Lead-free, potassium sodium niobate piezoelectric ceramics substituted with lithium (K0.5−x∕2,Na0.5−x∕2,Lix)NbO3 or lithium and tantalum (K0.5−x∕2,Na0.5−x∕2,Lix)(Nb1−y,Tay)O3 have been synthesized by traditional solid state sintering. The compositions chosen are among those recently reported to show high piezoelectric properties [Y. Saito, H. Takao, T. Tani, T. Nonoyama, K. Takatori, T. Homma, T. Nagaya, and M. Nakamura, Nature (London) 42, 84 (2004); Y. Guo, K. Kakimoto, and H. Ohsato, Appl. Phys. Lett. 85, 4121 (2004); Mater. Lett. 59, 241 (2005)]. We show that high densities and piezoelectric properties can be obtained for all compositions by pressureless sintering in air, without cold isostatic pressing, and without any sintering aid or special powder treatment. Resonance and converse piezoelectric (strain-field) measurements show a thickness coupling coefficient kt of 53% and converse piezoelectric coefficient d33 around 200pm∕V for the Li-substituted ceramics, and a kt of 52% and d33 over 300pm∕V for the Li- and Ta-modified samples. The unipolar strain-field hysteresis is small and comparable to that measured under similar conditions in hard Pb(Zr,Ti)O3. A peak of piezoelectric properties can be noted close to the morphotropic phase boundary. These ceramics look very promising as possible, practicable, lead-free replacements for lead zirconate titanate.

Compositional inhomogeneity is examined in Li-and Ta-modified potassium sodium niobate (K,Na)NbO 3 perovskite ceramics. The inhomogeneous distribution of the A-site (K and Na) and B-site (Nb and Ta) cations is found to be correlative. The inhomogeneity could not be eliminated by prolonged hightemperature annealing in samples prepared by direct mixing of alkaline carbonates and Ta and Nb oxides. In contrast, the precursor method, where a (Nb x Ta 1Àx ) 2 O 5 solid solution was first formed, led to a considerably improved compositional homogeneity and appreciably enhanced dielectric, ferroelectric, and piezoelectric properties. Our results suggest that more attention needs to be paid toward controlling the compositional fluctuation of this complex solid solution system.

No abstract

A composition-temperature phase diagram of the system ͑1−x͒͑K 0.5 Na 0.5 ͒NbO 3-xLiNbO 3 is presented for 0 ഛ x ഛ 0.1. Using dielectric and piezoelectric resonance measurements, and Raman spectroscopy, ceramic samples containing 2%-10% LiNbO 3 were studied over a temperature range of 7-770 K showing a complex sequence of phase transitions. Analysis of the different Raman, piezoelectric, and dielectric data shows distinct transitions from cubic to tetragonal to orthorhombic to rhombohedral phase for x = 0.02-0.05. The symmetries of the phases were assigned using analogy to phase diagram of ͑K 0.5 Na 0.5 ͒NbO 3 single crystals and ceramics ͑x =0͒. At x Ͼ 0.07 only one transition between ferroelectric phases occurs where tetragonal phase transforms to another phase, possibly of rhombohedral, orthorhombic, or monoclinic symmetry. In the region between x = 0.05 and x = 0.08, the phase transition sequence is more complex. Below 100 K this phase with unidentified symmetry creates a vertical boundary with the rhombohedral phase present near 5% Li. A triple point between the tetragonal, orthorhombic, and the new phase is identified.

No abstract