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Room temperature ferromagnetic (neither ferrimagnetic nor canted antiferromagnetic) spin ordering is here designed in a predictive way and demonstrated experimentally for the first time with Curie temperature T C(FM) > 450K within synthesizable d5-d3/d0 solid solution double-perovskite oxides. Meanwhile, a Volcano-shaped trend between Weiss temperature Θ and μ × r A /r B ensemble descriptor (μ, r A and r B are reduced mass of primitive cell, A-site cation radius and B-site cation radius, respectively) was observed and able to quantitatively predict chemical composition of double-perovskite oxide for various spin ordering including ferromagnetism, spin frustration and ferrimagnetism. Lattice symmetry, optical bandgap and insulating properties were also characterized experimentally and prospect for room temperature ferromagnetic semiconductor discussed.

A nanoscale core–shell NiO@CoO composite is prepared on flexible carbon cloth for electrodes in supercapacitors and overall water splitting. The needle-like CoO nanowires with NiO nanosheets as the active materials improve the elemental constituents as well as surface area. The NiO@CoO electrode boasts a capacity of 2.87 F cm−2 (1024.05 F g−1) at 1 A g−1 current density, and even at a large current density of 20 A g−1 the retention ratio is 80.9% after 5000 cycles. The excellent specific capacity with high rate capability can be ascribed to the unique structure which increases the area of the liquid–solid interface and facilitates electron and ion transport, improving the utilization efficiency of active materials. The asymmetric hybrid supercapacitor prepared with the core–shell electrode shows the energy output of 40.3 W h kg−1 at 750 W kg−1 with a better retention (71.7%) of specific capacitance after 15 000 cycles. In addition, linear sweep voltammetry is performed to assess the performance of the electrode in water splitting and the electrode shows excellent activity in the OER as manifested by a Tafel slope of 88.04 mV dec−1. Our results show that the bifunctional structure and design strategy have large potential in energy applications.

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.

This work was performed under contract to the European Space Agency under the Thermoelectric Converter for Small-Scale RTG programme, 23026/10/NL/AT.

A new photoanode material, Cu(II) silicotungstate — K 6 SiW 11 O 39 Cu(H 2 O)·xH 2 O modified TiO 2 (SiW 11 Cu/TiO 2 ), has been successfully prepared by a combined hydrolysis and hydrothermal method. When SiW 11 Cu/TiO 2 is mixed with P25 powder in a weight ratio of 1:9 as a photoanode in dye-sensitized solar cell, an improved power conversion efficiency of 7.90% is achieved, significantly higher than that of the cell using P25 as a photoanode (6.16%). The results show that the lifetime and the injection ability of electrons are improved after the modification of the photoanode. The large improvement in device performance is ascribed to the decrease of the charge carrier recombination and the enhanced injection ability of electrons. Furthermore, the introduction of SiW 11 Cu can synchronously extend the absorption to the visible region, which should also be beneficial to the increase of the device performance.

The production of nanostructured bulk materials from silicon powders has been well documented as being one way of bringing down the thermal conductivity of silicon while still maintaining its high power factor. This reduction of thermal conductivity is predicted to lead to significant increases in its figure-of-merit, ZT. The size of the starting particles has a major effect on the nanostructuring and grain size of the final silicon-based materials. Using particles of differing size and distribution, pellets were produced using spark plasma sintering. The results show a significant lowering in the thermal diffusivity as the particle size in the powders is decreased. As the starting particle size deceases from 1 μm to 60 nm, we see a tenfold decrease in the thermal diffusivity at 300 K, from 20 mm2 S−1 to 2 mm2 S−1. Both these show a significant decrease from the thermal diffusivity of 88 mm2 S−1 observed from bulk silicon. A further decrease to 1 mm2 S−1 is observed when the particle size of the starting material is decreased from 60 nm to sub-10 nm. The results also highlight the potential of using particles from solution approaches as a potential starting point for the prediction of nanostructured bulk materials.

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.

Magnesium tin silicide based thermoelectrics contain earth abundant and non-toxic elements, and have the potential to replace established commercial thermoelectrics for energy conversion applications. In this work, porosity was used as a means to improve their thermoelectric properties. Compared to dense samples of Sb doped Mg2Si0.5Sn0.5 with a maximum zT of 1.39 at 663 K, porous samples (37% porosity) prepared by a pressure-less spark plasma sintering technique showed significantly lower thermal conductivity and higher Seebeck coefficient, resulting in an increased maximum zT of 1.63 at 615 K. The possible origins of the enhanced Seebeck coefficient can be attributed to a change of carrier concentration and modification of the band structure, produced by microstructural engineering of the surface composition and particle–particle contacts.

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.

XNiSn (X = Ti, Zr and Hf) half-Heusler alloys have promising thermoelectric properties and are attracting enormous interest for use in waste heat recovery. In particular, multiphase behaviour has been linked to reduced lattice thermal conductivities, which enables improved energy conversion efficiencies. This manuscript describes the impact of spark plasma sintering (SPS) on the phase distributions and thermoelectric properties of Ti0.5Zr0.5NiSn based half-Heuslers. Rietveld analysis reveals small changes in composition, while measurement of the Seebeck coefficient and electrical resistivities reveals that all SPS treated samples are electron doped compared to the as-prepared samples. The lattice thermal conductivities fall between 4 W·m−1·K−1 at 350 K and 3 W·m−1·K−1 at 740 K. A maximum ZT = 0.7 at 740 K is observed in a sample with nominal Ti0.5Zr0.5NiSn composition.

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 the surfaces of SrBi4Ti4O15 above 700 °C, while reacting zones of intermediate phase(s) with less Bi(3+) 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.

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.

In the frame of the current research, thep-type Bi2Te3doped (GeTe)0.95(Bi2Te3)0.05alloy composed of hot pressed consolidated submicron structured powder was investigated. The influence of the process parameters (i.e., powder particles size and hot pressing conditions) on both reduction of the lattice thermal conductivity and electronic optimization is described in detail. Very high maximalZTvalues of up to∼1.6 were obtained and correlated to the microstructural characteristics. Based on the various involved mechanisms, a potential route for further enhancement of theZTvalues of the investigated composition is proposed.

Bi2Te3 based alloys play a dominant role in commercial applications in the fields of thermoelectric energy generation and solid state cooling. By combining the densification of nanostructured powders followed by a two-step hot forging process, hierarchical nanostructured p-type Bi0.5Sb1.5Te3 alloys with good preferred orientation were successfully fabricated. The Seebeck coefficient in the direction perpendicular to the pressing force, which is highly anisotropic, is much greater than that of the material sintered via one-step sintering. Meanwhile, the nanostructure and crystal defects produced during hot forging also contribute to both higher Seebeck coefficient, and lower thermal conductivity due to more effective and preferential scattering of phonons than electrons. As a result, a 50% enhancement of ZT value (from 1 to above 1.5) in the orientated, hierarchical, nanostructured alloys was obtained.

The perovskite-like layered structure ( PLS ) A 2 B 2 O 7 compound Sr 2 Nb 2 O 7 was doped with donor dopants CeO 2 and WO 3 to explore their doping effect on its A and B site, respectively. The doped ceramics were prepared by Spark Plasma Sintering. For Ce doping on the A site, single phase was maintained up to 5 mol% Ce ( x =0.05 in (Sr 1− x Ce x ) 2 Nb 2 O 7 ). For W doping on the B site, single phase was maintained at 2.5 mol% W ( x =0.025 in Sr 2 (W x Nb 1− x ) 2 O 7 ). The cerium and tungsten doping both inhibited grain growth and changed the grain morphology, leading to less anisotropic grains. The Curie point T c was obtained by measuring the temperature dependence of the dielectric constant and it was found to reduce for both Ce and W doped SNO. The W doped ceramics showed a diffuse ferroelectric phase transition at the Curie point. The DC resistivity of tungsten and cerium doped SNO increased compared to undoped SNO at temperatures below 700 °C. These results showed that both Ce and W had a strong influence on the dielectric and electrical properties of the Sr 2 Nb 2 O 7 ceramics.

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.

Spark plasma sintering (SPS) is a versatile and effective rapid processing technique to sinter nanostructured thermoelectric (TE) materials. This paper presents a combined experimental and modelling investigation of the Peltier effect during SPS processing of TE materials. The Peltier effect is related to the transport of heat by an electric current and produces a temperature gradient. This heat flow manifests itself at the junction between two dissimilar materials at which heat is evolved or absorbed. During SPS processing, the Peltier effect could manifest itself as a significant temperature gradient across the sample. The Peltier effect was found to produce a temperature gradient of 55 °C during the SPS processing of MgSi2 (ZT = 1.4, 3 mm sample thickness) and 60 °C for MnSi1.4 (ZT = 0.4, 3 mm sample thickness). The overall developed experimental and numerical methodology allowed us to confirm the existence of the Peltier effect and to quantify its magnitude. This enabled us to determine a correlation between the SPS process parameters and temperature gradient across the processed TE materials.

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.

Radioisotope fuels produce heat that can be used for spacecraft thermal control or converted to electricity. They must retain integrity in the event of destruction or atmospheric entry of the parent spacecraft. Addition of a metal matrix to the actinide oxide could yield a more robust fuel form. Neodymium (III) oxide (Nd 2 O 3 ) – niobium metal matrix composites were produced using Spark Plasma Sintering; Nd 2 O 3 is a non-radioactive surrogate for americium (III) oxide (Am 2 O 3 ). Two compositions, 70 and 50 wt% Nd 2 O 3 , were mechanically tested under equibiaxial (ring-on-ring) flexure according to ASTM C1499. The addition of the niobium matrix increased the mean flexural strength by a factor of about 2 compared to typical ceramic nuclear fuels, and significantly increased the Weibull modulus to over 20. These improved mechanical properties could result in reduced fuel dispersion in severe accidents and improved safety of space radioisotope power systems.

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).

Ferritic steels strengthened with Ti-Y-O nanoclusters are leading candidates for fission and fusion reactor components. A Fe-14Cr-0.4Ti + 0.25Y2O3 (14YT) alloy was fabricated by mechanical alloying and subsequently consolidated by spark plasma sintering (SPS). The densification of the 14YT alloys significantly improved with an increase in the sintering temperature. Scanning electron microscopy and electron backscatter diffraction revealed that 14YT SPS-sintered at 1150 °C under 50 MPa for 5 min had a high density (99.6%), a random grain orientation and a bimodal grain size distribution (2Ti2O7, FeO, and chromium carbides, while transmission electron microscopy and atom probe tomography showed uniformly dispersed Y2Ti2O7 nanoclusters of 23 m-3. Due to the very much shorter consolidation times and lower pressures used in SPS compared with the more usual hot isostatic pressing routes, SPS is shown to be a cost-effective technique for oxide dispersion strengthened (ODS) alloy manufacturing with microstructural features consistent with the best-performing ODS alloys.

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.

In this work, a silica-based glass–ceramic coating has been deposited on higher manganese silicide (HMS) thermoelectrics in order to provide oxidation protection at high temperature (up to 600 °C). The coating showed good densification behaviour and an excellent thermomechanical and chemical compatibility with the HMS substrate, suppressing the silica scale formation on the HMS at high temperatures. The thermoelectric properties of the glass–ceramic-coated HMS showed excellent stability after oxidation at 600 °C in air for 500 h.

Piezoelectric materials are used as sensors or actuators in many devices. Here, the authors demonstrate that semiconducting ZnO particles embedded into a Bi0.5Na0.5TiO3-based matrix improve its piezoelectric properties, promising an alternative to presently used lead-based materials.

Tellurium based glasses have interesting thermoelectric characteristics. However, their high electrical resistivity is still an obstacle to considering them for thermoelectric applications. In this work, the (Te85Se15)60 − 0.6xAs40 − 0.4xCux glass system was studied. This revealed that Cu can act as glass former and increase both glass thermal stability and electrical conductivity. The best candidate, (Te85Se15)45As30Cu25, was chosen to prepare composites with Bi0.5Sb1.5Te3 using spark plasma sintering. These glass ceramic samples exhibited a much better thermoelectric performance. Glass ceramics with 50 mol. % of Bi0.5Sb1.5Te3 show a maximum ZT value equal to 0.37 at 413 K. Meanwhile, the advantages of glass including low sintering temperature and high formability are well maintained.

The Cuprous oxide (CuO) thin film was prepared on texturized Si wafer by a simple sol–gel method to fabricate p-CuO/n-Si heterojunction photoelectric device. The novel sol–gel method is very cheap and convenient. The structural, optical and electrical properties of the CuO film were studied by X-ray diffraction (XRD), Scanning Electron Microscope (SEM), X-ray photoelectron spectroscopy (XPS), UV–Vis spectrophotometer and Hall effect measurement. A good nonlinear rectifying behavior is obtained for the p-CuO/n-Si heterojunction. Under reverse bias, good photoelectric behavior is obtained.

Commercial lead-based piezoelectric materials raised worldwide environmental concerns in the past decade. Bi₀.₅Na₀.₅TiO₃-based solid solution is among the most promising lead-free piezoelectric candidates; however, depolarization of these solid solutions is a longstanding obstacle for their practical applications. Here we use a strategy to defer the thermal depolarization, even render depolarization-free Bi₀.₅Na₀.₅TiO₃-based 0-3-type composites. This is achieved by introducing semiconducting ZnO particles into the relaxor ferroelectric 0.94Bi₀.₅Na₀.₅TiO₃-0.06BaTiO₃ matrix. The depolarization temperature increases with increasing ZnO concentration until depolarization disappears at 30 mol% ZnO. The semiconducting nature of ZnO provides charges to partially compensate the ferroelectric depolarization field. These results not only pave the way for applications of Bi₀.₅Na₀.₅TiO₃-based piezoceramics, but also have great impact on the understanding of the mechanism of depolarization so as to provide a new design to optimize the performance of lead-free piezoelectrics.

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.

et al., Cost-effective highly efficient nanostructured n-type Mg2Si 1-x Sn x and p-type higher manganese silicide (HMS) compositions were prepared for the development of practical waste heat generators for automotive and marine thermoelectric applications, in the frame of the European Commission (EC)-funded PowerDriver project. The physical, mechanical, and structural properties were fully characterized as part of a database-generation exercise required for the thermoelectric converter design. A combination of high maximal ZT values of ~0.6 and ~1.1 for the HMS and Mg2Si1-x Sn x compositions, respectively, and adequate mechanical properties was obtained. © 2013 TMS., The work was supported by the EC FP7 Power-Driver Project.

The nanostructured ZnO film was prepared on a texturized Si wafer by a simple sol–gel method to fabricate n-ZnO/p-Si heterojunction photoelectric device. The novel sol–gel method is cheap and convenient. The structural, optical and electrical properties of the nanostructured ZnO film were studied by XRD, SEM, XPS, PL, UV–Vis spectrophotometer and Hall effect measurement. The current–voltage (I–V) curve of nanostructured ZnO/p-Si heterojunction device shows good rectifying behavior. Good photoelectric behavior is obtained.

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.

The microscopic origin of the grain size effects on the dielectric, piezoelectric, and thermal depoling properties of Aurivillius phase Bi3.15Nd0.85Ti3O12 was investigated. Using atomic force microscopy, domain walls were observed in micrometer grain size ceramics, but gradually disappeared with reducing grain size and were not found in ceramics with 90 nm grain size. In strain-electric field butterfly loops, the strain decreased with decreasing grain size indicating a decreasing contribution of non-180° domain walls switching to the strain. Lattice distortion (a−b)/b decreased with decreasing grain size. The thermal depoling resistance decreased with decreasing grain size, due to increasing internal mechanical stresses.

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 effect of strontium substitution on structural, magnetic, and dielectric properties of a multiferroic Y-type hexaferrite (chemical formula Ba2−xSrxMg2Fe12O22 with 0 ≤ x ≤ 2) was investigated. Y-type hexaferrite phase formation was not affected by strontium substitution for barium, in the range 0 ≤ x ≤ 1.5, confirmed by x-ray diffraction and Raman spectroscopy measured at room temperature. Two intermediate magnetic spin phase transitions (at tempertures TI and TII) and a ferrimagnetic-paramagnetic transition (at Curie temperature TC) were identified from the temperature dependence of the magnetic susceptibility. Magnetic transition temperatures (TI, TII, and TC) increased with increasing strontium content. Magnetic hysteresis measurements indicated that by increasing strontium concentration, the coercivity increases, while the saturation magnetization decreases. The 57Fe NMR spectrum of the Y-type hexaferrite measured at 5 K and in zero magnetic field showed remarkable differences compared to that of o...

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.