Hirotaka Hida

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.

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.

Early characterization method of plant root adaptability to soil environments

This paper reports an on-chip analytical method for studying physical mechanisms of plant root growth in soil environments. To quantitatively evaluate physical interaction between root and soil, we developed a silicon-based microchannel device integrated with force displacement sensor which mimics a barrier in soil. By using developed microsystem, we successfully characterized the driving forces of root growth in three kinds of plants including Arabidopsis thaliana, which is known as a model organism. This analytical method allows us to efficiently characterize potential adaptability of root system to soil environments at early growth stage. The gaining knowledge might contribute for breed improvement, in terms of increasing crop productivity and plant biomass.

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.

High-productive fabrication method of flexible piezoelectric substrate

This paper reports a simple and high-productive fabrication method of flexible piezoelectric substrate. To achieve a low-cost fabrication process of the substrate, we developed a novel transfer technique of piezoelectric thin films which were formed on temporary stainless-steel substrate to target PDMS (Poly-dimethylsiloxane) substrate by using wet etching process. We experimentally clarified that Pb(Zr,Ti)O3 (PZT) thin films transferred on PDMS substrate, which is developed flexible piezoelectric substrate, have piezoelectric properties by evaluating crystal structures and an inverse piezoelectric effect. This fabrication method might allow us to efficiently develop novel MEMS (micro-electro-mechanical systems) devices such as wearable sensors and artificial muscle in large quantities at a low cost.

Multilayer La-modified PbTiO 3 capacitors via RF magnetron sputtering

In this study, multilayer ceramics (MLCs) composed of La-modified PbTiO3 layers with internal Pt electrodes were fabricated by radio frequency (RF) magnetron sputtering. Multiple (Pb0.9,La0.1)Ti0.975O3 (PLT) layers with thicknesses of approximately 500xa0nm were deposited on Pt/Ti-coated Si substrates through a square movable shadow mask. The isolated internal Pt electrodes were prepared by sliding the movable shadow mask. The MLCs composed of one, three, and five PLT layers were fabricated by alternate sputtering deposition of Pt electrodes and PLT ferroelectric layers with the movable shadow mask. The MLCs had a smooth and crack-free surface, and dense PLT layers could be prepared between the internal Pt electrodes. The MLCs exhibited excellent dielectric properties and their capacitances increased with the number of PLT layers, while the dielectric losses were lower than 3.5xa0%. The P–E hysteresis loops of MLCs showed symmetric loops because of the alternating direction of external electric field on the ferroelectric PLT layers.

Piezoelectric characterization of Pb(Zr,Ti)O3 thin films deposited on metal foil substrates by dip coating

We fabricated the piezoelectric bimorphs composed of Pb(Zr,Ti)O3 (PZT) thin films on metal foil substrates. To efficiently inexpensively manufacture piezoelectric bimorphs with high flexibility, 1.2-µm-thick PZT thin films were directly deposited on both surfaces of 10- and 20-µm-thick bare stainless-steel (SS) foil substrates by dip coating with a sol–gel solution. We confirmed that the PZT thin films deposited on the SS foil substrates at 500 °C or above have polycrystalline perovskite structures and the measured relative dielectric constant and dielectric loss were 323–420 and 0.12–0.17, respectively. The PZT bimorphs were demonstrated by comparing the displacements of the cantilever specimens driven by single- and double-side PZT thin films on the SS foil substrates under the same applied voltage. We characterized the piezoelectric properties of the PZT bimorphs and the calculated their piezoelectric coefficient |e 31,f| to be 0.3–0.7 C/m2.

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.