Ferroelectric Domain Structures and Electrical Properties of Fine‐Grained Lead‐Free Sodium Potassium Niobate Ceramics

Piezoelectric ceramics of the composition Na 0.5 K 0.5 NbO 3 (NKN) with grain sizes in the range of 0.2–1 μm were fabricated by spark plasma sintering. Ferroelectric domain size decreases with decreasing grain size and non‐180° ferroelectric domains walls were still visible in 200 nm sized grains. The Curie point of the ceramics was grain size independent. This suggests that the critical grain size for a single domain single grain structure for NKN is <200 nm. Optimized processing conditions enabled ceramics of high densities (>99.5% theoretical density) to be made at T ≥850°C. For the dense ceramics (grain size ≥350 nm), the room temperature dielectric constant and coercive field increased with decreasing grain size. The remnant polarization was grain size independent. The material sintered at 850°C is a very good candidate for lead‐free piezoelectric applications because of its high piezoelectric constant ( d 33 =160 ± 2 pC/N).