Titanate nanotubes are tubular nano materials for variety of applications. However, because of their large structural aspect ratio, it has been difficult to control aggregation and orientation states for further functionalization. In this review article, summary of our recent achievement on the preparation and functionalization of titanate nanotubes with controlled aggregation or orientations. Titanate nanotube films in which nanotubes oriented to perpendicular to the substrate show adhesive superhydrophobility. On the other hand, hierarchically porous titanate nanotube monoliths are prepared under hydrothermal treatment of condensed titania emulsion which show a large capacity and fast ion exchange capabilities. These achievements clearly indicate that the titanate nanotubes with controlled aggregation/orientation exhibit great potentiality as highly functional materials.

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Microstructural evolution during the sintering of nanoceramics was investigated in the compacts prepared by gelcasting of tetragonal zirconia nano-slurry with the solid loading of 7 vol %. The initial compact prepared by the gelcasting in this study had the densely packed structure with a narrow pore-size distribution and high connectivity in the particle network. Special attentions were paid to the evolutions of pore structures in the initial and intermediate stage sintering. It was found that the coarsening of the pores, which is thought to be the important obstacle of the densification in nanoceramics, was not remarkable in the sintering of the densely packed compact. The suppression of the pore coarsening resulted in the lowering the sintering temperature. Almost fully dense sample with 99.5% of theoretical density was successfully obtained at the sintering temperature of 1373 K for 2 h. The sample sintered at 1373 K maintained the nanometric structure with the grain size of 79 nm and exhibited the optical translucency.

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The effects of Al addition to Si–Ni flux on the flux-mediated pulsed laser deposition of SiC thin films were investigated. A small amount of 3 at.% Al addition was found to cause the dominant growth of a polytype of 6H–SiC (0001) at an early stage of the deposition on a 4H–SiC (0001) on-axis substrate even at a low growth temperature of 1160°C, at which a single polytype of 3C–SiC (111) film is thermodynamically favored. The direct observation of the interface between SiC and Si–Ni based flux with our originally developed high-vacuum laser microscope revealed that the dissolution behavior of SiC into Si–Ni–Al flux becomes milder than that before Al addition to Si–Ni flux. This feature is not inconsistent with the commonly accepted surface flattening effect of Al addition to the Si–Cr solvent in the flux growth of SiC bulk single crystals, and might be partially responsible for the initial preference of the 6H–SiC (0001) polytype growth.

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Ta₂O₅ thin films were deposited at 155–400°C on amorphous SiO₂ and Pt/TiO_x/SiO₂/Si substrates by metal organic chemical vapor deposition (MOCVD) from tri(tert-butoxy)(isopropylimido)tantalum [Ta[NCH(CH₃)₂][OC(CH₃)₃]₃]–O₂, tri(tert-butoxy)(tert-butylimido)tantalum [Ta[NC(CH₃)₃][OC(CH₃)₃]₃]–O₂ and pentaethoxy tantalum [Ta(OC₂H₅)₅]–O₂ systems. Temperature dependency of the deposition rate was almost the same for the films from Ta[NCH(CH₃)₂][OC(CH₃)₃]₃–O₂ and Ta[NC(CH₃)₃][OC(CH₃)₃]₃–O₂ systems irrespective of the kinds of substrates, and the deposition rate of these two systems was higher than that of Ta(OC₂H₅)₅–O₂ system for all deposition temperature range within the present study. Dielectric properties of Ta₂O₅ thin films deposited on Pt/TiO_x/SiO₂/Si substrates at 200°C from Ta[NC(CH₃)₃][OC(CH₃)₃]₃–O₂ system and at 300°C from Ta(OC₂H₅)₅–O₂ system were also investigated. In spite of their 100°C lower deposition temperature, the Ta₂O₅ films from Ta[NC(CH₃)₃][OC(CH₃)₃]₃–O₂ system showed lower leakage current density and almost the same dielectric constant with those from Ta(OC₂H₅)₅–O₂ system.

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Polycrystalline Mn–Al co-doped ZnO ceramics were prepared by sintering in air. Increasing Mn dopant concentration led to an increase of the ZnO lattice constants indicating substitution of Mn on the Zn sublattice. A Mn-rich spinel secondary phase was also detected within grains and at grain boundaries. The addition of the Al dopant led to an initial reduction in grain size while the addition of Mn led to an increase in grain size. Thermoelectric property measurements revealed that Al doping led to a large initial increase in electrical conductivity and increasing Mn dopant addition resulted in electrical conductivity reduction. The absolute value of the Seebeck coefficient was largely unaffected at low Mn doping levels; however, an increase was observed for high Mn doping levels. The thermal conductivity showed a marked decrease with increasing Mn concentration, indicating the effectiveness of Mn dopant substitution for Zn to reduce the thermal conductivity by point defect scattering. Overall, the maximum ZT was achieved for the Zn_0.99Al_0.01O sample (ZT = 0.07 at 750°C) and increasing Mn doping led to a decrease of ZT due to the increased resistivity caused by alloy scattering and secondary phase formation.

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Piezoelectric Ca₃TaGa₃Si₂O₁₄ (CTGS) single crystals are grown by the Czochralski technique and investigated in detail for high temperature sensor applications. The influence of the oxygen partial pressure during the growth on material properties is investigated by the use of Ir crucibles, as well as Pt ones. Colorless crystals are obtained, indicating that the typical yellowish coloration of CTGS crystals is related with the use of Ir-crucibles. Colored crystals present three absorption bands at 340, 450 and 1790 nm, however, related crystal defects do not affect the electrical properties of grown crystals. Dielectric and piezoelectric properties do not vary significantly with the growth conditions. Instead, it is found that the electrical resistivity depends on the oxygen partial pressure and it is higher for CTGS crystals grown under oxygen-less atmosphere.

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In this study, indium tin oxide (ITO) and indium zinc oxide (IZO) films were deposited on quartz glass through alternating sputtering. The optical and electrical properties of these films were characterized through spectroscopic ellipsometry (SE). The optical constants of ITO and IZO films were accurately estimated using a model that combined the Tauc–Lorentz, Gauss, and Drude models. However, the estimated resistivity was close to the Hall measurements only when the film thickness was higher than 250 nm. Anisotropy caused by the preferential orientation of crystallites increased the difficulty of the SE fitting process. Furthermore, a k-factor defined on the basis of the estimated optical constants was used to examine the Zn content of the IZO films. Variation in the k-factor was consistent with the band gap of the IZO films.

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We succeeded in fabricating light-emitting amorphous oxide (LEAO) thin films by doping Eu³⁺ ions to amorphous In–Mg–O deposited at room temperature using (In,Eu)₂MgO₄ polycrystalline targets by pulsed laser deposition. Clearly visible photoluminescence (PL) with a peak at 615 nm was observed at room temperature by 270 nm light excitation even without post-deposition thermal annealing. The unusual variation of PL intensity with Eu concentration is understood in terms of non-radiative Auger recombination via conduction carriers. The PL intensity also exhibited maximums with respect to oxygen pressure during deposition (PO₂) and annealing temperature (T_a), which are explained by recombination centers generated by oxygen deficiency-related defects at low PO₂, by excess/weakly-bonded oxygen at high PO₂, and by hydrogen depletion at high T_a. Since the LEAO films can be obtained even by room-temperature deposition, it is expected to have flexible optoelectronic applications using plastic substrates.

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Lu₃Al₅O₁₂ (LuAG) single crystals co-doped with Nd and Ce were grown by the Floating Zone (FZ) method to evaluate their scintillation properties particularly in the near infrared (NIR) wavelength. When X-ray was irradiated, the scintillation due to Nd³⁺⁴F_3/2→⁴I_11/2 transition was observed at 1064 nm. By changing the irradiation dose from 10–1000 mGy, we evaluated the relation between the scintillation intensity and the X-ray dose, and the relation was approximately linear. The scintillation decay time profiles were approximated by two or three exponential decay functions. The observed decay times were 100 ns due to Ce³⁺, 0.6–1.0 µs due to the garnet host and few µs due to Nd³⁺.

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Bulk SiO₂ glasses have been prepared by spark plasma sintering and characterized towards dosimeter applications. The temperature dependence studies of radioluminescence suggest that there are two possible groups of luminescent centres, which show emissions at 385 and 511 nm. At room temperature, the former emission is predominant while the latter emission significantly increases at low temperatures, especially below 200 K. The origins of these emissions are attributed to a pair of silylene centres and dioxasilirane and silylene, respectively. After irradiating the sample, thermally-stimulated luminescence (TSL) is observed around 160°C and above 300°C, in which the emission is mainly the 385 nm component. The TSL intensity is linearly proportional to the radiation dose delivered, and a linear dynamic range has been demonstrated over 10⁰–10⁴ mGy.

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Eu-doped SrCO₃ (Eu:SrCO₃) ceramics are prepared by a spark plasma sintering (SPS) technique. In photoluminescence (PL), emissions around 615 nm due to 4f–4f transition of Eu³⁺ appear under several excitation wavelengths. The PL decay time constants are 314 ns due to the host and 400–600 µs due to Eu³⁺. The X-ray induced scintillation spectra show an intense emission peak and some small bands around 615 nm and scintillation decay time constants are 210–224 µs. In thermally stimulated luminescence (TSL) after X-ray irradiation, a strong TSL glow peak is observed in 450°C and TSL intensity increase in association with Eu concentration.

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In this study, we developed Li₃PO₄–Al(PO₃)₃ glasses doped with different concentrations (0, 0.1, 0.2, 0.5 and 1.0%) of Ce and studied the optical, scintillation and dosimeter properties. The scintillation and photoluminescence (PL) emissions showed an intense emission due to the 5d–4f transitions of Ce³⁺. In addition, the scintillation and PL decay times of Ce³⁺ were obtained for 25–32 and 26–29 ns, respectively. These results were typical for Ce-doped scintillator. Moreover, as a dosimeter property, 0, 0.1, 0.5 and 1.0% Ce-doped glass samples showed thermally stimulated luminescence (TSL) around 150–200 and 350–400°C with a good liner response over 0.5–10 Gy.

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Photoluminescence (PL) properties of Sn-doped zinc phosphate (Sn:xZP) glasses with different chemical compositions are investigated. Sn exists only in the form of Sn²⁺ in Sn:xZP glasses, which were prepared by a conventional melt-quenching method in Ar atmosphere. The glass-transition temperatures, T_g, of the Sn-doped glasses are lower than that of Sn-free glasses, and the change in the T_g values of Sn-doped glasses increases with increasing ZnO amount. Since the relative ratio of 4-fold coordinated Sn²⁺ becomes higher with increasing ZnO amount, the coordination state is found to depend significantly on the network structure and not on the optical basicity of the Sn:xZP glasses. The PL bandwidth, indicating the site distribution, becomes narrower as the ZnO amount increases. In addition, the PL decay lifetime in the Zn-rich glass is shorter compared to that of the P₂O₅-rich glasses, implying that the distribution of Sn²⁺ is also associated with the network structure. Thus, the network structure of the host matrix rather than the average optical basicity significantly influences the local structure of Sn²⁺ emission center.

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We have investigated the photoluminescence (PL), scintillation and thermally-stimulated luminescence (TSL) dosimeter properties of MgO transparent ceramics doped with different concentrations of Mn²⁺ ion (0.001, 0.01, 0.1 and 1%). The samples were synthesized by a Spark Plasma Sintering (SPS) technique. All the samples show a PL band emission around 730 nm, and the intensity increases with the dopant concentration. The PL decay time constants were a few milliseconds which is on the typical order for Mn²⁺ ion. In contrast, the scintillation spectrum under X-rays shows additional emissions at 340, 620 and 760 nm. After the samples are irradiated by X-rays, the TSL is observed with a main glow peak appearing around 140°C. The TSL response is linearly related to the irradiation dose at least over the dose range from 0.1 to 1000 mGy.

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We have investigated the optical and scintillation properties of La₂Zr₂O₇ transparent ceramics doped with 0–0.46% Bi. In the photoluminescence (PL), an intense broad emission was observed around 600 nm, and the primary PL decay time was 1.2 µs. The origin of the emission was attributed to the Bi³⁺ related luminescence. The 0.10% Bi doped sample showed the highest PL quantum yield (70%) among the samples measured. In the X-ray induced scintillation spectra, a similar emission features to those observed in PL spectra were observed. The scintillation emission was not observed at low temperatures below 200 K and Bi emission around room temperature was promoted by the thermal activation.

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We investigated photoluminescence (PL) and scintillation properties of 0.8 mol % Ce-doped (Y, Gd, Lu)₃Al₂Ga₃O₁₂ (Ce:YAGG, Ce:GAGG and Ce:LuAGG) transparent ceramics, which were fabricated by the vacuum sintering method. The obtained samples showed a strong and broad PL emission in the wavelength range from 470 to 600 nm owing to 5d–4f transitions of Ce³⁺. The PL and scintillation decay curves consisted of two or three exponential components, and the primary components ranged around 35–43 ns. Besides, the scintillation spectra excited by X-rays showed similar features with those in PL. In the X-ray induced afterglow measurements, the Ce:LuAGG sample exhibited the weakest afterglow intensity. The pulse height spectra of these samples showed clear photoabsorption peaks. Among those samples, Ce:LuAGG sample showed the highest scintillation light yield of 18,700 ph/MeV.

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The properties of Ce:Y₃Al₅O₁₂ single-crystal powder phosphors, obtained by powdering single-crystals, are investigated as a function of the average grain size. It is found that single-crystal powders maintain the outstanding properties of bulky crystals, namely a high internal quantum efficiency and thermal stability. Furthermore, single-crystal powders exhibit a very high homogeneity and reproducibility, which contrasts with the large variation of emission intensity in reference ceramic powders of highest quality. The effective absorption of single-crystal phosphors increases with the grain size and it is enhanced by a treatment with HF. An effective absorption comparable with the one of reference ceramic powder phosphor is obtained, even though the Ce concentration in single-crystal phosphor powders is about one order to magnitude lower.

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Strontium titanate (SrTiO₃; STO) and barium titanate (BaTiO₃; BTO) powders were prepared using the solid-state reaction method. To study the tendency for nitrogen monoxide (NO) to adsorb on STO and BTO surfaces, we observed the morphology of the powders, calculated the specific surface areas, and made adsorption/desorption observations using scanning electron microscopy (SEM), nitrogen gas adsorption/desorption isotherms under 77 K, temperature programmed desorption (TPD), and diffuse reflectance fourier transformed spectroscopy (DRIFTs). The average STO and BTO powder particle size was 0.2 µm. The specific surface areas of the non-porous STO and BTO powders were 3 and 4 m² g⁻¹, respectively. TPD and DRIFTs measurements revealed the NO adsorption properties of the STO and BTO powders in the presence of oxygen gas. Additionally, it was considered that the adsorbed NO was formed as nitrates [M(NO₃)₂, M = Sr, Ba] on surface of each powder.

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We report on relationships between interlayer distances and electronic structures of hexagonal boron-nitride (h-BN) bilayers based on first-principles density-functional calculations. The energy-band structures and the band-gap values are calculated under uniaxial compressive strains along the direction perpendicular to the h-BN bilayer sheets. It is found that the band gap can be tuned by applying compressive strains uniaxially. More specifically, the band-gap value decreases with decreasing the interlayer distances between two layers of the h-BN bilayer. The behaviors of the band gaps under uniaxial strains are explained in terms of the energy-band structures.

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High density monolithic boron carbide specimens with grain size of 3–5 µm were consolidated by spark plasma sintering using identical heating conditions and Ar or N₂ atmospheres. The effect of impurities from two different raw powders on the flexural strength was revealed. The increase in flexural strength was observed up to 1600°C. Specimens consolidated in nitrogen had a higher strength than that consolidated in argon. Samples had the room temperature strength ranging from 350 to 550 MPa. The high temperature strength of our samples exceeding 400 MPa is higher than that previously reported for polycrystalline monolithic B₄C. The supporting cracking/strengthening mechanism was discussed and proposed.

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CaZrO₃ powders were synthesized by a combined molten salt and microwave heating technique, i.e., microwave–assisted molten salt synthesis (MMSS) technique, using ZrO₂ and CaCl₂ as the main raw materials, and molten LiCl and Na₂CO₃ as a reaction medium. Phase pure CaZrO₃ powders can be synthesized at 900°C for 3 h by using the MMSS method. This synthesis temperature was at least 100°C lower than that required by the conventional molten salt synthesis method. Microstructures and phase compositions of as-prepared CaZrO₃ powders were examined by field emission–scanning electron microscopy (FE–SEM) and transmission electron microscope (TEM), revealing that the crystalline size of the as-prepared CaZrO₃ powders was about 100–300 nm, and the “template formation” mechanism played a dominant role in the synthesis process of MMSS.

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X-ray photoelectron spectra of S_2p and P_2p were obtained for the xLi₂S·(100 − x)P₂S₅ (x = 70, 75 and 80 mol %) glasses prepared by mechanochemical treatment. A peak deconvolution technique was used to separate the S_2p spectra into three sulfur components of bridging sulfur (S⁰), non-bridging sulfur (S⁻) and sulfide ion (S²⁻) with different photoelectron binding energies. The fractions of sulfurs at different electronic states were obtained from the area ratio of each peak. No apparent difference in the P_2p spectra was observed in the glasses. In the (100 − y)(0.7Li₂S·0.3P₂S₅)·yLiBr (y = 0, 5, 10 and 20 mol %) glasses, the fractions of S⁰ and S⁻ were almost the same in each glass regardless of the amount of added LiBr content. The addition of LiBr did not affect the electronic state of sulfur in the Li₂S–P₂S₅–LiBr glasses.

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BiFeO₃ thin films were fabricated by a simple spin-coating method as an electron transport layer in CH₃NH₃PbI₃-based photovoltaic devices. Single-phase BiFeO₃ was obtained, which was confirmed by X-ray diffraction. To characterize photovoltaic properties, BiFeO₃/CH₃NH₃PbI₃-based photovoltaic devices were fabricated by different preparation steps for CH₃NH₃PbI₃ layer, and current density–voltage characteristics of the photovoltaic devices showed rectifying behaviors. Comparing with the BiFeO₃/CH₃NH₃PbI₃ photovoltaic device fabricated by a 1-step method, the conversion efficiency of the device fabricated by a 2-step method was improved. Bandgaps of the BiFeO₃ and CH₃NH₃PbI₃ layers were also determined from optical absorption measurements.

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Tin-phosphate (SnO–P₂O₅) glasses with high SnO contents have a high potential for new lead-free low-melting sealing frits, optical materials, and anode active materials for rechargeable lithium ion batteries (LIBs). The local structure (Raman scattering and ¹¹⁹Sn Mössbauer spectra), some physical properties (specific heat and thermal conductivity), and elastic properties (Young’s modulus and Vickers hardness) of 72SnO.28P₂O₅ glass were characterized to clarify the features of SnO–P₂O₅ glasses. The glass showed unique properties when compared with other phosphate glasses, i.e., small thermal diffusivity and conductivity, small Young’s modulus, intense Boson peak, and unique local structure. 72SnO.28P₂O₅ glass has an open structure with a low packing density, in which a large number of Q¹ ((PO_3.5²⁻) and Q⁰ (PO₄³⁻) units are formed. These structural features would be one of the main reasons for the fast diffusion of Li⁺ ions and result in the excellent LIB performance as anode materials of the glass.

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Li₄Ti₅O₁₂/C nano-powders were successfully synthesized at 500°C through the pulse jet spray pyrolysis with use of a citric acid aqueous solution. As-prepared Li₄Ti₅O₁₂/C nano-particles had spherical and irregular shapes with an average size of 50 nm. As-prepared Li₄Ti₅O₁₂/C nano-particles had a spinel phase with high crystallinity and a homogeneous chemical composition. The carbon content was 10 wt % in the Li₄Ti₅O₁₂ nano-powders. A 2032 type coin cell was used to examine electrochemical properties of the Li₄Ti₅O₁₂/C anode. The discharge capacity of Li₄Ti₅O₁₂/C anode exhibited 170 at 1 C and 120 mAh·g⁻¹ at 20 C, respectively. The discharge capacity of the Li₄Ti₅O₁₂/C anode maintained 96% of the initial discharge capacity after 500th cycle at 10 C and room temperature. The discharge capacity of the Li₄Ti₅O₁₂/C anode exhibited 160 mAh·g⁻¹ at 10 C and 50°C and maintained 98% of the initial discharge capacity after 100th cycle.

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Mordenite was synthesized using chemical reagents of amorphous SiO₂ and sodium aluminate without the addition of a template agent and a seed powder. The single phase of the mordenite was obtained for the Al/Si ratios of 0.085–0.120 as the starting material in the reaction temperature range of 170–200°C for 24 h. The cation exchange capacity (CEC) was ca. 180–210 cmol/kg and it increased with the Al/Si ratio. The Cs⁺ adsorption ability from 100 ppm Cs⁺ 100 mL solutions of water and seawater for the mordenite (1.0 g) was >99% and ca. 85–92%, respectively. A composite material consisting of the mordenite and magnetite (10, 20, 30 wt %) was synthesized for the magnetic collection. The total Cs decontamination rates using the magnetic collection material after the Cs⁺ adsorption in water were also higher than 95% for all the examined magnetite-containing composite materials.

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SiC ceramics doped with five ratios of ZrO₂ short fibers (0, 5, 10, 15, 20 vol.%) were fabricated via hot pressing at 1800°C for 60 min under a pressure of 50 MPa. The bending strength and fracture toughness were improved by ZrO₂ short fibers additives except SiC/20 vol.% ZrO₂. Moreover, the SiC/15 vol.% ZrO₂ possessed the bending strength of 553 ± 30 MPa and fracture toughness of 7.4 ± 0.4 MPa·m^−1/2, respectively. However, the relative density decreased. The relative density of pure SiC sample was 94.18%, and the hardness decreased from 13.01 to 12.13 GPa with the increase of the ZrO₂ short fiber additive. The ZrC peak was detected in X-ray diffraction patterns, which is the result of the reaction between ZrO₂ fibers and SiC.

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