Gye-Won Hong

Top-seeded melt growth of Y-Ba-Cu-O superconductor with multiseeding

Y-Ba-Cu-O superconductors were prepared by the top-seeded melt growth process with multiseeding technique. By using several seeds at the same time, large samples could be fabricated in a short time with simple heat treatment. However, the samples fabricated by the normal multiseeding technique show a rapid decrease of trapped magnetic field value across the grain boundaries because of the residual liquid layer. To remove the residual liquid layer, various types of multiseeding were newly suggested. The optimum result was obtained when the seeds were arranged without spacing. The individual grains were combined as a single domain and did not show any deterioration of magnetic properties at the boundary. The formation mechanism of a well-combined single domain by the multiseeding technique is discussed.

Defect formation, distribution and size reduction of in melt-processed YBCO superconductors

High critical current density in high temperature superconducting melt-processed Y-Ba-Cu-O (YBCO), which consists of a superconducting (Y123) phase matrix containing discrete (Y211) inclusions, can be achieved by control of the grain microstructure during processing. Y123 grain growth rate, Y211 particle size, morphology, distribution and density and the chemistry of the peritectically molten liquid are key parameters in the processing of this material and effectively determine its microstructure. In addition, YBCO processed with excess Y211 generates defects at the Y123/Y211 interface which form effective flux pinning sites and hence enhance . The effect of addition of Y211 on the sample microstructure, its distribution and refinement by chemical doping are discussed in detail in this review.

Y2BaCuO5 morphology in melt-textured Y-Ba-Cu-O oxides with PtO2·H2O/CeO2 additions

Abstract Microstructures related to the 2-1-1 morphology, and the effect of PtO 2 ·H 2 O and CeO 2 additions on 2-1-1 refinement were examined in melt-textures Y 1 Ba 2 Cu 3 O 7− y and Y 1.6 Ba 2.3 Cu 3.3 O 7− y systems. It is found that the characteristics of 2-1-1 nucleation and growth is considerably affected by the additives and the microstructure prior to peritectic decomposition. In the Y 1 Ba 2 Cu 3 O 7− y system, without an additive, block-like 2-1-1 particles are produced when the 1-2-3 phase was decomposed into 2-1-1 and a Ba-Cu-O liquid phase. PtO 2 ·H 2 O addition changes the 2-1-1 shape to be highly anisotropic while CeO 2 addition reduces the 2-1-1 size as well as making it anisotropic. In the Y 1.6 Ba 2.3 Cu 3.3 O 7− y system where excess 2-1-1 particles are present prior to melting, the excess 2-1-1 acts as a grain-growth inhibitor for the 1-2-3 phase at the sintered state and thus results in a fine-sized 1-2-3 grain structure which provides nucleation sites for formation of the 2-1-1 phase. Also, the excess 2-1-1 particles appear to act as heterogeneous nucleation sites for the decomposed 2-1-1. Without the additives, equiaxed 2-1-1 particles are produced while with additives, fine granular or plate-like 2-1-1 particles are produced. The resulting 2-1-1 shape is fairly dependent on the shape of the 2-1-1 particles formed at the sintered state, indicating the heterogeneous nucleation of the 2-1-1 on the prior 2-1-1.

Multiseeding with (100)/(100) grain junctions in top-seeded melt growth processed YBCO superconductors

Multiseeding with (100)/(100) grain junctions of top-seeded melt growth (TSMG) processed YBCO superconductors was studied. Multiple seeding shortened the processing time for the fabrication of TSMG-processed YBCO superconductors. The relationship among the number of seeds, the levitation forces and the trapped magnetic fields of the TSMG-processed YBCO samples is reported. The characteristic of the (100)/(100) grain junction is discussed in terms of a wetting angle of a melt.

Microstructure of melt-textured Y - Ba - Cu - O oxides with addition and the formation mechanism of the Ba - Cu - O platelet structure

Melt-textured (Y123) containing fine particles of (Y211) has been prepared from Y123/Y211 powder that was attrition milled with 1 wt% addition, and the microstructure has been examined. Fine and spherical Y211 particles (less than 1 m in size) are found to be homogeneously dispersed within the melt-textured Y123 domain. Many dislocations are observed to be formed around the trapped Y211 and the inclusions, which were formed as a result of addition and introduction from the jar and ball used for attrition milling. CuO stacking faults were also observed around the trapped Y211; these were initiated at the Y123/Y211 interface and extended into the Y123 matrix. Each stacking fault has a lenticular shape, with a width of a few tens of nanometres and a length of a few hundred nanometres, and the faults developed along the [100] and [010] directions of the Y123. The formation mechanism of the stacking fault was discussed together with the formation of the platelet structure (the elongated Ba - Cu - O phase) on the basis of an oxygenation-induced decomposition of the Y123 phase. It is concluded that the prolonged oxygenation heat treatment producing the tetragonal-to-orthorhombic phase transformation is responsible for the formation of the platelet structure and possibly for the formation of the stacking faults.

New method of producing fine Y2BaCuO5 in the melt-textured Y-Ba-Cu-O system: attrition milling of YBa2Cu3Oy-Y2BaCuO5 powder and CeO2 addition prior to melting

A new process yielding fine Y211 particles in melt-textured Y-Ba-Cu-O was developed by combining attrition milling of Y123-Y211 powder and CeO2 addition prior to the partial melting. Beneficial points of the attrition milling are as follows: (1) it allows uniform distribution of the Y211 refiner (CeO2) in the Y123-Y211 powder mixture, (2) it produces nanocrystalline Y123 powder with large surface area per unit volume which increases the possibility of Y211 nucleation during incongruent melting and (3) it controls the number and the size of Y211 particles preformed at the calcination/sintering stage from the off-stoichiometric powder of Y1.8 composition. The Y211 particles of the melt-textured Y123 sample which was prepared from the attrition-milled powder were of submicrometre scale and their distribution was quite uniform. The critical current density (Jc) at 77 K of the melt-textured Y1.8 sample containing fine Y211 (less than 1 mu m) was 2.0*104 A cm-2 and 1.16*104 A cm-2 at 1 T and 2 T, respectively, which is twice the Jc of 1.1*104 A cm-2 (at 1 T) and 0.56*104 A cm-2 (at 2 T) of the Y1.8 sample containing coarse Y211 ( approximately 5 mu m in average size).

The effect of diluent gases on the growth behavior of CVD SiC films with temperature

Silicon carbide films have been grown onto graphite substrates by low pressure chemical vapor deposition using MTS (CH3SiCl3) as a source precursor and H2 or N2 as a diluent gas. The experiments were performed at fixed conditions of a flow rate of 100 sccm for each MTS and carrier gas, a flow rate of 300 sccm for diluent gas addition, and a total pressure of 5 torr. The effect of temperature from 900°C to 1350°C and the alteration of diluent gas species on the growth rate and structure of deposits have been studied. The experimental results showed that the deposition rate increased with increasing deposition temperature irrespective of diluent gases and reactant depletion effect increased especially at H2 diluent gas ambient. At MTS-H2 system, the deposition mechanism changed from chemical reaction to mass transfer controlled reaction with temperature. Otherwise, For MTS-H2-N2 system, surface chemical reaction controlled the growth process at whole deposition temperature ranges. For N2 addition, surface morphology of leaf-like structure appeared, and for H2, faceted structure at 1350°C. The observed features were involved by crystalline phase of β-SiC and surface composition with different gas ambient.

Characteristic tests of a 1 MVA single phase HTS transformer with concentrically arranged windings

A 1 MV A single phase high temperature superconducting (HTS) transformer was manufactured and tested. The rated voltages of primary and secondary of the HTS transformer are 22.9 kV and 6.6 kV respectively. BSCCO-2223 HTS tape was used for HTS windings of 1 MV A HTS transformer. In order to reduce AC loss generated in the HTS winding, the type of concentric arrangement winding was adopted to a 1 MV A 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 of 1 MV A HTS transformer were performed such as no load test, short circuit test and several insulation tests at 65 K using sub-cooled liquid nitrogen. From the results of tests, the validity of design of HTS transformer was ascertained.

Microstructure, microhardness, and superconductivity of CeO2-added Y–Ba–Cu–O superconductors

The CeO 2 -added Y–Ba–Cu–O oxides were prepared by two different processes, the conventional solid-state reaction process and the partial melt process using powders, to examine the effect of the dopant on microstructure, microhardness, and superconductivity. In the solid-state reacted sample, most of the added CeO 2 was converted to a form of BaCeO 3 , but some might enter into the 1-2-3 phase, resulting in the orthorhombic-to-tetragonal phase transition that accompanied the disappearance of twin structure in 1-2-3 grains. In the partially melted sample, however, the phase change was not observed up to 5 wt. % of CeO 2 . All the added CeO 2 in these samples was consumed to form only BaCeO 3 which was finely dispersed in large 1-2-3 grains during the peritectic reaction stage. The zero resistance temperature (T c ) of the solid-state reacted sample gradually decreased with increasing CeO 2 content due to the phase change and the formation of BaCeO 3 , whereas the T c of the partially melted sample was nearly constant regardless of CeO 2 content up to 5 wt. %, owing to the separation of the second phase from the 1-2-3 superconducting phase. Microhardness of the partially melted sample increased with increasing CeO 2 content. The strengthening effect appears to come from the composite system where the fine BaCeO 3 particles are dispersed in a 1-2-3 matrix.

Effects of the seed distance on the characteristics of the (100)/(100) junctions of top-seeded melt growth processed YBCO superconductors using two seeds

Abstract The effects of the distance ( d ) between two Sm 1.8 Ba 2.4 Cu 3.4 O x seeds on the characteristics of the (100)/(100) junctions of top-seeded melt growth (TSMG) processed YBCO superconductors were investigated. The trapped magnetic field at the grain junction and the levitation force of the top surface decreased with increasing d value. The degradation of the properties was discussed in terms of the presence of the residual melt-forming phases (CuO and BaCuO 2 ) at the grain junction.