• Proceedings of the Korean Vacuum Society Conference
• /
• 2012.02a
• /
• pp.128-128
• /
• 2012

We investigated the atomic and electronic properties of graphene grown by Pd silicidation and intercalation using LEED, STM, and ARPES. Pd was deposited on the 6H-SiC(0001) surface at RT. The formation of Pd silicide gives rise to breaking of Si-C bonds of the SiC crystal, which enables to release C atoms at low temperature. The C atoms are transformed into graphene from $860^{\circ}C$ according to the LEED patterns as a function of annealing temperature. Even though the graphene spots were observed in the LEED pattern and the Fourier transformed STM images after annealing at $870^{\circ}C$, the topography images showed various superstructures so that graphene is covered with Pd silicide residue. After annealing at $950^{\circ}C$, monolayer graphene was revealed at the surface. The growth of graphene is not limited by surface obstacles such as steps and defects. In addition, we observed that six protrusions consisting of the honeycomb network of graphene has same intensity meaning non-broken AB-symmetry of graphene. The ARPES results in the vicinity of K point showed the non-doped linear ${\pi}$ band structure indicating monolayer graphene decoupled from the SiC substrate electronically. Note that the charge neutrality of graphene grown by Pd silicidation and intercalation was sustained regardless of annealing temperature in contrast with quasi-free- standing graphene induced by H and Au intercalation. Further annealing above $1,000^{\circ}C$ accelerates sublimation of the Pd silicide layer underneath graphene. This results in appearance of the $(6r3x6r3)R30^{\circ}$ structure and dissolution of the ${\pi}$ bands for quasi-free-standing graphene.

• Journal of the Korean Society for Heat Treatment
• /
• v.20 no.1
• /
• pp.17-21
• /
• 2007

In this study, brazing characteristics of the dissimilar Ti and Cu metals using a Zr-base amorphous filler ($Zr_{41.2}Ti_{13.8}Cu_{12.5}Ni_{10.0}Be_{22.5}$ in at.%) have been investigated for various bonding temperatures. In the sample brazed at $790^{\circ}C$ for 10 min., the Ti-rich phases in the joint were observed, while the Cu-rich phases were obtained in the sample brazed at $825^{\circ}C$ for 10 min.. Such a different microstructure and composition in the joints could be explained by the degree of the dissolution reaction. At $790^{\circ}C$, the reaction between the Zr-rich liquid phase and the Ti base metal was actively occurred to form Ti-rich liquid phase in the joint. As the temperature increased to $825^{\circ}C$, however, the reaction between the Ti-rich liquid phase and the Cu base metal was promoted to form the Cu-rich liquid phase in the joint finally. Such a different interface reaction is attributed to the reactivity or solubility between the Zr as a main element in the filler and the Ti and Cu as a base metal element.

• Korean Journal of Mineralogy and Petrology
• /
• v.36 no.4
• /
• pp.289-302
• /
• 2023

The study area was Gangnim-myeon, Hoengseong-gun, Gangwon-do, composed of the Chiaksan gneiss complex, and it was revealed that the concentrations of uranium (U) and thorium (Th) within the groundwater of the study area exceeded their water quality standards. Hence, artificial weathering experiments were conducted to elucidate mineralogically the mechanisms of their leaching using drilling cores obtained from the corresponding groundwater aquifers. First of all, the mineralogical compositions of core samples were observed, and the results indicated that the content of clinochlore, a member of the chlorite group of minerals that can form through low- and intermediate-temperature metamorphisms, was relatively higher. In addition, the Th concentration was measured ten times higher than that of U. The results of artificial weathering experiments suggested that the Th concentrations gradually increased through the dissolution of radioactive-element-bearing minerals up to the first day, and then they tended to decrease. It could be attributed to the fact that Th was leached with the dissolution of thorite, which might be a secondary mineral, and then dissolved Th was re-precipitated as the various forms of salt, such as sulfate. Even though the U content was lower than that of Th in the core samples, the U concentration was one hundred times higher than that of Th after the weathering experiments. It is likely caused by the gradual dissolution and desorption of U included in intensively weathered thorite or adsorbed as a form of UO₂²⁺ on the mineral surface. In addition, the leaching tendency of U and Th was positively correlated with the bicarbonate concentration. However, the concentrations between U and Th in groundwater exhibited a relatively lower correlation, which might result from the fact that they occurred from different sources, as aforementioned. Among various kinetic models, the parabolic diffusion and pseudo-second-order kinetic models were confirmed to best fit the dissolution kinetics of both elements. The period that would be taken for the U concentration to exceed its drinking-water standard was inferred using the regressed parameters of the best-fitted models, and the duration of 29.4 years was predicted in the neutral-pH aquifers with relatively higher concentrations of HCO₃, indicating that U could be relatively quickly leached out into groundwater.

• Journal of Korean Society of Environmental Engineers
• /
• v.27 no.7
• /
• pp.697-703
• /
• 2005

Iron-coated sand(ICS) was prepared with variation of particle size of Joomoonjin sand, primary and secondary coating temperature, coating time, and dosage of initial Fe(III). An optimum condition of the preparation ICS was selected from the coating efficiency, stability of coated Fe(III), and removal efficiency of As(V). Coated amount of Fe(III) increased as primary coating temperature increased with smaller particle size of sand. Coating efficiency was quite similar over the investigated secondary coating temperature and time, while adsorption efficiency of As(V) onto ICS was severely reduced with ICS prepared at higher secondary coating temperature. By considering these results, an optimum secondary coating temperature and time for the preparation of ICS was selected as $150^{\circ}C$ and 1-hr, respectively. Coating efficiency increased us the dosage of initial Fe(III) up to 0.8 Fe(III) mol/kg sand and then no distinct increase was noted. Maximum As(V) adsorption was observed at 0.8 Fe(III) mol/kg sand. Secondary coating temperature and time were important parameters affecting stability of ICS, showing decreased dissolution of Fe(III) from ICS prepared at higher coating temperature and at longer coating time. From anionic type adsorption of As(V) onto ICS, it is possible to suggest the application of ICS for the removal of As(V) contaminated in acidic water system.

• Journal of the Korean Society of Groundwater Environment
• /
• v.7 no.2
• /
• pp.73-88
• /
• 2000

The geochemistry of the $CO_2$-rich waters ($Pco_2\leq$about 1 atm) in NE part of the Kangwon province was investigated. The $CO_2$-rich waters can be divided to three types based on chemical compositions: Na-$HCO_3$, Ca-Na-$HCO_3$and Ca-$HCO_3$types. The water chemistry indicates that these type waters were evolved through reaction with host rocks by supply of deep-seated $CO_2$during deep circulation, and their geochemical environments in depth might have been different each other. The dissolution process of plagioclase is important in water/granite interactions and its solubility change according to reaction temperature played an important role in the determination of chemical compositions. The higher reaction temperature coincides with the lower different in solubility between albite and anorthite. It means that calcium is mainly released to the water in the lower temperature, whereas sodium in the higher temperature due to high Na/Ca ratio in plagioclase. The application of various chemical geothermometries on the $CO_2$-rich waters shows that the calculated reservoir temperature of Na-$HCO_3$type (about 15$0^{\circ}C$) is higher than those of Ca-$HCO_3$type. Therefore, we now interpret the recognized chemical difference was mainly due to the difference of reaction temperature. Considering normal thermal gradient, we can understand that the Na-$HCO_3$type was evolved from deeper crustal depth than the Ca-$HCO_3$type.

• Journal of Korea Foundry Society
• /
• v.5 no.4
• /
• pp.243-257
• /
• 1985

Many authors have studied the solidification process of cast iron and the effect of grain boundaries in austenite shell on the growth of spheroidal graphite. But, the studies on the melting morphology of cast iron are rare and the effect of grain boundaries in austenite shell on the melting procedure of spheroidal graphite cast iron is unknown. Therefore, in this work, the melting procedure of cast iron and the role of grain boundaries in austenite shell on the melting of spheroidal graphite have been studied. The main results are summarized as follows. 1. In white cast iron containing silicon, melting initiates at the interface between austenite matrix and temper carbon which was decomposed from $Fe_3C$ during heating. 2. In gray cast iron, melting initiates at the boundary of eutectic cell where elements with low melting temperature are condensed. The dissolution of kish graphite is difficult. 3. In spheroidal graphite cast iron containing little phosphor, melting initiates at the outer region of austenite shell in which silicon is condensed. In this case, grain boundaries in austenite shell give little effect on the melting procedure of spheroidal graphite. 4. In spheroidal graphite cast iron containing phosphor above 0.3 wt%, its melting phenomena are changed with heating rate due to the existence of steadite. In this case, it can be concluded that liquid phase of steadite, which segregated on outer region of austenite shell, moves to spheroidal graphite-austenite interface along the grain boundaries in austenite shell.

• Bulletin of the Korean Chemical Society
• /
• v.31 no.2
• /
• pp.309-314
• /
• 2010

The surface modified $LiMn_2O_4$ materials with Li-Fe composites were prepared by a sol-gel method to improve the electrochemical performance of $LiMn_2O_4$ and were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS), and transmission electron microscopy (TEM)-EDS. XRD results indicate that all the samples (modified and pristine samples) have cubic spinel structures, and XRD, XPS, and TEM-EDS data reveal the formation of $Li(Li_xFe_xMn_{2-2x})O_4$ solid solution on the surface of particles. For the electrochemical properties, the modified material demonstrated dramatically enhanced reversibility and stability even at elevated temperature. These improvements are attributed to the formation of the solid solution, and thus-formed solid solution phase on the surface of $LiMn_2O_4$ particle reduces the dissolution of Mn ion and suppresses the Jahn-Teller effect.

• Transactions of the Korean Society of Pressure Vessels and Piping
• /
• v.7 no.2
• /
• pp.26-33
• /
• 2011

To evaluate steam oxidation behaviours of Alloy 617 and Haynes 230, oxidation test were performed at $900^{\circ}C$ in steam and $steam+20\;vol.-%\;H_2$ environments. Oxidation rate in steam condition was similar to that in air for Alloy 617, while it was slightly lower for Haynes 230. When hydrogen was added to steam, oxidation rate was enhanced. Isolated $MnTiO_3$ particle were formed on $Cr_2O_3$ oxide layer and sub layer $Cr_2O_3$ were formed in steam and $steam+20\;vol.-%\;H_2$ for Alloy 617. On the other hands, $MnCr_2O_3$ layer were formed on top of $Cr_2O_3$ oxide layer for Haynes 230. The extensive sub layer $Cr_2O_3$ formation was resulted from the oxygen inward diffusion in such environments. When hydrogen was added, the oxide morphology was changed from polygonal to platelet because of the accelerated diffusion of cations under the oxide layer. In addition, decarburized zone was extended as hydrogen participated into the reactions causing carbide dissolution.

• Journal of the Korean institute of surface engineering
• /
• v.37 no.2
• /
• pp.110-118
• /
• 2004

The corrosion of pure titanium (CP- Ti Grade 2) and titanium alloy (Ti6Al4V ELI) were studied under various conditions of simulated body fluids. The static immersion test and the electrochemical test were performed in accordance with ISO 10271 : 2001. For the electrochemical test, the open circuit potential was monitored as a function of time, and the cyclic polarization curve was recorded. The corrosion resistance was evaluated from the values of corrosion potential, passivation current density, breakdown potential, and the shape of hysteresis etc. The effects of alloy type, surface condition, temperature, oxygen, and constituents in the fluids such as acid, chloride were estimated. Both specimens had extremely low dissolution rate in the static immersion test. They showed strong passivation characteristics in the electrochemical test. They maintained negligible current density throughout the wide anodic potential range. The passive layer was not broken up to 2.0 V (vs. SCE). The hysteresis and the shift of passivation potential toward the anodic direction was observed during the reversed scan. The passivation process appeared to be accelerated by oxygen in air or that dissolved in the fluids. The passivation also proceeded without oxygen by the reaction of constituents in the fluids. Acid or chloride in the fluids, specially later weakened the passive layer, and then induced higher passivation current density and less shift of passivation potential in the reversed scan. CP-Ti Grade 2 was more reactive than Ti6Al4V ELI in the fluids containing acid or chloride, but thicker layer produced on its surface provided higher corrosion resistance.

• Journal of the Korean Crystal Growth and Crystal Technology
• /
• v.10 no.2
• /
• pp.166-170
• /
• 2000

Amorphous calcium carbonate was prepared by wet chemical method and performed the control of their shape using crystalline calcium carbonate that prepared from oyster shell by the $CO_2$ gas blowing method. As a result, amorphous calcium carbonate was obtained by the dissolution process of crystalline calcium carbonate in the dil-HC1 solution, mixing of sodium carbonate solution, precipitation and filtering with high speed. And using the amorphous calcium carbonate of mud type, crystalline calcium carbonates with cubic, needle, spindle, spherical and plate shape were obtained in the temperature rang of 2~$85^{\circ}C$ and reaction time range of 5~60 minute.