• Journal of the Korean Vacuum Society
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• v.17 no.5
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• pp.381-386
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• 2008

Adatom kinetics on the surfaces of Co overlayers, prepared on the W(110) surface, was studied with scanning tunneling microscopy. By counting the number-density of the adatom-islands, we estimated the ratio of adatom diffusion coefficients. The ratio $D_{W(110)}:D_{1ML\;Co}:D_{2ML\;Co}$ was measured to be 1 : 125 : 33000 at room temperature, where $D_{W(110)},\;D_{1ML\;Co}$, and $D_{2ML\;Co}$ are the diffusion coefficients on bare W(110) surface, on one-monolayer Co overlayer, and on two-monolayers Co overlayers, respectively. An increased diffusion coefficient on two-ML Co overlayers, relative to that on one-ML Co overlayers, was explained with the heteroepitaxial strain effect.

• Journal of Radiation Protection and Research
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• v.20 no.4
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• pp.265-274
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• 1995

Diffusivity of ions of radioactive species is an important factor for designing radwaste repositories. Clay minerals are used as a backfill material. In this study, diffusion of Co-60 ions through the bentonite having various densities has been studied, using a diffusion cell. The measured diffusivities of Co-60 ions decreased as the density of bentonite increased. The diffusivity of Co-60 ion decreased from $8.79{\times}10^{11}m^2/s$ to $6.82{\times}10-13m^2/s$ as the clay dry bulk density increased from 0.41 to 2.03g/cm3. The diffusivity of Co ion was larger than that of Sr ion at low density, but the diffusivity of Co ion decreased rapidly as the density of clay increased and became smaller than that of Cs ion at high density. This phenomenon is thought to be caused by the rapid decrease of the fraction of mobile cation since the chemical combination of Co ions with oxygen or oxide on clay surface and the entrance of Co ions into the crystal structure of clay increase as the clay density increases. This change should be considered especially in designing the clay back fill for low and intermediate radwaste disposal facilities.

• Journal of the Korea institute for structural maintenance and inspection
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• v.14 no.1
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• pp.141-147
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• 2010

The rate of carbonation is usually low in the natural environment due to the low $CO_2$ concentration in the atmosphere. Therefore, investigation of carbonation is usually conducted under accelerated testing conditions so as to speed up the process. This study is to predict carbonation progress by mathematical model, based on the diffusions of $CO_2$ and its reaction with $Ca(OH)_2$ in carbonation progressing region, in the atmosphere. To predict of carbonation progress in the atmosphere, we adopted a diffusion coefficient of $CO_2$ that agreed well the experimental value obtained by the accelerated carbonation test. Consequently the model can predict the rate of carbonation of concrete exposed in the atmosphere regardless of finishing materials.

• Nuclear Engineering and Technology
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• v.29 no.6
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• pp.17-28
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• 1997

콘크리트를 통한 핵종의 확산계수는 처분장으로 부터의 핵종 유출을 평가하는데 중요한입력 인자이다. 본 연구에서는 콘크리트에서의 핵종확산 연구 현황 및 핵종화산에 미치는 주요 인자들의 영향 등이 조사되었고, 주요 핵종의 확산계수가 직접 측정되었다. 내부확산법으로 측정된 확산계수값은 시료의 물과 시멘트 비 (W/C)가 증가할수록 증가하였으나 거의 같은 승수내에 있었으며, 공극확산이 핵종이동을 지배하였다. Cs 과 I 의 겉보기 확산계수는 순수 시멘트에서 각각 $1.0{\times}10^{-12}~1.0{\times}10^{-11}m^2/s$ 및 $3.0{\times}10^{-14}~1.0{\times}10^{-13}m^2/s$, 몰타르에서는 각각 $3.0{\times}10^{-12}~9.0{\times}10^{-11}m^2/s$ 및 $3.0{\times}10^{-11}m^2/s$의 범위에 있었다. 이와 병행하여 시멘트, 몰타르 및 콘크리트에서의 주요 방사성핵종의 확산계수 값을 문헌으로부터 수집, 정리하였다. 대상 핵종은 Cs, I, Sr, C, Co, H, Am, PU, Ni, Mn, Fe, Nb 및 Tc로서 수집된 핵종확산계수 값은 시료의 조건 (공극률 밀도, W/C 비, 온도 등) 에 따라 큰 편차를 보여주고 있다.

• Journal of Radiation Protection and Research
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• v.16 no.2
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• pp.27-39
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• 1991

The diffusion of Sr-85, Cs-137, Co-60 and Am-241 in compacted domestic bentonite was studied, using a diffusion cell unit in which diffusion took place axially from the center of cylindrical bentonite sample body. The effects of compaction density and heat-treated bentonite on diffusion were analysed. And the diffusion mechanism of radionuclide was also analysed by evaluating the measured diffusivity of anion Cl-36. The apparent diffusivities obtained for Sr-85, Cs-137, Co-60 and Am-241 were $l.07{\times}10^{-11},\;6.705{\times}10^{-13},\;l.226{\times}10^{-13}\;and\; l.310{\times}10^{-14}m^2/sec$, respectively. When the as-pressed density of bentonite increased from $1.8\;to\;2.0g/cm^3$, the apparent diffusivity of Cs-137 decreased by quarter. In the case of bentonite heat-treated to $150^{\circ}C$, no significant change in diffusivity was observed, which showed the possibility that the domestic bentonite could be used as a chemical barrier to retard the radionuclide migration at below $150^{\circ}C$. From the calculated pore and surface diffusivity, the surface diffusion due to the concentration gradient of radionuclide sorbed on the solid phase was found to dominate greatly in total transport process.

• Journal of the Korea Concrete Institute
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• v.22 no.2
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• pp.209-217
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• 2010

Recently, some mathematical models for the prediction on progress of carbonation of concrete were reported. These models take account for $CO_2$ diffusion and chemical reaction between $Ca(OH)_2$ and $CO_2$. Based on the assumption that $CO_2$ diffuses in the carbonation zone and reacts with $Ca(OH)_2$ at the outer face of carbonation zone and non-carbonation zone. In this study, a mathematical model to predict the progress of carbonation of concrete has been established based on the reducing concentration of $Ca(OH)_2$ in the carbonation progress zone, where $Ca(OH)_2$ reacts with $CO_2$ and $Ca(OH)_2$ and $CaCO_3$ coexist. Also, the prediction model of carbonation progress rate of concrete using the air permeability coefficient regarding to $CO_2$ diffusion is developed. As a result of this study, an expression, the model equation is obtained for the prediction of carbonation based on the time and interaction velocity between $CO_2$ and Ca(OH)$_2$ dependent air permeability coefficient. The prediction by the model satisfied the experimental data of the accelerated carbonation for painted concrete. Consequently, the model can predict the rate of carbonation and the potential service life of concrete structure exposed to atmosphere.

• Journal of the Korean Chemical Society
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• v.41 no.8
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• pp.385-391
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• 1997

Diffusion Coefficient$(D_0)$ and electrode reaction rate Constant$(K_0)$ of Co$(dimethyl bipyridine)_3(ClO_4)_2$ were determined by cyclic voltammetry and chronoamperometry. It was also investigated that the effects of solvent, concentration, and scan rate, etc. on the diffusion coefficient and the temperature effect on the rate constant. The peak currents and diffusion coefficients were dcreased as increasing the viscosity of solvent. Diffusion coefficient was $5.54{\times}10^{-6 }cm^2/sec$ and the reaction rate constant was $2.39{\times}10^{-3 }/s$ at 25$^{\circ}C$. The thermodynamic parameters such as ${\Delta}G^{\neq},\;{\Delta}H^{\neq},\;and\;{\Delta}S$ were calculated from plotting the reaction rate constants versus the solution temperatures. This compound was shown the catalytic effect on the oxygen reduction that the reduction peak current of oxygen was greatly enhanced and the peak potential was shifted to +0.2 volt.

• Journal of the Korea Concrete Institute
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• v.15 no.3
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• pp.443-453
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• 2003

The diffusivity of carbon dioxide in concrete is very important in that it directly affects the degree of carbonation in concrete structures. The purpose of the present study is to explore the diffusivity of carbon dioxide and to derive a realistic equation to estimate the diffusion coefficient of carbon dioxide in concrete. For this purpose, several series of concrete specimens have been tested. Major test variables were the water-cement ratios. The total porosities and the diffusion coefficients of carbon dioxide were measured for the specimens. The present study indicates that the measured porosities agree well with the calculated ones. The effects of porosity and relative humidity on the diffusion coefficient of carbon dioxide were examined. A prediction equation to estimate the diffusion coefficient of carbon dioxide was derived and proposed in this study. The proposed equation shows reasonably good correlation with test data on the $CO_2$ diffusion coefficient of concrete

• Journal of the Korea Concrete Institute
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• v.15 no.6
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• pp.778-784
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• 2003

The carbonation of concrete is one of the major factors that cause durability problems in concrete structures. The rate of carbonation depends largely upon the diffusivity of carbon dioxide in concrete. The purpose of this study is to identify the diffusion coefficients of carbon dioxide for various concrete mixtures. To this end, several series of tests have been planned and conducted. The test results indicate that the diffusion of carbon dioxide reached the steady-state within about five hours after exposure. The diffusion coefficient increases with the increase of water-cement ratio and decreases with the increase of relative humidity at the same water-cement ratio. The content of aggregates also influences the diffusivity of carbon dioxide in concrete. It was found that the diffusion coefficient of cement paste is larger than that of concrete or mortar. The experimental study of carbon dioxide diffusivity in this study will allow more realistic assessment of carbonation depth in concrete structures.

• Analytical Science and Technology
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• v.9 no.3
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• pp.292-301
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• 1996

The synthesis of poly(ether)tailed bipyridine complex as redox reaction probes has advanced attempts to interpret very slow diffusion and heterogeneous electron transfer. Diffusion coefficients as low as $1.5{\times}10^{-15}cm^2/s$ have been observed for the oxidation of neat $Co(bpy(ppgm)_2)_{3^-}(ClO_4)_2$ with $LiClO_4$ electrolyte. Heterogeneous electron transfer rate constants of materials were found to vary with diffusion coefficient. The decrease in k as the diffusion coefficient decreases was actually caused by the decreasing D. Diffusion coefficient for compound of strong ion pairing anion($ClO{_4}^-$) was much smaller than the diffusion coefficient for compound of weak ion pairing anion($CF_3COO^-$).