Influence of compositional ratio K/Na on physical properties in (KxNa1-x)NbO3 ceramics

Lead-free KxNa1-xNbO3 (KNN) ceramics with x=0.10-0.80 were prepared by a conventional solid-state reaction method to investigate the influence of the K/Na ratio on their physical properties. It has been found that the KNN ceramics with x=0.45-0.60 exhibit quite different microstructures from those with other alkali compositional ratios. The lattice parameter discontinuity previously reported at x=0.475 has not been confirmed. The existence of a change in the line slopes of lattice parameter variation with x is verified in the vicinity of x=0.35. The permittivity epsilon', piezoelectric constant d(33), and planar electromechanical coupling coefficient k(p) show broad peaks in the compositional range of x=0.40-0.60, where d(33) and k(p) are nearly independent of the K/Na ratio and all the ceramics have the high piezoelectric values of d(33)similar to 125 pC/N and k(p)similar to 0.41, respectively. From the analyses, we suggest that the high piezoelectric properties for the KNN ceramics near x=0.50 are closely related to their microstructures of well-faceted and uniformly distributed large grains rather than the widely believed morphotropic phase boundary effect of lattice parameter discontinuity. (C) 2008 American Institute of Physics.