Thermodynamic Analysis of the Hysteresis Offsets from Polarization Graded Ferroelectric Materials
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U3.10.1
Thermodynamic Analysis of the Hysteresis Offsets from Polarization Graded Ferroelectric Materials Z.-G. Ban and S. P. Alpay Department of Metallurgy and Materials Engineering and Institute of Materials Science University of Connecticut, Storrs, CT 06269 J. V. Mantese Delphi Research Laboratories, Shelby Township, MI 48315 ABSTRACT Polarization graded ferroelectrics exhibit unconventional electrical properties that are not usually observed from homogenous ferroelectrics. Systematic spatial variations in the polarization in a ferroelectric material can be achieved by composition, temperature, and stress gradients; resulting in the displacement of the polarization vs. electric field hysteresis curve along the polarization axis. In this paper, this unusual phenomenon of hysteresis offset has been examined in BaTiO3 and BaxSr1-xTiO3 material systems using a Landau-Ginzburg phenomenological model for the first time. It is shown that the spatial non-uniformities can give rise to non-uniformities in polarization with corresponding spatial variations. This non-uniform polarization results in asymmetrical hysteresis with “up” or “down” charge offsets which are strongly dependent upon the magnitude and direction of the temperature, composition and stress gradients for three graded ferroelectrics. INTRODUCTION In recent years, there has been great interest in graded ferroelectrics as they exhibit behaviors and properties that are not observed from homogenous bulk or thin film ferroelectrics. Polarization graded ferroelectrics are distinguished from homogenous ferroelectrics by a spatial variation of the electric dipole moment in the graded ferroelectrics [1]. In addition, unlike homogenous ferroelectrics (which are characterized by symmetric hysteresis loops with respect to the polarization and applied field axes) graded ferroelectric devices display strikingly new behavior; the most notable being a translation of the hysteresis loop along the polarization axis [2-4]. The shifted charge-voltage hysteresis (“up” and “down”) loops are attributed to “built-in” potentials, in analogy with the asymmetric current-voltage characteristics resulting from the “built-in” potential across chemically doped regions in semiconductor diode junctions. The graded structures, therefore, have given rise to a new class of trans-capacitive ferroelectric devices (or “transpacitors”), having potential applications in infrared detection, actuation, and energy storage. However, while much is known experimentally concerning graded ferroelectric structures, theoretical descriptions and analysis of these or other transpacitive devices have been significantly lacking. This paper attempts to develop a common theoretical description for graded ferroelectric devices and structures. Starting from a generalized Landau-Ginzburg model we show that we can provide a quantitative theoretical analysis of the offset hysteresis behavior of polarization graded ferroelectric materials.
U3.10.2
THEORY A systematic variation in the polarization can be achieved (
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