4PO6-40 to 4PO6-45

4PO6-40 Active Random Composites. made of Superconducting Phase Bi-2212 and Tin or Lead. Percolation Conductions of Normal Electrons and Cooper pairs

Abdelkrim Ouammou, Département de chimie,Faculté des Sciences Dhar Mehraz, Université Sidi Mohamed Ben Abdellah, B.P. 1796 Atlas, Fès, Morocco, and Carel Claude, Groupe de Cristallochimie et Biomatériaux, UMR du CNRS 6511, Université de Rennes1- CS 74205, 35042 Rennes Cedex, France

Presenting Author: A. Ouammou

The fabrication and characterization of granular composites made of a high T_c superconductor and a low melting metal is synthetized with adjustable mechanical and electrical properties in order to optimize interface effects and switching properties.

The first part concerns the synthesis and the study of the chemical and physical properties of pure ceramic Bi-2212 obtained from a method based on gel precipitation and the composites of it and the binding agent (tin or lead). The second part concerns the 3d-percolation thresholds of conduction (normal electrons and Coper pairs) by fitting the electrical characteristics R(T).

Stress-strain curves based on bresilien compression tests at room temperature are studied as a function of the composition. All the curves resemble patterns of elastic deformation. The maximum compressive strength as well as the elastic modulus increase with tin content.

In order to model the percolation transitions in the different composites, the effective medium approximation (E.M.A.) is used. It fits very well the experimental variation at 300 K of the resistivity in pressed composites with tin and in sintered composites with Sn and Pb.

4PO6-41 Effect of Ultra-fine MgO Doping on Flux Pinning Properties of Bi-2223/Ag Superconducting Tapes

Ling Hua 1, Guansen Yuan 1, Jaimoo Yoo 2, Haidoo Kim 2, Hyungsik Chung 2, and Guiwen Qiao 3. 1 General Research Institute for Nonferrous Metals, Beijing 100088, China. 2 Korea Institute of Machinery and Materials, Changwon, South Korea 641-010. 3 Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110015, China.

Presenting Author: L. Hua

The ultra-fine MgO particles were doped into Bi-2223 precursor powder. The effect of MgO doping on flux pinning properties of Bi-2223/Ag tapes fabricated by powder-in-tube (PIT) technique was investigated. The transport critical current density (J_ct) of 1 wt% MgO doped samples sintered at 835 ° C and/or 839 ° C is improved significantly in comparison with the undoped samples. The 1 wt% ultra-fine MgO doped samples show superior J_ct-B behavior to the undoped samples. At 77 K, a J_ctof 33.6 kA/cm² in self-field, and 11.2 kA/cm² at 0.1 T (H//c-axis) has been achieved in the 1 wt% MgO doped samples. However, excessive doping (³ 3 wt%) and high sintering temperature are verified to damnify the transport properties because of severe agglomeration of MgO particles and the growth of residual secondary phase.

4PO6-42 Effect of Pb Content Variation on Microstructure and Critical Current Density of Bi-2223/Ag Superconducting Tapes

Presenting Author: L. Hua

The effect of Pb content variation (x = 0.3 ~0.4 in Bi_1.8Pb_xSr₂Ca_2.2Cu₃O_y) on microstructure, phase evolution, Bi-2223 grain growth and critical current density of silver sheathed Bi-2223 tapes has been studied by a combination of XRD, SEM and EDX analysis. The results indicate that high Pb content samples have higher conversion rate of Bi-2212 to Bi-2223 than low Pb content samples. However, it has been observed that too high Bi-2223 formation rate is not beneficial to Bi-2223 grain growth and thus the achieved critical current density in the tapes. By fine tuning the Pb content and controlling the relevant process parameters, a reduced volume fraction of secondary phases and a further increase in Bi-2223 grain size may be obtained. The influence of coarse particles in the precursor powder on the phase formation has also been investigated in this work.

4PO6-43 A review of elastic behavior of HTSC materials

P. Venugopal Reddy, Department of Physics, Osmania University, Hyderbad-500 00, India

Presenting Author: P.V. Reddy

The author of the research paper along with his group of students, has been working, on various aspects of the elastic behavior of High T_c superconductors, over a period of more than a decade. As a Principal Investigator of a Dept. of Science and Technology (DST) project in the subject, under National Superconductivity Program (NSP), considerable amount of work has been carried out. During this period, the author has been trying to understand mainly the origin of the elastic anomalies especially above the superconducting transition temperature T_c. Efforts were also made to investigate possibility of arriving at a relationship between high value of T_c and the observed elastic anomalies above T_c. For this purpose, a variety of HTSC materials numbering around 100 prepared by ceramic, sol-gel, co-precipitation routes, were used. With view to standardize the observed data, a few more materials prepared by the well known Melt Powder Melt Growth (MPMG) and Oxygen Controlled Melt Growth (OCMG) techniques by International Superconductivity Technology Center (ISTEC)

Japan, which are known for their high values of J_c and T_c, were also used.

A close examination of the elastic behavior all the types materials mentioned above clearly indicate that the elastic anomalies among ceramic superconductors prepared by solid state, sol-gel and co-precipitation techniques especially in the temperature region 100 - 250K are very common. However, it has been observed that in the case of MPMG materials, depending on their microstructure, these anomalies are found to be in the process of phasing out slowly. Finally, in the case of OCMG materials, which are supposed to have a structure very much close to that of a single crystal, the elastic anomalies are totally found to be absent. A comprehensive explanation of all the three different and distinct types of behaviors may be given on the basis of a qualitative model. According to this model, the mechanical properties of a ceramic material in general and oxide superconductors in particular are very sensitive to their microstructure. The elastic anomalies are likely to be present only when the grains of a sintered HTSC material are grown to a sufficient size, and the phenomenon may not take place in a material with fine grains (< 4 m meters ) and higher density (> 90%). Therefore, the grain size of the ceramic superconductors is very important micro structural parameter and its contribution to the elastic anomalies cannot be ignored. Thus occurrence of elastic anomalies in some of the HTSC materials under investigation may be attributed to thepresence of a large number of coarse grains and low contact area among the grains of a sintered HTSC material.

4PO6-44 The interaction of phase slip centers in mesoscopic aluminum line

Xueqiang Zhang 1, Lifang Xu 1, Yusheng He 1, Zhengxiang Gao 2, Shiguang Wang 2, and Yuandong Dai 2. 1 Institude of Physics & Center for Condensed Matter Physics, CAS, P.O. Box 2711, Beijing, 100080, P.R. China. 2 Department of Physics, Peking University, Beijing, 1000871, P.R. China.

Presenting Author: Y.S. He

The superconducting properties of mesoscopic aluminum structures have been studied widely in recent years. Many new anomalous features have been found. The temperature dependence of resistance curve shows a peak near the superconducting transition temperature; In the I-V characteristics there is a voltage peak near the critical current. These phenomena could be explained in terms of the charge imbalance associated with phase slip center (PSC). In this paper, the property of a mesoscopic aluminum line sample was studied. We found step structures in the I-V characteristics, which we believe are due to the formation of phase slip centers. With decreasing temperature, the step structure is replaced by peak structure. The anomalous behavior was studied in weak magnetic fields. We find they are very sensitive to the fields. Even a small field of 1 mT can suppress the peaks. The temperature and magnetic field dependence of the charge imbalance length was analyzed. We propose the interaction between the PSCs results the anomalous peaks in the I-V characteristics

4PO6-45 Effect of g -irradiation on superconducting transition temperature and resistive transition in polycrystalline YBa₂Cu₃O_7-_d

Boris I. Belevtsev 1, Irina V. Volchok 2, Nina V. Dalakova 1, Vladimir I. Dotsenko 1, Leonid G. Ivanchenko 1, Anatoly V. Kuznichenko 3, and Igor I. Logvinov 1. 1 B. Verkin Institute for Low Temperature Physics & Engineering, Kharkov, 310164, Ukraine. 2 Kharkov State Technical University of Agriculture, Kharkov, 310002, Ukraine. 3 Kharkov State University, Kharkov, 310077, Ukraine.

Presenting Author: I.V. Volchok

The bulk polycrystalline sample of YBa₂Cu₃O_(7-_d₎ (d » 0.1) has been irradiated by g -rays with 60C source. Non-monotonic behavior of T_c (defined as the temperature at which normal resistance is halved) with increasing irradiation dose F (up to about 220 MR) is observed: T_c decreases at low doses (F £ 50 MR) from initial value (» 93 K) by about 2 K and then rises, forming minimum. At highest doses (F ³ 120 MR) T_c goes down again. The temperature width, d T_c, of resistive transition increases rather sharp with dose below 75 MR and somewhat drops at higher dose. We believe that this effect is revealed for the first time at g -irradiation of HTSCs. The cross sections for the displacement of lattice atoms in YBCO by g -rays due to the Compton process were calculated, and possible dpa values were estimated. The results obtained are discussed taking into account that sample studied is granular superconductor and, hence, the observed variations of superconducting properties should be connected primarily with the influence of g -rays on intergrain Josephson coupling.