T. Haugan

Predictive model for critical current density of Ag-sheathed Bi2Sr2Ca1Cu2O8 composite tapes with fabrication defects

Abstract Fabrication of long Ag-sheathed Bi 2 Sr 2 Ca 1 Cu 2 O 8 composite tapes is sometimes accompanied by the distribution of large bubble-type defects located at relatively uniform intervals along the length of the conductor. The size, shape and number of these defects dictate the critical current density and eventually the overall quality of the tape. The effect of defect size and concentration on the overall critical current density is theoretically modelled. Using experimental data on the transport properties of short conductor samples with a defect of known dimensions, the model can be used to predict the critical current density of a long conductor tape by visual inspection of the number, size and location of these defects along the conductor length.

Recent status on high temperature superconducting Bi 2 Sr 2 CaCu 2 O 8+x wire development at NYSIS: 1-90 meter length J c s and 3 meter diameter ring furnace design

The status of long length, Bi2Sr2CaCu2O8+x (Bi-2212) wire development at the New York State Institute on Superconductivity (NYSIS) is reviewed and updated. Transport Jcs (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 Jc as the tape length was increased from 15 to 90 meters was attributed to the (measured) sensitivity of Jc 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 (∼13w × 10h × 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 90-m length Bi2Sr2CaCu2O8 + x Ag-sheathed superconducting tapes

Abstract Initial results of a 90-m length Ag-sheathed Bi 2 Sr 2 Ca 1 Cu 2 O 8+ x (Bi-2212) tape prepared by the oxide-powder-in-tube method are described. The transport J c of one 90-m length tape measured in 10-m length sections was ~20,000 A/cm 2 at 4.2 K. The J c of this tape was reduced to ~40% of expected values because the tape was processed in close contact with a ceramic cloth, rather than freestanding. Transport J c s of 10-m sections of the 90-m length tape were uniform within ~10% variation. Unique problems that must be addressed to obtain uniform property, 90-m length tapes include: (i) bubble defect formations, (ii) ~2 cm interval “kinks”, and (iii) effusion of melted oxides out of the Agsheathed tape. Possible solutions to these problems are suggested.

Deformation of silver-sheathed oxide superconducting tapes by trapped gases

Abstract A simple model was developed to describe the deformation in long silver-sheathed oxide superconducting tapes caused by the thermal expansion of air trapped within the conductor and by carbon dioxide released by the decomposition of SrCO 3 in the precursor powder during heat treatment. The analysis presented can be extended to include other carbon-containing compounds, impurities and gases released during the annealing process. The model assumes a zero-strain deformation process of the thin silver sheath in response to the gas pressure developed within the conductor due to heating. Results show that swelling (uniform deformation along the conductor length) increases linearly with temperature. For a carbon-free tape, swelling increases with decreasing precursor powder packing density. The reverse is true for tapes that contain carbon which is distributed uniformly within the tape. A carbon content of ≈0.001% by weight produces swelling similar to that produced by air. Increasing carbon content increases swelling. Carbon content in excess of 0.5% by weight will deform the conductor to the extent that elastic/plastic deformation can occur. Discrete SrCO 3 particles dispersed among the precursor powder produce bubbles (localized deformation). The size and concentration of these bubbles depend on the SrCO 3 particle size and concentration. For a given carbon content, small SrCO 3 particles form a large number of bubbles, which merge together resulting in swelling along the conductor length. Large SrCO 3 particles, on the other hand, form only a few discrete bubbles whose size increases with particle size.

A study of bubble-defect formations in Ag sheathed Bi2Sr2CaCu2O8+x (Bi-2212) superconducting tapes

Abstract Bubble-defect formations were studied in Ag sheathed ∼ 60 μm thick single-filamentary Bi 2 Sr 2 CaCu 2 O 8+ x (Bi-2212) oxide-powder-in-tube (OPIT) conductors. Bubble defects were affected by the powder-processing method, spray-roasted (SR) or solid-state (SS) and annealing conditions, removing or not removing the rolling lubrication, exposing precursor powder to air, and ball-milling. The effect of powder carbon content on bubbling was studied for SR powder (370–4010 ppmw) and SS powder (220–590 ppmw). It was possible to eliminate bubbles using SS powder, appropriate pre-annealing conditions, and processing in air for a wide range of processing conditions and conductor lengths (up to 90 m). The method of eliminating bubble defects in SS powder tapes was also compatible with high critical current density ( J c ) processing.

Effect of non-uniform core thickness on critical current density of silver-sheathed oxide superconducting tapes

Abstract The fabrication of long silver-sheathed oxide superconducting tapes by the powder-in-tube technique tends to lead to non-uniformity in the core thickness during mechanical deformation and densification. This non-uniformity restricts current flow and reduces the critical current density. Using a simple model, the detrimental effects of various types of core thickness non-uniformity on the critical current density are investigated. Also the degree to which such non-uniformity needs to be controlled, based on the temperature of application of these conductors, is qualitatively inferred.

Pinhole defects in Ag sheath of PIT Bi-2212 tapes

Abstract Pinhole defects have been observed in the silver sheath of powder-in-tube (PIT) fabricated Bi-2212 tapes. Material from the oxide core exit through these pinholes. A relatively large area of (Sr, Ca) x oxide condensate was found to surround the pinholes on the outer surface of the conductor following partial-melt-growth (PMG) heat treatment. The formation of these pinholes is attributed to the PIT mechanical deformation of the precursor powders, which contain a few relatively large (> 30 μm) and hard (relative to silver) particles/aggregates. Results indicate that these pinholes, which are more pronounced in thin tapes ( I c . This paper attempts to quantify the effects of these pinhole defects.

Optimization studies of high current Bi2Sr2CaCu2Oxsuperconducting tape systems

A review and comparison of different type systems used to produce high critical current density (Jc), high critical current Bi2Sr2CaCu2Ox (2212) tapes is described. Four different tape configurations have been investigated: 2212/Ag ‘‘uncovered’’ single layer, Ag/2212/Ag ‘‘covered’’ single layer, [2212/Ag]×N multilayer tapes, and powder‐in‐(Ag tube) wires and tapes. Each type of system has advantages and disadvantages that affect the growth mechanisms and performance of the tapes in meeting engineering design criteria. Critical current densities, magnetic field properties, examples of growth and the advantages and disadvantages of each tape system are reviewed and updated.

Effect of bending on Jc-ϵ characteristic of silver-sheathed oxide superconducting tape with sausaging

Abstract Sausaging in the core of silver-sheathed powder-in-tube generated superconducting tapes restricts current flow and lowers the effective critical current density J c . Although generally detrimental, such sausaging may indeed (a) enhance the J c tolerance to bending strain e relative to the zero-strain J c ; and (b) be partially responsible for the irreversibility limit observed in the J c - ϵ dependence. Using a simple model, the effects of sausaging on the J c - ϵ characteristic are qualitatively investigated.

Growth of Bi2Sr2CaCu2Ox/Ag single layer and multilayer superconducting tape systems

The effect of processing parameters on the growth of Bi2Sr2CaCu2O8+x/Ag tapes and how the possible ranges of processing parameters can influence methods for scaling up fabrication of long length conductors was examined. Tape systems compared include 2212/Ag ‘‘uncovered’’ single layer, Ag/2212/Ag ‘‘covered’’ single layer and [2212/Ag] – N multilayer tapes. Specific studies presented include: i) general results of melt‐quench experiments used to characterize liquid‐solid transformations during partial‐melt texturing; and ii) the sensitivities of different tape systems to variations in melting conditions.