Microstructural effects on dielectric and piezoelectric behavior of calcium-modified lead titanate ceramics
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This work presents an analysis on the main microstructural parameters that affect the dielectric and piezoelectric behavior of ceramics of calcium-modified lead titanate with Ca/Pb = 26/74. To this aim, ceramics were prepared under different sintering conditions to get a series of materials with different microstructures. Compositional and microstructural characterization was achieved by x-ray diffraction, energy dispersion spectroscopy, and optical microscopy. Computerized image analysis was carried out on the micrographs to determine grain and pore size distributions. These distributions were thoroughly analyzed using probability plots. Electromechanical coupling factors and piezoelectric coefficients were measured by the resonance method on thickness poled thin disks and rectangular bars. A similar combined effect of the grain size and the percentage of porosity on the inverse of the permittivity, the coupling factors at room temperature, and the temperature behavior of the electromechanical coupling factor k-n is found.
I. INTRODUCTION The calcium1 and rare earths2 modified lead titanate ceramics were proposed as materials of interest for ultrasonic transducer applications, due to their unusually large ratio between thickness and planar electromechanical coupling factors, k, and kp, respectively. Since the first report1 of the disappearance of kp as a function of the poling field in calcium-modified lead titanate ceramics, these materials have been the focus of extensive research3"6 to understand their high electromechanical anisotropy. The temperature3 and frequency6'7 behavior of the complex dj,\ coefficient, directly related to kp, in Ca and Sm-modified lead titanate ceramics has been explained on the basis of the existence of competing intrinsic and extrinsic contributions to the piezoelectric effect. The 90° domain relaxational behavior accounts8 for part of such extrinsic contributions. It is known that the mobility of the 90° domain walls is affected by the ceramic microstructure9 through the intergranular stresses acting on them. Previous works have shown a clear dependence of the behavior of the planar electromechanical coupling factor on the processing conditions. 410 Therefore, it seems that a precise knowledge of the microstructure of these ceramics is necessary to fully explain their properties. Although there are some examples of microstructural studies on modified lead titanates at high resolution microscopic scale,11'12 scarce attention has been paid to the study of their microstructure at mesoscopic scale, i.e., at an intermediate scale between atomic resolution and macroscopic scale. It is the aim of this study to carry out a detailed microstructural characterization of a series of calciummodified lead titanate ceramics at mesoscopic scale by 3194 http://journals.cambridge.org
J. Mater. Res., Vol. 10, No. 12, Dec 1995 Downloaded: 22 Mar 2015
the combination of optical microscopy and computerized image analysis. The piezoelectric and dielectric behavior of these ceramics has been widely char
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