1PO5-30 Characterization of Shock-compacted Bulk YBCO by Magnetic Torque Measurements, X-ray Diffraction Analysis and AFM-Image

H. Kezuka, M. Kikuchi, Y. Syono, S. Kawamata, and K. Okuda, Tokyo University of Technology, 1404-1 Katakura, Hachioji 192-0982, Japan

Presenting Author: H Kezuka

The shock compaction [1] effects on crystal structures of high-T_c YBCO particles are investigated. Superconducting YBCO particles [2] with an average size of 1000nm were prepared by the dry-type jet mill. The shocked specimen was compacted in plate-like grains with a high crystal density over 97%. The as-shocked specimen showed an oriented crystal structure with the lattice constants of the orthorhombic cell with a=0.38298nm, b=0.38992nm and c=1.1693nm for as-shocked bulk YBCO calculated by X-ray diffraction analysis just after the shock compaction under 5.71GPa. From the analysis of AFM(Atomic Force Microscope)-image, the surface of the specimens for as-shocked YBCO has large grains of 4 - 15 micro m in length partly with large growth-steps caused mainly by heats generated in shock compaction and the vertical distance of about 30 - 80 nm is obtained around the edge area in annealed YBCO grains. Also the oriented crystal structure in which c-axis is perpendicular to the pellet is investigated by the magnetic torque measurement in the superconducting mixed state under magnetic field of 8 kOe.

[1] H. Kezuka, M. Kikuchi, E. Ohshima, H. Hikosaka and Y. Syono, Physica C 282-287(1997)521

[2] Kezuka, H., Zhonghe, Xi, Miyanaga, Y., and Kan-nari, C., Thin Solid Films 281/282 (1996)584

1PO5-31 Stress and Strain Due to Flux Pinning in Bulk Superconductors

Tom H. Johansen, Department of Physics, University of Oslo, Norway

Presenting Author: T.H. Johansen

The giant magnetostriction in superconductors was discovered a few years ago by Ikuta et al., who observed wide hysteresis loops in dilatation versus magnetic field in Bi-2212 single crystals. The phenomenon was immediately recognized as a general manifestation of the flux-pinning forces, f = J_cB. As single-domain RE-123 bulks steadily grow in size and increase their J_c, the understanding of pinning-induced deformations becomes increasingly important. In this work, a full 3-dimensional magneto-elastic analysis is presented for the case of a cylindrical superconductor placed in a parallel magnetic field. Exact expressions are derived for all stress and strain components in terms of the flux distribution B(r) in a given magnetized state. Hence, the formulas are valid for any B-dependence of J_c. We determine in particular the stress-strain behavior during field cycles commonly used to activate RE-123 bulks as trapped-field magnets, i.e., (i) freezing-in an applied field which subsequently is removed, and (ii) zero-field cooling followed by an applied field pulse. For the two activation processes the magnitude of maximum tensile stress, and also where it occurs in the cylinder, are determined. It is shown how these parameters, which are important to predict crack developments, depend on the activation field.

1PO5-32 The effect of hot-pressing pressure and annealing atmosphere on critical current density of (Bi,Pb)₂Sr₂Ca₂Cu₃O_y current leads

X.K. Fu, Y.C. Guo, H.K. Liu and S.X. Dou. Institute for Superconducting and Electronic Materials, University of Wollongong, Wollongong, NSW 2500, Australia

Presenting Author: X.K. Fu

The effect of hot-pressing pressure and annealing atmosphere on critical current density of (Bi,Pb)₂Sr₂Ca₂Cu₃O_y current leads has been investigated in this paper. Current leads were prepared by a combination of CIP (Cold Isostatic Press) and hot-pressing method. The hot-pressing pressures were varied from 2 Mpa and 10 Mpa have been applied to fabricate current leads. The samples were annealed in air and low oxygen partial pressure atmosphere. The transport critical current densities of (Bi,Pb)₂Sr₂Ca₂Cu₃O_y current leads made by different processing schemes were compared. The highest critical current and critical current density have exceeded 500 A and 1000 A/cm² at 77 K and self-field. The results revealed hot-pressing pressure and annealing atmosphere both affected critical current densities significantly.

1PO5-33 Electrochemically oxygenated superconducting La₂CuO_4+d single crystal

Fang Zhou 1, Wen-jie Zhu 2, Yu-zhen Huang 1, Zu-gang Li 2, Zhong-xian Zhao 1, and Pei-herng Hor 2. 1 National Laboratory for Superconductivity, Institute of Physics, Chinese Academy of Sciences, P.O. Box 603, Beijing 100080, China. 2 Texas Center for Superconductivity at the University of Houston, Houston, TX 77204-5932, USA.

Presenting Author: F. Zhou

Electrochemical method was applied successfully in intercalating extra oxygen into non-superconducting La₂CuO₄ single crystals grown by flux method. The electrochemical experiments of both the galvanostatic and potentiostatic modes were carried out in NaOH electrolyte at, usually, room temperature; the electrochemical cell may also be heated (up to about 70 °C in this work) to facilitate the oxygen intercalation. The magnetic measurements showed that only a single superconducting phase was detected in these electrochemically oxygenated single crystal samples, with the transition temperatures ranging from 10K to 20K; while no antiferromagnetic phase was observed. The maximum superconducting volume fraction estimated from Meissner effect is 18%. A discussion about possible causes for the low superconducting transition temperature is given.

1PO5-34 Quenching effect on Superconducting (Pb_0.5Ba_0.5)Sr₂(Y_0.5Ca_0.5)Cu₂O_y

Qing-rong Feng, Jian-dong Guo, Han Zhang, and Shi-jie Li, Department of Physics, Peking University, Beijing 100871, P.R. China

Presenting Author: Q.-R. Feng

The specimens of (Pb_0.5Ba_0.5)Sr₂(Y_0.5Ca_0.5)Cu₂O_y has been synthesized in air atmosphere. Some times it showed T_c=70K, and in the other time it showed about 30K. In order to understand what reason it was, the quenching experiment have been done on it. For the samples that were finished the high temperature sintering and began to cool were quenched in different temperature in air and liquid nitrogen. It was found that the T_c values and the unit cell parameters changed with the variation of quenching temperature. The smaller the unit cell parameters, the higher the T_c value. The quenched samples showed the highest T_c of 58K (in air at 850° C) and 55K (in liquid nitrogen at 800° C) respectively. While their unit cell parameters approached the smallest values. The mechanism of quenching effect is discussed.

1PO5-35 Annealing effect on Superconducting (Pb_0.5Ba₀.₅)Sr₂(Y_0.5Ca_0.5)Cu₂O_y

Qing-rong Feng 1, Wu-zong Zhou 2, and Wei-yao Liang 2. 1 Department of Physics, Peking University, Beijing 100871, P.R. China. 2 IRC in superconductivity, University of Cambridge, Lensfield Road, Cambridge CB3 OHE, United Kingdom.

Presenting Author: Q.-R. Feng

Two specimens of (Pb_0.5Ba_0.5)Sr₂(Y_0.5Ca_0.5)Cu₂O_y has been synthesized in flowing oxygen atmosphere. Their original T_c values were 69K(sample A) and 32K(sample B). The samples were annealed in flowing argon atmosphere foe 13 hours and both of them became nonsuperconducting. The samples were then annealed at 620° C in flowing oxygen for 3, 6, 8, 11, and 13 hours. It was found that the T_c values and the unit cell parameters of A and B changed with the variation of annealing time, although no phase change was observed. The smaller the unit cell dimensions, the higher the T_c value. The samples after annealing for 8 hours showed the highest T_c of 38K (A) and 36K (B) respectively. While their unit cell parameters approached the smallest values. The mechanism of annealing effect is discussed.

1PO5-36 Intermediate steps during Hg-2212 synthesis by in-situ X-ray synchrotron diffraction

Pierre Toulemonde 1, Sylvie Le Floch 1, Pierre Bordet 1, Jean-Jacques Capponi 1, Philippe Odier 1, and Mohamed Mezouar 2. 1 Laboratoire de Cristallographie, CNRS, 25 avenue des martyrs, BP166, 38 042 Grenoble cedex 09, France. 2 ESRF, BP220, 38 043 Grenoble cedex, France.

Presenting Author: P. Odier

The synthesis of Hg₂Ba₂YCu₂O_8-d (Hg-2212) phase requires high pressure - high temperature process. It is then quite difficult to establish its reaction path, an important information required to optimize the final compound. In-situ X ray characterization is an extremely powerful technique now available at synchrotron facilities. At ESRF - Grenoble (ID30 beamline), in-situ characterizations can be made under high pressure - high temperature (1 - 8 GPa, up to 1500 °C), a range that covers the experimental conditions employed in the case of Hg-2212.

The synthesis is followed at 4 GPa from room temperature to 900 °C. We observe an intermediate step depending of the type of precursors that we used. When the precursor is a mixture of oxides (BaO₂, Y₂O₃, CuO) and Cu, we detect a transient phase composed at least of Ba, Hg and O. This phase is then converted into the final Hg-2212 product. On the other hand, when we start from the pre-reacted precursor "Ba₂YCu₂O_y" mixed with HgO, Hg-2212 grows directly.

This in-situ technique clearly shows its efficiency in identifying quickly a phase that would have been difficult to observe ex-situ. We expect this approach to be very powerful in other fields as well.

1PO5-37 Technological peculiarities of precursors for Hg-containing HTSC-samples synthesis procedure

R.V. Lutciv, L.Y. Tkachuk, M.V. Matviyiv, and I.V. Lazaryuk, Chair of Radioelectronics Materials Sciences, Physical Department of L'viv State University, Dragomanova, 50 Str., 290005, L'viv, Ukraine

Presenting Author: L.Y. Tkachuk

HgBa₂Ca_(n-1)Cu_nO_2n+2+d , having the highest critical state parameters (T_cH_c1g, H_c2g) and being very attractive with the point of viev of numerous practice applications are very exacting from the point of viev of single phase materials preparetion. We solved those problems in different ways:

· using of arc-melting for precursor preparation with following termal treatments in different atmospheres.

· using of specifical synthesis procedure which takes us the possibility to avoid free CaO in precursor with the aim to eliminate HgCaO₂ in resultant materials;

· using of as-prepared precursors for synthesis of Hg-containing HTSC allows us to receive near single phase material (it confirmed by X-ray analysis, magnetic measuring and respective temperature dependencies of resistivity) in short time (comparing with traditional procedure of precursor preparation) in enough quantities for industrial production and using enough technological BaCO₃ as a starting material instead of hygroscopic BaO.

1PO5-38 Synthesis of (Hg,Re)-1223 by sol-gel technique

A. Sin 1, P. Odier 1, and M. Núñez-Regueiro 2. 1 Laboratoire de Cristallographie-CNRS, 25 Av. Des Martyrs, Bp166, F38042 Grenoble CEDEX09, France. 2 Centre de Recherche sur les Très Basses Températures-CNRS, 25 Av. des Martyrs, Bp166, F38042 Grenoble Cedex09, France.

Presenting Author: A. Sin

The mercury-cuprates are generally synthesised by the sealed quartz tube technique in a two step process: the formation of a precursor first and then its mercuration. Starting with a high degree of homogenisation of all precursor oxides is essential to obtain superconducting ceramics of high quality.

Wet chemistry and particularly sol-gel processing is one of the most promising way to get an excellent homogenisation at an atomic scale of the elements and a high reactivity of the precursors.

We have studied a sol-gel method for processing the precursor "RexBa₂Ca₂Cu₃O_y" using the gelification of a solution by an in-situ polymerisation of acrylamide monomers. This method is much more rapid and safer than the so-called nitrate-citric route that involves nitrates, citrates and also ethylene glycol. We have succeeded to extend the cation concentration range previously available to that technique by using proper complexing agents. Precursors of mercury cuprates have been prepared in this way with excellent reactivity as proved by their easy combination with HgO to form performing superconductors. In-situ pressure measurements (TBA) were carried out to monitor the synthesis of the superconductor samples.

1PO5-39 Energy Transfer Between HTS Trapped Field Magnets

P.T. Putman and K. Salama, Texas Center for Superconductivity, University of Houston, Houston, TX

Presenting Author: P.T. Putman

Studies of HTS trapped field magnets usually focus on the peak force produced. In some applications, such as linear motors and accelerators, energy transfer is equally important. In this paper, we have studied the maximization of the kinetic energy of a YBCO rod pulled into a YBCO ring. A numerical model was developed to estimate the optimal parameters for maximum transferred energy. Experiments were then conducted employing top-seeded, melt-textured YBCO disks with varying lengths and radii. The rod and ring were field cooled so that current flowed in the same direction throughout each. To determine the energy transfer, force vs distance as the rod approached the ring was measured. Initial and final fields were measured using a scanning hall probe. Under these conditions, several factors are found to influence the transfer of energy, the most important of which are the energy stored in the magnetic field initially, the energy remaining when the rod is centered, the flux linkage, and flux drag losses. For better agreement with experiment, the model was refined to include the dependence of J_c on magnetic field.