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Chung, Kook-Chae,Yoo, Jai-Moo,Ko, Jae-Woong,Kim, Young-Kuk,Wang, X.L.,Dou, S.X. The Korean Society of Superconductivity and Cryoge 2007 한국초전도저온공학회논문지 Vol.9 No.2
The influence of water partial pressure in Metal-organic Deposition (MOD) method was investigated on the texture and the morphology of $YBa_2Cu_3O_{7-x}$ (YBCO) films grown on the buffered metal tapes. The water partial pressure was varied from 4.2% up to 10.0% with the other process variables, such as annealing temperature and oxygen partial pressure, kept constant. In this work, the fluorine-free Y & Cu precursor solution added with Sm was synthesized and coated by the continuous slot-die coating & calcination step. The next annealing step of the YBCO films was done by the reel-to-reel method with the gas flowed vertically down. From the x-ray diffraction analysis, the un-reacted phase like $BaF_2$ peak was found at the water partial pressure of 4.2%, but $BaF_2$ peak intensity is much reduced as the water partial pressure is increased. However, the higher water partial pressure of about 10% in this experiment leads to the poor crystallinity of YBCO films. The morphologies of the YBCO films were not different from each other when the water partial pressure was varied in this work. The maximum critical current density of 3.8MA/$cm^2$ was obtained at the water partial pressure of 6.2% with the annealing temperature of 780$^{\circ}C$ and oxygen partial pressure of 500ppm.
Wang, Mengjing,Kim, Jung Han,Han, Sang Sub,Je, Minyeong,Gil, Jaeyoung,Noh, Chanwoo,Ko, Tae-Jun,Lee, Kyu Seung,Son, Dong Ick,Bae, Tae-Sung,Ryu, Hyeon Ih,Oh, Kyu Hwan,Jung, YounJoon,Choi, Heechae,Chung, American Chemical Society 2019 The Journal of Physical Chemistry Part C Vol.123 No.45
<P>Benefiting from a large density of layer edges exposed on the surface, vertically aligned two-dimensional (2D) molybdenum disulfide (MoS<SUB>2</SUB>) layers have recently harvested excellent performances in the field of electrochemical catalysis and chemical sensing. With their increasing versatility for high-temperature, demanding applications, it is vital to identify their thermally driven structural and chemical stability, as well as to clarify its underlying principle. Despite various ex situ and in situ characterizations on horizontally aligned 2D MoS<SUB>2</SUB> layers, the direct in situ heating of vertically aligned 2D MoS<SUB>2</SUB> layers and the real-time observation of their near-atomic-scale dynamics have never been approached, leaving their thermal stability poorly understood. Moreover, the geometrical advantage of the surface-exposed vertically aligned 2D MoS<SUB>2</SUB> layers is anticipated to unveil the structural dynamics of interlayer van der Waals (vdW) gaps and its correlation with thermal energy, unattainable with 2D MoS<SUB>2</SUB> layers in any other geometry. Herein, we report a comprehensive in situ heating TEM study on cleanly transferred, vertically aligned 2D MoS<SUB>2</SUB> layers up to 1000 °C. Several striking phenomena were newly observed in the course of heating: (1) formation and propagation of voids between the domains of vertical 2D MoS<SUB>2</SUB> layers with distinct grain orientations starting at ∼875 °C; (2) subsequent decompositions of the 2D MoS<SUB>2</SUB> layers accompanying a formation of Mo nanoparticles at ∼950 °C, a temperature much lower than the melting temperature of their bulk counterpart; and (3) initiation of decomposition from the surface-exposed 2D layer vertical edge sites, congruently supported by molecular dynamics (MD) simulation. These new findings will offer critical insights into better understanding the thermodynamic principle that governs the structural stability of general vdW 2D crystals as well as providing useful technological guidance for materials design and optimization in their potential high-temperature applications.</P> [FIG OMISSION]</BR>
A Geometric Compression Method Using Dominant Points for Transmission to LEO Satellites
Ko, Kwang Hee,Ahn, Hyo-Sung,Wang, Semyung,Choi, Sujin,Jung, Okchul,Chung, Daewon,Park, Hyungjun The Korean Society for Aeronautical and Space Scie 2016 International Journal of Aeronautical and Space Sc Vol.17 No.4
In the operation of a low earth orbit satellite, a series of antenna commands are transmitted from a ground station to the satellite within a visibility window (i.e., the time period for which an antenna of the satellite is visible from the station) and executed to control the antenna. The window is a limited resource where all data transmission is carried out. Therefore, minimizing the transmission time for the antenna commands by reducing the data size is necessary in order to provide more time for the transmission of other data. In this paper, we propose a geometric compression method based on B-spline curve fitting using dominant points in order to compactly represent the antenna commands. We transform the problem of command size reduction into a geometric problem that is relatively easier to deal with. The command data are interpreted as points in a 2D space. The geometric properties of the data distribution are considered to determine the optimal parameters for a curve approximating the data with sufficient accuracy. Experimental results demonstrate that the proposed method is superior to conventional methods currently used in practice.
Fabrication and superconducting property of $MgB_2$ tape with Al metal powder addition
Ko, Jae-Woong,Yoo, Jai-Moo,Chung, Kuk-Chae,Kim, Young-Kuk,Wang, Xiaolin,Dou, Shi Xue,Yoo, Sang-Im,Chung, Woo-Hyun The Korean Society of Superconductivity and Cryoge 2007 한국초전도저온공학회논문지 Vol.9 No.2
The sub micron sized spherical $MgB_2$ powders were synthesized by spray reaction method. $MgB_2$ tapes with Al addition were fabricated by Powder in Tube (PIT) method. The superconducting property and microstructure of Al doped $MgB_2$ tapes were characterized by X-ray diffraction, optical microscopy and transport measurement under magnetic field. The $J_c$ value of $MgB_2$ tapes was increased with 10 vol. % Al addition. The $J_c$ value of 5,500 A/$cm^2$ and 11,000 A/$cm^2$ at 4.2 K and 5 T were obtained for the $MgB_2$ tape and 10 vol. % of Al added $MgB_2$ tape without heat treatment, respectively. The $J_c$ value of 8,000 A/$cm^2$ and 33,000 A/$cm^2$ at 4.2 K and 5 T were obtained for the $MgB_2$ tape and 10 vol. % of Al added $MgB_2$ tape with heat treatment, respectively. The $J_c$-B curves show enhancement in $J_c$ (B), which suggests that the microstructure and transport properties of $MgB_2$ tapes have been improved with Al addition.
Fabrication of $MgB_2$ tape with metal powder addition
Ko, Jae-Woong,Yoo, Jai-Moo,Kim, Young-Kuk,Chung, Kook-Chae,Yoo, Sang-Im,Wang, Xio Lin,Dou, Shi Xue The Korean Society of Superconductivity and Cryoge 2006 한국초전도저온공학회논문지 Vol.8 No.1
The $MgB_2$ tapes with several metal powder addition were fabricated by PIT method with or without heat treatment. The $J_c$ value of $5.600A/cm^2$ and $16.000A/cm^2$ at 4.2 K and 5 T were obtained for the $MgB_2$ tape and 10 vol % of Cu added $MgB_2$ tape without heat treatment respectively. The $J_c$ value of $8.000A/cm^2$ and $35,000A/cm^2$ at 4.2 K and 5 T were obtained for the $MgB_2$ tape and 10 vol. % of Al added $MgB_2$ tape with heat treatment, respectively. The $J_c-B$ curve shows enhancement in $J_c$ under magnetic field. which suggests enhancement in workability and grain connectivity with several metal powder addition.
Chung, Kook-Chae,Yoo, Jai-Moo,Ko, Jae-Woong,Kim, Young-Kuk,Wang, X.L.,Dou, S.X. 한국초전도저온학회 2006 한국초전도저온공학회논문지 Vol.8 No.1
Optimization of process variables, including oxygen and water partial pressure and also an nesting temperature, was performed in batch-type process to fabricate YBCO films on LaAlO3 single crystal. In this work, YBCO oxide powder was used as a starting precursor for metal-organic deposition(MOD)method. The precursor films were fabricated in batch furnace and they were converted to the epitaxial YBCO films at the same furnace with varying the process variables. The oxygen partial pressure was varied from 100ppm to 2000ppm and the water partial pressure from 1.2% to 12.2%. The window for optimal P(O2) was narrow about 700ppm for batch-type process. YBCO films in bathc-thype MOD process were optimized at 740-770oC and P(H2O) of 2.3%-7.3%.
A Geometric Compression Method Using Dominant Points for Transmission to LEO Satellites
Kwang Hee Ko,Hyo-Sung Ahn,Semyung Wang,Sujin Choi,Okchul Jung,Daewon Chung,Hyungjun Park 한국항공우주학회 2016 International Journal of Aeronautical and Space Sc Vol.17 No.4
In the operation of a low earth orbit satellite, a series of antenna commands are transmitted from a ground station to the satellite within a visibility window (i.e., the time period for which an antenna of the satellite is visible from the station) and executed to control the antenna. The window is a limited resource where all data transmission is carried out. Therefore, minimizing the transmission time for the antenna commands by reducing the data size is necessary in order to provide more time for the transmission of other data. In this paper, we propose a geometric compression method based on B-spline curve fitting using dominant points in order to compactly represent the antenna commands. We transform the problem of command size reduction into a geometric problem that is relatively easier to deal with. The command data are interpreted as points in a 2D space. The geometric properties of the data distribution are considered to determine the optimal parameters for a curve approximating the data with sufficient accuracy. Experimental results demonstrate that the proposed method is superior to conventional methods currently used in practice.
Design of steel and composite beams with web openings - Verification using finite element method
K. F. Chung,C. H. Ko,A. J. Wang 국제구조공학회 2005 Steel and Composite Structures, An International J Vol.5 No.3
This paper presents the findings of a design development project for perforated beams fully integrated with building services. A unified design approach for both steel and composite beams with large rectangular web openings is proposed which is based on plastic design methods and formulated in accordance with analytical structural design principles. Moreover, finite element models are established after careful calibration against test data, and comparison on the predicted ultimate loads of two composite beams with rectangular web openings from the finite element models and the proposed design method is also presented. It is demonstrated that the proposed design method is able to predict the ultimate loads of composite beams with rectangular web openings against ‘Vierendeel’ mechanism satisfactorily.