Electric Field Induced Strain And Er³⁺ Doped Upconversion Luminescence Property Near MPB Of Bismuth Sodium Titanate Based Ceramics

Piezoelectric ceramic is a class of functional materials that is widely used in sensors,actuators,filters,transducers,etc.,applying in daily life,manufacturing,transmission and communication,aerospace and other fields.However,lead in materials has a very bad impact on human health and the environment.With the increasingly great attention paid to this issue,more and more scholars are committed to research and develop high-performance lead-free piezoelectric materials to replace traditional lead-based ceramics.Bismuth titanate(Bi_0.5Na_0.5TiO₃)is known as one of the most promising alternatives to lead-based ceramics due to its good ferroelectricity and high Curie temperature.The researchers have found that the BNT-based materials with the morphotropic phase boundary?MPB?exhibited excellent properties.On the other hand,with the development of multifunctional devices and the trend of device miniaturization,rare earth doped piezoelectric multifunctional ceramics with optical-mechano-electrio conversions have been widely concerned.In this thesis,therefore,traditional solid phase reaction method was used to prepare BNT-based ceramics by incorporating other components and doping Er³⁺ions,and the structures and properties of ceramics were studied in various aspects.The main contents are as follows:1.The influence of Ti⁴⁺non-stoichiometric ratio on the structure and properties of BNT-BaTiO₃ solid solution((Bi_0.5Na_0.5)_0.93Ba_0.07Ti_1+xO₃)and its microscopic mechanism were analyzed.The experimental results showed that all the compositions?x=-0.02-0.01?suggested the coexistence of rhombohedral?R3c?and tetragonal?P4bm?phases,and the weight fraction of P4bm symmetry reduced initially from76.76%to 49.36%and then increased to 67.13%with increasing the Ti amount from deficiency?x=-0.02?to excess?x=0.01?.After polarization,the tetragonal phase of the x=0 sample increased significantly from 49.36%to 68.27%,indicating the existence of a large number of tetragonal nanodomains.Accordingly,the sample of x=0 showed good fatigue resistance and relatively low ferroelectric-relaxation phase transition temperature/depolarization temperature at⁹0 ^oC.Although high depolarization temperature of¹60 ^oC was detected in the compositions with whether Ti excess or high Ti deficiency,high electric conductivity with Ea¹ eV and significant fatigue behavior were found,which is suggested to be ascribed to large amounts of incipient Ti_Ti^?3+??4+-(1^??defects.2.B-site-complex perovskite A(Mg_1/3Nb_2/3)O₃?AMN,A=Ca,Sr,Ba?was used to modify BNT and the effect of CMN?SMN?BMN complex perovskite on structure and performance of BNT-xAMN ceramics was studied.The study showed that when the complex perovskite composed of alkali soil metal ions with small ion radius in A site was dissolved in solid solution with BNT,there was small amount of the second phase as x>0.06.As the AMN content increased,the temperature of ferroelectric-relaxation transition gradually decreased.The optimal piezoelectric constant d₃₃ of 86pC/N,92pC/N and 122pC/N for the BNT-xCMN,BNT-xSMN and BNT-xBMN ceramics were obtained at x=0.022,0.04 and 0.044,respectively,mainly due to irreversible domain swithching.In addition,the maximum normalized strain of 244pm/V,288pm/V and 493pm/V for the BNT-xCMN,BNT-xSMN and BNT-xBMN ceramics were obtained at x=0.04,0.08 and 0.052,respectively,mainly contributed to reversible relaxation-ferroelectric phase transition.In particular,the BNT-xBMN solid solution has a high d₃₃ and large electric field-induced strain compared to the BNT-xCMN and BNT-xSMN systems,which is attributed to the high polarizability of Ba²⁺ions.3.0.94BNT-0.06BT was modified by Er³⁺doped SBT solid solution(Sr_0.7Bi_0.18Er_0.02TiO₃,SBET).The effects of SBET solid solution components on?0.94-x?BNT-0.06BT-xSBET ceramics in ferroelectric,piezoelectric and luminescence properties were studied under the condition of determining Er³⁺doping.The experimental results showed that the SBET relaxor could form a single perovskite structure with 0.94BNT-0.06BT solid solution and the ferroelectric-relaxor phase transition temperature decreased from 96 ^oC to below room temperature with increasing SBET content.Moreover,the optimal piezoelectric property of 175 pC/N and the maximum strain of 0.33%with normalized strain of550 pm/V were achieved at x=0.04 and 0.055,respectively.The up-conversion green?⁵31nm and⁵52nm?and red?⁶63nm?light emssion of BNT-BT-0.055SBET ceramic was based on the three-photon process,and the green emission exhibited maximum optical thermal sensitivity of 0.0055 at 522K.This indicates that the composition is very promising for multifunctional applications in actuators and temperature sensing device.