Noriyuki Kaneko

Crystal Structure Analysis of Hydrothermally Synthesized Epitaxial (KxNa1-x)NbO3 Films

Epitaxial (KxNa1-x)NbO3 films with various compositions x were deposited on Nb-doped (100)SrTiO3 single crystal substrates at 240 °C by the hydrothermal method. Detailed crystal structures were analyzed by X-ray diffraction. All the (KxNa1-x)NbO3 films consisted of a perovskite phase without any impurity phase. High-temperature XRD data showed that (KxNa1-x)NbO3 films with x = 0.03 and 0.86 transformed to a single cubic phase at 600 °C through the intermediate phase. On the other hand, (KxNa1-x)NbO3 films with x = 0.24 showed two diffraction peaks at 600 °C originating from individual phases having different unit cell volumes. The result shows that these films consisted of a mixture of two perovskite phases with K- and Na- rich (KxNa1-x)NbO3 composition.

Ferroelectric and piezoelectric properties of KNbO3 films deposited on flexible organic substrate by hydrothermal method

KNbO3 films were synthesized at 120–240 °C by a hydrothermal method. The deposition amount of KNbO3 films decreased with decreasing deposition temperature because the deposition rate decreased together with the increase in the starting time of precipitation. The KNbO3 phase was ascertained at temperature as low as 120 °C in films deposited on (100)cSrRuO3//(100)SrTiO3 substrates. On the other hand, powders prepared at 150 °C included the Nb2O5 phase as well as the KNbO3 phase. In addition, powders prepared at 120 °C consist of the K3Nb7O19 phase instead of the KNbO3 phase. KNbO3 films were deposited at 150 °C on LaNiO3/Pt/Ti/polysulfone substrates. Films consisting of the polycrystalline KNbO3 phase were obtained, and their ferroelectricity and piezoelectricity were observed. The remanent polarization and piezoelectric coefficient were 4.2 µC/cm2 and 20–32 pm/V, respectively.

Structural and dielectric properties of BaTiO3–Bi(Mg1/2Ti1/2)O3 thin films fabricated by chemical solution deposition

Thin films of the BaTiO3–Bi(Mg1/2Ti1/2)O3 (BT–BMT) solid-solution system were fabricated with the aim of achieving a stable temperature coefficient of capacitance (TCC) favorable for high-temperature electronics. A single perovskite phase with pseudocubic symmetry was obtained for the films fabricated by chemical solution deposition on (111)Pt/TiO2/(100)Si substrates in the composition range of x = 0–0.80 for (1 − x)BT–xBMT. BMT added to the BaTiO3-based films enhanced the crystallinity of the perovskite phase and resulted in saturated P–E hysteresis behavior with remanent polarization of up to 13 µC/cm2. BMT addition led to gradual dielectric relaxation, which also resulted in stable TCC behavior with a relative dielectric constant of approximately 400 in the temperature range of RT − 400 °C, especially for the BT–BMT films with x = 0.20–0.40.

Vibration-energy-harvesting properties of hydrothermally synthesized (K,Na)NbO3 films deposited on flexible metal foil substrates

Energy-harvesting properties were investigated for the flexible piezoelectric devices, Pt/hydrothermally synthesized (K0.88Na0.12)NbO3/SrRuO3/metal foil, fabricated at low process temperatures below 300 °C. Fabricated devices had high flexibility, and cracking and peeling were not observed under the bending condition with the maximum curvature of 5.2 mm. The estimated Youngs modulus of the fabricated flexible devices was 37 GPa. The pulse poling treatment with AC voltage above 150 V enhanced the energy-harvesting properties. Although the dielectric constant was almost unchanged (er = 120 at 200 Hz), the maximum output voltage measured at an acceleration of 10 m/s2 was observed at a resonance frequency of 126 Hz, and this voltage increased from 7.2 to 11 V after pulse poling treatment at 200 V. The maximum output power was 7.7 µW at a load resistance of 560 kΩ. The calculated Q and K2 values were 30 and 0.0014, respectively. The power density was 1.8 µW/(G2mm3), which is higher than the previous reports for films and sintered bodies of (KxNa1−x) fabricated above 500 °C.

Effects of heat treatment on electrical and electromechanical properties of hydrothermally synthesized epitaxial (K0.51Na0.49)NbO3 films

Effects of heat treatment on the electrical and electromechanical properties of hydrothermally synthesized epitaxial (K0.51Na0.49)NbO3 films were investigated. The film composition and crystal structure did not markedly change with heat treatment from 240 to 600 ?C in O2 atmosphere. However, the leakage current density drastically decreased from the order of 101 to 10?5 A/cm2 at 50 kV/cm after heat treatment at 240 ?C. The relative dielectric constant, remanent polarization, and transverse piezoelectric constant increased with increasing heat treatment temperature from 300 to 600 ?C (from 330 to 750 in ?r, from 5.4 to 14 ?C/cm2 in remanent polarization, and from 41 to 56 pm/V in transverse piezoelectric response). H2O desorption of 12 ppm was detected in epitaxial (K0.51Na0.49)NbO3 films by thermal desorption spectroscopy measurement. Such H2O desorption can be related to the improvement of the electrical and electromechanical properties of the hydrothermally synthesized epitaxial (K0.51Na0.49)NbO3 films.