Antiferroelectric (AFE) materials have attracted considerable attention for applications in energy-storage dielectric capacitors. From the viewpoint of energy efficiency, AFEs with slim polarization (P)–electric field (E) hysteresis are desirable. Herein, we focused on tetragonal tungsten bronze-type K₂NdNb₅O₁₅ as a candidate for such a material and aimed to control its ferroelectric (FE)–AFE phase transition temperature to obtain the AFE phase at room temperature. In this study, K₂Nd(Nb_1−xTa_x)₅O₁₅ (x = 0–0.20) ceramics were prepared using a solid-state reaction method, and their dielectric properties were evaluated. It was established that the incorporation of Ta lowered the FE-AFE phase transition temperature, and the AFE phase was obtained at room temperature for x = 0.15 and 0.20. Through P-E measurements, narrow P-E double hysteresis loops were observed for x = 0.15 and 0.20. These results provide insight into controlling the temperature range of the AFE phase of K₂NdNb₅O₁₅ to around room temperature.

Semiconductor photocatalysis utilizing visible-light is a promising approach toward our current environmental and energy challenges. Among various visible-light-driven photocatalysts, bismuth vanadate (BiVO₄) is recognized as an attractive material due to its various benefits such as low cost, non-toxicity, narrow bandgap (2.4 eV) with a good response to green light in the visible region, and oxidation ability. Many approaches have been adopted to improve the photocatalytic oxidation activity of BiVO₄-based photocatalysts/photoanodes, however, it is still challenging to further improve its activities. In this review, we provide an overview of our latest progress for photocatalytic and photoelectrochemical O₂ production on particle size and crystallinity control by introducing a newly developed, environmentally-friendly aqueous-based synthetic process. In addition, by specific surface modification, we have demonstrated activity enhancements in the decomposition or selective oxidation of organic molecules on the BiVO₄ photocatalysts. This review provides valuable insights into not only the preparation of highly-crystalline BiVO₄ nanoparticles but also activation strategies for efficient oxidation reactions using BiVO₄.

Fine and homogeneous ceramic powders of bismuth sodium titanate, (Bi_0.5Na_0.5)TiO₃ (BNT), were prepared via a hydrothermal synthesis process by using the Bi₂O₃ and TiO₂ as starting raw materials. In particularly, hydrothermal conditions such as hydrothermal temperature, time and NaOH-aq concentration were controlled and optimized to obtain the fine and homogeneous BNT particles with single perovskite structure. Then, the appropriate hydrothermal conditions were optimized and obtained as NaOH-aq concentration of 12 M, and 240 °C for 24 h in hydrothermal process. Synthesized BNT powders under these conditions shows a single perovskite structure and have uniform spherical shape with the fine particle size of 200 nm in diameter. By using those hydrothermal BNT powders, dense BNT ceramics with more than 95 % of relative density could be obtained after the sintering at 1050 °C for 2 h. Then, the BNT ceramic shows a homogeneous microstructure with 2.8 µm of the grain size from the SEM observation. An electromechanical coupling coefficient k₃₃ = 0.44 and piezoelectric constant d₃₃ = 83 pC/N were calculated from the resonance and anti-resonance method on the BNT ceramic synthesized by the hydrothermal process and sintered at 1050 °C for 2 h. These values are almost comparable with the k₃₃ and d₃₃ values on the conventional BNT ceramics by the dry process sintered at 1140 °C for 2 h.

Lead-free ferroelectric and piezoelectric ceramics, (Bi_0.5Na_0.5)TiO₃ (BNT) ceramics doped with 1.0 wt % CuO (BNT + CuO) were prepared by the controlling the sintering temperature from 890 to 1140 °C. CuO is widely used as a sintering aid because of the formation of a liquid phase in various dielectric ceramics. In this study, we examined the electrical properties of BNT + CuO and discussed the low-temperature sintering mechanism. In addition, the role of CuO was clarified. As a result, the relative density of BNT + CuO sintered at 940 °C was 98 %. In addition, the electrical properties such as electromechanical coupling coefficient k₃₃, piezoelectric constant d₃₃, and depolarization temperature T_d were 0.46, 75 pC/N, and 170 °C, respectively. These values are comparable to those of BNT ceramics sintered at 1140 °C, which is a typical sintering temperature. Therefore, the 1.0 wt % CuO addition enabled BNT ceramics to be sintered at an approximately 200 °C lower temperature. Moreover, to clarify the role of CuO, the compositions of BNT + CuO ceramics sintered at 940 °C were analyzed by scanning electron microscopy/energy-dispersive X-ray spectroscopy (SEM/EDS). A Cu-derived component was observed at the triple point of grains and these components contained Cu, Na, Ti, and O ions rather than Cu alone. That is, the melting point of CuO is lowered by its combination with Na and Ti ions and then the Na–Cu–Ti–O compound forms a liquid phase. Thus, by the addition of CuO, the formation of liquid-phase Na–Cu–Ti–O compounds led to sintering BNT ceramics at 200 °C lower temperature.

We investigated the influence of the local crystal structure and microstructure on the electronic transport properties of (La_1/2K_1/2)TiO₃ (LKT) ceramics as an n-type thermoelectric material. Undoped and Nb-doped LKT ceramics were fabricated via the conventional solid-state reaction route. X-ray pair distribution function (PDF) of the undoped LKT ceramic was analyzed by reverse Monte Carlo (RMC) fitting using a supercell model containing 12 × 12 × 12 perovskite unit cells. The PDF analysis results demonstrated that the local structure of LKT resembled that of SrTiO₃ (ST), implying that LKT possessed comparable electron mobility with ST. Comparing the electrical conductivity and Seebeck coefficient between two Nb-doped LKT ceramics with different grain sizes showed that grain boundaries strongly inhibited the electronic transport in LKT ceramics. These findings suggest that grain coarsening is a key to achieving high electrical conductivity in LKT-based thermoelectric ceramics.

Crystal orientation is the key aspect of epitaxially grown ceramic thin films because it significantly affects their properties. In this study, the evaluation of thin-film orientation using scanning transmission electron microscopy moiré fringes (SMF) is demonstrated. The SMF result from the combination of the crystal lattice fringes in the bright-field image and raster of the scanned electron beam. As the SMF reflect the crystal orientation against the scanning lines, it is possible to observe the crystal orientation distribution in epitaxial thin films. The accuracy of the orientation degrees measured by the SMF is quantitatively evaluated and discussed. As a typical example of an epitaxial film, an yttria-stabilized zirconia (YSZ) film on a Si(001) substrate is subjected to the SMF method to analyze the misorientation degree through the cross section and plan view observations.

Ferroelectric lead zirconate titanate Pb(Zr,Ti)O₃ (PZT) films were deposited on ITO/glass substrates by chemical solution deposition (CSD) technique with processing temperature below the grass-transition point. (100)_c-oriented PZT films were successfully grown on the ITO/glass substrates with the buffer layer of calcium niobate Ca₂Nb₃O₁₀ nanosheets (ns-CN), together with enhanced crystallinity with increasing PbTiO₃ content in PbZrO₃–PbTiO₃ system, i.e., x in Pb(Zr_1−xTi_x)O₃. Moderate remanent polarization of ∼30 µC cm⁻² and relatively high optical transparency of approximately 80 % were accomplished for the PZT films on ns-CN/ITO/glass with x = 0.80 even at the crystallization temperature of 500 °C, whereas it exhibited only lower piezoelectric constant d₃₃ of 22 pm V⁻¹.

Tungsten bronze oxide solid solution Ba_6−xCa_xTi₂Nb₈O₃₀ with space group P4bm (No. 100) was prepared by solid state reaction method. Rietveld refinements revealed a decrease in the volume of the unit cell and distortion of the Ti(Nb)O₆ octahedra with increasing Ca substitution. Ca substitution increased ferroelectric transition temperature T_C and coercive electric field E_C of the samples, decreased their dielectric losses D, and gradually declined the frequency dependence of the dielectric constants.

Wurtzite-type aluminum scandium nitride [WZ-(Al_1−xSc_x)N] thin films were grown on 0.5 wt % Nb-doped SrTiO₃(111) (Nb:STO) single crystal substrates using the radio frequency reactive magnetron sputtering method with Al and Sc targets. WZ-(Al_1−xSc_x)N thin films with 0 ≤ x ≤ 0.49 were epitaxially grown on Nb:STO substrates. Films with x ≤ 0.3 exhibited multi-domain in-plane orientation. The coexistence of two rotation domains, (Al,Sc)N[100]//Nb:STO[110] and (Al,Sc)N[100]//Nb:STO[112], was observed. The abundance of these two domains varied with x, and films with x > 0.3 were single-crystal-like single-domain films. Although the lattice parameters and domain structure intricately changed with x, the calculated unit cell volume was in accordance with the Vegard’s law. These results indicate that the unit cell volume is determined by the chemical composition.

Mn-added (K,Na)NbO₃-based composite lead-free piezoelectric ceramic containing KTiNbO₅ phase was fabricated for potential application to high-power fields. The resulting (K,Na)NbO₃-based composite ceramic with 2 mol % MnO₂ exhibited piezoelectric properties (mechanical quality factor Q_m = 650, piezoelectric constant d₃₃ = 140 pC/N, electromechanical coupling coefficient k_p = 0.42, dielectric constant ε₃₃^T/ε₀ = 750, dielectric loss tan δ = 0.5 %), and adequate heat resistance (Curie temperature T_c = 320 °C). It exhibited an internal electric field, as indicated by a shift of the polarization–electric field hysteresis loop in the direction of the electric field. High-power characteristics of bolt-clamped Langevin transducers prepared using the (K,Na)NbO₃-based composite ceramic was superior to those of a Pb(Zr,Ti)O₃ ceramic at a vibration velocity under high input voltage. The (K,Na)NbO₃-based composite ceramic has good high-power properties and adequate heat resistance, dense microstructure, and process stability, and is considered to be a promising candidates as lead-free piezoelectric ceramics for high-power devices, such as ultrasonic motors, transformers, and transducers.

When exploring new ferroelectric materials computationally, it is necessary to consider the temperature dependence and physical properties of the structure. In the present study, we used a classical molecular dynamics simulation to investigate phase transitions during high-temperature annealing of KNbO₃. The results revealed that a supercell larger than 8 × 8 × 8 is required to reproduce the phase transitions clearly. We also attempted to establish a method for obtaining the interatomic potential for any alloy composition. For PbTiO₃, this was achieved using genetic algorithm fitting, and a phonon dispersion curve similar to that simulated by first-principles calculation was obtained.

As potential candidates for neutron scintillators, divalent rare-earth-doped Li₂CaSiO₄ (Ln:LCS) phosphors were synthesized and their photoluminescence (PL) properties were evaluated. LCS is as light as LiCaAlF₆ and has a low density of 2.86 g/cm³ and a small effective atomic number of 14. In this study, a ferrosilicon powder was used as a safe and convenient reductive medium for the reduction of Ln³⁺ to Ln²⁺. Sm, Eu, and, Yb were doped into LCS, where they occupied the Ca²⁺ sites. Among the phosphors synthesized by embedding in the ferrosilicon powder, Eu:LCS, Yb:LCS, and Eu,Yb:LCS exhibited luminescence arising from 5d–4f electronic transitions of the divalent emitter ion. All three divalent ion-doped phosphors had slower PL decay components with lifetimes of the order of tens of microseconds. Although Ln:LCS appears to be unsuitable for neutron detection under high doses owing to the slow decay of the doped Ln²⁺ emission, the phosphors, notably Eu²⁺ phosphors, have relatively high emission intensities and can potentially be applied as high-temperature neutron scintillators.

Piezoelectric single crystal (SC) plates of ternary system 0.24Pb(In_1/2Nb_1/2)O₃-0.46Pb(Mg_1/3Nb_2/3)O₃-0.30PbTiO₃ manufactured by the continuous-feeding Bridgman method with excellent uniformity were subjected to direct current poling (DCP) and alternating current poling (ACP) processes. And the relationship between their piezoelectric properties and microstructures was investigated using a scanning electron microscope. The dielectric constant (ε₃₃^T/ε₀) and piezoelectric constant (d₃₃) of the DCP SC were 5500 and 1800 pC/N, respectively, while the microstructure was a uniform random domain structure. Under optimum ACP conditions, the SC had ε₃₃^T/ε₀ of 7500 and d₃₃ of 2300 pC/N. Four types of 109° domains with stripes ranging from 1 to 3 µm were observed. The under-poled SC had ε₃₃^T/ε₀ of 1700–6000 and, d₃₃ of 600–2000 pC/N, and a large abnormal microstructure of approximately 25 × 150 µm² was observed along with 109° stripe-like fine domains. Also, in all SC samples, unclear microstructure layers of approximately 20 µm were observed in the area directly below the top and bottom electrodes. These results indicate that the microstructure of piezoelectric SC is non-uniform between small areas (less than 20 µm² in a plate), and that precision is essential when discussing the correlation between piezoelectric properties and the microstructure of SCs.

Grain boundary and local structure of Sr-doped Li-excess garnet Li_7+xLa_3−xSr_xZr₂O₁₂ (Sr-doped LLZO) with enhanced Li-ion conductivity were examined in this study. Cross-sectional observation by scanning electron microscopy (SEM) and time-of-flight secondary ion mass spectrometry (TOF-SIMS) revealed that the formation of the grain boundary structure is suppressed by the addition of Sr to LLZO. Local structural analysis on La and Zr sites by X-ray absorption near edge structure (XANES) revealed an actively changed structure around the Zr site, which was not observed around the La site where Sr is substituted. Furthermore, the XANES spectral changes suggest two types of Li conduction path structures in Sr-substituted LLZO.

A simulation program for phase separation using the phase-field method was developed to simulate the spinodal decomposition process in the growth of Sr excess SrTiO₃ films using dynamic aurora pulsed laser deposition (PLD). Effects of magnetic field application of dynamic aurora PLD and growth processes were introduced into the phase-field model. Consequently, spontaneous superlattice formation in film growth was reproduced. Furthermore, the validity of this simulation was discussed based on the relation between the superlattice period and the amount of cation impingement, and growth temperature. The simulation results reproduced the experimentally obtained data, showing no superlattice formation when the amount of impinging cations was low. The activation energy for diffusion was obtained from the simulation results as 0.49 eV, which is similar to the 0.25 eV obtained from the experimentally obtained data. Results obtained from simulations are qualitatively appropriate.

Sodium-ion batteries promise to be a low-cost, environmentally-friendly alternative to lithium-ion batteries, as the latter present a number of problems in terms of safety, cost, and limited mineral resources that need to be overcome for batteries to be used widely in vehicles and stationary energy storage systems. Newly developed cathode material Na₄Co₃(PO₄)₂P₂O₇ is attracting attention for use in sodium-ion secondary batteries because of its high rate capability, high capacity, and high voltage compared to other candidate materials. We performed first-principles molecular dynamics simulations of the system Na_xCo₃(PO₄)₂P₂O₇ for 0 < x < 4 using the GGA+U formalism of density functional theory, examining in detail the Na-ion migration mechanism and other atomic-level features. Local electronic and crystal structure changes during Na removal and insertion confirm that Na ions migrate via a 3D conduction pathway with low activation barriers of 0.10–0.17 eV within the orthorhombic lattice. Na ions spend most of their time migrating in the 2D main channels, occasionally traversing the short distance between channels in the a direction. These low values help explain why this material is able to support high charge/discharge rates, making it a promising cathode material for Na-ion battery systems.

The amorphous grain boundary phase contents of aluminum nitride (AlN) was estimated by the linear combination fitting method of XAFS, which is new approach. C-axis lattice parameters of the AlN crystals and median diameters of the AlN grains were also measured and correlations with thermal conductivities of the ceramics were analyzed. As a result, the amorphous grain boundary phase contents showed a negative correlation. On the other hand, the c-axis parameters and the median diameters showed positive correlations. Furthermore, a multiple regression analysis showed that the three properties could approximately explain the thermal conductivity and that the amorphous grain boundary phase contents and the median diameters dominated.

Alkali sodalites containing boron (B) as the main framework cation are scarce. A cubic lithium sodalite containing B in the sodalite framework, Li₈B₆Si₆O₂₄Cl₂, whose lattice parameter [7.8856(1) Å] is the smallest of the known alkali sodalites, has been discovered, and Li₈B₆Si₆O₂₄Cl₂-based glass-ceramics have been prepared from the Li₂O–B₂O₃–SiO₂–LiCl quaternary system. The relative density of the resulting glass-ceramics and weight fraction of Li₈B₆Si₆O₂₄Cl₂ in their crystalline part reached ∼0.98 and ≳0.9, respectively. The dc conductivity recorded at 60 °C with non-blocking Li–Au electrodes was ∼1.5 × 10⁻⁶ and ∼2.3 × 10⁻⁷ S cm⁻¹ in partially and highly crystallized samples, respectively, and the activation energy of conductivity was ∼0.5–0.6 eV. The glass-ceramics were stable in contact with Li metal and the transport number of Li⁺ ion was ∼0.95.

The surface of NASICON-type solid electrolyte Li_1.3Al_0.3Ti_1.7(PO₄)₃ (LATP) particles was modified with Co ions as a sintering aid by a wet chemical method. The effects of the inert and atmospheric sintering conditions on the sinterability of the Co element-modified LATP (Co-LATP) were investigated. With Co addition at x = 0.5 wt %, the densities of Co-LATP sintered at 800 °C in air and argon by the conventional sintering (CS) method were 81 and 66 %, respectively. In the case of the CS method in an air atmosphere, the reaction phases such as LiCoPO₄ (LCP) and lithium phosphate were formed and acted as sintering aids. For the case of the conventional method in Ar, LCP was not formed, but reduction of the Ti of LATP and reduced TiO_2−x were formed. These results suggested that the reaction mechanism of Co-LATP differs under each atmosphere in the conventional method. In conclusion, we succeeded in densifying Co-LATP by a two-step sintering (TSS); the first-step is sintering at 400 °C in air and the second-step is sintering at 800 °C in Ar. Co-LATP (x = 0.0–2.0 wt %) sintered under the optimized TSS condition, Co-LATP at x = 0.5 wt % showed the highest electrical conductivity of 1.3 × 10⁻⁴ S/cm at room temperature.

The morphology- and size-controlled syntheses of KNbO₃ nanoparticles, which are demanded by ferroelectric and piezoelectric applications, have been attempted by hydrothermal and solvothermal methods in the typical Nb₂O₅–KOH reaction systems, but the resultant KNbO₃ nanoparticles generally had a wide particle size distribution and formed severe aggregations. In this study, dispersible homogeneous KNbO₃ nanocubes were synthesized from layered potassium niobate, K₄Nb₆O₁₇, by the solvothermal method. The K₄Nb₆O₁₇ microsheets could be completely converted into KNbO₃ at the lower K/Nb atomic ratio as compared to the Nb₂O₅ nanoparticles because it requires a relatively minor rearrangement of NbO₆-unit network in K₄Nb₆O₁₇ to form the KNbO₃ nuclei. The homogeneous KNbO₃ nanocubes with the ferroelectric orthorhombic phase were obtained at the higher K/Nb atomic ratio, and their colloidal suspensions were stabilized by the electrostatic repulsion between the negatively charged surfaces of the KNbO₃ nanocubes.

B₂O–SiO₂ borosilicate glasses in a specific composition range show a spinodal-type phase separation by heat-treatment at around those glass transition temperatures, and porous glasses or porous glass-ceramics can be easily obtained via an acid treatment after the phase separation. We have studied preparation conditions of SrTiO₃-precipitated porous glass-ceramics for a photocatalyst of hydrogen generation using solar energy. It was found that the SrTiO₃ crystal was observed for a SrO–BaO–TiO₂–SiO₂ glass, while the SrTiO₃ crystal disappeared from SrO–TiO₂–B₂O₃–SiO₂ glasses, which contain the B₂O₃ component necessary for the phase separation. For this problem, we found that co-doping of K₂O and Al₂O₃ components is effective, and a SrTiO₃ containing porous glass-ceramic was successfully obtained by the co-doping. Molecular dynamics (MD) simulations were performed to understand the migration of Sr²⁺ ions in SrO–TiO₂–B₂O₃–SiO₂ glasses with K₂O and Al₂O₃ components. This paper is the first to describe the dynamics of alkali metal/alkaline earth metal ions in those glasses and the mechanism of SrTiO₃ precipitation in borosilicate glasses.

A correlation between the elastic modulus and photoelastic constant (PEC) was studied in binary Sn²⁺- and Bi³⁺-containing silicate glass. The Young’s and shear moduli decrease with the increase in the refractive index, whereas they increase with the increase in the PEC. The former is directly related to the strain-optic coefficient, and the latter is inversely related to the elasto-optic coefficient. A working hypothesis for obtaining highly refractive and very small photoelastic silicate glass is proposed based on the short-range structures of SnO- and Bi₂O₃-silicate glass that have been elucidated using ²⁹Si magic angle spinning-nuclear magnetic resonance.

Solid oxide fuel cells (SOFCs) are power generation systems that exhibit high energy conversion efficiency. Biogas is a renewable energy source generated by the fermentation of waste and its application as a fuel for SOFCs has attracted much attention to improve its effective utilization rate. Inhibiting carbon deposition from fuel on the anode is a critical problem in the development of SOFCs fueled with biogas. The electrochemical performance and ability to inhibit carbon deposition of nickel-copper alloy (Ni_1−xCu_x; x = 0.0, 0.2, 0.4, 0.6)/gadolinium-doped ceria (Ce_0.9Gd_0.1O_1.95; GDC) cermet anodes were investigated for intermediate-temperature solid-oxide fuel cells (IT-SOFCs) at 700 °C using humidified (3 vol % H₂O) simulated biogas with a molar ratio of CH₄/CO₂ = 2/1 as the fuel. The Ni_1−xCu_x/GDC anodes effectively inhibited carbon deposition on the anodes with increasing Cu content, although cell performance deteriorated. Moreover, the Ni_0.6Cu_0.4/GDC and Ni_0.4Cu_0.6/GDC anodes led to stable outputs of the cells during the 24 h electrochemical tests with minimal change in cell performance. In conclusion, the Ni_0.6Cu_0.4/GDC and Ni_0.4Cu_0.6/GDC anodes were found to be promising for biogas fuel at an intermediate temperature of 700 °C. These results provide a guideline for the practical application of IT-SOFCs directly fueled with actual biogas.

Sol–gel synthesis of TiO₂/SiO₂ multilayers was studied for application to structural color materials. The purpose of this study is to achieve crack-free thick films, which are indispensable for high reflectance and color vividness. We investigated the effect of polymer additives and heating conditions on the fracture of TiO₂ gel films. The critical cracking thickness increased more than three-fold at maximum by adding poly(vinyl pyrrolidone) (PVP) to precursor sols and by using a low heating rate. From stress measurement based on Stoney’s model, we found that the addition of PVP extensively reduced residual stress, proving the mechanism suggested so far that PVP hinders the evolution of the Ti–O network and reduces drying stress. Under the restriction of the critical thickness, TiO₂/SiO₂ multilayered films and flakes were prepared as a feasibility test of structural coloration. A high-order interference condition was used for the optical design, which allows us to obtain narrow spectral peaks and the primary colors of light. Theoretically expected reflection spectra that corresponded to blue and green colors were attained for five-layered films.

In this paper, we have successfully prepared porous chromium carbide ceramics by a facile one-step method. The synthesis reaction is carried out in an autoclave using waste polyethylene as a carbon source. Generally, the as-obtained chromium carbide is characterized by X-ray diffraction, scanning electron microscope (SEM) and transmission electron microscope. X-ray diffraction pattern shows that the as-obtained sample is cubic phase chromium carbide (Cr₂₃C₆). The SEM results show that the sample is composed of porous particles with an average pore size of 200 nm. In addition, the oxidation resistance of the as-obtained Cr₂₃C₆ sample was studied by thermogravimetric analyzer. The resource utilization of waste plastic can solve the problems of energy shortage and environment pollution simultaneously to achieve sustainable development.

In this study, three types of Al₂O₃ powders were used as raw materials to fabricate and evaluate the performance of all-alumina multi-layer filters, and an attempt was made to fabricate an ultrafiltration filter that separates and removes proteins. When bovine serum albumin (BSA, molecular weight 66,000) was used as the protein, a very high rejection rate of 91.9 % was obtained even under mild conditions with a differential pressure of 1 atm or less.