We investigated the influence of CuO amount (0.5-3.0 mol%), sintering temperature (900°C-1000°C), and sintering time (2-6 h) on the low-temperature sintering behavior of CuOadded Bi 0.5 (Na 0.78 K 0.22 ) 0.5 TiO 3 (BNKT22) ceramics. Normalized strain (S max /E max ), piezoelectric coefficient (d 33 ), and remanent polarization (P r ) of 1.0 mol% CuO-added BNKT22 ceramics sintered at 950°C for 4 h was 280 pm/V, 180 pC/N, and 28 lC/cm 2 , respectively. These values are similar to those of pure BNKT22 ceramics sintered at 1150°C. In addition, we investigated the performance of multilayer ceramic actuators made from CuO-added BNKT22 in acoustic sound speaker devices. A prototype sound speaker device showed similar output sound pressure levels as a Pb(Zr,Ti)O 3 -based device in the frequency range 0.66-20 kHz. This result highlights the feasibility of using low-cost multilayer ceramic devices made of lead-free BNKT-based piezoelectric materials in sound speaker devices.

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