Kyeongsoon Park

Phosphate phosphors for solid-state lighting

Introduction.- Basic Mechanisms of Photoluminescence.- Synthesis of Phosphate Phosphors.- Methods of Measurements (Instrumentation).- Some Orthophosphate Phosphors.- Some Halophosphate Phosphors.- Some Novel Phosphate Phosphors.- Current Progress in Solid State Lighting.

Structure-tunable synthesis of titanate nanotube thin films via a simple hydrothermal process

In this study, titanium metal acts as the titanium source to prepare oriented titanate nanotube thin film. The effects of preparation parameters, such as reaction temperature, duration and post-treatment conditions, on the film morphology and stability have been examined. A general formation mechanism for oriented titanate nanotube thin film is proposed on the basis of detailed observations of the products using two-dimensional surface SEM studies and TEM images. The overall formation of titanate nanotubes can be summarized as a sequence of four steps: (a) titanium dissolution and alkali titanate hydrogel formation; (b) alkali titanate hydrogel dissolution, increased TiO32−, TiO2(OH)22− or TinO2n+m2m− concentration and layered Na2Ti3O7 formation; (c) layered Na2Ti3O7 growth; (d) nanotube formation via Na2Ti3O7 splitting and the multilayer scrolling process. The Na2Ti3O7 lamellar structures split between the (010) planes into nanosheets.

Thermoelectric properties of p-type Te doped Bi0.5Sb1.5Te3 fabricated by powder extrusion

Abstract The p -type Te doped Bi 0.5 Sb 1.5 Te 3 compounds were fabricated by powder extrusion in the temperature range 340–460xa0°C. The Seebeck coefficient, electrical resistivity and thermal conductivity were measured as a function of extrusion temperature to calculate the figure of merit. The highest bending strength (61.2 MPa) and the highest figure of merit (2.78×10 −3 K −1 ) were obtained at 420xa0°C because of a low porosity and fine grain size. It is proposed that a noble, new powder extrusion process, for the fabrication of high-performance Bi 2 Te 3 -based thermoelectric materials, be a useful technique.

PTCR characteristics in porous (Ba,Sr)TiO3 ceramics produced by adding partially oxidized Ti powders

Abstract Porous and fine-grained (Ba,Sr)TiO 3 ceramics were fabricated in vacuum by the addition of partially oxidized Ti powders. The positive temperature coefficient of resistivity (PTCR) characteristics in the (Ba,Sr)TiO 3 ceramics appeared after paste-baking treatment at 580°C in air due to the adsorption of oxygen at grain boundaries, in the same behavior as ordinary air-sintered BaTiO 3 ceramics. It was found that the phase transition temperature from the tetragonal to cubic structure of (Ba,Sr)TiO 3 ceramics depends on the oxygen content and grain size. The electrical properties of the (Ba,Sr)TiO 3 ceramics were discussed, based on the microstructure, resistivity of grain boundaries, donor concentration of grains, and the electrical potential barrier of grain boundaries. The (Ba,Sr)TiO 3 ceramics containing the 5 vol.% partially oxidized Ti powders sintered at 1450°C showed a large PTCR effect greater than six orders of magnitude.

Thermoelectric properties of Ca0.8Dy0.2MnO3 synthesized by solution combustion process.

High-quality Ca0.8Dy0.2MnO3 nano-powders were synthesized by the solution combustion process. The size of the synthesized Ca0.8Dy0.2MnO3 powders was approximately 23 nm. The green pellets were sintered at 1150-1300°C at a step size of 50°C. Sintered Ca0.8Dy0.2MnO3 bodies crystallized in the perovskite structure with an orthorhombic symmetry. The sintering temperature did not affect the Seebeck coefficient, but significantly affected the electrical conductivity. The electrical conductivity of Ca0.8Dy0.2MnO3 increased with increasing temperature, indicating a semiconducting behavior. The absolute value of the Seebeck coefficient gradually increased with an increase in temperature. The highest power factor (3.7 × 10-5 Wm-1 K-2 at 800°C) was obtained for Ca0.8Dy0.2MnO3 sintered at 1,250°C. In this study, we investigated the microstructure and thermoelectric properties of Ca0.8Dy0.2MnO3, depending on sintering temperature.

Enhanced photoluminescence properties of red emitting Ba1−xAl2Si2O8:xEu3+ phosphors by charge compensation

Ba1−xAl2Si2O8:xEu3+ (0.03 ≤ x ≤ 0.15), Ba0.82Al2Si2O8:0.12Eu3+, and Ba0.76Al2Si2O8:0.12Eu3+,0.12M+ (M+ = Li+, Na+, K+) phosphors were prepared by the solution combustion method. All the prepared phosphors have a hexagonal crystal structure, belonging to the P6/mmm space group. In this study, in order to improve the photoluminescence properties of the Ba1−xAl2Si2O8:xEu3+ phosphors, two different approaches for the charge compensation were applied, i.e., self-charge compensation and the addition of charge compensators such as Li+, Na+, and K+. These two approaches significantly improved both the excitation and emission properties. In particular, among the charge compensation approaches, the Na+ ion as a charge compensator substantially enhanced the emission intensity and optical purity. The emission intensity of the Ba0.76Al2Si2O8:0.12Eu3+,0.12Na+ phosphor from the electric dipole transition was approximately five times higher than that of the Ba0.88Al2Si2O8:0.12Eu3+ phosphor.

Effect of atmosphere on the PTCR characteristics of porous (Ba,Sr)TiO3 ceramics

Porous and fine-grained (Ba,Sr)TiO3 ceramics were fabricated by adding partially oxidized Ti powders. During heating and cooling, the electrical resistance was measured in air, O2, N2, and H2 atmospheres and in H2 followed by air atmosphere. The effect of atmosphere on the PTCR characteristics and the role of oxygen on the grain-boundaries in the PTCR characteristics were investigated. The porous (Ba,Sr)TiO3 ceramics showed a large PTCR effect, which was strongly sensitive to atmosphere. This effect was increased in oxidizing atmosphere mainly because of an increase in potential barrier height whereas decreased in reducing atmosphere mainly because of a decrease in potential barrier height.

Effect of Synthesis Method on Photoluminescence Properties of Na2Sr2Al2PO4Cl9:Ce3+ Nanophosphor

We have synthesized a series of single-composition emission Na2Sr2Al2PO4Cl9:Ce3+ phosphor by traditional solid state reactions and novel combustion method. Formation of compound was confirmed by X-ray diffraction analysis. The photoluminescence (PL) emission spectra were observed at 376 nm when excited around 322 nm for the various concentrations in both cases. The PL emission spectra of phosphors showed strong Ce3+ emission due to the 5d→4f transition of Ce3+ ions. The Ce3+ emission intensity in Na2Sr2Al2PO4Cl9:Ce3+ prepared by combustion was higher than that of solid state reactions.

Effect of reoxidation on the PTCR characteristics of porous (Ba, Sr)TiO3

Abstract Semiconducting porous (Ba,Sr)TiO3 ceramics were prepared by sintering at 1350xa0°C in vacuum followed by reoxidation at 400–800xa0°C in air. The effect of reoxidation on the PTCR characteristics of the porous (Ba,Sr)TiO3 ceramics was investigated. It was found that the PTCR effect of the (Ba,Sr)TiO3 ceramics was developed at high reoxidation temperatures (≥550xa0°C), and also the magnitude of the PTCR effect increased with increasing reoxidation temperature. The role of oxygen on the grain boundaries in the PTCR characteristics was also discussed.

Effect of alkaline metal ions on the photoluminescence properties of Eu3+-doped Ca3Al2O6 phosphors

Abstract Ca 3– x Al 2 O 6 : x Eu 3+ (0.02≤ x ≤0.10) and Ca 2.92 Al 2 O 6 :0.04Eu 3+ ,0.04M + (M + =Li + , Na + , and K + ) phosphors were prepared by sol-gel method and their photoluminescence properties were investigated. The crystal structure of the annealed Ca 3– x Al 2 O 6 : x Eu 3+ and Ca 2.92 Al 2 O 6 :0.04Eu 3+ ,0.04M + phosphors was analyzed with X-ray diffraction (XRD). The size and morphological characteristics of the annealed phosphors were investigated with a field emission scanning electron microscope (FE-SEM). The photoluminescence properties were evaluated with a spectrofluorometer. The phosphors were effectively excited by the light of 396 nm and showed intense red emission at 616 nm, which originated from the 5 D 0 → 7 F 2 transition of Eu 3+ . Among the prepared phosphors, the Ca 2.96 Al 2 O 6 :0.04Eu 3+ phosphor showed the strongest emission intensity and the highest color purity. The addition of alkaline metal ions (Li + , Na + , and K + ) to Ca 2.96 Al 2 O 6 :0.04Eu 3+ phosphor degraded the emission intensity and color purity.