Flexible Ferroelectric Thin Films And Their Energy Convertion Applications

Flexible electronic technology is bringing a revolution to the electronics,which has opened up many innovative electronic products and applications,making them to be a new economic growth point.However,with the development of flexible electronic devices towards wearable,miniaturization and functional diversification,the challenge of power supply and device cooling are emerging.Ferroelectric materials,which possess piezoelectricity(i.e.,capability to convert mechanical energy to electric energy)and electrocaloric effect(i.e.,capability to change temperature of material with the variation of electric field),have the potential to address the concerned challenge.However,typical high performance inorganic ferroelectric materials are usually brittle and rigid,and ferroelectric thin films on rigid substrates are also hard to bend.It remains a big challenge to integrate them into flexible wearable electronics.In this thesis,we first realize the direct fabrication of flexible ferroelectric thin films based on widely used PbZr0.52Ti0.48O3(PZT),via unique two-dimensional mica substrates.The application of flexible ferroelectric thin films in the field of piezoelectric energy conversion and electrocaloric cooling devices are also investigated.Firstly,we successfully fabricate PZT thin films with large area and high quality directly on mica substrates by a sol gel method.The PZT films have an excellent flexibility with a bending curvature radius down to 2.2 mm.In addition,we found that the properties of ferroelectric thin films are enhanced by mica substrate,which physical mechanism was also discussed.The high surface quality of mica can optimize the microstructure of the PZT films,and the flexibility of mica has a function to release the resistance stress caused by the switch and deformation of the ferroelectric domain,and thus resulting in better ferroelectric and piezoelectric performances.Secondly,toward the power supply problem of wearable electronic devices,high performance PZT flexible piezoelectric energy harvesters(FPEHs)have been designed and fabricated.The FPEHs exhibit an excellent output performance with an open-circuit voltage up to 120 V,a short-circuit current of 0.28 μA and a current density of 150 μA cm-2.The maximum power density reaches to 42.7 mW cm-3.In addition,the FPEHs can be continuously bended under a high strain for 40000 times without obvious decline of output performance.Finally,toward the cooling problem of wearable electronic devices,high-quality flexible Pb0.82Ba0.08La0.1Zr0.9Ti0.1O3 ferroelectric thin films are fabricated.The maximum adiabatic temperature change and isothermal entropy change reach to 22.5 K and 25.9 J K-1 kg-1 at room temperature,respectively,better than previously reported flexible inorganic electrocaloric materials.In addition,the electrocaloric property keeps stable in the bending state.To sum up,in this thesis,we systematically study the direct fabrication of high-quality flexible ferroelectric thin films and explore their applications in the field of energy conversion,such as piezoelectric energy harvesting and electrocaloric cooling device.Our results provide feasible solutions for the power supply and cooling problems of wearable electronic devices,which promote the connotation and application of flexible electronics and wearable devices.