Obtaining and characterization of single crystals and ceramics of PZT

The electromechanical properties of lead zirconate titanate PbZrxTi1-xO3 (PZT) materials achieve maxima values in the morphotropic phase boundary. The present research was stimulated by the interest on knowing the causes for these maxima and their dependence on the microstructure. To know the causes that provoke the appearance of a maximum in the electromechanical properties, it was necessary to study ceramics with compositions around the morphotropic phase boundary. The ceramics were characterized from the dielectric and structural viewpoints and the maxima in the morphotropic phase boundary were confirmed, as well as their dependence on the grain size. The position of the maximum of the permittivity is related to the point where the phase transition from the rhombohedral structure to tetragonal one occurs in single crystals. For understanding the intrinsic properties of the PZT it was necessary to grow single crystals with suitable dimensions and adequate quality for measurements of the properties. In the present work, PZT single crystals were grown with sufficient size and quality, using a high temperature solution method, with a flux of [PbO-KF-PbCl2]-B2O3 in a molar ratio of 60/40 between PZT and flux. Initially, the conditions of processing were optimized, followed by different profiles of temperature and flux amounts, to promote the crystal growth and quality of the PZT crystals. The conditions identified as best were used to grow PZT crystals. The crystals obtained showed a cubic morphology with typical size of  3  3  3 mm. The morphotropic region comprises the rhombohedral and the tetragonal fields and the boundary between them. Its characterization was performed mainly on analysing five crystals, by selecting two of them of the rhombohedral field, other two of tetragonal field and the other one with the chemical composition of the boundary between those two fields. PZT crystals were characterized using dielectric, ferroelectric and piezoelectric measurements, which were carried out either parallel to c axis ([001] direction) or perpendicular to the plane ab (110) (in accordance with the x-rays diffraction analysis). The x-rays diffraction measurements were carried out at 180 K for all the crystals and also at 300 K whenever necessary. In this study, two modifications of the phase diagram were introduced. The low-temperature rhombohedral phase (R3c) does not appear for Zr composition less than 65% and, on the other hand, the morphotropic region does not exist in single crystals. Saturated hysteresis loops were observed when an alternative electric field was applied perpendicular to the ab plane of the crystal. The spontaneous polarization in the direction of the ferroelectric axis was estimated to be  40 C/cm for a crystal with 50% of Zr. The piezoelectric coefficient d33 was measured in the [001] direction (polar axis) and is  20 pm/V for a crystal with 50% in Zr. The domain distribution was studied and it was observed that the number of domains increases as the size decreases near the morphotropic boundary. The domain pattern varies from a state of definite visibility for high Ti concentration, to a complete disappearance on approaching the centre of the morphotropic region. The maximum of the electromechanical properties of this material is explained in this work based on a decrease of the distortion of the tetragonal structure which becomes unstable near the centre of the morphotropic region and where a small electric field changes the structure from one phase to another. This fast change between phases provokes great variations in the polarization and hence increases the dielectric and electromechanical properties. The present work also studied the possibility of using PZT crystals as seeds to induce the grain growth in PZT ceramic by the seeded polycrystalline conversion method (SPC). PZT ceramic with large grains were produced using seeds with average size of ~ 30  30  30 m. A small amount of seeds was dispersed in a fine particle matrix of PZT, containing an excess of liquid phase. The effects of the different parameters such as composition, excess of liquid phase and sintering conditions were studied with the purpose of obtaining dense ceramic with large grains.