Ki-Baik Kim

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.

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.

Entrapment of Ba-Cu-O liquid phase during growth of a Y1Ba2Cu3O7−y domain

Abstract Entrapment of Ba-Cu-O liquid phase during the growth of a 123 domain was investigated in the melt-textured Y-Ba-Cu-O oxide. From the microstructural examination of the growth front of the 123 domain, it is found that liquid phase is entrapped in the channels between adjacent 123 platelets formed ahead of the 123 domain, owing to the anisotropic growth of the 123 platelet, the difference between the growth rate of the 123 platelet and the dissolution rate of 211 in the liquid phase, and the inhomogeneous distribution of 211 particles at the peritectic reaction front. Dissolution of 211 particles in the reaction region, which supplies an yttrium source to the liquid channels, allows the entrapped liquid phase to solidify to 123 phase. After the peritectic reaction, the liquid channels turn into plate boundaries containing residual liquid phase as evidence of the entrapment. Isolation of the entrapped liquid phase from the yttrium source (211 particles) by the solidified 123 phase appears to be responsible for the presence of the residual liquid phase.

Microstructure change during oxygen annealing and the effect on the levitation force of melt-textured Y - Ba - Cu - O superconductors

The influence of oxygen annealing on the levitation force was studied for melt-textured multidomain YBCO samples cooled with and without magnetic field (field cooled (FC) and zero-field cooled (ZFC)). For both the FC and ZFC samples the repulsive force rapidly increased with increasing oxygen annealing time, reaching a maximum value at a certain annealing time and then slightly decreasing when the samples were further annealed. The attractive force of the FC samples followed a similar pattern to that of the repulsive force while the attractive force of the ZFC samples was small and nearly constant regardless of annealing time. The variation of the levitation force with oxygen annealing time is explained in terms of the microstructure developed during oxygen annealing.

refinement via an attrition milling and addition in Sm-Ba-Cu-O systems melt-textured at oxygen partial pressures of 0.001 - 1 atm

(Sm211) refinement was achieved in Sm-Ba-Cu-O systems melt-textured at various oxygen partial pressures ( of 0.001 - 1 atm) via 1 wt% addition and size control of a precursor powder (a mixture of (Sm123) plus Sm211 powder) by means of attrition milling. The resulting Sm211 of the (Sm1.8) sample melt-textured without addition was as large as a few tens of while the Sm211 of the Sm1.8 sample melt-textured from the attrition-milled powder with 1 wt% addition was as small as 1 - , with uniform Sm211 distribution. The Sm211 refinement achieved is ascribable to both the controlled Sm211 nucleation during incongruent melting and the growth inhibition of the Sm211 in the partial melting state. Furthermore, the Sm211 shape was changed from equiaxed granular in the undoped sample to anisotropic in the samples containing . Effects of oxygen partial pressure on the Sm211 size and the shape was not significant in either the undoped Sm1.8 sample and the -added Sm1.8 sample.

Microstructure of the domain boundary and the effect of excess CuO in the melt-textured YBaCuO oxides

Abstract Microstructures near the domain boundary were investigated in the YBa 2 Cu 3 O 7− y (Y123) samples melt-textured in air atmosphere. It is found that the free CuO phase is often observed near the domain boundaries of the Y123 samples, probably due to the variation of the composition of the finally solidified liquid phase. Also, the effect of excess CuO on the microstructure of the domain boundary was investigated in the Y123 melt-textured with 5 wt.%–20 wt.% CuO addition. When the amount of the CuO is small, CuO phase exists as a discrete form at the domain boundaries. As the CuO content increases, spherical CuO pockets and large CuO pools containing unreacted Y211 particles are developed. In addition, the CuO addition changes the growth morphology of the Y123 domain; from a planar mode to a Y123/CuO cellular structure with increasing CuO content.

Role of PtO2 on the refinement of Y2BaCuO5 second phase particles in melt-textured YBaCuO oxides

Abstract In order to understand the mechanism of the Y 2 BaCuO 5 (Y211) refinement, the effect of PtO 2 addition on the microstructure of each reaction state, i.e., the solid state, the Y211 plus liquid state and the peritectic growth state of the melt-texture YBa 2 Cu 3 O 7− y (Y123), was investigated. When heating a mixture of a Y123 powder and PtO 2 powder below the peritectic temperature of YBaCuO, the PtO 2 reacts with a Y123 phase and then forms of Pt-bearing compound via the solid state reaction. As a result of this reaction, free CuO is produced and it leads to the p1 pseudo-peritectic reaction of Y123 + CuO → Y211 + liquid. The produced Y211 particles by the p1 pseudo-peritectic reaction were acicular in shape. They serve as a seed for the growth of the Y211 formed by m1 peritectic melting of Y123 → Y211 + liquid, resulting in the formation of acicular Y211 particles at the Y211 plus liquid state. A liquid infiltration experiment with/without PtO 2 addition into a Y211 compact at 1100°C showed that Y211 coarsening in the liquid phase containing PtO 2 was less significant than that in the liquid containing no PtO 2 . It is concluded that PtO 2 addition suppresses the Y211 growth in the liquid phase as well as changes its morphology.