Interface engineering for high-performance, environmentally friendly piezoelectric materials
We determine the mechanism of significant piezoelectricity and ferroelectricity in 0.3BaTiO3–0.1Bi(Mg1/2Ti1/2) O3–0.6BiFeO3 ceramic with a perovskite-type pseudo-cubic symmetry with the Bi ions off-centered at six sites in the cubic <100> directions.
Piezoelectric materials have attracted much attention for a wide range of applications, including electrical, mechanical and medical devices, and, for the last three decades, high-performance, environmentally friendly (i.e., Pb-free) piezoelectric materials have been an important research target. The piezoelectric constant (d33) can be maximized at the Curie temperature (TC) ; this is because materials can be in their softest state due to the coexistence of two different phases. The depth analysis of microstructure at TC has shown the existence of numerous nano-ordered interfaces, and the structure-gradient epitaxial interfaces are much softer compared to non-interfacial regions . The material softness (elastic compliance) at room temperature is inversely related to TC, but Pb(Zr,Ti)O3 (PZT) with morphotropic phase-boundary composition is a very soft material, despite its high TC of 350 ˚C, due to numerous heteroepitaxial interfaces at room temperature . Moreover, Pb(Zn1/3Nb2/3)O3-PbTiO3 (PZN-PT) single crystals with engineered domain configurations have shown ultrahigh d33 values, over 2,000 pC/N, owing to their highly dense domain walls . Thus, high-performance, environmentally friendly (without Pb) piezoelectric materials should be composed of nano-ordered ferroelectric domains with fixed (static) electric polar vectors (interface engineering). First, we focused on the development of perovskite-type BaTiO3-Bi(Mg1/2Ti1/2)O3 (BT-BMT, Pb-free) relaxor-like materials (TC over 200 ˚C) composed of polar nanoregions with flipping (dynamic) polar vectors . The origin of the polar vectors is the off-centering of Bi3+ ions because of (1) the coexistence of large Ba2+ and small Bi3+ ions at the same A-site position in perovskite-type ABO3, and (2) strong covalent bonding between Bi3+ and O2- ions. To fix their dynamic polar vectors, nano-ordered electric or stress bias fields are required. We proposed the hypothesis that the introduction of three or more B-site ions with different valences and sizes in ABO3 can lead to nano-ordered bias fields because of the heterogeneous distribution of the various B-site ions in nano-sized regions, even though the distribution is homogeneous at the macroscopic scale. Therefore, we introduced BiFeO3 (BF; TC of 830 ˚C) in the BT-BMT system so that there were three cations with different valences and sizes [Mg2+ (72 pm), Fe3+ (64.5 pm) and Ti4+ (60.5 pm)] at the B-site. As a result, the studied BT-BMT-BF materials exhibited nanodomains, with sizes of only a few nm, and static polar vectors. The ultrahigh lattice strain of around 0.15 % under an applied electric field of 50 kV/cm is related to their softness. For more information, please see our recent publication in Communications Materials: Piezoelectricity in perovskite-type pseudo-cubic ferroelectrics by partial ordering of off-centered cations.
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