Enhancement of Density and Piezoelectric Properties of 0.96(K0.456Na0.536)Nb0.95Sb0.05-0.04Bi0.5(Na0.82K0.18)0.5ZrO3 Lead-Free Piezoelectric Ceramics through Two-Step Sintering Method

In this study, we investigated the microstructure and piezoelectric properties of 0.96(K0.456Na0.536)Nb0.95Sb0.05-0.04Bi(0.5)(Na0.82K0.18)(0.5)ZrO3 (KNNS-BNKZ) ceramics based on one-step and two-step sintering processes. One-step sintering led to significant abnormal grain (AG) growth at temperatures above 1,085 degrees C. With increasing sintering temperature, piezoelectric and dielectric properties were enhanced, resulting in a high d(33) = 506 pC/N for one-step specimen sintered at 1,100 degrees C (one-step 1,100 degrees C specimen). However, for one-step 1,115 degrees C specimen, a slight decrease in d(33) was observed, emphasizing the importance of a high tetragonal (T) phase fraction for superior piezoelectric properties. Achieving a relative density above 84 % for samples sintered by the one-step sintering process was challenging. Conversely, two-step sintering significantly improved the relative density of KNNS-BNKZ ceramics up to 96 %, attributed to the control of AG nucleation in the first step and grain growth rate control in the second step. The quantity of AG nucleation was affected by the duration of the first step, determining the final microstructure. Despite having a lower T phase fraction than that of the one-step 1,100 degrees C specimen, the two-step specimen exhibited higher piezoelectric coefficients (d(33) = 574 pC/N and k(p) = 0.5) than those of the one-step 1,100 degrees C specimen due to its higher relative density. Performance evaluation of magnetoelectric composite devices composed of one-step and twostep specimens showed that despite having a higher g(33), the magnetoelectric composite with the one-step 1,100 degrees C specimen exhibited the lowest magnetoelectric voltage coefficient, due to its lowest k(p). This study highlights the essential role of phase fraction and relative density in enhancing the performance of piezoelectric materials and devices, showcasing the effectiveness of the two-step sintering process for controlling the microstructure of ceramic materials containing volatile elements.