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To develop a better understanding of the mechanism responsible for topochemical microcrystal conversion (TMC) from Aurivillius SrBi4Ti4O15 precursors to perovskite SrTiO3 microplatelets, compositional/structural evolutions, morphological development, and reaction interface evolution of the (001) oriented SrBi4Ti4O15 microplatelets were investigated during the conversion process. The results show that multiple topotactic nucleation events of SrTiO3 occurred directly on die surfaces of SrBi4Ti4O15 above 700 °C, while reacting zones of intermediate phase(s) with less Bi3+ contents were observed to form in the interior of SrBi4Ti4O15. Extensive exfoliation of the precursors occurred generally parallel to the (001) surfaces above 775 °C. At 950 °C, the original single-crystal SrBi4Ti4O15 platelet was replaced by a polycrystalline aggregate consisting of (001) aligned SrTiO3 crystallites and poorly crystallized intermediate phase(s). With further increasing the temperature or holding time, the SrTiO3 phase formed from related intermediate phase(s), and the aligned crystallites were sintered to form dense SrTiO3 with strong (001) orientation. The obtained SrTiO3 microplatelets preserved the shape of SrBi4Ti4O15 and show high chemical and phase purity. This TMC mechanism has general applicability to a variety of compounds and will be very useful for the design and synthesis of novel anisotropic perovskite crystals with high quality in the future. [ABSTRACT FROM AUTHOR]

The perovskite-like layer structured strontium niobate (Sr2Nb2O7) and calcium niobate (Ca2Nb2O7) are good candidates for high-temperature piezoelectric applications because of their high Curie points. Single-phase dense Sr2Nb2O7 and Ca2Nb2O7 ceramics with grain-oriented microstructures were prepared by spark plasma sintering using a two-step method. The Lotgering orientation factors were 0.86 and 0.70 for Sr2Nb2O7 and Ca2Nb2O7, respectively. The Tc of Sr2Nb2O7 and Ca2Nb2O7 were measured as 1327±5°C and above 1525°C, respectively. The dielectric, piezoelectric, ferroelectric properties, and direct-current electrical conductivity are higher along the directions perpendicular to the pressing direction compared with parallel. The piezoelectric constant d33 for Sr2Nb2O7 was 2.8±0.2 pC/N. [ABSTRACT FROM AUTHOR]

The perovskite-like layer-structured (PLS) Nd2Ti2O7 and La2Ti2O7 have possibly the highest Curie points of any materials. To pole these ceramics, highly textured, dense ceramics with high DC electrical resistivity are required. The ferroelectric and piezoelectric properties of lead-free Nd2Ti2O7 and La2Ti2O7 grain-oriented ceramics prepared by spark plasma sintering using a two-step method are reported. The Tc of Nd2Ti2O7 and La2Ti2O7 are 1482±5° and 1461±5°C, respectively. The measured piezoelectric constant of the textured La2Ti2O7 was d33=2.6 pC/N. These results now open up the possibility of studying the ferroelectric/piezoelectric properties of the PLS family of ceramics with super-high Curie points. [ABSTRACT FROM AUTHOR]

Single-phase bismuth tungstate (Bi2WO6) ceramics with high relative density (>99%) were fabricated by spark plasma sintering. Ferroelectric, dielectric, and piezoelectric properties of Bi2WO6 ceramics were investigated. Almost saturated polarization–electric field ( P– E) hysteresis loops with a remanent polarization ( Pr) of ∼16.1 μC/cm2 and a coercive field ( Ec) of ∼3.7 kV/mm were obtained. Curie point and second phase transition temperatures were observed at 937°±5° and 665°±5°C, respectively. The average piezoelectric constant ( d33) of this high Curie point ceramic is 15±0.2 pCN−1. [ABSTRACT FROM AUTHOR]

Aurivillius phase, bismuth layer structured ferroelectric Bi3.15Nd0.85Ti3O12 (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 degC 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. [ABSTRACT FROM AUTHOR]

Bulk 0.7 PbZr0.52Ti0.48O3-0.3NiFe2O4 nanoceramic composites with a grain size of ~50 nm were fabricated using chemical synthesis, high-energy ball-milling, and spark plasma sintering. The composite produced broad dielectric constant peaks and frequency dispersion, similar to relaxor ferroelectrics, and was ferroelectric and ferromagnetic. The dielectric and piezoelectric properties of nanoceramics are lower than those of microceramics due to grain-size effects. 'Butterfly'-type magnetoelectric (ME) loops indicate that direct stress coupling between magnetostrictive and piezoelectric phases is still active in composite ceramics when the grain size is ~50 nm. [ABSTRACT FROM AUTHOR]