F. Wong

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.

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.

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.

In Situ Dynamic Measurement of Polydisperse Chain Aggregate Aerosols Using Photocorrelation Spectroscopy Technique

An experimental technique was developed to perform in situ photo correlation spectroscopy measurements on length distribution of dynamic polydisperse iron oxide aerosols with diameters ranging from 0.01 to 0.05 μm. The cumulant analytical method is used to determine the average length of the chain aggregate. The experimental results show that the average length of the iron oxide aerosols obtained by the photo correlation spectroscopy technique is basically consistent with the results obtained by TEM. Results indicate that the length of the iron oxide samples increases as the liquid carbonyl temperature increases and the gas flow dilution ratio decrease.

Alternating current losses in Bi2Sr2Ca1Cu2O8+δ/Ag tapes at power frequencies

Measurements of transport alternating‐current (ac) losses in Bi2Sr2CaCu2O8+x powder‐in‐tube Ag‐sheathed tapes at 4.2 and 65 K and at 20, 60, 200, and 400 Hz were carried out and comparisons to the theoretically predicted hysteretic loss plus ohmic loss were made. The measured loss, albeit larger than predicted, was found to agree with Bean’s critical state model. The loss, having a linear dependence on the frequency of the alternating transport current and increasing with decreasing critical current density, confirms hysteresis as the dominant source of power dissipation at both 4.2 and 65 K. However, at 4.2 K, the loss varies as the square of the amplitude of the transport current, which is not consistent with the model.

Development of bismuth‐2212 conductors

The influence of processing parameters on the fabrication of conductors of the Bi‐2212 oxide superconductor are discussed. Critical currents and critical current densities of short samples have reached acceptable levels for 4.2–20 K applications. However, problems related to reproducibility and uniformity of transport properties along the conductor length still exist. Possible methods to circumvent these and other associated problems are presented.

Computer simulation of thin film growth with thermal hopping of atoms

A three‐dimensional computer simulation that manipulates individual particles has been developed to model the early phases of the deposition of epitaxial films of a material with a specified lattice structure. The model is implemented with the simple cubic structure and the on‐lattice positioning scheme, and calculations have been performed for a variety of experimental conditions, including temperature and flux. A comparison has been made between this method, which uses the fixed time stepping (FTS) method of Outlaw and Heinbockel, and a more accurate, more efficient method that uses an event based scheme. Agreement between the methods is good. Three dimensional particle migration and surface evaporation have been included in the FTS model, but have been found unimportant to its final results. The model shows a transition similar to the one predicted by Muller, but at more realistic temperatures, due to an improvement in the description of the surface migration energy barrier. Packing densities and surface roughnesses have also been calculated.A three‐dimensional computer simulation that manipulates individual particles has been developed to model the early phases of the deposition of epitaxial films of a material with a specified lattice structure. The model is implemented with the simple cubic structure and the on‐lattice positioning scheme, and calculations have been performed for a variety of experimental conditions, including temperature and flux. A comparison has been made between this method, which uses the fixed time stepping (FTS) method of Outlaw and Heinbockel, and a more accurate, more efficient method that uses an event based scheme. Agreement between the methods is good. Three dimensional particle migration and surface evaporation have been included in the FTS model, but have been found unimportant to its final results. The model shows a transition similar to the one predicted by Muller, but at more realistic temperatures, due to an improvement in the description of the surface migration energy barrier. Packing densities and surfa...