T.P. Beales

Effect on the phase formation of Bi-2223 in some Ag-alloy sheathed PIT tapes

Powder-in-tube mono- and multifilamentary tapes with sheaths of Ag, AgCu0.02, Ag(AgCu0.02), AgAl0.25, Ag(AgAl0.25), AgNi0.25Mg0.25, AgTi0.25Mg0.25, and AgTi0.25 alloys have been fabricated and their physico-chemical properties and effect on the phase formation of Bi-2223 determined. Alloying was found to have a significant effect on phase formation of the Bi-2223 phase formation linked to the alloying elements reactivity to oxygen Cu<Ni<Al<Mg<Ti. Multifilamentary tapes, composed of an inner Ag sheath and an outer Ag alloy sheath showed no such effects. The resistivity and mechanical properties of the various Ag alloys used as sheathing materials are also reported on and their possible use in improving the performance of PIT tapes.

Performance of a 1 MV A high temperature superconductors-enabled saturable magnetic core-type fault current limiter

A 1 MV A single-phase DC saturated iron core-type fault current limiter (FCL) employing DC coils manufactured from Bi-2223/Ag high temperature superconductor has been designed, constructed, and tested. The device, configured to respond to a fault occurring on either the positive or negative wave cycle, exhibited instantaneous reaction and reset times. It could clip a continuous applied steady state fault current of 1200 A, and limited a transient peak fault current of 2200 A to the specified current of 100 A. The operation of the device did not produce any significant harmonic contribution from the clipping action, as under fault conditions, 99.96% of the harmonic power spectrum of the clipped waveform contributed to the fundamental, and the non-fault inductance of the FCL was <0.01 mH.

A new method to substantially reduce the processing time of Ag/Bi-2223 tapes

Abstract Multi-stage thermal cycling is commonly used in the processing of Ag/Bi-2223 tapes requiring a total annealing time often in excess of 100 h, which is very undesirable for large-scale production. A new process was investigated that reduced the heat treatment time for Ag/Bi-2223 tapes. In this process, the tape was quenched at the end of the 1st thermal cycle. Following either pressing or rolling, it was rapidly heated to the sintering temperature. In doing so, the usual decomposition and recovery of Bi-2223 during cooling and heating was eliminated, along with a reduced formation time for Bi-2223. By using this new procedure, the zero-field J c and the Bi-2223 volume fraction in tapes sintered for a total time of 20–30 h were comparable to those treated for 100 h using a conventional process. The results further confirm that a low-temperature annealing at 825°C in the end of the last thermal cycle eliminates residual 2201, hence improving J c . It was also found out that tapes containing a large number of filaments had a higher Bi-2223 volume fraction and exhibited a better J c vs. H performance, than for tapes with a small number of filaments, when both were processed for the same time period.

Development of an HTS inductor for an electronic high voltage generator

A power electronic circuit has been built and identified to be able to generate high voltage which can be increased rapidly to a value many times greater than the input low voltage dc source. A prototype device has been built and studied, which is able to generate 1.2 kV from a 12 V dc battery source, and theoretical voltage generation with a HTS inductor can reach very high values. Bi-2223/Ag wire has the capability to make the inductor to reduce the circuit resistance, and therefore a higher voltage can be generated. This high voltage generation method, HTS inductor, test result, and analysis will be presented in the paper.

A high gradient magnetic separator fabricated using Bi-2223/Ag HTS tapes

Bi-2223/Ag HTS wire provides a new opportunity to build an HTS magnet for use in a high separation efficiency, low operational cost, high gradient magnetic separator. A magnet has been designed using HTS wires and the results analyzed for use in this application. The magnet configuration consisted of 12 units and generated 3 T magnetic field. The capability of the Bi-2223/Ag HTS wire for this application was analyzed with consideration of its critical transport current density, conductor filling factor, and magnet field distribution.

Optimal reduction in rolling Ag-sheathed bi-2223 multifilamentary tapes

The difference in mechanical deformation of the cladding material and ceramic superconductor during cold working of Ag-sheathed Bi-based superconducting composite tapes has been analysed. From this, a optimal rolling procedure has been developed in order to improve the uniformity of the tapes by gradually increasing the reduction per pass during rolling. Using this technique, a uniform stress was achieved within the tapes which improved both the transverse and longitudinal filament interfacial homogeneity and reduced the sausaging effect. An improved and more uniform was observed in long-length tapes fabricated using this procedure than in those fabricated using a non-optimized route.

Current and field distribution within multifilamentary Bi2223/Ag tapes

The magnetic self-flux distribution around high current mono- and multi-core BSCCO 2223 tapes with various filament configurations has been measured with a scanning micro-Hall probe. Using a simple model, the measured field component perpendicular to the tapes was deconvoluted to give an estimate of the way the transport current distributes itself across the width of the tape. From the changes in flux profile as the applied transport current is varied, the current distribution across the width of the tape can be inferred. In the untwisted tapes, the outer filaments tend to screen the inner ones and carry the bulk of the current; in a twisted sample, the current is shared more equally between filaments.

Critical role of phase transformation during processing of Ag/Bi:2223 tapes

Phase transformation during the final stage of Ag/2223 tape processing has been investigated through quenching and normal cooling in a specially-designed two-step sintering process. It was found that the phase assemblage in the final tape was determined by the equilibrium composition at the sintering temperature and also from any phase transformation which occurred on cooling. A two-stage sintering procedure in the final thermal cycle was found to be effective in transforming the liquid (or amorphous phase) and residual 2201 into 2212 and 2223. However, the annealing temperature during the final step was critical to the final phase assemblage. The optimal annealing temperature of the second step in the two-step process was around 825/spl deg/C, where all the low T/sub c/ phases and impurities were at a minimum. Annealing at temperatures below 810/spl deg/C, resulted in a substantial increase in 3221, and a lower J/sub c/. Annealing above 825/spl deg/C led to a large 2212 fraction with a small amount of 2201 because of insufficient time needed to convert these phases into 2223 on normal cooling.

Optimization of processing to improve critical current density of Ag/Bi-2223 tapes

Several recent developments in powder-in-tube (PIT) processing are presented. A cryogenic deformation process has been developed, involving rolling or pressing the wires and tapes in supercold conditions, such as in liquid nitrogen. Cryogenic deformation has been found to improve the density, grain alignment and Ag-oxide core interface and to increase dislocation density, thereby enhancing and flux pinning. By incorporating Pb into Bi-2212 phase the sintering temperature can be raised above , resulting in a significant reduction of total sintering time from several hundred hours to 100 h. Recently, a new process to eliminate the decomposition and recovery of Bi-2223 during cooling and heating has been developed that further reduces the heat treatment time for Ag/Bi-2223 tapes to 20-30 h, with and Bi-2223 volume fraction in the tapes comparable with those in tapes treated for 120 h. A two-stage annealing procedure in the final thermal cycle has been used to eliminate residual amorphous phase and Bi-2201, which has been identified to be one of the major causes of weak links in PIT tapes. By incorporating hot deformation in a two-step process not only can Bi-2201 be eliminated, but also texture and density are improved, resulting in a very high at 77 K in multifilamentary tapes.

Grain connectivity and flux pinning in Bi-2223/Ag PIT tapes

Abstract A hot-pressing technique was used to prepare Ag-sheathed Bi-2223 multifilamentary tapes in order to study its effect on grain connectivity and flux pinning. The self field critical current density (Jc) after hot-pressing was significantly enhanced, the maximum increase reached more than double than before. Hot pressing raised the Jc (B=0) from 22 000 A/cm2 (Ic=18 A) to 56 800 A/cm2 (Ic=36.5 A) for 81-filament tape and from 26 000 A/cm2 (Ic=36.2 A) to 56 000 A/cm2 (Ic=56.6 A) for 19-filament tape. But, the Jc in applied magnetic field was not improved or degraded by hot-pressing. The Jc retention in magnetic field of 1 T decreased from 33% for the original tape to 21% for the hot-pressed 81-filamentary tapes and remained the same (24%) for 19-filamentary tapes before and after hot-pressing. The effect of cooling rate on connectivity and flux pinning in hot-pressed Bi-2223 PIT tape has been investigated. The fraction of strong links (IC0s/IC0) was improved from 64% to 73% for 81-filament tape due to hot pressing and slow cooling. However it was reduced to 42% for hot-pressed and normally cooled tape. Microstructural analysis showed that hot-pressing chiefly improved grain connectivity, increased core density, improved crystal alignment, recovered micro-cracks and reduced secondary phase impurities. Hot-pressing technique reduced some defects, but at the same time, it created different type of defects, so the whole amount of defects was not much changed. Therefore, hot pressing caused different influence on flux pinning for different original tapes. These results suggest that limitation of Jc in Bi-2223 tapes is multiple and potential for further improvement of Jc is great.