• Journal of the Korean institute of surface engineering
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• v.39 no.4
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• pp.179-189
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• 2006

High velocity oxygen fuel(HVOF) thermal spray coating of WC-Co powder is one of the most promising candidate for the replacement of the traditional hard chrome plating and ceramics coating because of the environmental problem of the very toxic $Cr^{6+}$ known as carcinogen and the brittleness of ceramics coating. WC-Co micron and nano powder were coated by HVOF thermal spraying method for the study of durability improvement of the high speed spindle. Coatings were planned by Taguchi program for the four spray parameters of spray distance, flow rates of hydrogen, oxygen and powder feed rate. Optimal coating process was obtained by the studies of coating properties such as porosity, surface roughness, micro hardness, and micro structure. WC-Co micron and nano powder were coated on the Inconel 718 substrate by the optimal coating process obtained in this study. The wear behaviors were studied by the sliding wear tester at room temperature and at an elevated temperature of $500^{\circ}C$ for the application to high speed spindle. Sliding wear test was carried out for four most promising hard coatings of chrome coating, ceramics coatings such as $A1_2O_3,\;Cr_2O_3$ and HVOF Co-alloy T800 for the comparison of their wear behaviors. HVOF WC-Co coating was better than other coatings showing highest micro hardness of 1400 Hv and comparable friction coefficients with others. HVOF WC-Co coating is a strong candidate for the replacement of the traditional hard chrome plating for the high speed spindle.

• Journal of Korea Soil Environment Society
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• v.3 no.1
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• pp.11-20
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• 1998

Inorganic nitrogens resulting from fertilizers are possible contaminants of bank-filtered ground water, which is an alternative source of safe drinking water. We conducted a laboratory study to determine the fate of inorganic nitrogens in the near field of bank filtration sites and we consequently illustrated the effects of these possible contaminants on the water quailty of bank filtration. Soil properties were found to be well equivalent to those of other cultivated field soils in Korea. Surface soil pH was about 4.3 which is slightly lower than average level. Overall, diverse concentrations of $NO_3$-N and $NH_3$-N were measured, and $NO_2$-N was nearly undetected. $NH_3$-N level in the field decreased, while $NO_3$-N increased along with increasing depth of unsaturated zone. Numbers of viable and nitrifying bacteria ranged from 6.73$\times$$10^{6}$to 10.7$\times$$10^{6}$ and 0.44$\times$$10^{4}$ to 5.21$\times$$10^{4}$ respectively, and both of them were highly correlated ($R^{2}$=0.992). Nitrifying potential assayed by batch test showed relatively lower than other reported results. The oxygen uptake potential of surface and subsurface soil was measured in the presence of ${(NH_4)_2}$$SO_4$. The results demonstrated that surface soil required shorter lag period and uptaked more oxygen than subsurface's.

• Resources Recycling
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• v.30 no.2
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• pp.68-74
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• 2021

This study investigated formaldehyde (HCHO) removal by preparing porous supports using domestic low-grade silica coated with Co-ZSM5 and Cu-ZSM5 as the catalysts. First, the sample of the raw material for the support contained 90% silica with quartz crystal phase, which was confirmed as low-grade silica. According to Energy-dispersive X-ray spectroscopy (EDS) and Fourier-transform infrared spectroscopy (FT-IR) analyses, the catalysts, Co-ZSM5 and Cu-ZSM5, were successfully coated on the surface of the porous silica supports. During the removal test of HCHO using the prepared Co-ZSM5 and Cu-ZSM5 coated beads, depending on the reaction temperature, the Co-ZSM5 coated beads exhibited higher removal efficiencies (>97%) than the Cu-ZSM5 beads at 200 ℃. The higher efficiency of the Co-ZSM5 coating may be attributed to its superior surface activity properties (BET surface area and pore volume) that lead to the favorable HCHO decomposition. Therefore, Co-ZSM5 was determined to be the suitable catalyst for removing HCHO as a coating on a porous support fabricated using domestic low-grade silica.

• The Journal of Korean Academy of Prosthodontics
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• v.51 no.3
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• pp.167-174
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• 2013

Purpose: This study was conducted to evaluate the roughness and surface alternations of three differently blasted titanium discs treated by $Nd:YVO_4$ Laser irradiation in different conditions. Materials and methods: Thirty commercially pure titanium discs were prepared and divided into three groups. Each group was consisted of 10 samples and blasted by $ZrO_2$ (zirconium dioxide), $Al_2O_3$ (aluminum oxide), and RBM (resorbable blasted media). All the samples were degreased by ultrasonic cleaner afterward. Nine different conditions were established by changing scanning speed (100, 300, 500 mm/s) and repetition rate (5, 15, 35 kHz) of $Nd:YVO_4$ Laser (Laser Pro D-20, Laserval $Korea^{(R)}$, Seoul, South Korea). After laser irradiation, a scanning electron microscope, X-ray diffraction analysis, energy dispersive X-ray spectroscopic analysis, and surface roughness analysis were used to assess the roughness and surface alternations of the samples. Results: According to a scanning electron microscope (SEM), titanium discs treated with laser irradiation showed characteristic patterns in contrast to the control which showed irregular patterns. According to the X-ray diffraction analysis, only $Al_2O_3$ group showed its own peak. The oxidation tendency and surface roughness of titanium were similar to the control in the energy dispersive X-ray spectroscopic analysis. The surface roughness was inversely proportional to the scanning speed, whereas proportional to the repetition rate of $Nd:YVO_4$. Conclusion: The surface microstructures and roughness of the test discs were modified by the radiation of $Nd:YVO_4$ laser. Therefore, laser irradiation could be considered one of the methods to modify implant surfaces for the enhancement of osseointegration.

• Journal of the Korea Concrete Institute
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• v.15 no.1
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• pp.95-101
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• 2003

Drying shrinkage cracking which may be caused by the relatively large specific surface is a matter of grave concern for latex modified concrete(LMC) overlay and rapid-setting cement latex modified concrete(RSLMC) overlay. LMC and RSLMC were studied for field applications very actively in terms of strength and durability in Korea. However, there were no considerations in drying shrinkage. Therefore, the purpose of this dissertation was to study the drying shrinkage properties of LMC and RSLMC with the main experimental variables such as cement types(ordinary portland cement, rapid setting cement), latex contents(0, 5, 10, 15, 20%) and curing days at a same controlled environment of 60% of relative humidity and $20^{\circ}C$ of temperature. The drying shrinkage for specimens was measured with a digital dial gauge of Demec. The test results showed that the drying shrinkage of LMC and RSLMC were considerably lower than that of OPC and RSC, respectively. This might be attributed to the interlocking of hydrated cement and aggregates by a film of latex particles, water retention due to hydrophobic, and colloidal properties of the latexes resulting in reduced water evaporation.

• The Journal of the Korea Contents Association
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• v.18 no.3
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• pp.542-549
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• 2018

In this study BGn-incorporated non-fluoride release of pit and fissure sealant $Concise^{TM}$ was developed to improve the mechanical properties and promote antibacterial effect of fit and fissure sealant with the original material. The mechanical properties and antibacterial activity of BGn incorporating vary-ing amounts bioactive glass nano particles(BGn) (0,0.5,1.0 and 2.0 wt% in sealant) were characterized composition of the resulting were investigated. The solubility to aid absorption was calculated by weighing specimens with a diameter of 10 mm and a thickness of 2 mm according to ISO 4049 (2009). The antimicrobial effect was evaluated using three strains of S. mutans, S. aureus and E. coli. The absorbance of the test results was as high as the addition of BGn increased, and the lower the solubility as the solubility was added(p<0.05). Adhesion experiment results S. mutans in contrast to the control group $Concise^{TM}$, BGn-added experimental group showed a somewhat lower adherent surface but no statistically significant difference was observed (p<0.05). However S. aureus and E. coli statistical analysis indicated a significant difference for antibacterial agents between control and BGn containing(p<0.05). It seems that this BGn proved that even a antibacterial effect was demonstrated. Therefore, it was suggest that the additional effects of BGn and research on a wide range of substances.

• Journal of the Korea Concrete Institute
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• v.22 no.5
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• pp.633-640
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• 2010

This study was performed to get basic information about properties of high blaine cement for shotcrete use. Particle size distribution, setting time and compressive strength test, analysis like as SEM, DSC thermal analysis, XRD was carried out to investigate principle properties of high blaine cement. Setting time of high blaine cement was shorter and compressive strength was higher than those of ordinary portland cement (OPC). Results of analysis showed early hydration products of high blaine cement is smaller and spread widely due to increased specific surface. From the SEM observation and analysis of DSC and XRD results, it was seen that the aluminates accelerators promoted calcium aluminium hydrates while the alkali free accelerators increased ettringite and monosulfates formation. Strength and setting time measurement of cement paste with aluminate accelerator is more effective than the alkali free accelerator in reducing the setting time and increasing early strength while alkali free accelerator is more effective in increasing the strength after 7 days.

• Polymer(Korea)
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• v.25 no.1
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• pp.133-141
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• 2001

The friction and wear properties of carbon-carbon composites made with different weight percent of $MoSi_2$ as an oxidation inhibitor were investigated using a constant speed wear test apparatus in an oxidation environment. The results indicated the carbon-carbon composites undergoing an abrupt transition of friction coefficient, from low-friction behavior(${\mu}$=0.15~0.2) during normal wear regime to the high-friction behavior(${\mu}$=0.5~0.6) during dusting wear regime at the frictional temperature range of 150~180${\circ}C$. The existence of temperature-dependent friction and wear regimes implied that the performance of specimen made with carbon-carbon composites was markedly affected by the thermal properties of the composites. The carbon-carbon composites filled with MoSi2 exhibited two times lower coefficient of friction and wear rate in comparison with the composites without $MoSi_2$. Especially, the composites containing 4wt% $MoSi_2$ filler showed a significantly improved activation energy for wear due to the reduction of both the porosity and powdery debris film formation on sliding surface when compared to those without $MoSi_2$.

• Korean Journal of Materials Research
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• v.4 no.2
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• pp.194-205
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• 1994

In this research, we attempt to fabricate an excellent CFRC(Carbon Fiber Reinforced Carbon), which has good thermal and mechanical properties, with 8H/satin woven fabric prepreg, high modulus and high strength type continuous carbon fiber and raw coal tar pitch(RCTP) matrix or THF soluble fraction(THFSP) matrix which has good graphitizability. Green bodies were fabricated with hot press molding technique and CFRC samples were made after carbonization, impregnation, recarbonization and graphitization steps. For the purpose of characterization of the physical properties, SEM, polarized light microscope, TGA were observed, and tested flexural strength, modulus and ILSS. After heat treating the THFSP matrix up to $2300^{\circ}C$, the value of $C_0$/2 was 3.380$\AA$, which is analogous to the structure of natural graphite and the value of 2$\theta$ is $26.276^{\circ}$ approached to the Bragg's angle of natural graphite. As a result of TGA to test the high temperature air oxidation, the THFSP matrix, graphitized up to $2300^{\circ}C$, exhibited the best air oxidation resistance. And mechanical properties were increased up to 65~70% as fiber volume fraction increased. Because of the good orientation graphitizability, the fracture surface of THFSP matrix CFRC is very good.

• Journal of Adhesion and Interface
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• v.9 no.3
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• pp.20-26
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• 2008

It is well know that the structure of shape memory alloy (SMA) can change from martensite austenite by either temperature or stress. Due to their inherent shape recovery properties, SMA fiber can be used such as for stress or cure-monitoring sensor or actuator, during applied stress or temperature. Incomplete superelasticity was observed as the stress hysteresis at stress-strain curve under cyclic loading test and temperature change. Superelasticity behavior was observed for the single-SMA fiber/epoxy composites under cyclic mechanical loading at stress-strain curve. SMA fiber or epoxy embedded SMA fiber composite exhibited the decreased interfacial properties due to the cyclic loading and thus reduced shape memory performance. Rigid epoxy and the changed interfacial adhesion between SMA fiber and epoxy by the surface treatment on SMA fiber exhibited similar incomplete superelastic trend. Epoxy embedded single SMA fiber exhibited the incomplete recovery during cure process by remaining residual heat and thus occurring residual stress in single SMA fiber/epoxy composite.