(V) Exploring the Density Improvement Mechanism of (K,Na)NbO3

At the beginning of the 21^st century, global efforts to reduce the use of lead (Pb) in electronic devices and packagings has led to extensive research in the area of lead-free piezoelectrics. In the present study, the densification process of the most promising lead-free piezoelectric ceramic, i.e., potassium sodium niobate (K_0.50,Na_0.50)NbO₃, hereafter KNN, was studied. The double-calcined KNN powder at 625 °C was pressed into a pellet without using any binder. Prior to sintering, a pre-heating process at 200 °C for 2h was applied for all the samples. In addition, the effects of different pre-annealing temperatures at 400 °C, 600 °C and 800 °C for 5 h on the density of the samples sintered at 1100 °C for 2 h were investigated. The Archimedes bulk density of the sintered samples that underwent only the pre-heating process was measured to be 92.2 ± 0.1 % TD (theoretical density), while the density reached 95.0 ± 0.2 %, 95.3 ± 0.1 % and 95.6 ± 0.2 % TD for those samples sintered at different pre-annealing temperatures at 400 °C, 600 °C and 800 °C for 5 h, respectively. The results of this study showed that by introducing a pre-annealing step at 800 °C for 5 h in the sintering regime of the KNN ceramic, a ~ 3 % increase in the bulk density of the sintered samples could be achieved. Taking into account that there are rarely density reports higher than 94 % TD in the Na₂O-K₂O-Nb₂O₅ ternary system, this significant increase in density through a simple and conventional solid-state synthesis and sintering process is very promising in order to find a reliable and easy-to-handle method for the fabrication of KNN lead-free piezoelectric ceramics.

Keywords: Piezoelectric ceramics, Potassium sodium niobate, Solid-state synthesis and sintering, Bulk density.