• Progress in Superconductivity
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• v.6 no.1
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• pp.19-23
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• 2004

YBCO do superconducting quantum interference device (SQUID) magnetometers based on bicrystal Josephson junctions on 10 mm ${\times}$ 10 mm $SrTiO_3$ substrates have been fabricated. The pickup coil of the device was designed to have 16 parallel loops with 50-fm-wide lines. We could obtain optimised direct coupled YBCO SQUID magnetometer design with field sensitivity $B_{N}$ $\Phi$/ of $4.5 nT/\Phi_{0}$ and magnetic field noise $B_{N}$ of about $22 fT/Hz^{1}$2/ with an I/f corner frequency of 2 Hz measured inside a magnetically shielded room. Preliminary results of magnetocardiograph measurement using the HTS SQUID magnetometers show signal to noise ratio of about 110, which is comparable to the quality of a commercial MCG system based on Nb-SQUIDs.

• Progress in Superconductivity
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• v.1 no.1
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• pp.20-25
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• 1999

We constructed a high-$T_c$ scanning SQUID microscope (SSM) operating in the liquid nitrogen. We used a washer-type YBCO SQUID with inner and outer dimensions of $12{\mu}m$ and $36{\mu}m$, respectively, which was grown on the $SrTiO^3$ bicrystal substrate. The sample, rather than SQUID, was scanned using two stepping motors. We also developed readout electronics, stepping motor controller, and the software for system control and data display. We took images of various samples using our SSM and found that the spatial resolution is about $40{\mu}m$ and noise level is lower than $10^{-7}T/{\surd}Hz$ at 100 Hz and higher at lower frequencies. The noise level was much higher than that of a typical SQUID due to the other coupling from the electric parts. We present a simple argument on the inductive coupling between the sample and the SQUID which should be under-stood for the proper interpretation of the obtained images. By comparing the measured data with the simulation results the gap between the SQUID and the sample is estimated to be $40{\mu}m$.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.33 no.1
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• pp.60-68
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• 2009

The steam reformer for hydrogen production from methane is studied by a numerical method. Langmuir- Hinshelwood model is incorporated for catalytic surface reactions, and the pseudo-homogeneous model is used to take into account local equilibrium phenomena between a catalyst and bulk gas. Dominant chemical reactions are Steam Reforming (SR) reaction, Water-Gas Shift (WGS) reaction, and Direct Steam Reforming (DSR) reaction. The numerical results are validated with experimental results at the same operating conditions. Using the validated code, parametric study has been numerically performed in view of the steam reformer performance. As increasing a wall temperature, the fuel conversion increases due to the high heat transfer rate. When Steam to Carbon Ratio (SCR) increases, the concentration of carbon monoxide decreases since WGS reaction becomes more active. When increasing Gas Hourly Space Velocity (GHSV), the fuel conversion decreases due to the heat transfer limitation and the low residence time. The reactor shape effects are also investigated. The length and radius of cylindrical reactors are changed at the same catalyst volume. The longer steam reformer is, the better steam reformer performs. However, system energy efficiency decreases due to the large pressure drop.

• Proceedings of the Materials Research Society of Korea Conference
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• 1994.11a
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• pp.118-118
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• 1994

Processings of the P $b_2$S $r_2$( $Y_{1-x}$ C $a_{x}$) C $u_3$ $O_{8+{\delta}}$ (2213)system for x=0.4-0.6 to control deleterious oxidative decomposition have been studied. Our results show that comounds are stable at both low p $O_2$ and high p $O_2$ if they are suitably oxidized. Various oxidation and deoxidation procedures have been investigated in order tp determine the optimum hole concentration in the Cu $O_2$layers for the maximum $T_{c}$. In cases x=0.5 and x=0.6, the optimum hole concentration in the 2213-phase is achieved, but with accelerated oxidative decomposition. Destite this, the maximum $T_{c}$~80-83K for the 2213-phase can be deduced when x=0.5 to 0.6 to 0.6

• Progress in Superconductivity and Cryogenics
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• v.7 no.1
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• pp.13-16
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• 2005

High temperature superconductor, $\YBa_2Cu_3O_{7-x}$ (YBCO) and ferroelectric, $\Ba_{0.1}Sr_{0.9}TiO_{3}$ (BST) multilayer thin films were deposited using on MgO(100) substrates pulsed laser deposition. The thin films exhibited only (001) peaks of YBCO and 1357 The HTS thin films demonstrated excellent zero resistance temperature of 92.5 K. We designed and fabricated HTS ferroelectric phase shifter using high frequency system simulator and standard photolithography method, respectively The HTS phase shifter shows a low insertion loss (2.97 dB) and large phase change ($\162^{circ}$) with 40 V do bias at 10 GHz. The HTS phase shifter shows 54 of figure of merit. These results can be applicable to phased anay antenna system for satellite communication services.

• Publications of The Korean Astronomical Society
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• v.8 no.1
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• pp.243-263
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• 1993

One hundred forty two digital spectra of 110 standard stars with 1 A resolution in ${\lambda}{\lambda}\;3000\;A\;{\sim}4150\;A$ region were analyzed to determine the spectral indices sensitive to the atmospheric parameters. The standard stars cover the temperature in the range of $T_eff$ from ${\sim}4000^{\circ}$ to ${\sim}7000^{\circ}\;K$ and the surface gravity in the range of log g from 0 to 5 and the metal abundance [Fe/H] from -2.7 to 0.4. We have derived physical parameters, theta(=T/5040), log g, and [Fe/H] for the standard stars using indices, P(38/93), Fe I(A), Fe I(B), SrII, and $H{\delta}$, with overall accuracies of 0.067, 0.617, and 0.466, respectively. However for some region of spectral type and metallicity, those can be obtained with better accuracies. Those indices are found to be useful especially for stellar gravity determination.

• Horticultural Science & Technology
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• v.17 no.4
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• pp.473-475
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• 1999

This study was carried out to restore the fertility by suppression of male sterility-induced gene using an antisense construct. Tobacco (cv. Petit Havana SR1) was transformed with the binary vector containing a GBAN215-6 promoter, an antisense diphtheria toxin (DTx-A) gene (pKDA215b) and a hygromycin resistant gene. Seventy-six confirmed transgenic plants regenerated from leaf disks were designated as the $R_0$ generation and selfed to produce the $R_1$ generation. From the inheritance study, five $R_1$ lines with multiple copies of the antisense construct were selected and selfed to identify homozygosity for the antisense construct. In order to restore fertility and finally to select restore lines, five $R_2$ lines with multiple copies of the antisense construct were crossed with male sterile plants. From these crosses, three different phenotypes have been observed: completely restored, partially restored, and not restored pollens, and otherwise tobacco plants were phenotypically same as normal plants. These plants were scored for the degree of restoration and selected for further study.

• Proceedings of the Korean Vacuum Society Conference
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• 2000.02a
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• pp.69-69
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• 2000

(Ba, Sr)TiO3 (BST)[1-3] 박막은 유전상수가 크고 고주파에서도 유전특성 저하가 적기 때문에 ULSI DRAM(Dynamic Random Access Memory)에 응용 가능한 물질로 최근 각광을 받고 있다. 하지만, 아직 BST 박막을 DRSM에 바로 적용하기 위해선 몇 가지 문제점이 있다. 그 중 누설전류 문제는 디바이스 응용시 매우 중요한 요소이다. 특히, DRAM에서 refresh time와 직접적인 관련이 있어 디바이스 내의 신뢰도 및 전력소모를 결정하는 주된 인자가 된다. 지금까지, BST 박막의 인가전업, 온도, 그리고 전극물질에 따른 누설전류 현상들이 고찰되었고, 이에 관한 많은 전도기구 모델들이 제시되었다. Schottky emission, Poole-Frenkel emission, space charge limited conduction 등이 그 대표적인 예이다. 하지만 아쉽게도 BST 박막의 정확한 누설 전류 전도 기구를 완전히 설명하는데는 아직 한계가 있다. 따라서 본 연구에서는 제작된 BST 커패시터 내의 기본적인 전기적 성질을 조사하고, 정확한 누설전류 기구 규명에 초점을 두고자 한다. 이를 위해 기존의 여러 기구들과 비교 분석할 것이다. 하부전극으로 사용하기 위해 스퍼터링 방법으로 p-Si(100) 기판위에 RuO2 박막을 약 120nm 증착하였다. 증착전의 chamberso의 초기압력은 5$\times$10-6 Torr이하의 압력으로 유지시켰다. Ar/O2의 비는 이전 실험에서 최적화된 9/1로 하였다. BST 박막 증착 시 5분간 pre-sputtering을 실시한 후 하부전극 기판위에 BST 박막을 증착하였다. 증착이 끝난 후 시편을 상온까지 냉각시킨 후 꺼내었다. 전기적 특성을 측정하기 상부전극으로 RuO2와 Al 박막을 각각 상온에서 100nm 증착하였다. 이때 hole mask를 이용하여 반경이 140um인 원형의 상부전극을 증착하였다. BST 박막의 증착온도가 증가하고 Ar/O2 비가 감소할수록 제작된 BST-커패시터의 전기적 성질이 우수하였다. 증착온도 $600^{\circ}C$, ASr/O2=5/5에서 증착된 막의 누설전류는 4.56$\times$10-8 A/cm2, 유전상수는 600 정도의 값을 나타내었다. 인가전압에 따른 BST 커패시터의 transition-current는 Curie-von Schweider 모델을 따랐다. BST 박막의 누설전류 전도기구는 기존의 Schottky 모델이 아니라 modified-Schottky 무델로 잘 설명되었다. Modified-Schottky 모델을 통해 BST 박막의 광학적 유전율 $\varepsilon$$\infty$=4.9, 이동도 $\mu$=0.019 cm2/V-s, 장벽 높이 $\psi$b=0.79 eV를 구하였다.

• Proceedings of the Korean Magnestics Society Conference
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• 2012.05a
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• pp.121-122
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• 2012

Iron-excess gallium ferrite, $Ga_{0.6}Fe_{1.4}O_3$ (GFO), is known to have room-temperature ferromagnetic phases and potentially exhibit ferroelectricity as well [1]. But, leaky polarization-electric field (PE) hysteresis curves of the GFO thin film are hurdle to prove its spontaneous polarization, in other words, ferroelecticity. One of the reasons that the GFO films have leaky PE hysteresis loop is carrier hopping between $Fe^{2+}$ and $Fe^{3+}$ sites due to oxygen deficiency. We focus on reducing conducting current by substituting divalent cations at $Fe^{2+}$ sites. GFO thin films were grown epitaxially along b-axis normal to $SrRuO_3/SrTiO_3$ (111) substrates by pulsed laser deposition. Current density of the ion-substituted GFO thin films was reduced by $10^3$ or more. Ferroelectric properties of the ion-substituted GFO thin films were measured using macroscopic and microscopic schemes. In particular, local ferroelectric properties of the GFO thin films were exhibited and their remnant polarization and piezoelectric d33 coefficient were obtained.

• Korean Journal of Materials Research
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• v.32 no.2
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• pp.66-71
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• 2022

In this article, Pb₂Ba_1.7Sr_0.3Ca₂Cu₃O_10+δ superconductor material was synthesized using conventional solid-state reaction method. X-ray diffraction (XRD) analysis demonstrated one dominant phase 2223 and some impurities in the product powder. The strongest peaks in the XRD pattern were successfully indexed assuming a pseudo-tetragonal cell with lattice constants of a = 3.732, b = 3.733 and c = 14.75 Å for a Pb-Based compound. The crystallite size and lattice strain between the layers of the studied compound were estimated using several methods, namely the Scherrer, Williamson-Hall (W.H), size-strain plot (SSP) and Halder Wagner (H.W) approach. The values of crystallite size, calculated by Scherrer, W.H, SSP and H.W methods, were 89.4540774, 86.658638, 87.7555823 and 85.470086 Å, respectively. Moreover, the lattice strain values obtained by W.H, SSP and H.W methods were 0.0063240, 0.006325 and 0.006, respectively. It was noted that all crystallite size results are consistent; however, the best method is the size-strain plot because it gave a value of R² approaching one. Furthermore, degree of crystallites was calculated and found to be 59.003321%. Resistivity analysis suggests zero-resistance, which is typical of superconducting materials at critical temperature. Four-probe technique was utilized to measure the critical temperature at onset T_c(onset), zero resistivity T_{c(off set)}, and transition (width ΔT), corresponding to temperatures of 128 K, 116 K, and 12 K, respectively.