Piezoelectric ceramics of the composition Na 0.5 K 0.5 NbO 3 (NKN) with grain sizes in the range of 0.2-1 lm were fabricated by spark plasma sintering. Ferroelectric domain size decreases with decreasing grain size and non-1801 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 o200 nm. Optimized processing conditions enabled ceramics of high densities (499.5% theoretical density) to be made at T 8501C. 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 8501C is a very good candidate for lead-free piezoelectric applications because of its high piezoelectric constant (d 33 5 16072 pC/N).
Aurivillius phase, bismuth layer structured ferroelectric Bi(3.15)Nd(0.85)Ti(3)O(12) (BNdT) ceramics with average grain sizes from 90 nm and high densities (>97%) were fabricated by spark plasma sintering. Decreasing grain size produced a diffuse ferro-paraelectric phase transition and a decrease in the Curie point. Compared with BNdT ceramics with grain sizes of micrometre scale, nanograined BNdT ceramics exhibit a depression of the dielectric maximum at the Curie point, enhanced dielectric constant from room temperature to 350 degrees C and dramatically decreased losses. Although ferroelectric switching was greatly inhibited in nanograined ceramics, both ferroelectric and piezoelectric measurements still clearly showed that BNdT ceramics with 90 nm average grain sizes are ferroelectrically switchable. This is the first reported evidence that nanoscale Aurivillius phase ceramics are ferroelectrically active.
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