Dielectric and piezoelectric properties of the KNN ceramic compound doped with Li, La and Ta

Abstract

With the purpose of improving the dielectric and piezoelectric properties of (K_0.5Na_0.5)NbO₃ (KNN), a multiple doping strategy was tested in this research. Piezoceramics with composition [(K_0.5Na_0.5)_0.94Li_0.06]_0.97La_0.01(Nb_0.9Ta_0.1)O₃ were prepared by the traditional ceramic method. The calcined powders were sintered in their own atmosphere at 1,100 °C for 1.0, 1.5 and 2.5 h. X-ray diffraction analysis showed that the Li⁺, La³⁺ and Ta⁵⁺ cations diffuse into the KNN structure to form a perovskite-structured solid solution. For 1 h sintering time, a dominant orthorhombic phase is obtained, whereas for the longer times, the dominant phase was tetragonal. The presence of a tetragonal tungsten–bronze minority second phase is confirmed. Scanning electron micrographs show rectangular-shaped grains with a mean size of 1.1 ± 0.2 μm. The existence of pores and traces of a liquid phase favoring grain growth and homogeneity is also observed. Experimental results show an enhancement of the permittivity associated with the enlargement of the c parameter of the cell that increases with sintering time. Li⁺ incorporation into the structure is made evident by its transition temperature at 400 °C different from those of KNNLaTi (81–110 °C) and KNNLaTa (340 °C). An analysis of the phase transition of the samples indicates a normal rather than a diffuse transition. The electromechanical parameters k _p, Q _m, σ _p, s ₁₁, d ₃₁ and g ₃₁ are determined and compared to those of commercial PZT ceramics.