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The ferroelectric compound calcium orthovanadate Ca3(VO4)2 was prepared as a ceramic by a mixed oxide route and conventional sintering. The single phase of the calcined powder and sintered ceramic was confirmed by X-ray diffraction. The Curie point Tc was obtained by measuring the anomaly of temperature dependence of the dielectric constant and loss tangent, and also by Differential Scanning Calorimetry (DSC). The Tc values derived from the two techniques were 1094 ± 5°C and 1097 ± 5°C, respectively. Ferroelectric domain switching was not observed even at 200°C, which was attributed to the large coercive field of the compound.

In this work, a large size lead-free piezoelectric single crystal, (K,Na,Li)(Nb,Ta,Sb)O3 (KNLNTS) with the dimensions of 8.5 � 8.5 � 13.5 mm 3 was successfully grown by the top-seeded solution growth method. This KNLNTS single crystal with high compositional homogeneity is in the tetragonal phase at room temperature. The Curie temperature TC of the tetragonal-cubic phase transition temperature is 210 1C. The piezoelectric coefficients and electromechanical coupling factors of the [001]C oriented KNLNTS single crystal are d33¼172.55 pC/N, d31 ¼� 71.90 pC/N, k31 ¼0.327, k33¼0.523, and kt¼0.541. In addition, the crystal shows good thermal stability so that it can be used for making high temperature electromechanical devices.

The ferroelectric properties of 2-layer Dion–Jacobson compounds ABiNb2O7 (A = Rb and Cs) were studied. Ferroelectricity and piezoelectricity of CsBiNb2O7 were demonstrated for the first time. The ferroelectric domain structure of Dion–Jacobson compounds were imaged using PFM. The Curie points of RbBiNb2O7 and CsBiNb2O7 are 1098 ± 5 and 1033 ± 5 °C, respectively. The piezoelectric constant of RbBiNb2O7 and CsBiNb2O7 are approximately 5 and 8 pC N−1. Thermal depoling was also studied to confirm the Curie temperature and the stability of the piezoelectricity.

Er-doped 0.94Bi0.5Na0.5TiO3-0.06BaTiO3 (BNT-6BT: xEr, x is the molar ratio of Er 3+ doping) lead-free piezoceramics with x = 0–0.02 were prepared and their multifunctional properties have been comprehensively investigated. Our results show that Er-doping has significant effects on morphology of grain, photoluminescence, dielectric, and ferroelectric properties of the ceramics. At room temperature, the green (550 nm) and red (670 nm) emissions are enhanced by Er-doping, reaching the strongest emission intensity when x = 0.0075. The complex and composition-dependent effects of electric poling on photoluminescence also have been measured. As for electrical properties, on the one hand, Er-doping tends to flatten the dielectric constant-temperature (er-T) curves, leading to temperature-insensitive dielectric constant in a wide temperature range (50°C–300°C). On the other hand, Er-doping significantly decreases the ferroelectric-relaxor transition temperature (TF–R) and depolarization temperature (Td), with the TF–R decreasing from 76° Ct o 42°C for x= 0– 0.02. As a result, significant composition-dependent electrical features were found in ferroelectric and piezoelectric properties at room temperature. In general, piezoelectric and ferroelectric properties tend to become weaker, as confirmed by the composition-dependent piezoelectric coefficient (d33), planar coupling factor (kp), and the shape of polarization-electric field (P–E), current-electric field (J–E), bipolar/unipolar strain-electric field (S–E) curves. Furthermore, to understand the relationship between the TF–R/Td and the electrical properties, the composition of x = 0.0075 has been intensively studied. Our results indicate that the BNT-6BT: xEr with appropriate Er-doping may be a promising multifunctional material with integrated photoluminescence and electrical properties for practical applications.

The ferroelectric and dielectric properties of cerium (Ce) substituted La2Ti2O7 (LTO) have been investigated. Single phase, dense La2-xCexTi2O7 (x=0.15, 0.25, 0.35) ceramics were prepared by spark plasma sintering. The solubility limit of Ce in La2-xCexTi2O7 was found to be between 0.35 and 0.5. The a-, b- and c-axes of the unit cell decrease with increasing Ce substitution. The Curie point (Tc) of La2-xCexTi2O7 (x=0, 0.15, 0.25, 0.35) decreases and dielectric constant and loss increase with increasing Ce substitution. Cerium can increase the d33 of La2Ti2O7. The highest d33 was 3.9±0.1pC/N for La1.85Ce0.15Ti2O7 textured ceramic.

Different levels of isovalent dopant vanadium were doped on the B site of Sr2Nb2O7 to explore the doping effect on its dielectric and ferroelectric properties. A second phase could be detected with above 1 mol% V2O5 doping. The grains after doping remained anisotropic and plate-like. The samples are not dense, as there are pores present at the grain boundaries. The corrected dielectric constant for 0.1 mol%, 0.2 mol%, and 0.3 mol% V doped SNO at 1 MHz is around 46, 48 and 49, respectively, which indicates the effect of V doping on the increase of dielectric constant of SNO (er is about 40 at 1 MHz). The Curie point Tc increased with the increase in the V doping level. The relatively high d33 (2.3 pC/N) measured from the non-dense and untextured samples indicates the potential effect of V2O5 doping on the improvement of piezoelectricity of SNO.

AbstractThe perovskite-like layer structured praseodymium titanate (Pr2Ti2O7; PTO) is a good candidate for high temperature piezoelectric application because of its high Curie point. Single phase, dense, textured PTO ceramic was prepared by spark plasma sintering using a two-step method for the first time. The Lotgering orientation factor was 0·87. The Curie point of PTO is >1555±5°C. The dielectric constant increases with grain size due to the decreasing effect of grain boundaries and increasing extrinsic contribution from the ferroelectric domain wall movement. For the first time, PTO was shown to be ferroelectric because it showed piezoelectric activity after poling (d33 = 0·5±0·1 pC N–1).

This work revealed that the solid solution compounds of Sr 2-xBaxNb2O7 are promising lead-free materials for high-temperature piezoelectric sensor application. These compounds were confirmed as ferroelectric materials with high Curie points (> 900°C) by their piezoelectric activity after poling, ferroelectric domain switching in their P-E hysteresis loops and thermal depoling behavior. The effect of Ba substitution on the structure and properties of Sr 2-xBaxNb2O7 (x < 1.0) was investigated. The solid solution limit of Sr2-xBaxNb 2O7 was determined by XRD as x < 0.6. The a-, b-, c- axes, and cell volume increase with Ba addition. The textured ceramics of Sr2-xBaxNb2O7 were prepared for the first time. The highest d33 was measured as 3.6 ± 0.1 pC/N for Sr1.8Ba0.2Nb2O7. © 2012 The American Ceramic Society.

Bulk 0.7PbZr0.52Ti0.48O3–0.3NiFe2O4 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.

Triangular voltage waveform was employed to distinguish the contributions of dielectric permittivity, electric conductivity and domain switching in current-electric field curves. At the same time, it is shown how those contributions can affect the shape of the electric displacement — electric field loops (D–E loops). The effects of frequency, temperature and microstructure (point defects, grain size and texture) on the ferroelectric properties of several ferroelectric compositions is reported, including: BaTiO3; lead zirconate titanate (PZT); lead-free Na0.5K0.5NbO3; perovskite-like layer structured A2B2O7with super high Curie point (Tc); Aurivillius phase ferroelectric Bi3.15Nd0.5Ti3O12; and multiferroic Bi0.89La0.05Tb0.06FeO3. This systematic study provides an instructive outline in the measurement of ferroelectric properties and the analysis and interpretation of experimental data.

Ferroelectrics exhibit bulk photovoltaic effect, of which spontaneous electric polarization separates photo-excited carriers and results in above-bandgap open-circuit voltage. Compared with organometal halide perovskites, ferroelectric oxide perovskites are much more stable in a wide range of mechanical, chemical and thermal conditions and able to fabricate using low-cost facilities. The bottlenecks for ferroelectric photovoltaic applications are their poor optical absorption of visible light and high electric resistivity owing to wide bandgap. In this work, bandgap of BiFeO3-based oxide perovskites was judiciously tuned in a range of 1.10-2.06 eV through B-site chemistry, which makes ferroelectric semiconducting oxide perovskites feasible for photovoltaic solar cell applications with wide light absorption and high efficient energy conversion. In particular, La and Mn co-substituted BiFeO3 solid solution exhibits a narrow direct bandgap around 1.20 eV, which extends photon absorptions over ∼80% sunlight spectrum covering ultraviolet-visible-infrared region and increases electrical conductivity (σdc) up to about 10-5 S/m. The electronic subshell configuration, the reduced mass of unit cell, and the tolerant factor/octahedral factor related to ionic size were attempted as descriptors to classify and map out the relationship between compositions and bandgap properties of BiFeO3-based oxide perovskites. This essay paves the road for ferroelectric semiconducting oxide perovskites to make photovoltaic solar cells with power conversion efficiency promising beyond the Shockley-Queisser limit of conventional p-n junction solar cells.

Abstract The Curie point T c of ferroelectrics sets the upper temperature limit for their application as piezoelectrics. Some Aurivillius phase and perovskite-like layer structured (PLS) materials are good candidates for high temperature piezoelectric applications because of their high T c. The Aurivillius phase CaBi2Nb2O9 and PLS Sr2Nb2O7 were densified by ordinary sintering and/or spark plasma sintering (SPS). All of the materials were single phase, and their grains were plate-like. Their density was >95%. The spark plasma sintered ceramics had grain oriented, textured microstructures. The Curie point of CaBi2Nb2O9 was measured as 943±5°C. Ferroelectric switching in CaBi2Nb2O9 was observed at room temperature. The d 33 of the textured CaBi2Nb2O9 materials was nearly three times that of ordinary sintered materials. The Curie point of Sr2Nb2O7 was measured as 1327±5°C. Ferroelectric and piezoelectric (2·8 pC N−1) properties were observed for spark plasma sintered textured Sr2Nb2O7 ceramics.

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.

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.

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.

A series of ceramics with a general formula, La 2 Ti 2-x Ta x O 7, in which x = 0.05, 0.1, 0.2, and 0.3, were prepared by Spark Plasma Sintering (SPS). Effects of tantalum substitution for titanium on the structure, dielectric, and piezoelectric properties were studied. Results revealed that the structure changed gradually from 4-layer to 3-layer due to the higher valence of Ta . The solid solution limit of tantalum in La 2 Ti 2 O 7 lattice was in the proximity of x = 0.2. The ferroelectric Curie temperature (Tc) decreased with increasing tantalum doping content. dc resistivity reached a maximum value at x = 0.2 with a value of ∼ 1.0 × 108Ω ⋅ cm at 600°C. The influence of texture on the piezoelectric properties of La 2 Ti 2-x Ta x O 7 ceramics was also investigated. A maximum d33 value ∼2.1 pC/N was obtained at x = 0.2.

In 0.95[0.94Bi0.5Na0.5TiO3-0.06BaTiO3]-0.05CaTiO3 ceramics, the temperature TS (dielectric permittivity shoulder at about 125 °C) represents a transition between two different thermally activated dielectric relaxation processes. Below TS, the approximately linear decrease of the permittivity with the logarithm of frequency was attributed to the presence of a dominant ferroelectric phase. Above TS, the permittivity shows a more complicated dependence of the frequency and Raman modes indicate a sudden increase in the spatial disorder of the material, which is ascribed to the presence of a nonpolar phase and to a loss of interaction between polar regions. From 30 to 150 °C, an increase in the maximum polarization with increasing temperature was related to three possible mechanisms: polarization extension favoured by the simultaneous presence of polar and non-polar phases; the occurrence of electric field-induced transitions from weakly polar relaxor to ferroelectric polar phase; and the enhanced polarizability of the crystal structure induced by the weakening of the Bi-O bond with increasing temperature. The occurrence of different electric field induced polarization processes with increasing temperature is supported by the presence of additional current peaks in the current-electric field loops.

Dense BiFeO3 nanoceramics (∼40 nm) were prepared by spark plasma sintering nanopowders synthesized via a sol–gel method and high energy ball milling. The Neel temperature decreases from 340 °C to 265 °C because of enhanced geometry-driven frustration of spin ordering in finer particles. The Curie point shifts to lower temperature (from 697 °C to 655 °C) due to internal stress and a single ferroelectric domain structure. Piezoelectricity in polycrystal ceramics indicates that multiferroic BiFeO3 is still ferroelectrically active at ∼40 nm scale.

The effects of temperature and electric field-induced structural modifications on the energy storage properties of 0.95[0.94Bi0.5Na0.5TiO3–0.06BaTiO3]–0.05K0.5Na0.5NbO3 (BNT–BT–5KNN) ceramics were investigated. X-ray diffraction performed on unpoled and poled ceramics in the temperature range 25–500 °C suggested an increment in the rhombohedral phase intensity peaks and in the tetragonal distortion after electrical poling. The rhombohedral phase content reduced with increasing temperature in both unpoled and poled ceramics. In the unpoled ceramic, the rhombohedral phase eventually disappeared, while it survived in the poled specimen up to 500 °C. The stabilization of the rhombohedral ferroelectric phase by dc poling produced remarkable differences in the temperature dependence of permittivity, loss, current–polarization–electric field loops and energy density. As a consequence of a reversible transition induced by an alternating electric field, competitive energy densities (0.39–0.51 J cm−3 in the range 25–175 °C) with those of lead-based and lead-free bulk ceramics recently developed was obtained, indicating bismuth-based perovskites as potential lead-free systems for energy storage applications.

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.

A single-phase Bi0.91La0.05Tb0.04FeO3 polycrystalline ceramic was fabricated by spark-plasma-sintering the precursor powder prepared by a sol-gel method. Temperature-dependent properties of polycrystalline Bi0.91La0.05Tb0.04FeO3 were characterized by x-ray diffraction, dielectric, and piezoelectric measurement. The x-ray diffraction results revealed a phase transition near 700 °C. Especially, temperature-dependent dielectric behavior demonstrated that there was a dielectric abnormal peak at about 697 °C, in addition to those two well-known dielectric abnormal peaks at 337 °C (Neel temperature) and 831 °C (Curie temperature). The observations, together with thermal depoled behavior, suggest a ferroelectric-ferroelectric phase transition from R3c to Pbnm at around 700 °C.