• Transactions on Electrical and Electronic Materials
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• v.18 no.4
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• pp.207-210
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• 2017

The suspension arrangement of insulators provides flexibility and assists in power transmission in transmission lines. The performance of the insulator string is strongly influenced by the environmental conditions to which it is exposed, its shape and the inherent material properties of suspension-type insulators. The suspension-type insulators are mostly made from glass, porcelain and ceramic material due to their high resistivity. Irregularity in charge distribution throughout the porcelain insulator may lead to accelerated aging and electrical breakdown. A very high and steep lightning impulse voltage may also cause breakdown of suspension-type insulators. We investigated various material characteristics such as alumina addition, surface morphology, x-ray diffraction pattern and relative density of suspension porcelain insulators manufactured in 1989 (36,000 lbs.), 1995 (36,000 lbs.) and 2001 (36,000 lbs.) by the KRI Company for use in 154 kV high power transmission lines. We compared the material characteristics of these porcelain insulators with that of the top-of-the-line porcelain insulators (36,000 lbs.) manufactured by the NGK Company in 2000. These suspension-type porcelain insulators were exposed to arc and flashover tests to examine their electrical and mechanical strength. It was noted that alumina addition (17 wt.%) for K-2001 was one of the major contributors to the enhancement of the performance of the porcelain insulators and to their ability to withstand very high current generation during the arc test. The porcelain insulators manufactured during 2001 also showed the highest relative density of 95.8% as compared to the other insulators manufactured in 1989 and 1995 respectively 94.2% and 91.5%. We also discuss reports of various failure modes of suspension-type porcelain insulators.

• Journal of the Korean Society for Heat Treatment
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• v.31 no.4
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• pp.171-179
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• 2018

Copper powder dispersed with 4 vol.% of $Cr_2O_3$ was successfully produced by a simple milling at 210 K with a mixture of $Cu_2O$, Cu and Cr elemental powders, followed by Hot Pressing (HP) at 1123 K and 50 MPa for 2h to consolidate the milled powder. The microstructure of the HPed material was characterized by standard metallographic techniques such as XRD (X-ray Diffraction), TEM and STEM-EDS. The results of STEMEDS analysis showed that the HPed materials comprised a mixture of nanocrystalline Cu matrix and $Cr_2O_3$ dispersoid with a homogeneous bimodal size distribution. The mechanical properties of the HPed materials were characterized by micro Vickers hardness test at room temperature. The thermodynamic considerations on the heat of formation, the incubation time to ignite MSR (Mechanically induced Self-sustaining Reaction), and the adiabatic temperature for the heat of displacement reaction between the oxide-metal are made for the delayed formation of $Cr_2O_3$ dispersoid in terms of MSR suppression. The results of TEM observation and hardness test indicated that the relatively large dispersoids in the HPed materials are attributed to the significant coarsening for the high temperature consolidation; this leads to the low Vickers hardness value. Based on the thermodynamic calculation for the operating processes with a limited number of parameters, the formation kinetics and coarsening of the $Cr_2O_3$ dispersoid are discussed.

• Korean Journal of Metals and Materials
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• v.50 no.11
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• pp.801-808
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• 2012

TiAlCN coatings with various C contents were synthesized by unbalanced magnetron sputtering. The characteristics, the crystalline structure, surface morphology, hardness, and friction coefficient of the coatings as a function of the C content were investigated by X-ray diffraction (XRD), atomic force microscopy (AFM), a microhardness tester, and a wear test. In addition, their corrosion behaviors in a deaerated 3.5 wt% NaCl solution at $40^{\circ}C$ were investigated by potentiodynamic polarization tests. The results indicated that the $Ti_{14.9}Al_{15.5}C_{30.7}N_{38.9}$ coating had the highest hardness, elastic modulus, and a plastic deformation resistance of 39 GPa, 359 GPa, and 0.55, respectively, and it also had the lowest friction coefficient of approximately 0.26. Comparative evaluation of the TiAlCN coatings indicated that a wide range of coating properties, especially coating hardness, could be obtained by the synthesis methods and processing variables. The microhardness of the coatings was much higher than that from previously reported coating using similar magnetron sputtering processes. It was almost as high as the microhardness measured from the TiAlCN coatings (~41 GPa) synthesized using an arc ion plating process. The potentiodynamic test showed that the corrosion resistance of the TiAlCN coatings was significantly better than the TiAlN coatings, and their corrosion current density ($i_{corr}$), corrosion potentials ($E_{corr}$) and corrosion rate decreased with an increasing C content in the coatings. The much denser microstructure of the coatings due to the increased amount of amorphous phase with increasing C contents in the coatings could result in the the improved corrosion resistance of the coatings.

• Journal of the Korean institute of surface engineering
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• v.52 no.6
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• pp.283-288
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• 2019

MCrAlY overaly coatings are used as oxidation barrier coatings to prevent degradation of the underlying substrate in high temperature and oxidizing environment of the hot section of gas turbines. Therefore, oxidation resistance in high temperature is important property of MCrAlY coatings. Also, coefficients of thermal expansion (CTE) of MCrAlY have middle value of that of Ni-based superalloys and oxides, which have the effect of preventing the delamination of the surface oxides. Cyclic oxidation test is one of the most useful methods for evaluating the high temperature durability of coatings used in gas turbines. In this study, NiCoCrAlY overlay coatings were formed on Inconel 792(IN 792) substrates by vacuum plasma spraying process. Vacuum plasma sprayed NiCoCrAlY coatings and IN 792 susbstrates were exposed to 1000℃ one-hour cyclic oxidation environment. NiCoCrAlY coatings showed lower weight gain in short-term oxidation. In long-term oxidation, IN 792 substrates showed higher weight loss due to delamination of surface oxide but NiCoCrAlY coatings showed lower weight loss. X-ray diffraction (XRD) analysis showed α-Al2O3 and NiCr₂O₄ was formed during the cyclic oxidation test. Through cross-section observation using scanning electron microscopy (SEM) and electron back scatter diffraction (EBSD) analysis, thermally grown oxide (TGO) layer composed of α-Al₂O₃ and NiCr₂O₄ was formed and the thickness of TGO increased during 1000℃ cyclic oxidation test. β phase in upper side of NiCoCrAlY coating was depleted due to oxidation of Al and outer beta depletion zone thickness also increased as the cyclic oxidation time increased.

• Journal of the Mineralogical Society of Korea
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• v.29 no.4
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• pp.179-190
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• 2016

X-ray diffraction analysis, chromaticity measurement, execution and evaluation by Dancheong artisan, accelerated weathering test, and fire resistance test were conducted to test the applicability as a Dancheong pigment raw materials of Korean low grade kaolin in cultural properties. The ores that feldspar rich and composed of fine particles (< $38.1{\mu}m$) showing advantageous for the inherent purpose of the white pigment than that of high grade kaolin. And the test of whiteness, concealment force, outdoor exposure durability and fire resistance shows similar or better result than existing products (Hobun and Sanhwa jidang). In conclusion, it is expected that the use of fine feldspar rich white soil and low-grade kaolin can be used as a white pigment raw materials which have similar to better material properties and economic efficiency than existing products.

• Solar Energy
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• v.12 no.2
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• pp.62-78
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• 1992

The solar cells of $ITO_{(n)}/Si_{(p)}$, which are ITO thin films deposited and heated on Si wafer 190[$^{\circ}C$], were fabricated by two source vaccum deposition method, and their electrical properties were investigated. Its maximum output is obtained when the com- position of the thin film consist of indium oxide 91[mole %] and thin oxide 9[mole %]. The cell characteristics can be improved by annealing but are deteriorated at temperature above 600[$^{\circ}C$] for longer than 15[min]. Also, we investigated the spectral response with short circuit current of the cells and found that the increasing of the annealing caused the peak shifted to the long wavelength region. And by experiment of the X-ray diffraction, it is shown to grow the grains of the thin film with increasment of annealing temperature. The test results from the $ITO_{(n)}/Si_{(p)}$ solar cell are as follows. short circuit current : Isc= 31 $[mW/cm^2]$ open circuit voltage : Voc= 460[mV] fill factor : FF=0.71 conversion efficiency : ${\eta}$=11[%]. under the solar energy illumination of $100[mW/cm^2]$.

• Journal of the Korea Concrete Institute
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• v.29 no.4
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• pp.407-413
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• 2017

The fundamental property of magnesia phosphate cement (MPC) for concrete repair material was investigated in this research. For mechanical properties, setting time, compressive strength and tensile/flexural bond strength were measured, and hydration products were detected by X-ray diffraction. The specimens were manufactured with dead burnt magnesia and potassium dihydrogen phosphate was admixed to activate the hydration of magnesia and a borax was used as a retarder. To observe the pore structure and ionic permeability of MPC mortar, mercury intrusion porosimetry was performed together with rapid chloride penetration test (RCPT). As a result, time to set of Fresh MPC mortar was in range of 16 to 21 min depend on the M/P ratio. Borax helped delaying setting time of MPC to 68 min. The compressive strength of MPC with M/P of 4 was sharply developed to 30 MPa within 12 hours. The compressive strength of MPC mortar was in range of 11.0 to 30.0 MPa depend on the M/P ratio at 12 hours of curing. Both tensile and flexural bond strength of MPC to old substrate (i.e. MPC; New substrate to OPC; Old substrate) were even higher than ordinary Portland cement mortar (i.e. [OPC; New substrate] to [OPC; Old substrate]) does, accounting 19 and 17 MPa, respectively. The total pore volume of MPC mortar was lower than that of OPC mortar. MPC mortar had the entrained air void rather than capillary pore. The RCPT showed that total charge passed of OPC mortar had more than that of MPC mortar, which can be explained by the pore volume and pore distribution.

• Journal of Korean Dental Science
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• v.11 no.2
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• pp.71-81
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• 2018

Purpose: The purpose of this study was to investigate the corrosion behaviors of dental implant alloy after microsized surface modification in electrolytes containing Mn ion. Materials and Methods: $Mn-TiO_2$ coatings were prepared on the Ti-6Al-4V alloy for dental implants using a plasma electrolytic oxidation (PEO) method carried out in electrolytes containing different concentrations of Mn, namely, 0%, 5%, and 20%. Potentiodynamic method was employed to examine the corrosion behaviors, and the alternatingcurrent (AC) impedance behaviors were examined in 0.9% NaCl solution at $36.5^{\circ}C{\pm}1.0^{\circ}C$ using a potentiostat and an electrochemical impedance spectroscope. The potentiodynamic test was performed with a scanning rate of $1.667mV\;s^{-1}$ from -1,500 to 2,000 mV. A frequency range of $10^{-1}$ to $10^5Hz$ was used for the electrochemical impedance spectroscopy (EIS) measurements. The amplitude of the AC signal was 10 mV, and 5 points per decade were used. The morphology and structure of the samples were examined using field-emission scanning electron microscopy and thin-film X-ray diffraction. The elemental analysis was performed using energy-dispersive X-ray spectroscopy. Result: The PEO-treated surface exhibited an irregular pore shape, and the pore size and number of the pores increased with an increase in the Mn concentration. For the PEO-treated surface, a higher corrosion current density ($I_{corr}$) and a lower corrosion potential ($E_{corr}$) was obtained as compared to that of the bulk surface. However, the current density in the passive regions ($I_{pass}$) was found to be more stable for the PEO-treated surface than that of the bulk surface. As the Mn concentration increased, the capacitance values of the outer porous layer and the barrier layer decreased, and the polarization resistance of the barrier layers increased. In the case of the Mn/Ca-P coatings, the corroded surface was found to be covered with corrosion products. Conclusion: It is confirmed that corrosion resistance and polarization resistance of PEO-treated alloy increased as Mn content increased, and PEO-treated surface showed lower current density in the passive region.

• The Journal of Advanced Prosthodontics
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• v.13 no.6
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• pp.385-395
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• 2021

PURPOSE. To characterize the microstructure of three yttria partially stabilized zirconia ceramics and to compare their hardness, indentation fracture resistance (IFR), biaxial flexural strength (BFS), and fatigue flexural strength. MATERIALS AND METHODS. Disc-shaped specimens were obtained from 3Y-TZP (Vita YZ HT), 4Y-PSZ (Vita YZ ST) and 5Y-PSZ (Vita YZ XT), following the ISO 6872/2015 guidelines for BFS testing (final dimensions of 12 mm in diameter, 0.7 and 1.2 ± 0.1 mm in thicknesses). Energy-dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD) and scanning electron microscopy (SEM) analyses were performed, and mechanical properties were assessed by Vickers hardness, IFR, quasi-static BFS and fatigue tests. RESULTS. All ceramics showed similar chemical compositions, but mainly differed in the amount of yttria, which was higher as the amount of cubic phase in the diffractogram (5Y-PSZ > 4Y-PSZ > 3Y-TZP). The 4Y- and 5Y-PSZ specimens showed surface defects under SEM, while 3Y-TZP exhibited greater grain uniformity on the surface. 5Y-PSZ and 3Y-TZP presented the highest hardness values, while 3Y-TZP was higher than 4Y- and 5Y-PSZ with regard to the IFR. The 5Y-PSZ specimen (0.7 and 1.2 mm) showed the worst mechanical performance (fatigue BFS and cycles until failure), while 3Y-TZP and 4Y-PSZ presented statistically similar values, higher than 5Y-PSZ for both thicknesses (0.7 and 1.2 mm). Moreover, 3Y-TZP showed the highest (1.2 mm group) and the lowest (0.7 mm group) degradation percentage, and 5Y-PSZ had higher strength degradation than 4Y-PSZ group. CONCLUSION. Despite the microstructural differences, 4Y-PSZ and 3Y-TZP had similar fatigue behavior regardless of thickness. 5Y-PSZ had the lowest mechanical performance.

• Journal of the Korean Recycled Construction Resources Institute
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• v.11 no.1
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• pp.89-96
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• 2023

In this paper, a ferrosilicon by-product was evaluated to confirm the feasibility of recycling it as supplementary cementitious material of ordinary Portland cement in concrete. Three different levels of replacement ratio (10 %, 20 % and 30 % of total binder) were applied to find which is the most beneficial to be used as a binder. Ferrosilicon concrete was initially assessed at setting time and compressive strength. Durability was evaluated by the resistance to chloride penetration test(RCPT) and alkali-silica reaction(ASR) with a comparison to silica fume concrete due to their similarity in chemical composition. The porosimetry and X-ray diffraction analysis along with energy dispersive X-ray spectroscopy give information on the microstructural characteristics of the ferrosilicon concrete. It was found that 10 % ferrosilicon concrete has higher strength while 20 %, 30 % have lower strength than OPC concrete. However, chemical resistance to chloride attack is higher when replacement is increased. Compared to silica fume, the durability of ferrosilicon might be less efficient however, it is obviously beneficial than OPC. High SiO₂ content in ferrosilicon results in producing more C-S-H gel which could make denser pore structure. Most of the risk of alkali silica reaction to silicate binders through length change tests was less than 0.2 %, and both mortar using ferrosilicon and silica fume showed better resistance to alkali silica reaction as the substitution rate increased.Reuse of industrial waste rather than producing highly refined additives might reduce environmental load during manufacture and save costs.