Study On Modification And Properties Enhancement Of CaBi₂Nb₂O₉ Bismuth Layer-structured Hxgh-temperature Piezoelectric Ceramics

Bismuth layer-structure ferroelectric?BLSF?materials have significant application value in high-temperature piezoelectric sensors,ferroelectric memories owing to their large spontaneous polarization,high Curie temperature,low dielectric loss,good fatigue resistance and low aging rate and other advantages.The structure and chemical composition of BLSFs are closely related to their electrical properties.For example,the ferroelectricity of BLSF is mainly derived from the deviation of B-site cation in the perovskite-like layer(A_m-1B_m O_3m+1)^2-,the oxygen octahedron tilting along the c axis and rotation in a-b plane.However,the piezoelectric property of BLSF ceramics is not high because the rotation of spontaneous polarization is restricted to a-b plane within the crystal structure.It was reported that the electrical properties of ferroelectric materials can be effectively improved by doping elements with similar valence or ionic radius to adjust their chemical composition and microstructure.According to the substitution of elements in perovskite-like layers,A-site,B-site,A/B-site compound modifications are normally involved.In this work,bismuth layer-structure piezoelectric ceramic CaBi₂Nb₂O₉ with high Curie temperature?T_c⁹43??,low piezoelectric constant(d₃₃⁶pC/N)was selected as matrix materials to investigate the enhancement of properties such as piezoelectric property,high-temperature electrical resistivity and the temperature stability by modification.Firstly,bismuth layer-structure piezoelectric ceramics of CaBi₂Nb₂O₉+xwt%Bi₂O₃?CBN+xwt%Bi,x=0,1,2,3?ceramics were synthesized by solid-state reaction method.The effect of Bi excess content on the structure,microstructure,and electric properties of the ceramics was investigated.The results indicate that the excess introduction of Bi does not change the orthorhombic bismuth layer-structure,and the grain size of the ceramic decreases and becomes more uniform by appropriate addition of excess Bi.The piezoelectric,electromechanical,dielectric properties and density of the ceramic are significantly improved,and the dielectric loss is reduced.The sample with x=1 exhibited the optimum properties with good thermal and frequency stability:d₃₃=6.4pC/N,k_p=11%,Q_m=7890,?^T₃₃/?₀=93.5,tan?=0.24%,T_c=940.Secondly,basing on the composition of CaBi₂Nb₂O₉+1wt%Bi₂O₃,the rare earth element Ce of suitable for twelve coordination was introduced to the A-site of the crystal lattice of the ceramics,i.e.,Ca_1-x-x Ce_x Bi₂Nb₂O₉?CCBN,x=0.00,0.02,0.03,0.04,0.06,0.08?ceramics were synthesized by solid-state reaction method.The effect of Ce doping on the structure,microstructure,piezoelectric,dielectric properties and conductivity of the ceramics was investigated.The results indicate that all the ceramic samples have a single bismuth layer-structure.The plate-like grain growth of the ceramic was inhibited by appropriate addition of Ce,and the piezoelectric,dielectric properties,density of the ceramic were significantly improved,as well as the dielectric loss and sintered temperature were reduced.The sample with x=0.04 exhibited the optimum properties:d₃₃=10.3 pC/N,k_p=11.5%,?^T₃₃/?₀=106,tan?=0.28%.The resistivity at 500?was still higher than10⁷?·cm,showing good thermal and frequency stability.Finally,basing on the composition of Ca_0.96Ce_0.04Bi₂Nb₂O₉,the transition metal element W of suitable for six coordination was introduced to the B-site of the crystal lattice of the ceramics,i.e.,Ca_0.96Ce_0.04Bi₂Nb_2-x-x W_x O₉?CCBNW,x=0.00,0.01,0.02,0.03,0.05,0.07?ceramics were synthesized by solid-state reaction method.The effect of Ce and W doped on the structure,microstructure,piezoelectric,dielectric properties and conductivity of the ceramics was investigated.The results indicate that all the ceramic samples have a single bismuth layer-structure.The grain growth of the ceramic along the a-b plane direction was promoted by appropriate addition of Ce and W.The sample with x=0.02 exhibited the optimum properties with good thermal stability:d₃₃=13.4 pC/N,k_p=12.2%,?^T₃₃/?₀=111,tan?=0.13%,and the resistivity at 500?is as high as 3.7×10⁹?·cm.