| Processing and Characterization of Reduction-Resistant Lead-Free Piezoelectric (Ba,Ca)TiO3 Ceramics and Their Grain Growth Control |
| Wataru SAKAMOTO |
| Nagoya University |
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Keywords: Barium titanate, Reduction-resistant, Lead-free piezoelectric, Grain-oriented, Electrical properties
Pb(Zr,Ti)O3 (PZT)-based oxides have been widely applied as piezoelectric materials because of their excellent electrical properties. However, these materials contain toxic PbO as a major component. Recently, because of growing concerns about the effects of toxic lead on the global environment, the development of materials which do not contain toxic elements is strongly required. Therefore, the research and development of lead-free piezoelectric materials have been carried out extensively all over the world. Among several ferroelectric alternatives to PZT, BaTiO3 has been receiving much attention as a potential candidate. In BaTiO3-based materials, the phase diagram and the characteristic giant electromechanical response of Ca-substituted BaTiO3 following the application of an electric field have been reported [1].
In this study, processing of reduction-resistant (Ba,Ca)TiO3 ceramics as lead-free piezoelectric materials was investigated. Nonreducible (Ba,Ca)TiO3 ceramics were fabricated by appropriately setting the chemical composition, such as the Ba and Mn concentrations in Mn-doped (Ba,Ca)TiO3 ceramics. Among (Ba,Ca)TiO3 ceramics with various CaTiO3 contents, the nonreducible (Ba0.85Ca0.15)TiO3 exhibited the highest field-induced strain coefficient (estimated from the slope of an unipolar strain loop) of 260 pm/V at room temperature. To improve their electrical properties, (100),(001)-oriented (Ba0.85Ca0.15)TiO3 ceramics were fabricated by the reactive templated grain growth (RTGG) method using a mixture of platelike CaTiO3 and BaTiO3 particles. The platelike CaTiO3 and BaTiO3 particles were prepared through a topochemical microcrystal conversion process [2] using CaBi4Ti4O15 and BaBi4Ti4O15 plate-like precursor crystals. The {100} orientation degree of the grain-oriented (Ba0.85Ca0.15)TiO3 ceramics was 92%, as estimated by Lotgering’s equation. In addition, 1 mol% Ba excess and 1 mol% Mn-doped (Ba0.85Ca0.15)TiO3 sintered bodies, which were sintered at 1350 oC in an Ar flow containing H2 (0.3%), had sufficient resistivity to allow the characterization of electrical properties utilizing a high applied field. The ferroelectric and field-induced strain properties of the (Ba0.85Ca0.15)TiO3 ceramics, sintered in the reducing atmosphere (oxygen partial pressure below 0.1 Pa (10-6 atm)), were markedly improved as a result of fabricating grain-oriented samples [3]. The field-induced strain coefficient of the nonreducible (100),(001)-oriented (Ba0.85Ca0.15)TiO3 ceramics reached 570 pm/V, which was higher than that of polycrystals (260 pm/V) with no preferential orientation.
Although further enhancements of the piezoelectric properties are still necessary, the reduction-resistant grain-oriented (Ba,Ca)TiO3-based ceramics are expected to be promising candidates for lead-free ceramic materials for multilayer-type piezoelectric applications.
References:
[1] D. Fu, M. Itoh, S. Koshihara, T. Kosugi, S. Tsuneyuki, Phys. Rev. Lett. 100 (2008) 227601.
[2] Y. Saito, H. Takao, T. Tani, T. Nonoyama, K. Takatori, T. Hommma, T. Nagaya, M. Nakamura, Nature 432 (2004) 84.
[3] H. Ichikawa, W. Sakamoto, Y. Akiyama, H. Maiwa, M. Moriya, T. Yogo, Jpn. J. Appl. Phys. 52 (2013) 09KD08.
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