Abstract
Here, we demonstrate the high electromechanical strain and enhanced temperature characteristics in the c-axis-oriented lead-free (Na,Bi)TiO3–BaTiO3 (NBT–BT) polycrystalline thin film prepared on Si substrates by rf magnetron sputtering. The effective transverse piezoelectric coefficient, e31*, estimated from the electromechanical strain measured under high electric field, reaches a high level of −12.5 C/m2, and is comparable to those of conventional Pb(Zr,Ti)O3 films. In-situ X-ray diffraction measurement and electron diffraction analysis revealed the electromechanical strain of the NBT–BT film to originate predominantly in elongation of the tetragonal (P4bm) crystal lattice in the c-axis direction. In addition to the large e31*, the NBT–BT film exhibits enhanced permittivity maximum temperature, Tm, of ~400 °C and no depolarization below Tm, as compared to bulk NBT–BT having Tm ≈ 300 °C and a depolarization temperature of ~100 °C. We conclude that the enhancement of temperature characteristics is associated with the distorted P4bm crystal lattice formed by deposition-induced stress and defects. We believe that the present study paves the way for practical applications of lead-free piezoelectric thin films in electromechanical devices.
Highlights
Much attention has been paid to lead-based material, Pb(Zr,Ti)O3 (PZT), for such applications due to its large piezoelectric properties and stable temperature properties
The pseudocubic in-plane and out-of-plane lattice parameters, estimated from the (101) and (001) X-ray diffraction (XRD) peaks, were apc = 0.391 nm and cpc = 0.393 nm, respectively, for the NBT–BT film. These structural results demonstrate that the NBT–BT film is of c-axis preferred orientation with a single tetragonal lattice that shows tetragonal distortion (c/a)pc value of 1.004
This feature is a characteristic of bulk NBT–BT samples with a P4bm space group[28,29], being the 1/2[ooe] super spot attributed to its in-phase oxygen octahedron tilts in perovskite cell[30]
Summary
Much attention has been paid to lead-based material, Pb(Zr,Ti)O3 (PZT), for such applications due to its large piezoelectric properties and stable temperature properties. The phase with a P4bm space group is recently reported to reversibly/irreversibly transforms into the strongly polar ferroelectric tetragonal phase with P4mm space group on applying an directional electric (E) -field more than a threshold[16,17,18,19] This phase transition is accompanied by a large lattice elongation/polarization extension, resulting in a very high electromechanical strain (d33* reaches up to 2500 pm/V)[18]. The obtained film shows a high electromechanical strain with e31* = −12.5 C/m2 and enhanced temperature characteristics wherein the Tm is increased to ~400 °C from ~300 °C seen in bulk NBT–BT and no depolarization occurs below Tm. We discuss the origin of the properties of NBT–BT film, especially based on the film’s individual properties as related to the deposition process
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