Effect of CuO on the ferroelectric and piezoelectric properties of lead-free KNbO3 ceramics

• CuO-added KNbO3 ceramics were sintered at 960 °C by liquid phase sintering mechanism.
• CuO-added KNbO3 ceramics exhibited a double P-E loop and a sprout-shaped S-E loop.
• The 1.0 mol% CuO-added KN ceramic exhibited a very high Qm value of 3005.
• The 1.0 mol% CuO-added KN ceramic shows a large unipolar strain (0.15% at 8.0 kV/mm).

CuO assisted the densification of KNbO3 (KN) ceramics sintered at 960 °C through the formation of a CuO-related liquid phase. Most of the Cu2+ ions entered the Nb5+ sites of the KN ceramics and acted as a hardener. In addition, the CuO-added KN (CKN) ceramics exhibited a double polarization versus electric field (P-E) loop and a sprout-shaped strain versus electric field (S-E) loop. The formation of these was attributed to the presence of defect dipoles formed by acceptor ions (Cu2+) and oxygen vacancies. In fact, owing to the high orthorhombic-to-tetragonal transition temperature of the CKN ceramics, the double P-E and sprout-shaped S-E loops developed without an aging process. Furthermore, the 1.0 mol% CuO-added KN ceramic exhibited a very high Qm value of 3005 with a large unipolar strain of 0.15% at 8.0 kV/mm.