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F-free Y & Cu 전구용액 적용에 의한 YBCO coated conductors의 MOD 공정 개선
김영국,유재무,정국채,고재웅,조윤숙,허은옥,Kim, Y.K.,Yoo, J.M.,Chung, K.C.,Ko, J.W.,Cho, Y.S.,Heo, E.O. 한국초전도학회 2006 Progress in superconductivity Vol.8 No.1
Total Fluorine content in the precursor solution for MOD processing of YBCO coated conductors can be significantly reduced by synthesizing precursor solution with F-free Y & Cu precursor and Barium trifluoroacetate(TFA). It was shown that crack-free and uniform precursor films were formed after calcinations in humidified oxygen atmosphere. Less than 2 hours are required to finish the calcination process, and XRD measurement shows that $BaF_2,\;CuO,\;and\;Y_2O_3$ are major constituent of calcined precursor films. Film thickness after calcination was measured to be ${\sim}2.8$ um by applying slot-die coating method. In particular, addition of Samarium shows critical current of Ic=226 A/cm-w($Jc=3.4\;MA/cm^2$). Also discussed are recent developments in the reel-to-reel processing using F-free Y & Cu precursor solutions. It is shown that uniform and fast processing route to YBCO coated conductor with high Ic can be provided by employing F-free Y & Cu precursor solutions in MOD process.
분무 열분해법으로 제조된 미세 Bi-2223 전구분말의 특성
김성환,유재무,고재웅,김영국,Kim S. H.,Yoo J. M.,Ko J. W.,Kim Y. K. 한국초전도학회 2005 Progress in superconductivity Vol.6 No.2
Homogeneous and fine powders for Bi-2223 tape were prepared by ultrasonic spray pyrolysis (SP) method from an aqueous solution of metal nitrates. Bi-2223 precursor powders were synthesized with various solutes concentration and pyrolysis temperature. The synthesized precursor powders had a narrow particle size distribution and an average particle size was $\~{\cal}um$. The reactivity of precursor powder by SP method is very high, attributed to the fine and narrow particle size distribution. Bi-2223/Ag tape was prepared using PIT method and followed by various sintering conditions. The precursor powder by SP method promoted a very quick formation of the Bi-2223 phase for short sintering time while the secondary phase such as large AEC phase and $Ca_2PbO_4$ were minimized for SP tapes.
화학적으로 변형된 전구용액을 이용한 YBCO 박막 제조
김영국,유재무,정국채,고재웅,김영준,한봉수,Kim, Y.K.,Yoo, J.M.,Chung, K.C.,Ko, J.W.,Kim, Y.J.,Han, B.S. 한국초전도학회 2006 Progress in superconductivity Vol.7 No.2
Superconducting YBCO films were successfully fabricated by MOD process using chemically modified precursor solution. In this study, a chemically modified precursor solution for MOD processing was synthesized using metal-organic salts and organic additives. It was shown that crack-free and uniform precursor films were formed after calcination in humidified Oxygen atmosphere. Less than 3 hours are required to finish the calcination process. XRD measurement shows that $BaF_2,\;CuO,\;Y_2O_3$ are major constituent of precursor films. Furthermore, YBCO films without any secondary phases were successfully fabricated after annealing in wet $Ar/O_2$ atmosphere. The YBCO film prepared on a $LaAlO_3$ single crystal substrate ($10mm{\times}10mm$) gives transport $I_c$ of 10A at 77K. This chemical modification approach is a possible candidate for improving MOD-processing of YBCO coated conductor.
IBAD 기판을 적용한 산화물 전구체 MOD 공정에 의한 YBCO Coated Conductor의 제조
김영국,유재무,고재웅,허순영,홍계원,이희균,정형식,Kim Y. K.,Yoo J. M.,Ko J. W.,Heo S. Y.,Hong G. W.,Lee H. G.,Chung H. S. 한국초전도학회 2005 Progress in superconductivity Vol.6 No.2
YBCO coated conductors have been fabricated with a newly developed oxide-precursor-based MOD process. The precursor solution was synthesized with low cost YBCO powders as starting materials and YBCO coated conductors have been deposited on IBAD substrate ($CeO_2$/IBAD-YSZ/SS). YBCO coated conductor prepared by dip coating shows transport $I_c$ of 15A/cm-w at 77 K. Microstructural and crystallographic analyses indicate that YBCO layer was grown in epitaxy with substrate and exhibited well-developed dense microstructure. Also discussed are processing and analysis of YBCO coated conductor by slot-die web coating method. It was shown that this oxide-based MOD process provides a low cost route to coated conductor with high $J_c$.
이영선(Y. S. Lee),권용남(Y. N. Kwon),김상우(S. W. Kim),유재무(J. M. Yoo) 한국소성가공학회 2013 한국소성가공학회 학술대회 논문집 Vol.2013 No.5
Reducing friction between tool and workpiece is very important to enhance the forming limit on both sheet and bulk forming. Various kinds of lubricant and coating are used to reduce the friction coefficient on metal forming. Friction has to be reduced because sheet forming of Mg alloy sheet is usually performed on warm temperature. Various lubricants are almost used on metal forming process. However, it is difficult to use the lubricant on multi-stage sheet forming process, especially, on warm temperature. Therefore, tool coating is better than lubricant on Mg ally sheet warm forming. The importance of friction effect has been studied through real formed part of AZ31 sheet according to the different lubricant. Two different non-metallic coating layers are compared with no-coating tool by pin-on-disc friction testing. Friction coefficients for different coating layers are measured. The materials of pin and disc are SKD61 and AZ31, respectively. The coefficients are measured at room temperature, 170 and 250℃. Until now, tool coating for Mg warm forming is not effective and coulomb friction coefficient is 0.4 in ambient.