Modeling ferroelectric hysteresis dynamics in lead-free materials

Zizhao Zhao, Purdue University

Abstract

A Landau-Ginzburg-Devonshire phenomenological theory was developed and numerically implemented by explicitly defining the underlying coefficients as a function of observable experimental hysteresis loop data. For polycrystalline materials, spatially resolved numerical simulations identify and quantify the appearance of three polarization populations, namely, simple switching, negatively switching and pinned domains, to describe the microscopic switching of ferroelectric domains. The electric load ratio, i.e., RE = Emin/E max, is defined to describe different sesquipolar cycling conditions. In comparison to the RE = -1 (bipolar) regime, tensile stresses are minimized in the -1 < RE < 0 (sesquipolar) regime by 33% and compressive stress minimized by 38%, while the maximum strain output decreases by only 1%. Additionally, calculations predict that a first-order transition from ferroelectric to relaxor develops at σc = 15 MPa. Results further demonstrate that a metastable state develops at zero polarization when the sample is subject to external compressive stress, which is a result of a local free energy minimum. The frequency dependent hysteresis response shows that a transition between relaxor ferroelectric and antiferroelectric develops at a critical cycling frequency, in agreement with previous publications. Numerical simulations predict that the built-in electric field increases quadratically as a function of applied compressive stress fields, in agreement with experiments. Such a trend is a result of thermal expansion and piezoelectric contributions for small compressive stresses, and a result of electrostrictive contributions and stress-induced phase transformations for large stresses. Finally, the effect of temperature on the electrical and electromechanical response demonstrates a phase transition occurs at Td = 294 K for BNT-BT lead free system. Frequency dependent hysteresis behavior show that a maximum strain output is obtained at T ≈ 320 K in a finite frequency range of 0.1Hz to 1Hz. A composition-temperature phase diagram that extends the published work is proposed, in which the morphotropic phase boundary extends to included x = 0.07 composition and the KNN modification suppresses the transition temperature from relaxor to antiferroelectric regime, in agreement with published work. The calculated composition-temperature hysteresis behavior demonstrates a strong frequency dependence, which can be tuned to meet electromechanical device specifications.

Degree

Ph.D.

Advisors

Garcia, Purdue University.

Subject Area

Materials science

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