Hybrid Atmosphere Processing of Lead-Free Piezoelectric Sodium Potassium Niobate-Based Ceramics

K0.5Na0.5NbO3-based ceramics, a promising group of lead-free piezoelectrics, are challenging to sinter dense while avoiding alkali evaporation. This work explores hybrid atmosphere processing, a new approach where reducing atmospheres is used during heating to avoid coarsening from alkali carbonates and hydroxides, and oxidizing atmospheres is used during sintering to avoid alkali evaporation. Discs of Li-0.06(K0.52Na0.48)(0.94)Nb0.71Ta0.29O3 with 0.25 mol% Mn (KNNLTM) were sintered in air, N-2, 9% H-2 in N-2, or 9% H-2 in N-2 during heating and air during sintering (hybrid atmosphere processing). The highest density was obtained by sintering in 9% H-2 in N-2, but resulted in high alkali loss and decomposition of the surface, followed by low piezoelectric response. However, with the hybrid H-2/air processing it was possible to both avoid surface decomposition and leakage currently associated with alkali evaporation during sintering in H-2, and to obtain a denser, more phase-pure and small-grained KNNLTM ceramic with a higher piezoelectric response than obtained by sintering in air or N-2.