Microstructure development and piezoelectric properties of highly textured CuO-doped KNN by templated grain growth

This paper demonstrates the production of 〈00l〉-oriented CuO-doped (K_0.476Na_0.524)NbO₃ (KNN) piezoelectric ceramics with a polymorphic phase transition (PPT) temperature greater than 180 °C by templated grain growth (TGG) using high aspect ratio NaNbO₃ template particles. A novel (to the KNN system) two-step sintering and annealing process combined with CuO doping is demonstrated to improve density and maximize texture quality (F_00l = 99% and rocking curve FWHM = 6.9°) in textured KNN ceramics. The best electromechanical properties (k_p ≈ 0.58, k₃₁ ≈ 0.33, d₃₃ ≈ 146 pC/N, T_o-t ≈ 183 °C, T_c ≈ 415 °C, ε_r = 202, and tan δ = 0.016) are achieved in 1 mol% CuO-doped KNN with F_00l = 99% and a relative density of 96.3%. The values of d₃₃, k_p, and k₃₁ are 70–90% higher than randomly oriented ceramics and are obtained without a significant reduction in the PPT temperature, resulting in stable piezoelectric performance over a wide temperature range (−50 to 180 °C). These results show that high-quality textured KNN can be obtained by TGG and that a reactive matrix is unnecessary.