Pyroelectricity versus conductivity in soft lead zirconate titanate (PZT) ceramics
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The electrical behavior of modified soft lead zirconate titanate (PZT) ceramics has been studied as a function of temperature at different direct current (dc) electric fields and grain sizes. As ferroelectrics, such as PZT, are highly polarizable materials, poling, depolarization, and electric conduction contribute to the total electrical current, which leads to anomalous electrical behavior as a function of temperature. The PZT appeared to have a high pyroelectric coefficient, and it was found that the displacement current hides the conduction current near room temperature. The (long-time) steady-state electrical resistivity of the soft PZT used has a typical, relatively high value of 3.6 × 1012 ⍀·cm near room temperature. The resistivity above the Curie temperature was two orders of magnitude lower than the room temperature. The resistivity decreases with increasing grain size probably due to the increased Pb vacancy concentration resulting as a consequence of a higher sintering temperature. The values of activation energies suggest that the charge carriers at high temperature are mainly oxygen vacancies. At intermediate temperature, the electrical behavior is controlled by the counteracting effect of depolarization and conduction. Considering the pyroelectric effect and the conduction, it was thus possible to explain the electrical behavior of this soft PZT ceramic over the temperature range considered. I. INTRODUCTION
High insulation resistance is desirable for many technological applications of lead zirconate titanate (PZT) piezoceramics. The electrical behavior of inorganic materials is generally controlled by several effects, the most important being the conduction, the degradation of the resistivity, and, if appropriate as in the case of PZT, the pyroelectric effect. We briefly discuss these effects first. A. Doping-dependent resistivity
In ceramics, defects—which can serve as donor or acceptor—contribute to the extrinsic electrical conduction. The major defects in PZT ceramics are lead and oxygen vacancies caused by the volatility of lead oxide during sintering.1–3 Gerson4 explained the conduction mechanism in PZT ceramics. The p-type conduction is caused by the Pb vacancies, and the charge carriers (holes) are generated by the ionization of Pb vacancies.4 Alternatively, doping is another mechanism for introducing defects. When lower valence ions enter the lattice, the charge deficit is compensated for by oxygen vacan-
a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2007.0438 3448
http://journals.cambridge.org
J. Mater. Res., Vol. 22, No. 12, Dec 2007 Downloaded: 26 Jun 2014
cies,5–7 which are also responsible for the resistance degradation.8,9 When higher valence ions enter the lattice, excess charge is compensated for by vacancies in the perovskite structure. This process eventually leads to a reduction in the charge carrier concentration, and accordingly the resistivity is enormously increased.4–7,10,11 B. Resistance degradation
Resistance degradation is ch
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