Ferroelectric Domain Inversion in LiNbO 3 and its Application to High-Precision Piezoelectric Actuators
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ABSTRACT Heat treatment of bare single-domain LiNbO3 plates near the Curie temperature induces a local polarization reversal, thereby yielding a ferroelectric inversion layer. For a sufficiently long heat treatment the inversion layer thickness becomes equal to one half of the plate thickness. Similar domain inversion is also induced by heat treatment of protonexchanged LiTaO3. These domain inversion phenomena and a model for explaining the mechanism are reviewed. The inversion layer has piezoelectric constants opposite in sign to those of the uninveiled domain. Therefore, a LiNbO 3 plate with such an inversion layer can be used as a piezoelectric actuator similar to the bimorph, because bending motion or torsional motion can be piezoelectrically produced, depending on the plate orientation. The measured characteristics of these actuators are presented and demonstrated to exhibit excellent linearity and no hysteresis. INTRODUCTION Lithium niobate (LiNbO 3 )l, 2 is widely used in various piezoelectric devices because of
its high electromechanical coupling factors. It is also important as a substrate material for optical waveguide devices. Most practical LiNbO 3 crystals are composed of a single ferroelectric domain. The Curie temperature of this crystal grown from congruent melts is as high as about 11 50°C, and it is not easy to reverse the spontaneous polarization at room temperatures. Ohnishi 3 reported that heat treatment at about 10000 C of a LiNbO 3 plate etched in a mixture of HF and H-N0 3 created a 1-15 jpm thick domain with a reversed polarization at the +c surface. Nakamura et al.4-6 found that domain inversion took place when unetched bare LiNbO 3 plates underwent heat treatment at temperatures above I1070°C, and showed that the domain boundary of the resulting ferroelectric inversion layer ultimately stopped at the median plane of plate for sufficiently long heat treatment. Similar domain inversion was observed at the -c face of LiTaO 3 by proton-exchange followed by heat treatment near the Curie temperature. 7-9 These domain inversion phenomena occur in local regions of crystals under no applied electric field. The inverted domain with a reversed spontaneous polarization Ps has piezoelectric, pyroelectric, electro-optic, and 2nd-order nonlinear optic constants opposite in sign to those of the uninverted region. This character of inverted domains can be utilized for various
devices. 10
For example, the strains piezoelectrically generated in the two regions are 21 Mat. Res. Soc. Symp. Proc. Vol. 360 @1995 Materials Research Society
opposite in sign to each other. This fact enables us to obtain bending actuatorsII and torsional actuators12 consisting of a single plate. This paper reviews the domain inversion induced by heat treatment and describes applications of the resulting domains to high-precision piezoelectric actuators. DOMAIN INVERSION INDUCED BY HEAT TREATMENT Heat treatment of unetched bare LiNbO 3 plates at temperatures higher than about 10700 C induces local domain inversion, thereby yiel
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