Reduced atmosphere firing of (Na0.5K0.5)NbO3 (NKN) ceramics opens up an important opportunity for lead‐free piezoelectric materials; one of the promising applications is base metal cofired multilayer actuators. Here, we extend an investigation on the effects of reduced atmosphere firing (PO2 = 10−10 atm) on the piezoelectric properties and associated point defects in NKN–LiF ceramics. To access the domain dynamics contribution to the converse piezoelectric properties and consider point defects in the sintered ceramics, Rayleigh analysis and thermal stimulated depolarization current (TSDC) were used. The piezoelectric properties and Rayleigh analysis of NKN–LiF ceramics demonstrated that the intrinsic and extrinsic piezoelectric response is essentially independent of firing atmosphere. Furthermore, reduced fired ceramic was found to have much higher resistivity than in the air‐fired case. From the TSDC analysis, the space charge in the reduced fired ceramic was considered to be much smaller than in the air‐fired ceramic, implying that the alkali volatilization was suppressed in reduced fired NKN–LiF ceramic.

A Miniature cantilever-type ultrasonic motor was fabricated using multilayer piezoelectric ceramics (MLPC) made of (Sr,Ca)2NaNb5O15 (SCNN), a Pb-free piezoelectric material, and the electrical driving properties were investigated. The displacement of SCNN-MLPC was 177 nm at 100 V, which was 10-fold the voltage of MLPC made of Pb(Zr,Ti)O3–Pb(Ni,Nb)O3–Pb(Zn,Nb)O3 (PZT). The ultrasonic motor using SCNN-MLPC started to rotate at 3.8 Vp–p, and showed the following characteristics: a revolution speed of Ω0=517 rpm, a torque of T0=1.4 µN m, and an efficiency of η=7% at 13.4 Vp–p. The power consumption, in this case, was 0.3 mW, which is not so high. On the other hand, an ultrasonic motor of the same form made of PZT-MLPC showed Ω0=610 rpm, T0=1.6 µN m, and η=9% at 1.9 Vp–p, and the same power consumption as the SCNN motor. That is, it was found that the SCNN motor was not so different from the PZT motor in terms of their characteristics; however, the SCNN motor needed 10 times as high a voltage as the PZT motor in electrical driving. In conclusion, it can be said that we succeeded in realizing a miniature cantilever-type ultrasonic motor using Pb-free MLPC.

We investigated why the nonlinear behaviors of (Bi,Na,Ba)(Ti,Mn)O3 (BNBTM) ceramics and (Sr,Ca)2NaNb5O15 (SCNN) ceramics occur in comparison with Pb(Zr,Ti)O3-based (PZT) ceramics by evaluating their high-power properties with the distinction between mechanical nonlinearity and temperature dependence of properties. The equivalent stiffness of PZT decreased with increasing vibration velocity and sample temperature. However, the equivalent stiffnesses of BNBTM and SCNN did not depend on vibration velocity, they decreased and increased linearly with an increase in sample temperature, respectively. The nonlinear behavior of PZT originated from the soft-spring effect through which the mechanical nonlinearity was induced and the temperature dependence of stiffness was enhanced. On the other hand, it appeared that the nonlinear behaviors of BNBTM and SCNN originated from the apparent soft- and hard-spring effects through which the temperature dependence of stiffness was induced. The temperature characteristic of elasticity is important for controlling the high-power properties.

Multilayer (Na0.5K0.5)NbO3 (NKN) ceramics are considered promising candidates for lead-free piezoelectric actuators. The possibility of cofiring a nickel inner electrode in an NKN–LiF ceramic was investigated by evaluating the electrical properties and microstructures. The fabricated Ni inner electrode multilayer actuator sintered in a reduced atmosphere exhibited comparable dielectric properties to those of bulk NKN ceramics. The electric-field-induced strain was approximately 210 pm/V. Analysis based on the Rayleigh model showed that the extrinsic non-180° domain wall motion was suppressed in the multilayer structure. The microstructure of the NKN ceramic was not affected by cofiring with nickel, and the NKN/Ni interface was both compositionally and structurally sharp. These results imply that the NKN–LiF ceramic can be cofired with nickel without any deterioration of its properties.