비스트리플루오메탄설포닐이미드 또는 비스플루오설포닐이미드를 음이온으로 가지는 2-알콕시 피롤리늄 이온성액체의 합성과 LiFePO4 리튬이온전지에서의 물성연구

학위논문 (석사)-- 서울대학교 대학원 : 화학생물공학부, 2012. 2. 김영규.리튬 이차전지의 전해액으로 사용하기 위한 이온성 액체(IL)를 합성하고 물리화학적, 전기화학적 성능을 평가하였다. 기존 이미다졸륨계 양이온의 불안정한 C-2위치의 수소로 인해 환원 불안정성이라는 단점을 가지고 있다. 과거, 이를 해결하기 위하여 포화된 고리형 양이온인 피롤리디늄과 피페리디늄을 사용하여 연구한 결과, 환원 불안정성을 대폭 향상시킬수가 있었다. 하지만, 높은 점도와 낮은 전도도가 또다른 문제로 남았다. 그리하여 알콕시 치환체를 가진 피롤리늄 양이온을 도입하게 되었다. 즉, 피롤리늄의 이중결합은 양이온의 구조를 점도면에서 유리한 평평한 구조로, 알콕시 치환체는 낮은 점도와 전기화학적 안정성을 도모하기 위해 도입되었다. 지금까지 bis(trifluoromethanesulfonyl)imide (TFSI) 가 주로 연구되어왔는데, 이는 다소 높은 점도와 녹는점이라는 단점을 가지고 있다. 그래서 우리는 bis(fluorosulfonyl)imide (FSI) 를 도입하여 보다 더 낮은 점도와 전도도 뿐만 아니라 낮은 녹는점을 가진 이온성액체를 얻을 수 있었다. 우리는 여러가지 피롤리늄계 이온성액체를 합성하였는데, 그중에서 [A(OEt)]Pyrl-FSI (16)이 가장 낮은 점도 (22 cP)와 높은 전도도 (14.3 mS cm-1)를 보여주었다. 또한, 1M LiFSI가 녹은 [A(OMe)]Pyrl-FSI (12)이 4.4 V로 가장 넓은 전위창을 보여주었고, 1M LiTFSI가 녹은 [E(OMe)]Pyrl-FSI (10)과 [P(OMe)]Pyrl-FSI (11) 는 LiFePO4 양극에서 17 싸이클 동안 하프셀 충-방전테스트를 한 결과 초기용량대비 99%의 높은 수준의 가역 용량이 유지되었다. 이와 같은 연구를 통하여 기존 이온성 액체들이 높은 점도, 낮은 전도도, 전기화학적 불안정성 및 낮은 효율성 등으로 인해 리튬 이차전지에 적용되기 어려웠던 여러 문제점에 대한 새로운 해결책을 제시 할수 있었다.Ionic liquids (ILs) are salts with low melting point which is below 100 oC and composed of ionic species such as cations and anions. ILs have been issued since it has high potential to be applicable for lithium secondary batteries (LIBs) due to its unique properties such as wide liquid range, high ionic conductivity, wide voltage window, non-volatility, and non-flammability. One of the well known ionic liquids is imidazolium-based ILs which have low viscosity and high ionic conductivity compared to other conventional ILs. However, there still are some defects as electrolyte of lithium secondary batteries. That is, imidazolium-based ILs were found electrochemically unstable at low potential called cathodic polarization, which is mainly because of the decomposition of acidic proton at C-2 position of imidazolium ring. Few years ago, we reported saturated cyclic ammonium-based ILs called pyrrolidinium and piperidinium-based ILs to see the stability at cathode. As we expected, it showed more stable behavior compared to imidazolium-based ILs, but viscosity and ionic conductivity problems happened. To deal with it, using heteroatom substituents such as ether and sulfide is one of the solutions due to the fact that hetroatom containing substituents such as methoxymethyl group have positive effects on not only cathodic stability, but also viscosity and ionic conductivity. Therefore, in this work, we tried to overcome the defects that conventional ionic liquids have by using pyrrolinium cation with an alkoxy substituent which accompanies with low viscosity. Bis(trifluoromethanesulfonyl)imide (TFSI) have been widely used as an anion of ionic liquids. However, it contains several defects such as somewhat high viscosity and high melting point. Therefore, we modified an anion of pyrrolinium-based ILs for bis(fluorosulfonyl)imide (FSI) that gives low viscosity and melting point. We synthesized pyrrolinium FSI ionic liquids with several different types of substituents including an alkoxy group and characterized its electrochemical and physicochemical properties. Among them, 1-allyl-2-ethoxy-pyrrolinium bis(fluorosulfonyl)imide (16) ([A(OEt)]Pyrl-FSI) showed the lowest viscosity of 22 cP and high conductivity of 14.6 mS cm-1 and we were able to see the widest electrochemical window of 4.4 V from [A(OMe)]Pyrl-FSI (12). In addition, [E(OMe)]Pyrl-FSI (10) and [P(OMe)]Pyrl-FSI (11) have shown good cycleability on the LiFePO4-based half cell (capacity retention ratio after 17 cycles: 99%) From these results, we believe that ILs with enhanced physicochemical and electrochemical properties can provide a lot of applications as electrolytes in LIBs.Maste

(A) Study on Delayed Feedback Controller Design for Uncertain Time-Delay Systems

Docto

압입시험 및 표면 변위 분석을 이용한 재료의 항복 특성 평가

학위논문 (박사) -- 서울대학교 대학원 : 공과대학 재료공학부, 2020. 8. 권동일.Assessment of reliability for structural materials requires various information such as operating conditions, applied stresses, defect characteristics, mechanical properties, etc. The operating conditions and applied stress can be estimated by the design stress, and defect characteristics can be measured by nondestructive tests such as acoustic emission and ultrasonic test. However, it is difficult to evaluate the mechanical properties in the field using the conventional mechanical tests. Mechanical properties are basic data to analyze the reliability of the structure, and are measured with a sample specified in international standards. Since international standard tests require sample with specific shapes and sizes, it is impossible to evaluate the mechanical properties of structures in the field, and it is difficult to apply them to materials with small scales and complex shapes. Therefore, a nondestructive way to measure in-service material properties is highly desirable to assess the reliability of structural materials. Instrumented indentation testing (IIT), developed from conventional hardness testing, provides not only hardness and elastic modulus but also high-level information such as tensile properties, residual stress and fracture toughness through elasto plastic analysis of the continuously measured load and depth curves. In addition, it can be used at various scales from micro-scale to nano-scale by controlling the size of the indenter and the load or depth range. Therefore, many studies have been conducted for decades now on IIT to evaluate mechanical properties as well as hardness and elastic modulus. Especially, the yield strength among the mechanical properties is defined as the limit stress at which permanent deformation occurs, so yield strength is one of the most important parameters used in reliability assessment to predict material plastic deformation and fracture. Previous studies have suggested experimental or analytical relationships between yield strength and spherical indentation parameters, and were successfully verified for material with various strain-hardening behaviors. However, the indentation yield strength models proposed in the previous studies have some limitations. First, since directionality was not considered in many spherical indentation models, there is a limitation that it cannot be applied to materials with anisotropy such as rolled material, extruded material and 3D printing material, etc. Seconds, research on the uncertainty in yield strength is needed because the stress variation at the yield point is more severe than at the necking point, increasing the standard uncertainty. Moreover, so far the uncertainty different in the yield strength as measured by the spherical indentation and uniaxial tensile testing remains unclear and it is still unknown how uncertainty sources quantitatively affect the uncertainty of indentation yield strength. In this study, in terms of applying the spherical indentation to more diverse materials, we attempted to derive an indentation yield strength model considering directionality using dimensional analysis of single spherical indentation. In order to obtain the basic data of the modeling, FEA simulation was performed on various imaginary materials which were made by giving different yield strength, direction ratio of yield strength and strain-hardening exponent. Applying the -theorem in dimensional analysis, we obtained a model of indentation yield strength considering directionality. Subsequently, we analyzed the displacement distribution around the impression for all imaginary materials, from which a phenomenological model was derived to evaluate the direction ratio of yield strength. The model proposed in this study was verified through surface displacement analysis of metallic materials with Digital Image Correlation (DIC). In addition, in terms of the effect of the data deviation of spherical indentation, we proposed a method for accurately evaluating the uncertainty of indentation yield strength calculated from the modified Meyer relation as a mathematical function of the measurement, taking into account Type A and Type B uncertainty. Using this method, we investigated the effect of the major sources of uncertainty in the indentation system to interpret the difference in uncertainty between indentation and uniaxial tensile testing. Finally, acceptable uncertainty sources are proposed that give the indentation similar uncertainty to uniaxial tensile testing.구조재료에 대한 건전성 평가를 위해서는 가동 환경, 작용 응력, 결함 특성, 기계적 특성 등과 같은 다양한 정보가 필수적이다. 가동 환경과 작용 응력의 경우 설계 정보로부터 대략적인 예측이 가능하고, 결함 특성의 경우 음향 방출 시험 및 초음파 시험으로부터 산업 현장에서 측정이 가능하다. 반면, 재료의 고유한 기계적 특성은 전통적인 파괴적 평가 방법으로 현장에서 측정이 불가능하고, 표준화된 시험 절차에 의해서만 측정이 가능하다. 또한, ASTM (American Society for Testing Materials) 과 같은 재료 시험에 관한 기준은 특정 형상과 크기를 규정하기 때문에, 미세 구조물 혹은 복잡한 형상을 갖는 구조물의 기계적 특성 평가에 큰 어려움이 있다. 따라서, 가동 중 구조물에 대해 건전성 수준을 진단하거나, 복잡∙다양해지는 최첨단 산업 제품의 건전성을 평가하기 위해서는 기존 시험법의 한계점을 극복할 수 있는 평가 방법이 필요하다. 기존 경도 측정법으로부터 발전된 연속압입시험법은 기본적으로 경도와 탄성계수를 측정할 수 있을 뿐만 아니라, 하중 및 변위곡선의 탄∙소성 해석을 통해 인장물성, 잔류응력, 파괴인성과 같은 고급 물성을 측정할 수 있는 방법이다. 또한, 압입자의 크기 혹은 하중 및 변위 범위를 조절하여 마이크로부터 나노 스케일 하에서 기계적 특성에 대한 측정이 가능하다. 이와 같은 특징과 장점으로 연속압입시험법에 관한 연구가 활발히 이어져 오고 있다. 특히, 다양한 기계적 특성 중 항복강도는 재료의 파손 예측에 가장 기본이 되는 정보이기 때문에, 연속압입시험법과 하중 및 변위곡선을 활용하여 항복강도를 예측하기 위한 많은 연구들이 수행되고 있다. 기존 연구들은 실험적 혹은 해석적 방법을 통해 압입파라미터와 항복강도를 상관시킬 수 있는 모델을 정립하였고, 다양한 가공경화 거동을 갖는 재료에 대하여 제안된 모델의 신뢰성을 검증하였다. 하지만, 기존 연구에 의해 제안된 대부분의 모델은 등방성을 갖는 재료로 가정하여 유도되었기 때문에 방향성 에 관한 정보가 배제되었다. 따라서, 냉간/열간 압연과 같은 소성 변형 공정에 의해 제작된 제품과 최근 이슈가 되고 있는 3D 프린팅 재료와 같이 방향성을 나타내는 대상물에 대해서는 적용이 어려울 것으로 예상된다. 또한, 기존 연구들은 연속압입시험에 의해 예측된 항복강도의 정확도에 집중하였을 뿐, 데이터의 변동성과 신뢰성에 대해서는 거의 고려되지 않았다. 특히, 항복강도는 재료의 소성 변형의 시작점이기 때문에 네킹점 에서의 응력보다 변동성이 매우 심하다는 특징을 갖는다. 이러한 데이터의 변동성에 대한 심도 있는 연구가 수행되어야만 시험법의 국제적인 표준화 및 보편화 이루어질 수 있다. 본 연구에서는 방향성이 고려된 항복강도 평가 모델을 유도하기 위해 복잡한 현상을 수학적 방법으로 단순화할 수 있는 차원해석이 도입되었다. 이에 대한 기본적 데이터를 얻기 위해서, 항복강도, 항복강도의 방향비, 가공경화지수를 변경하여 100 가지의 경우에 대한 유한요소해석을 수행하였다. 이를 통해 얻어진 하중 및 변위 데이터를 활용하여 차원해석이 수행되었고, 항복강도의 방향비가 고려된 압입항복강도 평가 모델이 제안되었다. 또한, 압흔 주변에 발생하는 변위 분포를 분석하여 항복강도의 방향비를 평가할 수 있는 모델이 제안되었다. 이는 이미지 분석을 통해 압흔 주변 변위 분포의 해석이 가능한 Digital Image Correlation (DIC) 를 통해 실험적 검증까지 수행되었다. 연속압입시험에 의해 얻어지는 데이터 및 결과의 변동성에 관련해서는 ISO (International Organization for Standardization) 과 같은 국제 표준에 기반하여 불확도 평가 연구가 수행되었다. 불확도 평가를 위한 항복강도 계산 모델은 개선된 Meyer relation 이 사용되었으며, 이에 대해 A형과 B형 불확도가 고려되었다. 최종적으로, 압입 항복강도의 불확도 평가 방법이 제안되었고, 확률 및 통계적 접근을 통해 연속압입시험의 주요 불확도 인자인 시편의 표면 거칠기 및 표면과 압입축 간 각도 기준이 제안되었다.Chapter 1. Introduction 1 1.1 Objective of the Thesis 2 1.2 Organization of the Thesis 10 References 11 Chapter 2. Research Background 14 2.1 Contact Mechanics 15 2.1.1 Elastic Contact 15 2.1.2 Elastic-Plastic Contact 21 2.2 Instrumented Indentation Technique 28 2.3.1 Instrumented Indentation Technique (IIT) 28 2.3.2 Basic properties 29 2.3.3 Advanced properties 30 References 51 Chapter 3. Evaluation of indentation yield strength for anisotropic materials 55 3.1 Motivation and research flow 56 3.2 Evaluation of indentation yield strength considering directionality 60 3.2.1 Theoretical modeling 60 3.2.2 Computational verification 66 3.3 Evaluation of directionlaity of yield strength 67 3.3.1 Phenomenological modeling 67 3.3.2 Experimental and computational verification 70 References 95 Chapter 4. Improvement of evaluating indentation yield strength 98 4.1 Motivation and research flow 99 4.1.1 Evaluation of indentation yield strength 102 4.1.2 Possible uncertainty sources in indentation test 107 4.2 Uncertainty of indentation yield strength 109 4.2.1 Uncertainty evaluation method of indentation yield strength 110 4.2.2 Uncertainty of indentation test and uniaxial tensile test 115 4.3 Effect of uncertainty sources on uncertainty of indentation yield strength 118 4.4 Issues and limitations 127 References 139 Chapter 5. Conclusion 143 List of Publications 150Docto

Anti-windup Controller Design for Linear Tracking System with Saturation Nonlinearity

Maste

전기장 인가에 따른 폴리락틱산/그래핀 나노플레이트 복합체의 유변학적 및 전기적 특성에 관한 연구

학위논문 (박사)-- 서울대학교 대학원 : 공과대학 화학생물공학부, 2018. 2. 이승종.The goal of this thesis is to examine effect of electric field-induced graphene nanoplatelet (GNP) networks on rheological and electrical properties of poly(lactic acid) (PLA)/GNP composites and correlating between the composite properties and the GNP networks depending on concentration of GNPs or application intensity/time of the electric field. For melt-compounded PLA composites containing GNPs at various, low GNP volume fractions ≤ 0.34 vol%, this thesis focused on correlation between the composite properties and a large-scale structure of GNPs formed by a fixed, strong AC electric field (with the frequency and intensity of 60 Hz and 1.75 kV/mm, respectively). Optical microscopy, transmission electron microscopy, and 2D wide-angle X-ray diffraction measurements revealed that almost randomly oriented, thin stacks of GNPs in the as-fabricated composites were aligned by the electric field in the field direction and further organized into a column of stacks. This columnar structure bridged two parallel plate electrodes when the GNP volume fraction was above a threshold value ~ 0.17 vol%. Corresponding to this structural change, the liquid-like rheological response of the as-fabricated composites became the solid-like response (for the composites having GNP volume fractions ≥ 0.17 vol%), after application of the electric field. In contrast, the loss modulus G" was insensitive to the electric field and remained almost proportional to low angular frequency, suggesting that the columnar structure formed by the field was an elastic structure bridging the parallel plates. This bridge also served as an electrically conductive path between the plates so that an insulator-conductor transition occurred when the composites with GNP volume fractions ≥ 0.17 vol% were subjected to the electric field. Quantitative analysis of the equilibrium modulus and conductivity of the composites suggested that the field-induced columnar structure was not a rigid single slab but included the junctions being much softer and less conductive compared to the body of GNP. Moreover, analysis of the GNP volume fraction dependence of the equilibrium modulus and static electrical conductance after the transition (GNP volume fraction ≥ 0.17 vol%) suggested that an elastically inert secondary structure was formed by the GNP stacks remaining out the columnar structure (primary structure). This secondary structure appeared to be in soft contact with the primary structure to provide an extra conducting path. Meanwhile, for a melt-mixed composite of PLA with GNPs having a fixed, low volume fraction = 0.34 vol%, this thesis also examined growths of mechanical and electrical properties under an AC electric field of various intensities E for various times tE, focusing on field-induced GNP structures governing those properties. A fraction of randomly oriented GNPs was aligned by the field and then connected into columns, as suggested from optical microscopy. This structural evolution led to qualitatively similar growths of low-frequency storage modulus and static electrical conductivity. The key quantity for understanding this growth was a time tE* for occurrence of short circuit that detected formation of GNP columns conductively bridging the electrodes. The growths of both modulus and conductivity for various E were summarized as functions of a reduced variable, tE/tE*, confirming the growths commonly reflected the evolution of the GNP columns. However, the modulus grew fast and leveled off by tE/tE* ~ 1, whereas the conductivity kept growing gradually even at tE/tE* > 1. This difference was discussed in relation to the matrix chains and residual GNPs out the column. This thesis, which focused on the interrelation between the composites properties and the field-induced GNP structure, highlights the importance of controlling the microstructure of GNPs particularly for their orientation in order to realize the excellent intrinsic properties of GNPs still in the composites.Chapter 1. Introduction 1 1.1. General introduction 2 1.2. Overview of thesis 7 Chapter 2. Background 9 2.1. Graphene nanoplatelet 10 2.2. Control of microstructure: Dispersion 12 2.3. Control of microstructure: Orientation 15 2.4. Objectives of thesis 21 Chapter 3. Experimental methods 22 3.1. Sample preparation 23 3.2. Application of electric field to composites 25 3.3. Visualization and X-ray scattering 27 3.4. Rheological measurement 29 3.5. Electrical measurement 31 Chapter 4. Results and discussion 33 4.1. Concentration-dependent changes by electric field 34 4.1.1. Electric field-induced structural changes 34 4.1.2. Rheological properties 45 4.1.3. Electrical properties 52 4.1.4. Correlation between structure and properties 61 4.2. Time/intensity-dependent changes by electric field 70 4.2.1. Structure evolution under an electric field and evaluation of percolation time tE* 70 4.2.2. Rheological properties 75 4.2.3. Electrical properties 81 4.2.4. Similarity and difference of the elasticity and conductance growths 87 4.2.5. Further examination of time scale of structure formation 92 Chapter 5. Summary 113 국문 초록 117 References 121Docto

클라우드 저장소에서 공유되는 동적 데이터에 대한 안전하고 효율적인 검증 기법

학위논문(석사) - 한국과학기술원 : 전산학과, 2014.2, [ v, 28 p. ]With popularization of cloud storage services, there appears to share outsourced data by multiple users for cooperative work with minimum storage costs. For data integrity and consistency in the cloud service, the audit mechanisms were proposed. However, existing approaches have some security vulnerabilities and require a lot of computational overheads. This paper proposes a secure and efficient audit mechanism for dynamic shared data in cloud storage. The proposed scheme prevents a malicious cloud service provider from deceiving the auditor. Moreover, it devises a new index table management method and reduces the auditing costs by employing less complex operations. We prove the resistance against some attacks and show less computational costs and shorter time for auditing to compare with conventional approaches. The results present that the proposed scheme is secure and efficient for cloud storage services managing dynamic shared data.한국과학기술원 : 전산학과