Two-step sintering affecting the escaping of Na and K and its impact on dielectric properties and morphology of lead-free Na_0.92K_0.08NbO₃ ceramics

Abstract

Lead-free Na_xK_1–xNbO₃ (NKN) ceramics with x = 0.08 were prepared by a conventional solid-state reaction method. A two-step sintering technique was used for the sintering of ceramics. In this study, the influence of escaping of Na and K on the physical properties of prepared samples is investigated. It has been observed that the NKN ceramics with sintering temperature 1150 °C, 1200 °C and 1250 °C exhibit quite different microstructures from those with single sintering temperature. The lattice parameter discontinuity previously reported at x = 0.475 has not been confirmed. The permittivity, piezoelectric constant d₃₃, show broad peaks at the sintering temperature 1200 °C, where permittivity and d₃₃ are maximum and dependent on the escaping of Na and K. Further, all the ceramics sintered at 1200 °C have the high permittivity (є_c = 3175) and piezoelectric values (d₃₃ = 192 pm/V). From the analyses, we suggest that the high permittivity and piezoelectric properties for the NKN ceramics near 1200 °C are closely related to their escaping of Na and K and microstructures of well-faceted and uniformly distributed large grains rather than the widely believed morphotropic phase boundary effect of lattice parameter discontinuity.

Keywords:

• Two-step sintering
• microstructure
• grain size
• escaping of Na and K
• dielectric properties