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K0.5Na0.5NbO3 (KNN) crystals have great application prospects due to their excellent piezoelectric properties, high Curie temperature, and environmental friendliness. Currently, KNN crystals grown by the seed-free solid-state crystal growth (SFSSCG) method exhibit relatively high dielectric loss. In order to reduce their dielectric loss, Cu, Bi and Mn co-doped KNN lead-free piezoelectric single crystals were prepared by the SFSSCG method. The effects of the CuO doping content on the structure and electrical properties of (1 − x)(0.996KNN–0.004BiMnO3)–xCuO crystals were systematically studied. The results show that an appropriate doping amount of CuO is beneficial for the growth of the crystals and the improvement of electrical properties. When the diameter of the billets is 40 mm, the maximum crystal size of 25 × 21 × 2 mm3 is obtained. All crystals have a typical perovskite structure almost with a single orthorhombic phase and are very compact almost without holes. The minimum average domain width of 276 nm is obtained for the crystal at x = 0.005. The KNN crystals exhibit normal ferroelectric characteristics and a space-charge-limited-conduction (SCLC) mechanism. The introduction of Cu2+ ions reduces the dielectric loss of the crystal and increases its piezoelectric and dielectric properties. The Cu, Bi and Mn co-doped KNN crystals achieve excellent comprehensive properties: d33 = 314 pC N−1, Pr = 29.9 μC cm−2, Ec = 10.1 kV cm−1, = 869 pm V−1, Qm = 56.8, Kt = 0.38, tan δ = 0.013 and TC = 409 °C. This provides a route to improve the electrical properties of KNN-based crystals. [ABSTRACT FROM AUTHOR]

Lead-free has been a goal that people pursue due to the environmental pollution caused by lead in lead-based materials. K0.5Na0.5NbO3(KNN) is one of the most promising lead-free piezoelectric materials. Theoretically, the piezoelectric properties of single crystals are much better than those of polycrystalline materials. However, it is difficult to fabricate uniform KNN-based crystals by the high-temperature melt method because of volatilization of elements. This method also needs complicated equipment and is expensive. Here, a seed-free, solid-state crystal growth (SFSSCG) method was used to prepare Mn, Li, and Bi co-doped KNN crystals, which show an ultrahigh piezoelectric coefficient of about 1050 pC N−1, around one order of magnitude higher than that of pure KNN ceramics. Furthermore, the effective piezoelectric coefficient d33* of this crystal can reach 2290 pm V−1 at a unipolar electric field of 10 kV cm−1. This crystal also shows a high Curie temperature and good ferroelectricity. It is expected that the lead-free KNN-based single crystals fabricated by this simple method show promise for high performance ferroelectric and piezoelectric applications in industry and are environment-friendly. [ABSTRACT FROM AUTHOR]