Effects of post-annealing on electrical properties of CuF2·xH2O-doped KNN ceramics for rotary-linear ultrasonic motors

Lead-free (K0.5Na0.5)NbO3 (KNN) ceramics were prepared using a novel sintering aid (CuF2·xH2O (x≈2)) to investigate the effects of post-annealing temperature and atmosphere on oxygen vacancies, microstructure, and electrical properties. Post-annealing (K0.5Na0.5)NbO3 + 1.5 mol% CuF2·xH2O (KNNCH) ceramics at 800 °C under argon was shown to increase the bulk relative density to 97% through the formation of a homogeneous microstructure with liquid phase. The resulting samples presented the following excellent piezoelectric properties: kp：34.1% (±2%); kt：45.3% (±2%); Qm：3170 (±2%); Rz：8.6 Ω (±3%); and tanδ：0.1%. Our results clearly demonstrate that annealing under argon can produce oxygen vacancies in ceramics, which has a significant influence on the stability of domain structures of the samples. Piezoelectric motors fabricated using these ceramics achieved a velocity of 4.5 mm/s, vertical velocity of 3.02 ·mm/s, and output power of 2.93 mW with a negligible increase in temperature and high stability while driven.