Microstructure of Nb 3 (AlSiB) superconducting tapes with extremely high critical current densities
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Tariq Manzur Department of Metallurgy, University of Connecticut, Storrs, Connecticut 06268 (Received 6 October 1988; accepted 27 December 1988) Alloys of Nb73Al12Si14 5B05 were rapidly solidified into amorphous ribbons using the melt spinning technique. These were isothermally annealed at temperatures ranging from 660 to 780 °C. The A15 phase began to crystallize at 700 °C and small amounts of second phases appeared at the higher temperatures. Crystallization was dependent on quenching rate as well as annealing conditions. Below 750 °C nucleation was nonuniform and was enhanced by surfaces and quenched-in nuclei. Above 750 °C nucleation became more uniform and completely crystalline ribbons with equiaxed grains —30 nm in diameter were obtained. These ultra fine grained ribbons had extremely high superconducting critical current densities of 8 X 1010 A/m 2 and 5 x 1010 A/m 2 at magnetic fields of 0.5 and 15 tesla, respectively, at 4.2 K.
I. INTRODUCTION A new processing technique has been developed to produce A15 tapes with improved superconducting and mechanical properties.1"3 The basic approach was to start with the A15 compound of interest, to rapidly solidify it into an amorphous ribbon using the melt spinning technique, and then to anneal to form an ultra fine grained A15 microstructure. This technique applied to Nb73Al12Si145B05 showed that this alloy could have extremely high critical current densities Jc at very high magnetic fields, 3 X 1010 Amps/m2 at 20 tesla.3 Since the key to enhanced Jc is the microstructure, it is essential to understand how to control the crystallization of the A15 from the amorphous state. This has been the focus of this work. II. EXPERIMENTAL PROCEDURE Alloys of Nb73Al12Si145B05 were melt spun into amorphous ribbons 0.5 mm wide and 10 /JL thick. Details of the melt-spinning procedure are given elsewhere.1'2'4'5 The appearance and surface quality of the ribbons were determined with an optical microscope. Prior to annealing, the samples were cleaned in acetone and alcohol, wrapped in Nb foil, and sealed in a quartz tube under a vacuum of 10~6 Torr; annealing temperatures and times varied from 620 to 780 °C and 12 to 96 h. Crystal structure was determined using an x-ray diffractometer; six 1 cm long pieces of ribbon were positioned side by side on a glass slide. Critical current densities were measured as a function of transverse magnetic field up to 15.2 tesla at 4.2 K. For a given field, current was passed through the sample and the voltage recorded. The critical current was defined as the current at which a voltage of 2 (JLV appeared across 1 cm of the superconductor at a specific value of magnetic 526
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J. Mater. Res., Vol. 4, No. 3, May/Jun 1989
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field. Currents as high as 450 Amps were used. Under these conditions values of Jc can be limited by heating problems due to contact resistance between the sample and current leads. To avoid this problem, the following mounting technique was used. The amorphous ribbons were lightly
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