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

Lead-free CuF2-doped NKN ceramics (NKNCFx, x = 0–2.5 mol%) were prepared using a conventional mixed oxide method to investigate the effects of CuF2 additives on the microstructure, oxygen vacancies, and electrical properties of these materials. The addition of CuF2 enabled 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 (TC). Compared to pure NKN ceramics sintered at 1100 °C, the proposed NKNCFx ceramics sintered at 1000 °C with x = 1.5 exhibited excellent piezoelectric properties: kp: 39.5% (34.3%); kt: 51% (42%); Qm: 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. •CuF2 effectively reduced the sintering temperature from 1100–950 °C.•Fluorine doping lowers the dielectric loss due to the charge balance.•The dielectric constant of the samples is high as the temperature nears TC.•NKNCFx ceramics exhibited excellent piezoelectric properties.