Takakiyo Harigai

Piezoelectric Properties of Lead-Free (Na,Bi)TiO3?BaTiO3 (001) Epitaxial Thin Films around the Morphotropic Phase Boundary

The piezoelectric properties and the crystallographic nature of (1-x)(Na,Bi)TiO3–xBaTiO3 (NBT–BT) thin films around the morphotropic phase boundary (MPB) composition (x=0.05–0.10) were studied. NBT–BT thin films were grown epitaxially on Pt(100)/MgO(100) substrates by RF magnetron sputtering and exhibited highly (001)-oriented single-phase perovskite with a tetragonal structure. A maximum piezoelectric coefficient e31 of -14.4 C/m2 was obtained at the x = 0.07 composition of the tetragonal side near MPB (x = 0.06). These results indicate that NBT–BT thin films around the MPB are a promising lead-free replacement for Pb(Zr,Ti)O3 (PZT) based applications.

Vibration energy harvesting using highly (001)-oriented Pb(Zr,Ti)O3 thin film

Energy conversion from mechanical vibration into electric power was investigated using the piezoelectric Pb(Zr,Ti)O3 lead zirconate titanate (PZT) thin film with highly (001)-orientation. The piezoelectric d31 constant was found to be as large as −150 pC/N. The generated electric voltage under vibration acceleration of 45 m/s2 at 2450 Hz for the PZT/Si cantilever exceeded 2 V and the operation of light emitting diode lighting was demonstrated. The average output power of 100 μW was obtained at the impedance-matched load resistance of 2.2 kΩ. The generated power density of 50 μW/mm3 was much larger than that of conventional piezoelectric harvesters.

Large Transverse Piezoelectricity in Strained (Na,Bi)TiO3–BaTiO3 Epitaxial Thin Films on MgO(110)

Large transverse piezoelectricity has been demonstrated in lead-free epitaxial (Na,Bi)TiO3–BaTiO3 (NBT–BT) thin films grown on MgO(110) substrates. Through the internal strain caused by the difference in thermal expansion between NBT–BT and MgO, the crystal structure of the films was distorted to orthorhombic lattice, which does not form in bulk NBT–BT. The films showed a planar anisotropic nature where the effective transverse piezoelectricity along the orthorhombic b-axis was much larger than that along the orthorhombic a-axis. For the NBT–BT film with 9% BaTiO3, transverse piezoelectric coefficient d31* along the orthorhombic b-axis reached as high as -221 pC/N.

Preparation and piezoelectric properties of self-polarized (Na,Bi)TiO3–BaTiO3 thin films on Si substrate

The c-axis oriented polycrystalline thin film of (Na,Bi)TiO3–BaTiO3 (NBT–BT) around the morphotropic phase boundary (MPB) was prepared on an LaNiO3-buffered Si substrate by rf magnetron sputtering. The NBT–BT film showed a large crystal lattice distortion in the out-of-plane direction, and a voltage shift of the hysteresis loop along the negative field axis as large as −200 kV/cm, which indicates that the film is internally biased and strongly self-polarized towards the top electrode. Owing to the large internal bias field, the NBT–BT film exhibited a linear piezoelectric response with the piezoelectric coefficient, , reaching −4.8 C/cm2 in the unipolar excitation and a low dielectric permittivity of 230. The temperature-dependent dielectric properties revealed that the permittivity maximum temperature, Tm, of the NBT–BT film was significantly enhanced to ~550 °C from the ~300 °C of bulk NBT–BT, accompanied by the disappearance of the depolarization temperature, Td, which is confirmed by the structural data where the crystal lattice remained unchanged up to ~400 °C. These stable temperature properties would lead to an expansion of the temperature range of use of NBT–BT film.

High electromechanical strain and enhanced temperature characteristics in lead-free (Na,Bi)TiO 3 –BaTiO 3 thin films on Si substrates

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.