Ag-clad Tapes Research Articles

Partial melting and HIP processing of Bi(2223): bulk and tapes

A detailed investigation of the equilibrium between the Bi,Pb(2223) phase and the melt was carried out by in-situ high temperature neutron powder diffraction both on sintered bulk samples and Ag-clad tapes. Stability, decomposition and re-formation of Bi,Pb(2223), as well as the evolution of secondary phases, were studied and the effect of the oxygen partial pressure was investigated. Bi,Pb(2223) melts incongruently to (Sr,Ca)/sub 14/Cu/sub 24/O/sub 41/, (Sr,Ca)/sub 2/CuO/sub 3/, and a Bi,Pb-rich liquid, and no precipitation of Bi(2212) was observed at this stage. Direct formation of the Bi,Pb(2223) phase from the melt was observed, opening up the possibility of processing Ag-sheathed tapes from a partial melting route. The possibility of reforming the Bi,Pb(2223) phase from the melt proved to be extremely sensitive to temperature and strongly dependent on the Pb-losses. The study of the mass losses due to Pb-evaporation was complemented by thermogravimetric analysis which proved that the Pb-losses are responsible for moving away from the equilibrium and therefore hinder the Bi,Pb(2223) to reversibly form from the melt. Hot isostatic pressure has proved to be an effective remedy for preventing volatile elements from evaporating and for increasing the density of the tapes. The first annealing stage of the Ag-Bi,Pb(2223) processing was performed under isostatic pressure after making the Ag-sheath airtight. Positive effects of pressure on filament density, formation kinetics and critical current were observed at 10 MPa. We have extended our search for an alternative processing route to the Pb-free Bi(2223) phase. High purity Bi(2223) bulk samples and Ag-sheathed tapes were successfully prepared from pre-reacted powders. Very large Bi(2223) grains (up to /spl sim/500 /spl mu/m) were observed to grow at the Ag-ceramic interface.

Bi,Pb(2223) equilibrium and decomposition: in situ high-temperature neutron diffraction study

The non-equilibrium reaction path which is commonly used to form the Bi,Pb(2223) phase inside Ag-clad tapes involves a small amount of a local transient liquid. New high-temperature reaction paths involving a larger amount of a stable liquid that are closer to equilibrium are needed to improve the Bi,Pb(2223)–Ag tape performance and achieve their high expected potential. In order to understand the reactions involved at high temperature during the Bi,Pb(2223) decomposition, an in situ neutron diffraction study has been carried out. Both Bi,Pb(2223) pellets and Ag-clad tapes have been melted and slowly cooled at different rates under various oxygen partial pressures. A phase analysis from the full pattern profile refinement has been performed. Bi,Pb(2223) decomposes into a Bi,Pb-rich liquid and (Ca,Sr) 2CuO 3 which is found to be the main phase at high temperature regardless of the oxygen partial pressure. Direct crystallisation of Bi,Pb(2223) from the melt has been observed in Ag-sheathed tapes and the critical current can be partially recovered after melting and slow cooling.

Preparation of Ag–Cu/Ni/Ag–Cu clad tapes for YBCO superconducting tape and its textured properties

The Ag clad tape consisting of Ag–0.1%Cu and some Ni alloys was prepared. The textured property, the proof stress at high temperature and the superconductivity of YBa 2Cu 3 (YBCO) film deposited by PLD were studied. The uniform clad tape with a length of 10 m and a thickness of 0.1 mm was obtained and it showed the textured structure of (2 1 0) 〈1 2 0〉 after annealing at 700°C. YBCO film deposited on the textured clad tape was aligned in-plane. From two to four times higher proof stress σ 0.2 at 700°C than that of Ag was obtained by cladding. High J c value of about 10 5 A/cm 2 was obtained on Ag–0.1%Cu/Ag–10%Ni clad tape.

Heat transfer and oxygen diffusion in Ag-clad BSCCO superconducting tapes

The quality of Ag-clad BSCCO superconducting tapes is a function of their processing conditions. Two important processing parameters are the cooling rate from the annealing condition, and the oxygen partial pressure in the furnace. Although a link between oxygen content and critical current density has been established, apparently no attempts have been made to predict the temperature and oxygen content distributions within a Ag-clad BSSCO tape. This study presents a combined thermal and mass transfer model which enables the calculation of the oxygen content distribution, and the results indicate a slow cooling rate, and relatively high oxygen partial pressure, leading to a greater and more uniform oxygen concentration within the tape, and thus potentially superior superconducting properties.

(Bi,Pb)-2223 phase formation in Ag clad tapes

The most promising material for fabricating long-length conductors is (Bi,Pb)-2223 produced by the powder-in-tube process in Ag clad tapes. Intensive studies have shown that the critical current density of fully processed tapes is affected by a large variety of processing parameters, but the nature of the (Bi,Pb)-2223 phase formation is still controversial. In this study certain steps of the 2223 phase formation were investigated at different stages of sintering under the influence of various temperature/time schedules of thermomechanical processing and the important role of Pb is shown. The main effects of Pb in the phase forming process are: (i) Acceleration of the 2223 formation via a rapid generation of (Bi,Pb)-2212; (ii) introduction of grain growth and texture; (iii) increase of the temperature window for the 2223 formation; (iv) decrease of reaction temperature; (v) adjustment of the hole concentration of the 2223 phase; (vi) operation as buffer for charge carriers via the temperature dependent equilibrium between Pb 4+ and Pb 2+; and (vii) creation of Pb containing pinning centres.

Advances in the development of high-temperature superconducting Bi-2223/Ag pancake coils for electric power applications

Abstract High-temperature superconducting (HTS) double-pancake coils have been fabricated using (Bi,Pb)2Sr2Ca2Cu3O10+ x (Bi-2223) multifilamentary (MF) Ag-clad tapes. The thermal-mechanical processing of Bi-based composites has shown an improvement in the transport properties of composite tapes by optimizing powder stoichiometry, deformation rate and reduction, sintering temperature and duration, and adjusted rate of heat treatment. Ag-clad Bi-2223 MF tapes up to 55 m have been fabricated using the powder-in-tube technique. An overall Ic of ∼7.5 A with a fluctuation along the length of less than 12% was achieved. An improvement in the electrical transport properties of ∼12 mT Bi-2223/Ag wind-reacted HTS double-pancake coils prepared from long partially processed tapes was achieved by means of post-sintering.

Connectivity and flux pinning improvements in Ag-clad BSCCO-2223 tapes produced by changes in the cooling rate

The rate at which Ag-clad (Bi, Pb)2Sr2Ca2Cu3Ox tapes are cooled from their final reaction heat treatment influences both the intergranular connectivity and intragranular flux pinning strength of the polycrystalline filaments. As the cooling rate from 825 °C to 730 °C in 7.5% O2 was decreased over a range of 5 °C/min to 0.005 °C/min, Jc (77 K, 0 T) increased from ∼8 to ∼24 kA/cm2, and the irreversibility field increased from, ∼120 to, ∼200 mT. The Jc (4.2 K, 0 T) increased in a similar fashion. Cooling slowly also sharpened the critical temperature transition and increased the critical onset temperature from 107 K to 109 K. These improvements in the superconducting properties occurred despite partial decomposition of the (Bi, Pb)2Sr2Ca2Cu3Ox phase into non-superconducting impurity phases during the slow cooling. A microstructural basis for these multiple effects is described.

The effect of wire drawing and cold rolling on the precursor 2212 phase texture and its subsequent influence on the (Bi,Pb)/sub 2/Sr/sub 2/Ca/sub 2/Cu/sub 3/O/sub x/ texture in Ag-clad tapes

We have studied the effect of wire drawing and cold rolling on the texture development of precursor (Bi,Pb) <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Sr <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> CaCu <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> 0 <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> (2212) phase and its influence on the subsequent (Bi,Pb) <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Sr <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Ca <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Cu <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> 0 <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">y</sub> (2223) phase texture in Ag-clad tapes. A mixture of (Bi,Pb) <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Cu0 <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> and SrCaCu0 <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> powders having an overall 2223 composition was reacted to form the 2212 phase at different stages of deformation. X-ray diffraction peak height analysis was used to quantify the 2212 texture F <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2212</sub> = I(OOS)/[I(OO8)+I(115)] at various oxide filament thickness. Wires containing powder reacted to the 2212 phase prior to loading into a Ag tube had F <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2212≈0.3 after drawing. This texture increased to F₂₂₁₂=0.7 upon rolling. Two other composites reacted to the 2212 phase immediately after drawing, and after being drawn and partially rolled, had worse initial textures, but improved during rolling to FzZl2≈0.7. Samples reacted to 2212 after rolling was complete had F₂₂₁₂≈0.35. The results indicate that the 2212 texture reaches a maximum value of F₂₂₁₂≈0.7 and remains at that level upon subsequent rolling. Samples of filament thickness ≈80 μm were then reacted to form 2223. For the samples which contained 2212 that had received at least some mechanical deformation, the 2223 phase formed with a texture factor F₂₂₂₃ &gt; 0.9, while the tape that contained 2212 formed after rolling developed a 2223 texture of only F₂₂₂₃≈0.75. The results show that there is a substantial texture enhancement upon reaction to 2223, which could be the result of either the 2223 formation mechanism or geometrically constrained grain growth.</sub>

A study of microstructural variations in BSCCO Ag-clad tapes and a method to improve grain alignment

The microstructural variations in BSCCO Ag-clad tapes obtained by a conventional fabrication procedure were studied by SEM and microhardness tester. A method to improve grain alignment in the tape by treatment of the magnetic field with mechanical vibration was developed and investigated.

- B - T surface of high- tape

Critical current density of an Ag-clad tape has been studied in the temperature range 7 - 77 K and magnetic field For , and the pinning force density show scaling behaviour with the scaling field corresponding to the field of maximum . At intermediate fields , whereas well below and above , shows the asymptotic variations with and 3 respectively. The thermally activated flux motion and the thermal smearing of the pinning potential describe equally well the temperature dependence of for K. These findings show that in finite fields well-prepared tape behaves as homogeneous high- superconductor (HTS). The implications of these results for the understanding of in HTSs and the applications of tapes are briefly discussed.

Development of high-T c superconducting Bi-2223 tape joints

High-T{sub c} superconducting joints between Ag-clad Bi-2223 tapes have been developed for persistent current applications. Two presintered tapes with one side of the silver stripped were lapped and then wrapped by a silver foil. The complex was uniaxially pressed followed by appropriate sintering to form a high-T{sub c} superconducting tape joint. It was found that the ratio of critical currents through the joint to that of the tape, I{sub cj}/I{sub c}, depended on the uniaxial pressure and the sintering conduction. At liquid-nitrogen temperature 77K, I{sub cj}/I{sub c} = 99% has been achieved. Persistent current loops formed by Bi-2223 tapes have also been fabricated and tested. Joint resistance of a loop was determined to be {approximately}4 {times} 10{sup {minus}13} {Omega} between the decay time of 120 and 3600 sec.

The use of nanophase powders for high Jc superconductor tape production

Advances in processing of superconductors has demonstrated that ceramic powders with exact composition, super-homogeneity, and extremely fine particle size can be prepared by chemical processing to achieve high density and uniform microstructure. The chemical processing routes have great advantage in preparing powder with high reactivity thus promoting solid state reactions required to produce ‘purer’ superconducting phase. We have studied extensively the coprecipitation technique for the preparation of superconducting ceramic powders. Chemical solution methods have been used in order to model several superconducting systems and to predict optimum conditions for obtaining exact composition. Several improvements on the control of precipitation reaction have been implemented in order to obtain powders of few nanometers in size. This has been done by carrying out the reaction in the bulk of a solution or in microemlusion systems. The corecipitated precursor powder has been used for production of 20− 30 meter long Ag-clad tapes. These tapes require considerably reduced processing times. By controlled heat treatment conditions, these tapes have been processed to carry transport critical current density, Jc <4 × 104 A/cm2 at 77K in zero applied field.

Sandwich Roiling, Jc, and pinning energy in Ag-sheathed BPSCCO superconducting tapes

A “sandwich rolling” process was developed to prevent the formation of sausaging and cracks in the longitudinal direction. The stress-strain state of the tape in “sandwich” rolling is the same as that of uniaxial pressed tape because the deformation of steel sheets is negligible in comparison to that of Ag-clad tape. Critical current densities of 3.2 × 104 A/cm2 at 77K and 2.7 × 105 A/cm2 at 4.2K and zero field Ag-sheathed Bi-based 2212 tapes have been achieved using a melt and atmosphere-controlled process. The comparison of pinning potential U0(B) < U(T-0, B) for Bi-2212 tape and Bi-2223 tapes consisting of a different fraction of 2212 phase as well as Bi-2212 and Bi-2223 thin films shows that for the same fields, the Uo for good quality 2223 tapes is at least 1.3 times that for the best 2212 tape and epitaxial thin films after taking into account the difference of the Tc between 2223 tape and 2212 tape, indicating that in BSCCO compounds, in addition to anisotropy, the specific pinning centers such as dislocations, introduced during processing, affect the flux motion at lower B.

Mechanism of deformation and “sandwich” rolling process in Ag-clad Bi-based composite tapes

Abstract The mechanism of deformation was studied by stress-strain state analysis in Ag-clad Bi-based superconducting composite tapes prepared by the powder-in-tube method. The sausaging morphology was investigated and compared in tapes produced by different deformation processes. A new deformation process called “sandwich” rolling was developed to prevent the formation of sausaging and cracks in the longitudinal direction. In this “sandwich” rolling, the tape is deformed between two steel sheets. The stress-strain state of the tape in “sandwich” rolling is the same as that of uniaxial pressed tape because the deformation of steel sheets is negligible in comparison to that of Ag-clad Bi-2223 tape. The stress-strain distribution of the superconducting oxide core can be controlled by the curvature and hardness of the steel sheets. The critical current density of the sandwich rolled tape is improved in comparison with the flat rolled tape.

High-temperature ultrasonic characterization of Ag-clad superconductor tapes

An ultrasonic nondestructive evaluation technique was developed to monitor liquid-phase evolution during heat treatment of high-T/sub c/ superconductors. The liquid phase is essential for microstructural and phase development of superconductors and is important in fabricating conductors with high critical current density, J/sub c/. Tapes were fabricated by a powder-in-tube process and then placed in a controlled-atmosphere furnace. During heat treatment, a magnetostrictive transducer launches a 140-kHz wave into the sample, and the relative change in acoustic velocity is then measured. A significant decrease in velocity was seen at the incongruent melting temperatures of Bi-based superconductors. In addition, Ag-clad tapes with Al/sub 2/O/sub 3/ and NaCl cores were used to validate the measurement technique. Advantages of this approach over other thermal analysis methods include in-situ analysis of final tape form, monitoring of isothermal liquid evolution, control of volatile species such as Tl and Pb.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

The effects of stoichiometry in the (Tl,Pb)/sub 1/(Sr,Ba)/sub 2/Ca/sub 2/Cu/sub 3/O/sub x/ system on the transport properties of Ag-clad tapes

Evidence shows that texturing is necessary to achieve high current densities and strong links in the Tl-based superconductors. Conventional sintering is not sufficient to achieve strong links, but instead, melt processing is required to produce platelets which can develop into a textured structure. Changes in the stoichiometry of the precursor powders can lead to enhanced melt-phase formation at lower processing temperatures. An experimental matrix was designed with variations in the Tl/Pb ratio, the total Tl/Pb content, and the Ca/Cu level to change liquid formation and subsequent grain growth. Thirteen compositions were prepared in both pellet form and as Ag-clad tapes to investigate grain development over a range of thermal processing conditions. Tapes were produced using unreacted powders by the powder-in-tube method and processed by conventional furnace annealing. We report here on the transport properties of these tapes, their strong link behavior, and microstructural development.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Effect of silver on phase formation and superconducting properties of Bi-2223/Ag tapes

The effect of Ag on the phase formation and superconducting properties of Ag-clad Bi-2223 tapes has been studied. TEM studies reveal that the grains adjacent to the Ag-sheath are nearly single 2223 phase with their c-axis perpendicular to the Ag-sheath, but in the interior of the superconductor core, an intergrowth of the 2223/2212 polytypoids within the grain and misalignment become evident. Ag addition in the Bi-2223 tapes lowers the annealing temperature and accelerates the formation of 2223 phase. However, its undesirable shape and large size cause grain misorientation and hence leads to a decrease of J/sub c/. The improvement in bend strain is attributable to the interfacial layer adjacent Ag sheath. The results of magnetoresistance of a well defined tape after correction for the parallel conduction through the Ag-sheath and the core show that the effective activation energy for the flux creep is larger than those for the single crystals and thin films. But the dissipative behaviour is similar to single crystals and the epitaxial thin films. >

Nanophase precursors for high-Jc Ag-clad superconducting BiPbSrCaCuO tapes

A controlled coprecipitation technique has been used to synthesize precursors of superconducting BiPbSrCaCuO ceramics. The oxalate precursors were nano-sized powders with high homogeneity, low crystallinity, and well-defined stoichiometry. These precursors have been used to fabricate superconducting tapes by the powder-in-tube technique, which involves the use of Ag sheaths. The nano-sized homogeneous powders require considerably reduced tape-processing (sintering) times for Bi-2223 formation, and provide 'clean' intergrain boundaries within the sintered tapes. As a result, high critical transport current density, Jc, can be obtained even in multiphase composites that do not necessarily exhibit a high degree of texture. To gain further insight into the role of microstructural details at each step of the tape fabrication, extensive X-ray diffraction analyses and scanning electron microscopy measurements have been made. The influence of homogeneity and grain size distribution on the superconducting properties of the Ag-clad tapes is discussed. We have reproducibly achieved, within 60 h of the thermal processing, Jc values as high as 4.4*104 A cm-2 at 77 K in zero applied magnetic field.

Anisotropic transport properties of BSCCO single crystalsand Ag-clad tapes in high magnetic fields

We report measurements of resistivity and critical current density on single crystals of Bi2212 with current flop both parallel and perpendicular to the CuO 2 planes and in magnetic fields to 18 T. For comparison, similar measurements pere performed on samples of Bi-2223 tape conductors with the Ag cladding removed. For current flop along the c-axis of the crystalsthe semiconducting behavior (resistivity increasing with decreasing temperature) is extended to belop 50 K in a field of 18 T, folloped by a sharp transition to the superconducting state. From I - V curves of c-axis transport pe extract the magnetic field and temperature dependencies of the interlaper critical current density J c , c and define a decoupling line H D ( T ) above phich J c , c vanishes. H D ( T ) lies close to but above the irreversibility H irr ( T ) obtained from magnetic measurements. pe also observe a decoupling induced belop H D ( T ) by the application of a magnetic field starting from the pero field cooled state, ZFC. The resulting “pancake vortex” density gradient causes a loss of c-axis phase coherence and produces a caxis resistance that relaxes logarithmicallp with time as the magnetipation decaps. These results are contrasted with similar measurements performed on highlp textured Bi-2223 tapes. For current flop along the tape normal (nominal c-axis) the resistivity is metallic and its temperature and field dependence scales with those for in-plane current flop. Similarlp critical currents for the two directions shop similar field and orientation dependence. These results are consistent with predominant percolative current paths along crpstallographic ab planes even for transport normal to the tape plane.

Hot rolling of powder-in-tube Li-doped Bi(2212) tapes

Powder-in-tube Li-doped Bi(2212) tapes with high critical currents have been obtained by a hot rolling process. Hot rolling was accomplished in the final rolling stage after partial melting by preheating the rolls with infrared heaters to various temperatures ranging from 300/spl deg/C to 410/spl deg/C and then rolling the tapes directly from a 750/spl deg/C furnace. T/sub c/ decreased after hot rolling and final heat treatment. Microstructural analysis shows improved grain alignment with increased hot rolling temperature. Due to the small thermal capacities of these Ag-clad BSCCO tapes, as well as the difference in temperature between the rolls and the furnace, the remnant liquid phase that termed from partial melting was melted during the 750/spl deg/C preheat and subsequently formed a glassy phase upon contact with the heated rolls. The amount of glassy phase was reduced with increasing roll temperature. Results show that the enhancement of J/sub c/ was due to an improvement of grain alignment and reduction in the amount of secondary phases. >