Sintering temperature affects the insulation resistance degradation of (K,Na)NbO3-based lead-free piezoelectric multilayer ceramics with Ni internal electrodes

We fabricated piezoelectric multilayer ceramics using (K,Na)NbO 3 (KNN)-based ceramics with Ni internal electrodes and investigated the effects of sintering temperature (1020-1040 °C) on their electrical properties and reliability.Overall, sintering at 1020 °C yielded the highest piezoelectric strain response (406 pm/V) and the highest roomtemperature volume resistivity (4 × 10 10 Ω ･ m), although these values did not significantly differ from samples prepared at 1030 and 1040 °C.However, in time-dependent insulation resistance measurements at 150 °C and 5 kV/mm, the degradation was significantly accelerated as the sintering temperature increased, thereby shortening the operational lifetime from 12 h (sintered at 1020 °C) to 6 h (sintered at 1040 °C).Microstructural analysis confirmed that a K-rich secondary phase formed near the grain boundaries, and the K concentration in this region increased with sintering temperature.This secondary phase may be a cause of the rapid insulation degradation, particularly for KNN-based ceramics with Ni internal electrodes sintered at higher temperatures.