Low-temperature–sintered CuF₂-doped NKN ceramics with excellent piezoelectric and dielectric properties

Lead-free CuF₂-doped NKN ceramics (NKNCF_x, x = 0–2.5 mol%) were prepared using a conventional mixed oxide method to investigate the effects of CuF₂additives on the microstructure, oxygen vacancies, and electrical properties of these materials. The addition of CuF₂enabled a reduction in sintering temperature from 1100–950 °C, and doping with appropriate quantities of fluorine reduced the dielectric loss, due to a charge balance of electrons and holes. Under low frequencies at high temperatures, the dielectric constant and space charge effect gradually increased with the addition of CuF₂. At x > 0.5, the dielectric constant was very high (>250,000) when the heating temperature was close to the tetragonal–cubic phase transition temperature (T_C). Compared to pure NKN ceramics sintered at 1100 °C, the proposed NKNCF_xceramics sintered at 1000 °C with x = 1.5 exhibited excellent piezoelectric properties: k_p: 39.5% (34.3%); k_t: 51% (42%); Q_m: 2331 (163); d₃₃: 96 pC/N (81 pC/N); and tanδ: 0.1% (5%) as well as the high thermal stability required for most practical applications.