Piezoelectric properties of (Na1−xKx)NbO3‐based lead‐free piezoelectric ceramics and their application in knocking sensor

Piezoelectric knocking sensors with a dense microstructure were fabricated at 960°C for 2 hours using various CuO‐added (Na0.5K0.5)NbO3 (NKN)‐based piezoelectric ceramics. The practical sensitivity (SP) of the knocking sensor, which is the ability to detect the knocking of a car engine, was influenced by the g33 × kp value of the piezoelectric ceramics, indicating that the g33 × kp can be considered a figure of merit of the piezoelectric ceramics used in the knocking sensor. The knocking sensor synthesized using the CuO‐added 0.95(Na0.5K0.5)(Nb0.95Sb0.05)O3–0.05CaTiO3 (CNKNS–CT) ceramic, which showed a g33 of 25.7 Vm/N and kp of 0.46, exhibited a high SP of 119 mV/g at the resonance frequency. The SP of the commercial knocking sensor, which was synthesized using the Pb(Zr,Ti)O3 (PZT)‐based ceramic, was 112 mV/g at the resonance frequency. Hence, the knocking sensor fabricated using the CNKNS–CT piezoelectric ceramic can be used to replace the commercial PZT‐based knocking sensor.