Preparation And Properties Of Rare Earth-doped Lead-based Titanate Piezoelectric Ceramics

As ferroelectric material with tetragonal perovskite phase structure, lead titanate (PbTiO₃, PT) has many excellent physical and electric properties, including high Curie Temperature, high c/a ratios, low dielectric constant, large piezoelectric anisotropy and well stability etc, which make it very attractive for a number of different applications as significant piezoelectric and pyroelectric materials. However the large tetragonality results in a large stress within lattice, it is not easy to prepare pure and dense PT ceramics. In addition, it is difficult to pole these ceramics due to low resistivity. Therefore a wide variety of applications have been greatly limited to some extent. In order to improve the properties of PT, a lot of studies have been reported. There are two methods, one is to minish the grain size of PT, the other is to modified PT.Compared with these two methods, the properties of rare earth modified PT fine powders can be enhanced greatly. Modified PT which is characterized by remarkable ferroelectric, piezoelectric and pyroelectric properties has highly versatile applications especially in high temperature and high frequency transducer applications for example as piezoelectric transducers, pyroelectric detectors, infra-red sensors, sonar aspect superior and piezoelectric ceramic step-down transformers etc.Recently, a number of techniques have been developed to prepare lead-based titanate powders and ceramics such as conventional solid state reaction, sol-gel process, chemical coprecipitation process, chemical vapor deposition as well as hydrothermal process etc. Among them, hydrothermal process provides an inexpensive, low temperature and environmentally friendly route, because the crystalline powders are directly formed in the hydrothermal treatment. During this processing such low temperature can not only reduce the loss of lead but also eliminate the calcination step, the resulting aggregation and the subsequent millingprocess. So in this paper we introduce hydrothermal process to synthesize rare earth modified PT powders, and then prepare ceramics by conventional sintering method.The main content in this paper includes:1. Preparation of La-doped, Ce-doped PT fine powdersHydrothermal synthesis has been utilized for the preparation of La-doped, Ce-doped PT powders respectively. The influences of dopant content, reaction time and the concentration of NaOH on the grain size, morphology of PT powders have been studied. It can be seen from the results of the XRD analysis that neither La nor Ce did enter the PT lattice. It is proposed that on this condition La and Ce was dispersed around the grain boundary in amorphous form. However, there are some differences between La-doped and Ce-doped. From SEM and calculation of grain size associating with XRD, the breakdown of conventional ferroelectric phase transition behaviors in lanthanum modified lead titanate ceramics has been correlated with the appearance of domain structures exhibiting texture on a nanometer scale, while in Ce-doped process this phenomenon did not happen. Moreover the gain size reaches minimum at 5 mol% lanthanum composition.2. Preparation of La-doped, Ce-doped PT fine ceramicsWe obtained La-doped PT fine ceramics using conventional sintering method. The borderline reaction conditions, such as the sintering temperature, the sintering time and dopant content for the formation of La-doped, Ce-doped PT ceramics and the effects on the phase, morphology and crystal lattice were investigated. The results reveal that La has been solved into the lattice deriving PLT ceramics by sintering. Meanwhile more and more La enters the lattice if we increase the sintering temperature and prolong the sintering time. As the same time Pb will volatize when the temperature reaches 1000V and the production changes into rutile TiO₂.Whereas on this condition Ce still has not entered the lattice. Due to Ce-dopedthere were remarkable changes to morphology and it appeared more and more deepholes with the increasing of the Ce concentration.