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  • Author: B.-W. Kim x
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Open access

X.-K. Huynh, B.-W. Kim and J.S. Kim

Abstract

The in situ formation of TiB2 particulates via an interface reaction between Ti and FeB powders was studied. The effects of mechanical activation by high-energy milling on the decomposition of TiH2 and the interface reactions between Ti and FeB powders to form TiB2 were investigated. Powder mixtures were fabricated using planetary ball-milling under various milling conditions. The specific ball-milling energy was calculated from the measured electrical power consumption during milling process. High specific milling energy (152.6 kJ/g) resulted in a size reduction and homogeneous dispersion of constituent powders. This resulted in a decrease in the decomposition temperature of TiH2 and an increase in the formation reaction of TiB2 particulates in the Fe matrix, resulting in a homogeneous microstructure of nanoscale TiB2 evenly distributed within the Fe matrix. In contrast, the powder mixture milled with low specific milling energy (36.5 kJ/g) showed an inhomogeneous microstructure composed of relatively large Fe-Fe2B particles surrounded by a thin layer of Fe-TiB2 within a finely dispersed Fe-TiB2 matrix region.

Open access

K.-W. Kim, K.-S. Oh, H. Lee, B.-S. Kim and T.-J. Chung

Abstract

Two-step sintering route was applied for the densification SiC by promoting the role of liquid phase. The specimen contained 8 vol% of liquid phase composed of Al2O3 and Y2O3. The heating schedule consisted with initial rapid heating to 2000°C and immediate quenching to 1700 or 1750°C. By heating at elevated temperature, even distribution of the liquid phase was intended. The heat treatment at reduced temperature was to suppress the evaporative loss of the liquid and to secure the time for densification. The two-step sintering effectively suppressed loss of mass and coarsening. The resultant SiC was thus dense and was composed of fine grains exhibiting hardness of 2321 kgf/mm2.

Open access

K. S. Kang, B. H. Cheon, S. U. Han, C. S. Kim and W. Y. Choi

Abstract

Genetic gain and diversity were estimated in a 13- year old Quercus serrata breeding seed orchard under three selection (rouging) methods. The selections were based on individual selection, family selection, and family plus within family selection. Genetic gain was for stem volume and gene diversity was estimated by status number concept. Both estimated genetic gain and gene diversity were compared to those before selection and among selection scenarios. Estimated genetic gain for tree volume ranged from 4.0% to 9.1% for three selection methods under 50% selection intensity. Individual selection was better than family selection for retaining higher genetic gain and status number. Family plus within family selection was the best selection method, while individual selection was more efficient at the strong selection intensity. An optimal point, which maximized gain and diversity, was occurred at 50% selection intensity that would be applied for genetic thinning in the breeding seed orchard of Quercus serrata. The effect of genetic relatedness among families and possible pollen contamination on both genetic gain and gene diversity, although were not studied but their impact, are discussed. The selection method and intensity level applied should be chosen after careful consideration of the impacts on both genetic gain and diversity for seeds produced from the seed orchard.

Open access

S.H. Choi, B. Ali, S.K. Hyun, J.J. Sim, W.J. Choi, W. Joo, J.H. Lim, Y.J. Lee, T.S. Kim and K.T. Park

Abstract

Combustion synthesis is capable of producing many types of refractory and ceramic materials, as well as metals, with a relatively lower cost and shorter time frame than other solid state synthetic techniques. TiO2 with Mg as reductant were dry mixed and hand compacted into a 60 mm diameter mold and then combusted under an Ar atmosphere. Depending on the reaction parameters (Mg concentration 2 ≤ α ≤ 4), the thermocouples registered temperatures between 1160°C and 1710°C · 3 mol of Mg gave the optimum results with combustion temperature (Tc) and combustion velocity (Uc) values of 1372°C and 0.26 cm/s respectively. Furthermore, this ratio also had the lowest oxygen concentration in this study (0.8 wt%). After combustion, DC plasma treatment was carried out to spheroidize the Ti powder for use in 3D printing. The characterization of the final product was performed using X-ray diffraction, scanning electron microscopy, energy dispersive spectroscopy, and N/O analysis.

Open access

S.H. Choi, B. Ali, K.S. Choi, S.K. Hyun, J.J. Sim, W.J. Choi, W. Joo, J.H. Lim, T.H. Lee, T.S. Kim and K.T. Park

Abstract

Although TiNb2O7 is regarded as a material with high application potential in lithium-ion batteries (LIBs) and solid-oxide fuel cells (SOFCs), it has been difficult to find suitable cost-effective conditions for synthesizing it on a commercial scale. In this study, TiNb2O7 compounds were synthesized by a solid state synthesis process. For stoichiometrically precise synthesis of the TiNb2O7 phase, the starting materials, TiO2 and Nb2O5 were taken in a 1:1 molar ratio. Activation energy and reaction kinetics of the system were investigated at various synthesis temperatures (800,1000,1200, and 1400°C) and for various holding durations (1,5,10, and 20 h). Furthermore, change in the product morphology and particle size distribution were also evaluated as a function of synthesis temperature and duration. Additionally, quantitative phase analysis was conducted using the Rietveld refinement method. It was found that increases in the synthesis temperature and holding time lead to increase in the mean particle size from 1 to 4.5 μm. The reaction rate constant for the synthesis reaction was also calculated.

Open access

Myong Gi Chung, M. G. Chung, E. R. Myers, J. M. Chung, K.-J. Kim, C.-W. Park, B.-Y. Sun and J.-H. Pak

Abstract

Quercus acutissima is a large deciduous tree of hillsides ranging from South Korea to Japan. It occurs in many plant communities, often as a pioneer species in monospecific stands which colonized gaps after gap formation. This study used multilocus allozyme genotypes mapped from two disturbed populations near farm houses in southern Korea to compare our results with previous studies conducted on undisturbed populations of trees in southern Korea. Coancestry measures (fij), RIPLEY’s L-statistics, and WRIGHT’s F-statistics were then calculated to examine the distribution of individuals and spatial genetic structure both within and between populations. RIPLEY’s Lstatistics indicated significant aggregation of individuals at interplant distances. A weak but significant positive fine-scale genetic structure at 10 m distance was detected in the two disturbed populations, which is consistent with the structure found in an inland, disturbed population in southern Korea. Estimates of near-distance fij in the two populations (0.020 and 0.036) were considerably lower than that expected for half-sibs (0.125) under random mating, suggesting secondary seed dispersal and substantial overlap of seed shadows. The levels of genetic diversity within the two disturbed populations of Q. acutissima were found to be comparable to the within-mean for populations of other oak species. Significant deficits of heterozygosity were detected in both populations, probably due to several parent-offspring and sib matings. Finally, a significant but low differentiation between the two disturbed populations of Q. acutissima was found, which is likely to be attributable to long-distance pollen movement by wind, which should enhance homogeneity of allele frequencies between adjacent local oak populations.

Open access

J.K. Park, J.H. Lee, S.Y. Shin, J.H. Yi, W.H. Lee, B.J. Park, J.H. Choi, N.Y. Kim and Y.G. Choi

Abstract

Chalcogenide glass in the ternary Ge-Sb-Se system is inherently moldable, thus being considered as a strong candidate material for use in infrared-transmitting lens applications from the viewpoint of thermal and mechanical stability. In an effort to experimentally determine compositional region suitable for the molded lens applications, we evaluate its compositional dependence of hardness. Among the constituent atoms, Ge content turns out to exert a most conspicuous correlation with hardness. This phenomenological behavior is then explained in connection with the structural evolution that Ge brings about.