Key Engineering Materials Vol. 616

Abstract: Ce and Ca co-doped LiGdF₄-LiF eutectics were grown and their directionally solidified eutectic (DSE) system has been investigated. In these eutectics, apart from the expected 305 nm emission of the regular Ce³⁺, 310 nm emission of the Gd³⁺ and Ce³⁺-perturbed emission around 320-380 nm was observed. Ce³⁺-perturbed emission at 340 nm shows the intrinsic decay time of about 27 ns. In photoluminescence mechanism the observed decay time shortening of the regular Ce³⁺ and Gd³⁺ centers point to the energy transfer in the sequence the regular Ce³⁺→ (Gd³⁺)_n→ the perturbed Ce³⁺ sites.

Abstract: c-BN films were deposited on Si (100) substrates by magnetron sputtering followed by annealing at high temperatures. The effect of annealing temperature on the structure, surface morphology and mechanical property of the films were investigated. The structural characterization by FTIR showed an increasing in c-BN content with annealing temperature, and the surface morphology of the c-BN films became more homogeneous. The stress produced in the films was relaxed after annealing, and the hardness was increased with the increasing of c-BN content. At the optimum annealing temperature of 900°C, the hardness reached the maximum value of 13.32 GPa, where the films had the highest c-BN content of 64.3%.

Abstract: Lead-free Li_0.06(Na,K)_0.94NbO₃ (LNKN) piezoelectric ceramics were sintered by a new process, “two-step mixing process”, in which a part of alkali source powders (R_A) was initially preserved and mixed with the rest matrix powders after the calcinations step, in order to improve bulk density. Sintering of the LNKN ceramics was improved by this process, and the sample with R_A = 5% had the highest bulk density of 4.34 g/cm³ (ca. 95%TD). The pore shape and the fracture morphology of the samples suggested the formation of a liquid phase. The present results indicated that the two-step mixing process was an universal approach for improvement in the densification of alkali niobate ststem.

Abstract: Hydroxide ion conductors containing KOH were prepared for application in an all-solid-state Fe–air battery. ZrO₂ and Mg–Al layered double hydroxide (LDH) were employed as the matrix materials. The ionic conductivity and conducting ion species were evaluated by impedance and electromotive force measurements. Repeated charge and discharge were achieved by using negative electrodes composed of the solid electrolyte and iron oxide-supported carbon.

Abstract: Chemical reactivity and cathode properties of LaCoO₃ were investigated using new oxide ion conductor, lanthanum silicate oxyapatite. The LaCoO₃ is found to be good candidate for cathode of the lanthanum silicate oxyapatite solid-electrolyte since no chemical reaction occurred between the LaCoO₃ and lanthanum silicate oxyapatite heating at 1273 K for 60 h in air. Based on electrochemical measurements, lower overpotential between the LaCoO₃ and lanthanum silicate oxyapatite was confirmed compared to the overpotential at YSZ/LaCoO₃ interface. From analysis on the extended interfacial conductivity as function of oxygen activity at the triple phase boundary at fixed temperature, the overpotential evaluated by impedance spectra is the rate limiting process by oxygen diffusion on the LaCoO₃ surface. Comparing to the bulk conductivity of LaCoO₃, the electrode resistance evaluated by impedance spectra was confirmed to be different from the electrical transport properties of the LaCoO₃ bulk.

Abstract: Visible-light responsive photoanodes for water splitting into hydrogen and oxygen were fabricated via NiTi-layered double hydroxide (NiTi-LDH, [Ni_1-xTi_x(OH)₂]^2x+(A^n-)_2x/n・mH₂O]) and NiTi-LDH/graphene composite prepared by homogeneous precipitation methods. The NiTi-LDH and NiTi-LDH/graphene films were formed on transparent fluorine-doped tin oxide (FTO) glass substrates by electrophoretic deposition (EPD). The EPD films showed the pattern of hydrotalcite structure under x-ray diffraction, indicating that NiTi-LDH has been deposited on FTO by EPD without degradation. NiTi-LDH powder was obtained from formic acid intercalated LDH by homogeneous precipitation in formamide with hexamethylenetetramine, and NiTi-LDH/graphene composite was obtained from graphene and NiTi-LDH aqueous suspension with urea. The formed NiTi-LDH and NiTi-LDH/graphene composite films have different morphologies. Both NiTi-LDH and NiTi-LDH/graphene films prepared showed photovoltage of 15-55 mV under visible-light irradiation. Their photoelectrochemical properties were affected by the film morphology.

Abstract: Supercapacitors have been emerged as an important energy storage device, owing to their high power density, excellent cycle ability, fast charge-discharge processes and low self-discharging. In this work, we have developed a facile soft-templated hydrothermal procedure to produce hollow mesoporous carbon spheres, which can deliver a relatively high capacitance (~190 F/g) and an energy density of ~26.38 Wh/kg at the discharging current of 1 A/g. The hollow cavity and micropore volume are shown to be the main factors that contribute to the good capacitive behavior.

Abstract: (004)-oriented γ-LiAlO₂ films were prepared on poly-crystalline AlN substrates by laser chemical vapor deposition at deposition temperature (T_dep) of 1100–1250 K, molar ratio of Li/Al (R_Li/Al) of 1.0–10 and low total pressure (P_tot) of 100–200 Pa. The (004)-oriented γ-LiAlO₂ films consisted of pyramidal grains with a columnar structure. The deposition rate of (004)-oriented γ-LiAlO₂ films reached to 65–72 μm h^-1.

Abstract: The effects of ZnO addition and sintering temperature on dielectric properties of MgTiO₃ ceramics were investigated. The results showed that addition ZnO not only reduce the sintering temperature but also improve the density and dielectric properties of MgTiO₃ ceramics. (Mg_1-xZn_x)TiO₃ (MZT) ceramics shows microwave dielectric properties in a wide range of sintering temperature when x = 0.1, 0.2, 0.3. At x ≥ 0.4, the density of ceramics decreased and porosity, defects and glass phase increased with increased doping of ZnO, resulting in the increase of dielectric loss of MZT and reduce of quality factor.

Abstract: Highly pure transparent single-crystalline BaTi₂O₅, 6 mm in diameter and 15 mm in length, was prepared by a floating zone (FZ) method using high purity BaCO₃ as a source material. The as-prepared crystal was partially dark blue and became transparent by annealing at 773 K in air. The maximum value of permittivity (ε_max) was 36000 at the frequency of 100 kHz and the Curie temperature (T_C) was 757 K.