• Proceedings of the Korean Institute of Electrical and Electronic Material Engineers Conference
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• 2005.07a
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• pp.52-53
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• 2005

$Y_2O_3$ film was directly deposited on Ni-3at%W substrate using DC reactive sputtering technique. Metallic yttrium was used for DC sputtering target and water vapor was used for oxidizing the deposited metallic Yttrium atoms on the substrate. The window of the water vapor turned out to be broad. The minimum partial pressure of water vapor was determined by sufficient oxidation of the $Y_2O_3$ film, and the maximum partial pressure of water vapor was determined by the non-oxidation of the target surface. As the sputtering power was increased, The deposition rate increased without narrowing the window. The fabricated $Y_2O_3$ films showed good texture qualities and surface morphologies. The YBCO film deposited directly on the $Y_2O_3$ buffered Ni-3at%W substrate showed $T_c$, $I_c$ (77 K, self field), and $J_c$ (77 K, self field) of 89 K, 64 A/cm and 1.l $MA/cm^2$, respectively.

• Proceedings of the Korean Institute of Electrical and Electronic Material Engineers Conference
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• 2006.06a
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• pp.243-244
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• 2006

As an preliminary study for the quench protection of high temperature superconducting (HTS) cable using superconducting fault current limiter (SFCL), experimental research was carried out. The test circuit was composed of Bi-2223/Ag HTS tape and a SFCL made of YBCO thin films. In the normal state, the applied current of 56 A, which was critical current of HTS tape, could be flown through the circuit without resistive loss. Increasing the currents, the quench development of both materials was investigated from the voltage signal acquired from the resistance of the quenched superconductor. Up to around 10 times of the critical current was applied to the HTS tape and the current limiting characteristics of SFCL were investigated. In addition, for the finding out the optimal operating condition of SFCL such as the numbers of elements, a shunt resistor was applied to the SFCL and quench characteristics were analyzed as well.

• Bulletin of the Korean Chemical Society
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• v.12 no.5
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• pp.499-504
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• 1991

EPR spectra of the high $T_c$ superconductor $YBa_2Cu_3O_{7-y}$ (YBCO) doped with $Pd^{2+} or Zn^{2+}$ have been measured at several temperatures and dopant concentrations. The spectral intensity of $YBa_2({Cu_{1-x}}{Pd_x})_3O_{7-y}$ is proportional to the dopant concentration. The behavior of $YBa_2(Cu_{1-x}Zn_x)_3O_{7-y}$ is quite different: the spectral intensity remains almost constant up to x=0.10 and then increases rapidly above x=0.10. The results are interpreted in terms of localized and antiferromagnetically spin-paired d holes in both CuO chain and planes. The $Pd_{2+}$ ion substitutes on the CuO chain consisting of "CuOCu dimers", and a $Cu_{2+}$ ion with an unpaired spin is gene rated for each $Pd_{2+}$ ion substituted. On the other hand, $Zn_{2+}$ substitutes on the CuO planes, and all or most of the spins in the two-dimensional plane manage to pair up in the region of low dopant concentration. When the dopant concentration exceeds a certain limit, it becomes more difficult for the spins to find partners, and the number of unpaired spins increases rapidly with increasing dopant concentration. The $Zn_{2+}$ ion is more effective than the $Pd_{2+}$ ion in suppressing the superconductivity of YBCO. This is attributed to the fact that $Zn_{2+}$ substitutes on the CuO planes which are mainly responsible for the superconductivity, while $Pd_{2+}$ substitutes on the CuO chain which is of secondary importance in the superconductivity.

• Progress in Superconductivity and Cryogenics
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• v.11 no.1
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• pp.1-4
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• 2009

Mechanical and electrical properties of silver stabilizer layer of coated conductor, which as prepared with nano silver paste as starting materials, have been investigated, Nano silver paste was coated on a YBCO film by dip coating process at a diping speed of 20m/min. Coated film was dried in air and heat treated at $400{\sim}700^{\circ}C$ in an oxygen atmosphere. Adhesion strength between YBCO and silver layer was measured by a tape est(ASTM D 3359). Hardness and electrical conductivity of the samples were measured by pencil hardness test (ASTM D 3363) and volume resistance test by LORESTA-GP (MITSHUBISHD, respectively. The sample heat-treated at $500^{\circ}C$ showed poor adhesion 1B, but samples heat treated at higher than $600^{\circ}C$ showed enhanced adhesion of 5B. The silver layer heat-treated at $700^{\circ}C$ showed the high hardness value larger than 9 H, low volume resistance, surface resistance value as well as superior current carrying capacity compared to sputtered silver. SEM observations showed that a dense silver layer was formed with a thickness of about $2{\mu}m$. Dip coated silver layer prepared by using nano silver paste showed superior electrical and mechanical characteristics.

• Progress in Superconductivity and Cryogenics
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• v.9 no.1
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• pp.22-26
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• 2007

Superconductor is weak in AC condition. Bifilar geometry provides a solution to reduce AC loss. Bifialr geometry is piled up or wound with more than two layers. When a layer of superconductor abuts on other layers, AC loss is affected by not only self-field, but also magnetic field induced by adjacent layers. In this study, two superconductors are piled up as a series connection so that current flows in different directions. By this method, magnetic field is cancelled. If magnetic field is cancelled, AC loss is reduced. To compare AC loss with respect to piling method, we measured the AC loss difference between the case facing each other with substrate side and the case facing with YBCO side. Measured AC loss is compared with one-way current flow single layer AC loss. In addition, we analyzed how much AC loss was increased, or reduced. All results were compared with those calculated with Norris equation. By this experiment, we concluded that distance between two wires is the important cause of AC loss. The distance between two wires affects magnetic field reduction in YBCO and induced current flow on substrate side.

• Progress in Superconductivity and Cryogenics
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• v.9 no.3
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• pp.5-8
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• 2007

All three buffer layers of $Y_2O_3$, YSZ, and $CeO_2$ have been deposited on the biaxially textured metal substrates using rf-sputtering method, The first 50-70nm thick $Y_2O_3$ films were grown epitaxially on biaxially textured metal substrates as a seed layer and followed by the diffusion barrier ${\sim}100nm$ thick YSZ and subsequent capping layer ${\sim}200nm$ thick $CeO_2$ deposited epitaxially on top of $Y_2O_3$ seed layer. The epitaxial orientation of all three layers were all (100) grown with rocking curve Full Width at Half Maximum(FWHM) of $4-5^{\circ}$ and in plane phi-scan FWHM of $6-8^{\circ}$ using X -ray diffraction analysis. The NiO phases formed during the $Y_2O_3$ seed layer deposition seem to degrade the crystallinity and roughen the surface morphology of the following layer observed by AFM(Atomic Force Microscopy). The buffered tapes were used as substrates for long length YBCO coated conductors with high critical current density $J_c$. The five multi-turn of metal tapes was employed to increase the thickness of films and production rate to compensate the low growth rate of rf-sputtering method.

• Progress in Superconductivity and Cryogenics
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• v.10 no.2
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• pp.16-19
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• 2008

Angular dependence of critical current density of SmBCO coated conductor fabricated by co-evaporation method was investigated. For comparison, three samples were fabricated by a co-evaporation method and one sample was fabricated by a pulsed laser deposition process. The deposition system, named EDDC (Evaporation using Drum in Dual Chambers), is a batch type co-evaporation system, which is composed of reaction chamber and evaporation chamber. The normalized critical current density ratio ($I_c/I_c$(H//ab-plane)) of EDDC-SmBCO samples was found to be higher than that of PLD-YBCO sample in the whole range of angle. While the EDDC-SmBCO samples evidently had a peak at the angle of H//c-axis in the plot of the angular dependence of critical current, the normalized critical current of PLD-YBCO sample decreased monotonically without any peak as angle increased. The field dependence of critical current under the magnetic field parallel to the normal direction of those samples showed similar aspect in the range of $0\;G{\sim}5000\;G$.

• Proceedings of the Korea Association of Crystal Growth Conference
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• 1996.06a
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• pp.258-292
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• 1996

In the last year great interest appears to YBCO thin films preparation on different substrate materials. Preparation of epitaxial film is a very difficult problem. There are many requirements to substrate materials that must be fullfilled. Main problems are lattice mismatch (misfit) and similarity of structure. From paper [1] or follows that difference in interatomic distances and angles of substrate and film is mire important problem than similarity of structure. In this work we present interatomic distances and angle relations between substrate materials belonging to ABCO4 group (where A-Sr or Ca, B-rare earth element, C-Al or Ga) of different orientations and YBCO thin films. There are many materials used as substrates for HTsC thin films. ABCO4 group of compounds is characterized by small dielectric constants (it is necessary for microwave applications of HTsC films), absence of twins and small misfit [2]. There most interesting compounds CaNdAlO4, SrLaAlO4 and SrLaGaO4 were investigated. All these compounds are of pseudo-perovskite structure with space group 14/mmm. This structure is very similar to structure of YBCO. SLG substrate has the lowest misfit (0.3%) and dielectric constant. For preparation of then films of substrates of this group of compound plane of <100> orientation are mainly used. Good quality films of <001> orientations are obtained [3]. In this case not only a-a misfit play role, but c-3b misfit is very important too. Sometimes, for preparation of thin films substrates of <001> and <110> orientations were manufactured [3]. Different misfits for different YBCO faces have been analyzed. It has been found that the mismatching factor for (100) face is very similar to that for (001) face so there is possibility of preparation of thin films on both orientations. SrLaAlO4(SLA) and SrLaGaO4(SLG) crystals of general formula ABCO4 have been grown by the Czochralski method. The quality of SLA and SLG crystals strongly depends on axial gradient of temperature and growth and rotation rates. High quality crystals were obtained at axial gradient of temperature near crystal-melt interface lower than 50℃/cm, growth rate 1-3 mm/h and the rotation rate changing from 10-20pm[4]. Strong anisotropy in morphology of SLA and SLG single crystals grown by the Czochralski method is clearly visible. On the basics of our considerations for ABCO4 type of the tetragonal crystals there can appear {001}, {101}, and {110} faces for ionic type model [5]. Morphology of these crystals depend on ionic-covalent character of bonding and crystal growth parameters. Point defects are observed in crystals and they are reflected in color changes (colorless, yellow, green). Point defects are detected in directions perpendicular to oxide planes and are connected with instability of oxygen position in lattice. To investigate facets formations crystals were doped with Cr3+, Er3+, Pr3+, Ba2+. Chromium greater size ion which is substituted for Al3+ clearly induces faceting. There appear easy {110} faces and SLA crystals crack even then the amount of Cr is below 0.3at.% SLG single crystals are not so sensitive to the content of chromium ions. It was also found that if {110} face appears at the beginning of growth process the crystal changes its color on the plane {110} but it happens only on the shoulder part. The projection of {110} face has a great amount of oxygen positions which can be easy defected. Pure and doped SLA and SLG crystals measured by EPR in the<110> direction show more intensive lines than in other directions which allows to suggest that the amount of oxygen defects on the {110} plane is higher. In order to find the origin of colors and their relation with the crystal stability, a set of SLA and SLG crystals were investigated using optical spectroscopy. The colored samples exhibit an absorption band stretching from the UV absorption edge of the crystal, from about 240 nm to about 550 m. In the case of colorless sample, the absorption spectrum consists of a relatively weak band in the UV region. The spectral position and intensities of absorption bands of SLA are typical for imperfection similar to color centers which may be created in most of oxide crystals by UV and X-radiation. It is pointed out that crystal growth process of polycomponent oxide crystals by Czochralski method depends on the preparation of melt and its stoichiometry, orientation of seed, gradient of temperature at crystal-melt interface, parameters of growth (rotation and pulling rate) and control of red-ox atmosphere during seeding and growth (rotation and pulling rate) and control of red-ox atmosphere during seeding and growth. Growth parameters have an influence on the morphology of crystal-melt interface, type and concentration of defects.

• Transactions on Electrical and Electronic Materials
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• v.9 no.6
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• pp.260-264
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• 2008

The operational characteristics of superconducting amplifier using vortex flux flow were analyzed from an equivalent circuit in which its current-voltage characteristics for the vortex motion in YBCO microbridge were reflected. For the analysis of operation as an amplifier, dc bias operational point for the superconducting amplifier is determined and then ac operational characteristics for the designed superconducting amplifier were investigated. The variation of transresistance, which describes the operational characteristics of superconducting amplifier, was estimated with respect to conditions of dc bias. The current and the voltage gains, which can be derived from the circuit for small signal analysis, were calculated at each operational point and compared with the results obtained from the numerical analysis for the small signal circuit. From our paper, the characteristics of amplification for superconducting flux flow transistor (SFFT) could be confirmed. The development of the superconducting amplifier applicable to various devices is expected.

• Transactions on Electrical and Electronic Materials
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• v.9 no.6
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• pp.255-259
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• 2008

The magnetic field application effect on resistance of a high-$T_c$ superconducting (HTSC) element comprising a flux-lock type superconducting fault current limiter (SFCL) was investigated. The YBCO thin film, which was etched into a meander line using a lithography, was used as a current limiting element of the flux-lock type SFCL. To increase the magnetic field applied into HTSC element, the capacitor was connected in series with a solenoid-type magnetic field coil installed in the third winding of the flux-lock type SFCL. There was no magnetic field application effect on the resistance of HTSC element despite the application of larger magnetic field into the HTSC element when a fault happened. The resistance of HTSC element, on the contrary, started to decrease at the point of four periods from a fault instant although the amplitude of the applied magnetic field increased.