Transparency of K_0.5N_0.5NbO₃–Sr(Mg_1/3Nb_2/3)O₃ lead-free ceramics modulated by relaxor behavior and grain size

High transparency was obtained in (1−x)(K_0.5Na_0.5)NbO₃–xSr(Mg_1/3Nb_2/3)O₃ (x=0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08) lead-free ceramics by pressure-less sintering procedure. The effects of Sr(Mg_1/3Nb_2/3)O₃ content on the microstructure, phase transition, optical properties and electrical properties were studied in detail. The X-ray diffraction results showed that the crystal structure of ceramics gradually transformed from orthorhombic phase into pseudo-cubic phase with doping of Sr(Mg_1/3Nb_2/3)O₃. The fine grain microstructure with clear grain boundary was observed in all compositions, while the grain size exhibited significant composition dependence. It was found that a more uniform distribution with smaller grain size was favorable to high optical transmittance, owing to the decreased scattering by grains and grain boundaries. In addition, a strong diffuse phase transformation in KNN-based ceramics induced by Sr(Mg_1/3Nb_2/3)O₃ doping, causing the ceramics become more relaxor-like and transparent. The transmittance and electric properties results indicated that the 0.95(K_0.5Na_0.5)NbO₃–0.05 Sr(Mg_1/3Nb_2/3)O₃ ceramics exhibited higher transmittance (60% in the near-IR region) accompanied with better electrical properties (ε_m=2104, P_r=5.0 μC/cm², d₃₃=92 pC/N).