Spark plasma sintering of (K0,5Na0,5)(Nb1-xTax)O3 lead free perovskite ceramics: a composite microstructure to improve piezoelectric properties

Sodium potassium niobate (K,Na)NbO3 (KNN)lead free perovskite ferroelectrics have focused broad interest since early 2000 in order to replace in certain applications the lead-based ferroelectrics, predominantly Pb(Zr,Ti)O3.... [ view full abstract ]

Sodium potassium niobate (K,Na)NbO₃ (KNN)lead free perovskite ferroelectrics have focused broad interest since early 2000 in order to replace in certain applications the lead-based ferroelectrics, predominantly Pb(Zr,Ti)O₃. Clearly densifications and piezoelectric properties of pure KNN do not reach yet those of PZT and chemical modifications are necessary to become competitive_. In this is study K_0,5Na_0.5Nb_1-xTa_xO₃ (KNNT) (with x = 0.00, 0.05, 0.10, 0.20, 0.30, 0.50 and 1) ceramics are prepared by ball milling and calcined at 830 °C for 5 hours to obtain XRD pure phase. Subsequent sintering is studied using conventional and spark plasma sintering techniques. X-Ray diffraction and Raman spectroscopy phase identification reveal orthorhombic to tetragonal phase transitions occurring at about x = 0.50, associated to chemical disorder. Scanning electron microscope observations associated with energy dispersive X-ray spectroscopy analyzes reveal some composite aspect of the ceramics. Substitution of niobium by tantalum decreases significantly the grain size while enhancement of the piezoelectric properties is observed. Thanks to parameters optimization of the spark plasma sintering process, temperature-time-pressure, and subsequent annealing to recover fully oxygen perovskite, significant improvement of the relative density, over 96 %, is obtained for all the compositions sintered between 920 and 960 °C, under 50 MPa, for 5 to 10 minutes with heating rates of 100° C/min. High relative permittivity (ε_r = 1027), piezoelectric charge coefficient (d₃₃ = 160 pC/N) and piezoelectric coupling factor (k_p = 46 %) are obtained for some specific spark plasma sintering conditions. Thus, tantalum single element substitution on niobium site, combined with spark plasma sintering, is revealed to be a powerful combination for the optimization and the reliability of piezoelectric properties in KNN system, that might be transferable to an industrial process.