Structure And Electrical Properties Of Strontium Niobate Perovskite Layer Structured Piezoelectric Ceramics

The perovskite layer structured?PLS?compounds have been paid more and more attention as one of the most promising candidates for high temperature piezoelectric vibration sensors due to their extremely high Curie points,very high DC resistivity and excellent temperature stability.Sr₂Nb₂O₇ as one of the most typical PLS compounds possess ultra high Curie point of 1342oC and high resistivity of 10⁷?·cm at 600oC.These superior electrical properties of Sr₂Nb₂O₇ enable it to be used for sensors in aerospace,nuclear plants and automotive industries for structural control and health monitoring.However,the major problems concerning Sr₂Nb₂O₇ system are its low bulk density which makes it impossible to achieve the efficient poling and very small piezoelectric constant(d₃₃ of 1³pC/N even with textured techniques).In present paper,aimed at the problems above,we studied processing technologies,compositions and grain size effect on sinterability,crystal structure,and modified electrical properties.The first principle and variable temperature XRD were used to analyze the paraelectric-ferroelectric phase transition,which dicussed the origin of perovskite layer structured ferroelectric spontaneous polarization.In addition,the antiferroelectric-like behavior of PbO doped Sr₂Nb₂O₇ ceramics was obtained,and the internal mechanism of antiferroelectric-like effect was proposed.The introduction of CuO addition has significant effects on the sinterability,microstructure and electrical properties of Sr₂Nb₂O₇ ceramics prepared by solid state reaction method.The 0.5wt%CuO low melting fluxes is beneficial to the liquid phase bridge formation at sintering process,leading to lower sintering temperature of 1180oC,better sinterability and higher bulk density up to 98%.The T_c values of Sr₂Nb₂O₇-0.5wt%CuO ceramics are?1340±2?°C and the d₃₃ is?1.1±0.1?pC/N.Sr₂Nb₂O₇-xwt%CuO?x=0.3,0.5 and 0.7?ceramics shows enhanced ferroelectric properties with a larger P_r of⁴.1?C/cm² and a smaller E_c of⁶3.1kV/cm.It was found that dielectric breakdown strength increases up to 258.8kV/cm and then decreases gradually with the increase of CuO content.The?1-x?Sr₂Nb₂O₇-x(Na_0.5Bi_0.5)TiO₃?SNO-NBT?ceramics prepared by solid state reaction method were firstly fabricated.The addition of NBT is beneficial to speed up mass transfer and particle rearrangement during sintering,leading to better sinterability and higher bulk density up to 96.8%.The solid solution limit x in the SNO-NBT system is below 0.03,over which Ti⁴⁺is preferable to aggregate and results in the generation of secondary phase.After the modification by NBT,all SNO-NBT ceramics have a Curie temperature T_c up to over 1300°C and piezoelectric constant d₃₃ about 1.0pC/N.The breakthrough of piezoelectricity can mainly be attributed to rotation and distortion of oxygen octahedron as well as higher poling electric field resulting from the improved bulk density.Grain size effect on piezoelectric properties and thermal stability of perovskite layer structured Sr₂Nb₂O₇ ceramics are investigated.The Sr₂Nb₂O₇ ceramics with different average grain sizes from 1.2?m to 3.6?m were prepared in different sintering temperature by solid state reaction method.The average grain size increases,accompanied by a higher relative density up to 96%.Pure Sr₂Nb₂O₇ ceramics with larger grain size show a remarkable d₃₃ of?1.7±0.1?pC/N while still with a very high T_c of?1340±4?°C.The thermal depolarization temperature of samples with large grain sizes reach over 1200°C and the thermal stability is increased with increasing of grain size.The ferroelectric domains structure was observed by PFM and lager grain is easy to form ferroelectric domain then enhance piezoelectric properties.Further study showed the temperature dependence of unit cell parameters a,b,c and volume V changed in three stages by variable temperature XRD.By comparing the crystal structure of ferroelectric and paraelectric phase,it can be found the NbO₆oxygen octahedron rotated significantly along the c axis.According to Glazer's law,the rotation mode of oxygen octahedron is a⁰b⁰c⁺,which indicate a⁰b⁰c⁺the phase transition pathway of paraelectric-ferroelectric phase transition and origin of the ferroelectricity of Sr₂Nb₂O₇.Meaningwhile,the origin of ferroelectricity of Sr₂Nb₂O₇ can be investigated by density functional theory?VASP,PAW,GGA+PBEsol?and phonon mode calculation?Phonopy,DFPT,2×2×2 supercell?.Theoretical spontaneous polarization values were calculated to be 7.75?C/cm²,slightly lower than experimental values of 10?C/cm².The contribution of atomic displacement to ferroelectric soft mode and polarizability and the phase transition pathway of Cmcm-Cmc2₁ phase transition were determined.Finally,a novel perovskite layered structured Sr₂Nb₂O₇ ceramic by PbO doping was successfully synthesized via solid solution reaction.Double P-E hysteresis loop suggested antiferroelectric-like nature.A combination of low frequency and terahertz band dielectric measurements under applied DC field reveal field induced transition from antiferroelectric to ferroelectric structure.All the results show antiferroelectric-like behavior be related to the electron structures of Pb²⁺with a 6s² lone pair.Our results gave us a new concept provide possibilities for the design of antiferroelectric-like materials in layer structured compounds with super high ferroelectric Curie point.