리튬이차전지용 NiSi₂음극의 전기화학적 특성연구

학위논문(석사)--서울대학교 대학원 :재료공학부,2003.Maste

경량 금속에서의 통전 소성 변형 및 메커니즘 규명에 대한 연구

학위논문 (박사)-- 서울대학교 대학원 : 재료공학부, 2016. 2. 한흥남.Electrically-assisted forming is a promising alternative forming technique, in which the mechanical property of a metal alloy is altered by simply applying electricity to the target alloy during deformation. The reduced flow stress and increased ductility, which are often called the electroplastic effect, are generally observed in electrically-assisted deformation. Even though a number of researches have been conducted on the effect of electric current on deformation, the amount of experimental data to clearly understand the phenomenon of electroplasticity is not sufficient yet. Also, the underlying mechanism of electroplasticity is still unclear. The objective of present study is mainly consisted of two part. Firstly, the effect of electric current on lightweight alloy is investigated based on microstructural perspective. Through the understanding of electric current-assisted phenomenon, finally, the underlying mechanism of electroplasticity is investigated. Firstly, electroplasticity in non-heat treatable aluminum alloy was investigated with subsequent microstructural analysis. Al-Mg alloy was selected as non-heat treatable aluminum alloy. The elongation of both as-received (H32 treated) and cold-rolled specimens increases drastically with softening of flow stress by applying electric current during deformation. Recrystallization and grain growth were observed after fracture from the pulsed tensile test and it could be expected that thermal effect would be dominant after severe necking. However, increase of formability before severe necking was still observed strongly and it could not be explained by usual thermal effect. It was confirmed that the recovery occurs at a given electric pulsing condition comparing with nonpulsed tensile test. This study proves that the electric current could induce annealing as a distinct role from Joule heating. Secondly, electroplasticity in precipitation hardened aluminum alloy was investigated with subsequent microstructural analysis. Al-Mg-Si alloy was selected as precipitation hardened aluminum alloy. Specimens with three different heat treatment conditions, solution treated, naturally aged, and artificially aged (as-received) conditions, are prepared. In solution treated specimen, the elongation and flow stress increase by applying pulsed electric current during plastic deformation compared to the result of non-pulsed tension. The Portevin-Le Chaterlier (PLC) phenomenon, which is clearly observed in non-pulsed tensile test, nearly disappears by applying electric current during deformation. For the naturally aged specimen, the flow stress decreases while the elongation significantly increases under a pulsed electric current compared to the result of non-pulsed tensile test. In case of artificially aged specimen, both elongation and flow stress decrease under a pulsed electric current. From XRD analysis, it was observed that thermal and electric current-induced annealing occur in all the specimen under the electric current. Also, the formation of early stage of precipitation from a supersaturated state might be accelerated by applying electric current with a distinct effect of Joule heating, which causes the increase of flow stress and the disappearance of PLC phenomenon in the solution treated specimen. In addition, the microstructural observation shows that electric current accelerates the formation of microvoid around the precipitates at grain boundary, which results in earlier fracture in the artificially aged specimen. Lastly, underlying mechanism of electroplasticity was investigated based on effect of electric current on elastic modulus. From the results discussed in Al-Mg alloy and Al-Mg-Si alloy, it was suggested that electric current can enhance the atomic diffusion with a distinct effect of Joule heating. For diffusion, bonding energy of atoms with neighbors is an important factor to activate the diffusion. Bonding energy is closely related to the lattice potential energy. When material is excited by external energy, it will cause the changes in lattice potential energy. Elastic modulus is closely related to the atomic potential energy, which is derived from the second derivative of the potential energy-atomic distance curve. Therefore, the change in potential energy can be expected by measuring an elastic modulus. Laser ultrasonic method was used as a non-contact with high accuracy measuring technique to detect change in elastic modulus. For aluminum and magnesium alloy, electric current can induce additional decrease in elastic modulus more than Joule heating effect. Therefore, it can be suggested that electric current may induce the additional change in lattice potential energy with a discrete effect of Joule heating, which can cause the enhance of diffusion by weakening the atomic bonding force. Also, effect of grain boundary on electric current itself effect in electroplasticity was studied for each alloy with different grain size of specimen. It was confirmed that the decrease in elastic modulus by applying electric increases with increasing the fraction of grain boundary in both aluminum and magnesium alloy. From these results, it can be said that the grain boundary is dominantly influenced by applying electric current to decrease in elastic modulus. This study provides an important insight to apply electric current-assisted forming. As a positive aspect, recovery occurs due to the electric current-induced annealing as well as the thermal one by applying electric current. This can enlarge the capacity for deformation. In addition, electric current-assisted aging occurs during deformation with less time and lower temperature compared to conventional aging. However, electric current can accelerate the formation of microvoid around the particles, which results in earlier fracture as a negative way in formability. Therefore, electric current should be carefully applied to material considering the microstructural features to obtain enhanced formability without degradation of mechanical property. From this study, electroplasticity in lightweight alloy was investigated well. Also, the underlying mechanism of electroplasticity, which has not been clear up to now, is suggested. The discussion on electroplasticity and suggested underlying mechanism can provide insight to apply electrically-assisted manufacturing in real industry as well as academic interests for electroplasticity.Chapter 1. Introduction 1 1.1 Lightweight alloy 1 1.2 Electrically-assisted manufacturing (EAM) 3 1.3 Electroplasticity 7 1.4 Thesis motivation 10 1.5 References 12 Chapter 2. Electroplasticity in non-heat treatable aluminum alloy (Al-Mg alloy) 16 2.1 Introduction 16 2.2 Experimental procedures 18 2.2.1 Specimen preparation 18 2.2.2 Experimental set-up 20 2.2.3 Microstructural observation 23 2.3 Effect of electric current on mechanical behavior and microstructure 24 2.3.1 Electroplasticity in as-received specimen 24 2.3.2 Electroplasticity in cold-rolled specimen 30 2.4 Electric current-induced annealing 34 2.5 Summary 44 2.6 References 46 Chapter 3. Electroplasticity in precipitation-hardened aluminum alloy (Al-Mg-Si alloy) 48 3.1 Introduction 48 3.2 Experimental procedures 49 3.2.1 Specimen preparation 49 3.2.2 Experimental set-up 52 3.2.3 Microstructural observation 54 3.3 Basic properties: Mechanical behavior and microstructure 56 3.3.1 Mechanical behavior 56 3.3.2 Microstructure 58 3.4 Effect of electric current on uniaxial quasi-static tensile behavior 63 3.5 Annealing during pulsed tension 73 3.6 Effect of electric current on fracture behavior 80 3.7 Summary 85 3.8 References 87 Chapter 4. A study on underlying mechanism of electroplasticity 89 4.1 Introduction 89 4.2 Motivation 91 4.2.1 Diffusion kinetics 91 4.2.2 Measurement of elastic modulus using a laser based ultrasonic technique 98 4.3 Experimental procedures 102 4.3.1 Specimen preparation 102 4.3.2 Experimental set-up 103 4.4 Effect of electric current on elastic modulus 106 4.4.1 Temperature dependence on elastic modulus 106 4.4.2 Electric current itself effect in electroplasticity 111 4.5 Origin of decrease in elastic modulus in electroplasticity 119 4.5.1 Grain boundary and elastic modulus 119 4.5.2 Effect of grain boundary on temperature dependence on elastic modulus 124 4.5.3 Electric current itself effect in electroplasticity under the different grain size 127 4.5.4 Effect of grain boundary in electroplasticity 132 4.6 Summary 136 4.7 References 139 Chapter 5. Total conclusion 141 국문 초록 147Docto

개인화 TV 프레임워크에서 표적 광고를 위한 사용자 프로파일 추론에 관한 연구

학위논문(석사) - 한국정보통신대학교 : 공학부, 2006, [ ix, 79 p. ]The traditional broadcasting services such as terrestrial, satellite and cable broadcasting have been unidirectional mass media regardless of TV viewer``s preferences. Recently rich media streaming has become possible via the broadband networks. Furthermore, since bidirectional communication is possible, personalcasting such as personalized TV services has been an emerging technology by taking into account the user``s preference on content genres, viewing times, and actors/actresses, etc. Accordingly, personal media becomes an important means for content provision service in addition to the traditional broadcasting service as mass media. The personal media means contents considering individual preferences. In order to provide media services in personalized ways, TV personalization framework and its related technologies are essential. In the thesis, we propose a TV personalization framework which includes pEPG (personalized Electronic Program Guide) generation based on user preferences, target advertisement service, and content mobility service for consuming contents in some personalized ways. In the thesis, a user profile reasoning method for target advertisement is developed in the TV personalization framework which supports the content mobility service for multi-users. The user profile reasoning method predicts unknown TV viewer``s gender and ages by analyzing TV program viewing history. Based on the estimated user``s gender and ages, targeted advertisement becomes possible. The target advertisement is developed based on such a user profile reasoning algorithm. In the proposed TV personalization framework, pEPG can be generated based on the user preference values that are calculated by the user preference learning algorithm. Users can browse, select, and consume their preferred TV program contents from the given pEPG. Also, the content mobility is possible based on user preference via various kinds of user terminals in the TV personalizatio...한국정보통신대학교 : 공학부

Consumer Movement in the U.S.A

This study explores the main characteristics and limits of consumer movement in the United States of America. We begin with a historical investigation of how the American consumers organized themselves to protect their interest, which is followed by an examination of current structure and status of consumer organizations. The development of U.S. consumer movement has been the configurative result of macro variables such as the political structure, economic situation, and consumption patterns and macro variables such as individual leadership and voluntary action. We argue that the American consumer movement today can be characterized by the concepts of institutionalization and bifurcation. The former refers to the cooptation of the movement into the legal and governmental system, while the latter represents the differing trajectories of consumer organizations in terms of the fund, audience, and personnel. The consumers have won many battles leading to governmental gurantee of various consumer rights thanks to the active lobbying by professional consumerists. However, quite ironically, the institutionalization of consumer rights led to the removal of voluntariness and participation from consumer movment. In consequence, (over)institutionalized U.S. consumer movement is facing a major crisis in current political economic context, which emphasizes the small government, market, and business competitiveness

TARGET ADVERTISEMENT SYSTEM OF COMMUNICATION NETWORK AND ITS METHOD

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