| Piezoelectric material is an important functional material for converting mechanical energy and electrical energy.It is widely used in the manufacture of sensors,actuators,ultrasonic transducers,resonators,filters and other electronic components,and has important applications in national economy,science and technology,modern national defense and other fields.In recent years,with the rapid development of the aerospace and nuclear industries,the demand for sensors that can operate at 500? or higher temperature with a long mean time between failures has increased dramatically.However,conventional PZT-based piezoelectric ceramics and currently widely studied lead-free piezoelectric ceramic systems(BCZT,KNN,BNT,etc.)are limited by lower Curie temperatures and are not suitable for use under high temperature conditions.Therefore,the development of new high-temperature piezoelectric materials and devices is an urgent task.Bismuth layer structure ferroelectrics(abbreviated as BLSFs)were discovered by Aurivillius et al.in 1949,and their structures are arranged by regularly alternating intervals of a two-dimensional perovskite layer and a bismuth oxide layer along the c-axis direction.Bi4Ti3O12(BIT)is a typical three-layered bismuth-structured ferroelectric,which is formed by interlacing a(Bi2O2)2+layer and a pseudo-perovskite layer(Bi2Ti3O10)2-.Due to its structural characteristics,bismuth titanate has a high ferroelectric phase transition temperature(675?)and strong ferroelectricity,which has received extensive attention and research in high temperature piezoelectric applications.However,due to its own structural characteristics,ferroelectric polarization inversion is limited to the ab plane,resulting in very low piezoelectric performance(the d33 of the BIT ceramic prepared by the conventional solid phase reaction method is around 8pC/N);Secondly,subject to the higher content of bismuth in the composition,oxygen vacancies are easily generated during high-temperature sintering,resulting in low material resistivity,affecting the stability and sensitivity of charge output at high temperatures.Based on this background,this thesis regulates the oxygen vacancy concentration by B-site doped mixed valence ions,so as to optimize the piezoelectric properties and electrical properties of BIT-based piezoelectric ceramics.In this thesis,BIT-based piezoelectric ceramics were prepared by solid-phase reaction method.By studying the influence of ion valence state,ion radius,electron configuration,electronegativity and other factors under mixed valence ion doping on the micro structure,lattice distortion,defect state and conductive mechanism of BIT-based ceramics,master the regulation methods of Curie temperature,piezoelectric activity and resistivity of BIT-based ceramic doped with mixed valence ions.Based on mastering the laws of physical properties,the preparation has both high Curie temperature(TC>675?),high piezoelectric activity(d33>30 pC/N),and high resistivity at high temperature(resistivity ?>106 ?·cm at 500?)of high performance ceramics.1.The BIT-based piezoelectric ceramics doped with Cu2+/Nb5+,Cu2+/Ta5+,Cu2+/Sb5+,Cu2+/W6+,Cu2+/Mo6 mixed valence ions were prepared by immobilizing low-valent cation Cu2+,to explore the influence mechanism of high-priced cations doped with mixed valence ions on the piezoelectric activity,Curie temperature and resistivity of ceramics.The experimental results show that the B-site doped mixed valence ions significantly improve the piezoelectric activity of BIT-based piezoelectric ceramics.The piezoelectric properties of BIT-based ceramics doped by Cu2+/Nb5+ are as high as 38 pC/N,which is the maximum value of the BIT-based ceramic reported at present.Compared with the undoped,d33 increased by 5 times;Based on the analysis of resistivity and complex impedance measurement proved that oxygen vacancy in BIT-based piezoelectric ceramic dominant role in conductive behavior,and compared with the undoped modified BIT-based piezoelectric ceramics,500 ? resistivity ? improved 2?3 orders of magnitude,especially Cu2+/Sb5+ doping modification of BIT-based ceramic 500? resistivity when ?p=1.40×107 ?·cm.The resistivity ?=1.40×107 ?·cm is the maximum value of the currently BIT-based ceramics;the bipolar fatigue is greatly improved and the comprehensive performance is improved,which is closed related to the decrease of oxygen vacancy concentration;while the piezoelectric properties and resistivity are significantly improved,the Curie temperature Tc is maintained above 672?,and there is no significant deterioration(TC?675? for pure BIT ceramics).2?Take the optimal performance of the above BIT-based piezoelectric ceramics(Cu2+/Nb5+),fix the high-valent cation Nb5+,and prepare Ni2+/Nb5+,Mn2+/Nb5+,In3+/Nb5+,Cr3+/Nb5+mixed valence ion doped BIT-based piezoceramics,the piezoelectric properties was attempted to break through 40 pC/N,and the influence mechanism of low-valent cation doped with mixed valence ions on the piezoelectric activity,Curie temperature and resistivity of ceramics was investigated.The results show that after changing the low-valent cations,the B-site doped mixed valence ions can also greatly improve the piezoelectric activity of BIT-based piezoelectric ceramics.The piezoelectric properties of Mn2+/Nb5+doped BIT-based ceramics can ce as high as 34 pC/N;After changing the low-valent cations,the mixed valence ion doping is performed.The resistivity at 500? is increased by 1?2 orders of magnitude,which is slightly inferior to the previous experimental scheme;When annealing at 600?,the piezoelectric properties can be maintained above 90%;B-doped mixed valence ions have little effect on Curie temperature. |
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