REEL-TO-REEL TEXTURE ANALYSIS OF HTS COATED CONDUCTORS USING A MODIFIED GADDS SYSTEM

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1 Copyright JCPDS - International Centre for Diffraction Data 2003, Advances in X-ray Analysis, Volume REEL-TO-REEL TEXTURE ANALYSIS OF HTS COATED CONDUCTORS USING A MODIFIED GADDS SYSTEM J.L. Reeves and V. Selvamanickam SuperPower, Inc., Schenectady NY R.L. Snyder Materials Science and Engineering Dept., The Ohio State University, Columbus OH ABSTRACT High temperature superconducting (HTS) materials need a high degree of texture over kilometerlong lengths in order to carry the maximum amount of supercurrent for electric power industry applications. One way to achieve the high degree of texture is with the coated conductor architecture. The coated conductor is comprised of the HTS layer epitaxially grown on a textured oxide buffer layer on a metal tape which gives flexibility and strength to the multilayer conductor. Traditionally a 360º phi scan is conducted on a small centimeter-wide sample to determine the (111) texture of the buffer layer. The pole figure full-width at half-maximum (FWHM) is calculated for the (111) peaks to quantify the texture of the buffer layer. This pole figure FWHM value is a critical quality control parameter, since the current-carrying capability of the HTS material increases as the FWHM decreases. The texture of the HTS coated conductors was characterized using a Bruker AXS General Area Detector Diffraction System (GADDS) which was modified to analyze meter-long lengths of conductor. The unique reel-toreel sample holder design, coupled with the parallel beam produced by the cross-coupled Göbel mirrors in the GADDS system, allows fast and accurate pole figure FWHM determination at selected points along meter-long samples. System modifications and texture results for samples up to 10 meters long are described. INTRODUCTION The high temperature superconductor YBa 2 Cu 3 O x (YBCO) has great potential for use in electrical power generation and transmission applications [1,2]. For YBCO to be a cost-effective replacement for conventional materials, the critical current (I c ) carrying capability must be increased. Other researchers have shown that high angle grain boundaries in the superconductor reduce I c [3,4]. At low misorientation angles, the current is relatively unaffected by the grain boundary. As the grain boundary misorientation angle increases, the current able to flow across the grain boundary decreases exponentially. To retain high I c in polycrystalline superconductors, the grain boundary angles must remain small; therefore, a well-textured superconductor is necessary for high I c. The coated conductor architecture schematically shown in Figure 1 makes it possible to produce long lengths of well-textured HTS. The superconductor gets its texture from the buffer layer, which also acts as a barrier to prevent diffusion from the substrate. The substrate is inexpensive metal chosen for strength and flexibility. The top layer of silver protects the superconductor during handling. To make long lengths of YBCO superconductor with only low

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3 Copyright JCPDS - International Centre for Diffraction Data 2003, Advances in X-ray Analysis, Volume substrate buffer HTS silver Figure 1. Coated conductor architecture showing the flexible metal substrate, ceramic oxide buffer layer, YBCO HTS layer, and protective silver layer. angle grain boundaries, the superconductor needs a well-textured template to grow on. In our coated conductor architecture, the ceramic buffer layer is textured by an ion-beam assisted deposition process [5]. The buffer layer grows with its [001] direction perpendicular to the sample surface, and its [110] direction parallel to the tape length direction. The YBCO grows epitaxially on the textured buffer layer. To quantify the texture of the buffer layer, small samples can be rotated 360 in phi ( ), and the full-width at half maximum (FWHM) of the (111) pole figures can be calculated. Compared to conventional X-ray diffraction systems, measuring 360 scans on short samples is relatively fast (<15 minutes) with the General Area Detector Diffraction System (GADDS). However, we need to check the texture on meter-long lengths of buffered tape to confirm the quality of the buffer layer before subsequent YBCO deposition. RESULTS AND DISCUSSION As shown in Figure 2, the Bruker GADDS system [6] was modified to measure texture of the buffer layer over meter-long lengths. Modifications to the GADDS system included adding the reel-to-reel spooling capability, with software-controlled motors that move the tape through the X-ray beam to a given position for measurement. At a fixed position, the tape stops; then the sample holder is rotated from = -25 to = +25 in 2 intervals. At each angle, a single frame of data is collected for 10 seconds, then the sample is rotated 2, data is collected for another 10 seconds, etc. Then the motors engage to move the sample to the next position, where the rotation is again completed. The laser and video camera are used to verify that the sample is at the proper height for each measurement. Figure 3 shows typical frames of data taken with the GADDS system at various angles at a single tape position. In each frame, the [002] diffraction direction from the buffer layer and partial Debye rings from the polycrystalline substrate are visible. As is increased from -15 (Figure 3a) to -5 (Figure 3b), the intensity of the buffer layer (111) peak increases in the boxed area. The (111) peak is most intense near =0 and decreases again as continues to increase. The intensity of the data in the boxed area is integrated as a function of and shown as a quadrant of a pole figure, where each slice in the pole figure is from a single frame of data taken at a specific angle (Figure 3c). The pole figure data is interpolated and the FWHM of the bright (111) peak is calculated.

4 Copyright JCPDS - International Centre for Diffraction Data 2003, Advances in X-ray Analysis, Volume Figure 4a shows the results of reel-to-reel ex-situ buffer texture measurements over a 2.5 meterlong length where the average texture =13. Subsequent growth of superconductor on this buffer layer leads to high I c. Buffer layer texture is also relatively uniform over 10m lengths as shown in Figure 4b. Pay-out reel Area detector Laser for sample height adjustment Area detector Collimator Take-up reel sample Video camera Figure 2. Modifications to the Bruker GADDS system to for reel-to-reel sample measurements. (a) = -15 (b) = -5 (c) pole figure substrate [002] substrate [002] [111] [111] Figure 3. Single frames of diffraction data taken at (a) =-15, (b) =-5. The intensity of the [111] diffraction peak is integrated as a function of and plotted in a pole figure (c).

5 Copyright JCPDS - International Centre for Diffraction Data 2003, Advances in X-ray Analysis, Volume (111) PF FWHM (degrees) Position (cm) Position (m) Figure 4. Buffer layer texture uniformity over (a) 2.5 meters and (b) 10 meters. (111) PF FWHM (degrees) While the ex-situ reel-to-reel texture measurements are very useful as a quality control tool for determining buffer layer texture (and thus predicting the quality of the subsequent superconductor layer), the next step is to use texture measurements as an on-line, in-situ tool for buffer layer deposition. SUMMARY High I c performance of YBCO coated conductors depends on the texture of the buffer layer. A GADDS system was modified to measure texture on the buffer layer at multiple points over meter-long lengths. Ex-situ measurements show uniform buffer layer texture over 2-10 meters. Future plans are to construct an in-situ XRD system to monitor the buffer layer texture during processing. ACKNOWLEDGEMENTS The authors would like to thank Rich Pasquini, Tim Morris, Michael Jones, and Katy Likes at SuperPower for their help building the reel-to-reel equipment and measuring the buffer layer films; Trygve Haglund at Bruker AXS for his help while modifying the GADDS system; Jay Burdett and Huapeng Huang at X-Ray Optical Systems for their discussions about ex-situ and insitu system design; and Dave Karcher at DK Design for his help with the design drawings. REFERENCES [1] Norton, D.P., Annu. Rev. Mater. Sci., 1998, 28, [2] DOE report #GO , July [3] Dimos, D.; Chaudhari, P.; Mannhart, J., Phys Rev B, 1990, 41, [4] Heinig, N.F.; Redwing, R.D.; Nordman, J.E.; Larbalestier, D.C., Phys Rev B, 1999, 60, [5] Arendt, P.N.; Foltyn, S.R.; Groves, J.R.; DePaula, R.F.; Dowden, P.C.; Roper, J.M.; Coulter, J.Y., Appl. Supercond. 1996, 4, 429. [6]