Densification of (Na,K)NbO₃ piezoelectric ceramics by two-step mixing process

Lead-free Na_0.5K_0.5NbO₃ (NKN) piezoelectric ceramics were sintered with a new process, “two-step mixing process,” in which a part of alkali source powders was initially preserved and mixed with the rest matrix powders after the calcinations step. As a result, the sintering of NKN ceramics was improved, and the sample sintered at 1082 °C with the initial preservation ratio (R _A) of 5% demonstrated the highest density of ρ = 4.38 g/cm³ (97.1% of the theoretical density), compared with ρ = 4.36 g/cm³ (96.7% of the theoretical density) for the non-preservation specimen (R _A = 0%). The former sample showed the best piezoelectric constant of d ₃₃ = 125 pC/N and electromechanical coupling coefficient of k _p = 0.42, while the latter sample had d ₃₃ = 116 pC/N and k _p = 0.37. These results indicated that the two-step mixing process was effective for the sintering of lead-free NKN ceramics, despite no sintering additive and cold isostatic pressing were used.