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Comparison of in-situ $MgB_2$ Superconducting Properties Under Different Annealing Environment
정국채,장세훈,김정호,Chung, K.C.,Sinha, B. B.,Chang, S.H.,Kim, J.H.,Dou, S. X. The Korean Superconductivity Society 2012 Progress in superconductivity Vol.14 No.2
Effect of mixed gas and additional Mg powder in an annealing process of the $MgB_2$ is investigated. Four different type of samples were prepared, each in different annealing environment of Ar, $Ar+4%H_2$, Ar with Mg powder and $Ar+4%H_2$ with Mg powder. Different annealing environment did not affect the electron-phonon interaction which is reflected from the same superconducting transition of 36.6 K for all samples. The reducing effect of hydrogen is clearly depicted from the presence of excess Mg in sample synthesized in $Ar+4%H_2$ gas implying the reduced rate of reaction between Mg and B. This has manifested itself in terms of slightly increased high-field critical current density of the sample. In contrast, the sample synthesized in $Ar+4%H_2$ with Mg powder, has shown overall enhancement in the superconducting properties as presented by higher diamagnetic saturation and critical current density.
CeO2 완충층의 결정성장 특성 및 금속 유기물 증착법으로 제조된 초전도 YBCO층에 미치는 영향
정국채 ( K. C. Chung ),김영국 ( Y. K. Kim ),X. L. Wang,S. X. Dou 대한금속·재료학회 2009 대한금속·재료학회지 Vol.47 No.10
CeO2 buffer layers have been deposited on YSZ single crystal substrates via a radio-frequency sputtering method. We focused on the texture development of CeO2 with out-of-plane alignment and its effects on a superconducting YBCO layer, which was deposited by metal organic deposition. CeO2 layers were grown epitaxially on single crystal YSZ substrates and subsequent YBCO layers were also grown epitaxially from CeO2 layers. It was observed that the intensity of CeO2(200) decreased with deposition temperature. θ-2θ scan FWHM values of CeO2(200) were inversely proportional to the peak intensities of CeO2(200). The sample with the lowest CeO2(200) intensity and poor out-of-plane alignment showed a strong reaction with the MODYBCO layer resulting in a thicker BaCeO3 layer. The texture and superconducting property of the YBCO layer were affected indirectly by the formation of a BaCeO3 layer at the interface between the CeO2 and YBCO layers.
Improved Critical current Density in $MgB_2$ by Graphene nano-platelets
정국채,장세훈,김정호,Sinha, B.B.,Chung, K.C.,Chang, S.H.,Kim, J.H.,Dou, S.X. 한국초전도학회 2012 Progress in superconductivity Vol.14 No.1
The effect of graphene inclusion in the ex-situ $MgB_2$ was analyzed with the help of resistivity behavior and critical current density studies. Amount of graphene was systematically varied from 0% for pristine sample to 3% by the weight of $MgB_2$. Graphene that is considered as a good source of carbon was found to be intact without any significant carbon doping in $MgB_2$ structure as reveled by XRD measurements. There was no signature of graphene inclusion as far as the superconducting transition is concerned which remained same at 39 K for all the samples. The transition width being sensitive to defect doping remained more or less about 2 K for all the samples showing no variation due to doping. Although there was no change in the superconducting transition or transition width, the graphene doped sample showed noticeable decrease in the overall resistivity behavior with respect to decrease in temperature. The graphene inclusion acted as effective pinning centers which have enhanced the upper critical field of these samples.
Mg과 B 혼합분말을 이용하여 분말압연 공정으로 제조된 $MgB_2$ 초전도 판재연구
정국채,장세훈,김정호,Chung, K.C.,Chang, S.H.,Sinha, B.B.,Kim, J.H.,Dou, S.X. 한국초전도학회 2012 Progress in superconductivity Vol.13 No.3
$MgB_2$ superconducting sheets have been fabricated by powder rolling method using mixture of Mg and B powders. Sheet-type $MgB_2$ bulk samples of ~10 mm width and 50-100 mm long were squeezed out after compacted by two rotating rolls of 130 mm diameter with gap distance of 0.5 mm and speed of ~40 cm/min (~1 rpm). The nominal composition of Mg, which is ductile metal, was added up to 30% to facilitate forming the $MgB_2$ sheets. The annealed samples at $900^{\circ}C$ and 3 hrs showed superconducting transition temperature of ~32 K and critical current densities at zero fields were ${\sim}10^5A/cm^2$ at 5 K and ${\sim}5{\times}10^4A/cm^2$ at 20 K.
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.
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.
SmBCO 초전도 선재 특성에 대한 Sm:Ba:Cu 조성비의 영향
김호섭,오상수,장세훈,민창훈,하홍수,하동우,고락길,염도준,문승현,정국채,Kim, H.S.,Oh, S.S.,Jang, S.H.,Min, C.H.,Ha, H.S.,Ha, D.W.,Ko, R.K.,Youm, D.J.,Moon, S.H.,Chung, K.C. 한국초전도학회 2011 Progress in superconductivity Vol.13 No.1
조성경사법을 이용하여 LMO/IBAD-MgO template 상에 EDDC 증착법을 이용하여 $Sm_{1+x}Ba_{2-x}Cu_{3+y}O_{7-d}$ 초전도 박막을 증착하였다. 테이프 형상의 기판상에 길이방향으로 $Sm_{1+x}Ba_{2-x}Cu_{3+y}O_{7-d}$ 박막의 조성비가 연속적으로 변하는 샘플을 얻을 수 있었고, 비접촉 Hallprobe를 이용하여 임계전류를 측정한 결과 조성비에 대응하는 연속적인 임계전류분포를 측정할 수 있었다. 본 조성비 영역 중 Sm:Ba:Cu = 1.01:1.99 :4.87의 조성비에서 최대의 임계전류를 보였다. 이 조성비에서 SmBCO 박막표면에 전체적으로 roof tile 형상의 결정립들이 관찰되었으며 국소적으로 2차상들이 관찰되었다. 조성 경사법을 이용하면 광범위한 조성비영역에서 신뢰도 높은 초전도특성, 결정배향성, 박막표면 형상 등을 조사할 수 있을 것으로 예상된다. The effects of Sm:Ba:Cu composition ratio in SmBCO coated conductor on their superconducting properties were investigated. The SmBCO coated conductors were fabricated by reactive co-evaporation method using EDDC(Evaporation using Drum in Dual Chamber) system. In this system, we could obtain various samples with different composition ratios in a batch by the technique providing composition gradient at deposition zone. From the specimens prepared by EDDC system, we found that composition ratio is uniform parallel to the drum axis, but gradient along the circumferential direction of the drum. We installed a shield having parallelogram open area between the deposition chamber and the evaporation chamber in EDDC system, and attached a 30 cm long template, which is parallel to drum axis, onto the drum surface. In this configuration, we could obtain SmBCO coated conductors having a gradient composition along the length of template. We measured the composition ratios and surface morphologies with periodic interval by SEM and EDAX, and confirmed the profile of composition ratio. We also measured critical current using non-contact Hall probe critical current measurement system and thereby could plot composition ratio vs. critical current. The maximum critical current was obtained, and the surface morphology with the shape of roof tile was observed at the corresponding composition ratio of Sm:Ba:Cu = 1.01:1.99:4.87. It was also found that composition ratio had an effect on not only critical current but also surface morphology.