The Electromechanical And Pyroelectric Properties Of Oriented PMN-PT Ferroelectric Thin Films Tuned By The Misfit Strain

Ferroelectric thin films with excellent physical properties satisfy the increasingly miniaturization of the device,and they are widely used in optoelectronics,microelectronics,and micro-electromechanical and other fields.The performances of thin films affect the performances of the devices.The different oriented ferroelectric thin films have different properties,and the misfit strain between the film and the substrate has important influence on the phase structures of thin films.Therefore,it is expected that the electromechanical and pyroelectric properties of different oriented ferroelectric thin films can be tuned by misfit strain.As an effective method for the study of ferroelectric thin films,the thermodynamic theory has been well applied in the study of(001)oriented ferroelectric thin films,but the most of studies are with the assumptions of single domain in ferroelectrics.The study on other orientations and multi-domain ferroelectric films is rather few,the development of thermodynamic theoretical framework for other orientations and multi-domain thin films is important.Additionally,the(1-x)PMN-xPT bulk materials show excellent electromechanical properties,but the enhancement and optimization the electromechanical and pyroelectric properties of(1-x)PMN-xPT thin films is rather lacking.Thus,in this thesis,we have constructed the thermodynamic theoretical framework for different oriented single-domain thin films and the multi-domain thin films,then we choose the(1-x)PMN-xPT thin films as an example,and study and analyze its electromechanical and pyroelectric properties.After that,we propose some strategies for enhancing performance of(1-x)PMN-xPT thin films tuned by misfit strain.The main works and the conclusions are as follows:(1)The thermodynamic theoretical framework and the calculation method of electromechanical and pyroelectric properties of(001),(110)and(111)oriented single-domain thin film are established by taking the coordinate transformation of the order parameters and taking into account the misfit strain subjected to the substrate.The combination of different domains to form the multi-domain structures and considering the mechanical and electrical boundary conditions at the global coordinate system and local domain walls,we establish the thermodynamic theoretical framework of the multi-domain ferroelectric thin films and the calculations method of the electromechanical and pyroelectric properties of the multi-domain ferroelectric thin films.(2)The strain-temperature phase diagrams,electromechanical and pyroelectric properties of the different oriented single-domain(1-x)PMN-xPT thin films are calculated and analyzed based on the thermodynamic framework of the different oriented single-domain ferroelectric thin films.The results show that the symmetry of the phase structures of(1-x)PMN-xPT films decrease with order from(111),(001)to(110)orientated films.At the boundary of the phases transformation,the different oriented single domain(1-x)PMN-xPT thin films exhibit good dielectric and piezoelectric properties,and(001)and(110)orientated single domain 0.7PMN-0.3PT and 0.58PMN-0.42 PT thin films exhibit better dielectric and piezoelectric properties than that of(111)orientated(1-x)PMN-xPT at the same composition.And the(110)and(111)oriented single domain(1-x)PMN-xPT thin films exhibit better than pyroelectric properties than that of(001)oriented(1-x)PMN-xPT thin films.(3)The strain-temperature phase diagrams,electromechanical and pyroelectric properties of the multi-domain(1-x)PMN-xPT thin films are calculated and analyzed based on the thermodynamic framework of the multi-domain thin films.The results show that the phases of the multi-domain(1-x)PMN-xPT thin films have more complex phase structures than that of single domain thin films.With the increasing composition x,the area of Te(a)/Te(c)and M(aa*)/M(ca*)increased.At the phase boundary,the multi-domain(1-x)PMN-xPT thin films exhibit good dielectric and piezoelectric responses.The multi-domain 0.7PMN-0.3PT thin films doesn't have the out-of-plane pyroelectric coefficients in the wide strain region,but near the phase boundary it exhibits better pyroelectric properties than the other composition,includeing x=0.42 and 0.7.