1PO5-20 The Changes of Microstructure During Solidification in Bi-2212/Ag Tapes

A. Matsumoto, H. Kitaguchi, H. Kumakura, and K. Togano, National Research Institute for Metals, 1-2-1, Sengen, Tsukuba, Ibaraki 305-0047, Japan

Presenting Author: A. Matsumoto

Bi2212/Ag tapes fabricated by melt-solidification process have high critical current density (J_c) because of the excellent grain alignment and strong grain coupling. It is reported that the highest local J_c in Bi-2212/Ag is obtained for the region near the oxide/silver interface. However, the microstructure changes during the heat treatment have not well understood so far particularly at the oxide/silver interface. We studied the microstructure changes in the oxide part and the oxide/silver interface during solidification process of Bi-2212/Ag tapes.

We prepared the sample by a dip-coating technique. The oxide layers of these samples partially melted at maximum processing temperature Tmax, and then slowly cooled at the rate of 5° C/h. The samples were oil-quenched at various temperatures during solidification process. We removed the silver substrates by the reaction with mercury. The microstructure changes were examined by using SEM observation and XRD analysis

At Tmax, impurity phases such as Bi-free and Cu-free solid phases exist in the liquid. The solidification of Bi-2212 phase starts near the free surface and the oxide/silver surface. No obvious evidence can be confirmed for Bi-2212 nucleation at the second phase/liquid interface. No relation can be observed between Bi-2212 grain growth and grain boundaries of silver substrates. Bi-2212 grains grow along silver interface or free surface, and then grow into the inside of molten oxide region randomly. These growth mechanisms give an explanation for the excellent grain alignment at the oxide/silver interface and the free surface and the poor grain alignment in the middle of oxide part.

1PO5-21 YBaCuO solidification window determination from magnetic susceptibility

Eric Beaugnon, Xavier Chaud, Dirk Isfort, and Robert Tournier, CNRS, laboratoire de Cristallographie - CRETA, BP166 38042, Grenoble Cedex 09, France

Presenting Author: E. Beaugnon

Succeeding in growing large YBaCuO monodomain from the top seeding melt texturing growth requires an accurate knowledge of the solidification window which is defined as the temperature range between the onset of the nucleation from the seed and the parasitic nucleation from the bulk.

Magnetic susceptibility measurements at high temperature during an actual process allows the monitoring of all phase changes in the YBaCuO system: this technique, based on a Faraday balance, gives real time information about the transformations in the bulk of the material.

The width of the solidification window and the effect of composition, precursors origin and overheating prior to the peritectic recombination have been studied. It is shown that the growth of monodomains can be achieved only in a very narrow temperature range and that the temperature of the nucleation from the bulk is strongly dependent upon these parameters. These results confirm efficient ways to enlarge the solidification range and hence help for the growth of very large monodomains.

1PO5-22 Flux Motion of melt processed YBCO in pulsed magnetic fields

Takeshi Kono 1, Masaru Tomita 2, Masato Murakami 2, and Takeo Takizawa 1. 1 Department of Physics., College of Humanities and Sciences, Nihon University, 3-25-40 Sakura-Josui, Setagaya-ku, Tokyo 156-8550, Japan. 2 Superconductivity Research Laboratory, ISTEC, 1-16-25 Shibaura, Minato-ku, Tokyo 105-0023 Japan.

Presenting Author: T. Kono

Flux motion has been observed using Hall probe sensors which are placed on the surface of high quality melt-processed YBCO disk with applying pulse magnetic fields. The effect of bias static magnetic fields was also studied. We found that fluxoids tend to move more easily as the static magnetic field is increased, presumably due to the fact that the pinning force is depressed. This phenomenon is explained by magnetic field dependence of critical current density (J_c) obtained from V-I characteristics. It was also found that the flow resistivity (· _f) increased with increasing magnetic field.

1PO5-23 Texturing process, superconducting and mechanical properties of Ag doped top seeded melt grown YBCO pellets

Christelle Leblond-Harnois 1, Renaud Caillard 2, Isabelle Monot-Laffez 1, Gilbert Desgardin 1, Bernard Raveau 1. 1 CRISMAT Laboratory, UMR 6508 CNRS/ISMRA, 6 Bd Marechal Juin, 14050 CAEN Cedex, France. 2 LERMAT Laboratory, ISMRA, 6 Bd Marechal Juin, 14050 CAEN Cedex, France.

Presenting Author: C. Leblond-Harnois

Well textured pellets of YBCO with large diameter are now routinely obtained using the top seeded melt textured growth process. But, the low mechanical properties of this material represent a limiting factor to the development of practical applications. Silver additions have been shown to improve mechanical properties. The obtention of textured pellets of Ag doped YBCO is then of great interest.

We focus this study on cerium doped YBCO with 5 and 10 wt% of silver. DTA measurements on Ag+Ce doped YBCO have revealed a decrease of the YBCO decomposition temperature of 70°C and 50°C in comparison to pure YBCO and Ce doped YBCO respectively, allowing to texture samples with a maximum temperature of 1000°C. The nucleation temperature windows were determined. A shift of 20°C was found between the 5 wt% and 10 wt% Ag doped mixtures. Pellets of the two compositions were successfully textured.

Although TGA curves reveal a relatively low oxygenation rate, the Bean critical current density of a 50 h annealed sample reaches 50 kA/cm² in self field at 77K and the annealing optimisation is still in progress.

The mechanical properties of these materials have also been investigated, showing the improvements brought by this addition.

1PO5-24 Uranium Fission Fragment Pinning Centers in Melt-Textured YBCO

Ravi-Persad Sawh 1, Roy Weinstein 1, Yanru Ren 1, Victor Obot 2, and Harald W. Weber 3. 1 Department of Physics and TCSUH, University of Houston, Houston TX 77204. 2 Department of Mathematics, Texas Southern University, Houston TX 77004. 3 Atomic Institute of the Austrian Universities, A-1020 Vienna, Austria.

Presenting Author: R. Sawh

In the U/n process YBCO powders are doped with U, textured, and then irradiated with thermal neutrons. Deposits of (U_0.6Pt_0.4)YBa₂O₆ (= Y-5) 300nm in size are formed during texturing. Prior to irradiation Y-5 deposits increase J_c about 10% per 0.1% M_U, where M_U is the % by weight of uranium. We find the number of Y-5 deposits µ M_U. Hence, inter-deposit spacing, S µ M_U^-1/3. Irradiation of uranium-doped YBCO with neutrons (~95% thermal) produces fission fragment pinning centers. The effective length, d , of these pinning centers was estimated theoretically to be 2µm to 4µm. The chemical and irradiation pinning centers increase J_c(B_A,T) by factors of 14-40. Peak J_c depends only on F_n X M_U(235), where F_n is the number of neutrons per square centimeter. In the work reported here, M_U is varied from 0.0375% to 1.0%, with M_U(235) = 0.025%, thus varying S by a factor of 3. The enhancement of trapped field, R_M, decreases from 500% for 0.15% M_U to 400% for 0.0375% M_U, indicating d ~ 2.7µm. Anisotropy decreases monotonically from 3 to 2 with increasing F_n. Optimum Fn for various M_U(235) is discussed. Residual radioactivity a few months after irradiation is also reported.

1PO5-25 Improvement of the mechanical properties and durability of bulk-superconductors with resin impregnation

Masaru Tomita and Masato Murakami, Superconductivity Research Laboratory, 1-16-25, Shibaura, Minato-ku, Tokyo 105-0023, Japan

Presenting Author: M. Tomita

Large single-grain bulk RE-Ba-Cu-O (RE: rare earth elements) superconductors can trap large fields exceeding several teslas and thus can function as very strong quasi-permanent magnets, however, the maximum trapped field is essentially limited by the mechanical strength of bulk-superconductors. The stress produced by refrigeration sometimes causes cracking. A large electromagnetic force will also act on the superconductors, when they trap large magnetic fields, which occasionally leads to the fatal failure. We have recently found that epoxy resin can penetrate into the bulk superconductors under proper conditions. Microstructural observation revealed that microcracks as well as macrocracks and porosities can be impregnated with epoxy resin, which greatly improves the mechanical properties of bulk RE-Ba-Cu-O and thus results in the improvement of field trapping capability.

This work was supported by New Energy and Industrial Technology Development Organization (NEDO).

1PO5-26 Factors Affecting the Joining of Superconducting Materials

C. Vipulanandan, W. Lu, and A.N. Iyer, Materials Engineering Labotatory, TCSUH, Department of Civil and Environmental Engineering, University of Houston, Houston, TX 77204-4791

Presenting Author: C. Vipulanandan

Methods of joining BSCCO superconducting bulk and tapes were investigated. Superconducting monolithic and composite Bi(2223) bulk rectangular prism specimens with and without silver addition were fabricated using a combination of high temperature sintering and cold isostatic pressing. Silver powder addition increased the critical current at early stages of the processing. Various butt joint configurations (by varying the contact angle and surface area) were investigated to optimize the joint section. Silver paste was used to further improve the joining process. Of the various thermomechanical method investigated, hot pressing followed post sintering resulted in the highest joint efficiency. Hot pressing between 810 and 850 ° C under a uniaxial pressure of 35 MPa for 1 hour was the most efficient way to form the Bi(2223) bulk joint. Microstructural development of the joint was investigated using SEM. The maximum joint efficiency of 100% was achieved by optimizing the joining process.

Monofilament Bi(2223) tapes were fabricated using the powder-in-tube method and groove rolling with critical current in the range of 50 to 70 A. Lap and butt joint configurations were investigated for the green tape. Various thermomechanical processes were investigated by changing the pressure, temperature and processing time. Maximum joint efficiencies for butt and lap joints achieved were 30% and 80% respectively. The phase and microstructural development at the joints were investigated using XRD and SEM.

1PO5-27 Fabrication of and Transport Studies on Doped PrBa₂Cu₃O_7-d (PBCO)

Tar-Pin Chen, John L. Wagner, Udom Tipparach, and Xiaochu Yuan, Physics Dept., University of North Dakota, P.O. Box 7129, Grand Forks, ND 58203-7129

Presenting Author: T.P. Chen

The lattice parameters, fabrication conditions, and thermal expansion coefficient of PBCO are almost the same as those of YBa₂Cu₃O_7-d . This makes PBCO an ideal buffer layer for YBCO/PBCO multilayers/superlattices if PBCO has high enough electrical resistivity to insulate adjacent YBCO layers. Experimental results, on the other hand, indicated that the PBCO buffer layer in multilayers/superlattices behave more like a metal than an insulator. This behavior can be explained if some of the Ba has not been replaced by Pr and still remains in the Ba sites. To improve the dielectric properties but retain other excellent buffer layer properties of PBCO, we fabricated and studied a series of PrBa₂(Cu_1-xT_x)₃O_7-d (where T is a transition metal element). X-ray powder diffraction measurements show that the doped samples have same crystal structure as the PBCO and the lattice parameters for the doped samples also match excellently to the lattice parameters of YBCO. However, transport studies show that some of the dope materials has resistivity orders of magnitude higher than that of PBCO and should be ideal as a buffer layers for YBCO superconducting electronics and superlattices.

1PO5-29 Increase in T_c of YBa₂Cu₃O_y by Oxygen Plasma Treatment

Wu. M. Chen 1,2, Shu. S. Jiang 1, Yuan C. Guo 2, Hua K. Liu 2, and Shi X. Dou 2. 1 National Laboratory of Solid State Microstructures, Nanjing University, Nanjing 210093, P.R. China. 2 Institute for Superconducting and Electronic Materials, University of Wollongong, Wollongong, New South Wales 2522, Australia.

Presenting Author: S.X. Dou

Oxygen annealed YBCO samples were treated with oxygen plasma at room temperature for 40 minutes. Oxygen content of the samples was determined by an accurate iodometric titration method. X-ray diffraction analysis was carried out in order to determine their structure. Superconducting transition temperature, T_c, was determined by a conventional four-probes method. The T_c of the annealed sample with zero resistance is determined as 91.5K for the untreated sample. After 40 minutes-plasma treatment, the values of T_c increase from 91.5K to 96.0K, and the room temperature resistance also increases from 19ÊÍ to 24ÊÍ. On weak-coupling BCS theory, superconducting transition temperature, T_c, is proportional to the energy gap 2D. After oxygen plasma treatment, the value of T_c increases, i.e. energy gap D increases, too. Believed, to reveal the mechanism of oxygen plasma enlarging energy gap D and rising T_c is significant for experimental and theoretical researches.