Chan-Joong Kim

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.

Formation of BaCeO 3 and its influence on microstructure of sintered/melt-textured Y-Ba-Cu-O oxides with CeO 2 addition

Formation of BaCeO[sub 3] and its effects on microstructure were studied in sintered/melt-textured Y--Ba--Cu--O oxides containing 5 wt.% CeO[sub 2] and various amounts of Y[sub 2]Ba[sub 1]Cu[sub 1]O[sub 5]. The added CeO[sub 2] was converted to fine particles of BaCeO[sub 3] near 930 [degree]C which is the conventional sintering temperature for Y--Ba--Cu--O. Y[sub 2]Ba[sub 1]Cu[sub 1]O[sub 5] and CuO are formed as by-products of the reaction between CeO[sub 2] and Y[sub 1]Ba[sub 2]Cu[sub 3]O[sub 7[minus]] [sub [ital y]] phase. The CeO[sub 2] addition reduced the particle size of Y[sub 2]B[sub 1]Cu[sub 1]O[sub 5] which was trapped in the Y[sub 1]Ba[sub 2]Cu[sup 3]O[sub 7[minus]][sub [ital y]] matrix after the melt-texture growth. During the peritectic decomposition stage of Y[sub 1]Ba[sub 2]Cu[sub 3]O[sub 7[minus]][sub [ital y]] phase into Y[sub 2]B[sub 1]Cu[sub 1]O[sub 5] and liquid phase, the morphology of the decomposed Y[sub 2]Ba[sub 1]Cu[sub 1]O[sub 5] was changed from a blocky shape in the undoped sample to an acicular shape of high anisotropy in the CeO[sub 2]-added sample. The formation of the highly anisotropic Y[sub 2]Ba[sub 1]Cu[sub 1]O[sub 5] particles appears to be responsible for the refinement of Y[sub 2]Ba[sub 1]Cu[sub 1]O[sub 5] particle after the melt-texture processing.

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.

Effective carbon incorporation in MgB2 by?combining mechanical milling and the glycerin treatment of boron powder

A combined process of a mechanical ball milling and liquid glycerin (C3H8O3) treatment of boron (B) powder has been conducted to enhance the superconducting properties of MgB2. The individual aims of the mechanical milling and the glycerin treatment were to reduce the grain size of the MgB2 and to achieve homogeneous carbon (C) incorporation into the MgB2, respectively. Four kinds of B powders of as-received, glycerin treated, 2 h milled and 2 h milled + glycerin treated were prepared. MgB2 bulks were fabricated by an in situ process using the prepared B powders. According to our experimental results, the mechanical ball milling was effective for grain refinement, and a lattice disorder was easily achieved by glycerin addition. It was found that the critical current density (Jc) values were enhanced in the samples with milled B or glycerin-treated B only. In the MgB2 bulk prepared with both milled and glycerin-treated B, the Jc was further increased to 1.27 × 104 A cm−2 at 5 K and 8 T due to a higher grain boundary density and a greater C substitution. The upper critical field and irreversibility field were also further enhanced by a combined treatment process of the B powder.

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).

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 Pb content on the formation of the high-Tc phase in the (Bi, Pb)-Sr-Ca-Cu-O system

Formation of the high-Tc phase in the (Bi, Pb)-Sr-Ca-Cu-O system was studied using specimens with various Bi/Pb ratios. The synthesis temperature was suppressed as the Pb/(Pb+Bi) was increased. The lattice constant of the c axis of the low-Tc phase (Tc~80 K) and high-Tc phase (Tc~105 K) increased with increasing Pb content. Substitution of 30% Pb for Bi was found to be most preferential for the formation of the high-Tc phase.

Synthesis of Cu–Ni Alloy Powder Directly from Metal Salts Solution

Cu–Ni alloy powders were synthesized directly from metal nitrate solution. Combustion, ultrasonic mist combustion and ultrasonic pyrolysis processes were applied and Cu–Ni alloy powder was successfully synthesized by mist combustion and ultrasonic pyrolysis of nitrate salts in a reducing atmosphere. X-ray diffraction data showed that the copper and the nickel atoms were completely mixed. For Cu–Ni alloy powder prepared by ultrasonic mist combustion, powder was a hollow sphere and consisted of nano-sized particles. For Cu–Ni alloy powder prepared by ultrasonic pyrolysis, particles consisted of nano-scale particles loosely coagulated. The synthesis temperature was 800°C, which is much lower than the liquidus of a Cu–Ni binary system. Metal alloying by mist pyrolysis, named chemical alloying, has many advantages: (1) no crucible, ball or jar is needed, (2) low synthesis temperature below the liquidus of the alloy system, (3) no extraction step is needed, (4) no cation contamination, (5) direct synthesis of fine powder from metal salts and (6) a simple and inexpensive process. The disadvantage is the contamination of organic elements from the solvent and the salt.