• Design & Manufacturing
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• 제14권1호
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• pp.49-55
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• 2020

In this study, the fatigue behavior and fatigue life characteristics of PA2200 specimens fabricated by SLS 3D printer were studied. Fatigue tests were performed according to the standard specification (ASTM E468) and fatigue life curves were obtained. In order to perform the fatigue test, mechanical properties were measured according to the test speed of the simple tensile test, and the self-heating temperature of the specimen according to the test speed was measured using an infrared temperature measuring camera in consideration of heat generation due to plastic deformation. There was no significant difference within the set test speed range and the average self-heating temperature was measured at 38.5 ℃. The mechanical strength at the measured temperature showed a relatively small difference from the mechanical strength at room temperature. Fatigue test conditions were established through the preceding experiments, and the loading conditions below the tensile strength at room temperature 23 ℃ were set as the cyclic load. The maximum number of replicates was less than 100,000 cycles, and the fracture behavior of the specimens with the repeated loads showed the characteristics of Racheting. It was confirmed that SLS 3D printing PA2200 material could be applied to the Basquin's S-N diagram for the fatigue life curve of metal materials. SEM images of the fracture surface was obtained to analyze the relationship between the characteristics of the fracture surface and the number of repetitions until failure. Brittle fracture, crazing fracture, grain melting, and porous fracture surface were observed. It was shown that the larger the area of crazing damage, the longer the number of repetitions until fracture.

• 센서학회지
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• 제32권5호
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• pp.290-294
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• 2023

Among various types of harmful gases, hydrogen sulfide is a strong toxic gas that is mainly generated during spillage and wastewater treatment at industrial sites. Hydrogen sulfide can irritate the conjunctiva even at low concentrations of less than 10 ppm, cause coughing, paralysis of smell and respiratory failure at a concentration of 100 ppm, and coma and permanent brain loss at concentrations above 1000 ppm. Therefore, rapid detection of hydrogen sulfide among harmful gases is extremely important for our safety, health, and comfortable living environment. Most hydrogen sulfide gas sensors that have been reported are electrical resistive metal oxide-based semiconductor gas sensors that are easy to manufacture and mass-produce and have the advantage of high sensitivity; however, they have low gas selectivity. In contrast, the electrochemical sensor measures the concentration of hydrogen sulfide using an electrochemical reaction between hydrogen sulfide, an electrode, and an electrolyte. Electrochemical sensors have various advantages, including sensitivity, selectivity, fast response time, and the ability to measure room temperature. However, most electrochemical hydrogen sulfide gas sensors depend on imports. Although domestic technologies and products exist, more research is required on their long-term stability and reliability. Therefore, this study includes the processes from electrode material synthesis to sensor fabrication and characteristic evaluation, and introduces the sensor structure design and material selection to improve the sensitivity and selectivity of the sensor. A sensor case was fabricated using a 3D printer, and an Ag reference electrode, and a Pt counter electrode were deposited and applied to a Polytetrafluoroethylene (PTFE) filter using PVD. The working electrode was also deposited on a PTFE filter using vacuum filtration, and an electrochemical hydrogen sulfide gas sensor capable of measuring concentrations as low as 0.6 ppm was developed.

• 대한치과보철학회지
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• 제57권3호
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• pp.211-218
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• 2019

목적: 본 연구에서는 CAD/CAM 기술로 제작한 4가지 금속하부구조물의 변연 및 내면 적합도를 비교하여 정확도 및 임상적 효용성을 알아보고자 한다. 재료 및 방법: 상악 중절치 레진모형치아를 삭제한 뒤 복제하여 Ni-Cr 합금 표준 모형을 제작하였다. 이를 공초점 현미경방식의 구강 스캐너를 이용해 12개의 STL 파일을 얻었다. CAD 프로그램 상에서 $50{\mu}m$의 시멘트 공간을 부여한 두께 0.5 mm의 금속하부구조물을 디자인하였다. Co-Cr 금속하부구조물은 다음 4가지 방법으로 제작하였다: Wax pattern milling & Casting (WM), Resin pattern 3D Printing & casting (RP), Milling & Sintering (MS), Selective laser melting (SLM). 변연 및 내면 적합도를 측정하기 위해 실리콘 복제법을 이용하였다. 측정한 결과값은 SPSS 통계 프로그램을 이용하여 일원배치분산분석(one-way ANOVA)으로 통계처리하고, 사후검정으로 Scheffe test를 시행하였으며, 5% 유의수준으로 평가하였다(${\alpha}=.05$). 결과: 변연 적합도는 WM군($27.66{\pm}9.85{\mu}m$)과 MS군($28.88{\pm}10.13{\mu}m$)이 RP군($38.09{\pm}11.14{\mu}m$)에 비해 통계적으로 유의하게 작았다. 치경부 적합도는 MS군이 RP군에 비해 통계적으로 유의하게 작았다. 축면 적합도는 WM군과 MS군이 RP군과 SLM군 보다 통계적으로 유의하게 작았다. 절단면 적합도는 RP군이 통계적으로 유의하게 작았다. 결론: Wax pattern milling & Casting, Milling & Sintering법으로 제작한 Co-Cr coping의 변연과 축면에서의 적합도가 더 우수하였다. 모든 군의 Co-Cr coping의 변연, 치경부, 축면 적합도는 임상적으로 허용할만한 범위 안에 있었다.

• 한국정보디스플레이학회:학술대회논문집
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• 한국정보디스플레이학회 2009년도 9th International Meeting on Information Display
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• pp.995-997
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• 2009

$In_2O_3$ thin-film transistors (TFTs) were fabricated on various dielectrics [$SiO_2$ and self-assembled nanodielectrics (SANDs)] by spin-coating a $In_2O_3$ film precursor solution consisting of methoxyethanol (solvent), ethanolamine (EAA, base), and $InCl_3$ as the $In^{3+}$ source. Importantly, an optimized film microstructure characterized by the high-mobility $In_2O_3$ 004 phase, is obtained only within a well-defined base: $In^{3+}$ molar ratio. The greatest electron mobilities of ~ 44 $cm^2$, for EAA:$In^{3+}$ molar ratio = 10, $V^{-1}s^{-1}$, is measured for $n^+$-Si/SAND/$In_2O_3$/Au devices. This result combined with the high $I_{on}:I_{off}$ ratios of ~ $10^6$ and very low operating voltages (< 5 V) is encouraging for high-speed applications.

• 한국진공학회:학술대회논문집
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• 한국진공학회 2015년도 제49회 하계 정기학술대회 초록집
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• pp.82-82
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• 2015

Industrial crystalline silicon (c-Si) solar cells with using a screen printing technology share the global market over 90% and they will continue to be the same for at least the next decade. It seems that the $2^{nd}$ generation and the $3^{rd}$ generation technologies have not yet demonstrated competitiveness in terms of performance and cost. In 2014, new world record efficiency 25.6% (Area-$143.7cm^2$, Voc-0.740V, $Jsc-41.8mA/cm^2$, FF-0.827) was announced from Panasonic and its cell structure is Back Contact $HIT^*$ c-Si solar cell. Here, amorphous silicon passivated contacts were newly applied to back contact solar cell. On the other hand, 24.9% $TOPCon^{**}$ cell was announced from Fraunhofer ISE and its key technology is an excellent passivation quality applying tunnel oxide (<2 nm) between metal and silicon or emitter and base. As a result, to realize high efficiency, high functional technologies are quite required to overcome a theoretical limitation of c-Si solar cell efficiency. In this presentation, Si solar cell technology summarized in the International Technology Roadmap for Photovoltaics ($^{***}ITRPV$ 2014) is introduced, and the present status of R&D associated with various c-Si solar cell technologies will be reviewed. In addition, national R&D projects of c-Si solar cells to be performed by Korea University are shown briefly.

• KEPCO Journal on Electric Power and Energy
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• 제7권1호
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• pp.129-135
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• 2021

Recently, additive manufacturing (AM) technology such as powder bed fusion (PBF) and directed energy deposition (DED) are actively attempted as consumers' needs for parts with complex shapes and expensive materials. In the present work, the effect of processing parameters on the mechanical properties of 316L stainless steel coupons fabricated by PBF and DED AM technology was investigated. Three major mechanical tests, including tension, impact, and fatigue, were performed on coupons extracted from the standard components at angles of 0, 45, 90 degrees for the build layers, and compared with those of investment casting and commercial wrought products. Austenitic 316L stainless steel additively manufactured have been well known to be generally stronger but highly vulnerable to impact and lack in elongation compared to casting and wrought materials. The process-induced pore density has been proved the most critical factor in determining the mechanical properties of AM-built metal parts. Therefore, it was strongly recommended to reduce those lack of fusion defects as much as possible by carefully control the energy density of the laser. For example, under the high energy density conditions, PBF-built parts showed 46% higher tensile strength but more than 75% lower impact strength than the wrought products. However, by optimizing the energy density of the laser of the metal AM system, it has been confirmed that it is possible to manufacture metal parts that can satisfy both strength and ductility, and thus it is expected to be actively applied in the field of electric power section soon.

• 한국분말재료학회지
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• 제27권3호
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• pp.219-225
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• 2020

The directed energy deposition (DED) process of metal 3D printing technologies has been treated as an effective method for welding, repairing, and even 3-dimensional building of machinery parts. In this study, stainless steel 316L (STS316L) and Inconel 625 (IN625) alloy powders are additively manufactured using the DED process, and the microstructure of the fabricated STS316L/IN625 sample is investigated. In particular, there are no secondary phases in the interface between STS316L and the IN625 alloy. The EDS and Vickers hardness results clearly show compositionally and mechanically transient layers a few tens of micrometers in thickness. Interestingly, several cracks are only observed in the STS 316L rather than in the IN625 alloy near the interface. In addition, small-sized voids 200-400 nm in diameter that look like trapped pores are present in both materials. The cracks present near the interface are formed by tensile stress in STS316L caused by the difference in the CTE (coefficient of thermal expansion) between the two materials during the DED process. These results can provide fundamental information for the fabrication of machinery parts that require joining of two materials, such as valves.

• Transactions on Electrical and Electronic Materials
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• 제4권4호
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• pp.22-25
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• 2003

High series resistance due to the presence of glass frit is one of the major problems for screen printed silicon solar cells. Cells having electrical parameters below the prescribed values are usually rejected during solar module fabrication. Therefore, it is highly desirable to improve the electrical parameters of the silicon solar cells and thereby to increase the overall production yield. It was observed that, the performance of low quality mono-crystalline silicon solar cells made by standard screen printing technology could be improved remarkably by novel surface treatment. We have chemically treated the surface using sodium hydroxide (NaOH) and silver nitrate ($AgNO_3$) solutions. NaOH treatment helps to reduce the series resistance by decreasing the presence of excess glass frit on the top silver grid contact. The $AgNO_3$ treatment is used to reduce the series resistance comes from the deposition of silver on the grids by filling the holes present (if any) within the grid pattern.

• Corrosion Science and Technology
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• 제17권4호
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• pp.166-175
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• 2018

In this study, the electrochemical properties of CP-Ti (commercially pure titanium) and Ti-64 (Ti-6Al-4V) were evaluated and the effect of hydrofluoric acid on corrosion resistance and electrochemical properties was elucidated. Additive manufactured materials were made by DMT (Directed Metal Tooling) method. Samples were heat-treated for 1 hour at $760^{\circ}C$ and then air cooled. Surface morphologies were studied by optical microscope and SEM. Electrochemical properties were evaluated by anodic polarization method and AC-impedance measurement. The oxide film formed on the surface was analyzed using an XPS. The addition of HF led to an increase in the passive current density and critical current density and decreased the polarization resistance regardless of the alloys employed. Based on the composition of the oxide film, the compositional difference observed by the addition of HF was little, regardless of the nature of alloys. The Warburg impedance obtained by AC-impedance measurement indicates the dissolution of the constituents of CP-Ti and Ti-64 through a porous oxide film.

• Journal of Welding and Joining
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• 제35권2호
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• pp.6-12
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• 2017

Ti-alloys have high specific strength and are widely used for the filed of space aeronautics plant. However, it is difficult to process Ti-Alloys due to its high yield strength and it cannot raise the machining speed because it has a possibility of catching fire while processing. In order to reduce the number of processes for the Ti-alloys, the researches related to Additive Manufacturing(AM) have been actively carried out at the moment. As for the initial stage of AM market related to Ti-alloys, it started to use the raw material of powder metal, and it is currently being developed based on welding. In this study, Interpass peening reduced the size of the primary ${\beta}$ grain in the z-axis direction, increased the nucleation site of ${\alpha}-colony$, and decreased the length and width of ${\alpha}$ laths as though interpass rolling. Interpass peening leads to an increase in yield/ultimate tensile strength without decrease elongation, resulting decrease in anisotropy of the material.