Ferroelectric Behavior and Microstructure of Calcium-Modified Lead Titanate Ceramics
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Zhiqiang Zhuang*, G.A. Kulesha,*" H. Du,** and B. Gallois'* " South China University of Technology, Department of Inorganic Materials Science and Engineering, Guangzhou, China 510641 "*Stevens Institute of Technology, Department of Materials Science and Engineering, Hoboken, NJ 07030, ABSTRACT
Calcium-doped lead titanate ceramics exhibiting piezoelectric anisotropy were fabricated for applications in ultrasonic transducers, infrared detectors and surface acoustic wave devices. Transmission electron microscopy and X-ray diffraction techniques were used to characterize the development of the piezoelectric anisotropy and the dependence of the piezoelectric anisotropy on microstructure. Electron diffraction patterns indicated that most of the ferroelectric domains in samples with [CaA] up to 30 mole% were 900 domains. The size of the ferroelectric domains was not related to piezoelectric anisotropy. Ferroelectric domains could not be detected above this concentration. Measurements of the temperature dependence of dielectric properties at different frequencies did not show any ferroelectric relaxor behavior. INTRODUCTION
Piezoelectric lead zirconate titanate (PZT) ceramics are widely used in ultrasonic transducers," 2 surface acoustic-wave devices3 and medical imaging systems.4'5 In applications such as non-destructive testing of metals, medical diagnostic procedures using phase-array devices
and hydrophones, piezoelectric transducers which exploit the thickness vibration mode should exhibit an electromechanical coupling ratio, k/kp, as large as possible to avoid coupling of transverse to thickness vibrations.' Materials with high piezoelectric anisotropy are also needed for very high frequency applications. Intensive studies of materials which exhibit piezoelectric anisotropy have been carried out in the last ten years.67 Even though some compositions with very high piezoelectric anisotropy have been developed, the effects of microstructural features on piezoelectric anisotropy have remained elusive. No mechanistic understanding has been achieved of the piezoelectric anisotropy induced by dopants in solid solutions of lead titanate (PT), lead zirconate and PZT. This research was carried out to study the effects of composition and microstructure on the piezoelectric and dielectric properties of calcium-doped lead titanate ceramics. Materials showing high piezoelectric anisotropy and a pure thickness vibration mode have been produced which are ideally suited to the fabrication of ultrasonic transducers for the measurement of the thickness of metals. *
A visiting professor at Stevens Institute of Technology
9 Mat. Res. Soc. Symp. Proc. Vol. 360 01995 Materials Research Society
EXPERIMENTAL PROCEDURE An oxide-mixing technique was employed to prepare the calcium-doped lead titanate (CPT) ceramic samples. The starting chemicals were reagent-grade lead oxide, titanium oxide and zirconium oxide. The weighted oxide powders were mixed with distilled water in an alumina ball-mill, dried, cold-pressed into pellets and sinte
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