Microtexture and mesotexture in high-Jc Bi-2223

The microstructure of high-J_c Bi-2223 powder-in-tube tapes was studied using x-ray and electron diffraction. Although the c-axis is nominally aligned perpendicular to the tape surface (FWHM∼20‡), x-ray phi scans and pole figures show no evidence of any in-plane texture, either macroscopically or locally. Electron backscatter diffraction patterns acquired in a scanning electron miscroscope (SEM) were used to measure individual grain orientations. Grain boundary misorientation between adjacent grains was described by rotation angles and axes (i.e. the disorientation) and compared with theoretical values of ideal coincidence site lattices (CSLs). Data collected from over 113 spatially correlated grains resulting in 227 grain boundaries, show that over 40% of the boundaries are small angle. In addition, 8% of the boundaries were found to be within the Brandon criterion for CSLs (larger than ⌆1 and less than ⌆50). Grain boundary “texture maps” derived from the SEM image and orientation data reveal the presence of percolative paths between low energy boundaries.     

Processing and transport properties of high-Jc silver-clad Bi-2223 tapes and coils

The powder-in-tube process has been used to fabricate long lengths of flexible, high-J_c, silver-clad Bi-2223 HTS conductors. By improving thermomechanical processing and precursor powder preparation, we have succeeded in achieving J_c values of≥4×10⁴ A/cm² at liquid nitrogen (77K) temperature and >10⁵ A/cm² at liquid helium (4.2K) and liquid neon (27K) temperatures in short tape samples. Detailed measurements with high applied magnetic fields are reported. Several long tapes up to 10 m in length have also been fabricated and cowound into small superconducting pancake coils by the “wind-and react” approach. Transport measurements at 77 and 4.2K for these coils are also reported.     

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 U_o 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 T_c 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.     

Explosive Consolidation of (Bi,Pb)-Sr-Ca-Cu-0 Superconductor Powders

Superconducting (2212 phase) Bi-Sr-Ca-Cu-0 (BSCCO) and BSCCO-Ag composites were explosively consolidated in silver tubing and then drawn and rolled into tapes. These silver-sheathed tapes were then subjected to repeated cycles of pressing and heat treatment, which resulted in enhanced texturing and grain growth and a subsequent increase in critical current density (J_c). The effect of Ag powder additions to the superconducting powder further increased texturing and J_c, with optimal properties occurring with 10 vol.% silver powder additions. Scanning electron microscopy indicated that grain alignment increased and grain structure became refined after the thermomechanical treatment. X-ray diffraction studies indicated that grain orientation and conversion of 2212 phase to 2223 phase are improved when explosive consolidation is introduced before the drawing step in the powder-in-tube process.     

Microstructures and superconducting properties of V doped Bi2223 tapes

We prepared Ag-sheathed tapes by the powder-in-tube method using V doped Bi2223 powder. We found that V doping into Bi oxide superconductors greatly enhances the Bi2223 phase formation of the tapes without any degradation of the superconducting critical temperature T_c. The doped V is not included in the Bi2223 (or Bi2212) grains, but instead exists as Sr₆V₂O₁₁ particles with diameters of 0.1 to several micrometers, in which the larger plate-like particles play an important role to promote the formation of the Bi2223 phase. With V doping, the optimum critical current density J_c value is obtained for a short time heat treatment.     

Microstructural development and critical currents in long-length (Tl,Bi)−(Sr,Ba)−Ca−Cu−O wires

Synthesis of long Tl−Ba−Ca−Cu−O wires has been achieved byin situ synthesis techniques. Substitutions of Bi for Tl and of Sr for Ba were found to greatly enhance formation of the superconducting phase. Detailed microstructural characterization was performed on wires processed under various conditions in order to elucidate the reaction mechanisms. In addition, microstructural studies were carried out to investigate the beneficial effects of uniaxial compression on critical current density. It was found that thein situ reaction follows a “natural” two-powder process with early formation of Tl-1212. This reaction pathway provides a mechanism for sintering and crack healing and protects existing Tl-1223 for regrowth. Intermediate heat treatment allows more complete advantage to be taken of the above processes, while subsequent pressing increases the density and promotes grain alignment with minimum damage. Further heat treatment sinters grains and allows completion of the transformation to Tl-1223. Elements of this process have been incorporated in a sandwich-type geometry using plate-like grains to promote additional grain alignment.     

Influences of the precursor powder on the microstructure of HTSC-tapes

Wires and tapes are the most promising application of high temperature superconductor material for the future. For their preparation both the doctor-blade process (DBP) and the powder-in-tube((PIT) method with silver as a matrix material are mainly used. To obtain good electrical properties, the preparation had to start from a precursor material instead of fully reacted, phase pure superconducting powder. Tapes of Bi(Pb)−Sr−Ca−Cu−O 2212 and 2223 were fabricated by the DBP or PIT methods, respectively. We compared the suitability of precalcined precursor powders prepared by different routes and investigated their microstructure in the reacted wire or tape. Homogeneity, phase purity, and carbon content were investigated. The precursor powder prepared by a modified coprecipitation exhibited improved properties in comparison to other routes.     

Effect of lead content on Bi-2223 formation by a two-powder process

Decreasing the total lead content from Pb_0.4 to Pb_0.3 significantly stunts Bi-2223 phase development and greatly reduces the critical current density (J_c) of powder-in-tube tapes made by a two-powder process. This effect can be explained on the basis of the solubility limit for lead in 2212. Pb_0.4 samples exceed the solubility limit for lead in 2212, so lead is rejected to create a lead-rich liquid that enhances the kinetics of 2223 formation during heat treatment. By contrast, a liquid does not form in the Pb_0.3 sample because its lead content is below the solubility limit. As a result, 2223 formation is much slower and J_c is much lower in the Pb_0.3 sample.     

Recent status on high temperature superconducting Bi2Sr2CaCu2O8+x wire development at NYSIS: 1-90 meter length Jcs and 3 meter diameter ring furnace design

The status of long length, Bi₂Sr₂CaCu₂O_8+x (Bi-2212) wire development at the New York State Institute on Superconductivity (NYSIS) is reviewed and updated. Transport J_cs (4.2K, 0 T) of Bi-2212/Ag oxide powder-in-tube singlefilamentary tapes have reached 70,000-80,000, 50,000-60,000, and 30,000–40,000 A/cm² for 1, 4–15, and 40–90 meter length tapes, respectively. The decrease in J_c as the tape length was increased from 15 to 90 meters was attributed to the (measured) sensitivity of J_c to temperature nonuniformities (±3‡C) in the box-type furnace used for annealing. To reduce this problem, a ringtype high-temperature furnace (∼3 meter diameter) was designed and constructed which provides a large-volume (∼13^w × 10^h × 1000¹ cm) processing zone with expected excellent temperature uniformity (±0.5‡C). The advantages of the ring-type furnace for processing of kilometer-length conductors are described.     

Fabrication of Bi2Sr2CaCu2O x and Bi2Sr2Ca2Cu3O x Superconductor Thick Films by Using Cu-Free Precursors on Ni Substrates

We studied the formation of Bi₂Sr₂CaCu₂O _x (Bi-2212) and Bi₂Sr₂Ca₂Cu₃O _x (Bi-2223) thick films in a heat treatment process of the Ni-sheathed Bi-Sr-Ca-Cu-O (BSCCO) system. Cu was electrodeposited initially on the Ni substrates (Cu/Ni). Well-oriented Bi-2212 superconductor thick films were formed successfully on Ni tapes by liquid reaction between Cu-free precursors and Cu/Ni tapes. However, only a small amount of Bi-2223 was formed. Thick films were prepared by screen-printing with Bi₂O₃, SrCO₃, and CaCO₃ powders on Cu/Ni tapes and heat treating them. Heat treatment was performed in the temperature range of 750–850°C in a tube furnace for several minutes to hours. The phases and the microstructures of the high temperature superconductor thick films were analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. Electrical properties were examined by the standard four-probe method. At the heat treatment temperature, the specimens were in a partially molten state during reaction between the oxidized copper layer and the screen-printed precursors on the Cu/Ni tapes.     

Reactive sintering of Bi2Sr2CaCu2Ox superconductors

Three phase assemblages were used to produce Bi₂Sr₂CaCu₂O_x (2212) during sintering: a mixture of 20% Ca-rich 2212+80%2212, partially synthesized 2212, and Bi₂Sr₂CuO_x (2201)+(1/2 Ca₂ CuO₃+1/2 CuO) (denoted 0011). The mixture of 2201+0011 produced highly pure 2212 within 50 h of heating in air at ≈850°C. Ag tubes were filled with a mixture of 2201+0011 and worked into tapes by a powder-in-tube process. Heat treatments produced microstructures consisting of small, highly textured 2212 grains. T_c values were ≈70K. Transport J_c values at 4.2K were ≈10⁴ A/cm².     

Fabrication of Ag-Sheathed Bi-2223 tapes using powders produced by aerosol spray pyrolysis

Bi-2223 tapes were manufactured from a fine “two-powder” product produced by using an aerosol spray pyrolysis technique. Critical current density of 22000 A/ cm2 at 77K and 0 T was achieved. Nondestructive transmission x-ray diffraction study indicated good alignment of the superconducting grains. The texturing process of the superconducting phase was found to be nearly complete after the first 24 h of heat treatment for the samples studied. Pressing was found to play little role in the texturing process. The texturing can be enhanced by Ag-doping. J_c, however, was not found to be improved significantly, presumably due to the reduced effective cross-sectional area. A new phase, Bi-4435, was identified which may play a significant role in the formation of 2223. On leave from Northeastern University, Shenyang, P.R.C. On leave from Kobe Steel Ltd., Kobe, Japan     

Role of silver addition on mechanical and superconducting properties of high-Tc superconductors

The effect of silver additions on the mechanical and superconducting properties of sintered bulk YBa₂Cu₃O_δ (YBCO), Bi₂Sr_1.7CaCu₂O_δ (BSCCO-2212), and Bi_1.8Pb_0.4Sr_2.2Ca₂Cu₃O_δ (BSCCO-2223) has been evaluated. Strength and fracture toughness of YBCO and BSCCO bars increased with increasing Ag content up to 30 vol.% Ag. Addition of 30 vol. % Ag to YBCO increased strength from 87 to 136 MPa and fracture toughness from 1.82 to 3.9 MPa√m. Addition of 30 vol.% Ag to 2212 and 2223 increased strength from 58 to 107 and 41 to 90 MPa, respectively. Corresponding increases in fracture toughness were from 1.89 to 2.79 and 1.09 to 1.94 MPa√m, respectively. These improvements in strength and fracture toughness are believed to be due to the presence of Ag particles that may induce compressive stresses in the superconducting matrix and resist crack propagation by pinning the propagating cracks. The values of strength and fracture toughness of BSCCO-30 vol.% Ag specimens are comparable to those of monolithic BSCCO obtained by sinter forging, hot pressing, and hot isostatic pressing. On the other hand, the hardness of YBCO and BSCCO decreased with increasing Ag contents because of the lower hardness of Ag. Addition of Ag showed no adverse effects on superconducting properties (J_c and T_c) of YBCO or BSCCO superconductors.     

Finite element modeling of the powder-in- tube process for manufacture of BSCCO-2212 superconducting wires

High-temperature superconductors have recently attracted a great deal of attention owing to their potential use in a variety of applications including power generators, superconducting magnets for mine sweepers or ship propulsion motors, and magnetic levitation transportation systems. The powder-in-tube (PIT) process has emerged as one of the most promising and economically feasible techniques to produce long lengths high-T_c oxide based superconducting wires. The PIT method involves multi-pass wire drawing followed by rolling and heat treatment. This work focuses on the development of finite element models to simulate the PIT drawing process for fabrication of silver sheathed Bi-2212 superconducting wires. The numerical models were used to predict the density of the oxide powder, the wire drawing forces, and the silver-oxide ratio during drawing. A cap-type pressure dependent constitutive equation was implemented in the model to simulate the powder behavior. The model incorporated experimentally obtained material data for the silver and powder. Data from wire drawing experiments were used to verify model predictions.     

Effect of site-selective substitution in bulk superconducting Tl2Ba2CaCu2O8

Magnetization measurements were carried out on bulk Tl₂Ba₂CaCu₂O₈ (referred to as Tl-2212) and on various site-selective substituted Tl-2212 samples. At 5K between 0 and 4.5 T, the 5 at. % Mg-doped Tl-2212 (Tl,Mg-2212) samples displayed enhanced pinning as demonstrated by a field dependent increase of the magnetic critical-current density J_c by 18 to 25 percent over that of pristine Tl-2212. Excess Mg (10–15 at. %), however, is deleterious. Rietveld refinement of the x-ray diffraction pattern showed Mg on the Tl sites. Auger electron spectroscopy analysis showed part of the Mg on grain boundaries. The flux-creep activation energies are higher for flux expulsion than for flux penetration in both Tl-2212 and Tl,Mg-2212 samples; the latter displays higher individual values. Our results demonstrate an increase in the number density of flux lines as a result of increased density of atomic-size-structural, defects by Mg (5 at. %) doping. In the Tl_2−yBa₂(Ca_1−zY_z)Cu₂O_8−x(z=0–0.3; single phase; x and y represent oxygen and thallium vacancies) system also studied, the T_c decreases as z increases. At z=0.3, the sample becomes an antiferromagnetic semiconductor.     

Engineered flux-pinning centers in Bi2Sr2CaCu2Ox and TIBa2Ca2Cu3Ox superconductors

Three methods were used to introduce flux-pinning centers into Bi₂Sr₂CaCu₂O_x (Bi-2212) and TlBa₂Ca₂Cu₃O_x (Tl-1223) samples. It was found that carbon induced local decomposition, that nanosized Al₂O₃ additions created stable reaction products, and that second phases could be isolated in Tl-1223 during synthesis. Each of these defects enhanced flux pinning and was of most benefit at temperatures ≤ 35K.     

Long lengths of silver-clad Bi2223 superconducting tapes with high current-carrying capacity

Long lengths of silver-clad (Bi,Pb)₂Sr₂Ca₂Cu₃O₁₀ (Bi2223) high-T_c multifilamentary tapes were produced using the powder-in-tube (PIT) technique followed by a thermomechanical process. The relationships between microstructure and electrical, magnetic and mechanical properties of the heat treated tape were evaluated from the critical current density measurements, irreversibility magnetic field determination and mechanical bending tests. Emphasis was stressed on the J_c behavior in magnetic fields at different temperatures. A J_c of 10,000 A/cm² at 77 K in a zero field for a 10 m tape and 75,000 A/cm² at 23 K in a field of 3 T for a short tape was achieved. The results obtained showed that Bi2223/Ag high-T_c composite tapes are a potential alternative to conventional low-T_c superconductors in magnetic levitation (MAGLEV) applications.     

Recent issues in fabrication of Ag-clad BSCCO superconductors

Long lengths of mono-and multifilament Ag-clad BSCCO superconductors were fabricated by the powder-in-tube technique. Critical current density (J_c) up to 12,000 A/cm² has been achieved in an 850 m long multicore conductor. Long length conductors were formed into pancake-shaped coils by the wind-and-react approach. Test magnets were then fabricated by stacking the pancake coils and connecting them in series. The magnets were characterized as a function of applied magnetic field at various temperatures. A test magnet, fabricated with ≈770 m of BSCCO tape, generated fields of ≈1 T at 4.2K and ≈ 0.6 T at 27K, both in an applied background field of 20 T. Additionally, the strain tolerance of both mono-and multifilament conductors at 77K in 0.5 T applied field has been studied. We observed that multifilament conductors have better strain tolerance than monofilament tapes, retaining more than 90% of the initial critical current (at 0.5 T) with strain ≥1%.     

Fabrication and characteristics of tapes and test magnets made from Ag-Clad Bi-2223 superconductors

Pb_0.4Bi_1.8Sr₂Ca_2.2Cu₃O_x (Bi-2223) precursor powder was prepared by a solid-state reaction of carbonates and oxides of lead, bismuth, strontium, calcium, and copper, and the powder was then used to fabricate silver-clad tapes by the powder-in-tube technique. Transport critical current density (J_c) values>4×10⁴ A/cm² at 77K and 2×10⁵ A/cm² at 4.2 and 27K have been achieved in short tape samples. Long lengths of tape were tested by winding them into pancake coils. Recently, we fabricated a test magnet by stacking ten pancake coils, each containing three 16m lengths of rolled tape, and tested it at 4.2, 27 and 77K. A maximum generated field of 2.6 T was measured in zero applied field at 4.2K and the test magnet generated significant self-field in background fields up to 20 T. The results are discussed in this paper.     

Processing and physical properties of single phase Tl2Ba2CaCu2O8 and (Tl0.5Pb0.5)Sr2−xBaxCa2Cu3O9 superconductors

Single phase Tl₂Ba₂CaCu₂O₈ (2212) superconductors with T_c of 97 K were obtained by a two-step synthesis. Prolonged annealing at 860°C in the second step of the synthesis resulted in a higher T_c, at the expense of the growth of an impurity trilayer 2223 phase in the form of stacking faults in the 2212 phase. Nearly single phase (Tl_0.5Pb_0.5)Sr_2−xBa_xCa₂Cu₃O₉) (1223) samples were obtained in a single air-sintering step. Replacement of Sr by Ba is necessary to grow the 1223 phase. Samples with x=0.75 displayed T_c as high as 117 K with a superconducting volume fraction over 50%.