Isolation and characterization of Korean domestic microalgae for a biodiesel feedstock : 토착 미세조류의 분리 및 특성분석을 통한 바이오 디젤 가능성 탐색

장지원 경북대학교 대학원 2013 국내석사

세계적인 에너지 사용량의 급증과 한정된 석유 매장량으로 인하여 인류는 에너지 부족의 시대에 직면해 있다. 대체에너지를 개발하기 위한 범세계적인 연구가 진행 중에 있으며, 이는 비단 새로운 에너지의 개발이라는 명목뿐 아니라 세계 각 국의 주권 향상과도 관련이 된다. 전기에너지와는 달리 석유기반의 액체연료 시스템을 그대로 사용할 수 있는 바이오 에너지는 이미 상당한 연구 중에 있으며, 그 효율성으로 인하여 많은 관심을 받고 있다. 미세조류는 광합성을 하는 미분화된 단세포들을 통칭하는 말로써 제 3세대 바이오 에너지원의 유력한 후보로 각광받고 있다. 1, 2세대의 에너지원들이 식량문제 및 토지효율적인 면에서 치명적인 약점을 드러냄에 따라 미세조류의 강점은 더욱 부각된다. Sheehan(1998)에 따르면 당시에 알려진 미세조류만 112,000여 종이 있고, 자연적인 광합성으로 인한 이산화탄소의 감축, 빠른 증식속도, 높은 지질함량 등은 미세조류를 이용한 바이오 연료의 당위성을 더욱 높여준다. 본 연구는 연료생산에 적합한 토착균주를 발굴하고 보다 효율적인 에너지원으로 사용될 수 있도록 각 균주의 특성을 개발하는 것에 주안점을 두었다. 실험에 사용된 모든 균주 (남조 Limnothrix redekei KNUA002, Phormidium autumnale KNUA026; 규조 Nitzschia palea KNUA022; 녹조 Scenedesmus obliquus KNUA025)는 항생제를 이용한 순수분리 기법을 통하여 분리되었으며, 균주 확인의 정확성을 높이기 위하여 형태학적, 계통학적, 분자적인 방법들이 사용되었다. 특히, 오랜 기간 보전되어온 미세조류의 염기서열은 다양한 미세조류를 확인함에 있어 강점을 지닌다. 바이오 연료의 원천이 되는 지질성분 측정 (GCMS)과 기본적인 생리실험 (온도, pH, salinity tolerance, N, P, CO2)이 일차적으로 진행되었으며, 각 균주의 특성에 따른 탄화수소 생산 (alkane productivity), 저온내성, 중성지방산 (FAMEs)의 증가, 배양방식의 개선 등 추가적인 이차 실험이 병행되었다. 이 연구를 통해 대한민국에서 미세조류를 이용한 바이오 연료의 생산이 더욱 활성화되고, 다양한 미세조류 균주의 발굴에 도움이 될 것이라고 기대한다.

Electrochemical characterization of defects and interfaces in solid-state perovskite solar cells

서갑석 성균관대학교 일반대학원 2017 국내박사

본 연구에서는 최근 차세대 태양전지로 주목 받고 있는 양자점과 페로브스카이트(CH3NH3PbI3) 태양전지를 제작하고 전지의 광전변환 효율에 영향을 주는 결함 및 계면의 특성과 그에 따른 영향에 대한 연구를 진행 하였다. 최근 많이 연구되고 있는 양자점과 페로브스카이트 물질은 긴 전하 이동 거리, 매우 높은 흡광계수, 작은 엑시톤 결합 에너지에 의한 높은 개방 전압을 기반으로 높은 광전변환 효율과 안정성으로 태양전지 연구 분야에서 많은 관심을 끌고 있다. Shockley-Quisser의 보고에 의하면 pn 접합을 가지는 태양전지의 경우 이론 광전변환 효율이 30%에 이른다고 한다. 그러나, 최근에 보고된 양자점과 페로브스카이트 태양전지의 경우 각각 13.4%와 22.1%의 높은 광전변환 효율을 보고 하고 있다. 이러한 높은 효율에 도달하였음에도 불구하고, 이론 효율에는 도달하지 못하고 있다. 이러한 이유에는 아직 밝혀지지 않은 많은 문제점이 있는데, 그 중에서도, 태양전지의 광전변환 효율을 저해하는 요소 중 가장 큰 영향을 주는 태양전지가 갖고 있는 결함에 대해서는 많이 연구가 진행되지 않았다. 그리하여 우리는 양자점과 페로브스카이트 태양전지의 계면에 대한 연구와 태양전지에서 만들어지는 다양한 결함의 종류와 에너지 그리고 생성된 결함의 위치에너지를 알아보고 그에 따른 태양전지의 광전변환 효율에 대한 영향을 알아보고자, 전기화학 임피던스 분광법, 시분해 광루미네선스 분광법, 깊은레벨 순간 분광법 등과 같은 전기적 측정 분석이 사용되었다. 첫째, 황화납 양자점의 강한 표면 결합에 의하여 생성되는 결함으로 인한 태양전지의 낮은 에너지 효율을 향상 시키고자 양자점 표면에 페로브스카이트 막을 형성하였다. 이러한 새로운 개념의 페로브스카이트의 쓰임새는 양자점의 표면 결함의 감소를 가져오고 전하의 분리를 빠르게 하여 전하의 재결합을 감소 시켜 단락전류의 증가와 소자의 안정성을 가져오는 역할을 하였다. 이는 임피던스 분광법과 시간분해분광법을 사용하여 특성을 분석하였다. 둘째, 기존에 잘 알려진 페로브스카이트 태양전지의 원스텝과 투스텝으로 제작된 페로브스카이트 태양전지의 효율을 저하시키는 깊은레벨에 존재하는 결함에 대하여 깊은레벨 순간 분광법을 사용하여 태양전지에 바이어스 전압을 가하여 온도에 따른 정전용량의 변화에 따른 깊은레벨에 생성된 결함의 밀도와 에너지를 확인하고 광전변환 효율과의 연관성을 확인 하였다. 특히나, 다른 제작 방법에 의해 생성된 페로브스카이트는 각기 다른 모양을 하고 있으며, 이로 인하여 표면의 거칠기와 광학적/전기적 특성을 갖고 있다. 그로인한 페로브스카이트가 갖는 밴드갭의 크기 또한 바뀐다는 것을 확인 하였다. 이러한 특징들은 페로브스카이트가 갖고 있는 정전용량의 변화를 가져왔고 더하여 깊은 레벨에 생성되는 결함의 차이로 인한 전지의 광전변환 효율의 차이를 가져왔다. 깊은 레벨에 존재하는 결함에 대한 연구는 고효율화가 기대되는 페로브스카이트 태양 전지의 새로운 길을 제시할 것으로 기대된다. 앞서 서술한 바와 같이 본 연구는 양자점과 페로브스카이트 태양전지를 제작하여 전지가 갖고 있는 결함 및 계면의 특성과 영향이 전지의 효율에 주는 영향 연구에 초점을 두었으며 이는 양자점과 페로브스카이트 태양전지뿐만 아니라, 다양한 LED, 유기 태양전지, 물 분해 등의 다양한 응용분야에 적용할 수 있는 가능성을 제시 하였다. In this dissertation, we investigated the characteristics and presence of defects and interface the photon energy conversion efficiency in the quantum dots and perovskite solar cells, which are attracting attention as next generation solar cells. Quantum dots and perovskite materials, which have been studied extensively in recent years, that are attracting attention in the field of solar cell research due to their high photon energy conversion efficiency and stability based on long charge transfer distance, high extinction coefficient, and high open-circuit voltage. However, Shockley-Quisser reports that the theoretical conversion efficiency of solar cells with pn junctions reaches 30%. The most recently reported quantum dots and perovskite solar cells have reported photon energy conversion efficiencies 13.4% and 22.1%, respectively. However, despite these high efficiencies, there are still many problem remains. Many kinds of defects, which have the greatest influence on the photon energy conversion efficiency in the solar cells, but have not been studied much. Therefore, we investigated quantum dots, perovskite solar cells, by using electrical measurements such as electrochemical impedance spectroscopy (EIS), time-resolved photoluminescence spectroscopy (TRPL), and deep level transient spectroscopy (DLTS). First, we introduce the perovskite shell layer was formed on the surface of the quantum dots in order to improve the energy efficiency. Due to surface defects caused by strong surface dangling bond. These defects act as a recombination center and reducing the efficiency and stability. New concept of perovskite shell layer reduces the surface defects of the PbS quantum dots and accelerates the charge separation, thereby reducing the carrier recombination rate, increasing the short-circuit current and stabilizing the device. It is characterized by impedance spectroscopy and time-resolved spectroscopy. Second, deep level transient spectroscopy was used to investigate defects in deep levels that have not yet been clarified, among many factors that reduce the efficiency of the perovskite solar cell produced by one-step and two-step methods. Deep level transient spectroscopy confirmed the relationship between photon conversion efficiency and the defect density and activation energy of defects generated at a deep level according to the change of capacitance and temperature by applying a bias voltage to the solar cells. In particular, the perovskites produced by different fabrication methods have different shapes, different surface roughness and optical/electrical properties. As a result, perovskite materials bandgap was also changed. These characteristics lead to a change in the capacitance of the perovskite and also to a difference in the photon conversion efficiency of the cell due to the difference in defects generated at the deep level. Deep level defects are expected to prove a new path for perovskite solar cells, which are expected to be highly efficient. As described above, this study focused on the characteristics and effects of defects in quantum dots and perovskite solar cells. This is not only the quantum dots and perovskite solar cells, but also various LEDs, organic solar cells, and the possibility of applying it to various application fields.

Graphene Based Nanomaterials for High-Performance Supercapacitors

Jian Chang 성균관대학교 일반대학원 2015 국내박사

Graphene nanosheets have recently drawn much attention due to their unique two dimentional (2D) structure and excellent properties such as high intrinsic electrical conductivity, ultrahigh theoretical surface area of 2630 m2 g-1, high theoretical capacitance of 550 F g-1 and high mechanical flexibility. Therefore, graphene based nanomaterials might be employed as excellent electrodes for electrochemical capacitors. Electrochemical capacitors, also known as supercapacitors, have been widely used in numerous areas such as hybrid electric vehicles, mobile electronics devices, military device and memory backup systems due to their ultrahigh power density, long cycling stability, wide operation temperature and improved safety. However, they suffer from a lower energy density, which has restricted their potential applications. Therefore, the improvement of energy density of supercapacitors is crucial to meet the future energy demands. For this purpose, organic electrolyte/ionic liquid based supercapacitors can effectively increase the operation voltage but require moisture-free environment and take a risk of explosion. Alternately, aqueous electrolyte based asymmetric supercapacitors (ASCs) have been developed for the purpose of simultaneously reaching high energy density and high power density. In this thesis work, the operation voltage of ASCs is maximized by choosing two metal oxides with the largest work function difference. We have successfully developed an ASC using graphene/MnO2 nanospheres and graphene/MoO3 nanosheets as positive and negative electrode, respectively. The operation voltage of ASCs is expanded to 2.0 V in spite of the use of aqueous electrolyte, revealing a high energy density of 42.6 Wh k g−1 at a power density of 276 W k g−1 and a maximum specific capacitance of 307 F g−1, consequently giving rise to an excellent Ragone plot. The hybridized nanostructure electrode materials and cell configuration of this ASC will strongly impact on improving energy density of not only generalized supercapacitors but also solid state flexible supercapacitors with high power density in future. Continuous miniaturization of portable electronics with enhancing functionality requires self-sustainable energy storage devices. One solution can be solid-electrolyte microbatteries which are commercially available but suffer from sluggish rate capability and limited cycling stability. All-solid-state microsupercapacitor could be a promising alternative with high rate capability but simultaneously reaching high volumetric energy density remains challenging. Here we propose, inspired by natural vein-textured leaves, two-dimensionally nanochannelled graphene film to facilitate high rate capability while maintaining high energy density. These 2D nanochannels play a role of pathways for facile ion diffusion parallel to the graphene plane. Interdigitated electrodes for microsupercapacitors fabricated by photolithography patterning of nanochannelled graphene facilitate ion diffusion in parallel to the film to maintain high rate capability, regardless of the film thickness. High volumetric energy density of 6.7 mWh cm−3 was obtained at a volumetric power density of 0.1 W cm−3, with a slowly reduced energy density of 3.8 mWh cm−3 at a high power density of 20 W cm−3. Our 2D architectured graphene film intercalated by nanochannelled solid electrolyte can be a platform of microsupercapacitors for self-sustainable portable electronics and microelectromechanical systems with high volumetric energy density while retaining high power density.

Network Analysis of Power Grids: Synchronization Stability and Sustainability

김희태 성균관대학교 일반대학원 2016 국내박사

In this thesis, we apply network theory for studying power grid problems, particularly for synchronization stability and sustainability. We focus on the synchronization phenomenon of alternating current at power-grid nodes in terms of the functional stability of power grids. In power grids, the transition pattern of synchronization stability varies according to the nodes’ network characteristics. We introduce transition window and community consistency representing the nodal synchronization and community char- acteristics. We find that the transition patterns of synchronization stability are correlated with the consistent community membership of power-grid nodes, which we call community consistency. We also investigate the functional form of the synchronization stability transition. As the building blocks of power grids, we extensively investigate the all isomorphically distinct networks having two, four, and six nodes. We classify the transition forms into four patterns based on the basin stability at three different transmission strength values—three dimensional investigation. In addition, we seek to integrate network theory into sustainability analysis. Network theory evaluates the functional centrality of nodes, which can directly be used for environmental impact assessment. We allocate the greenhouse gas emissions of electricity transmission loss according to the functional load of power-grid nodes. As a case study, we estimate the greenhouse gas emissions caused by electricity transmission loss of provinces in Chile. We conclude that the network theory is the crucial complement for the power-grid analysis. 전기 에너지는 개인의 일상 깊은 곳까지 침투하며, 현대 사회에서 매우 중요한 에너지원으로 자리매김하고 있다. 기술의 발전에 힘입어 스마트폰, 드론, 생체 신호 계측기 등 다양하고 새로운 전자 제품들이 개발되어 일상생활에 융합되고 있으며, 전기 에너지의 압도적인 사용 편의성은 조명, 난방, 조리, 수송 등 전통적으로 화석 연료를 사용하던 영역마저도 전자제품의 영역으로 흡수하게 하였다. 사물인터넷, 인공지능, 거대 정보 처리 기술, 그리고 인터넷의 발달을 체감하고 있는 현시점에서, 전기는 에너지원 일 뿐만 아니라, 전자 제어로 작동하는 모든 기술의 근본적인 신호 전달 물질로서도 사용되고 있다. 바야흐로 전기는 이제 단순한 에너지원을 넘어 탄소 연료 이후의 시대를 주도하는 중요한 역할을 감당하게 되었다. 전력 수요가 급격하게 늘어나고 전력 산업이 점점 고도화됨에 따라, 전력의 안정적인 생산과 분배의 중요성 또한 더욱 커지고 있다. 일부 지역의 단선이나 과부하가 걷잡을 수 없는 연쇄작용을 거쳐 대규모 정전을 촉발하기도 하며, 이에 따라 전력망을 큰 연결망의 관점에서 이해하려는 시도가 대두하였다. 본 학위 논문에서는 최근 비약적으로 발전한 연결망 이론을 활용하여 전력망에서 일어나는 복잡한 상호작용을 분석하고자 한다. 실제 사례 연구로서는 칠레 전력망의 동기화 안정성과 지속가능성을 분석하였다. 전력망을 연결망 이론으로 분석하려면 우선 전력망을 분석에 적합한 구조로 가공해야 한다. 본 논문의 2장에는 행렬을 활용하여 전력망을 표현하는 기본적인 방법부터 회로 이론을 바탕으로 불필요한 연결점을 제거하거나 연결선 가중치를 계산하는 방법 등 연결망 구조화에 필수적인 이론을 정리하였다. 이와 함께 현재 가용한 국가별 전력망 자료도 망라하였다. 3장에서는 전력망을 분석하는 다양한 이론적 방법을 다룬다. 특히, 기능적 안정성과 지속가능성을 분석하는 방법으로서, 동기화 안정성과 전과정평가를 중점적으로 정리하였다. 일반적으로, 교류 전력망에서 각각의 연결점들은 적절한 위상각과 위상각속도를 유지하며 다른 연결점들과 동기화되어 있다. 연결점이 외부로부터 가해지는 위상 충격에 대응하여 얼마나 동기화를 잘 회복하는지를 살펴보면, 그 연결점의 동기화 안정성을 분석할 수 있다. 이 동기화 안정성은 연결점들 사이의 연결 강도에 따라 달라지는데, 연결 강도가 강해짐에 따라 동기화 안정성이 변화하는 형태는 연결점 별로 다르다. 4장과 5장의 사례 연구에서는 이 동기화 안정성이 변하는 형태와 특성을 이해하기 위해, 연결점의 연결망 특성과 동기화 안정성 그리고 공동체 일관성을 서로 연관 지어 분석하였고, 그 사이의 상관관계를 밝혔다. 또한, 다양한 동기화 안정성의 변화 형태를 분석하기 위해서, 2개, 4개, 6개의 연결점으로 구성된 모든 연결망 구조를 광범위하게 조사하였다. 모두 596가지의 연결망을 서로 다른 세 가지 결합 강도를 기준으로 분석하여, 동기화 안정성의 변화 양상을 총 4가지 형태로 구분하였다. 분석 결과, 동기화 안정성의 형태 변화는 연결점의 중간중심성에 비례하는 것으로 나타났다. 한편, 전력망의 지속가능성을 분석하고자 기존 전과정평가 방법론에 연결망 이론을 적용하였다. 연결망 이론은 기본적으로 연결점의 기능적 중심성을 분석할 수 있게 해주는데, 이는 환경 영향 평가에 직접 활용될 수 있는 여지가 많다. 예를 들면, 송전 과정에 발생한 온실가스를 전력망 연결점들의 중심성을 기준으로 할당할 수 있다. 6장에서는 실제로 칠레 전력망의 송전 과정에 발생한 온실가스를 연결망 중간중심성에 따라 각 지역에 배분하였다. 이를 통해, 실제로 각 소비 주체가 전력망에 부하를 발생시킨 정도에 따라서 온실가스 발생량을 할당할 수 있었다. 전력망에 연결망 이론을 적용하는 시도는 아직 몇 가지 주제를 대상으로 제한적으로 이뤄지고 있다. 향후 신재생에너지를 포함하여 전력 생산 방법이 다양해지고, 전력 생산원이 소규모 다점식으로 분포되며 생산과 소비의 유동성이 커질 것으로 예상된다. 이에 따라, 전통적인 방법으로 전력 시스템을 분석하거나 예측하는 것에 한계가 따를 것으로 보인다. 본 학위 논문을 통해 연결망 이론이 다각화되는 전력 에너지 시스템을 분석하는데 매우 유용한 이론적 기반을 제공할 수 있음을 확인하였다.

A study on Synthesis and Electrochemical Property of Nano-sized Lithium Manganese Oxide Deposited on Carbon nanotubes

최아름 성균관대학교 일반대학원 2012 국내석사

Electrochemical energy storage system offer an enormous potential for meeting future energy demands, such as renewable energy, electric vehicles, uninterruptible power supplies and portable electronic devices. These applications require both high specific energy and high specific power. Lithium ion batteries and electrochemical capacitors are among the leading electrochemical energy storage systems. Electrical energy can be stored either by a chemical or physical mechanism on the interface between electrolyte and electrode materials. In general, considering their operating principle, lithium ion batteries have a high specific energy but low specific power. Thus, the current technology of electrochemical energy storage systems can not satisfy the future demand. Therefore, great efforts have been devoted to identifying alternative and inexpensive electrode materials with high specific energy and power for electrochemical energy storage systems. This thesis reported synthesis and electrochemical properties of various lithium manganese oxide/ carbon nanotube (CNT) nanocomposite for lithium-ion batteries. The LMO nanoparticles were successfully synthesized and well dispersed on entangled carbon nanotube by microwave-hydrothermal process for electrochemical energy storage devices. LMO/CNT nanocomposite showed excellent cycleability as well as good structural electrochemical properties of nano-sized material were attributed to shorter lithium diffusion length, good dispersion and large ionic supply high electrode/electrolyte interfacial area by introducing entangle carbon nanotubes. This synthesis storage opens a new rout for the synthesis of lithium metal oxides on the entangled carbon nanotubes web in successful and efficient way.

Network Analysis of excitation energy transfer in bacterial photosystem complexes

이은 성균관대학교 일반대학원 2013 국내석사

We explored the correlations of the excitation energy transfer network (EET-NET) structure of photosystem II (PSII) core complex (Protein Data Bank 1W5C) in Thermosynechococcus elongates. We regard main pigements − chlorophylls and pheophytins − and EET rates (EET-RATE) between two main pigments as nodes and links of the established network, respectively. Here we estimated EET-RATE by a simple dipole-dipole approximation (Förster theory) using on the coordination and spatial orientation of photosynthetic pigments such as chlorophylls and pheophytins in PSII core (EET-dynamics; EET-DYN). In this work we ignored other photosynthetic pigments like carotenoids and quinones, rather we concentrated on the EETs among main pigments in PSII core. Using the network analysis of EET-NET, we constructed the simplified (rather arbitrary) network model and explored the question how much of excited energy in chlorophylls could reach to the reaction centers. We focus on the following network parameters: degree centrality (DC), betweenness centrality (BC), and the clustering coefficient (CC) originated from the theoretical network analysis. Our simulation results show the close correlations between chlorophyll network properties and functionality. Specifically they also show the functionality of individual pigments of special pairs in the EET-DYN with EET-NET. The special pair chlorophylls (node 11, 12 and 21, 22 for D1 and D2 proteins, respectively) in the reaction center (RC) are characterized by a high BC and a low CC. Remarkably chlorophylls in active branch (node 11, 21) has even larger BC than those in inactive branch (node 12, 22) of the PSII core. This simulation results imply that chlorophylls in active branch (node 11, 21) can deliver more excitation energy to reaction centers with high probability than those in the other branch of special pair. In this work, we found that several significant principles of EET-DYN network in whole PSII core complex can be even re-emphasized in our simplified network of EET-NET. This network can be applicable to analyze the functionality of the EET-DYN in PSII core and the mathematical topology of chlorophyll pigments in EET-NET.

The Relation between Defect Chemistry and Thermoelectric Properties in n-type Bi2Te2.7Se0.3 Thermoelectric Materials

김민영 성균관대학교 일반대학원 2017 국내석사

Thermoelectric (TE) materials can generate electricity from waste heat and be used as Peltier cooler. The energy conversion systems are potentially useful for waste heat recovery as renewable energy source. However, the performance of thermoelectric materials has a crucial limitation that is a very low energy conversion efficiency below 10%. This problem provokes the development of high performance thermoelectric materials, which is prerequisite for the efficient thermoelectric device and system. Recently, the highest performance in p-type Bi0.5Sb1.5Te3 was developed, showing the zT value of 1.86 ± 0.15 (@320 K) by the formation of dislocation arrays at the grain boundary to significantly reduce the lattice thermal conductivity. On the other hand, the performance of n-type Bi2Te2.7Se0.3, which is the counterpart of p-type Bi0.5Sb1.5Te3, is very poor. Even though the tremendous efforts including doping and nanostructuring strategies have been applied, the performance of n-type Bi2Te2.7Se0.3 has not been enhanced to the level of p-type materials. Importantly, the reproducibility of thermoelectric transport properties for n-type Bi2Te2.7Se0.3 has been considered as a main issue to be solved for the highly efficient thermoelectric devices based on Bi-Te systems In this thesis, the defect chemistry of n-type Bi2Te2.7Se0.3 has been systemically studied in order to solve the reproducibility issue and to enhance the TE properties. In details, the relation between defect chemistry and thermoelectric properties in n-type Bi2Te2.7Se0.3 by comparing fabrication processes such as conventional melt-solidification, ball milling, spark plasma sintering and newly developed metal nanoparticle decorating process. The Cu0.01Bi2Te2.7Se0.3 fabricated by the newly developed Cu nanoparticle decorating process shows a high Seebeck coefficient value of -206.03 μV/K (@ 360 K) and the reduced lattice thermal conductivity of 0.63 W/mK (@ 360 K), leading to the zT value of 0.96 (@ 360 K). In case of heavily Cu-doped Bi2Te2.7Se0.3, it is suggested that the Cu atom is located between quintuple layers of Bi2Te2.7Se3 structure acting as intercalator, which is verified from the increased c-axis from 31.122 Å to 31.177 Å. In addition, the lattice thermal conductivity is reduced to the value of 0.3 W/mK (@ 300 K) due to the enhanced phonon scattering. Further, it is found that the Cu atom in n-type Bi2Te2.7Se0.3 is electron donor, increasing the electrical conductivity to 1693.42 S/cm (@ 300 K) and carrier mobility to 130.9cm2/Vs. 열전 기술(thermoelectric technology)은 열과 전기에너지의 상호 가역적 변환이 가능하여 신재생 에너지원으로 각광 받고 있다. 그러나 열전 소재 성능의 한계로 인해 에너지 변환 효율이 낮아 상용화에 어려움을 겪고 있다. 그러므로 고효율 열전 시스템 개발을 위해서 고성능 열전소재 개발이 핵심이다. 최근 김상일 연구팀에서 p-type Bi0.5Sb1.5Te3 소재에 고밀도 입계 전위를 형성시켜 열전도도를 감소시킴에 따라 소재 성능(zT)을 세계 최고값인 zT = 1.86±0.15 (@320K)로 향상시킨 결과가 보고되었다. 반면에 n-type Bi2Te2.7Se0.3 소재의 성능은 Te,Se vacancy에 의한 재현성문제 등으로 인해 p-type보다 상대적으로 낮다. 본 연구는 기존 공정의 한계를 극복하고 새로운 합성법(Cu-decoration)으로 Bi2Te2.7Se0.3에 Cu를 첨가하여 Te,Se vacancy 형성을 억제 시켜 캐리어 농도를 제어하고 재현성을 확보하여 열전 성능 최적화를 진행하였다. Cu가 과량 도핑된 경우 Cu원자가 Bi2Te2.7Se0.3구조 내의 반 데르 발스 결합을 이루는 Te-Te layer 사이에 intercalation 되면서 격자가 31.122 Å 에서 31.177 Å로 증가한다. 또 전기전도도가 1693.42 S/cm(@300K)로 증가하고 포논의 점 결함 산란의 증가로 인해 격자 열전도도가 0.3 W/mK (@300 K)으로 낮게 유지된다. Cu0.01Bi2Te2.7Se0.3의 경우 Cu원자가 Bi자리와 치환 (Substitution)되면서 제벡 계수가 -206.03 μV/K(@360 K)로 증가하고 포논의 점 결함 산란으로 격자 열전도도가 0.63 W/mK(@360 K)로 감소하여 최대 zT 0.96(@360 K)까지 향상되었다.

Thermal Management and Neutronics Calculations for Medium to High Power Neutron Production Targets

Masoud Behzad 성균관대학교 일반대학원 2014 국내박사

Neutron sources can be applied for various purposes from nuclear medicine to material science, and they can be used for power generation and waste transmutation in the form of accelerator driven system in future. Although research reactors are the prime source of neutron generation, they need to address the major issues like proliferation risks, safety, aging, nuclear wastes, etc. These issues limit the future development of reactors as a clean, safe and sustainable solution. Developing accelerator-based production of neutrons can be one of the solutions to overcome the problems associated with research reactors. It is worth mentioning that one of the most critical component in the accelerator complex is the target irradiation system which is composed of different parts. Among all parts in the target irradiation system, there is a component that is bombarded by particles, referred to as the target. Despite all the progress in target technology, this area needs more research for new applications of neutron sources. Target study can be complex as several parameters like geometry, materials, cooling flow, particle interaction, etc. are involved in its design and testing. The cost of target station in an accelerator site is quite considerable, hence the target design as well as its operating condition should be treated properly to prevent any failure. The simulating tools in physics and engineering are employed in design and optimization stages. These simulation tools cover several aspects of target study from neutronics, safety, shielding, radiation protection, material damage to heat transfer. Among these, this thesis is focused on thermal management and neutronics characterization of several target systems for neutron production. The accelerator technology is well established for different types of applications; a general description of which is explained in Chapter 1. Chapter 2 briefly covers a few different applications of neutron sources. Potential materials for neutron source targets are also reviewed in Chapter 2. The majority of the beam power loss occurs in the target material. The heat is generated by the interaction of charged particles with target medium. Chapter 3 explains the physics of charged particle interaction with matter. The theory is accompanied by simulation of charged particle interaction with different target materials (as well as different geometry and projectile) in order to obtain the neutron yield and energy deposition in target volume for spallation process. It is also essential to consider an optimal cooling configuration for the targets. The theory of fluid mechanics, stress analysis and the related simulation tools are further discussed in Chapter 3. An alternative to reactor-based production of 99Mo (as one of the major application of neutron sources) is the accelerator-based method via 98Mo(n,γ)99Mo reaction. Neutrons are produced by bombarding targets with proton (or deuteron) beam. The total beam power considered for this work is 2 kW, and beryllium is selected as target material. The heat transfer analysis was done for two type of coolants; helium and water. Thermal analysis of a multi-channel helium cooled device is performed with the computational fluid dynamics code CFX. Different boundary conditions are taken into account in the simulation process and many important parameters such as maximum allowable solid target temperature as well as uniform inlet velocity and outlet pressure changes in the channels are investigated. The simulation has been carried out for water-cooled beryllium target as well. The temperature distribution in different components is obtained for the target bombarded by protons or deuterons. Stress analysis of the water-cooled beryllium target is also done in this work. The results of the simulation for beryllium target are given in Chapter 4. The idea of designing a high power (100 kW) portable neutron product target is the main theme of Chapter 5. Such device fits in several laboratories' scope which are planning to use neutron sources. Additionally neutron irradiation can be used for research in material science. Neutronic analysis as well as heat transfer analysis of the liquid metal target are highlighted in Chapter 5. Furthermore it has been tried to improve the primary design of the liquid metal target from fluid mechanics point of view. Chapter 6 lists and describes the challenges in studying neutron source targets from physics and engineering point of view. The proposal for future works is given in Chapter 6. Chapter 7 concludes the thesis.

Thermoelectric properties of pavonite Cu-Bi-S compounds

안준연 성균관대학교 일반대학원 2017 국내석사

Fossil fuels, including petroleum, coal and natural gas have been dominant source of energy as the world became industrialized. On a global scale, over 80% of fossil fuels are consumed for energy source per a year. However, the excess use of fossil fuels brings incidental side effects, from environmental and economic perspectives. Perhaps worst of all, fossil fuels are non-renewable source of energy. Accordingly, alternative energy sources have been considered steadily to reduce the dependency on fossil fuels and cope with the ongoing energy crisis. In this respect, thermoelectric energy conversion has been mentioned as one of the major solutions for energy issue. It is an environmentally friendly energy conversion technology with size variability in a wide temperature range. Thermoelectric (TE) conversion is a general term of the phenomena in which electricity is generated from heat or vice versa. The efficiency of TE conversion directly relies on the performance of the TE materials. Generally, TE materials are applied to the form of TE module in practical life. TE module is a circuit containing nand p-type semiconductors that are connected electrically in series connection and thermally in parallel. The array of semiconductors and their connecting pieces are soldered between two ceramic plates. The module is used to TE generator or thermoelectric cooler. TE generator converts temperature gradient between two junctions of TE module directly into electricity on the basis of the phenomenon, ‘Seebeck effect’. It can turn waste heat from exhaust gas into reusable energy source in the form of electricity in power plants, automobile engine and numerous machines. Conversely, TE cooler creates temperature difference between two junctions by insertion of electrical current according to the phenomenon, ‘Peltier effect’. When the electrical current flows through the junctions of two different semiconductors, heat is removed at one side of junction and cooling occurs. It can be used either for heating or cooling such as temperature controller and refrigerator with size variability. The performance of TE materials is represented as dimensionless figure of merit (ZT), which is defined as ZT=S2σT/κtot, where S is Seebeck coefficient, σ is electrical conductivity, κtot is total thermal conductivity and T is absolute temperature respectively. Additionally, the value of S2σ is called power factor (PF) and κtot is sum of electronic (κele, charge carriers) and lattice contributions (κlat, phonons) for thermal conduction. High ZT requires simultaneous high S and σ with low κtot. However trade-off relations between TE parameters limit ZT optimization and the applications of TE conversion, which have less than 20% of conversion efficiency compared to other technologies. Over the past two decades, there have been several approaches to maximize ZT. Especially, there are number of meaningful researches to get low κtot, from design and manipulation of a material for κlat reduction. Generally, reduction of κlat can be realized by promoting wide range of phonon scattering. As well as the phonon engineering, much effort has been focused on finding new TE materials with intrinsically low κlat. Many of the researched complex TE materials have κlat with half value of the commercial TE materials. The structural complexity is related to large unit cells with many constituent atoms, partial occupancy of atoms and existence of disordered sites. The Cu-Bi-S compound, (CBS) which fulfills many of the characteristics of complex TE materials, has substitutional and interstitial Cu sites as well as individual atomic sites with distinctive structural configurations in the unit cell. The average value of lattice thermal conductivity in CBS system was 0.5 W/mK, which is lower than those of state-of-the-art TE materials such as 1.0 W/mK of Bi-Te systems. Combining with the power factor, the highest ZT is 0.21 at 673K. Additionally, appropriate doping strategy could maximize the ZT of 0.27 in the same temperature.

Functional Nano-Carbon Hybrid Materials for the Energy Storage Device

유희준 성균관대학교 대학원 2015 국내박사

Today, the technologies on energy generation and storage are very important issue for humans, and continuously developments and studies for superior technologies have been strongly demanded. On these technologies, the novel materials design and characterization of performance should be importantly carried out, and they are decisive factor for the energy generation and storage technologies. On the other hand, the carbon-based nanomaterials based on graphene and carbon-nanotube have been received a great deal of attention as the key material for the energy storage device and in the various science and industry fields. These nano-carbons composed with single layered carbon structure of sp2-bonded carbon atoms, has good physical and chemical properties. In additionally, nano-carbon has high surface area and active sites, they can be act as excellent host materials. Owing to their unique properties, nano-carbons have been successfully utilized and adopted for various applications like nano-electronics, energy storage materials, sensing, and polymer composite materials. In addition, nano-carbon is very recommended materials for advanced energy storage materials because of its nature abundance, low density, low cost, and green material. This thesis carry out the study on nano-carbon composite for the energy storage device, representatively, electric double layer capacitor for high power applications on first part and lithium ion batteries for high energy applications on the second part. In the first chapter, we designed new procedure for the nitrogen doped porous graphene. The highly winkled and porous nitrogen doped reduced graphene oxide (NrGO) is prepared through a simple microwave irradiation reaction from graphene oxide (GO) and ammonium hydroxide (NH4OH). Microwave irradiation reaction with solvent assisted bubbling nitrogen source facilitates a safe-gram scale, nitrogen doped and porous reduced graphene oxide (rGO). The nitrogen doped amount and surface area of NrGO is increased as the reaction ratio of NH4OH. The highly nitrogen doped and porous NrGO exhibits superior capacitance performance. This chapter newly suggests effective mass-producible nitrogen doping porous graphene flake through simple microwave reaction. In the second chapter, we demonstrate a low resistant and high volumetric on-chip type solid-state supercapacitor (SSC) using hydrophilic surface modified multi-walled CNTs (MWCNTs) and reduced graphene oxides (RGOs). The hydrophilic surface modified carbon electrode is expected to show a good electrolyte affinity and a homogenous dispersibility in water, resulting in the low resistance and the uniform dense electrode to give high volumetric capacity. Especially, for the good accessibility of electrolyte into the carbon-based electrode, we carefully designed the surface modified MWCNTs with a hydrophilic sulfonate groups (R-SO3-), which allow a hydrophilic surface of the outer carbon nanotube, while keeping high electric conductivity through inner carbon nanotube. Different from previous results, this hydrophilic carbon composite film electrodes exhibit much lower series resistance and simultaneously lower charge-transfer resistance despite its high dense structure (over 0.9 g cm-3). The hydrophilic surface modified carbon electrode shows a good electrolyte affinity with homogeneous dispersibility in water, resulting in low ion-transfer resistance and a uniform and dense electrode to give a high volumetric capacitor. The hydrophilic carbon electrode exhibits a superior capacitance (58 F cm-3, 99.3 mF cm-2) and is stable up to 5000 cycles. In the third chapter, an efficient and scalable process applicable for mass production was developed to synthesize non-aggregated MoS2-intercalated three-dimensional (3D)-nanostructured graphite based on stress induced and microwave irradiation techniques in which exfoliated graphite, sulfur, and molybdenum chloride were used as the starting materials. X-ray diffraction, field emission scanning electron microscopy, and high-resolution transmission electron microscopy analysis demonstrated that the as-synthesized materials consisted of MoS2-intercalated 3D hybrid-nanostructured graphite platelets that had a multiply repeated MoS2/graphite/MoS2 structure. The obtained MoS2-graphites powder surpasses MoS2 as an anode material in terms of specific capacity, cycle stability, and rate performances at high current densities for Li-ion batteries. The electrochemical impedance spectroscopy demonstrated that the graphite sheets not just reduced the contact resistance in the electrode but also facilitated electron transfer in the lithiation/delithiation processes. The superior electrochemical performances of the as-synthesized MoS2-intercalated 3D-nanostructured graphite can be ascribed to its hybrid 3D architecture and synergetic effects between the layered MoS2 and graphite sheets. In the fourth chapter, we carried out a new unique synthesis method to produce a new type of hybrid nanostructure of MoS2-CNTs composites. We developed a novel strategy for the synthesis of cylindrical MoS2 directly grown on CNT composites without the use of any other additives, exhibiting super electrochemical performances as the anode material of lithium ion batteries via a microwave irradiation technique. We adopt a simple, step-by-step method: sulfur coating on CNTs and then reaction with a Mo source to synthesize hybrid cylindrical nanostructures of the MoS2-CNT composite. X-ray diffraction, field emission scanning electron microscopy, and high-resolution transmission electron microscopy analyses demonstrated that the as-synthesized MoS2-CNTs possessed a hybrid nanostructure in which MoS2 sheets were well attached to the CNTs. The directly attached MoS2 sheets on the CNTs showed superior electrochemical performance for anode materials of a lithium ion battery.