Structure-property relations in Sr, Nb, Ba doped lead zirconate titanate.

Abstract

rhombohedral or tetragonal forms or as mixture of the two (MPB), depending on Zi:Ti ratio. Zr:Ti ratio strongly affected d sub 3 sub 3 , which was maximised in the tetragonal phase close to, but not at, the MPB. Sr sup 2 sup + substitution on the A-site promoted tetragonality in PZT, greatly reducing T sub C , and broadening the dielectric maximum. As the Sr sup 2 sup + content was increased, Zr:Ti ratio was adjusted to maximise d sub 3 sub 3 and the optimised d sub 3 sub 3 values increased from 410 pC/N (Sr sup 2 sup + = 0) to 640 pC/N (Sr sup 2 sup + = 0.12), commensurate with a decrease in the T sub C. However, for ceramics where Sr sup 2 sup + > 0.12, optimised d sub 3 sub 3 decreased with respect to the values for ceramics where Sr sup 2 sup + = 0.12 even though T sub C was lowered. Electron diffraction patterns revealed superlattice reflections occurring at 1/2 left brace hkl right brace positions associated with rotations of oxygen octahedra in anti-phase. It was suggested that Sr sup 2 sup + substitution on the A-site decreased the tolerance factor t, resulting in the onset of oxygen octahedral tilting. Co-doping PZT with Sr sup 2 sup + and Ba sup 2 sup + on the A-site resulted in the disappearance of the 1/2 left brace hkl right brace superlattice reflections. However, the d sub 3 sub 3 was not improved. Evidence of relaxor behaviour revealed by TEM in Sr, Ba co-doped PZT was thought to be responsible for the deterioration in piezoelectric properties. Effect of sintering temperature on the decomposition of perovskite phase was also examined. PbO loss was detected in Sr-doped PZT (PSZT) at the sample surface >= 1170 deg C, which was accompanied by the formation of a second phase. The second phase was identified as monoclinic ZrO sub 2. An increase in degree of tetragonality was also observed in the perovskite matrix. Lead zirconate titanate (PZT) ceramics have been utilised for several decades to fabricate electromechanical sensors and actuators. Compositional modifications have led to the development of 'hard' and 'soft' PZT's. Soft PZT's are used in applications such as receiving transducers requiring high sensitivity, pulsed transmitting transducers with high acoustic outputs, high sensitivity receivers and actuators with large displacements. In this investigation a systematic study was performed on a range of soft PZT materials. Their structures, microstructures and domain structures were characterised using X-ray diffraction, scanning and transmission electron microscopy. This data was then used to interpret dielectric, ferroelectric and piezoelectric properties. The relative importance of three known softening methods in PZT was assessed: i) donor doping using Nb sup 5 sup + on the B-site, ii) proximity to a morphotropic phase boundary (MPB), and iii) lowering the paraelectric-ferroelectric phase transition temperature (T sub C) by substitution of Sr sup 2 sup + or Ba sup 2 sup + on the A-site. Nb sup 5 sup + substitution onto the B-site reduced the grain size although the domain structure (approx 100 nm wide) remained the same except finer-scale domains (approx 20 nm wide) and higher dislocations were present in ceramics with 7.2 mol% Nb sup 5 sup +. T sub C was lowered and the piezoelectric coefficient (d sub 3 sub 3) increased. Nb-doped PZT could be stabilised either i