Lead-free KNbO3 piezoceramics synthesized by pressure-less sintering

Dense KNbO3 ceramics have been successfully synthesized by pressure-less sintering under optimized heat-treatment conditions using a small amount of La2O3 and Fe2O3 additives. X-ray diffraction (XRD) analysis revealed that KNbO3 forms (K1−xLax)(Nb1−xFex)O3 solid solutions and changes in the crystal system, depending on the additive content, from orthorhombic to tetragonal at x of 0.020, and from tetragonal to cubic at x of 0.200 or higher. The x = 0.002 specimen showed orthorhombic symmetry and demonstrated the best saturated ferroelectric hysteresis loop with largest remanent polarization (Pr) of 18 μC/cm2, which is comparable to the value reported for pure KNbO3 ceramics fabricated by hot pressing. This specimen also showed a planar electromechanical coupling ratio (kp) of 0.17 and thickness mode (kt) of 0.48. The piezoelectric d33 constant was 98 pC/N.