Electrical Properties And Piezoelectric Mechanism Of Co-Doped BaTiO₃ Ceramics

Ba Ti O3(BT) ceramics are widely used in a variety of applications such as PTC thermistors, multi-layer ceramic capacitors(MLCC) and ferroelectric random access memories(FRAM), because of its outstanding dielectric and ferroelectric properties. However, there are still many solved problems for BT ceramics. First of all, it is very difficult to obtain high d33 values of more than 300 p C/N just by using ordinary raw materials and a conventional sintering method. Secondly, up to now, the basic approach to the design and optimization of new ABO3-related piezoceramics has been to induce a phase transition between two ferroelectric phases, as exemplified by morphotropic phase boundary(MPB) and polymorphic phase transition(PPT). The question now arises as to whether or not there is a new high piezoelectric mechanism waiting to be found. Finally, the BT composite ceramics will sacrifice their piezoelectric properties for the enhancement of mechanical properties in most cases. Thus there is an urgent need to develop BT ceramic with enhanced piezoelectric and mechanical properties.In the case of LAS/BT ceramics, Li+ and Al3+ ions, both of which have relatively small ionic radii, substitute the A-sites occupied by the Ba2+ ions of the ABO3-type perovskite structure, while Si4+ ions replace the B-sites occupied by Ti4+. Li+ and Al3+ ions maybe form Li+-Al3+ pairs within the same cell in order to minimize electrostatic energy, and these Li+-Al3+ pairs tend to adopt a preferential alignment such that most of them are parallel to the [001] direction. The piezoelectric properties and depolarization temperature(Td) of BT ceramics have been improved remarkably via LAS doping. The optimum piezoelectric properties, including d33=378p C/N, kp=0.41 and Qm=157, are obtained at the composition of x=7.5mol.% at the sintering temperature 1350 oC. The Td value Td of approximately 120 °C is very close to the Curie temperature of these ceramics. In addition, the phase transition behavior of BT ceramics has been modified by the introduction of LAS. To clarify the origin of the high piezoelectricity in Li+-Al3+ pairs doped BT ceramics, a possible piezoelectric mechanism is proposed.On the basis of the research work of LAS/BT ceramics, the effects of the excess Li and Al together with the excess Si on the phase, microstructure and electrical properties of BT ceramics were investigated and analyzed. It is found that the 2Pr value increases with the increasing Li and Al contents. As Si content increases, the 2Ec and 2Pr values of 96/4BLATSΔ ceramics increases. The piezoelectric constant of BT ceramcis can be improved by doping a small amount of Li, Al and Si.In the case of Al N/BT ceramics, Al3+ and N3- ions should form Al3+-N3- pairs in the same cell in order to minimize the electrostatic energy. The most energy-favored replacement way is Al3+ for Ba2+ sites and N3- for O2- sites. As x≤1.5mol.%, the Al3+-N3- pairs tend to adopt a distribution vertical to c axis. In addition, the preferential distribution of Al3+-N3- pairs perpendicular to c axis is on the decline as the sintering temperature increases. The optimum piezoelectric properties of d33=305 p C/N, kp=0.33 and Qm=135 are obtained for 1.5mol.%Al N/BT ceramics sintered at 1325 oC. The high piezoelectricity and high mechanical properties can be simultaneously enhanced by adding an appropriate Al N content. The BT-1.5%Al N ceramic sintered at 1325 oC exhibits a high piezoelectric coefficient of 305 p C/N and an improved Vickers hardness of 5.9 GPa.The solubility limit of Al N in BT lattice can be enhanced by Si O2 doping. The optimum electrical properties of 2Pr=20.0μC/cm2 and d33=297 p C/N are achieved at x=1.5mol.% and y=1.5mol.% due to the preferential distribution of Al3+-N3- pairs vertical to c axis.