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Melt-processed Bi/sub 2/Sr/sub 2/Ca/sub 1/Cu/sub 2/O/sub 8+x//Ag tapes are now commercially available in long lengths. The melting step, which is mostly important for tape performance is difficult to control on the industrial scale. A dynamic approach is proposed. The melting step and subsequent annealing (2212-formation) step are separated. For the melting step, the reel-to-reel transfer system through a 140-cm long furnace was developed. The maximum processing temperature in the hot zone can be controlled to within 1/spl deg/C; heating rate from 3000/spl deg/C/h to 30000/spl deg/C/h and cooling rate between 60/spl deg/C/h to 30000/spl deg/C/h. The effect of heat treatment parameters characterizing the melting step on the transport critical current densities at 4.2 K was studied and correlated to microstructure characterized by SEM/EDS and X-ray diffraction analyzes.

The influence of the current slope (dI/dt) and thermal exchanges on the magneto-thermal transition of textured bulk YBCO are analyzed within the framework of the resistive Fault Current Limiter (FCL). This study was carried out at 90.5 K both in pressurized liquid nitrogen, in vacuum and when the YBCO sample is in contact with alumina plates. The superconductor U(I) response is governed by a power law, depends on dI/dt and is influenced by the thermal surroundings. These phenomena can be justified if the dissipated energy during the transition is concentrated in a small volume of the conductor. The second part of this paper deals with the thermal recovery of bulk materials. Even in the case of film boiling where the exchanges are poor, a thermal gradient due to the weak thermal conductivity of HTc materials appears in the section of the conductor. This point and its consequences are described and discussed by using a finite element software.

The fault current limiter is a very attractive device for electric networks. Meander pattern conductors cut from bulk melt textured YBCO were studied for this application. The meanders are put in series and/or in parallel to match the required current and voltage. The YBCO materials are attractive because they show a very effective limitation with a relative low volume (high engineering current density and normal state resistivity). However, they are sensitive to hot spots. To avoid these destructive hot spots the operating temperature is chosen very close to the critical temperature (above 90 K). This temperature range is reached using a pressurised liquid nitrogen bath. Working close to Tc has two major advantages. The first is reduced values of Jc which limit the power dissipation. The Jc can be matched by changing the pressure on the nitrogen bath. The second is that the proximity of the normal state is favourable for homogeneous quenches along the whole meander as it can be experimentally recorded. Results obtained on single meanders and on the whole assembly are reported under steady state operation as well as during current limitation. Forty three meanders in series limited the current to 740 A (11 000 A unlimited value) under 1 kV.

The superconducting current limiter could bring a solution to a problem not satisfactory solved: the limitation of fault currents under high voltages. We carry out a research and development program based on bulk bismuth materials. They are attractive due to their homogeneous quench into the resistive state and after the normal state even if their critical current densities are not very high (several MAm/sup -2/ at 77 K, self field). They limit efficiently currents without any risk of burning (no hot spots) even under high electrical fields. These materials are elaborated in the form of Bi-2223 bars formed by an isostatic pressure and classical sintering. Sixty bars (/spl phi/=4 mm, /spl Lscr/=120 mm, I/sub c/=35 A) were assembled. This assembling was tested up to 1100 V at 1080 A instead of a theoretical value of 5000 A. The superconductor was exposed to a maximum power density of 12 GW/m/sup 3/ without damage. We present the measurements obtained on the bars and their assembly.

The superconducting (SC) current limiter is one of the most attractive applications of superconductivity because it is an innovative device without any conventional equivalence. Following the emergence of ultra low AC loss NbTi strands in the eighties, several devices have been designed and built using these superconductors. The discovery of high T/sub c/ materials has created numerous potential applications such as particularly for current limiters. The operation at higher temperatures and the very different nature of the SC oxides compared to low T/sub c/ strands deeply modify the limitation process. The authors have carried out some experiments to study the transition of high T/sub c/ samples fed by voltages with the objective of making a resistive limiter. The quench behaviour of sintered and textured Bi compounds have been studied and compared. Grain boundaries play a very important part in the limitation process. The resistive transition seems to begin at the grain boundaries. The complete resistive transition of the sample has never been recorded. The propagation of the transition from the resistive part to the entire sample is very low. To overdrive the quench, a magnetic field has been applied. In this paper, the results are reported to show the grain boundaries role in the transition.