Phase transitional behavior, microstructure, and electrical properties in Ta-modified [(K0.458Na0.542)0.96Li0.04]NbO3 lead-free piezoelectric ceramics

Lead-free [(K0.458Na0.542)(0.96)Li-0.04](Nb1-xTax)O-3 ceramics were prepared by ordinary sintering technique. The effects of Ta content on the phase transitional behavior, Raman spectrum, microstructure, and dielectric, piezoelectric, and ferroelectric properties of the ceramics were investigated. X-ray diffraction results indicate that the phase structure undergoes a transition from orthorhombic to tetragonal phase with the increase in x. The small and large peaks observed in the Raman spectra can all be attributed to the internal modes of the NbO6 octahedron, and both nu(5) and v(1) modes slightly shift to lower frequency numbers by increasing x. Ta substitution for Nb leads to the disappearance of the abnormal grain growth behavior, inhibits the grain growth, and improves the density of the ceramics. Increasing x leads to the different variations of dielectric constants before and after poling and makes the ceramics more relaxorlike. The proper substitution of Ta shifts the polymorphic phase transition (at To-T) to near room temperature, and a coexistence of orthorhombic and tetragonal phases is formed, which leads to significant enhancements of the electrical properties. For the ceramics with x=0.15, the electrical properties become optimum, which are as follows: piezoelectric coefficient d(33)=298 pC/N, electromechanical coupling coefficient k(p)=0.52, dielectric constant epsilon(r)=1195, dielectric loss tan delta=0.016, To-T= 35 C, Curie temperature T-C=366 degrees C, remanent polarization P-r= 28.68 mu C/cm(2), and coercive field E-c=7.14 kV/cm. These properties are much higher than those of previously reported (K,Na,Li)(Nb,Ta)O-3 systems with K: Na= 1:1. (c) 2008 American Institute of Physics.