Texture by the kilometer
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Texture by the kilometer E. D. Specht and F. A. List Metals and Ceramics Division, Oak Ridge National Laboratory Oak Ridge, TN 37831-6118 ABSTRACT A novel x-ray diffractometer has been used to characterize the texture of 2 km of textured tape in segments up to 20 m long. Techniques have been developed for the study of the uniformity of texture and for the detection of second phases, deviations from cube texture, and the sharpness of cube texture, in metal substrates, oxide buffer layers, and YBa2Cu3O7 (YBCO) superconductors. INTRODUCTION There is great interest in the development of superconducting tapes consisting of YBCO films grown on metal substrates (with or without buffer layers). High critical currents (Jc) have been demonstrated for such tapes over ~1 m lengths, persisting to high magnetic fields [1,2]. Most applications, however, will require production of ~km lengths. The requirements for scale-up are made particularly challenging by the intergranular transport properties of YBCO: Jc drops significantly when the grain boundary disorientation exceeds ~5°, so the texture of the YBCO must be uniform over the length of the conductor [3]. A single wall of high-angle grain boundaries could block the supercurrent. X-ray diffraction has been widely used to characterize ~cm lengths of superconducting tape. We have developed a novel x-ray diffractometer to completely characterize the phase content and texture of longer lengths. Preliminary results suggest that processing conditions can be kept sufficiently uniform to produce long-length superconducting tapes [4]. We report the techniques we have developed, and the range of texture variations which we have observed in the first 2 km of operation. EXPERIMENTAL DETAILS Up to 20 m of tape, typically 1 cm in width and 0.05 mm in thickness, is spooled on reels 10 cm in diameter (Fig. 1). These reels are mounted on a four-circle diffractometer [5]. The 40 cm inside diameter of the χ circle provides clearance for full rotation of χ and φ, with the restriction 2θ < 90°. One reel is controlled by a microstepping motor [6], the other is held at a constant torque of 0.2 N-m [7]. The sample slides across stainless steel guides machined to a 5 cm radius. The sample is irradiated between these guides, which hold it at a constant height. The smooth motion provided by a microstepper and the gentle bending applied by the large-radius guides prevent damage to the sample. X rays are provided by a 50 kV / 100 mA Cu rotating anode source[8]. A sagitally bent graphite (002) crystal selects Kα fluorescence and collimates the x rays in the vertical (out-of-plane) direction. A 75 cm flight-path followed by a 2.5 mm (wide) x 5 mm (high) slit collimates horizontally (in-plane). Diffracted radiation is collimated inE8.21.1
plane by soller slits (100 cm long, 0.7 mm pitch) and counted using a scintillation detector.
ω, 2θ Figure 1. Reel-to-reel tape handler mounted on a four-circle diffractometer. Reel diameter is 10 cm, chi circle inside diameter is 40 cm. Counting and control of reel motors
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