Effect of lead content on the structure and piezoelectric properties of hard type lead titanate–zirconate ceramics
Introduction
Solid solution of lead zirconate–lead titanate ceramics, known as PZT (Pb(Zr,Ti)O3 compounds modified with izo or aliovalent ions, in order to improve their basic properties, have become the most important materials in an increasing range of application as sensors and transducers. Their excellent electromechanical properties make them the key candidates for power ultrasonic transducers, actuators for fine displacements systems, piezoelectric motors, medical transducers for diagnosis and ultrasonic imaging.1 The performance requirements for transducers implies similar requirements for piezoelectric materials. Hence high care must be taken in controlling the material properties by a careful control of processing factors such as composition, structure and heat treatment. One of the problem encountered in obtaining high quality PZT materials is the compositional control and stability due to the fact that at high temperature, one of the basic component, PbO, volatilizes in a uncontrollably manner and so the stoichiometry of the final product is altered.2 Some studies regarding the effect of compositional fluctuations due to the PbO loss on material properties were already reported.3, 4 More recently Fernandez et al.5 studied in details the origin of the decomposition mechanism and its relationship with compositional fluctuations and the final properties of an iron-doped PZT material. They suggested that the dopant incorporation into the crystalline lattice of the material represents the main phenomenon which controls the nature and evolution of foreign phases formed by PbO loss and the change of the properties of iron-doped PZT. This process is temperature dependent, being more intense at high temperature. Unfortunately, there is not yet known a relationship between temperature and PbO loss in PZT type ceramics, because there are many other factors of influence such as the nature and amount of other additives used,6 the samples environments (sintering atmosphere) during sintering. There are a few ways to avoid as much as possible the PbO volatilization. The usual practice is to protect the samples during sintering either by covering them with a blanket of a similar presintered powder able to create a rich PbO atmosphere during sintering7 or by tightly sealing the crucibles in which the samples are sintered. Another way is to add small amounts of PbO in excess to the basic composition8, 9, 10, 11, 12 or to use sintering aids to lower, to some extent, the sintering temperature.13, 14, 15, 16, 17, 18, 19, 20, 21, 22 Unfortunately, though helpful, neither way can provide a satisfactory solution to the problem but from the practical point of view the best approach still remains the control of stoichiometry by adding a certain amount of PbO before or after calcining, as a means to find experimentally the optimum processing conditions for a given material.
In a previous paper9 we investigated the effect of small amount of lead oxide on the properties of a soft type PZT ceramics. In the present investigation we extended the study regarding the effect of lack and excess of PbO, compared to the stoichiometric formula, on the properties of a hard type PZT ceramics.
Section snippets
Experimental
The PZT material investigated has the general chemical formula: Pbx(Mn0.017Sb0.033Zr0.48Ti0.47)O3 with 0.96 ≤ x ≤ 1.04. We prepared samples with 4%, 3%, 2% and 1% lack of Pb and 1%, 2%, 3% and 4% excess Pb, respectively, with respect to the stoichiometric composition with x = 1. The processing procedure was the conventional mixing oxide route, using p.a. purity raw oxides. The weighted amounts of oxides were mixed for 2 h in a planetary ball mill using agate jars and methanol as wetting media. The
Results and discussion
The X-ray diffractograms of the ceramic samples, containing different amount of Pb, are shown in Fig. 1. One observes that for lead-deficient compositions, along with the known perovskite phase, another foreign phase appeared. This foreign phase may possible be pyrochlore phase. This phase decreased with increasing lead content and completely disappeared for stoichiometric composition and for those containing excess lead. In turn for excess lead compositions new peaks appeared, which were
Conclusion
The structure, ferroelectric, dielectric and piezoelectric properties of a hard type piezoceramic material were determined as a function of the lack and excess lead oxide within −4 up to 4% molar compositional range. The lead-deficient samples showed the presence of a pyrochlore type foreign phase while the excess lead ones showed only the PbO foreign phase. Maximum values for all materials parameters were recorded in all samples containing 2 mol% excess lead, which means that this amount of
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