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RF-스퍼터링법을 이용하여 Ni-W 금속기판에 연속공정으로 증착된 Y2O3 완충층 특성 연구
정국채,정태정,최규채,김영국,X.L.Wang,S.X.Dou 한국초전도학회 2010 Progress in superconductivity Vol.11 No.2
Reel-to-reel deposition of Y2O3 has been performed on Ni-5%W metal substrates using the RF-sputtering method. The epitaxial orientation of Y2O3 buffer layers to the base bi-axially textured substrate was well identified using θ-2θ,out-of-plane (ω), and in-plane (φ) scans in X-ray diffraction analysis. The optimization of Y2O3 seed layers in reel-to-reel fashion were investigated varying the deposition temperature, sputtering power, and pressure for its significant roles for the following buffer stacks and superconducting layers. Y2O3 were all grown epitaxially on bi-axially textured metal substrates at 380 watts and 5 mTorr in the temperature range of 600-740 oC with higher Y2O3 (400) intensities at ~710 oC. It was found that the Δω values were 1-2o lower but the Δφ values were above 1o higher than that of Ni-W substrates. As the sputtering power increased from 340 to 380 watts, Δω and Δφ values showed decreased tendency. Even in the small window of deposition pressure of 3-7 mTorr, the Y2O3 (400) intensities increased and Δω and Δφ values were reduced as sputtering pressure increased.
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.
정국채,유재무,고재웅,김영국,Xiaolin Wang,Shi Xue Dou 한국초전도.저온공학회 2007 한국초전도저온공학회논문지 Vol.9 No.2
The influence of water partial pressure inMetal-organic Deposition (MOD) method was investigated on the texture and the morphology of YBa2Cu3O7-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 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 BaF2 peak was found at the water partial pressure of 4.2%, but BaF2 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 2 was obtained at the water partial pressure of 6.2% with the annealing temperature of 780oC and oxygen partial pressure of 500ppm.
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.