Designing Of Bi_0.5Na_0.5TiO₃-based Ferroelectrics And Their Actuator Applications

With the impetus from the "China Intelligent Manufacturing 2025" strategy and the rapid development of artificial intelligence,the demand for ultra-precision feeding and positioning technologies in microelectronics engineering,nanotechnology and other fields has become increasingly prominent.With the characteristics of small size,good chemical resistance,and high precision,ceramic actuators have become important actuator components with great application prospects.As important candidate materials for actuators,relaxor ferroelectric ceramics are favored due to their advantages such as large strain and small hysteresis.At present,international companies represented by Japan's Fuji Corporation,Toshiba Corporation,and Kyocera Corporation have mature actuator production technology,and the actuators produced are widely used in civilian products such as oil nozzle of engines and inkjet printers.However,in deep well exploration,aerospace and other fields,the operating temperature of actuators often needs to reach 150?.Although the existing lead-based relaxor ferroelectric ceramics show excellent electrical properties and temperature stability,lead-based ceramics with lead content exceeding 60%will cause irreversible damage to the environment and human bodies,and have been restricted to be used in most countries and regions.In many lead-free relaxor ferroelectric ceramic systems,high-performance strain ceramics obtained through component design are very sensitive to temperature,only slight temperature fluctuation will lead to large changes in strain values,limiting their use in the application of actuators.Therefore,there is an urgent need to develop new lead-free relaxor ferroelectric ceramic system with high strain characteristics and excellent temperature stability to meet the application of actuators in extreme environmentsBased on the above background,this thesis focuses on Bi0.5Na0.5TiO3(BNT)based lead-free relaxor ferroelectric ceramics and systematically studied the room temperature strain performances and temperature stability of the strain characteristics The method to design a new type of temperature stable lead-free strain ceramic materials is also summarized in terms of component design.The main research contents and research results of the full text are summarized as follows1?Doping modifying(1)Doped with different molar content of Nb ions,we designed 0.76 Bi0.5Na0.5TiO3-0.24Bi0.5K0.5TiO3(0.76BNT-0.24BKT)ceramic system located in the tetragonal phase region.The experimental results show that the introduction of Nb ions break the long-range ferroelectric domain structure of 0.76BNT-0.24BKT ferroelectric ceramics,so that the 0.76BNT-0.24BKT binary ceramic system changes from P4mm ferroelectric phase to P4bm relaxor ferroelectric phase under the control of Nb ions.At the same time,at the critical component,5 mol%Nb-doped 0.76BNT-0.24BKT component the electrical properties of the dielectric constant is stable and the room temperature strain is 0.2%,and this strain value can be at RT?160? The change rate of less than 1%is maintained within the temperature range,which is better than most reported lead-free relaxor ferroelectric ceramic strain materials(2)We found for the first time that BNT-based relaxor ferroelectric systems with the room temperature phase structure of P4bm can maintain a stable electric field-induced phase structure change in a wide temperature range.This is because the BNT-based ceramic system with a P4bm phase structure at room temperature can maintain the same field-induced phase transition type in a wide temperature range,thereby reducing the influence of different field-induced phase transition types on the strain.The research content in this part provides a certain guiding significance for the component design of the new temperature-stable lead-free strained ceramic system2?Solution modification(3)For the first time,the BNT-based ceramic component located at the relaxor boundary(MRB)phase boundary was designed,that is,the solid solution of 0.8Bi0.5Na0.5TiO3-0.2Bi0.5K0.5TiO3-0.01NaNbO3 ceramic matrix with different molar content of SrTiO3(ST).A relaxor ferroelectric ceramic composition with MRB phase boundaries where R3c and P4bm polar nano-domains coexist at room temperature is obtained.The special structure with multiple phase structures coexisting at the MRB phase boundary ensures that the investigated material has a large strain value at room temperature,and at the same time,this large strain effect can maintain a stable change rate of less than 10%in a wide temperature range from room temperature to 140?.(4)Through the in-situ temperature-dependent Raman and Landau free energy calculations,we can find that:on the one hand,the polarization of a single cell is reduced at high temperature,and the strain value contributed by the intrinsic lattice strain shows a downward trend;on the other hand,as the temperature increases,the energy barrier between the relaxor ferroelectric phase and the ferroelectric phase transition induced by the electric field decreases,and the strain contributed by the electro-induced phase transition shows the upward trend.The mutual cancellation of two effects in the measured temperature range results in the temperature-stable strain performances.The experimental results confirm that the MRB phase boundary can greatly improve the room temperature strain properties of BNT-based relaxed ferroelectric ceramics.