INSTITUTE OF PHYSICS PUBLISHING

SUPERCONDUCTOR SCIENCE AND TECHNOLOGY

Supercond. Sci. Technol. 14 (2001) 717–721

PII: S0953-2048(01)26540-3

Section area and self-field dependence of critical current density in homogeneous bulk textured Bi-2223 V Garnier1 , C Goupil and G Desgardin Laboratoire CRISMAT, UMR 6508, ISMRA et Universit´e de Caen, 6 Boulevard du Mar´echal Juin, 14050 Caen Cedex, France E-mail: [email protected]

Received 4 June 2001 Published 22 August 2001 Online at stacks.iop.org/SUST/14/717 Abstract We report bulk ceramic Bi-2223 transport measurements for samples of various cross sections. The sample preparation process allows the synthesis of large and homogeneous samples. The measurements were carried out at 77 K in self-field (with currents up to 70 A) over a wide range of sample thickness and widths. 17 bars of various sizes, ranging from 0.5 to 3.5 mm width and 0.32 to 3.5 mm thickness, were prepared by sinter forging. The transport critical current density (JCT ) clearly decreases as the sample cross −1 section increases, and JCT varies linearly with thickness. This JCT variation cannot be attributed to differences in sample quality (texture, phases, etc) because of the careful synthesis procedure, and implies that the current does not flow uniformly through the samples. The magnetic field dependence of the critical current density (JCM ) was compared with the JCT values, allowing us to calculate the induced field, Bind , produced during the JCT measurement. This induced field contributes to a decrease in the measured JCT values. Therefore, the application of a correction factor obtained from Bind on the critical current density results in a value of JC independent of the cross section of the sample. These results question the significance of homogeneous bulk pinning and reveal the possible existence of surface and interface pinning processes.

1. Introduction Since the work of Michel et al [1] and Maeda et al [2] on superconductivity in the BSCCO system, important efforts have been undertaken to study its phase diagram [3]. In this system, the high-Tc phase (Bi-2223), has been and is still widely studied, and is believed to be the most promising phase for applications. In order to obtain improved superconducting properties, the Bi-2223 grains must be aligned. Different texturation techniques such as powder-in-tube (PIT) [4], hot isostatic pressing [5] and sinter forging [6] are employed to ensure good crystal orientation. The sinter-forging method provides massive and well textured ceramics. Bi-2223 bulk samples are more appropriate than tapes to study the influence of the sample thickness and/or width on the transport critical 1

Author to whom correspondence should be addressed.

current densities (Jc ). In fact, in order to perform such a study the sample must be completely homogeneous throughout the sample volume. In order to compare the Jc of several samples with various cross section areas one must be careful with the superconducting phase homogeneity. The resistivity in the normal conducting state and the critical temperature (Tc ) must be invariant with cross-sectional variation. In the silver sheathed tapes the Bi-2223 phase forms preferentially at the interface between the silver sheath and the oxide core, because the silver reduces the melting temperature of the oxide. Thus the microstructure near the interface is different from that at the centre of the oxide core [7] and the variations of the tapes thickness and/or width result in different superconducting phase homogeneities. It should also be noted that the oxygen stoichiometry must be consistent throughout the whole sample whatever the sample size. Since the oxygen diffusion during annealing from the

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surface into the inner part varies depending on the sample section, the annealing time must be sufficient to reach a total oxygen homogeneity in the sample. The sample texture must also be identical throughout the entire sample. It has been largely shown that the degree of texturing of the Bi-2223 phase in tapes was highest at the Ag interface and lowest at the centre [8, 9], because the Bi-2223 crystal growth aligns near the silver interface [10, 11]. The alignment of the crystals increases with the decreasing thickness of the Ag-sheathed tape. When the tape reduction ratio is increased or the thickness of the tape is decreased, the Jc increases due to improved grain alignment [12]. However, if the tape becomes thinner than a given thickness, the interface becomes non-uniform in shape, this effect is called sausaging, and the Jc abruptly decreases [9, 13]. Samples should also have the same density throughout the whole sample core cross section in order to compare the Jc of equal sample quality with various cross-sectional areas, because higher density results in higher Jc [14–16]. Contradictory results have been published concerning the dependence of Jc on the cross-sectional area in superconductors. Yamada et al [17] have shown that for YBa2 Cu3 O7−x , IC is approximately proportional to the sample cross section square area, while Pachla et al [18] report that the Jc in bulk Bi-2223 is more than three times lower than for a thin layer, and Noudem [19] has determined that Jc decreases with increasing bulk sample thickness. Alternatively, Murayama and Tor¨u [20] have shown that IC is approximately proportional to the cross-sectional area, and they conclude that transport current flows uniformly across the cross section of the sample. In the same way, Rouessac et al [21] found that Jc is nearly independent of the shape of the sample and of its crosssectional area, at least for cross sections lower than 3 mm2 . The aim of this present study is to first produce homogenized and well textured Bi-2223 superconducting bulk samples with various cross-sectional areas, and second, to compare their Jc ’s and discuss the observed differences.

2. Sample preparation Using the nominal stoichiometric composition Bi1.85 Pb0.35 Sr2 Ca2 Cu3.1 O10+y and the corresponding metal acetates, the powder precursor was prepared by a liquid-phase process— the polymer matrix method, detailed elsewhere [22]. After firing on a hot plate, the precursor powder was crushed and hand milled in an agate mortar, calcined at 820 ◦ C for 24 hours 718

Figure 2. (0 0 10) pole figure: (a) cumulative grain disorientation, (b) 3D representation.

Figure 3. SEM photomicrograph along the bulk sample thickness.

in air, milled again and pelletized (3 g, 16 mm diameter, 1.5 T cm−2 ). The pellets were sintered several times with intermediate millings [23]. After the last sintering step, each pellet was in turn directly sinter forged. The sinter-forging conditions are described in figure 1. Hot-forged discs of about 0.5 mm thickness were obtained. Pole figure measurements were performed on these discs with a Philips X’Pert x-ray diffractometer. The (0 0 10) pole figure (figure 2) revealed that 50% of the Bi-2223 grains are aligned within an angle of 5◦ , which correspond to a sharp texture. Eight discs of equal texture quality and phase homogeneity were joined together by an additional sinter-forging step at the same temperature and under the same stress as for each single previously forged disc. A thick Bi-2223 ceramic of about 2.6 mm thickness was obtained without any texture quality degradation [21, 24]. The thick disc was cut into two equal halves and joined together with another sinter-forging step. The resulting Bi-2223 sample had a thickness of 3.5 mm. If we compare this to the starting thickness of the eight discs (8 × 0.5 mm) cut into two parts (×2), the resulting total thickness reduction is more than 50%. This large thickness reduction allows the formation of a dense and homogeneous Bi-2223 bulk sample. Scanning electron microscopy (SEM) observations of this thick ceramic (figure 3) show that no interface between the starting discs could be detected, and the bulk is as homogeneous and textured as the thin starting discs. 17 bars of various sizes ranging from 0.5 to 3.5 mm in width and 0.32 to 3.5 mm in thickness were obtained by cutting the bulk with a wire saw.

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Larger samples were not synthesized due to our inability to measure their transport properties, since the power needed to measure larger samples exceeded the capabilities of our instrument (90 A). Contacts were applied to the samples using Ag paste, and the samples were then annealed to optimize the contacts. To prevent any improvement in the critical current, and to maintain a homogeneous oxygen distribution, the samples were annealed in air for a short period of time.

3. Critical current density Transport critical current density measurements were performed using the four-probe method from the dc current–

voltage curve using a 1 µV cm−1 criterion. The magnetic field dependence of the critical current density was determined using a SQUID magnetometer and Bean model calculation. In figure 4 the transport critical current density (JCT ) is plotted against the sample cross section. JCT clearly decreases as the sample cross section increases. These variations cannot be attributed to differences in sample quality (texture, phase) because of the careful synthesis procedure described previously. Figure 5 shows the JCT dependence against sample thickness at a constant width (3.45–3.53 mm) and against width at a constant thickness (3.48–3.58 mm). In both cases the JCT increases as the sample thickness or width decreases, 719

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of bars on top of one another and by carrying current parallel and antiparallel in the adjacent conductor. This technical configuration, which was difficult to accomplish, shows no difference between the JC obtained with the bar alone and with the antiparallel configuration (figure 8). However, for the parallel configuration, the samples’ JC decreased (>5%) compared to that of the single sample. These results are due to the high self-field generated at the bar edges, which induces vortex nucleation, and then decreases the JC value. The effect of the surface quality was also studied (figure 9). The JCT was measured on (i) an as-prepared sinter-forged sample (figure 10(a)), (ii) with one side of the sample’s surface sanded (figure 10(b)), (iii) with both sides sanded and (iv) with an etched surface. The JCT gradually decreases as the sample’s surface becomes degraded, showing a dependence of the JC with the surface roughness. This surface effect, which should not be confused with surface barrier effect, has been observed in many phases, such as Y-123 [25, 26] and PbIn [27], where it can explain most of the pinning contribution to the critical current. To our knowledge, this is the first observation of such a surface effect in Bi-2223.

4. Conclusion

(b) Figure 10. SEM photomicrographs: (a) a sanded surface and (b) an etched surface.

although, the JCT decreases more rapidly as the thickness increases. The magnetic critical current density (JCM ) against the applied field (H ) is shown in figure 6. This SQUID magnetometer measurement was performed with the sample’s c-axis (thickness) parallel to the applied field H . In this geometry and after zero field cooling, the geometric barrier of the sample gives rise to a vortice penetration with a strong gradient; this is also the case with the transport configuration. This strong gradient overcomes the vortice bending effects so that the component Bx is absent, and the Maxwell equation (µ0 Jy = (∂Bx /∂z)−(∂Bz /∂x)) becomes (µ0 Jy = −∂Bz /∂x) where the x component corresponds to the sample width (w). From this measurement, one can deduce (Bz = µ0 Jy w), which corresponds to the induced field, B, produced by the current during the JCT measurement with JCT = Jy . This induced field contributes to a decrease in the JCT measured value. Thus, the corrected JC (A cm−2 ) can be explained as: JC (0) = JCM (0) − JCM (B), where JCM (0) = 4150 A cm−2 . The corrected JC are plotted in figure 7 against the sample cross section. The Jc is constant against the sample cross section and is thus independent of the samples’ width and thickness. The exception is for the smaller cross section sample, which corresponds to the thinnest sample (0.32 mm), whose JC value is clearly above the JC average. This result may be due to a difference in the texture quality between this sample and the others. Considering that the other discs have a thickness of 0.7–0.8 mm, and the texture quality is better on both disc surfaces than inside, the thinner sample may have a better textured part; it will then have a higher JC value. Complementary results have been obtained by mounting pairs 720

The transport critical current density has been found to decrease as the sample cross section increases. This variation has been found to depend on the self-field, which is generated proportional to the current intensity. This decrease is slightly more important with increasing sample thickness. Magnetic field dependence measurements of the critical current density has allowed us to calculate the induced field, Bind , produced during the transport measurement. Considering this induced field contribution, corrected current density values were obtained and found to be constant with the sample’s cross section.

Acknowledgments The authors want to acknowledge J Lecourt for his help in samples preparation and A Ambrosini for fruitful discussions.

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