Fumiya Kurokawa

Lead-Free Piezoelectric MEMS Energy Harvesters of (K,Na)NbO3 Thin Films on Stainless Steel Cantilevers

We fabricated piezoelectric MEMS energy harvesters (EHs) of lead-free (K,Na)NbO3 (KNN) thin films on microfabricated stainless steel cantilevers. The use of metal substrates makes it possible to fabricate thin cantilevers owing to a large fracture toughness compared with Si substrates. KNN films were directly deposited onto Pt-coated stainless steel cantilevers by rf-magnetron sputtering, thereby simplifying the fabrication process of the EHs. From XRD measurement, we confirmed that the KNN films on Pt-coated stainless steel cantilevers had a perovskite structure with a preferential (001) orientation. The transverse piezoelectric coefficient e31f and relative dielectric constant er were measured to be -3.8 C/m2 and 409, respectively. From the evaluation of the power generation performance of a KNN thin-film EH (length: 7.5 mm, width: 5.0 mm, weight of tip mass: 25 mg), we obtained a large average output power of 1.6 µW under vibration at 393 Hz and 10 m/s2.

Composition Dependence of Piezoelectric Properties of Pb(Zr,Ti)O3 Films Prepared by Combinatorial Sputtering

We examined the composition and orientation dependences of the piezoelectric properties by combinatorial sputtering. PbTiO3 and PbZrO3 were simultaneously sputtered to prepare composition gradient Pb(Zr,Ti)O3 (PZT) films on Pt/Ti/Si substrates, and an almost linear composition gradient was observed in a range of Zr/(Zr+Ti) ratios from 0.33 to 0.76. Dielectric and piezoelectric properties attained maximum at the Zr/Ti composition close to the morphotropic phase boundary (MPB), which is consistent with that of PZT ceramics. 100-oriented PZT films showed a higher dielectric constant than the PZT films with the other orientations, while the PZT films with 111 + 100 mixed orientation showed larger values of e31f. These results suggest that the a-axis orientation is dominant in the 100-oriented PZT films. In this study, we demonstrated that the combinatorial sputtering method enables the precise evaluation of the composition dependence of piezoelectric and dielectric properties.

Comparison of effective transverse piezoelectric coefficients e31,f of Pb(Zr,Ti)O3 thin films between direct and converse piezoelectric effects

We evaluated the effective transverse piezoelectric coefficients (e31,f) of Pb(Zr,Ti)O3 (PZT) thin films from both the direct and converse piezoelectric effects of unimorph cantilevers. (001) preferentially oriented polycrystalline PZT thin films and (001)/(100) epitaxial PZT thin films were deposited on (111)Pt/Ti/Si and (001)Pt/MgO substrates, respectively, by rf-magnetron sputtering, and their piezoelectric responses owing to intrinsic and extrinsic effects were examined. The direct and converse |e31,f| values of the polycrystalline PZT thin films were calculated as 6.4 and 11.5–15.0 C/m2, respectively, whereas those of the epitaxial PZT thin films were calculated as 3.4 and 4.6–4.8 C/m2, respectively. The large |e31,f| of the converse piezoelectric property of the polycrystalline PZT thin films is attributed to extrinsic piezoelectric effects. Furthermore, the polycrystalline PZT thin films show a clear nonlinear piezoelectric contribution, which is the same as the Rayleigh-like behavior reported in bulk PZT. In contrast, the epitaxial PZT thin films on the MgO substrate show a piezoelectric response owing to the intrinsic and linear extrinsic effects, and no nonlinear contribution was observed.

Fabrication of High-Efficiency Piezoelectric Energy Harvesters of Epitaxial Pb(Zr,Ti)O3 Thin Films by Laser Lift-off

Abstract We fabricated piezoelectric vibration energy harvesters of c-axis-oriented epitaxial Pb(Zr,Ti)O3 (PZT) thin films on stainless steel (SS304) cantilevers in an effort to improve their power-generation efficiency and toughness. Using radio-frequency magnetron sputtering, we deposited the epitaxial PZT thin films on the MgO substrates, and then transferred the PZT films onto microfabricated SS304 cantilevers using laser lift-off (LLO). LLO did not degrade the transferred epitaxial PZT thin films, which exhibited a high piezoelectric coefficient (e31,f=–4.8 C/m2) and a low relative dielectric constant (εr=340), comparable to those of the original PZT thin film on MgO. At a resonance frequency of 143 Hz, the energy harvesters generated large output power of 1.8 μW at an acceleration of 1.0 m/s2, and the output power reached a maximum of 49 μW at an acceleration of 7.5 m/s2.

Compositional dependence of Pb(Mg1/3,Nb2/3)O3–PbTiO3 piezoelectric thin films by combinatorial sputtering

We evaluated the compositional dependence of Pb(Mg1/3,Nb2/3)O3–PbTiO3 (PMN–PT) polycrystalline thin films by combinatorial sputtering. We prepared compositional gradient (1 − x)PMN–xPT polycrystalline thin films with preferential orientation along the 〈001〉 direction in the composition range of x = 0–0.62. We determined that the morphotropic phase boundary (MPB) composition of PMN–PT polycrystalline thin film existed at around x = 0.35, from the X-ray diffraction (XRD) measurements. The maximum value of relative dielectric constants (er = 1498) was obtained at approximately x = 0.23. On the other hand, the piezoelectric coefficients (|e31,f| = 14.1 C/m2) peaked at the determined MPB composition of x = 0.35. From the results of the compositional dependence of dielectric and piezoelectric characteristics, the FOM () of the PMN–PT (x = 0.35) thin film reached 21 GPa, which is much higher than that of the other polycrystalline piezoelectric thin films. These results suggest that PMN–PT (x = 0.35) thin film is a promising material for high-efficiency piezoelectric MEMS energy harvesters.

Modeling of metal-based piezoelectric MEMS energy harvesters

We fabricated a piezoelectric MEMS energy harvester (EH) of Pb(Zr, Ti)O3 (PZT) thin film on stainless steel cantilever. The use of metal substrates makes it possible to fabricate thin cantilevers owing to a large fracture toughness compared with Si substrates. The PZT thin film was directly deposited onto 50-μm-thick stainless steel substrate by rf-magnetron sputtering. By attaching a tip mass (weight: 480 mg) to the substrate, the resonant frequency of the cantilever (length: 10 mm, width: 10 mm) was dropped to about 75 Hz. From X-ray diffraction (XRD) measurement, we confirmed that the PZT thin film on Pt-coated stainless steel substrate had a perovskite structure with a random orientation. The relative dielectric constant εr and transverse piezoelectric coefficient e31, f were measured to be 650 and -1.7 C/m2, respectively. From the evaluation of the power generation performance of the PZT thin-film EH, we obtained a large average output power of 1.1 μW under vibration at a low frequency of 75 Hz (acceleration amplitude: 5 m/s2, load resistance: 20 kΩ). Moreover, the experimental output voltages with open circuit state were in good agreement with the theoretical values calculated using theoretical equation.

UV-light driven piezoelectric thin-film actuators

We fabricated UV-light driven micro-actuators utilizing epitaxial piezoelectric thin films. We prepared Pb(Zr,Ti)O3 (PZT) epitaxial thin films on cantilever-shaped Pt/MgO substrates, and observed light-induced deformation. When the UV-LED light was irradiated to the surface of PZT thin film, the cantilever deflected due to the coupling of photovoltaic and piezoelectric properties. The deflection of the cantilever was proportionally increased with the UV-light intensity, while the time constant was less than 2 sec, which was much shorter than that of bulk ceramics. These results indicate that UV-light driven piezoelectric thin-film actuators will open a new avenue for remote controlled micro-actuators.