Field-induced strain associated with polarization reversal in a rhombohedral ferroelectric ceramic
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The polarization reversal process in a rhombohedral ferroelectric ceramic material was investigated using field-induced strain measurements and texture development. Special attention was focused on the difference in the field-induced strains between the first quarter cycle and subsequent loading conditions. Results show that the initial field-induced strain is about twelve times greater than the subsequent strain, which immediately suggests that mechanisms involved in these conditions during the polarization reversal process are different. The difference in the magnitude of field-induced strain is discussed in terms of 180° and non-180° domain reorientation processes. I. INTRODUCTION
Ferroelectic ceramics have found many technical applications due to the development of spontaneous polarization during the paraelectric (PE)-to-ferroelectric (FE) phase transformation. For most applications, the materials are made either of polycrystalline ceramics or thin layers that need to be electrically poled to be piezoelectrically active. It is well known that the poling process involves (i) a domain reorientation process to maximize the resolved dipole moment in the crystalline lattice to the applied field direction, (ii) a converse piezoelectric effect by stretching the dielectric dipole by the electric field, and (iii) a higher order nonlinear electrostrictive effect under extremely high field conditions. Because the third process is not pertinent to the subject, it will not be discussed in this study. When the material breaks the symmetry from a high temperature cubic PE phase to a low symmetry FE phase a spontaneous strain develops along the polarization direction. During the poling process, as the dipoles are reoriented, the material will be subjected to a deformation in which the field-induced strain parallel to the electric field direction increases [as illustrated by the longitudinal (//) strain loop at 75 °C in Fig. 1], while transverse to the field direction the strain decreases [compare to the transverse (⊥) strain loops at 25 or 105 °C in Fig. 1]. As a result of this deformation, a large stored elastic strain energy is developed in the partially electroded ceramic components, which makes these components more prone to cracking. 1,2 Data presented in Fig. 1 were collected when the material was first subjected to an electric field (sometimes these curves are called “virgin curves”). Notice the magnitude of the initial field-induced strain in the first quarter cycle, which is much greater than that of the rest of the cycle. The connection between this large initial J. Mater. Res., Vol. 18, No. 12, Dec 2003
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field-induced strain and cracking in electroceramic components was reported recently.3 The fundamental mechanism with respect to the field-induced strain between the initial and the subsequent polarization reversal is not fully understood and is a subject of this investigation. In this paper, we will review some important aspects of this process and present so
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