Texture formation and grain boundary networks in rolling assisted biaxially textured substrates and in epitaxial YBCO films on such substrates

PERGAMON

Micron 30 (1999) 463–478 www.elsevier.com/locate/micron

Texture formation and grain boundary networks in rolling assisted biaxially textured substrates and in epitaxial YBCO films on such substrates A. Goyal a,*, S.X. Ren a, E.D. Specht a, D.M. Kroeger a, R. Feenstra b, D. Norton b, M. Paranthaman a, D.F. Lee a, D.K. Christen b a

Metals and Ceramics Division, Chemistry and Analytical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6116, USA b Solid State Division, Chemistry and Analytical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6116, USA Received 16 November 1998; accepted 12 March 1999

Abstract Electron backscatter Kikuchi diffraction was used to study texture development and grain boundary networks in rolling-assisted-biaxiallytextured-substrates, and the transfer of such a biaxial texture and preferential grain boundary network to epitaxial YBCO films grown on such substrates. It was found that the rolling texture in the Ni substrate is highly complicated, with most of the grains having orientations at and between the “B” and “S” orientations. No cube nuclei could be discerned in the etched, as-rolled sample. On recrystallization, a sharp {001}k100l, cube texture was imparted to the Ni substrate. It was found that the substrate was percolatively connected within 38. Examination of epitaxial oxide layers on Ni showed that excellent epitaxy was obtained. At times, undesirable orientations nucleated during growth of the oxide layer, however, these were engulfed by the growing film (with correct orientation). Orientation image micrographs of an epitaxial YBCO film on a RABiT substrate with a Jc of 1.6 MA/cm 2 at 77 K showed that most of the film was percolatively connected within 28. A comparison of the spatially correlated and uncorrelated grain boundary misorientation distributions showed that local grain-to-grain correlations were present between the grains. Comparison of the data obtained for the YBCO film with those obtained for the underlying Ni substrate showed that both the macroscopic texture and the grain boundary networks were very similar in the two cases, implying excellent epitaxy of the multilayers. q 1999 Published by Elsevier Science Ltd. All rights reserved. Keywords: Rolling assisted biaxially textured substrates; Grain boundary misorientation distributions; Superconductors

1. Introduction Numerous applications of high temperature superconductors, such as transformers, generators and motors require high current carrying, flexible conductors that can sustain magnetic fields above 0.1 T. Due to the thermally activated flux flow, the critical current density of most of the highly anisotropic superconducting compounds, such as the Bibased compounds, rapidly drop at 77 K in the presence of an externally applied magnetic field. Hence, the development of a viable processing route based on (Y or Re)Ba2Cu3Ox (YBCO) is of great interest currently and forms a central research thrust in the area of high temperature superconductivity. YBCO compounds have favorable intrinsic properties. Epitaxial YBCO thin films on single-crystal * Corresponding author. Tel.: 1 1-423-574-1587; fax: 1 1-423-5747659. E-mail address: [email protected] (A. Goyal)

substrates yield critical current densities (Jc’s) in the range of 10 6 –10 7 A/cm 2 at 77 K, 0 T (Kormann et al., 1992). YBCO films also have a high irreversibility field of ,6 T at 77 K, and the reduction of Jc in an applied field is typically only a factor of 4 at 1 T (Matsuda et al., 1992). However, conventional ceramic fabrication methods which can be used to make a long, flexible conductor result in materials with weak, if any, macroscopic or microscopic biaxial texture. In particular, YBCO materials fabricated using conventional techniques invariably contain numerous high angle grain boundaries. High angle grain boundaries act as Josephson coupled weak-links leading to a significant field-dependent suppression of the supercurrent across the boundary. For clean stoichiometric boundaries, the grain boundary critical current density depends primarily on the grain boundary misorientation. The dependence of Jc(gb) on the misorientation angle was first determined by Dimos et al. (1988; 1990) in YBCO for grain boundary types which can be formed in epitaxial films of bicrystal substrates.

0968–4328/99/$ - see front matter q 1999 Published by Elsevier Science Ltd. All rights reserved. PII: S0968-432 8(99)00047-5

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Fig. 1. Schematic of the RABiTS process. Starting with a randomly oriented Ni bar/plate, cold-rolling is used to produce a distinct coppertype rolling texture (see schematic pole figure shown). This is followed by recrystallization to a cube texture. Epitaxial metal and/or oxide buffer layer(s) are then deposited on the textured Ni.

similar analysis in recrystallized, cube oriented Ni is summarized. Measurements of grain orientations and grain boundary misorientations of large, representative regions of the substrate show detailed characteristics of the welldeveloped biaxial texture. The orientation image micrographs, constructed from tens of thousands of spatially correlated measurements, show that the metal substrate is percolatively connected within 2–38. This analysis is then extended to buffer layers and epitaxial YBCO films. It is found that in films with very high Jc’s, the macroscopic texture and grain boundary misorientation distribution of the superconductor is similar to those in the starting Ni substrate. It is found that the film is percolatively connected within 2–38. It is believed that the high-Jc observed is a direct result of this strong biaxial texture and a near complete suppression of high angle grain boundaries.

2. Experimental These include [001] tilt, [100] tilt, and [100] twist boundaries. In each case high angle boundaries were found to be weak-linked. The low Jc observed in randomly oriented polycrystalline HTS fabricate using conventional methods can be understood on the basis that the population of low angle boundaries is small and that frequent high angle boundaries impede long-range current flow. Hence, controlling the grain boundary misorientation distribution towards low angles is key to fabricating high-Jc materials. Practically speaking, this limitation entails the fabrication of biaxially textured superconductors. Macroscopically, biaxially textured YBCO conductors have been fabricated by epitaxial deposition of YBCO on flexible substrates fabricated by two techniques: (a) an unoriented, polycrystalline metal substrate coated with an oxide buffer film(s) with a forced biaxial texture induced by ion-beam-assisted-deposition (IBAD), where an assisting noble gas beam extracted from an ion source is directed onto the growing film (Iijima et al., 1992; Reade et al., 1993; Wu et al., 1995). A similar biaxial texture is observed during oblique vapor deposition on an inclined polycrystalline substrate (Hasegawa et al., 1997); (b) a biaxially textured, metal based substrate formed by conventional thermomechanical processing, followed by epitaxial deposition of buffer layer(s) (Goyal et al., 1996a–c; Norton et al., 1996). This technique is referred to as rolling assisted biaxially textured substrates (RABiTS). Using both these techniques Jc’s over 1 MA/cm 2 at 77 K have been achieved. Fig. 1 shows a schematic of the RABiTS process along with a progression of (111) pole figures illustrating the development and transfer of the biaxial texture to all the deposited layers. In this paper, microstructural characteristics of RABiT substrates and epitaxial YBCO films on these substrates is reported. Starting from the macroscopic texture and grain boundary misorientation distributions in as-rolled Ni, a

Electron backscatter Kikuchi patterns (EBKP) were obtained on a Philips XL30-FEG, Field-Emission Scanning Electron Microscope (SEM) fitted with a fiber optically coupled, silicon-intensified-target (SIT) camera. The camera is mounted in a manner such that it makes a 108 angle with the horizontal axis of the microscope. Phosphors on the SIT are matched to give a maximum response to electrons with energies of 10 and 20 kV. Under normal operating conditions, EBKP’s with a solid angle corresponding to 60–708 were captured. A camera control unit (CCU) receives the signal from the SIT camera and provides a television output video signal. The CCU has controls for electronically altering the intensity gradient across the detector so that the raw image is as smooth as possible. The output signal is then sent to an image processing unit, which enhances the signal and corrects for any intensity gradients across the phosphor. This is done by first collecting a background signal from the material being analyzed. This image is then stored in a buffer. This buffer is then used to divide the live signal captured from the phosphor. This is done using a frame averaging scheme to decrease the noise in the system. Typically, 16 frames/s were averaged in the data reported here. A Silicon Graphics Workstation using the TexSem’s OIM software, version OIM 2.5 was used to analyze the data. Pattern center calibration was done using a silicon standard. To generate an orientation image micrograph (OIM), data are gathered in a hexagonal grid of spacing typically a fraction of the average grain size in the sample. At each point of the hypothetical hexagonal lattice, an EBKP pattern is captured, indexed and its orientation stored in a file. At the end of the scan, the output file contains information regarding the location and orientation of each point in the scan. Micrographs are constructed by placing a hypothetical hexagonal lattice of the hexagonal grid. Grain boundary misorientations are then calculated for all resulting grain

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Fig. 2. (a) Schematic illustration of the coordinate system used. (b) Calculated BSE trajectories observed from the perpendicular and and side views. (c) Calculated BSE trajectories observed from the top view. (d) Variation of calculated spatial resolution of BSE as a function of beam energy for a bulk Cu sample. (e) Variation of the calculated, lateral spatial resolution of BSE as functions of beam energy and cutoff energy for a bulk Cu sample.

boundaries. These are then superimposed on the scanned area to reveal the actual grain boundaries present. Pole figures and orientation distribution functions are calculated using the entire set of orientation data. 2.1. Resolution in the electron backscatter Kikuchi pattern measurements In an earlier work, spatial and depth resolution in electron

backscatter Kikuchi diffraction measurements were determined using a Monte Carlo Simulation (Ren et al., 1998). Fig. 2(a) shows a schematic illustration of the geometry used for the simulations. The specimen surface defines the XY plane and the incident electron beam is in the YZ plane. The 1 Y direction is defined as the forward scattering direction. The 1 X then defines a lateral direction. Note that the specimen is tilted at a high angle of 708, as is used for all experimental measurements in this work. The

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Table 1

Energy (keV)

20 15 10

Spatial resolution Longitudinal (nm)

Lateral (nm)

Depth (nm)

129 80 41

48 30 10

16 10 5

longitudinal spatial resolution of the backscattered electrons (BSE) is defined as the spatial extent of the detected BSE signal along the y-axis when viewing from the side direction. The lateral spatial resolution is defined as the spatial extent of the detected BSE signal along the x-axis when viewing from the perpendicular direction. And the depth spatial resolution is defined as the extent (maximum depth) of the BSE signal detected along the z-axis in the incident beam direction. In the simulation, the coordinates and energy of every electron were tracked and recalculated after each scattering event. All electron trajectories were initially plotted by viewing in a direction perpendicular to the specimen and from the side direction, with the energy of the scattered electrons color coded from red (incident beam energy) to dark blue (threshold energy, e.g. 0.5 keV). This approach offers a comprehensive picture of the specimen-electron beam interactions, including the interaction volume of all scattered electrons, the energy and angular distribution of BSEs, and the electron backscattered coefficient. Those trajectories associated only with the BSEs were extracted from the set of all electron trajectories and plotted. The interaction volume of BSEs could then be determined from the perpendicular and side views of BSEs trajectories. BSEs emerging at angles greater than 808 to the specimen normal were excluded, as these are not expected to impinge on the detector. The spatial resolution is defined as the smallest volume from which a certain percentage of the BSEs emerge. This percentage is defined by the appropriate cutoff energy with respect to the incident beam energy for the spectrum, which is relevant for producing the backscattered diffraction patterns (90–98%). Fig. 2(b) and (c) shows the calculated BSE trajectories

using the MC simulation observed from the perpendicular, side and top views for the interaction of a 20 kV incoming electron beam with a bulk Cu sample. As expected, a significant fraction of the BSEs exit in the forward beam direction with a high fraction of the incident energy, pointing to the importance of high tilts to obtain a good EBKP pattern. Fig. 2(d) shows the calculated longitudinal, lateral and depth resolution of BSE as a function of beam energy for Cu. Fig. 2(e) shows the lateral spatial resolution as a function of the cutoff energy in the range of 90–98% for a Cu sample. Data for Ni are similar. Hence, for a cutoff energy of 98%, the lateral spatial resolution for Ni or Cu is about , 15 nm at 20 kV incident beam energy. The resolution is improved significantly at lower kV. A similar MC calculation was performed for YBCO and the results are summarized in Table 1. The cutoff energy used here was 98%. Since it is not clear what the appropriate cutoff energy is, it can be expected that the actual resolution is within a factor of 2–3 of that reported in Table 1 for lower beam energies as illustrated by Fig. 2(e). 2.2. Electron backscatter Kikuchi patterns Fig. 3 shows three typical EBKP patterns obtained from the materials used for this study. Each of the materials is either cube textured or 458-rotated cube textured. Clearly, the average patterns from the Ni surface were the most well formed. However, patterns from the surfaces of oxide buffer layers (CeO2 or YSZ) and the YBCO film were also reasonably well formed. 3. Results and discussion 3.1. As-rolled Ni substrate Biaxially textured Ni substrates were formed by consecutive rolling of a polycrystalline, randomly oriented high purity (99.99%) bar to total deformations greater than 90%, followed by recrystallization (Goyal et al., 1996a,b). By controlling the surface condition of the work rolls, it was possible to obtain substrates with surfaces as smooth as those obtained by mechanical and chemical polishing. Average line scans in a 50 × 50 mm 2 region indicate an

Fig. 3. EBKP patterns from a cube textured Ni surface, a typical pattern from either epitaxial CeO2 or YSZ and a typical pattern from an epitaxial YBCO film on RABiTS.

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Fig. 4. (a) OIM micrograph of an as-rolled Ni sample deformed to greater than 90% total reduction. Gray level shading on the micrograph is an indication of the EBKP pattern contrast. Superimposed on the micrograph are only high angle grain boundaries greater than 108 denoted by red lines. (b) Log scale (100), (110) and (111) intensity pole figures from the data shown in Fig. 4(a).

rms roughness of , 10 nm (Goyal et al., 1997). The surface condition of a substrate can greatly affect the epitaxy and integrity of buffer layers, and hence the Jc of the superconducting film. Obtaining substrates with surfaces adequate for film growth without the need for a cumbersome polishing step is important for scale up to long lengths. Fig. 4 shows an OIM image of an as rolled Ni substrate which

Fig. 5. Idealized schematic representation of the rolling texture in fcc metals in the first subspace of Euler space.

was electropolished to remove , 5 mm of the surface layer. Very well-formed EBKP patterns were obtained despite the sample being heavily deformed. Data were obtained on a hexagonal grid at a spacing of 1 mm from a macroscopic region of the substrate of size , 370 × 250 mm 2. Gray level shading on the micrograph is a reflection of the pattern quality or intensity of the Kikuchi bands observed at each point. Indexing of the pattern at each location gave a unique measure of the orientation at that point. A hypothetical hexagonal lattice with a grain size of 1 mm was superimposed at each point from which a pattern was obtained. Grain boundary misorientations were then calculated for all the resulting boundaries using standard techniques. The micrograph was then regenerated with certain grain boundary criteria. In Fig. 4(a), only high angle grain boundaries greater than 108 and denoted by red lines are shown. Numerous low angle boundaries are also present within the regions defined by the high angle grain boundaries (shown in red) and are purposely not shown for clarity purposes. The orientation data obtained from this region is illustrated in interpolated, log-scale intensity (111), (100) and (110) pole figures in Fig. 4(b). Such pole figures are determined by first calculating the three-dimensional orientation distribution function from the measured experimental data and then deriving the individual pole figures. The pole

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Fig. 6. (a) OIM micrograph created by coloring grains in Fig. 4(a) with intensities along the b -fiber. Primarily, the “B” and the “S” orientations are present. (b) Corresponding color coded (100), (110) and (111) pole figures.

orientations which runs from {110} k112l, the brass point (B) at f 1 ˆ 358, f ˆ 458 and f 2 ˆ 908 through {123} k634l (S) at f 1 ˆ 598, f ˆ 378 and f 2 ˆ 638 to {112} k111l, the Copper point (C) at f 1 ˆ 908, f ˆ 358 and f 2 ˆ 458. This tube or fiber running from the B through the S point to the C point in Euler space is called the b -fiber. When an fcc metal is rolled appropriately to form an adequate precusor to the cube texture upon annealing, essentially no intensity is present along the a-fiber, which runs from the Goss point (G) at f 1 ˆ 08, f ˆ 458 and f 2 ˆ 908 to the brass point (B) at f 1 ˆ 358, f ˆ 458 and f 2 ˆ 908. The pole

figures shown are also in complete agreement with the Xray diffraction determined pole figures, as published elsewhere (Goyal et al., 1996a,b) and represent an idealized Cu-type rolling texture which is expected for Ni. The data shown in Fig. 4(b) are best understood by a three-dimensional texture representation in Euler space as opposed to a two-dimensional representation in the form of pole figures. Fig. 5 shows a schematic representation of the rolling texture in fcc metals in the first subspace of Euler Space. The most important observation from this figure is that the texture is now represented by a continuous tube or fiber of Table 2

Symbol

{hkl}

kuvwl

((((((f 1

((((((f

(((((f 2

Fraction intensity

Color

Cu “C”

112

111

“S”

123

634

Brass “B”

011

211

Cube Goss

001 011

100 100

39.2 90.0 52.9 59.0 27.0 121.0 54.7 35.2 0.0 0

65.9 35.3 74.5 36.7 57.7 36.7 90.0 45.0 0.0 45

26.6 45.10 33.7 63.4 18.4 26.6 45.0 0.0 0.0 90

0.029 0.030 0.013 0.050 0.132 0.096 0.244 0.315 0 0

Green Green Orange Orange Orange Orange Blue Blue Black Brown

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Fig. 7. Log scale, contour representation of the three-dimensional orientation distribution function for the as-rolled Ni sample calculated using data from the region shown in Fig. 4(a).

figures shown in Fig. 4(b) represent intensity solely along the b -fiber. Fig. 6(a) shows a color coded map of grains corresponding to the typically observed texture components in fcc metals. Each of the statistically symmetric orientations for each texture component illustrated along with its color designation and fraction intensity observed are shown in Table 2. The colors in Fig. 6(a) match the orientations shown in Table 2. Fig. 6(b) shows the corresponding color-coded (111), (100) and (110) pole figures, indicating the intensity along the fiber corresponding to the orientation in question. In Fig. 6(a), any grain with orientation within 308 of the idealized orientations shown above are colored with a given color. Should a given measurement point meet more than one orientation interval, then it is assigned a color corresponding to the orientation to which it more closely matches. The shading of a given color corresponds to its deviation from the exact orientation. A large orientation interval is necessary as the fibers shown in the schematic in Fig. 5 have a radius, typically in the range of about 308. Fig. 6(a) shows

that upon heavy deformation of Ni, bands corresponding to the “B” and “S” orientations form preferentially. The “C” orientation is interdispersed inhomogenously within the microstructure. Fig. 7 shows a contour representation of an orientation distribution function calculated from the measured data from the region in Fig. 4(a). Data is presented in constant sections of f 2. By placing successive sections on top of one another the threedimensional fiber shown in Fig. 7 can be compared to the schematic shown in Fig. 5. It can be seen that the much of the intensity is concentrated between the “B” and “S” orientations. Studies are ongoing to see how this grain orientation structure changes with deformation conditions in an effort to get a basic understanding of rolling-texture development. It is, however, important to note that no Cube or Goss oriented grains were observed in the region studied. This is important as one of the most commonly accepted theories for the formation of cube texture upon annealing an fcc metal with a rolling texture represented by Fig. 4(b), is rapid growth of prior cube nuclei. Hence, if such nuclei

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Fig. 8. (a) OIM micrograph from a macroscopic region of a RABiT substrate (see text for details). BKD patterns were obtained in a hexagonal grid with a step size of 3 mm. Variations in intensity is a reflection of pattern quality or intensity of the Kikuchi bands. Three types of grain boundaries are indicated in the figure, green boundaries denote boundaries with misorientations greater than 18 and less than 58, yellow lines denote boundaries with misorientations greater than 58 and less than 108 and red boundaries denote boundaries with misorientations greater than 108. Also shown in the figure are two line segments marked A and B. (b) Point-to-point and point-to-origin misorientation line plots for segment marked “A” in Fig. 8(a). (c) Point-to-point and point-to-origin misorientation line plots for segment marked “B” in Fig. 8(a). (d) Log scale (100), (110) and (111) intensity pole figures for the data gathered from the region shown in Fig. 8(a).

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Fig. 9. (a) OIM image of the region shown in Fig. 8(a), colored according to the deviation of the normal from point-to-point with respect to the [001] direction. (b) OIM image of the region shown in Fig. 8(a), colored according to the deviation of in-plane orientation from point-to-point with respect to the [100] direction; OIM image of the region shown in Fig. 8(a), colored according to the deviation of the measured orientation from point-to-point with respect to the {001} k100l, cube orientation (see text for further details).

are present, they are of a size scale below the resolution of the EBKP technique. 3.2. Annealed Ni substrate Subsequent annealing of the deformed Ni substrates in a wide temperature range in a vacuum of , 10 26 Torr results in the formation of a sharp {100} k100l cube texture. Typical samples have X-ray v - and f -scans with full-widthhalf-maximum (FWHM) of 6 and 78, respectively. The texture was found to be stable up to the melting point of

Ni. Fig. 8(a) shows an orientation image micrograph of a Ni substrate annealed at 10008C for 2 h. Extremely wellformed EBKP patterns were obtained from the recrystallized sample. Data were obtained on a hexagonal grid at a spacing of 3 mm from a macroscopic region of the substrate of size , 500 × 500 mm 2. Gray level shading on the micrograph is a reflection of the pattern quality or intensity of the Kikuchi bands observed at each point. Indexing of the pattern at each location gave a unique measure of the orientation at that point. As before, a hypothetical hexagonal lattice with a grain size of 3 mm was superimposed at

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Fig. 11. Schematic of multilayer structures used currently for fabricating YBCO on RABiTS.

each point from which a pattern was obtained and the resulting grain boundary misorientations calculated. In Fig. 8(a), green boundaries are boundaries with misorientation angles greater than 18 and less than 58. Yellow boundaries have misorientation angles greater than 58 and less than 108, and red boundaries have misorientations greater than 108. It is clear that low angle grain boundaries less than 58 are the most prevalent. After recrystallization at 10008C, the average grain size is approximately equal to the thickness of the substrate, which in this case is 125 mm. Thus, the substrate can be considered to consist of a columnar structure of grains, with the columns aligned with the (100) plane parallel to the surface of the columns and the [100] direction aligned along the rolling direction. Also shown in Fig. 8(a) are two line segments marked “A” and “B” respectively. The misorientations along the line segments A and B are shown in Fig. 8(b) and (c), respectively. Both the point-to-point (red) and point-toorigin (black) misorientations are plotted. In both the cases, the point-to-point (red) curves show that the actual misorientation between neighbouring grains is very small and marked by changes when crossing a grain boundary. The point-to-origin (black) curves show the orientation changes with respect to the starting point. An interesting observation for line segment marked A is that the starting grain and the terminating grain, both of which have similar gray level shading (determined by Kikuchi band intensity contrast) are similarly oriented. This may imply that some sort of channelling contrast is inherent in the gray level shading in Fig. 8(a). Fig. 8(d) shows (111), (100) and (110) pole figures derived from the region shown in Fig. 8(a). An extremely sharp cube texture is evident. In Fig. 9(a), those grains which have normals that correspond very closely to the [001] direction are colored dark red. Grains with a slight deviation away from [001] normal

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are shaded in a lighter red. In Fig. 9(b), those grains which have in-plane orientation that correspond very closely to the [100] direction are colored dark red. Grains with a slight deviation away from [100] in-plane direction are shaded in a lighter red. In Fig. 9(c) these effects are combined, and those grains which have an orientation that corresponds very closely to the cube orientation, {001} k100l are colored dark red. Grains with a slight deviation away from this orientation are shaded in a lighter red. In each case, color coded grain boundaries are superimposed corresponding to the criterion in Fig. 8(a). Comparison of these illustrations shows how the grains are misoriented with respect to one another in the sample. While it is clear that the gray level shading in Fig. 8(a) corresponds to the deviation away from the exact cube orientation, separating the effects of out-ofplane tilt and in-plane rotation is more complicated. In order to visualize the percolation of current flow, an epitaxial superconducting film should be grown on such a substrate. Fig. 10 shows the coloring of the same region shown in Fig. 8(a). In Fig. 10, grains have been colored according to the criterion that a single color represents a contiguous or percolative region of orientation changes less than 0.5, 1, 2, 2.5, 3, and 58, respectively. It can be seen that most of the substrate is percolatively connected within 2.58. 3.3. Epitaxial oxide buffer layers In order to grow high quality epitaxial superconducting films on the biaxially textured Ni substrate, a chemical and structural buffer layer is required. Typically, the desired buffer layers for 123 film growth are oxides. Hence, the task of fabricating a suitable substrate for epitaxial deposition of the superconductor involves epitaxial deposition of oxide buffer layers on Ni. This is difficult because of the ease of surface oxide formation on Ni under the typical oxidizing conditions required for oxide film growth. Although the surface oxide on (100) Ni can be epitaxial, it typically forms a (111) textured NiO layer, the orientation of which is unsuitable for fabricating high-Jc 123 films as many high angle boundaries are present. We found two methods that have proven successful in producing cubeon-cube epitaxial oxide buffer layer films on rolled and recrystallized Ni. The first involves epitaxial deposition of noble metal layers on Ni followed by deposition of oxides (Goyal et al., 1996a–c), and the second involves deposition of oxides directly on Ni under reducing conditions (Norton et al., 1996; Goyal et al., 1996a,b). In much of the ongoing work, however, the second method was chosen as reduction in the number of processing steps is desirable for a manufacturing process. Typically, two substrate configurations as illustrated in Fig. 11 are used. In both cases, epitaxial layers

Fig. 10. Orientation image micrographs shown in Fig. 8(a), colored with the criterion that a given color represents a percolative region within 0.5, 1, 2, 2.5, 3, and 58. Clearly most of the substrates are well connected within 38.

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Fig. 12. (a) OIM micrograph of a RABiT sample at the vicinity of the Ni/CeO2 interface. BKD patterns were obtained in a hexagonal grid with a step size of 30 nm. Variations in the intensity area, reflection of pattern quality or intensity of the Kikuchi bands. (b) Corresponding log scale (100), (110) and (111) intensity pole figures from the data in Fig. 12(a). (c) OIM micrograph in Fig. 12(a) colored according to the criterion that a given color represents a percolatively connected region within 18. Peaks marked with an asterisk correspond to reflections from the CeO2 layer.

of CeO2 and YSZ with a single 458 rotated cube-on-cube epitaxial orientation have been successfully deposited by laser ablation (Norton et al., 1996), sputtering (List et al., 1998) or e-beam evaporation (Paranthaman et al., 1997). Fig. 12(a) shows an EBKP scan on a cross-sectional sample in which CeO2 was deposited epitaxially on a cube textured Ni. The sample was prepared using standard crosssectional TEM sample preparation methods. Data were obtained on a hexagonal grid at a spacing of 30 nm at the region close to the interface between Ni and CeO2. Gray level shading on the micrograph is a reflection of the pattern quality or intensity of the Kikuchi bands observed at each point. Indexing of the pattern at each location gave a unique measure of the orientation at that point. As before, a hypothetical hexagonal lattice with a grain size of 30 nm was superimposed at each point from which a pattern was obtained and the resulting grain boundary misorientations calculated. In this image, the superimposed hexagonal lattice can actually be seen. The observed dominant grain boundary is at the interface and is a high angle grain boundary because of the 458 in-plane rotation of the CeO2 layer with respect to the underlying cube textured Ni substrate. Fig. 12(b) shows the corresponding pole figures for the orientations measured from Fig. 12(a). Points marked with

a red asterisk correspond to intensities from the CeO2 layer and the remaining points correspond to intensities from Ni. The pole figures show that the CeO2 layer is oriented exactly at an in-plane rotation of 458 from the Ni substrate. In fact, rotation of the cube points from the Ni in the pole figures by a 458 in-plane rotation will result in intensities at exactly the same locations as those measured for the CeO2 layer. This illustrates that very good epitaxy is obtained. Fig. 12(c) is a color coded map of Fig. 12(a) wherein a given color represents a percolatively connected region within 18. Several inferences can be drawn from Fig. 12(a) and (c). Firstly, it appears that the interface is not very flat on a macrosopic scale (on a scale much larger than that typically studied using TEM). Secondly, additional orientations nucleate randomly at some points on the sample during growth, but these were eventually engulfed by the growing film. It is likely that at these points second phase inclusions were embedded during growth of the film. Such observations have been made using the TEM and high resolution SEM on fracture specimens (Yang et al., 1998). Details of the local microstructure of CeO2 and YSZ layers grown by laser ablation have been studied using TEM and are reported elsewhere (Sun et al., 1998). TEM data for buffer layers grown using e-beam evaporation have also been reported

Fig. 13. OIM micrographs of a YBCO film with a Jc of 1.6 MA/cm 2 at 77 K, 0 T. Data were obtained on a hexagonal grid at a spacing of 1.1 mm from a macroscopic region of the substrate of size , 420 × 260 mm 2. In order to visualize the percolation of current flow through the YBCO film, grains are colored according to the criterion that a single color represents a contiguous or percolative region of orientation changes less than 1, 1.5, 1.8, 2, 5, and 78, respectively. It can be seen that most of the film is percolatively connected within 28.

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(Yang et al., 1998). It is found that the oxide layers for the most part have a columnar microstructure with no change in orientation across a column (Sun et al., 1998; Yang et al., 1998), consistent with the observations reported here.

477

or point-to-point correlation between grains, such that an Xray measured FWHM of , 88 can be accommodated in a manner such that percolation is achieved within 28 in most of the film.

3.4. Epitaxial YBCO films on RABiTS YBCO films on RABiTS have been grown successfully using two methods. The first is an in situ deposition using laser ablation (Norton et al., 1996; Goyal et al., 1996a–c; 1997; Mathis, 1998) and the second is an ex situ formation of a BaF2 precursor film deposited using electron beam coevaporation (Feenstra et al., 1998). YBCO films on RABiTS have been shown to have Jc’s approaching 3 MA/cm 2 (Mathis, 1998). EBKP data on a YBCO film fabricated using the ex situ method (Feenstra et al., 1998) are discussed here. Fig. 13 shows the orientation image micrographs of a YBCO film with a Jc of 1.6 MA/cm 2 at 77 K, 0 T (Feenstra et al., 1998). Data were obtained on a hexagonal grid at a spacing of 1.1 mm from a macroscopic region of the substrate of size , 420 × 260 mm 2. Indexing of the pattern at each location gave a unique measure of the orientation at that point. As before, a hypothetical hexagonal lattice with a grain size of 1.1 mm was superimposed at each point from which a pattern was obtained and resulting grain boundary misorientations calculated. In order to visualize the percolation of current flow through the YBCO film, Fig. 13 shows the coloring of the grains according to the criterion that a single color represents a contiguous or percolative region of orientation changes less than 1, 1.5, 1.8, 2, 5, and 78, respectively. It can be seen that most of the film is percolatively connected within 28. These data are in excellent agreement with similar data shown for the Ni substrate in Fig. 10, implying that the sharp biaxial texture of the Ni substrate is effectively transferred to the YBCO film. It can be further argued that the high-Jc observed in this sample is directly related to the local grain-to-grain correlations within 28 in reference to the strong dependence of Jc on misorientation angle. Fig. 14 shows the grain boundary misorientation distributions (GBMD) calculated from the region shown in Fig. 13. The black curve represents the experimentally measured or “correlated” GBMD and the red curve represents a calculated “uncorrelated” GBMD. The “uncorrelated” GBMD was calculated assuming no spatial correlation between adjacent points, and hence reflects the expected GBMD should no local effects or correlations be present. In other words, all possible misorientations between the points sampled were calculated. As the measurement points within a grain typically have very low misorientations, the correlated curve is somewhat exaggerated towards low angles. Nevertheless, the data clearly show the presence of a local

4. Summary Electron backscatter Kikuchi diffraction was used to study texture development and grain boundary networks in RABiTS and the transfer of such a biaxial textured and preferential grain boundary network to epitaxial YBCO films grown on such substrates. EBKP data show that the as-rolled Ni sample which upon annealing gives a sharp cube texture, has intensities predominantly at orientations corresponding to the “B” and “S” orientations. Orientation distribution functions clearly show how the texture can be characterized by localization of intensities at and between the “B” and “S” orientations. The corresponding grain structure in the sample consists of interweaving bands of these orientations. Numerous high angle grain boundaries exist throughout the sample, as expected at the interfaces between these bands. No grains with a cube orientation were found in the as-rolled sample. Upon recrystallization, a sharp cube texture is imparted to the Ni substrate. EBKP measurements show that the grain boundary networks formed upon recrystallization are predominantly low angle with most grain boundaries having misorientations less than 58. Point-to-point and point-toorigin misorientation line plots show how the orientation changes in a correlated and an uncorrelated manner, respectively. Maps of color coding of similarly oriented grains by plotting areas percolatively connected with a certain degree, show that the substrate is almost completely connected within 38. EBKP data from a cross-sectional sample in which CeO2 was deposited epitaxially on Ni, show that excellent epitaxy is obtained. It is found that the interface between the Ni and CeO2 is rough on a macroscopic scale. It was also observed that secondary or non-epitaxial orientations sometimes nucleate during growth but are finally engulfed by the growing film with the correct orientation. Orientation image micrographs of a YBCO film with a Jc of 1.6 MA/cm 2 at 77 K, show that most of the film is percolatively connected within 28. Comparison of the spatially correlated and uncorrelated GMBD shows that local grainto-grain correlations are present between the grains. Comparison of the data obtained for the YBCO film with that for the underlying Ni substrate show that both the macroscopic texture and grain boundary networks are very similar in the two cases, implying excellent epitaxy of the multilayers.

Fig. 13. (continued)

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Fig. 14. Grain boundary misorientation distributions calculated from the region shown in Fig. 13. The black curve represents the experimentally measured or “correlated” GBMD and the red curve represents a calculated “uncorrelated” GBMD. The “uncorrelated” GBMD was calculated assuming no spatial correlation between adjacent points, and hence reflects the expected GBMD should no local effects or correlations be present.

Acknowledgements Research sponsored by US Department of Energy, Office of Efficiency and Renewable Energy, Office of Utility Technologies—Superconductivity Program, and the Office of Energy Research, Basic Energy Sciences, managed by Lockheed Martin Energy Research Corporation for the US Department of Energy under contract DE-AC0596OR22464 i8s gratefully acknowledged. References Dimos, D., Chaudhari, P., Mannhart, J., LeGoues, F.K., 1988. Phys. Rev. Lett. 61, 219. Dimos, D., Chaudhari, P., Mannhart, J., 1990. Phys. Rev. B 41, 4038. Feenstra, R., Goyal, A., Christen, D.K., Paranthaman, M., Lee, D.F., Verebelyi, D., Specht, E.D., Budai, J.D., Norton, D.P., Kreger, D.M., 1999. Science, submitted for publication.

Goyal, A., Norton, D.P., Christen, D.K., Specht, E.D., Paranthaman, M., Kroeger, D.M.D.P., Budai, J.D., He, Q., List, F.A., Feenstra, R., Kerchner, H.R., Lee, D.F., Hatfield, E., Martin, P.M., Mathis, J., Park, C., 1996. Appl. Supercond. 69, 403–427. Goyal, A. et al., 1996b. US Patent No. 5, 739, 086; US Patent No. 5, 741, 377. Goyal, A., Norton, D.P., Kroeger, D.M., Christen, D.K., Paranthaman, M., Specht, E.D., Budai, J.D., He, Q., Saffian, B., List, F.A., Lee, D.F., Hatfield, E., Martin, P.M., Klabunde, C.E., Mathis, J., Park, C., 1997. J. Mater. Res. 12, 2924–2940. Goyal, A., Norton, D.P., Budai, J.D., Paranthaman, M., Specht, E.D., Kroeger, D.M., Christen, D.K., He, Q., Saffian, B., List, F.A., Lee, D.F., Martin, P.M., Klabunde, C.E., Hatfield, E., Sikka, V.K., 1996. Appl. Phys. Lett. 69, 1795. Hasegawa, K., Yoshida, N., Fujino, K., Mukai, H., Hayashi, K., Sato, K., Ohkuma, T., Honjo, S., Ishii, H., Hara, T., 1997. Proc. 16th Int. Cryogenic Mater. Conf. (ICMC), Elsevier, Amsterdam p. 1413. Iijima, Y., Tanabe, N., Kohno, O., Ikeno, Y., 1992. Appl. Phys. Lett. 60, 769. Kormann, G., Bilde, J.B., Sorensen, K., de Reus, R., Anderson, R.H., Vace, P., Freltoft, T., 1992. J. Appl. Phys. 71, 3419. List, F.A., Goyal, A., Paranthaman, M., Norton, D.P., Specht, E.D., Lee, D.F., Kroeger, D.M., 1998. Physica C 302, 87–92. Mathis, J.E., Goyal, A., Lee, D.F., List, F.A., Paranthaman, M., Christen, D.K., Specht, E.D., Kroeger, D.M., Martin, P., 1998. Jpn J. Appl. Phys. 37, L1379–L1382. Matsuda, H., Soeta, A., Doi, T., Aikhara, A., Kamo, T., 1992. Jpn J. Appl. Phys. 31, L1229. Norton, D.P., Goyal, A., Budai, J.D., Christen, D.K., Kroeger, D.M., Specht, E.D., He, Q., Saffian, B., Paranthaman, M., Klabunde, C.E., Lee, D.F., Sales, B.C., List, F.A., 1996. Science 274, 755. Paranthaman, M., Goyal, A., List, F.A., Specht, E.D., Lee, D.F., Martin, P.M., He, Q., Christen, D.K., Norton, D.P., Budai, J.D., 1997. Physica C 275, 266. Reade, R.P., Burdahl, P., Russo, R.E., Garrison, S.M., 1993. Appl. Phys. Lett. 74, 1905. Ren, S.X., Kenik, E.A., Alexander, K.B., Goyal, A., 1998. Microsc. Microanal. 4, 15–22. Sun, E.Y., Goyal, A., Norton, D.P., Park, C., Kroeger, D.M., Paranthaman, M., Specht, E.D., Christen, D.K., 1999. Physica C, to be published. Wu, X.D., Foltyn, S.R., Arendt, P.N., Blumenthal, W.R., Campbell, I.H., Cotton, J.D., Coulter, J.Y., Hults, W.L., Maley, M.P., Safar, H.F., Smith, J.L., 1995. Appl. Phys. Lett. 67, 2397. Yang, C.Y., Babcock, S.E., Goyal, A., Paranthaman, M., List, F.A., Norton, D.P., Kroeger, D.M., Ichinose, A., 1998. Physica C 307, 87–98.