No abstract

Dense ([Formula: see text][Formula: see text]NbO3 lead-free ceramics with the simple composition were prepared via sintering in low oxygen partial pressure (pO2, [Formula: see text][Formula: see text] atm) atmosphere and adding LiF. All the ceramics have pure orthorhombic structure. Compared to the LiF-added ([Formula: see text][Formula: see text]NbO3 ceramics sintered in air and the low pO2-sintered pure ([Formula: see text][Formula: see text]NbO3 ceramics without LiF addition, the present ceramics exhibit improved piezoelectric and ferroelectric properties. The piezoelectric constant [Formula: see text] is 125 pC/N, and the converse piezoelectric constant [Formula: see text]* is 186 pm/V. The dielectric constant and dielectric loss of the ceramics at room temperature and 1 kHz are 451 and 0.03, respectively. Under the measured electric field of 70 kV/cm, the remanent polarization is 25.9 [Formula: see text]C/cm2 and the coercive field is 13.9 kV/cm. Furthermore, if the base metals such as Cu and Ni powders were mixed into the green pellets and sintered in the low pO2 atmosphere, the base metals cannot be oxidized, suggesting possibility of using base metals as electrodes.

Dense K0.5Na0.5NbO3 lead‐free ceramics were successfully fabricated by combining cold sintering process (CSP) with annealing in air atmosphere or low pO2 atmosphere. Acetic acid was used as transition liquid. By optimizing the liquid content and pressure during CSP, the CSP samples with relative densities 74.2% were obtained with adding 15 wt% acetic acid under 800 MPa. The CSP samples were then annealed in air atmosphere between 1000 and 1100°C. The ceramics annealed at 1050°C exhibit excellent electrical properties with piezoelectric constant d33 = 125 pC/N, dielectric constant εr = 509, dielectric loss tan δ = 0.04, and remanent polarization Pr = 38.17 μC/cm2. Additionally, the mixture of K0.5Na0.5NbO3 and base metal Ni powders was also cold‐sintered and then annealed in the low pO2 atmospheres in order to detect whether Ni was oxidized or not. The optimized low pO2 atmospheres to protect the Ni from oxidation are pO2 = 10−10 atm followed by reoxidizing in pO2 = 10−6 atm. The K0.5Na0.5NbO3 fabricated by CSP with annealing in the optimized low pO2 atmospheres exhibits comparable electrical properties of the ceramics annealed in air atmosphere, providing an optional approach for using base metals as electrodes.