일개 건물 A형 간염 집단 발생의 위험 요인

Dept. of Medicine/석사ObjectivesOn April, 2009, a hepatitis A outbreak occurred in a building. The outbreak was investigated to understand the characteristics of the outbreak and to clarify the risk factors.MethodsThe cases were defined as workers who worked in the building from February to April, 2009, they had medical evidence of hepatitis and they were positive on anti-hepatitis A virus (HAV) IgM antibody tests. This case-control study was conducted with 55 subjects (11 cases and their matched controls). The controls are matched according to gender, age and the employing company among the workers who were negative for IgG and IgM anti-HAV. The progress of the outbreak is described. Stool specimens were tested genetically. General characteristics were compared between the case and control groups. Common candidate risk factors were evaluated between the groups. The menus of the cafeteria at the building and the foods at the restaurants around the building were analyzed, respectively. ResultsThe outbreak lasted from March 23 to April 1, 2009. HAV was detected in only one stool specimen. The differences of general characteristics between the groups were not significant. The case group worked overtime significantly more frequently (Odds Ratio (OR) =1.63) than did the control group, but the other differences among the common factors were not significant. The intake of each cafeteria menu didn’t differ between the groups. The case group took significantly more leafy vegetables for wrapping (OR=8.81) among the foods eaten outside the building. The frequency of recent overtime work and the intake of leafy vegetables outside the building were still significant according to the multivariate analysis.ConclusionA higher frequency of overtime work and a greater intake of leafy vegetables at restaurants around the building increased the risk of infection during the HAV outbreak.ope

Impedance measurements of PEDOT films on nanoscale using Fourier transform

Maste

The effect of operating pressure on the performance during power generation and water purging of PEM fuel cells

학위논문 (박사)-- 서울대학교 대학원 : 기계항공공학부, 2016. 2. 김민수.Polymer electrolyte membrane fuel cells (PEMFCs) are emerging due to their advantages of high energy density, high energy efficiency, low operation temperature and zero emission characteristics. Because these merits are good for automobile, so the fuel cell is spotlighted as a power source for vehicle. To generate more power within limited size condition, pressurized system is considered, but the previous studies did not explain the characteristics of pressurized system for various operating condition. Additionally, there is no effective purging method which can be applied to the pressurized system. To adapt the pressurized system for fuel cell in real world, an in-depth study is necessary. In this study, to analyze the performance of segmented fuel cell, 25 times segmented model is adapted. To evaluate the internal water distribution through the gas diffusion later and the channel, saturation profile model and water diffusion model in membrane is applied. From the simulation results, the internal water distribution is analyzed. For low RH condition, high pressure helps more vapor condensed, so fuel cell is humidified and current density becomes more homogeneous. For high RH condition, on the other hand, amount of water supplied to the fuel cell is decreased, so flooding problem can be solved. It is expected that experimental can be well analyzed using numerical analysis. To adapt the fuel cell as a power source for various applications, it is important to understand electrochemical characteristics such as current distribution and Ohmic resistance distribution were observed in order to validate the effect of operating pressure for various operating temperature, humidity, and flow rate of reactant gases conditions. 25 times segmented fuel cell is used to measure local current distribution and Ohmic resistance. The test was conducted in a constant current operation mode, and uneven current distribution was observed at high pressure condition. When the fuel cell was operated in a constant voltage operation mode, more electricity is generated for whole area because of high partial pressure of reactant gases. But current density distribution becomes inhomogeneous even at high temperature. Additionally, the results show that RH and flow rate is the main factor for current distribution. High flow rate with lower RH reactants make membrane of inlet area more dried out, so less electrochemical reaction occurs. When flow rate is low, capacity of evaporation is decreased, and effect of inlet RH condition is minimized. On the other hand, current distributions for various RH and flow rate conditions at 3 bar are relatively similar with each other. This is because pressurized volume acts as a damper, so effect of external conditions rarely affect the performance of fuel cell. When operating pressure is increased, relatively homogeneous current distribution is observed at low RH condition. That is, when the fuel cell is operated in high pressure, there is no need to supply lots of water to the fuel cell. Using the test results, dry start-up process is tested for normal pressure and high pressure. The results show that dry start-up performance is superior with high operating pressure condition. As polymer electrolyte membrane fuel cells are used for the power source of automobile, it is important to endure harsh conditions like sub-zero temperature. For successful cold start, the residual water in the cell should be minimized before start up. In this study, new effective purge method using a sudden pressure reduction is introduced. The internal ohmic resistance measurement using current interrupt method, dew point temperature measurement of exhausted gas in the cathode were carried out to verify the effectiveness of pressure reduction purge method. In addition, images of micro porous layer surface were obtained by microscope, and cold start experiments were done to check the usefulness of this purge method. The results show that most of water at catalyst layer and membrane electrolyte assembly is removed during pressure reduction purge process, compared with normal gas blowing purge process. Durability test was also conducted and no significant degradation during pressure reduction purge process was observed.Chapter 1. Introduction 1 1.1 Background of the study 1 1.2 Literature survey 4 1.3 Objectives and scopes 9 Chapter 2. Numerical study on segmented fuel cell 11 2.1 Introduction 11 2.2 segmented fuel cell model 13 2.2.1 Overall description of segmented fuel cell model 13 2.2.2 Flow channel model 16 2.2.3 Gas diffusion layer model 19 2.2.4 Water transport model in membrane 22 2.2.5 Voltage model 24 2.3 Analysis of simulation results 28 2.3.1 Validation of the model 28 2.3.2 Effect of operating pressure with temperature 28 2.3.3 Effect of operating pressure with relative humidity 33 2.3.4 Effect of operating pressure with flow rate 35 2.4 Summary 39 Chapter 3. Local electrochemical characteristic measurement for pressurized system of PEM fuel cells 40 3.1 Introduction 40 3.2 Experimental setup 42 3.2.1 Experimental apparatus 42 3.2.2 Compnents of segmented fuel cell 45 3.2.3 Measurement and evaluation of high frequency resistance 46 3.3 Local electrochemical characteristics for various operating pressure with temperature 51 3.3.1 Galvanostatic mode operation 51 3.3.2 Potentiostatic mode operation 62 3.3.3 High frequency resistance measurement 67 3.4 Local electrochemical characteristics for various operating pressure with humidity and flow rate 72 3.4.1 Middle SR operation 72 3.4.2 Low SR operation 85 3.4.3 High SR operation 86 3.5 Dry start-up performance 96 3.6 Summary 105 Chapter 4. Novel purging method using pressure reduction for effective water removal in polymer electrolyte membrane fuel cell 106 4.1 Introduction 106 4.2 Experimental setup 107 4.2.1 Experimental apparatus 107 4.2.2 Introduction of pressure reduction purge method 109 4.2.3 Experimental procedure 112 4.3 Water removal performance of new purging method 117 4.3.1 Feasibility of cathode purge 117 4.3.2 Internal Ohmic resistance measurement 119 4.3.3 Dew point temperature measurement 126 4.3.4 Imaging of MPL surface 127 4.3.5 Cold start experiments 135 4.4 Durability test 138 4.5 Summary 143 Chapter 5. Concluding remarks 144 References 149 Abstract (in Korean) 165Docto

개혁기 중국의 부패발생기제와 반부패 정책에 관한 연구

학위논문(석사)--서울대학교 대학원 :정치학과,2004.Maste

A Wide-Range Locking Multi-Phase Delay Locked Loop

DoctorIn this thesis, a 40-to-700MHz locking multi-phase DLL, an analysis and design methodology of resistor-based phase error averaging for multiphase generation, and a 110MHz-to-1.4GHz locking 40-phase all-digital DLL are proposed.Firstly, a delay matrix and a gradual switching of shunt capacitors are proposed for wide-range-locking multi-phase DLL. With a two-dimensional structure, the delay matrix achieves multiphase generation at high-frequency. The delay matrix employs a novel resistor network to improve the phase linearity by error averaging. For the wide-frequency-range lock, shunt capacitors in delay cells are gradually switched by a bias control scheme. The DLL was implemented in a 0.13？？m CMOS process. Monotonous 40 phases were achieved in a lock range of 40MHz to 700MHz. measured INL was 1.1LSB and DNL was 0.47LSB. The peak-to-peak jitter was 12ps, and power consumption was 43mW at the supply voltage of 1.2V.Secondly, a quantitative analysis and design methodology of resistor-based phase error averaging scheme for precise multiphase generation are performed. By taking the signal transition time into account, the analysis leads a different result from those of previously reported analyses which commonly state that more averaging achieves better linearity. The developed model shows a good agreement with a Monte-Carlo circuit simulation. A test PLL with a 32-phase two-dimensional ring VCO, implemented in a 0.18？？m CMOS, generates monotonous 32 phases, showing an INL of +0.27/-1.0LSB and a DNL of +0.37/-0.27LSB at 1.2GHz, and an INL of +0.23/-1.57LSB and a DNL of +0.44/-0.44LSB at 1.6GHz.Finally, an all-digital DLL is designed to generate low jittery 40 phases in a continuous lock range of 110MHz-to-1.4GHz. The DLL is driven by dual loops ？V one for phase lock and the other for offset calibration. The two loops are updated by a chopping PD which adaptively extracts valid information for each loop, one at a time. For the optimal 1b delay resolution in the entire lock range, a piecewise profiling of delay line is also proposed. The DLL, fabricated in a 0.13？？m CMOS, reveals the best linearity performance compared with previously reported works, showing a DNL of less than 0.3LSB and a INL of less than 0.8LSB in the entire lock range up to 1.4GHz. With the piecewise-fitted delay line, the amount of peak-to-peak and rms jitters induced by DLL operation is controlled to be less than 0.825-percent and 0.2-percent of the clock period, respectively. Power consumption was 74.4mW at the supply voltage of 1.2V

A-40-to-800MHz Locking Multi-Phase DLL

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A 5Gb/s 16-bit Transmitter with Segmented Group-Inversion Encoding

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A numerical simulation of two-phase flow with evaporating droplets using a Eulerian-Lagrangian approach

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A-40-to-800MHz Locking Multi-Phase DLL

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