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Choi, Hyunjin,Choi, Woohyuk,Lim, Jiseok,Choi, Jungwook Elsevier 2019 Sensors and actuators. A Physical Vol.298 No.-
<P><B>Abstract</B></P> <P>Light-emitting diodes (LEDs) are widely used in many industrial applications owing to their high performance and efficiency compared with conventional lighting systems. However, a considerable amount of input power is inevitably dissipated into heat at the LED junction, which can degrade the performance and reliability of the LED; thus, it is important to monitor the change in the junction temperature of the LED. In this study, we present a micro-temperature sensor-integrated surface-mounted device (SMD) for accurate and real-time measurement of the junction temperature of an LED. The LED is mounted on a microfabricated Pt sensor in a similar way to the typical SMD assembly. The heat generated at the LED junction is conductively transferred to the microsensor, increasing the temperature and changing its electrical resistance. In contrast to the conventional techniques for thermal characterization of LEDs, the integrated microsensor provides real-time information on the junction temperature with high precision, reproducibility, and simplicity. Additionally, the temperature of the solder, which is not easily accessible but is closely related to the reliability of the LED, can be estimated by analyzing the thermal resistance of the LED package. Experimental and numerical results indicate a linear correlation (R<SUP>2</SUP> = 0.988) between the junction and sensor temperatures, which is practically useful for the thermal management of the miniaturized SMD-LED.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Methodology for measuring LED junction temperature is proposed. </LI> <LI> Pt micro-temperature sensor-integrated SMD-LED is designed and fabricated for real-time measurement of junction temperature. </LI> <LI> Highly linear correlation of the temperature between the microsensor and the junction is experimentally found. </LI> <LI> Analysis of structure function, thermal resistance, and numerical calculation of SMD-LED validates measurement accuracy of Pt microsensor. </LI> </UL> </P> <P><B>Graphical abstract</B></P> <P>[DISPLAY OMISSION]</P>
Batch-processed carbon nanotube wall as pressure and flow sensor
Choi, Jungwook,Kim, Jongbaeg IOP Pub 2010 Nanotechnology Vol.21 No.10
<P>A pressure and flow sensor based on the electrothermal-thermistor effect of a batch-processed carbon nanotube wall (CNT wall) is presented. The negative temperature coefficient of resistance (TCR) of CNTs and the temperature dependent tunneling rate through the CNT/silicon junction enable vacuum pressure and flow velocity sensing because the heat transfer rate between CNTs and the surrounding gas molecules differs depending on pressure and flow rate. The CNT walls are synthesized by thermal chemical vapor deposition (CVD) on an array of microelectrodes fabricated on a silicon-on-insulator (SOI) wafer. The CNTs are self-assembled between the microelectrodes and substrate across the thickness of a buried oxide layer during the synthesis process, and the simple batch fabrication results in high throughput and yield. A wide pressure range, down to 3 × 10<SUP> − 3</SUP> from 10<SUP>5</SUP> Pa, and a nitrogen flow velocity range between 1 and 52.4 mm s<SUP> − 1</SUP>, are sensed. Further experimental characterizations of the bias voltage dependent response of the sensor as a vacuum pressure gauge are presented. </P>
Scalable and number-controlled synthesis of carbon nanotubes by nanostencil lithography
Choi, Jungwook,Koh, Kisik,Kim, Jongbaeg Springer 2013 Nanoscale research letters Vol.8 No.1
<P>Controlled synthesis and integration of carbon nanotubes (CNTs) remain important areas of study to develop practical carbon-based nanodevices. A method of controlling the number of CNTs synthesized depending on the size of the catalyst was characterized using nanostencil lithography, and the critical dimension for the nanoaperture produced on a stencil mask used for growing individual CNTs was studied. The stencil mask was fabricated as a nanoaperture array down to 40 nm in diameter on a low-stress silicon nitride membrane. An iron catalyst used to synthesize CNTs was deposited through submicron patterns in the stencil mask onto a silicon substrate, and the profile of the patterned iron catalyst was analyzed using atomic force microscopy. The feasibility toward a scalable, number-, and location-controlled synthesis of CNTs was experimentally demonstrated based on the diameter and geometry of the apertures in the stencil mask.</P>
Investigation of Interfacial Adhesion between the Top Ends of Carbon Nanotubes
Choi, Jungwook,Eun, Youngkee,Kim, Jongbaeg American Chemical Society 2014 ACS APPLIED MATERIALS & INTERFACES Vol.6 No.9
<P>Understanding the interfacial forces of carbon nanotubes (CNTs) is fundamental to the development of electromechanical systems based on the contact of CNTs. However, experimental studies on the adhesion properties between CNTs are scarce despite the remarkable contact quality of CNTs. Here, we present an experimental investigation of the adhesion between the top ends of aligned, self-adjusted CNTs using a CNT-integrated microelectromechanical actuator. The pull-out and pull-in behaviors of the contact as a function of the applied force by the actuator are precisely identified by measuring the contact resistance between the CNTs. The adhesion between the top ends of individual CNTs is extracted from the measured adhesive strength between the CNT arrays, and it agrees with the theoretical values of the van der Waals interactions. By exploiting the adhesion of the CNT-to-CNT contact, a programmable and reliable microelectromechanical switching device is demonstrated. Our results offer design strategies for diverse CNT-based nano- and microelectromechanical devices that need repeatable contacting interfaces.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/aamick/2014/aamick.2014.6.issue-9/am500252s/production/images/medium/am-2014-00252s_0007.gif'></P><P><A href='http://pubs.acs.org/doi/suppl/10.1021/am500252s'>ACS Electronic Supporting Info</A></P>
Choi, Sunghoon,Lee, Seungwan,Lee, Haenghwa,Lee, Donghoon,Choi, Seungyeon,Shin, Jungwook,Seo, Chang-Woo,Kim, Hee-Joung Elsevier 2017 Nuclear Instruments & Methods in Physics Research. Vol.848 No.-
<P><B>Abstract</B></P> <P>Digital tomosynthesis offers the advantage of low radiation doses compared to conventional computed tomography (CT) by utilizing small numbers of projections (~80) acquired over a limited angular range. It produces 3D volumetric data, although there are artifacts due to incomplete sampling. Based upon these characteristics, we developed a prototype digital tomosynthesis R/F system for applications in chest imaging. Our prototype chest digital tomosynthesis (CDT) R/F system contains an X-ray tube with high power R/F pulse generator, flat-panel detector, R/F table, electromechanical radiographic subsystems including a precise motor controller, and a reconstruction server. For image reconstruction, users select between analytic and iterative reconstruction methods. Our reconstructed images of Catphan700 and LUNGMAN phantoms clearly and rapidly described the internal structures of phantoms using graphics processing unit (GPU) programming. Contrast-to-noise ratio (CNR) values of the CTP682 module of Catphan700 were higher in images using a simultaneous algebraic reconstruction technique (SART) than in those using filtered back-projection (FBP) for all materials by factors of 2.60, 3.78, 5.50, 2.30, 3.70, and 2.52 for air, lung foam, low density polyethylene (LDPE), Delrin<SUP>®</SUP> (acetal homopolymer resin), bone 50% (hydroxyapatite), and Teflon, respectively. Total elapsed times for producing 3D volume were 2.92s and 86.29s on average for FBP and SART (20 iterations), respectively. The times required for reconstruction were clinically feasible. Moreover, the total radiation dose from our system (5.68mGy) was lower than that of conventional chest CT scan. Consequently, our prototype tomosynthesis R/F system represents an important advance in digital tomosynthesis applications.</P>
Choi, Jungwook,Lee, Jae‐,Ik,Eun, Youngkee,Kim, Min‐,Ook,Kim, Jongbaeg WILEY‐VCH Verlag 2011 ADVANCED MATERIALS Vol.23 No.19
<P><B>A highly reliable micromechanical contact based on aligned carbon nanotube (CNT) arrays</B> is demonstrated and characterized. The CNT arrays are synthesized directly on three‐terminal microelectromechanical systems, and they are adjusted and simultaneously preloaded during the synthetic process, making intimate micromechanical contacts. It is found that the CNT array‐to‐array contact improves upon conventional solid‐to‐solid micromechanical contact. </P>
최정욱 ( Choi Jungwook ),이웅종 ( Lee Woongjong ),김영진 ( Kim Youngjin ),배정주 ( Bea Jungju ) 한국구조물진단유지관리공학회 2022 한국구조물진단유지관리공학회 학술발표대회 논문집 Vol.26 No.2
국내·외 항만시설물의 재해 피해사례를 분석하고 이를 통하여 재해위험도 평가대상 시설물을 선정하였다. 선정된 재해-항만시설물(태풍-방파제 및 지진-안벽)의 파괴모드 및 파괴변수를 분석하고 평가지표별 성능수준에 따른 정량적 손상한계기준을 제시하였다.
최정욱 ( Choi Jungwook ),이웅종 ( Lee Woongjong ),김영진 ( Kim Youngjin ),배정주 ( Bea Jungju ) 한국구조물진단유지관리공학회 2022 한국구조물진단유지관리공학회 학술발표대회 논문집 Vol.26 No.2
국내·외 항만시설물의 재해 피해사례를 분석하고 이를 통하여 재해위험도 평가대상 시설물을 선정하였다. 선정된 재해-항만시설물(태풍-방파제 및 지진-안벽)의 파괴모드 및 파괴변수를 분석하고 평가지표별 성능수준에 따른 정량적 손상한계기준을 제시하였다.
최정욱(Jungwook Choi),고기식(Kisik Koh),김종백(Jongbaeg Kim) 한국생산제조학회 2011 한국생산제조시스템학회 학술발표대회 논문집 Vol.2011 No.4
In this research, we have fabricated the nano-scale apertures down to 40 nm on the low-stress silicon nitride membrane using a focused ion beam milling process. The apertures can be utilized as a mask for nanostencil lithography to fabricate nano-scale structures on an arbitrary substrate with batch-fabrication capability.