a negative ECE.On the other hand,the total polarization in the CZ/PMN-15PT heterojunctures films increases with temperature,possibly due to the transformation from the non-polar CZ layer into the polar CZ ferroelectric phase under the temperature field and the electric field.Therefore,the film exhibits a negative ECE.（4）PbZr_0.52Ti_0.48O₃/Ca_0.2Zr_0.8O_1.8/PbZr_0.52Ti_0.48O₃（AZA-type）and PbZr_0.3Ti_0.7O₃/Ca_0.2Zr_0.8O_1.8/PbZr_0.52Ti_0.48O₃（AZB-type）sandwich heterojuncture films were constructed by sol-gel method.The results show that both AZA-type and AZB-type sandwich heterojuncture films exhibit double hysteresis loops.The formation of double hysteresis loops is related to the build-in electric field in the interface of heterojuncture rather than the formation of antiferroelectric phase.Compared with PbZr_0.52Ti_0.48O₃ films,AZA-type sandwich heterojuncture films shows greater negative ECE,whose?T is about-16.5 K,and refrigerating temperature range is from 223 K to 360 K.The negative ECE can also be attributed to the pinning of the build-in electric field to the domain wall of the film and the transition of the CZ layer from non-polar to polar ferroelectric phase induced by the external electric field.Besides,AZA-type and AZB-type sandwich heterojuncture films with higher breakdown field strength(E_BDS)are 2305 k V/cm and 2130 k V/cm,which is 1.4 times and 1.3 times than that of PbZr_0.52Ti_0.48O₃films,respectively.The results show that the CZ layer is beneficial to obtain the breakdown field strength.In addition,under the electric field is～1150 k V/cm,the energy storage density of AZA-type and AZB-type sandwich heterojuncture films is 13.8 J/cm³ and 18.3 J/cm³,respectively.And the energy storage efficiency is 56.9%and 60.1%,respectively.