High temperature stable dielectric properties of (K_0.5Na_0.5)_0.985Bi_0.015Nb_0.99Cu_0.01O₃ ceramics with core-shell microstructures

New high temperature ceramic capacitors (K_0.5Na_0.5)_(1-x)Bi_xNb_(1-2x/3)Cu_2x/3O₃ (x = 0-0.03) with Bi substitution for the A site and Cu substitution for the B site were synthesized using a solid state reaction process. Owing to its inhomogeneous microstructure and diffuse phase transition, (K_0.5Na_0.5)_0.985Bi_0.015Nb_0.99Cu_0.01O₃ shows a stable permittivity (∼1350) over a broad temperature range (40-520 °C) and the capacitance variation is maintained smaller than ±15%, thereby paving the way for achieving high temperature capacitors in KNN based materials. Furthermore, the grain and grain boundary contributions to the capacitance have been separated using impedance spectroscopy, and the grain boundary effects show temperature independent capacitance, whereas grain effects show a capacitance maximum at the tetragonal to cubic phase transition.