Yeon Hwang

Preparation of TiB2 powders by mechanical alloying

Abstract TiB 2 powders with submicrometer size were synthesized by mechanical alloying (MA) the mixture of elemental Ti and B powders in argon atmosphere using a steel jar. The effect of heat of formation and the atomic size factor on MA as well as the phase change during MA were investigated. The amorphization reaction, a common process during MA, had not been observed and the starting raw materials directly converted into final stable phase. When Zr of which atomic radius was larger than that of Ti was substituted for Ti, the alloying time was greatly reduced. On the contrary, substitution of Ta for Ti prolonged the alloying time because of the less negative heat of formation of TaB 2 than that of TiB 2 .

Phonon dispersion of monolayer graphite on Pt(111) and NbC surfaces: bond softening and interface structures

Abstract The phonon dispersion relations of monolayer graphite on Pt(111), NbC(111) and NbC(001) were measured by electron energy loss spectroscopy. The monolayer graphite on Ni surfaces has been reported to show large softening, while that on Pt(111) was found to indicate little softening and rather resembled pristine graphite. Similarly, the monolayer graphite on NbC(111) indicated fairly large softening but that on NbC(001) did not. This difference is discussed in relation with the interface structure.

Ca segregation at the MgO(001) surface studied by ion scattering spectroscopy

Abstract The structure and the composition of the Ca-segregated MgO(001) surface were investigated by using time-of-flight impact-collision ion scattering spectroscopy and Rutherford backscattering spectroscopy. MgO including 210 ppm Ca impurities in bulk was heated at a temperature of 1400°C for 5 h in the oxygen atmosphere. It is found that Ca ions, being segregated with an amount of (2.14 ± 0.04) × 10 14 ions cm −2 , exist only in the outermost layer, and no Ca enrichment occurs in the deeper layers.

Surface phonon and electronic structure of a graphite monolayer formed on ZrC(111) and (001) surfaces

Abstract The surface phonon dispersion relationship and electronic structure of a graphite monolayer on (111) and (001) surfaces of ZrC were studied by high-resolution electron energy loss and angle-resolved ultraviolet photoelectron spectroscopy. The graphite monolayer on the (111) surface shows larger softening in phonon structures and decreased β band initial state energy than on the (001) surface. These phenomena were discussed in the viewpoint of charge transfer from the substrate to the graphite overlayer and it was revealed that there was much more charge transfer for the C/ZrC(111) system than for the C/ZrC(001) system.

Morphology, Physiological Characteristics, and Complete Sequence of Marine Bacteriophage ϕRIO-1 Infecting Pseudoalteromonas marina

ABSTRACT Bacteria of the genus Pseudoalteromonas are ubiquitous in the worlds oceans. Marine bacteria have been posited to be associated with a major ancient branch of podoviruses related to T7. Yet, although Pseudoalteromonas phages belonging to the Corticoviridae and the Siphoviridae and prophages belonging to the Myoviridae have been reported, no Pseudoalteromonas podovirus was previously known. Here, a new lytic Pseudoalteromonas marina phage, ϕRIO-1, belonging to the Podoviridae was isolated and characterized with respect to morphology, genomic sequence, and biological properties. Its major encoded proteins were distantly similar to those of T7. The most similar previously sequenced viruses were Pseudomonas phage PA11 and Salinivibrio phage CW02. Whereas many elements of the morphology and gene organization of ϕRIO-1 are similar to those of podoviruses broadly related to T7, ϕRIO-1 conspicuously lacked an RNA polymerase gene. Since definitions of a T7 supergroup have included similarity in the DNA polymerase gene, a detailed phylogenetic analysis was conducted, and two major DNA polymerase clades in Autographivirinae and several structural variants of the polA family represented in podoviruses were found. ϕRIO-1 carries an operon similar to that in a few other podoviruses predicted to specify activities related to γ-glutamyl amide linkages and/or unusual peptide bonds. Most growth properties of ϕRIO-1 were typical of T7-like phages, except for a long latent period.

Effects of surrounding ion on Eu3+ luminescence in glass

Abstract The energy relaxation of excited Eu 3+ in glass hosts is investigated in this study. Eu 3+ ions are incorporated in both melted glass and sol–gel derived glass. The strong Eu 3+ emission lines from 5 D 0 to 7 F level show that the glass hosts provide the effect of the random structure of the silica network on the Eu 3+ ion. The relative intensities of the Eu 3+ emission depend strongly on the host glass types and excitation wavelengths. The 5 D 0 emissions of Eu 3+ in the sol–gel derived glass are more efficient than in the melted glass due to an excitation to the charge-transfer-state at 250 nm. The promotion of the charge-transfer-state excitation in the sol–gel derived glass, which is annealed at 400°C, comes from the anions encapsulated during the sample preparation step. The energy relaxation processes are identified from the wavelengths of the excitation peaks of Eu 3+ . The roles of UV-absorbing cations, Ce 3+ and Ti 4+ , which are codoped in the glasses, are examined.

Effect of starting phase on microstructure and fracture toughness of hot-pressed silicon carbide

The effect of starting phase of raw material on the microstructure and the fracture toughness of silicon carbide was investigated. The starting phase was adjusted by varying the mixed ratio of α-SiC and β-SiC powders, and they were hot pressed with sintering aids. From the microstructure, some elongated grains were found among equiaxial grains. The anisotropic grain growth was resulted from phase transformation of the β-SiC to the α-SiC with 4H polytype during sintering or heat treatment. Volume fraction, maximum length, and aspect ratio of elongated grains were the highest at 50 vol.% β-SiC in the starting SiC powder, which showed the highest fracture toughness about 6.0 MPa m 1/2.

Pseudoalteromonas neustonica sp. nov., isolated from the sea surface microlayer of the Ross Sea (Antarctica), and emended description of the genus Pseudoalteromonas.

A Gram-stain-negative, facultatively anaerobic, rod-shaped and motile strain, designated PAMC 28425T, was isolated from a sea surface microlayer sample from the Ross Sea, Antarctica. Analysis of the 16S rRNA gene sequence of strain PAMC 28425T showed an affiliation with the genus Pseudoalteromonas. Phylogenetic analyses revealed that strain PAMC 28425T formed a clade with Pseudoalteromonas prydzensis MB8-11T and Pseudoalteromonas mariniglutinosaKMM 3635T with 16S rRNA gene sequence similarities of 98.3-98.6u2009%. Genomic relatedness analyses based on the average nucleotide identity and the genome-to-genome distance showed that strain PAMC 28425T is clearly distinguished from the phylogenetically close relatives. Cells of strain PAMC 28425T grew optimally at 25u2009°C and pH 7.5-8.5 in the presence of 1.0-3.0u2009% (w/v) sea salts. The major cellular fatty acids (>10u2009%) were C16u2009:u20091ω6c and/or C16u2009:u20091ω7c, C16u2009:u20090, and C18u2009:u20091ω6c and/or C18u2009:u20091ω7c. The genomic DNA G+C content was 39.7 mol%. On the basis of the phylogenetic, genomic, chemotaxonomic and phenotypic data presented, we propose the name Pseudoalteromonas neustonica sp. nov. with the type strain PAMC 28425T (=KCCM 43187T=JCM 31286T).

Carbothermal synthesis of β-SiC powders from silicon and SiO2-coated carbon powders

Abstract β-SiC powders were synthesized via a direct carbothermal reaction between solid silicon and SiO2-coated carbon powders. An SiO2 layer can be coated on carbon powders by appropriately controlling the ratios of 3-aminopropyltriethoxysilane (APTES) and tetraethoxysilane (TEOS). The mixture of silicon and SiO2-coated carbon was reacted at 1 500 °C for 1 h in an Ar atmosphere. The morphologies of the SiO2-coated carbon and the synthesized SiC powders were examined, and the phase evolution in the SiC powders during synthesis was analyzed. β-SiC powders with a particle size of around 200 nm were synthesized under SiO2-coating conditions of 0.2 – 0.6 mL of APTES and 0.8 – 0.4 mL of TEOS for 100 mg of carbon black powders. When no TEOS was used in the coating procedure, the β-SiC powders showed large and irregular shapes, while coating was not possible without the addition of APTES.

Effect of Additives on the Compressive Strength of Geopolymerized Fly Ash

Geopolymer cements and geopolymer resins are newly advanced mineral binders that are used in order to reduce the carbon dioxide generation that accompanies cement production. The effect of additives on the compressive strength of geopolymerized class-F fly ash was investigated. Blast furnace slag, calcium hydroxide(), and silica fume powders were added to fly ash. A geopolymeric reaction was initiated by adding a solution of water glass and sodium hydroxide(NaOH) to the powder mixtures. The compressive strength of pure fly ash cured at room temperature for 28 days was found to be as low as 291 , which was not a suitable value for use in engineering materials. On the contrary, addition of 20 wt% and 40 wt% of blast furnace slag powders to fly ash increased the compressive strength to 458 and 750 , respectively. 5 wt% addition of increased the compressive strength up to 640 ; further addition of further increased the compressive strength. When 2 wt% of silica fume was added, the compressive strength increased to 577 ; the maximum strength was obtained at 6 wt% addition of silica fume. It was confirmed that the addition of CaO and to the fly ash powders was effective at increasing the compressive strength of geopolymerized fly ash.