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이창혁(Changhyuk Lee),진재식(Jae Sik Jin),이준식(Joon Sik Lee) 대한기계학회 2009 대한기계학회 춘추학술대회 Vol.2009 No.5
The size effects on the figure of merit (FOM) of BiSb alloy film are analyzed using Boltzmann transport equation (BTE). To show seebeck coefficient change, a simple method and boundary conditions are suggested for BTE of number density form. The size effects due to boundary scatterings are treated by using the effective mean free path concepts. The smaller seebeck coefficient of thinner films can be explained by the ballistic transport phenomena. The effective mean free path of electrons in BiSb thin films is suggested which show good agreement between simulation and experimental results of Mallik et al. BTEs of energy density form for both electrons and phonons are solved to calculate the thermal conductivity. For the electron BTE, force term which contains electrical field is described by the velocity reduction caused by thermoelectric voltage, which means reduction in the thermal conductivity of electrons. Finally, the FOMs of BiSb thin films with respect to thickness of the films are predicted.
이지훈(Jihun Lee),김수진(Soojin Kim),이창혁(Changhyuk Lee) 대한전자공학회 2023 대한전자공학회 학술대회 Vol.2023 No.6
Current mode resistive sensing instrumentation with a high channel count, programmable gain, and wide bandwidth is scientifically essential. However, commercial instruments need more channel count and bandwidth at a reasonable cost. Another critical challenge to resistive sensor read-out systems is the need for the accessibility of resistive sensor arrays with low-shunt parasitic impedance and the simplicity of sensor packaging to switch sensors while reusing the instrumentation. This paper proposes a versatile readout circuit and system that provides plug-and-play ready commercially available packaging. The readout circuit has a wide tunability, including an extended range of gain/bandwidth settings. It also has a programmable voltage across individual resistive sensors, enabling channel-wise dynamic range adjustment. The low noise trans-impedance outputs from AFE are fed to an array of AAF and ADC, which then transmits digitally converted data to custom-designed graphic user interface software via an FPGA-based USB 3.0 port.
안중기(Joongki Ahn),이창혁(Changhyuk Lee),최진(Jin Choi),변종렬(Jongryul Byun) 한국추진공학회 2015 한국추진공학회 학술대회논문집 Vol.2015 No.5
이 논문은 이중모드 램제트의 천이특성 모델링 기법을 연구한 내용을 담고 있다. 준 1차원 방정식을 이용하여 램모드에서 스크램모드로 천이특성을 시뮬레이션 하였다. 천이특성 모델링에서 어려운 점은 thermal throat부근에서 해를 구하는 부분이다. 이곳에서는 흐름의 질식이 발생하여 흐름속도가 마하 1이 됨에 따라 방정식의 해는 특이상태가 된다. 본 연구에서는 해석적인 방법을 이용하여 마하 1영역에서의 해를 계산하였다. 계산결과, 안정적인 수렴성과 기존 연구결과에 비해 물리적으로 타당한 결과를 확인할 수 있었다. This paper presents the modelling technique of the transition of DMR(Dual Model Ramjet). The transition from ram to scram mode combustion for DMR is studied by a quasi 1-dimensional model. One of the challenges for the simulation of transition is to calculate the properties at the thermal throat. As the flow is choked and becomes Mach 1 at the thermal throat, the solution of equations becomes singular. In this paper, the solution at the sonic point is calculated by analytic way. The results show good convergence and reasonable characteristics at the sonic point.
박근홍(Geunhong Park),최진(Jin Choi),이창혁(Changhyuk Lee) 한국추진공학회 2015 한국추진공학회 학술대회논문집 Vol.2015 No.5
초음속 흡입구는 받음각에 따라 흡입유동이 민감하게 반응한다. 아음속 터보제트의 경우 압축기에 의해 유동의 불균일도가 감쇄하지만 램제트 같은 경우 기계적인 장치 없이 램압축이 압축기를 대신하므로 받음각에 따라 유동의 변화와 이에 따른 압력 회복률등이 크게 영향을 받게 된다. 이에 따라 본 연구에서는 동축형 초음속 흡입구에 대하여 받음각에 따른 내부 유동의 변화와 불균일성을 조사하고 전체 추진기관에 미칠 영향을 예측하였다. A Supersonic inlet is sensitive to change of angle of attack. For the air-intake of the subsonic turbojet engine, turbine compressor reduces the non-uniformity of internal flow. However, in the case of Ramjet, there is no mechanical compression, while only ram compression exists by supersonic free stream in front of inlet. In this study, internal flow properties of supersonic inlet were estimated by CFD analysis for basic study of supersonic inlet performance.
TCSPC 시스템의 피코초 레이저 광원과 광 검출기 간의 동기화를 위한 낮은 지터 신호 변환기
배효인(Hyoin Bae),이형민(Hyungmin Lee),이창혁(Changhyuk Lee) 대한전자공학회 2023 대한전자공학회 학술대회 Vol.2023 No.6
TCSPC (Time-Correlated Single Photon Counting) is a crucial method for Lidar, Fluorescent imaging, and quantum computing/sensing applications. A typical TCSPC system analyzes the statistical characteristic by repetitively measuring the emission and detecting pulsed light at the single photon level, which necessitates measurement reliability via low noise. This paper proposes a simple yet effective method connecting picosecond-level accurate time reference between diverse instrumentation standards. We employed a low-noise dual power supply and anti-reflection transmission line design to connect different signal standards without adding excessive noise (jitter). In general, picosecond laser light source outputs laser pulse locked current mode output (e.g., NIM, ECL) where SPAD, TDC, and TCSPC signal processing hardware inputs logic level signal below 3.3 V. By using a four-channel low jitter comparator we demonstrate a TCSPC capability via NIM – Voltage Logic Level. The resulting jitter performance of the proposed system is verified via a measurement of the Instrument Response Function of a SPAD 33.623 psec. This result verifies the efficacy of the TCSPC systems based on the proposed signal translator.