산화그래핀 여과막의 물여과 특성 평가 및 여과성능 향상을 위한 원자충증착법의 적용

Multi-layered GO membrane (GOM) that is composed of atomic thick (0.34nm) graphene oxide (GO) flakes are mostly studied by researchers due to its unique water transport system, high chemical and mechanical stability. However, their low salt rejection efficiency and significant decrease of water permeability limit their wide application in water desalination. This research aims to understand the water transport mechanism and enhancing GOM performance. To investigate GOM behavior during filtration, GOMs composed with different sized GO flakes were applied. Moreover, on the purpose of enhancing water purification performance, a novel surface nano-functionalization method, atomic layer deposition (ALD) was applied. It was observed that water molecule transportation through GOM is limited by enveloped region where edge and basal plane of each GO meets. Interlayer space of wet state GOMs by X-ray diffractometer revealed that smaller GO stacked membrane possesses more enveloped region. Due to the narrowed nanochannel of GOM, water filtration with larger GO stacked GOM showed two folds higher permeability than GOM containing smaller GO nanoflakes. This study suggests that any modification of GOM that could affect the microstructure of membrane should consider such invariance of structural defects of GOM. ALD allowed for the formation of an ultra-thin Al2O3 layer (1.44nm) on the surface of GOM, due to the strong chemisorption of Al2O3 precursor, trimethylaluminum to GO. Atomically thin coating of Al2O3 layer enhanced water permeability (from 40LMH/bar to 75LMH/bar) and NaCl rejection up to 67.4%, overcoming the trade-off between permeability and selectivity. This enhanced filtration performance is mainly attributed to the changes of surface electrochemical properties without affecting the GOM microstructure. X-ray diffraction, X-ray photoelectron spectroscopy, Fourier-transform infrared spectroscopy and water contact angle were performed to investigate the effect of ALD on the physico-chemical properties of GOM. The distinctive features of ALD to the GO membrane may contribute towards enhancing its wider application in water desalination.Chapter 1. Introduction 1 Chapter 2. Literature review 5 2.1 GO membranes behavior during water filtration 5 2.2 Atomic layer deposition on GO membrane 6 Chapter 3. Materials and methods 8 3.1 Size-differentiated GOM fabrication 8 3.2 Surface modification of GOMs by ALD 10 3.2.1 Physico-chemical properties of GO nanoflake 10 3.2.2 Water intercalation behavior inside the GOMs 11 3.2.3 Confirmation of ALD-Al2O3 on the surface of GOM 12 3.2.4 Physico-chemical properties of ALD-Al2O3 coated GOM 12 3.3 Membrane filtration performance evaluation 13 Chapter 4. Results and discussion 15 4.1 Physico-chemical properties of controlled size GO flakes 15 4.2 Water transport through GOMs during water filtration 21 4.3 Deposition of Al2O3 on the GOM by PEALD 23 4.3.1 Confirmation of ALD-Al2O3 on the surface of GOMs 24 4.3.2 Enhanced membrane performance of the ALD-Al2O3@GOM 26 4.4 Characterizations of the few cycle ALD-Al2O3@GOM 28 4.5 Effect of ALD cycles on GOM filtration performance 33 Chapter 5. Conclusion 37 References 39 Academic achievement 51Maste

병렬 그래프 알고리즘을 위한 계산 구조에 관한 연구

학위논문(박사) - 한국과학기술원 : 전산학과, 1986.8, [ iv, 60 p. ; ]In this thesis, computational structures applicable to graph problems and tree problems have been established and are used to develop parallel algorithms for typical graphs and trees. A breadth first search algorithm for general graphs and a depth first search algorithm for acyclic digraphs have been developed. These algorithms which are based on the shortest path algorithm by Dekel et al. runs in $O(\log^2n$) time using $O(n^3$) processors where d and n are the diameter and the number of vertices of graphs, respectively. Our breadth first search algorithm turned out to be the same as that of Ghosh and Bhattacharjee which is based on the parallel breadth first algorithm for trees. Maximum matching algorithm for bipartite graphs which runs in $O(n \ast \log\, n \ast \log \log\, n)$ time using $O(N^2 \ast [n/\log\,n])$ also has been developed. For tree problems, we have developed algorithms for tree orientation, for maximum matching, and for finding the diameter and the center of trees. These algorithms have the time complexity $O(\log^2n)$ using $O(n)$ processors. For maximum matchings in bipartite graphs, we were able to lower the time bound by using more processors. For other problems, a lower time bound is also achieved using fewer processors.한국과학기술원 : 전산학과