Bismuth layer-structured ferroelectrics with high Curie temperatures have recently attracted significant attention as promising candidates for high-temperature piezoelectric applications. However, the conventional solid-state reaction method entails high-temperature processing that induces bismuth volatilization, thereby degrading device reliability. In this study, we employed a co-precipitation method enabling atomic-level mixing to significantly lower the synthesis temperature of Nb/Tadoped Bi₄Ti₃O₁₂ ceramics compared to the solid-state reaction method. Experimental results demonstrated that the coprecipitation method yielded a pure single phase at 600℃ without intermediate phases. Furthermore, the synthesized nanopowders, with an average size of 100 nm, lowered the onset temperature of sintering shrinkage to 650℃, approximately 200℃ lower than that of the solid-state counterpart. The low-temperature synthesis process proposed in this work is expected to contribute to the performance enhancement of high-temperature piezoelectric devices by effectively suppressing bismuth volatilization and ensuring compositional stability.

In this paper, we investigated the effect of Co content on the microstructural and electrical properties of Ni_0.79Mn_2.21-xCo_xO₄ (x=0 to 0.25) specimens. Solid-state reaction was used to prepare the bulk specimens. XRD (X-ray diffraction) patterns showed that all compositions had a cubic spinel phase. As a result of the microstructural properties, FE-SEM(field-emission scanning electron microscopy) analysis showed a dense structure, and the mean grain size increased from 5.24 μm to 7.33 μm with an increase of Co content from x=0 to 0.25. All specimens exhibited the typical NTC thermistor characteristics as the electrical resistance exponentially decreased with increasing temperature. The resistivity and the B-value of Ni_0.79Mn_1.96Co_0.25O₄ were 2959 Ω·cm and 3719, respectively.

CaNb2O6:RE3+ (RE=Sm or Eu) phosphor powders were prepared with different contents of activatorions by using solid-state reaction method. The X-ray diffraction patterns exhibited that the phosphors synthesized with different activator ions showed an orthorhombic system with a main (131) diffraction peak. The maximum size of the grain particles, determined from the measurement of scanning electron microscopy, was observed at 0.05 mol of Eu3+ ions and at 0.01 mol of Sm3+. As for the Eu3+-doped phosphor powders, the excitation spectra were composed of a broad band peaked at 278 nm and several weak bands in the range of 350～500 nm, and the highest red emission spectrum was observed at 0.15 mol of Eu3+ ions. As for the Sm3+-activated phosphor powders, three strong emission bands under excitation at 273 nm were observed at 570, 612, and 659 nm, respectively. The intensities of all the emission bands approached maxima for 0.05mol of Sm3+ ions. The optical properties show that the Eu3+- or Sm3+-doped CaNb2O6 powders are promising red-orange emitting phosphor powders applicable to full-color photonic devices.

CaMoO4:Tb3+ green phosphor powders and thin films were successfully prepared by using the solid-state reaction method and the radio-frequency magnetron sputtering technique, respectively. The crystalline structure of all phosphor powders with different Tb3+ ion concentrations was found to be a tetragonal system with the maximum diffraction intensity at 28.58°, while that of the phosphor thin films, irrespective of the type of substrate, was amorphous. As for the phosphor powders, the grain particles showed the chain-like patterns with inhomogeneous size distribution, the excitation spectra were composed of a broad band peaked at 307 nm and two small narrow bands centered at 381 and 492 nm, and the highest green emission spectrum was observed at 0.01 mol of Tb3+ ions. As for the phosphor thin films, the average transmittance exceeding 85% was measured in the 400∼1,100 nm range and the optical band gap showed a significant dependence on the type of substrate.

Synthesis of Li2MnSiO4 was attempted by the conventional solid-state reaction method, and the phase formation behavior according to the change of the calcination condition was investigated. When the mixture of the three source materials, Li2O, MnO and SiO2 powders, were used for calcination in air, it was difficult to develop the Li2MnSiO4 phase because the oxidation number of Mn2+ could not be maintained. Therefore, two-step calcination was applied: Li2SiO3 was made from Li2O and SiO2 at the first step, and Li2MnSiO4 was synthesized from Li2SiO3 and MnO at the second step. It was easy to make Li2SiO3 from Li2O and SiO2. Li2MnSiO4 single phase was developed by the calcination at 900℃ for 24 hr in Ar atmosphere as the oxidation of Mn2+ was prevented. However, the Li2MnSiO4 was γ -Li2MnSiO4, one of the polymorph of Li2MnSiO4, which could not be used as the cathode materials in Li-ion batteries. By applying the additional low temperature annealing at 400℃, the single phase β -Li2MnSiO4 powder was synthesized successfully through the phase transition from γ to β phase.

Y1-xVO4:Eux 3+ red phosphors were synthesized with changing the mol ratios of Eu3+ ions by using the solid-state reaction method. The crystalline structure of phosphors was found to be a tetragonal system with the maximum diffraction intensity at 25.02°. The grain particles showed the truncated hexagonal patterns with a very homogeneous size distribution at 0.05 mol of Eu3+ ion. The excitation spectra of the phosphor ceramics were composed of a broad band centered at 303 nm and weak narrow multilines peaked in the range of 360-420 nm. The dominant emission spectrum was the strong red emission centered at 619 nm due to the 5D0→7F2 electric dipole transition. The experimental results suggest that the optimum doping mol ratio of Eu3+ ions for preparing the red phosphors is 0.10 mol with the asymmetry ratio of 5.21.

Gd(1-x)VO4:Eu(x)3+ red phosphors were synthesized with changing the concentration of Eu3+ ion by using a solid-state reaction method. The crystal structure, surface morphology, and photoluminescence and photoluminescence excitation properties of the red phosphors were measured by using X-ray diffractometer, field emission-scanning electron microscopy, and florescence spectrometer, respectively. The XRD results showed that the main peak of all the phosphor powders occurs at (200) plane. As for the photoluminescence properties, the maximum excitation spectrum occurred at 306 nm due to the charge transfer band from VO4(3-) to Eu3+ ions and the maximum emission spectrum was the red luminescence peaking at 619 nm when the concentration of Eu3+ ion was 0.10 mol.