From single crystals to ceramics on Pb[(Zn1/3Nb2/3)0.91Ti0.09]O3 composition

Recently, we found a giant electromechanical coupling factor of k31 mode over 80% and a piezoelectric d31 constant nearly −1700 pC/N in (1 0 0) plane Pb[(Zn1/3Nb2/3)0.91Ti0.09]O3 (PZNT91/09) relaxor single crystals in addition to the large k33 mode of over 92%. The origin of the giant k31 and d31 was due to realize the single-domain structure in the direction perpendicular to the poling field as well as in the direction parallel to the poling field. Through our studies, the poling field (E) dependence of ferroelectric properties and their domain structures was investigated. The single-domain structure was achieved at E = 1000–1500 V/mm with decreasing the frequency constant (fc31: half of the bulk wave velocity on k31 mode) and the giant k31 was obtained at the minimum fc31 of 522 Hz m. While the single-domain was divided into two domains over E = 1500 V/mm, the k31 decreased and fc31 increased. As the formation of a domain wall, which corresponds to a grain boundary in ceramics, caused the increase of the fc31 (Young's modulus), the material became mechanically hard. Moreover, we evaluated PZNT91/09 single crystals with (1 0 0), (1 1 0) and (1 1 1) planes for their aging properties. Large aging occurred in the crystals with (1 1 0) plane, on the other hand, little aging in (1 0 0) and (1 1 1) planes. Namely, the ferroelectric and their aging properties depended on the crystal planes. Since ceramics are composed of the small-size single crystals, our results were very helpful to understand the roles of grain boundaries, grain size, domain size and crystal planes in piezoelectric materials. Furthermore, from the relationships between coupling factors of k31, k33 and Young's modulus on various kinds of single crystals and ceramics, future research on the piezoelectric materials including lead free materials was proposed.