High Voltage Coefficient Piezoelectric Materials For Underwater Transducer Applications

An investigation into the piezoelectric voltage coefficient (gij) has been carried out from a fundamental materials science perspective. For single crystals, the tetragonality is found to be the structural driving force for high polarisation and gij coefficients. Textured ceramics also exhibit high gij values due to the degree of grain and polarisation orientation. For polycrystalline ceramics, the presence of grains and grain boundaries and the associated elastic stresses suppresses gij at high tetragonalities.
Landau-Devonshire theory has been applied to BiFeO3-PbTiO3 to highlight its potential as a high gij candidate. Initially the compositional dependence is modelled by applying Vegard's law between the Landau coefficients of each end member. Whilst this manages to replicate the experimentally constructed phase diagram, the spontaneous strain behaviour is incorrectly described. The second method models the compositional dependence as an external tensile stress which acts to elongate the unit cell within the tetragonal phase, and as a shear stress within the rhombohedral region to reduce the angle between the polarisation and [001]. Using this method, the g33, g31, g15 and gh for 0.70BiFeO3-0.30PbTiO3 are calculated to be 0.208, -0.061, 0.078 and 0.087 Vm/N, respectively; significantly larger than current military grade devices and previously reported bulk materials. Nb-doping of 0.65BiFeO3-0.35PbTiO3 has been carried out to reduce conductivity. For all doping regimes, the low-signal AC and DC conductivity decreases by up to an order of magnitude. Initially Nb-doping hinders the piezoelectric activity however once annealed, g33 values improve by up to 10 times for some compositions. Impedance spectroscopy, P/x-E loops and X-ray photoelectron spectroscopy show that the piezoelectric behaviour is dictated by the degree of defect distribution, with annealed samples possessing a more distributed defect network and an increase in domain wall mobility. A cooling rate study showed that whilst a more randomly distributed defect structure is achieved compared to unannealed samples, quenching decreases the piezoelectric activity due to the larger intergranular stress when transitioning through the Curie temperature.