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All sourcesNASA NTRSDOE OSTI
NASA NTRS1995

Partical Melting of bulk Bi-2212

Dense and textured Bi-2212 bulk samples have been produced by the partial melting process. The appropriate amount of liquid phase necessary for complete densification has been adjusted by controlling the maximum processing temperature. The maximum temperature itself has to be adapted to several parameters as powder stoichiometry, silver addition and oxygen partial pressure. Prolonged annealing at 850 and 820 C and cooling in N2 atmosphere led to nearly single phase material with T(sub c) = 92 K. Critical current densities j(sub c) of 2'200 A/sq cm at 77 K/0 T have been achieved in samples of more than 1 mm thickness. Reducing the thickness below 0.4 mm enhances j(sub c) considerably to values is greater than 4'000 A/sq cm. The addition of 2 wt% Ag decreases the solidus temperature of the Bi-2212 powder by 21 C. Therefore, the maximum heat treatment temperature of Ag containing samples can be markedly lowered leading to a reduction of the amount of secondary phases. In addition, Ag enhances slightly the texture over the entire cross section and as a result j(sub c) at 77 K/0 T.

Heeb, B.↗
NASA NTRS1995

Tape casting and partial melting of Bi-2212 thick films

To produce Bi-2212 thick films with high critical current densities tape casting and partial melting is a promising fabrication method. Bi-2212 powder and organic additives were mixed into a slurry and tape casted onto glass by the doctor blade tape casting process. The films were cut from the green tape and partially molten on Ag foils during heat treatment. We obtained almost single-phase and well-textured films over the whole thickness of 20 microns. The orientation of the (a,b)-plane of the grains was parallel to the substrate with a misalignment of less than 6 deg. At 77 K/0T a critical current density of 15, 000 A/sq cm was reached in films of the dimension 1 cm x 2 cm x 20 microns (1 micron V/cm criterion, resistively measured). At 4 K/0T the highest value was 350,000 A/sq cm (1 nV/cm criterion, magnetically measured).

Buhl, D.↗
NASA NTRS1995

The role of silver in the processing and properties of Bi-2212

The influence of the silver content and the oxygen partial pressure on the solidus temperature and the weight loss during melting of Bi2Sr2Ca1Cu2O(x) has been examined by means of DTA and TGA. By decreasing the oxygen partial pressure the solidus is lowered (e.g. del T = 59 C by decreasing pO2 from 1 atm to 0.001 atm) and the weight loss is increased. The addition of silver causes two effects: (1) the solidus is further decreased (e.g. 2 wt% Ag lower T (solidus) by up to 25 C, depending on the oxygen partial pressure); and (2) the weight loss during melting is reduced. Thick films (10-20 micron in thickness) with 0 and 5 wt% silver and bulk samples with) and 2.7 wt% silver were melt processed in flowing oxygen on a silver substrate in the DTA, allowing the observation of the melting process and a good temperature control. The critical current densities are vigorously dependent on the maximum processing temperature. The highest j(sub c) in thick films (8000 A/sq cm at 77 K, O T) was reached by melting 7 C above the solidus temperature. The silver addition shows no significant effect on the processing parameters or the superconducting properties. The highest j(sub c) for bulk samples (1 mm in thickness) was obtained by partial melting at 900 C or 880 C, depending on the silver content of the powder (0 or 2.7 wt%). The j(sub c) of the samples is slightly enhanced from 1800 A/sq cm (at 77 K, O T) to 2000 A/sq cm by the silver addition. To be able to reach at least 80% of the maximum critical current density, the temperature has to be controlled in a window of 5 C for thick films and 17 C for bulk samples.

Lang, TH.↗
NASA NTRS1995

Flux pinning characteristics and irreversibility line in high temperature superconductors

The flux pinning properties in high temperature superconductors are strongly influenced by thermally activated flux motion. The scaling relation of the pinning force density and the irreversibility line in various high temperature superconductors are numerically analyzed in terms of the flux creep model. The effect of two factors, i.e., the flux pinning strength and the dimensionality of the material, on these properties are investigated. It is speculated that the irreversibility line in Bi-2212 superconductors is one order of magnitude smaller than that in Y-123, even if the flux pinning strength in Bi-2212 is improved up to the level of Y-123. It is concluded that these two factors are equally important in determination of the flux pinning characteristics at high temperatures.

Matsushita, T.↗
NASA NTRS1995

Status of high temperature superconductor development for accelerator magnets

High temperature superconductors are still under development for various applications. As far as conductors for magnets are concerned, the development has just been started. Small coils wound by silver sheathed Bi-2212 and Bi-2223 oxide conductors have been reported by a few authors. Essential properties of high T(sub c) superconductors like pinning force, coherent length, intergrain coupling, weak link, thermal property, AC loss and mechanical strength are still not sufficiently understandable. In this talk, a review is given with comparison between the present achievement and the final requirement for high T(sub c) superconductors, which could be particularly used in accelerator magnets. Discussions on how to develop high T(sub c) superconductors for accelerator magnets are included with key parameters of essential properties. A proposal of how to make a prototype accelerator magnet with high T(sub c) superconductors with prospect for future development is also given.

Hirabayashi, H.↗
NASA NTRS1995

Correlation between fabrication factor and superconducting properties of the Tl-and-Bi-based high-Tc superconductor

Large critical current densities (J(sub c)) were obtained in c axis oriented Tl-1 223/Ag composite tapes fabricated by spraying methods without a vacuum. Transport measurements at 77K under a zero field indicated a J(sub c) of 9 x 10(exp 4) A/sq cm and 7 x 10(exp 3) A/sq cm at 1T for the tapes fabricated by spray pyrolysis. The novel GPM method was also applied for Bi-2212/Ag PIT composite wire, and found to be very effective for improving the distribution of voids, which caused from the melt-solidifying process. The GPM showed a marked effect for obtaining homogeneous long wire. A 1 T class coil was successfully fabricated with monocore wire.

Maki, Naoki↗