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Fabrication and X-ray characterization of biaxially textured thick film EuBa2Cu3O7−δ homo-substrates
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Journal of Crystal Growth 289 (2006) 527–533 www.elsevier.com/locate/jcrysgro
Fabrication and X-ray characterization of biaxially textured thick film EuBa2Cu3O7d homo-substrates S.Q. Wanga,, J.Z. Zhangb, R.S. Markiewiczb, B.C. Giessena a
Department of Chemistry and Barnett Institute, Northeastern University, Boston, MA 02115, USA b Department of Physics and Barnett Institute, Northeastern University, Boston, MA 02115, USA
Received 15 March 2005; received in revised form 10 November 2005; accepted 22 November 2005 Available online 20 February 2006 Communicated by R.S. Feigelson
Abstract To fabricate biaxially textured rare earth (RE) substituted RE-123 thick films as homo-substrates for growth of YBa2Cu3O7d (YBCO) superconductor epitaxial films, we have initially aligned polycrystalline EuBa2Cu3O7d Superconductor using a magnetic force and a mechanical force simultaneously. The structural properties of EuBa2Cu3O7d thick films uniaxially aligned by each single force and biaxially aligned by a combination of the two forces were studied for the first time using X-ray diffractometery. We report systematically the structural sample characterization of such EuBa2Cu3O7d thick films by X-ray diffraction (XRD), demonstrating the biaxially textured polycrystalline EuBa2Cu3O7d homo-substrates having substantial biaxial grain alignment with their grain [0 0 1] axes perpendicular to the surfaces of the thick films and [0 1 0] axes parallel to the magnetic field direction in the specimen plane. The biaxial textured thick film homo-substrates are thus formed. This can be seen as establishing a new paradigm in ceramics science. r 2005 Elsevier B.V. All rights reserved. PACS: 74.76.B; 61.10.N Keywords: A1. Biaxial texture; A1. Pole figure study; A1. X-ray diffraction; A2. Homo-substrate; A2. Magnetic field alignment; A2. Shape alignment; B2. EuBa2Cu3O7d thick film
1. Introduction The growth of high-quality epitaxial thin film of YBa2Cu3O7d (YBCO) superconductor for electric power and energy storage applications has attracted much attention. Most of the important techniques in developing YBCO superconductor films are based on the three critical parts: substrate, buffer layer and coated superconductor layer. The substrates, such as Ag, Ni and Ni-alloy were used as a carrier, the buffer layers, such as MgO, YSZ and CeO2 were employed as a texture base and a reaction barrier between the YBCO layer and the substrate, the Corresponding author. Current address: 80 Scott Drive, Air Force Research Lab, Hanscom AFB, MA 01731-2909, USA. Tel.: +1 781 3775588; fax: +1 781 3776948. E-mail address: [email protected] (S.Q. Wang).
0022-0248/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2005.11.093
coated superconductor layers function as the current carrier. The popular techniques for substrate texture generation are rolling assisted biaxial orientation [1,2], and ion beam assisted deposition [3] which are applied to make out-of-plane and in-plane orientation on metallic substrates and on the buffer layer itself, respectively. Many deposition techniques, such as chemical solution deposition, vapor deposition, pulsed laser deposition and pulsed electron deposition were developed for buffer layer and YBCO film fabrication. The YBCO coated conductors with a current density over 1 mA/cm2 have been demonstrated [2,4,5]. However, it has been reported that the Jc decreases rapidly as the thickness of an epitaxial YBCO layer increases either on a single-crystal substrate or a metallic substrate [6–9]. The reason for the Jc drop, which seems to be universal regardless of the deposition technique, is not clear. There are still numerous scientific
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and technological difficulties to be overcome before the thick-coated conductors of YBCO with high Jc are commercially fabricated. For the successful commercial development of coated conductor YBCO, it may be important to make thick YBCO film as a homo-substrate to completely eliminate the buffer layer effects and the lattice mismatch between the substrate, buffer layer and coated conductor layer for growth of YBCO epitaxial films. In order to fabricate biaxially textured YBCO homo-substrates, we have developed a combined magnetic–mechanical technique for simultaneously aligning superconductor grains along two independent crystalline axes [10–17], and have applied this technique for the first time to EuBa2Cu3O7d superconductor. This technique is based on the use of two perpendicular alignment forces: one of these is magnetic and applied horizontally, orienting a particular direction in the a–b plane; the other is mechanical and acts through a shape and deformation anisotropy of superconductor grains, orienting the c-axis along the direction perpendicular to the a–b plane. To align the a or b axes, it is necessary to substitute certain rare earth (RE) so-called normal aligners [18] into the superconductors, e.g., Eu, Yb, Er or Gd for Y in 123 [14,15]. When a cuprate superconductor is doped with a normal aligner RE element, the resulting magnetic anisotropy of the cuprate crystal is the result of the competition between the principal magnetic moment of the RE ion and the magnetic moment contributed by the Cu–O conducting planes. In this case, the principal magnetic moment of the RE ion lies normal to c-axis and is higher than that of the Cu–O plane which is parallel to c-axis, i.e. DwRE(?)4DwCuO (||). Therefore the resulting susceptibility will tend to align the crystal with its c-axis perpendicular (normal) to the magnetic field direction during magnetic alignment. We chose Eu as the RE component for biaxial alignment. This is critical, because among the four normal-aligning RE elements that can be incorporated into YBCO, Eu is the only one that produces a sufficiently large differentiation between the potential alignment direction in the a–b plane (/1 1 0S vs. [0 1 0] and [1 0 0]) [19,20]; therefore, it is useful for in-plane alignment. 2. Experimental procedure For biaxial alignment, Eu-123 particles with somewhat plate-like shape and a size range of 38–45 mm were dispersed in 2-pentanol alignment medium. A non-magnetic copper die with some pure alignment medium was frozen first in liquid nitrogen. The Eu-123-alcohol dispersion was then added and frozen layer by layer in the die. In a 1.5 T horizontal magnetic field, the die was placed on a support that was kept at about 30 1C. The top of the die was continuously cooled by covering it with a high thermal conduction container filled with liquid nitrogen. According to the temperature gradient that was established
in the die, the dispersion began to melt from bottom to top. The Eu-123 particles suspended in each alignment medium layer settled to the die bottom slowly because of the high viscosity of the 2-pentanol in the temperature range near its melting point [11]. During settling, the particles were aligned simultaneously by gravity and the horizontal field. After the aligned particles settled completely to the bottom of the die, the alignment medium was removed by gentle heating of the copper die at 110 1C. Finally, the aligned Eu123 particles were pressed in the die with a pressure of 9000 psi; this improved further the shape alignment obtained during the particle deposition. With this biaxial alignment technique, a free-standing thick film of 0.5 mm thickness and lateral dimensions of 30 10 mm was obtained. The biaxially aligned samples were pre-sintered at 400 1C for 2 h and then sintered at 970 1C for 12 h in oxygen. The Eu-123 thick films uniaxially aligned by each individual alignment force were also fabricated. A vertical superconducting magnet was used for magnetic field alignment. Eu-123 grains with a size range of 38–45 mm were mixed with a 5 min epoxy matrix in a volume ratio of 1:1 in a rectangular plastic box wrapped with aluminum foil. The mixture was then held in a vertical field of 10 T at room temperature to allow the grains to align until the epoxy resin became hard in the field. Rectangular shaped specimens with their flat bottoms and sides facing in the directions of and normal to the magnetic field B were obtained. For c-axis (shape) alignment, a dilute suspension of 38–45 mm Eu-123 grains in 2-pentanol medium [11] was poured into a 10-mm diameter circular copper die filled with 2-pentanol and placed in an ultrasonic vibration bath. After the Eu-123 grains have settled to the bottom of the copper die with some shape orientation via gravity due to the shape anisotropy of the grain, the top layer of fluid medium in the die was removed by gentle heating of the copper die at 110 1C. After that, the sample in the die was pressed, i.e., additional shape alignment was then provided with a uniaxial pressure of 9000 psi. A thick film of about 1 mm thickness was thus obtained and then sintered at 970 1C for 24 h in O2. The alignment characteristics or structural properties of both uniaxially and biaxially aligned Eu-123 thick films were studied by a four- circle X-ray diffractiometer with Cu Ka radiation. Modern X-ray analysis techniques, including X-ray diffraction (XRD) 2Y–Y scan, w-circle scan, f-circle scan and pole figure measurement, were used in this study.
3. Results and discussion In order to demonstrate clearly how the biaxial alignment technique works, The XRD analyses for Eu123 thick film samples uniaxially aligned by magnetic force and mechanical force, respectively, will be presented first.
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3.1. XRD results of Eu-123 single axis aligned by magnetic force This important alignment is presented in four different ways, partly as a demonstration, and partly to generate familiarity with pole figure interpretation in the reader. The first and third presentations are based on 2Y scans at different tilt angles w; the second and fourth ones use pole figures at constant Y. Fig. 1 shows reflection XRD 2Y scan spectra (taken at the w values indicated) of Eu-123 in epoxy aligned by a 10 T vertical magnetic field. The face of the test sample that upon mounting in the diffractometer becomes the reflecting sample surface was perpendicular to the field direction; this caused the normals to the reflecting planes (for w ¼ 01) to lie in the direction of the vertical magnetic field. When w ¼ 01, essentially only the (0 k 0) peaks show over a uniform background, accompanied with the (h 0 0) peaks due to twinning in Eu-123 grains [except for some (0 1 3)/ (1 0 3)/(1 1 0) reflections, see below]. The intensities of (0 k 0) peaks may involve a small degree of intensity contribution from (0 0 l) parasitical peaks because their Bragg angles differ only by a few hundredths of a degree; however the intensities contributed by such (0 0 l) peaks were low, otherwise the (0 0 5) and (0 0 7) peaks would appear at their respective positions. A relatively low contribution of (1 1 0), (0 1 3) and (1 0 3) (due to possibly parasitic misalignment) is observed. As expected for wellaligned grains, the intensity of the joint peak of these three
Fig. 1. X-ray Y–2Y reflection spectra (Cu-Ka radiation, taken at different angle w) of Eu-123 in epoxy aligned by a vertical magnetic field, showing a b-axis field alignment.
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planes becomes strong when w ¼ 451, since the angles between these planes and (0 k 0) plane equal about 45.51. The [0 1 0] axis (b-axis) is thus seen to be well aligned in the direction parallel to the applied field; this can also be demonstrated by a (0 1 3)/(1 0 3)/(1 1 0) pole figure analysis. For a uniaxially (b-axis) aligned sample, the (0 1 3) and (1 1 0) poles will be randomly distributed on a circle about [0 1 0] axis with angle w 451, the angle between the [0 1 0] and the [0 1 3] directions (or between [0 1 0] and the [1 1 0] direction). As shown in Fig. 2, this is the case. The figure is presented in spatial representation, tilted by 201. Nearly perfect b-axis field alignment is thus presented. A third demonstration of the b-axis aligned Eu-123 thick film uses 2Y scan. The reflection XRD 2Y scan diffractograms for the cross-section of uniaxially vertical-fieldaligned Eu-123 sample are given in Fig. 3. During field alignment, the sample surface was parallel to the field direction. This time, the sample was mounted for reflection such that the normals to the reflecting planes (at w ¼ 01) were perpendicular to the field direction, or expressed alternatively; this arrangement caused the reflecting planes to form a zone with [0 1 0] as a common axis. As result of the formation of this zone, the diffractograms in Fig. 3 show that only (h 0 l), (h 0 0) and (0 0 l) reflections presented at w ¼ 01 [except some parasitic (1 1 0) and twinning (0 1 3)]. It is of interest to note that w ¼ 451 scans for the arrangements in Figs. 1 and 3 become identical, because
Fig. 2. (0 1 3)/(1 0 3)/(1 1 0) pole figure of the field-aligned Eu-123 sample (in spatial representation, tilted by 201 about the projection axis [0 1 0]), indicating a uniaxially b-axis alignment. The contribution from (1 0 3) is thought to be negligible, but cannot be separated out due to the peak overlap.
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Fig. 4. (0 1 3)/(1 0 3)/(1 1 0) pole figure for the cross-section of the uniaxially vertical field aligned Eu-123, taken (0 0 1) as the projection plane (in contour line representation), showing a good b-axis field alignment.
Fig. 3. X-ray 2Y scan (Cu-Ka, taken at different angle w) reflection diffractograms for the cross-section of the uniaxially vertical field aligned Eu-123 sample.
for both cases, at w ¼ 451, the physical configurations of the sample in the pole figure device have become identical. A fourth demonstration of b-axis alignment of Eu-123 is contained in a pole figure of the (0 1 3)/(1 0 3)/(1 1 0) line cluster taken from the cross-sectional sample face used and discussed in the 2Y-scan of Fig. 3 ([0 1 0]||B||sample surface). Here the poles are randomly located on two arcs and one line, as shown in Fig. 4. The poles densities are not uniform, being higher near the center area (the line from j ¼ 01 to 1801) and lower in the edge area of the pole figure, which is caused by defocusing effects in this X-ray reflection technique. These defocusing effects will be discussed in detail in Ref. [21]. In order to understand the pole figure in Fig. 4 better, some background crystallographic relationships for b-axis aligned polycrystalline Eu-123 are provided in Fig. 5 which shows the principal poles for the Eu-123 crystal projected on its (0 0 1) plane or, in other words, shows a partial (0 0 1) projection of b-axis field aligned Eu-123. It is this plane, parallel to the cross-section of the sample, which was looked at by reflection XRD of the b-axis aligned sample, and thus the (0 1 3)/(1 0 3)/(1 1 0) pole figure was obtained (Fig. 4). In a pole figure of a completely random sample, the poles will be distributed uniformly over the whole area of the pole figure. But if uniaxial orientation (or single alignment) is present, for instance, in vertical field aligned polycrystalline Eu-123 where the b-axis of Eu-123 grains were aligned along the field direction, in the projection
Fig. 5. Partial (0 0 1) projection for a perfectly, biaxially aligned polycrystalline Eu-123.
plane, the [since the angle between any two planes (or directions) will remain unchanged] two arcs joining the {0 1 3} and {1 1 0} poles both must lie at E451 from {0 1 0}, predicting the upper and lower arcs in Fig. 4. For the same reason, the poles of the planes sharing the [0 1 0]—zone axis such as 103 and 1¯ 0 3 will lie in the equatorial region of the pole figure, Fig. 5; this explains the linear central ‘‘arc’’ of Fig. 4.
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It is useful to point out here that the dots plotted in Fig. 5 represent the poles for a perfectly, biaxially aligned Eu-123 polycrystalline sample that behaves as ‘‘a granular single crystal’’. The arcs in Fig. 4 were derived from the dots in Fig. 5 by the absence of simultaneous c-axis alignment (assumed to exist in Fig. 5); this missing c-axis alignment allowed the 0 0 1 poles to move along the equatorial plane and broadened the ‘‘dots’’ in Fig. 5 into the arcs in Fig. 4. A pole figure projection can also help to clarify the prior pole figure, Fig. 2, which is derived from an [0 1 0] axis projection (i.e., along the North–South axis of the previous pole figure, Fig. 5), as shown in Fig. 6. Here, a small ¯ and 110. ¯ circle joins the poles 0 1 3, 1 1 0, 0 1 3, In the absence of c-axis alignment (fixing the 0 0 1 pole) these poles will be randomly distributed on the small circle, as seen in Fig. 2.
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and 1, where the highest intensity of 2.5 is associated with the contours outlining the shaded areas. A strong c-axis alignment is seen. However, a low degree of c-axis misalignment also exists, as shown by the misorientation background observed at the (0 0 1) position, the center of the pole figure in Fig. 7. The crystallographic relation of the poles in this figure to each other is analogous to that assumed above and is demonstrated in Fig. 5 as a ring at E451 about (0 0 1). These results allow us to conclude that the sample exhibits a somewhat platy (or marginally tilelike) particle shape, where the short axis of the plates coincides with the direction of the applied force (perpendicular to the surface of the specimen).
3.2. XRD results of Eu-123 single axis aligned by mechanical force For a uniaxially, (c-axis) aligned polycrystalline Eu-123 sample produced by pressing, the reflection XRD (0 1 3)/ (1 0 3) pole figure measurements gave the same results for both top and bottom surfaces of the sample, showing the uniformity of the pressing alignment. These poles are randomly located on a small circle corresponding to an angle w ¼ 451, i.e., the angle between the [0 0 1] and the [0 1 3] or [1 0 3] directions; a corresponding pole figure showing this ring (in contour line representation) is given in Fig. 7. The contours represent the intensities 2.5, 2
Fig. 7. (0 1 3)/(1 0 3) pole figure of the aligned Eu-123 by pressing polycrystalline sample, taken (0 0 1) as the projection plane (in contour line representation), showing uniform ring of poles which indicates c-axis alignment due to the action of a mechanical force.
Fig. 6. Partial (0 1 0) projection for a perfectly, biaxially aligned Eu-123 polycrystalline sample.
Fig. 8. X-ray (0 0 6) w-circle scan diffractogram (Cu-Ka) of the biaxially aligned Eu-123, demonstrating a strong c-axis shape alignment due to the action of a mechanical force.
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3.3. XRD results of biaxially aligned Eu-123 The (0 0 6) pole figure analyses for the top and bottom surfaces of the biaxially aligned samples shows that the [0 0 1] axis, (c-axis), was oriented in the direction normal to the sample surface, documenting a very high degree of c-axis shape alignment (not shown). The localization of these poles remains within a small area; the full-width at half-maximum (FWHM) of (0 0 6) w-circle scan for the biaxially aligned Eu-123 is 4.71, as shown in Fig. 8. This indicates that the crystallites were strongly aligned, with their (0 0 l) planes parallel to the sample surface due to the action of a mechanical force on their anisotropic (somewhat platy) shape. This alignment can also be demonstrated by reflection XRD 2Y scan, which showed only the (0 0 l) peaks over a uniform background, with a small degree of {1 1 0} misalignment observed, as shown in Fig. 9. The fraction, F, of grains aligned [11] along [0 0 1] was found by calculation according to the intensity data in Fig. 9 to be about 97%. As mentioned above, a small degree of c-axis misalignment also remained in our sample. This may be caused by the relatively high uniaxial pressure used during the c-axis alignment (9000 psi), which must have broken some crystals already aligned along the b-axis. It was found that a high degree of c-axis alignment depends on finding a good combination of grain size and alignment force, or pressure. We turn now to the direct evidence for the presence and degree of biaxial alignment, demonstrated by the locations of (0 1 3) and (1 0 3) poles on their joint pole figure. (Because their Bragg angles differ only by D(2Y) ¼ 0.271, the (0 1 3) and (1 0 3) planes cannot be separated without unacceptable loss of intensity; therefore, both contribute to the pole figure.) The (0 1 3)/(1 0 3) pole figure (contour line representation) for the bottom surface of the biaxially aligned Eu-123 sample is shown in Fig. 10; the same result was obtained for the top surface. In this figure, the biaxial
Fig. 9. X-ray 2Y-scan spectrum (Cu-Ka) of the biaxially aligned Eu-123, demonstrating a strong shape-induced c-axis alignment.
Fig. 10. (0 1 3)/(1 0 3) pole figure of the biaxially aligned Eu-123 polycrystalline sample, taken (0 0 1) as the projection plane (in contour line representation), showing partial biaxial alignment due to a magnetic force applied normal to a mechanical force.
alignment is expressed in a clover-leaf arrangement of reflections. The contours represent intensities 4, 3, 2.5 and 1; where intensity 4 is associated with the contour outlining the black areas. When there is b-axis alignment in addition to c-axis alignment, the (0 1 3)/(1 0 3) ring is broken up into four arcs, with the length of the arcs being a measure of the degree of b-axis alignment. A f-circle scan of (0 1 3)/(1 0 3) of the biaxially aligned Eu-123 showed four peaks with FWHM of 471 (not shown here); the positions of these peaks located around f ¼ 01; 901; 1801 and 2701, respectively. For a perfectly, double aligned sample, the arcs would reduce to four points at f ¼ 01, 901, 1801 and 2701, as explained in Section 3.1 (see Fig. 5). The pole figure thus demonstrates the crucial fact that the a, b axes have adopted preferential orientations due to the action of the magnetic field. To have accomplished this degree of biaxial alignment is the central result of this work. The magnetic field direction B is along the North–South axis of Fig. 10. Because the b-axes of the Eu-123 grains line up parallel to B, the a-axes will be oriented at an angle of 901 to B; due to the overlap of (0 1 3) and (1 0 3), the pole figure assumes a pseudotetragonal appearance. A further contribution to the (1 0 3) reflections seen at the positions transverse to B comes from twinning in Eu123 grains. Twin boundaries bisect the angle between [1 0 0] and [0 1 0]; upon alignment, the majority of regions of each biaxially aligned, twinned crystallite will therefore force the twinned minority portion into an orientation such that the [1 0 0] axes of the latter coincide with the [0 1 0] axes of the former, i.e., they lie in the B direction. Therefore, the [0 1 3] directions of the minority portion coincide with the [1 0 3] direction of the majority and contribute to the transverse diffraction spot intensity. This will occur even if the separation of (0 1 3) and (1 0 3) were complete.
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4. Conclusions In order to grow high-quality epitaxial YBCO thick films on superconductor homo-substrates, we are conducting research on a two-step process which includes Eu-123 homo-substrate fabrication and YBCO film epitaxial deposition on the pre-made substrates. We have reported studies on the first step in this paper, the preparation of biaxially aligned Eu-123 substrate thick films using a combined magnetic–mechanical technique. However, the alignment factors of both out-of-plane and in-plane are only 33%, according to quantitative X-ray analysis [21]. It was found that a high degree of biaxial texture depends on a good combination of superconductor grain size, magnetic force and mechanical force. Work on the quantification and optimization of the process continues. Once the Eu123 homo-substrates are optimized with the biaxial texture, the next step will commence, epitaxial growth of YBCO thick films on these unique substrates. References [1] T.J. Doi, T. Yuasa, T. Ozawa, K. Higashiyama, Advance in Super conductivity VII, Springer, Tokyo, 1994. [2] A. Goyal, D.P. Norton, D.K. Christen, E.D. Specht, M. Paranthaman, D.M. Kroeger, J.D. Budai, Q. He, F.A. List, R. Feenstra, H.R. Kerchner, D.F. Lee, E. Hatfield, P.M. Martin, J. Mathis, C. Park, Appl. Supercond. 4 (1996) 403. [3] C.P. Wang, K.B. Do, M.R. Beasley, T.H. Geballe, R.H. Hammond, Appl. Phys. Lett. 71 (1997) 2955. [4] A. Goyal, D.P. Norton, J.D. Budai, M. Paranthaman, E.D. Specht, D.M. Kroeger, D.K. Christen, Q. He, B. Saffina, F.A. List, D.F. Lee, P.M. Martin, C.E. Klabunde, E. Hatfield, V.K. Sikka, Appl. Phys. Lett. 69 (1996) 1795. [5] X.D. Wu, S.R. Foltyn, P.N. Arendt, W.R. Blumenthal, I.H. Campbell, J.D. Cotton, J.Y. Coulter, W.L. Hults, M.P. Maley, H.F. Safar, J.L. Smith, Appl. Phys. Lett. 67 (1995) 2397.
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Journal of Crystal Growth 289 (2006) 527–533 www.elsevier.com/locate/jcrysgro
Fabrication and X-ray characterization of biaxially textured thick film EuBa2Cu3O7d homo-substrates S.Q. Wanga,, J.Z. Zhangb, R.S. Markiewiczb, B.C. Giessena a
Department of Chemistry and Barnett Institute, Northeastern University, Boston, MA 02115, USA b Department of Physics and Barnett Institute, Northeastern University, Boston, MA 02115, USA
Received 15 March 2005; received in revised form 10 November 2005; accepted 22 November 2005 Available online 20 February 2006 Communicated by R.S. Feigelson
Abstract To fabricate biaxially textured rare earth (RE) substituted RE-123 thick films as homo-substrates for growth of YBa2Cu3O7d (YBCO) superconductor epitaxial films, we have initially aligned polycrystalline EuBa2Cu3O7d Superconductor using a magnetic force and a mechanical force simultaneously. The structural properties of EuBa2Cu3O7d thick films uniaxially aligned by each single force and biaxially aligned by a combination of the two forces were studied for the first time using X-ray diffractometery. We report systematically the structural sample characterization of such EuBa2Cu3O7d thick films by X-ray diffraction (XRD), demonstrating the biaxially textured polycrystalline EuBa2Cu3O7d homo-substrates having substantial biaxial grain alignment with their grain [0 0 1] axes perpendicular to the surfaces of the thick films and [0 1 0] axes parallel to the magnetic field direction in the specimen plane. The biaxial textured thick film homo-substrates are thus formed. This can be seen as establishing a new paradigm in ceramics science. r 2005 Elsevier B.V. All rights reserved. PACS: 74.76.B; 61.10.N Keywords: A1. Biaxial texture; A1. Pole figure study; A1. X-ray diffraction; A2. Homo-substrate; A2. Magnetic field alignment; A2. Shape alignment; B2. EuBa2Cu3O7d thick film
1. Introduction The growth of high-quality epitaxial thin film of YBa2Cu3O7d (YBCO) superconductor for electric power and energy storage applications has attracted much attention. Most of the important techniques in developing YBCO superconductor films are based on the three critical parts: substrate, buffer layer and coated superconductor layer. The substrates, such as Ag, Ni and Ni-alloy were used as a carrier, the buffer layers, such as MgO, YSZ and CeO2 were employed as a texture base and a reaction barrier between the YBCO layer and the substrate, the Corresponding author. Current address: 80 Scott Drive, Air Force Research Lab, Hanscom AFB, MA 01731-2909, USA. Tel.: +1 781 3775588; fax: +1 781 3776948. E-mail address: [email protected] (S.Q. Wang).
0022-0248/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2005.11.093
coated superconductor layers function as the current carrier. The popular techniques for substrate texture generation are rolling assisted biaxial orientation [1,2], and ion beam assisted deposition [3] which are applied to make out-of-plane and in-plane orientation on metallic substrates and on the buffer layer itself, respectively. Many deposition techniques, such as chemical solution deposition, vapor deposition, pulsed laser deposition and pulsed electron deposition were developed for buffer layer and YBCO film fabrication. The YBCO coated conductors with a current density over 1 mA/cm2 have been demonstrated [2,4,5]. However, it has been reported that the Jc decreases rapidly as the thickness of an epitaxial YBCO layer increases either on a single-crystal substrate or a metallic substrate [6–9]. The reason for the Jc drop, which seems to be universal regardless of the deposition technique, is not clear. There are still numerous scientific
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and technological difficulties to be overcome before the thick-coated conductors of YBCO with high Jc are commercially fabricated. For the successful commercial development of coated conductor YBCO, it may be important to make thick YBCO film as a homo-substrate to completely eliminate the buffer layer effects and the lattice mismatch between the substrate, buffer layer and coated conductor layer for growth of YBCO epitaxial films. In order to fabricate biaxially textured YBCO homo-substrates, we have developed a combined magnetic–mechanical technique for simultaneously aligning superconductor grains along two independent crystalline axes [10–17], and have applied this technique for the first time to EuBa2Cu3O7d superconductor. This technique is based on the use of two perpendicular alignment forces: one of these is magnetic and applied horizontally, orienting a particular direction in the a–b plane; the other is mechanical and acts through a shape and deformation anisotropy of superconductor grains, orienting the c-axis along the direction perpendicular to the a–b plane. To align the a or b axes, it is necessary to substitute certain rare earth (RE) so-called normal aligners [18] into the superconductors, e.g., Eu, Yb, Er or Gd for Y in 123 [14,15]. When a cuprate superconductor is doped with a normal aligner RE element, the resulting magnetic anisotropy of the cuprate crystal is the result of the competition between the principal magnetic moment of the RE ion and the magnetic moment contributed by the Cu–O conducting planes. In this case, the principal magnetic moment of the RE ion lies normal to c-axis and is higher than that of the Cu–O plane which is parallel to c-axis, i.e. DwRE(?)4DwCuO (||). Therefore the resulting susceptibility will tend to align the crystal with its c-axis perpendicular (normal) to the magnetic field direction during magnetic alignment. We chose Eu as the RE component for biaxial alignment. This is critical, because among the four normal-aligning RE elements that can be incorporated into YBCO, Eu is the only one that produces a sufficiently large differentiation between the potential alignment direction in the a–b plane (/1 1 0S vs. [0 1 0] and [1 0 0]) [19,20]; therefore, it is useful for in-plane alignment. 2. Experimental procedure For biaxial alignment, Eu-123 particles with somewhat plate-like shape and a size range of 38–45 mm were dispersed in 2-pentanol alignment medium. A non-magnetic copper die with some pure alignment medium was frozen first in liquid nitrogen. The Eu-123-alcohol dispersion was then added and frozen layer by layer in the die. In a 1.5 T horizontal magnetic field, the die was placed on a support that was kept at about 30 1C. The top of the die was continuously cooled by covering it with a high thermal conduction container filled with liquid nitrogen. According to the temperature gradient that was established
in the die, the dispersion began to melt from bottom to top. The Eu-123 particles suspended in each alignment medium layer settled to the die bottom slowly because of the high viscosity of the 2-pentanol in the temperature range near its melting point [11]. During settling, the particles were aligned simultaneously by gravity and the horizontal field. After the aligned particles settled completely to the bottom of the die, the alignment medium was removed by gentle heating of the copper die at 110 1C. Finally, the aligned Eu123 particles were pressed in the die with a pressure of 9000 psi; this improved further the shape alignment obtained during the particle deposition. With this biaxial alignment technique, a free-standing thick film of 0.5 mm thickness and lateral dimensions of 30 10 mm was obtained. The biaxially aligned samples were pre-sintered at 400 1C for 2 h and then sintered at 970 1C for 12 h in oxygen. The Eu-123 thick films uniaxially aligned by each individual alignment force were also fabricated. A vertical superconducting magnet was used for magnetic field alignment. Eu-123 grains with a size range of 38–45 mm were mixed with a 5 min epoxy matrix in a volume ratio of 1:1 in a rectangular plastic box wrapped with aluminum foil. The mixture was then held in a vertical field of 10 T at room temperature to allow the grains to align until the epoxy resin became hard in the field. Rectangular shaped specimens with their flat bottoms and sides facing in the directions of and normal to the magnetic field B were obtained. For c-axis (shape) alignment, a dilute suspension of 38–45 mm Eu-123 grains in 2-pentanol medium [11] was poured into a 10-mm diameter circular copper die filled with 2-pentanol and placed in an ultrasonic vibration bath. After the Eu-123 grains have settled to the bottom of the copper die with some shape orientation via gravity due to the shape anisotropy of the grain, the top layer of fluid medium in the die was removed by gentle heating of the copper die at 110 1C. After that, the sample in the die was pressed, i.e., additional shape alignment was then provided with a uniaxial pressure of 9000 psi. A thick film of about 1 mm thickness was thus obtained and then sintered at 970 1C for 24 h in O2. The alignment characteristics or structural properties of both uniaxially and biaxially aligned Eu-123 thick films were studied by a four- circle X-ray diffractiometer with Cu Ka radiation. Modern X-ray analysis techniques, including X-ray diffraction (XRD) 2Y–Y scan, w-circle scan, f-circle scan and pole figure measurement, were used in this study.
3. Results and discussion In order to demonstrate clearly how the biaxial alignment technique works, The XRD analyses for Eu123 thick film samples uniaxially aligned by magnetic force and mechanical force, respectively, will be presented first.
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3.1. XRD results of Eu-123 single axis aligned by magnetic force This important alignment is presented in four different ways, partly as a demonstration, and partly to generate familiarity with pole figure interpretation in the reader. The first and third presentations are based on 2Y scans at different tilt angles w; the second and fourth ones use pole figures at constant Y. Fig. 1 shows reflection XRD 2Y scan spectra (taken at the w values indicated) of Eu-123 in epoxy aligned by a 10 T vertical magnetic field. The face of the test sample that upon mounting in the diffractometer becomes the reflecting sample surface was perpendicular to the field direction; this caused the normals to the reflecting planes (for w ¼ 01) to lie in the direction of the vertical magnetic field. When w ¼ 01, essentially only the (0 k 0) peaks show over a uniform background, accompanied with the (h 0 0) peaks due to twinning in Eu-123 grains [except for some (0 1 3)/ (1 0 3)/(1 1 0) reflections, see below]. The intensities of (0 k 0) peaks may involve a small degree of intensity contribution from (0 0 l) parasitical peaks because their Bragg angles differ only by a few hundredths of a degree; however the intensities contributed by such (0 0 l) peaks were low, otherwise the (0 0 5) and (0 0 7) peaks would appear at their respective positions. A relatively low contribution of (1 1 0), (0 1 3) and (1 0 3) (due to possibly parasitic misalignment) is observed. As expected for wellaligned grains, the intensity of the joint peak of these three
Fig. 1. X-ray Y–2Y reflection spectra (Cu-Ka radiation, taken at different angle w) of Eu-123 in epoxy aligned by a vertical magnetic field, showing a b-axis field alignment.
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planes becomes strong when w ¼ 451, since the angles between these planes and (0 k 0) plane equal about 45.51. The [0 1 0] axis (b-axis) is thus seen to be well aligned in the direction parallel to the applied field; this can also be demonstrated by a (0 1 3)/(1 0 3)/(1 1 0) pole figure analysis. For a uniaxially (b-axis) aligned sample, the (0 1 3) and (1 1 0) poles will be randomly distributed on a circle about [0 1 0] axis with angle w 451, the angle between the [0 1 0] and the [0 1 3] directions (or between [0 1 0] and the [1 1 0] direction). As shown in Fig. 2, this is the case. The figure is presented in spatial representation, tilted by 201. Nearly perfect b-axis field alignment is thus presented. A third demonstration of the b-axis aligned Eu-123 thick film uses 2Y scan. The reflection XRD 2Y scan diffractograms for the cross-section of uniaxially vertical-fieldaligned Eu-123 sample are given in Fig. 3. During field alignment, the sample surface was parallel to the field direction. This time, the sample was mounted for reflection such that the normals to the reflecting planes (at w ¼ 01) were perpendicular to the field direction, or expressed alternatively; this arrangement caused the reflecting planes to form a zone with [0 1 0] as a common axis. As result of the formation of this zone, the diffractograms in Fig. 3 show that only (h 0 l), (h 0 0) and (0 0 l) reflections presented at w ¼ 01 [except some parasitic (1 1 0) and twinning (0 1 3)]. It is of interest to note that w ¼ 451 scans for the arrangements in Figs. 1 and 3 become identical, because
Fig. 2. (0 1 3)/(1 0 3)/(1 1 0) pole figure of the field-aligned Eu-123 sample (in spatial representation, tilted by 201 about the projection axis [0 1 0]), indicating a uniaxially b-axis alignment. The contribution from (1 0 3) is thought to be negligible, but cannot be separated out due to the peak overlap.
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Fig. 4. (0 1 3)/(1 0 3)/(1 1 0) pole figure for the cross-section of the uniaxially vertical field aligned Eu-123, taken (0 0 1) as the projection plane (in contour line representation), showing a good b-axis field alignment.
Fig. 3. X-ray 2Y scan (Cu-Ka, taken at different angle w) reflection diffractograms for the cross-section of the uniaxially vertical field aligned Eu-123 sample.
for both cases, at w ¼ 451, the physical configurations of the sample in the pole figure device have become identical. A fourth demonstration of b-axis alignment of Eu-123 is contained in a pole figure of the (0 1 3)/(1 0 3)/(1 1 0) line cluster taken from the cross-sectional sample face used and discussed in the 2Y-scan of Fig. 3 ([0 1 0]||B||sample surface). Here the poles are randomly located on two arcs and one line, as shown in Fig. 4. The poles densities are not uniform, being higher near the center area (the line from j ¼ 01 to 1801) and lower in the edge area of the pole figure, which is caused by defocusing effects in this X-ray reflection technique. These defocusing effects will be discussed in detail in Ref. [21]. In order to understand the pole figure in Fig. 4 better, some background crystallographic relationships for b-axis aligned polycrystalline Eu-123 are provided in Fig. 5 which shows the principal poles for the Eu-123 crystal projected on its (0 0 1) plane or, in other words, shows a partial (0 0 1) projection of b-axis field aligned Eu-123. It is this plane, parallel to the cross-section of the sample, which was looked at by reflection XRD of the b-axis aligned sample, and thus the (0 1 3)/(1 0 3)/(1 1 0) pole figure was obtained (Fig. 4). In a pole figure of a completely random sample, the poles will be distributed uniformly over the whole area of the pole figure. But if uniaxial orientation (or single alignment) is present, for instance, in vertical field aligned polycrystalline Eu-123 where the b-axis of Eu-123 grains were aligned along the field direction, in the projection
Fig. 5. Partial (0 0 1) projection for a perfectly, biaxially aligned polycrystalline Eu-123.
plane, the [since the angle between any two planes (or directions) will remain unchanged] two arcs joining the {0 1 3} and {1 1 0} poles both must lie at E451 from {0 1 0}, predicting the upper and lower arcs in Fig. 4. For the same reason, the poles of the planes sharing the [0 1 0]—zone axis such as 103 and 1¯ 0 3 will lie in the equatorial region of the pole figure, Fig. 5; this explains the linear central ‘‘arc’’ of Fig. 4.
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It is useful to point out here that the dots plotted in Fig. 5 represent the poles for a perfectly, biaxially aligned Eu-123 polycrystalline sample that behaves as ‘‘a granular single crystal’’. The arcs in Fig. 4 were derived from the dots in Fig. 5 by the absence of simultaneous c-axis alignment (assumed to exist in Fig. 5); this missing c-axis alignment allowed the 0 0 1 poles to move along the equatorial plane and broadened the ‘‘dots’’ in Fig. 5 into the arcs in Fig. 4. A pole figure projection can also help to clarify the prior pole figure, Fig. 2, which is derived from an [0 1 0] axis projection (i.e., along the North–South axis of the previous pole figure, Fig. 5), as shown in Fig. 6. Here, a small ¯ and 110. ¯ circle joins the poles 0 1 3, 1 1 0, 0 1 3, In the absence of c-axis alignment (fixing the 0 0 1 pole) these poles will be randomly distributed on the small circle, as seen in Fig. 2.
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and 1, where the highest intensity of 2.5 is associated with the contours outlining the shaded areas. A strong c-axis alignment is seen. However, a low degree of c-axis misalignment also exists, as shown by the misorientation background observed at the (0 0 1) position, the center of the pole figure in Fig. 7. The crystallographic relation of the poles in this figure to each other is analogous to that assumed above and is demonstrated in Fig. 5 as a ring at E451 about (0 0 1). These results allow us to conclude that the sample exhibits a somewhat platy (or marginally tilelike) particle shape, where the short axis of the plates coincides with the direction of the applied force (perpendicular to the surface of the specimen).
3.2. XRD results of Eu-123 single axis aligned by mechanical force For a uniaxially, (c-axis) aligned polycrystalline Eu-123 sample produced by pressing, the reflection XRD (0 1 3)/ (1 0 3) pole figure measurements gave the same results for both top and bottom surfaces of the sample, showing the uniformity of the pressing alignment. These poles are randomly located on a small circle corresponding to an angle w ¼ 451, i.e., the angle between the [0 0 1] and the [0 1 3] or [1 0 3] directions; a corresponding pole figure showing this ring (in contour line representation) is given in Fig. 7. The contours represent the intensities 2.5, 2
Fig. 7. (0 1 3)/(1 0 3) pole figure of the aligned Eu-123 by pressing polycrystalline sample, taken (0 0 1) as the projection plane (in contour line representation), showing uniform ring of poles which indicates c-axis alignment due to the action of a mechanical force.
Fig. 6. Partial (0 1 0) projection for a perfectly, biaxially aligned Eu-123 polycrystalline sample.
Fig. 8. X-ray (0 0 6) w-circle scan diffractogram (Cu-Ka) of the biaxially aligned Eu-123, demonstrating a strong c-axis shape alignment due to the action of a mechanical force.
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3.3. XRD results of biaxially aligned Eu-123 The (0 0 6) pole figure analyses for the top and bottom surfaces of the biaxially aligned samples shows that the [0 0 1] axis, (c-axis), was oriented in the direction normal to the sample surface, documenting a very high degree of c-axis shape alignment (not shown). The localization of these poles remains within a small area; the full-width at half-maximum (FWHM) of (0 0 6) w-circle scan for the biaxially aligned Eu-123 is 4.71, as shown in Fig. 8. This indicates that the crystallites were strongly aligned, with their (0 0 l) planes parallel to the sample surface due to the action of a mechanical force on their anisotropic (somewhat platy) shape. This alignment can also be demonstrated by reflection XRD 2Y scan, which showed only the (0 0 l) peaks over a uniform background, with a small degree of {1 1 0} misalignment observed, as shown in Fig. 9. The fraction, F, of grains aligned [11] along [0 0 1] was found by calculation according to the intensity data in Fig. 9 to be about 97%. As mentioned above, a small degree of c-axis misalignment also remained in our sample. This may be caused by the relatively high uniaxial pressure used during the c-axis alignment (9000 psi), which must have broken some crystals already aligned along the b-axis. It was found that a high degree of c-axis alignment depends on finding a good combination of grain size and alignment force, or pressure. We turn now to the direct evidence for the presence and degree of biaxial alignment, demonstrated by the locations of (0 1 3) and (1 0 3) poles on their joint pole figure. (Because their Bragg angles differ only by D(2Y) ¼ 0.271, the (0 1 3) and (1 0 3) planes cannot be separated without unacceptable loss of intensity; therefore, both contribute to the pole figure.) The (0 1 3)/(1 0 3) pole figure (contour line representation) for the bottom surface of the biaxially aligned Eu-123 sample is shown in Fig. 10; the same result was obtained for the top surface. In this figure, the biaxial
Fig. 9. X-ray 2Y-scan spectrum (Cu-Ka) of the biaxially aligned Eu-123, demonstrating a strong shape-induced c-axis alignment.
Fig. 10. (0 1 3)/(1 0 3) pole figure of the biaxially aligned Eu-123 polycrystalline sample, taken (0 0 1) as the projection plane (in contour line representation), showing partial biaxial alignment due to a magnetic force applied normal to a mechanical force.
alignment is expressed in a clover-leaf arrangement of reflections. The contours represent intensities 4, 3, 2.5 and 1; where intensity 4 is associated with the contour outlining the black areas. When there is b-axis alignment in addition to c-axis alignment, the (0 1 3)/(1 0 3) ring is broken up into four arcs, with the length of the arcs being a measure of the degree of b-axis alignment. A f-circle scan of (0 1 3)/(1 0 3) of the biaxially aligned Eu-123 showed four peaks with FWHM of 471 (not shown here); the positions of these peaks located around f ¼ 01; 901; 1801 and 2701, respectively. For a perfectly, double aligned sample, the arcs would reduce to four points at f ¼ 01, 901, 1801 and 2701, as explained in Section 3.1 (see Fig. 5). The pole figure thus demonstrates the crucial fact that the a, b axes have adopted preferential orientations due to the action of the magnetic field. To have accomplished this degree of biaxial alignment is the central result of this work. The magnetic field direction B is along the North–South axis of Fig. 10. Because the b-axes of the Eu-123 grains line up parallel to B, the a-axes will be oriented at an angle of 901 to B; due to the overlap of (0 1 3) and (1 0 3), the pole figure assumes a pseudotetragonal appearance. A further contribution to the (1 0 3) reflections seen at the positions transverse to B comes from twinning in Eu123 grains. Twin boundaries bisect the angle between [1 0 0] and [0 1 0]; upon alignment, the majority of regions of each biaxially aligned, twinned crystallite will therefore force the twinned minority portion into an orientation such that the [1 0 0] axes of the latter coincide with the [0 1 0] axes of the former, i.e., they lie in the B direction. Therefore, the [0 1 3] directions of the minority portion coincide with the [1 0 3] direction of the majority and contribute to the transverse diffraction spot intensity. This will occur even if the separation of (0 1 3) and (1 0 3) were complete.
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4. Conclusions In order to grow high-quality epitaxial YBCO thick films on superconductor homo-substrates, we are conducting research on a two-step process which includes Eu-123 homo-substrate fabrication and YBCO film epitaxial deposition on the pre-made substrates. We have reported studies on the first step in this paper, the preparation of biaxially aligned Eu-123 substrate thick films using a combined magnetic–mechanical technique. However, the alignment factors of both out-of-plane and in-plane are only 33%, according to quantitative X-ray analysis [21]. It was found that a high degree of biaxial texture depends on a good combination of superconductor grain size, magnetic force and mechanical force. Work on the quantification and optimization of the process continues. Once the Eu123 homo-substrates are optimized with the biaxial texture, the next step will commence, epitaxial growth of YBCO thick films on these unique substrates. References [1] T.J. Doi, T. Yuasa, T. Ozawa, K. Higashiyama, Advance in Super conductivity VII, Springer, Tokyo, 1994. [2] A. Goyal, D.P. Norton, D.K. Christen, E.D. Specht, M. Paranthaman, D.M. Kroeger, J.D. Budai, Q. He, F.A. List, R. Feenstra, H.R. Kerchner, D.F. Lee, E. Hatfield, P.M. Martin, J. Mathis, C. Park, Appl. Supercond. 4 (1996) 403. [3] C.P. Wang, K.B. Do, M.R. Beasley, T.H. Geballe, R.H. Hammond, Appl. Phys. Lett. 71 (1997) 2955. [4] A. Goyal, D.P. Norton, J.D. Budai, M. Paranthaman, E.D. Specht, D.M. Kroeger, D.K. Christen, Q. He, B. Saffina, F.A. List, D.F. Lee, P.M. Martin, C.E. Klabunde, E. Hatfield, V.K. Sikka, Appl. Phys. Lett. 69 (1996) 1795. [5] X.D. Wu, S.R. Foltyn, P.N. Arendt, W.R. Blumenthal, I.H. Campbell, J.D. Cotton, J.Y. Coulter, W.L. Hults, M.P. Maley, H.F. Safar, J.L. Smith, Appl. Phys. Lett. 67 (1995) 2397.
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