| Piezoelectric materials are widely used in piezoelectric sensors,actuators,transducers and other fields due to the mutual conversion and coupling of electrical energy and mechanical energy.In recent years,with the broader application prospects of micro-electromechanical systems?MEMS?,piezoelectric devices have developed in the direction of intelligentization and miniaturization.The miniaturized piezoelectric devices made of piezoelectric thin films have higher mechanical precision and stability,so it can be applied to fields that require precise positioning/driving,such as micro-robots,scanning probe microscopes,hard disk drives,and micro-optical devices.Sodium niobate?Na Nb O3?is a very promising piezoelectric material with rich phase structure and excellent piezoelectric performance.Under external field induction,Na Nb O3 undergoes irreversible phase transition from antiferroelectric phase?Pbma?to ferroelectric phase?Pmc21?.It is reported that the the piezoelectric coefficient d33 of the ferroelectric phase structure?Pmc21?of Na Nb O3 ceramic reaches 40 p C/N at room temperature,the depolarization temperature reaches 300°C,and it has 0.2%strain value,which is potential for piezoelectric applications.However,the ferroelectric phase structure with superior piezoelectric properties is a metastable structure of Na Nb O3material.At room temperature,the Na Nb O3 material is a structure in which the antiferroelectric?Pbma?phase structure and the metastable ferroelectric phase coexist.Among them,the antiferroelectric phase structure dominates.Due to the small difference in free energy between the antiferroelectric and ferroelectric structure,there is relative instability between antiferroelectric and ferroelectric phase structure which is easily affected by nonstoichiometry,microstructure,external electric field,doping and other effects.Therefore,in the Na Nb O3 thin film,due to the volatility of the Na element and the instability of the phase structure,it is difficult to obtain the ferroelectric phase structure,which limits the application of the Na Nb O3 material in piezoelectric field.In addition,because the structure of Na Nb O3 is relatively complex and difficult to prepare,so far,there has been relatively little basic research on Na Nb O3 thin films.In addition,the intragranular stress of the Na Nb O3 material can significantly affect the relative stability of its antiferroelectric and ferroelectric phases,which indicates that potential factors?such as density,grain size,or cation vacancy?that affect the intragranular stress can affect the phase stability of the Na Nb O3 material.In particular,for the Na Nb O3 thin film,the Na deficiency caused by the volatilization of the Na element will definitely affect the phase stability,thereby affecting the antiferroelectric to ferroelectric phase transition process under the external field.However,the law and mechanism of how the Na nonstoichiometry affects the relative phase stability is still lacking.In order to stabilize the ferroelectric phase structure of the Na Nb O3 film under the external field,and to reveal the laws and mechanisms of how Na nonstoichiometry affecting phase stability,A series of research work has been carried out in three aspects:the effect of the nonstoichiometry ratio on the phase stability of the Na Nb O3 ceramic,the preparation of the Na Nb O3 thin films and the micro-region analysis of the transition from antiferroelectric to ferroelectric phase structure in the Na Nb O3 thin film A series of research work has been carried out to prove that by adjusting the oxygen pressure,the Na Nb O3 film can obtain a stable ferroelectric phase under external electric field,opening up the prospects of the Na Nb O3 thin film in piezoelectric applications.The main contents and conclusions are as follows:?1?A series of Na1+xNb O3 ceramics?targets?were successfully prepared by the solid-state reaction method.The effects of Na nonstoichiometry on microstructure,electrical properties,and antiferroelectric phase stability were systematically studied.Studies have shown that the antiferroelectric to ferroelectric phase transition electric field of Na-deficient Na Nb O3 ceramics significantly increased compared to that of pure component,which proves that the energy barrier of antiferroelectric to ferroelectric phase transition in Na deficient components is higher.Introducing appropriate A-site vacancies will increase the compressive stress in the crystal lattice,enhancing the antiferroelectric distortion and the energy barrier between the antiferroelectric P phase and the ferroelectric Q phase.?2?The pulsed laser deposition method was used to successfully prepare epitaxial,dense and flat Na Nb O3 thin film on Nb-doped Sr Ti O3 substrate with low dielectric loss.The effect of the preparation process on the microstructure and electrical properties of the Na Nb O3 thin film was studied.After process optimization,the optimal growth temperature,target-substrate distance,oxygen pressure and laser pulse frequency were determined to be 625°C,8 cm,0.15 mbar and 2 Hz,respectively.The roughness of the?001?Na Nb O3 epitaxial thin film grown under this condition is 0.174nm,and the dielectric constant and dielectric loss are 1350 and 0.05,respectively.?3?The oxygen pressure can not only affect the growth mode,surface structure and electrical properties of the Na Nb O3 film,but also affect the occurrence of antiferroelectric to ferroelectric phase transition by affecting the Na content in the thin film.The Na Nb O3 thin film had more Na volatilization under the lower oxygen pressure,which caused Na deficiency in the thin film and stabilized the antiferroelectric phase.The increase of the oxygen pressure can increase the Na content in the thin film,so that the film could undergo antiferroelectric to ferroelectric phase transition under the induction of electric field,thus obtaining the stable ferroelectric phase.?4?The microscopic mechanism of the domain change and strain change accompanied in the transition of the antiferroelectric to ferroelectric phase structure of the Na Nb O3 film was studied using a piezoelectric force microscope.Through the loading of different voltages,the microscopic evolution of ferroelectric domains and the electric field-induced phase transition process were obtained.The voltage of antiferroelectric to ferroelectric phase transition was determined and the existence of stable ferroelectric phase in sodium niobate was confirmed.Secondly,the strain contribution mechanism under different phase structure is obtained by judging the strain curve under the piezoelectric force microscope. |
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