The Preparation Of BiFeO₃-BaTiO₃ Ferroelectric Thin Films And Research On Multifunctional Properties

With the development of science and technology,functional materials have played a pivotal role in many fields such as information communication,network technology,etc.,due to their excellent electrical,magnetic,optical,thermal,acoustic,and mechanical properties.Therefore,it is important to study multifunctional materials that integrate multiple physical properties such as electrical magnetism.Multiferroic bismuth ferrite?BiFeO₃,abbreviated as BFO?materials,which possess high Curie temperature?T_C=850??and Neil temperature?T_N=370??,have attracted increasing interests in the past several years due to their fundamental physical properties and the potential applications.Moreover,it is reported that the spontaneous polarization of BFO can reach as high as 100?C/cm² and it has similar electron configuration(lone 6s²electron of Bi³⁺)to Pb²⁺ions,which means that BFO-based dielectric materials can be considered to be a potential candidate,similar to lead-based materials.Particularly,BiFeO₃-BaTiO₃?BF-BT?can form the Morphotropic phase boundary?MPB?similar to PZT.The BF-BT systems have rich physical properties and high Curie temperature,making it a very promising lead-free material system in recent years.With the advent of the new technological revolution in recent years,miniaturization,intelligence,and multi-functionalization have become the main directions for the development of current devices.Therefore,research on thin-film materials is urgent.Compared with traditional bulk materials such as ceramics and single crystals,thin-film materials always have higher breakdown field strength and are more conducive to the integration of micro-electromechanical systems?MEMS?.With the advent of new technology revolutions in recent years,miniaturization,intelligence,and multi-functionalization have become the main directions of device development.Therefore,the research on high-performance thin-film materials is urgent.However,compared with traditional material systems like PZT,there are few studies on BF-BT system films.This is due to the influence of factors such as the volatilization of Bi and the change of the valence state of Fe ions during the preparation of the BF-BT film,resulting in the prepared film having more defects and larger leakage current,which greatly limits the application of BF-BT film.Based on the above background,this thesis focuses on the BF-BT system material,and uses pulsed laser deposition?PLD?to prepare the thin film.The energy storage characteristics of the BF-BT thin film are also studied for the first time.At the same time,an excellent iron voltage is prepared by excess Bi in the target.The main research contents and results are as follows:?1?0.68BiFeO₃-0.32BaTiO₃:0.15wt.%MnCO₃ target material with well-developed and uniform grain excess of 5%Bi and 10%Bi excess was prepared by a conventional solid phase sintering method at an appropriate sintering temperature.Characterizing the ceramic dielectric properties of 5%Bi excess and 10%Bi excess,both have a high Curie temperature of about 450?.Characterize the ferroelectric and piezoelectric properties of both at room temperature and variable temperature.Ceramics with an excess of 10%Bi have good ferroelectric properties.The highest polarization strength P_max of the ceramics is 25?C/cm² and the strain is 0.15%,under the electric field of 7 kV/mm.Bi excess ceramic targets with high Curie temperature and good ferroelectric performance were successfully prepared,which laid a good foundation for the subsequent thin film preparation.?2?Using 0.68BiFeO₃-0.32BaTiO₃:0.15wt.%MnCO₃ target material to prepare BF-BT thin film on Pt/Ti/SiO₂/Si substrate,by changing the substrate temperature,oxygen pressure,laser energy,laser frequency and other process parameters Process Optimization.The prepared films were tested for dielectric and ferroelectric properties,and the effects of different preparation processes on the film properties were analyzed and summarized.Among the process parameters,the changes of substrate temperature and oxygen pressure have the greatest influence on the performance of the thin film,the performance of the film prepared under the conditions of 600? and 2 Pa is more excellent.Under the optimal conditions,high-quality BF-BT films were prepared.?3?A dense BF-BT thin film with a thickness of about 200 nm and no heterogeneous phase was successfully prepared on a Pt/Ti/SiO₂/Si substrate.The film has a low loss of 0.02 in the frequency range of 100 Hz¹00 kHz,and a lower leakage current(1×10^-4A/cm²)at room temperature.The energy storage characteristics of BF-BT films were studied for the first time.Under the electric field of 900 kV/cm,the calculated energy storage density of the film is 19 J/cm³,and the energy efficiency is 51%.It maintains high performance after 1×10⁷ cycles and has excellent fatigue durability.The results show that BF-BT thin film can be a candidate for energy storage applications.?4?Oriented epitaxial films were prepared on?001?Nb:SrTiO₃ single crystal substrates using targets with no excess Bi and 10%Bi excess,and had good density.Under the same conditions,the films prepared with 10%Bi excess target have lower roughness?R_a=0.67 nm?and more excellent ferroelectric properties.The spontaneous polarization intensity P_r of the prepared B_1.1F-BT film can be as high as 97?C/cm²,the highest polarization intensity P_max is as high as 123?C/cm²,and the unipolar strain value is as high as 1.27%.And the film has stable frequency stability and fatigue characteristics.