• Transactions of the Korean Society of Mechanical Engineers
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• v.17 no.2
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• pp.300-312
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• 1993

The material deterioration of service-exposed boiler tube steels in fossil power plant was evaluated by using the electrochemical technique namely, modified electrochemical potentiokinetic reactivation(EPR). It was focused that the passivation of Mo$_{6}$C carbide which governs the mechanical properties of Mo alloyed steels did not occur even in the passivity region of steel in sodium molybdate solution and the reactivation peak current (Ip) observed as the result of non-passivation indicating the precipitation of Mo$_{6}$C carbides. To obtain the optimal test conditions for the field test by using the specially designed electrochemical cell, the effects of scan rate, the surface roughness and the pH of electrolyte on Ip value were also investigated. Furthermore, the change of mechanical properties occurred during the long time exposure at high temperature was evlauated quantitatively by small punch(SP) tests and micro hardness test taking account of the metallurgical changes. It is known that reactivation peak current (Ip) has a good relationship with Larson-Miller Parameter(LMP) which represents the information about material deterioration occurred at high temperature environment. In addition it was possible to estimate the ductile-brittle transition temperature (DBTT) by means of the SP test. The Sp test could be, therefore, suggested as a reliable test method for evaluating the material degradation of boiler tube steels. From the good correaltion between the SP DBTT and Ip values shown in this study, it was knows that the change of mechanical properties could be evaluated non-destructively by measurring only Ip values.ues.

• Journal of Electrochemical Science and Technology
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• v.14 no.1
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• pp.75-84
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• 2023

The accumulation of spent carbide (YG8), not only pollutes the environment but also causes waste of tungsten, cobalt and other rare metal resources. To better address this issue, we proposed a combined electrochemical separation process of low-temperature aqueous solution and high-temperature molten salt for tungsten and cobalt. H₂WO₄ was obtained from spent carbide in an aqueous solution, and we calcined it to obtain WO₃, which was used as a raw material to obtain tungsten by using molten salt electrodeposition. The influence of the current efficiency and the electrochemical behavior of the discharge precipitation of W(VI) were also studied. The calcination results showed that the morphology of WO₃ was regular and there were no other impurities. The maximum current efficiency of 82.91% was achieved in a series of electrodeposition experiments. According to XRD and SEM analysis, the recovered product was high purity tungsten, which belongs to the simple cubic crystal system. In the W(VI) reduction mechanism experiments, the electrochemical process of W(VI) in NaCl-Na₂WO₄-WO₃ molten salt was investigated using linear scanning voltammetry (LSV) and chronoamperometry in a three-electrode system. The LSV showed that W(VI) was reduced at the cathode in two steps and the electrode reaction was controlled by diffusion. The fitting results of chronoamperometry showed that the nucleation mechanism of W(VI) was an instantaneous nucleation mode, and the diffusion coefficient was 7.379×10^-10 cm²·s^-1.

• Journal of the Korean Electrochemical Society
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• v.23 no.1
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• pp.11-17
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• 2020

The Prussian blue analogues of Fe₂(CN)₆ and Na_xFe₂(CN)₆ are prepared by precipitation method and evaluated the electrochemical characteristics as positive electrode materials for lithium-ion batteries (LIBs) because of their low cost. Fe₂(CN)₆ shows a low reversible capacity of 34.6 mAh g^-1, whereas sodium-containing Na_xFe₂(CN)₆ exhibits a reversible capacity of 107.5 mAh g^-1 when the discharge process proceeds first. When charging is first carried out to remove sodium in the structure, the reversible capacity of 114.1 mAh g^-1 is achieved and the cycle performance is further improved. In addition, Na_x-Fe₂(CN)₆ is synthesized at 0℃, room temperature (RT), and 60℃, respectively. Regardless of the synthesis temperature, Na_xFe₂(CN)₆ shows similar initial reversible capacity, but the crystallite size is formed smaller and improved cycle performance when synthetic temperature is lower. The sample synthesized at 0℃ shows a reversible capacity of 86.4 mAh g^-1 at the 120^th cycle and maintains 76.8% of the initial capacity.

• Bulletin of the Korean Chemical Society
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• v.32 no.8
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• pp.2683-2688
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• 2011

Silica-manganese oxides with a core-shell structure were synthesized via precipitation of manganese oxides on the $SiO_2$ core while varying the concentration of a precipitation agent. Elemental analysis, crystalline property investigation, and morphology observations using low- and high-resolution electron microscopes were applied to the synthesized silica-manganese oxides with the core-shell structure. As the concentration of the precipitating agent increased, the manganese oxide shells around the $SiO_2$ core sequentially appeared as $Mn_3O_4$ particles, $Mn_2O_3+Mn_3O_4$ thin layers, and ${\alpha}-MnO_2$ urchin-like phases. The prepared samples were assembled as electrodes in a supercapacitor with 0.1 M $Na_2SO_4$ electrolyte, and their electrochemical properties were examined using cyclic voltammetry and charge-discharge cycling. The maximum specific capacitance obtained was 197 F $g^{-1}$ for the $SiO_2-MnO_2$ electrode due to the higher electronic conductivity of the $MnO_2$ shell compared to those of the $Mn_2O_3$ and $Mn_3O_4$ phases.

• Journal of the Korean Ceramic Society
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• v.45 no.12
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• pp.755-759
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• 2008

New cathode materials $LaNi_{1-x}{Cu_x}{O_3}$ (typically $LaNi_{0.8}Cu_{0.2}O_3$) were synthesized using a co-precipitation method. The structure and morphology of the powders were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The composite material [$Ce_{0.8}Sm_{0.2}O_{2-\ddot{a}}$(SDC) and carbonate (${Na_2}{CO_3},{Li_2}{CO_3}$)], NiO and $LaNi_{1-x}{Cu_x}{O_3}$ were used as the electrolyte, anode and cathode, respectively. The electrochemical performance of La-Ni-Cu-O perovskite oxide at low temperatures ($400{\sim}550^{\circ}C$) was studied. The results showed that $LaNi_{0.8}Cu_{0.2}O_3$ precursor powder prepared through a co-precipitation method and calcined at $860^{\circ}C$ for 2 h formed uniform grains with diameters in the range of $400{\sim}500\;nm$. The maximum power density and the short circuit current density of the single cell unit at $550^{\circ}C$ were found to be $390\;mW/cm^2$ and $968\;mA/cm^2$, respectively.

• Bulletin of the Korean Chemical Society
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• v.35 no.2
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• pp.357-364
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• 2014

Size controlled, $LiNi_{0.8}Co_{0.15}Al_{0.05}O_2$ cathode powders were prepared by co-precipitation method followed by heat treatment at temperatures between 750 and $850^{\circ}C$. The synthesized samples are characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and electrochemical performance. The synthesized $LiNi_{0.8}Co_{0.15}Al_{0.05}O_2$ after calcined at $750^{\circ}C$ has a good electrochemical performance with an initial discharge capacity of $190mAhg^{-1}$ and good capacity retention of 100% after 30 cycles at 0.1C ($17mAg^{-1}$). The capacity retention of $LiNi_{0.8}Co_{0.15}Al_{0.05}O_2$ after calcined at $750^{\circ}C$ is better than that at 800 and $850^{\circ}C$ without capacity loss at various high C rates. This is ascribed to the minimized cation disorder, a higher conductivity, and higher lithium ion diffusion coefficient ($D_{Li}$) observed in this material. In the differential scanning calorimetry DSC profile of the charged sample, the generation of heat by exothermic reaction was decreased by calcined at high temperature, and this decrease is especially at $850^{\circ}C$. This behavior implies that the high temperature calcinations of $LiNi_{0.8}Co_{0.15}Al_{0.05}O_2$ prevent phase transitions with the release of oxygen.

• Korean Journal of Materials Research
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• v.18 no.4
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• pp.187-192
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• 2008

Multi-walled carbon nanotube (MWNT)/$SnO_2$ nano-composite (MSC) for the anode electrode of a Li-ion battery was prepared using a homogeneous precipitation method with $SnCl_2$ precursors in the presence of MWNT. XRD results indicate that when annealed in Ar at $400^{\circ}C$, $Sn_6O_4(OH)_4$ was fully converted to $SnO_2$ phases. TEM observations showed that most of the $SnO_2$ nanoparticles were deposited directly on the outside surface of the MWNT. The electrochemical performance of the MSC electrode showed higher specific capacities than a MWNT and better cycleability than a nano-$SnO_2$ electrode. The electrochemical performance of the MSC electrode improved because the MWNT in the MSC electrode absorbed the mechanical stress induced from a volume change during alloying and de-alloying reactions with lithium, leading to an increase in the electrical conductivity of the composite material.

• Advances in nano research
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• v.5 no.3
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• pp.215-230
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• 2017

The development of hybrid systems comprising nanoparticles and polymers is an opening pathway for engineering nanocomposites exhibiting outstanding mechanical, optical, electrical, and magnetic properties. Among inorganic counterpart, iron oxide nanoparticles (IONP) exhibit high magnetization, controllable surface chemistry, spintronic properties, and biological compatibility. These characteristics enable them as a platform for biomedical applications and building blocks for bottom-up approaches, such as the layer-by-layer (LbL). In this regard, the present study is addressed to investigate IONP synthesised through co-precipitation route (average diameter around 7 nm), with either positive or negative surface charges, LbL assembled with sodium sulfonated polystyrene (PSS) or polyaniline (PANI). The surface and internal morphologies, and electrochemical properties of these nanocomposites were probed with atomic force microscopy, UV-vis and Raman spectroscopy, scanning electron microscopy, cross-sectional transmission electron microscopy, and electrochemical measurements. The nanocomposites display a globular morphology with IONP densely packed while surface dressed by polyelectrolytes. The investigation of the effect of thermal annealing (300 up to $600^{\circ}C$) on the oxidation process of IONP assembled with PSS was performed using Raman spectroscopy. Our findings showed that PSS protects IONP from oxidation/phase transformation to hematite up to $400^{\circ}C$. The electrochemical performance of nanocomposite comprising IONP and PANI were investigated in $0.5mol{\times}L^{-1}$ $Na_2SO_4$ electrolyte solution by cyclic voltammetry and chronopotentiometry. Our findings indicate this structure as promising candidate for potential application as electrodes for supercapacitors.

• Journal of Electrochemical Science and Technology
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• v.5 no.1
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• pp.32-36
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• 2014

The $0.3Li_2MnO_3{\cdot}0.7LiMn_{0.55}Ni_{0.30}Co_{0.15}O_2$ cathode material was prepared via a co-precipitation method. The vapor grown carbon fiber (VGCF) was used as a conductive material and its effects on electrochemical properties of the $0.3Li_2MnO_3{\cdot}0.7LiMn_{0.55}Ni_{0.30}Co_{0.15}O_2$ cathode material were investigated. From the XRD pattern, the typical complex layered structure was confirmed and a solid solution between $Li_2MnO_3$ and $LiMO_2$ (M = Ni, Co and Mn) was formed without any secondary phases. The VGCF was properly distributed between cathode materials and conductive sources by a FE-SEM. In voltage profiles, the electrode with VGCF showed higher discharge capacity than the pristine electrode. At a 5C rate, 146 mAh/g was obtained compared with 232 mAh/g at initial discharge in the electrode with VGCF. Furthermore, the impedance of the electrode with VGCF did not changed much around $9-10{\Omega}$ while the pristine electrode increased from 21.5${\Omega}$ to $46.3{\Omega}$ after the $30^{th}$ charge/discharge cycling.

• Journal of the Korean Society for Nondestructive Testing
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• v.19 no.6
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• pp.411-419
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• 1999

It has been well recognized that a long term service at elevated temperature of $350^{\circ}C{\sim}550^{\circ}C$ induces embrittlement damage due to carbide precipitation and/or P, Sb and Sn segregation at grain boundaries and thereby deteriorates the grain boundary strength of heat resisting components in the energy-related plants. Therefore, it is very important to assess quantitatively the extent of embrittlement damage of heat resisting components to secure the reliable and efficient service condition and to prevent brittle failure in service. However, because fracture tests are limited in size and number of specimen obtained from the structural components, nondestructive test method is required. In this study, the optimum electrochemical parameters are investigated and discussed to evaluate nondestructive embrittlement damage for aged 2.25Cr-1Mo steels by means of electrochemical polarization test method (ECPTM) in proper corrosive environment. In addition, the electrochemical test results are compared with embrittlement degree evaluated by semi-nondestructive SP test.