The Performance Tuning And Microstructure Of Lead Free Ferroelectric Single Crystal

The ferroelectric and piezoelectric materials play important roles in national economy and national defense technology.The high piezoelectric performance,high temperature-resistance and environmentally friendly materials are the current development trend of ferroelectric materials.The current research focuses on new lead-free piezoelectric materials replacing traditional lead based PZT and PMN systems.NBT and KNN based materials are thought to be most promising lead-free ferroelectric materials because of their good ferroelectric,piezoelectric performance and high curie temperature.Therefore,this thesis focuses on NBT and KNN based piezoelectric materials,single crystals growth was conducted,and their electric,optic performance,hierarchical structure and properties tuning were studied.The effect of Fe³⁺doping on the piezoelectric and ferroelectric performance of0.95Na_1/2Bi_1/2TiO₃-0.05BaTiO₃?NBBT5?crystals has been researched.The results show that Fe³⁺doping can decrease effectively dielectric loss,increase piezoelectric performance,the piezoelectric constant d₃₃ of NBBT5 crystal and 0.2 at%Fe³⁺doped NBBT5 is 420 pC/N and 590 pC/N,respectively.After Fe³⁺doping,the larger coercive field and remnant polarization are attained with more saturate ferroelectric hysteresis loop.The ferroelectric domain based on PFM and TEM was studied,and the results show that domain size is smaller from several micrometers to several nanometers,with larger domain thickness after Fe³⁺doping.The thicker domain is beneficial to higher piezoelectric performance.The ferroelectric materials possess the structure with different dimensions and different levels,such as domain?phase and polar nano-region?PNR?.Therefore,studying the structure with different dimensions is essential to clarify high piezoelectric response of NBBT single crystal.The phonon modes with different space groups of NBBT were calculated based on group theory,and then Raman scattering experiments were conducted based on these results.The phase transitions of NBBT5 and 0.5%Fe³⁺doped NBBT5 crystals were studied by Raman spectrum.The results show that NBBT5 crystal is R3c phase at zero field with octahedral tilting a^-a^-a^-,which can suppress the domain wall evolution and is detrimental to obtain high piezoelectric response.However,0.5 at%Fe³⁺doped NBBT5 single crystal is P4bm phase with octahedral tilting a⁰a⁰c⁺and the external field can induce phase transition from P4bm to P4mm at 3kV/mm-4kV/mm.The Fe³⁺doped NBBT5 crystal with P4mm phase do not have antiphase boundaries?APB?regions.Therefore,P4mm phase induced by the external field is beneficial to higher piezoelectrical response.The KNN?Mn⁴⁺doped KNN and Er³⁺doped KNN single crystals were successfully grown by top seeded solution growth?TSSG?and the piezoelectric?dielectric performance were studied.The results show that the properties are lower than NBBT crystals.However,the Er³⁺doped KNN single crystals possess good up-conversion photoluminescence properties.The effect of different annealing conditions on up-conversion photoluminescence was studied,and results show that when annealing temperature is higher,or annealing oxygen concentration is more,the crystal has less oxygen vacancies and narrower defect level,which can induce less photon-generated carrier and lower conductivity.800?oxygen annealing can decrease conductivity from semiconductor to insulator.Oxygen vacancies can absorb980 nm photon,so the fluorescence intensity is less when oxygen vacancies are more,and especially Er³⁺doped KNN crystal of annealing at the vacuum under 500?losses up-conversion photoluminescence.Based on the XPS results of the different annealing conditions,the unannealed crystal has the coexistence of Nb⁴⁺and Nb⁵⁺,with only Nb⁵⁺existing in crystal after 800?oxygen annealing and more Nb⁴⁺after500?vacuum annealing.The Nb⁴⁺makes the crystal possess semiconductor behavior and the only Nb⁵⁺existing in crystal after 800?oxygen annealing make crystal possess insulator behavior.Based on the PL emission spectra of different temperature,the photoluminescence intensity decreases as the temperature increases,which is thermal quenching effect.The intensity ratio of 526 nm and 548 nm peak is the linear relation with 1/T,which could be used for fabricating temperature sensor.