Fabrication of Ag/Bi₂Sr₂Ca₁Cu₂O_8±x superconducting tapes by the oxidation and postoxidation (partial melt) annealing of malleable, metal-bearing precursors

Abstract

A solid metal-bearing precursor route has been used to fabricate Ag-sheathed Bi 2 Sr 2 Ca 1 Cu 2 O 8± x (2212) superconducting tapes. Intimately mixed Bi 2 O 3 -Sr-Ca-Cubearing powder was first synthesized by a two-stage, high-energy/low-energy mechanical alloying process. The powder was then packed into silver tubes and formed into tapes by drawing and biaxial (Turk's-head) rolling. The precursor tapes were oxidized and converted into Ag-sheathed 2212 by heat treatment at 350-860°C in oxygen. Subsequent annealing at 897°C, followed by slow cooling and subsolidus annealing at 850°C, yielded 2212 grains with enhanced c-axis alignment. The resulting tapes exhibited average and maximum transport J c (4.2 K, self-field) values of 3.2 × 10 4 and 6.8 × 10 4 A/cm 2 , respectively. desired superconducting phases can form with minimal carbon contamination via novel, low-temperature reaction sequences. Indeed, prior work has shown that mechanically alloyed, alkaline-earth-metal-bearing precursors can be converted into multicomponent ceramics at appreciably lower temperatures than alkaline-earthcarbonate-bearing precursors. For example, 26-28 have formed at 550-750°C during annealing of Y-Ba-Cu-bearing, Ba-Al-bearing, Ba-Fe-bearing, Bi 2 O 3 -Sr-Ca-bearing, and Ba-Pb-Ti-TiO 2 -bearing precursors, respectively, in oxygen. Although several papers have been published on the use of the powder metallurgical MPIT method to produce (Bi,Pb) 2 Sr 2 Ca 2 Cu 3 O 10±x / Ag 19,20,22 and (Y,Ca)Ba 2 Cu 4 O 8 /Ag 21 composite superconductors, detailed descriptions of the mechanical alloying, forming, and heat treatment conditions, and of the phase and microstructural evolution during oxidation and postoxidation annealing, were not provided. The fabrication of melt-processed, Ag-sheathed 2212 superconductors from mechanically alloyed, metal-bearing precursors has also not been reported.The purpose of the present paper is to demonstrate the feasibility of fabricating Ag-sheathed 2212 superconducting tapes by the oxidation and postoxidation (partial melt) annealing of mechanically alloyed, malleable, metal-bearing precursors. The mechanical alloying, deformation processing, and heat-treatment conditions used to produce such conductors will be discussed. A detailed investigation of the reaction sequence to 2212 is the subject of another paper. 29 II. EXPERIMENTAL PROCEDURES A. Fabrication of metal-bearing precursor tapesMetal-bearing precursors to Ag-sheathed, Bi-2212based tapes were fabricated by a powder metallurgical route. Intimate powder mixtures of Bi 2 O 3 , Sr, Ca, and Cu were first synthesized by mechanical alloying (ball milling). The milled, metal-rich powder was then sealed within silver tubes that were cold drawn into wires and cold rolled into tapes.

Formation of Bi2Sr2CaCu2Ox/Ag multifilamentary metallic precursor powder-in-tube wires

“…The strain non-uniformity of the MP core and Ag sheath due to different deformation resistance is one of the factors responsible for sausaging [53]. One of the causes of severe sausaging is the presence of brittle Bi particles and intermetallic Ca-Cu formed during milling, reducing the MP ductility [51,54,55]. Another cause may be the deformation parameters.…”

“…Thermodynamic data [58,59] between 298.15 and 1000 K indicates that the oxidation reactions to form Bi 2 O 3 , CaO, CuO, SrO and Cu 2 O are favored in a sequence of CaO>SrO>Bi 2 O 3 >Cu 2 O>CuO. Hence, the oxidation of Ca and Sr is strongly favored as well as displacement reactions between Ca/Sr and Bi 2 O 3 [51]. Thus, Bi and Cu remain metallic until the surrounding Sr and Ca powders are oxidized.…”

“…But the samples were only 10-20 mm in length and with one side of the Ag sheath removed. This investigation did not study the influence of a closed Agsheath or long length oxidation [51].…”

“…An alternative to the oxide precursor route, with the potential to address these issues, is metallic precursors (MPs) that are oxidized after wire deformation. Previously, MPs were studied extensively for Ag-sheathed Bi 2 Sr 2 Ca 2 Cu 3 O x (Bi2223) multifilamentary tapes and Bi2212 monofilamentary tapes [49][50][51]. Starting with ductile MPs, hot extrusion was used to form multifilamentary wires with fine filaments (as small as 5 μm) and, a relatively smooth interface between the filaments and Ag sheath [49,50].…”

“…After heat treatment, the Bi2223 tapes were flexible and ductile with the high strain tolerance required for large-scale use [49]. In addition, a monofilamentary Bi2212/Ag tape was made via solid metalbearing precursor route [51]. Mixed Bi 2 O 3 , Sr, Ca and Cu powders were synthesized by mechanical alloying and filled into a Ag tube.…”