• Journal of the Korean Chemical Society
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• v.52 no.2
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• pp.186-196
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• 2008

purpose of this study was to investigate college students' understanding of the thermodynamic aspects of the dissolution of solids and gases. The subjects were 34 juniors from the Seoul area who answered questionnaires composed of six items which asked the directions and reasons for the changes in enthalpy, entropy, and the solubility by temperature for the dissolution of solid sodium chloride and gaseous carbon dioxide into water. The results showed that the students understanding of the enthalpy change of dissolution was poor: many students answered that the dissolution of solids is an exothermic process because the dissolution occurs when the solute-solvent interaction is greater than the solute-solute interaction; the students also thought that the enthalpy should be reduced for spontaneous dissolution because the spontaneity depends on the enthalpy change only. For the entropy change, the students understanding was better and they explained it according to the meaning of disorder. For the temperature dependence of solubility, most students answered correctly regarding the direction, but only 25% of them explained the reason accounting for the enthalpy change. Many students who answered incorrectly on the enthalpy change could not explain the reason why.

• Korean Journal of Materials Research
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• v.14 no.8
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• pp.595-599
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• 2004

We investigated the structural, electrical and magnetic properties of Mn-doped $CuAlO_2$ delafossite ceramics ($CuAl_{1-x}Mn_{x}O_2,\;0\le\;x\;\le0.05$), synthesized by solid-state reaction method in an air atmosphere at a sintering temperature of $1150^{\circ}C$. The solubility limit of Mn ions in delafossite $CuAlO_2$ was found to be as low as about 3 $mol\%$. Positive Hall coefficient and the temperature dependence of conductivity established that non-doped $CuAlO_2$ ceramic is a variable-range hopping p-type semiconductor. It was found that the Mn-doping in $CuAlO_2$ rapidly reduced the hole concentration and conductivity, indicating compensation of free holes. The analysis of the magnetization data provided an evidence that antiferromagnetic superexchange interaction is the dominant mechanism of the exchange coupling between Mn ions in $CuAl_{1-x}Mn_{x}O$ alloy, leading to an almost paramagnetic behavior in this alloy.

• Journal of Ocean Engineering and Technology
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• v.23 no.2
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• pp.69-73
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• 2009

The objective of this study was to obtain a better understanding of the delayed hydride cracking (DHC) of Zr-2.5Nb alloy. The DHC model has some defects: first, it cannot explain why the DHC velocity (DHCV) becomes constant regardless of an applied stress intensity factor, even though the stress gradient is affected by the applied stress intensity factor at the notch tip. Second, it cannot explain why the DHCV has a strong dependence on the method of approaching the test temperature by a cool-down or a heating-up, even under the same stress gradient, and third, it cannot predict any hydride size effect on the DHC velocity. The DHC tests were conducted on Zr-2.5Nb compact tension specimens with the test temperatures reached by a heating-up method and a cool-down method. Crack velocities were measured in hydrided specimens, which were cooled from solution-treatment temperatures at different rates by being furnace-cooled, water-quenched, and liquid nitrogen-quenched. The resulting hydride size, morphology, and distributions were examined by optical metallography. It was found that fast cooling rates, which produce very finely dispersed hydrides, result in higher crack growth rates. This different DHC behavior of the Zr-2.5Nb tube with the cooling rate after a homogenization treatment is due to the precipitation of the $\gamma$-hydrides only in the water-quenched Zr-2.5Nb tube. This experiment will provide supporting evidence that the terminal solid solubility of a dissolution (TSSD) of $\gamma$-hydrides is higher than that of $\delta$-hydrides.

• Resources Recycling
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• v.17 no.6
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• pp.34-42
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• 2008

The properties of sodium silicate solution were surveyed by using the yellow silicomolybdic method, and the formation of silica sol from sodium silicate solution and the growth of silica sol were investigated in this study. The $SiO_2$ content of 2 wt% in sodium silicate solution was proper to oxidize sodium silicate with sulfuric acid. After the removal of sodium ions in sodium silicate solution, the pH of silicate solution had to be controlled above 9 for the stabilization of silicate solution. The condensation between silicic acid species and silica nuclei surfaces has been studied at $20{\sim}80^{\circ}C$ and pH 10 in silicate solutions with silica nuclei. The reaction falls into two kinetics regimes, limited at high silicic acid species concentration by polymerization, but at lower concentration by a process whereby deposited silicic acid species condenses further to silica. The overall condensation is first-order in silicic acid species concentration, proceeded toward to pseudo equilibrium concentration, $C_x$, rather than the solubility of amorphous silica. The heat of solution of amorphous silica was 3.34 kcal/mol and exhibits an Arrhenius temperature dependence with an apparent activation energy of 3.16 kcal/mol in the range of $20{\sim}80^{\circ}C$.

• Applied Chemistry for Engineering
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• v.18 no.3
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• pp.234-238
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• 2007

Highly conducting Polypyrroles soluble in organic solvents were synthesized using functional doping agents, such as mixed dopants [sodium di(2-ethylhexyl)sulfosuccinate (DEHSNa) Naphthalenesulfonic acid (NSA), DEHSNa Toluenesulfonic acid (TSA), DEHSNa Dodecylbenzensulfonic acid (DBSA)] and mixed oxidants [$(NH_4)_2S_2O_8{\cdot}FeCl_3$, $(NH_4)_2S_2O_8{\cdot}Fe_2(SO_4)_3$]. Ppy-DEHS powder using an oxidant, such as $(NH_4)_2S_2O_8$ (10 wt%/vol.) showed higher solubility than the mixed dopant (DEHSNa NSA, 3 wt%/vol.) and mixed oxidant [$(NH_4)_2S_2O_8{\cdot}Fe_2(SO_4)_3$, 4 wt%/vol.] in DMF solvent. But Ppy-DEHS free standing film using a mixed dopant, such as DEHSNa NSA (16 S/cm) and a mixed oxidant, such as $(NH_4)_2S_2O_8{\cdot}Fe_2(SO_4)_3$ (13 S/cm) cast from DMF solvent showed higher electrical conductivity than $(NH_4)_2S_2O_8$ (2 S/cm). For the Ppy-DEHS films using various condition cast from DMF solvent, three dimensional various range hopping model (3D VRH ; $\{{\sigma}_{dc}(T)={\sigma}_oexp[-(T_o/T)^{1/4}]\}$) provided fit to the results of temperature dependence of electrical conductivity measurement.