Fabrication and Properties of Reduction-Resistant Lead-Free NaNbO3–BaTiO3 Piezoelectric Ceramics

Lead-free NaNbO3–BaTiO3 piezoelectric ceramics were fabricated under a low oxygen partial pressure, which enables co-sintering with base metal Ni electrodes. The reducing atmosphere was accurately controlled by adopting a suitable mixture of Ar, H2, and CO2 gases. By adding MnCO3, ZrO2, and Li2CO3, resultant NaNbO3–BaTiO3 ceramics sintered in a reducing atmosphere have high resistivities, maintain high sintering densities, and exhibit piezoelectric constants equivalent or superior to samples prepared in air. The Curie temperatures of the prepared reduction-resistant NaNbO3-BaTiO3 samples depend on the amount of BaTiO3; they can be maintained at approximately 200 °C or higher by setting the BaTiO3 content to 10 mol%. Particularly, 5 mol% MnCO3, 2 mol% ZrO2, and 1 mol% Li2CO3-added (Na0.9Ba0.1)(Nb0.9Ti0.1)O3 ceramics sintered under a low oxygen partial pressure (PO2 = 10–8 atm at 1250 °C) showed a piezoelectric constant (d 33) of 135 pC N−1 and an electromechanical coupling coefficient (k p) of approximately 25%.