Hee-Gyoun Lee

Korea Polytechnic University, Sŏul, Seoul, South Korea

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Publications (50)53.55 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: NiO films have been epitaxially grown by metal-organic chemical vapor deposition (MOCVD) on a bi-axially textured Ni substrate using Ni(thd)2 as a precursor. The NiO film was deposited at 470°C for 10 min at a deposition pressure of 10 Torr and oxygen partial pressure of 0.91 Torr. SEM and AFM observations for the deposited NiO film showed a smooth and dense morphology. X-ray rocking curve and φ-scan showed that the NiO film has a bi-axial texture with a (100)<001> orientation. The out-of-plane and the in-plane deviations were measured to be 4.2° and 6~7° from the FWHM of (200) and (111) planes, respectively.
    Metals and Materials International 04/2012; 6(6):565-570. · 1.43 Impact Factor
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    ABSTRACT: Coated conductor by MOCVD shows the best Ic × L performance currently, but cost reduction is still ongoing issue. R&D effort for a process capable of utilizing cheap alternative precursors were tried by many research groups but few of them showed results having potential for replacing current MOCVD. Spray pyrolysis method adopting ultrasonic atomization was tried as one of the possible options. Y123 films have been deposited on LaAlO<sub>3</sub> (100) single-crystal and IBAD substrates by aerosol assisted spray pyrolysis method. Ultrasonic atomization was used in order to generate fine droplets of precursor solution made of Y, Ba, Cu nitrate. A pre-heater was located between spraying nozzle and substrate for fast drying and enhancing decomposition of precursors. SEM and XRD observation revealed that deposited films have smooth and dense microstructure. The influence of operating parameters such as cation stoichiometry, oxygen partial pressure, substrate temperature on the microstructure, formation of superconducting 123 phases and superconducting properties of deposited films were tested. Ex-situ conversion was tried to decrease the possible reactions between precursor compounds and buffer layer materials of metal substrate showed the possibility of adopting this technique for epitaxial growth of 123 phase on metal substrate.
    IEEE Transactions on Applied Superconductivity 07/2011; · 1.20 Impact Factor
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    ABSTRACT: It is well known that high temperature superconductors (HTS) are promising for a magnet application, but it has been almost impossible so far to apply them to the magnet in persistent current mode because of poor qualities of their superconducting joint at higher current as well as lower index values. In this paper, in order to get rid of the joints from an HTS superconducting magnet, a prototype HTS pancake coil using YBCO coated conductors by the technique of wind-and-flip pancake coil was made and will be tested later. This paper presents a design, fabrication, and thermal analysis of the prototype pancake coil. The coil will be cooled down around 20 K by conduction cooling using a GM cryocooler. The expected current drift rate for an MRI application has not been achieved, probably caused by large flux flow resistance of coated conductor at higher temperature, 77.3 K. A temperature dependency of temporal stability of the persistent current will be measured by monitoring the long-term drift of the magnetic field at different operating temperatures by Hall sensors.
    IEEE Transactions on Applied Superconductivity 07/2010; · 1.20 Impact Factor
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    ABSTRACT: It is well known that HTS (high temperature superconductor) is promising for magnet applications, but it has been almost impossible so far to apply them to magnets in persistent current mode because of poor qualities of their superconducting joints at higher current as well as their lower index values. To get rid of the non-superconducting joints from an HTS superconducting magnet, a wind-and-flip technique for demonstrating a persistent current mode operation of HTS magnet using YBCO coated conductors had been suggested. This paper presents experimental results of measured drift of a persistent current in the YBCO pancake coil which was made by using the wind-and-flip technique. The pancake coil was wound by YBCO coated conductor with a width of 12 mm, which was slit longitudinally at the center of the conductor along the conductor by a specially designed slitter. A stainless steel heater was attached on the one end of the YBCO conductor as a persistent current switch to energize the pancake coil. A temporal stability of the magnetic field was analyzed by monitoring the long-term drift of the magnetic field at 77.3 K by Hall sensors.
    IEEE Transactions on Applied Superconductivity 07/2009; · 1.20 Impact Factor
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    ABSTRACT: The effect of doping on the critical current density of YBCO film by TFA-MOD method was studied. doping was made by two method; one is direct addition of nano-sized powder prepared by citrate process followed by grinding with planetary ball mill for 10 hours. Another is addition of Ba-Ce precursor solution prepared with Ba-acetate and Ce acetate dissolved in TFA to the YBCO-TFA precursor solution. The film was made by standard dip coating and heat treatment process with conversion temperature of in 1000 ppm oxygen containing moisturized Ar gas atmosphere. The direct addition of powder resulted in YBCO film with good epitaxial growth and no evidence of second phase formation. The addition through precursor solution resulted in the increase of critical current density upto 30 at% doping and uniform dispersion of fine inclusion was confirmed by SEM-EDX.
    Progress in Superconductivity and Cryogenics. 01/2008; 10(1).
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    ABSTRACT: YBCO films were deposited on a moving substrate by a spray pyrolysis method using nitrate aqueous solution as a precursor. Deposition was done on LaAlO3(100) single crystal substrate by spraying the precursor mist generated by a concentric nozzle. The cation ratio of the precursor solution was Y:Ba:Cu = 1:2.65:4.5. The distance between the nozzle and the substrate was 15 cm. The substrate was transported with a speed ranging from 0.23 to 0.5 cm min−1. Films were deposited at a pressure of 10 and 20 Torr and the substrate temperature varied from 740 to 790 °C. The oxygen partial pressure was controlled between 1 and 5 Torr. Scanning electron microscope (SEM) and x-ray diffraction (XRD) observations revealed that the films are smooth and highly textured with (00l) planes parallel to the substrate. The highest Jc was 1.04 MA cm−2 at 77 K and self-field for film with a thickness of 0.5 µm.
    Superconductor Science and Technology 08/2007; 20(10):950. · 2.76 Impact Factor
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    ABSTRACT: The fluorine-free dichloroacetic-metalorganic deposition (DCA-MOD) method has been optimized and high-Jc YBa2Cu3Ox (YBCO) superconducting films over 1 MA cm−2 have been obtained. The effects of using a dilution solvent for the preparation of coating solutions were investigated for methanol and 2-methoxyethanol, respectively. The uniformity of the film thickness was improved by the use of 2-methoxyethanol. The two-step heat treatment for the preparation of the YBCO film led to a significant densification of the microstructure. Coated films were calcined at lower temperatures of up to 500 °C in a flowing oxygen atmosphere with a humidity of 7.2%. Conversion heat treatment was performed at various temperatures of 780–810 °C for 2 h in flowing Ar gas containing 1000 ppm oxygen with a humidity of 9.45%. SEM observations showed that the films have very dense microstructures with little pores. X-ray diffraction analysis showed that YBCO grains grew with a (00l) preferred orientation. A high critical current density (Jc) of 1.28 MA cm−2 (at 77 K and self-field) was obtained for the YBCO film prepared by the two-step heat treatment.
    Superconductor Science and Technology 03/2007; 20(5):428. · 2.76 Impact Factor
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    ABSTRACT: Persistent mode HTS pancake coil has been fabricated using a coated conductor by a "wind-and-flip" method. A coated conductor with the length of 1.2 meters was divided at the center along the length. The sliced coated conductor was wound on a pair of bobbins with a diameter of around 4 cm and two pancake coils connected superconductively without a resistive joint were prepared. By flipping one of the pancake coils, the magnetic field generated by each coil is to be aligned to the same direction and generate meaningful magnetic field while the magnetic fields of two spit coils are canceled without flipping. Permanent current was induced by flowing current to the coil immersed in liquid nitrogen pool using a power supply. A magnetic field of 48.8 Gauss was generated when 20 A of current was flowing in the pancake coils. The "Wind and flip" method can be applied for the fabrication of a long solenoid magnet by winding a sliced coated conductor on a cylindrical bobbin. It is also introduced that the construction of multiple sets of pancake (or solenoid) coils is possible by a "wind-and-flip" method using a wide coated conductor.
    Progress in Superconductivity and Cryogenics. 01/2007; 9(2).
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    ABSTRACT: We fabricated YBCO film using a new approach to the TFA-MOD method. In the fabrication process, Y2Ba1Cu1Ox and Ba3Cu5O8 powders were used as precursors (the so called '211 process'), instead of Y-, Ba-, and Cu-based acetates, and dissolved in trifluoroacetic acid followed by calcining and firing heat treatment. Consequently, we successfully synthesized YBCO film and evaluated the phase formation, texture evolution, and critical properties as a function of the calcining and firing temperature and humidity, in order to explore its possible application in coated conductor fabrication. The films were calcined at 430–460 °C and then fired at 750–800 °C in a 0–20% humidified Ar–O2 atmosphere. We observed that the amount of BaF2 phase was effectively reduced and that a sharp and strong biaxial texture formed under a humidified atmosphere, leading to increased critical properties. In addition, we found that the microstructure varied significantly with the firing temperature; the grains grew further, the film became denser, and the degree of texture and phase purity varied as the firing temperature increased. For the film fired at 775 °C after calcining at 460 °C, the critical current was found to be 39 A cm−1 width (the corresponding critical current density is 2.0 MA cm−2), which was probably attributable to such factors as the enhanced phase purity and out-of-plane texture, the moderate film density and grain size, and the crack-free surface.
    Superconductor Science and Technology 02/2006; 19(4):306. · 2.76 Impact Factor
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    ABSTRACT: YBa2Cu3Oy superconducting films were deposited on LaAlO3(1 0 0) single crystal substrates by spray pyrolysis method. Two types of ultrasonic and concentric nebulizer were used in order to generate fine droplets of metal-inorganic precursor solution. c-Axis oriented films were obtained at deposition temperature of 750–850 °C and at working pressures of 100 Torr and 500 Torr. In case of ultrasonic nebulizer, the films showed rough surface morphology due to the presence of enormous droplets, whereas smooth and dense films were obtained for concentric nebulizer. The good c-axis oriented YBCO films were formed at the wide range of the oxygen partial pressure. Oxygen which is generated via the decomposition of nitrate precursors is considered to participate in the formation reaction of YBCO film. Microstructures of YBCO films varied depending on oxygen partial pressure and rod-like grains were appeared when the oxygen partial pressure was lower than 30 Torr. YBCO films were deposited epitaxially on LAO(1 0 0) substrate. Δϕ of in-plane and Δω of out-of-plane texture were measured as 3.3° and 1.0°, respectively. A transport Jc value of 0.50 MA/cm2 at 77 K and self-field was achieved for the YBCO film deposited on LaAlO3(1 0 0) single crystal substrate.
    Physica C Superconductivity 01/2006; · 0.72 Impact Factor
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    ABSTRACT: Surface oxidation behavior of cube-textured polycrystalline nickel was investigated at various oxidation conditions. Cube-textured nickel substrate, which was prepared by rolling and texture anneal, was used. Cube-textured NiO film was formed on a cube-textured polycrystalline nickel regardless of oxidation conditions but different growth behavior of NiO crystals was observed depending on the oxidation conditions. The introduction of water vapor in O2 did not affect on the texture evolution but rough and porous microstructure was developed. Microstructure of NiO film tends to be denser as the oxygen partial pressure is increased. It is interesting that (1 1 1) oriented crystal grew favorably in the substrate oxidized in air atmosphere while (2 0 0) plane was major in the substrate oxidized in high purity argon gas. Other crystallographic planes than (2 0 0) plane were developed when N2O was used as an oxidant while only (2 0 0) plane crystal was formed in dry O2 atmosphere. The morphology and texture of NiO crystal were changed from smooth one into faceted one when oxidation was performed at 900 °C in dry O2 gas followed by the reduction under a reducing atmosphere. It is thought that absorption behavior of oxygen or OH varies depending on the oxidizing species.
    Physica C: Superconductivity. 01/2006;
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    ABSTRACT: Coated conductors suitable for the fabrication of permanent mode high Tc magnets are suggested and the fabrication method of permanent mode magnets using coated conductor are demonstrated. Permanent current was observed in a small piece of sliced coated conductor. Closed loop of coated conductor with a diameter of around 1cm was successfully prepared and was cooled with a magnetic field of about 500G in order to induce supercurrent. Coated conductor with an Ic of 100A/cm-width was used for the preparation of closed loop of coated conductor. Permanent current was confirmed by measuring the magnetic field generated from closed loop of coated conductor by using Gauss meter. Magnetic field of 4.4G was detected from the supercurrent of closed loop of coated conductor. It shows that superconducting joint of coated conductor is not a prerequisite for the construction of permanent mode magnets. It is thought that this work opens the possibility to use coated conductor for the construction of permanent mode high Tc magnets for MRI, NMR and magnetic separation applications.
    Physica C-superconductivity and Its Applications - PHYSICA C. 01/2006; 445:1099-1102.
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    ABSTRACT: We fabricated YBCO films on LAO substrates using the TFA-MOD method and evaluated the effects of the heat treatment temperature on the microstructure, degree of texture, and critical properties. The phase formation and microstructure were characterized by X-ray diffraction, Raman spectroscopy and scanning electron microscopy (SEM) and the degree of texture was evaluated by pole-figure analysis.The firing was performed in the temperature range of 750–800°C and we found that the phase purity, grain size and orientation, degree of texture, and oxygen content varied with the firing temperature. The films fired at 775°C showed the highest critical temperature (TC-onset) of 89.5K and critical current (IC) of 40A/cm-width, which corresponds to a critical current density (JC) of 1.8MA/cm2. According to the results of the XRD, pole-figure, SEM and Raman analyses, these optimum critical properties can probably be attributed to the formation of a pure YBCO phase, stronger c-axis orientation and higher oxygen content.
    Physica C Superconductivity 01/2006; 445:590-593. · 0.72 Impact Factor
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    ABSTRACT: Superconductive YBCO thin films have been prepared by trifluoroacetate (TFA) metalorganic deposition (MOD) method. Non-stoichiometric precursor solutions with cation ratios of Y:Ba:Cu = 1 + x:2:3 (x = 0, 0.05, 0.1, 0.15 and 0.2) have been prepared by dissolving high purity Y2O3, BaCO3 and CuO powders into trifluoroacetate. Coated film was calcined at lower temperature in a moisture-containing oxygen atmosphere. Superconducting YBCO films have been obtained by conducting a conversion heat treatment at higher temperature in a moisture-containing Ar (1000 ppm oxygen) atmosphere. Critical current density (Jc) of YBCO film was increased with the amount of excess yttrium in YBCO film and revealed maximum Jc for x = 0.15 and decreased again for x = 0.2. Four times higher Jc was obtained for the film with x = 0.15 compared to that of the film prepared from stoichiometric solution of Y:Ba:Cu = 1:2:3. It is considered that the densification of microstructure and absence of a-axis grains attributed to the enhancement of Jc in the film made of non-stoichiometric solution.
    Physica C Superconductivity 01/2006; · 0.72 Impact Factor
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    ABSTRACT: High quality REBa2Cu3Ox (RE = Y, Eu, Gd) films have been successfully fabricated by metalorganic deposition method using dichloroacetic acid. Metal acetates or oxide powders were dissolved in dichloroacetic acid and blue-colored get was obtained by drying. Coating solution was prepared by diluting blue gel in methanol. Coating solution was coated on single crystal (0 0 1) LaAlO3(LAO) by dip coating method. Processing parameters such as oxygen partial pressure, water vapor, ramping rate and pyrolysis temperature etc were controlled in order to obtain a good epitaxial film. 0.5 μm-thick film was obtained by single coating and with no crack after calcination heat treatment. Oxygen partial pressure was varied in the range of 100–1000 ppm and conversion heat treatment was carried out at the temperature range of 725–765 °C. A critical transition temperature (Tc0) of 90 K and a critical transport current density (Jc) of >0.5 MA/cm2 (77 K and self-field) were demonstrated for the GdBCO film grown on LAO single crystal substrate with a thickness of 0.5 μm. Ic was determined by utilizing a transport measurement. SEM observations showed that the growth behavior of Y123 grains is different from that of TFA–MOD process.
    Physica C Superconductivity 01/2006; · 0.72 Impact Factor
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    ABSTRACT: Long length MOD processing of YBCO conductor requires high performance precursor solution without degradation during the process. In this study, effect of chemical modification of precursor solution on MOD processing was investigated to meet the requirement. Highly stable YBCO precursor solution was synthesized with PVP (polyvinylpyrrolidone) and crack-free thick precursor film was successfully fabricated. However, annealed YBCO film shows porous microstructures originated from large HF gas generation from thick films and residual carbon from additives. Reduction of fluorine content is required to densify microstructures. In order to reduce fluorine content, F-free Cu-precursor solution was synthesized. YBCO film with Jc = 0.8MA/cm2 was grown on buffered RABiTS tape. Annealed YBCO film still contains pores and further reduction of pores is required to improve critical current (Ic)
    Journal of Physics Conference Series 01/2006; 43(1):211-214.
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    ABSTRACT: A 1 MVA single phase high temperature superconducting (HTS) transformer was manufactured and tested. The rated voltage of primary and secondary of HTS transformer are 22.9 kV and 6.6 kV respectively. BSCCO-2223 HTS tape was used for HTS windings of 1 MVA HTS transformer. In order to reduce AC loss generated in the HTS winding, the concentrically arranged winding was adopted to 1 MVA HTS transformer. Single HTS tape for primary windings and 4 parallel HTS tapes for secondary windings were used considering the each rated current of the HTS transformer. A core of HTS transformer was fabricated as a shell type core made of laminated silicon steel plate. And a GFRP cryostat with a room temperature bore was also manufactured. The characteristic tests such as no load test, short circuit test and several insulation tests were performed at 65 K using sub-cooled liquid nitrogen. Through these test results, the validity of design of HTS transformer was ascertained.
    IEEE Transactions on Applied Superconductivity 07/2005; · 1.20 Impact Factor
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    ABSTRACT: YBCO thin films were prepared on LaAlO<sub>3</sub> (100) single-crystal substrates using the metal-organic deposition of trifluoroacetates (TFA-MOD). Unlike the normal TFA-MOD process, TFA-based solutions were prepared dissolving YBCO powder into the trifluoroacetic acid solvent. The solutions were coated on the substrates by spin coating, fired at the temperature up to 400°C in 4.2% humid and oxygen atmosphere, fired again at the temperature region of 725-850°C for 2 h in 4.2% humidity and 1000 ppm oxygen partial pressure, and finally oxygenated at 450°C for 1 h. Similar to the normal TFA-MOD process, both microstructure and superconducting properties of YBCO films were highly sensitive to the firing temperature. Optimally processed YBCO films of ∼250 nm thickness exhibited T<sub>C,zero</sub> of 90 K and J<sub>C</sub> of 2.9 MA/cm<sup>2</sup> at 77 K in a self-field. The present results clearly evidence that our cost-effective modified TFA-MOD process is applicable to the fabrication of YBCO coated conductors.
    IEEE Transactions on Applied Superconductivity 07/2005; · 1.20 Impact Factor
  • 2nd International Energy Conversion Engineering Conference; 08/2004
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    ABSTRACT: An innovative method to fabricate biaxially textured nickel substrates applicable to coated conductor is developed. This method used the industrially scalable electrodeposition method combined with external magnetic field. The biaxially textured Ni layer was formed by electrodeposition process under external magnetic field. The electrodeposited Ni substrate has been characterized by means of X-ray diffraction and shows well-developed biaxial texture (Δθ∼7.29°, Δφ∼8.14°). A CeO2 buffer layer was epitaxially grown on the electrodeposited Ni substrates subsequently by thermal evaporation. In addition, a continuous processing of long tape was tried to test the compatibility of electrodeposition with further scale-up. The grown 20 cm-long Ni tape shows good crystallinity but non-uniform texture. Pinhole-free and smooth surface was obtained by additives and electropolishing. The magnetic texturing in the electrodeposition process offers a simple and economical way to quasi-single-crystalline metal substrates suitable for coated conductor.
    Physica C Superconductivity 01/2004; · 0.72 Impact Factor

Publication Stats

94 Citations
53.55 Total Impact Points

Institutions

  • 2004–2007
    • Korea Polytechnic University
      • Department of Advanced Materials Engineering
      Sŏul, Seoul, South Korea
  • 2006
    • Sungkyunkwan University
      Sŏul, Seoul, South Korea
  • 2003
    • Rochester Institute of Technology
      Rochester, New York, United States
  • 1998–2003
    • Chonbuk National University
      • • Department of Chemical Engineering
      • • Division of Advanced Materials Engineering
      Sŏul, Seoul, South Korea
    • Korea Institute of Energy Research
      Sŏul, Seoul, South Korea
  • 1995–2003
    • Korea Atomic Energy Research Institute (KAERI)
      • Department of Nuclear Fuel and Materials Development
      Daiden, Daejeon, South Korea