The Electrical Properties Of High Temperature Bismuth Layer-structured Piezoelectric Ceramics

Bismuth layer-structured piezoelectric materials have a wide application prospect in ferroelectric random-access memories (FeRAMs) because they possess a large remanent polarization and good fatigue properties. At the same time, these kinds of materials are extensive applications on high-temperature and high frequency fields owing to their predominant performance such as high Curie temperature, high resistance, low dielectric permittivity, notable anisotropic electromechanical coupling factor and low rate of aging. Representative bismuth layer-structured piezoelectric materials of CaBi2Nb2O9, Bi4Ti3O12and K0.5Bi4.5Ti4O15(m=2,3,4. m is perovskite layers) were investigated. Thereinto, CaBi2Nb2O9(m=2) has a high Curie temperature of940℃. Bi4Ti3O12(m=3) possesses excellent ferroelectric property. K0.5Bi4.5Ti4O15(m=4) has a high piezoelectric performance with bismuth layer-structured. High temperature Aurivillius ceramics were prepared by conventional solid-state reaction method. In order to get free piezoelectric materials of high Curie temperature and good piezoelectric properties, the microstructure, dielectric, piezoelectric and ferroelectric properties of the ceramics have been systematical studied. The main results are summarized as follows:(1) High temperature piezoelectric ceramics of A-site composite substitution CaBi2Nb2O9(CBN) were synthesized by standard solid state reaction technique, molecular formula was [Ca(1-x)(Na0.5Bi0.5)x]0.9(NaCe)0.05Bi2Nb2O9abbreviated to (CBN-NaBi)(when x=0,0.2,0.4,0.6,0.8and1.0). Piezoelectric properties were studied on A-site composite substitution. XRD showed that the peaks were found to shift to lower angle positions with the increasing of (NaBi)o5, revealing that the introduction of NaBi into CBN materials resulted in slight crystal distortion. Dielectric constant increased with the increasing of x at room temperature. A reduction in Curie point and an increase of a, b and c were evident in the A-site substitution of (NaBi)0.5for Ca. The donor-doped CBN-NaBi (x=1.0) had good piezoelectric properties of a higher piezoelectric constant (d33=24pC/N), mechanical quality factor Qm=3024. dielectric loss tanδ=0.21%, planar electromechanical coupling coefficient kp=9.8%, thickness electromechanical coupling coefficient kt=26.4%. The thermal annealing behavior showed the piezoelectric constant d33except x=0.6remains80%of room temperature. These results revealed that the (NaBi)0.5-modified CBN were very tolerant to thermal annealing, and can potentially be used in high-temperature piezoelectric devices.(2) The fabrication and investigation of substituting Ce for Bi3+ion on the A-site of La3+-doped Bi4Ti3O12structure to form Bi3.25-xCexLa0.75Ti3O12abbreviated to (BLT-Ce)(when x=0,0.005,0.01.0.02and0.03) ceramics were carried out using solid-state reaction method. XRD indicated all samples had an orthorhombic structure with lattice distortion. The value of d33increased with the increasing of CeO2The donor-doped BLT-Ce (x=0.01) had a higher piezoelectric constant (d33=22pC/N) compared with BIT (d33=3.5pC/N), dielectric loss tanδ was only0.21%, Qm got up to4150, Curie temperature Tc was about405℃. The thermal annealing behavior shows the piezoelectric coefficient d33remained almost unchanged (only decreased by10%) at temperatures below400℃. This indicates that the Ce-modified BLT possessed excellent thermal stabilities. The remanent polarization Pτ and the coercive field Ec of BLT-Ce (x=0.0) and BLT-Ce (x=0.01) were9.6and11.0μC/cm2,54.7and70.4kV/cm, respectively.(3) The cobalt-modified potassium bismuth titanate K0.5CoxBi4.5-xTi4O15ceramics abbreviated to (KBT-Co)(when x=0,0.03,0.04,0.05,0.06,0.08and0.1) were synthesized using conventional solid-state processing. XRD indicated all samples had an orthorhombic structure. The peaks for KBT-Co ceramics were found to shift to lower angle positions when compared to pure KBT ceramic, revealing that the introduction of Co into KBT material results in slight crystal distortion. The dielectric, piezoelectric and ferroelectric properties of KBT-Co ceramics were investigated. KBT-Co ceramics had a lower dielectric loss, curie temperature increased with the increasing Co content. The Co modification at B-site induces the distortion of oxygen octahedra, resulting in the enhanced piezoelectric activities. The piezoelectric constant of K.BT-Co5was found to be28pC/N, Curie temperature was575℃. The remanent polarization P, and the coercive field Ec were13.8μC/cm2and63.5kV/cm. Planar electromechanical coupling coefficient kp=3.8%, thickness electromechanical coupling coefficient kt=26.5%, thickness electromechanical coupling coefficientwas about seven times of planar electromechanical coupling coefficient kp, which indicated an evident anisotropic electromechanical coupling coefficient. Electromechanical coupling coefficient (kp, kt) as a function of temperature and thermal annealing analysis revealed the cobalt-modified KBT possesses excellent thermal stabilities.