Effects of MnO2-doping on growth, structure and electrical properties of lead-free piezoelectric K0.5Na0.5NbO3-BiAlO3 single crystals

Abstract

K0.5Na0.5NbO3 (KNN)-based lead-free piezoelectric single crystals have been prepared by the Seed-Free, Solid-State Crystal Growth (SFSSCG) method, and the effects of MnO2 doping on the structural and electrical properties were systematically investigated. The results showed that the MnO2-doping can promote the growth of K0.5Na0.5NbO3-BiAlO3 (KNN-BA) single crystals and enhance its electrical properties. Furthermore, the size of the (1 - x)( KNN-BA)-xMnO2 crystals reached the maximum value of about 18 × 17 × 2 mm3 at x = 0.005. Also, the color of the crystals did gradually change, from light yellow to dark brown, due to the addition of MnO2. The XRD and refinement results showed that the addition of MnO2, in the range of 0 ≤ x ≤ 0.009, did not change the crystalline structure of the perovskite crystals. A two-phase perovskite crystalline structure was present in all crystals: an orthorhombic and a tetragonal phase. The content of the orthorhombic phase decreased, while the content of the tetragonal phase increased, with an increase in the MnO2 content. In addition, the domains in the crystals were mainly composed of 180° ones and had a parallel lamellar structure. Also, the smallest average domain width was obtained for crystals with x = 0.003. These crystals had the largest piezoelectric constant of 568 pC/N and an excellent overall performance of d33 = 568 pC/N, Pr = 32.2 μC/cm2, Qm = 55.3, kt = 0.36, kp = 0.39, TC = 418 °C, εr = 430, and tan δ = 0.015.