INSTITUTE OF PHYSICS PUBLISHING

SUPERCONDUCTOR SCIENCE AND TECHNOLOGY

Supercond. Sci. Technol. 16 (2003) 820–826

PII: S0953-2048(03)61959-7

New understanding of the role of Ag at undeformed Ag/Bi,Pb(2223) interface: texturing and outgrowths ¨ V Garnier, R Passerini, E Giannini and R Flukiger DPMC, University of Geneva, 24, quai Ernest-Ansermet, CH-1211 Gen`eve, Switzerland E-mail: [email protected]

Received 28 March 2003, in final form 16 May 2003 Published 11 June 2003 Online at stacks.iop.org/SUST/16/820 Abstract The role of Ag at the Ag/Bi,Pb(2223) interface on the texture development was revisited using unusual experimental conditions. The Ag/BiSCCO interface of tapes was reproduced without the heavy deformation process commonly employed in the tape fabrication, in order to eliminate the pre-texturing of the Bi,Pb(2212) phase in the precursor powder. The observed Bi,Pb(2223) texture development was found to be exclusively ascribed to the presence of Ag. Even without mechanical pre-texturing of the Bi,Pb(2212) phase precursor, Ag was found to promote the formation of a textured layer of Bi,Pb(2212) at the early stages of sintering. This textured layer gives rise to the formation of aligned Bi,Pb(2223) grains, thus inducing the alignment of subsequent Bi,Pb(2223) platelets. The new finding is that the alignment reaches deep inside the ceramic core up to 10 µm depth from Ag surface. The influence of the individual Ag grain orientations on the Bi,Pb(2223) outgrowth formation was also investigated. The average length and misorientation angle of the outgrowths are not correlated to the Ag grain orientation. Their density and volume are also found to be independent of the orientation of neighbouring Ag grains. The influence of the Ag thickness on the outgrowth formation was studied using a stepped Ag foil with thickness ranging from 25 µm to 500 µm. The average length and angle of the outgrowths remain constant, regardless of the Ag thickness. However, the linear density and ratio of outgrowths located at the grain boundaries were strongly reduced when the Ag thickness was less than 100 µm.

Introduction The development of texture in highly anisotropic HTS materials is one of the key factors in achieving high transport critical currents needed for practical applications. The Bi,Pb(2223) wire and tapes produced with the standard powder in tube (PIT) technique exhibit well aligned platelets, with the c-axis around a direction perpendicular to the rolling direction. The thin layer of well aligned grains which form along the Ag sheath has been observed by Feng et al [1] and studied in detail by Merchant et al [2]. The evolution of the grain orientation from the interface inwards has been described and the texture of the precursor Bi,Pb(2212) and 0953-2048/03/070820+07$30.00

the growing Bi,Pb(2223) has been quantified by means of ex situ x-ray diffraction techniques [2]. The texture factors of Bi,Pb(2212) and Bi,Pb(2223) were found to be similar at the very beginning of the reaction treatment. As shown by Giannini et al [3], using in situ x-ray diffraction techniques, texturing of Bi,Pb(2212) in multifilamentary tapes occurs during heating and is clearly visible for temperatures above 750 ◦ C. Furthermore, several authors [4–6] have studied the development of Bi,Pb(2212) texture prior to transformation into Bi,Pb(2223) and have investigated the correlation to the initial texture of the Bi,Pb(2223) phase. Recently, magnetic investigations [7] have concluded that the texture development takes place in two steps: the first one being induced by the

© 2003 IOP Publishing Ltd Printed in the UK

820

New understanding of the role of Ag at undeformed Ag/Bi,Pb(2223) interface: texturing and outgrowths

mechanical treatment on the Bi,Pb(2212) platelets, and the second one occurring during the heat treatment leading to the Bi,Pb-2223 phase formation. Two processes can be responsible for texturing in Bi,Pb(2223) tapes: a ‘reaction induced texturing’ and a ‘deformation induced texturing’. The former was suggested by Merchant et al [2] and emphasizes the key role played by annealing in the presence of an Ag sheath, which participates in the Bi-2223 phase formation and favours the growth of aligned platelets. This texturing mechanism is supported by the results of Mao et al [8], which suggest a mechanism which implies a combined effect of preventing Pb loss by the Ag sheath and supplying a fast path for oxygen diffusion during 2212 to 2223 conversion, leading to plate-like grain growth. The second mechanism is supported by the observations of Grasso et al [9] which suggest a correlation between the pre-texture of Bi,Pb(2212), appearing during the different deformation steps, and the final texture of the Bi,Pb(2223) grains. The effect of intermediate mechanical deformation on the degree of c-axes texture has been studied by Li et al [10] by using x-ray diffraction performed at different depths in the filaments. Whereas they demonstrated some enhancement of texturing in the centre of the Bi,Pb(2223) core for high deformation ratio, they did not observe any variation of grain alignment at the interface with silver, which always exhibits a strong c-axis texture. Since the fabrication of dense and homogeneous tapes requires heavy deformation processes, an intrinsic deformation induced texture (pre-texture) of Bi,Pb(2212) is inevitably present in any green tape. Therefore, the real effect of annealing on the Bi,Pb(2212) cannot be deduced straightforwardly from experiments made with tapes. The use of bulk pellets in contact with Ag foils is therefore of great interest for studying the texturing mechanism at the interface with Ag. As the pre-texture of Bi,Pb(2212) is absent, the only factor that could influence the appearance of texture caused by the reaction treatment would be the Ag interface. In this paper we investigate the interface between BSCCO and Ag at different stages of the reaction treatment, performed with BSCCO pellets on silver foils of various thicknesses. The texturing mechanism as well as the formation of the outgrowths (misaligned Bi,Pb(2223) grains growing in the Ag matrix [11]) as a function of Ag thickness (ranging from 25 to 500 µm) is discussed. Furthermore, the relationship between Ag grain micro-texture and outgrowth characteristics (density, length, misorientation angle) has been extensively studied, and statistical results are presented.

1. Experimental details 1.1. Preparation of the precursor powder The precursor powder was prepared by a liquid-phase process, the sol-gel method [12], starting from oxides and carbonates with an overall nominal composition Bi1.85Pb0.34Sr2Ca2Cu3. Stoichiometric amounts of Bi2O3, PbO, SrCO3, CaCO3 and CuO were weighed and dissolved together in 65% nitric acid

Figure 1. SEM micrograph of precursor powder before the calcination process.

and distilled water. During dissolution, the evaporation of CO2 from carbonates occurs. In order to dissolve Bi2O3, excess acid is required. The final solution has a limpid pale blue colour, due to copper ions. A second solution was prepared by dissolving solid ethylenediaminetetracetic (EDTA) acid in ammonium hydroxide in a molar ratio 1:4. EDTA processing may be an effective method for synthesizing Bi-based high Tc superconducting ceramics because of the greater ability of the EDTA anions to chelate metal cations, forming very stable and soluble complexes [13]. The as-prepared EDTA basic solution was then poured into the dissolved precursor solution in the ratio of one EDTA formula for each metallic cation present. The solution became dark blue, the colour of chelated cupric ions. If the initial excess of nitric acid (added to dissolve Bi2O3) is too large, the mixed solution can precipitate, because a low pH value leads to EDTA dissociation; this white EDTA precipitate can be redissolved by adding ammonium hydroxide to reach a pH value between 5 and 10. The solution returns to dark blue and becomes limpid again. The pH can be set around a neutral value and the stability of metallic cation-EDTA complexes is high. Water is then evaporated from this limpid solution by heating. During evaporation, the pH is maintained above 6 by adding NH3 to prevent EDTA from precipitating. When the concentration, i.e. the viscosity, becomes higher (about 100 ml solution left for 50 g of initial precursors), the blue mixture turns to a dark green gel. Care should be taken at this step to avoid EDTA precipitation. Further heating turns the gel into a darkbrown foam and finally, after self-combustion with exhausting vapours (water, CO2, NOx), a dark-brown powder precursor is obtained. Figure 1 shows the SEM micrograph of this precursor; the resulting powder is made of small and porous agglomerates. The fired powder is then crushed by hand in an agate mortar. XRD measurements show (see figure 2(a)) a few crystalline phases but mainly Bi2CuO4 and CuO phases, as well as a small quantity of SrCO3 in an amorphous matrix. 1.2. Sample preparation The precursor powder was calcined in air using an optimized thermal profile, 820 ◦ C for 24 h [14]. Figure 2(b) shows the 821

Intensity (a.u.)

V Garnier et al

B

A

0

4

8

12

16

20

24

28

32

36

40

Angle 2θ (Cu Kα)

Figure 2. XRD pattern of (a) precursor powder and (b) calcined powder (820 ◦ C/24 h) with : Bi-2212, N: Ca2PbO4, : Ca2CuO3, ¨: CuO, ♦: Bi2Sr3O6, ⊔ ⊓: Bi2CuO4 and : SrCO3.

◦

•

Ca2PbO4 and CuO are also present. After calcination, the powder was milled by hand in an agate mortar and pelletized (200 MPa, 2 × 5 × 30 mm). Figure 3 shows the cross section of such a pressed pellet, in which small and randomly oriented grains are visible in the pellet core as well as at the pellet surface. The calcined pellet was then sandwiched between Ag foils and subjected to the reaction thermal treatment performed in air at 837.5 ◦ C under a very small uniaxial pressure (100 µm) whereas outgrowths become very scarce for thin Ag foils. The mean linear density of outgrowths for thick Ag is 30 mm−1 which is of the same order of magnitude as the values obtained in tapes (even if the tape sheath thickness is below 50 µm). Comparison between the outgrowth densities obtained in this experiment and the results from Ag-sheathed tapes previously reported [16] should be treated with caution because of the different powder precursors used. Nevertheless, this result emphasizes the role of reaction conditions in the development of outgrowths since even if the nuclei are much more numerous in this study, they do not result in a significant increase in outgrowth number. We also calculated the mean length and mean orientation angle of the outgrowths as a function of Ag thickness (figure 9). Clearly, length and orientation are not related to Ag thickness. The mean length, 7.5 µm, fits the values obtained in tapes after the same heat treatment. The mean angle also corresponds to values measured in tapes; it is very close to 45◦ which would correspond to a perfectly random distribution.

New understanding of the role of Ag at undeformed Ag/Bi,Pb(2223) interface: texturing and outgrowths

2

(a)

0.04

0.03

0.01

0.00

no data

0.02

100 310 210 320 110 331 221 332 111 322 321 211 311

4

no data

6

100 310 210 320 110 331 221 332 111 322 321 211 311

Mean length of outgrowths [µm]

8

Mean density of outgrowths [µm -1]

(a)

10

0

Ag grains orientation

Ag grains orientation

(b)

(b) 0.4

20

no data

30

10

0.3

0.2

0.1

0

Ag grains orientation Figure 10. (a) Mean length and (b) mean misorientation angle of outgrowths in each family.

100 310 210 320 110 331 221 332 111 322 321 211 311

no data

40

Mean length * mean density

50

100 310 210 320 110 331 221 332 111 322 321 211 311

Mean angle between outgrowths and the interface with Ag [°]

60

0.0

Ag grains orientation Figure 11. (a) Mean density of outgrowths and (b) product of the mean length and the mean density. This value is proportional to the volume of Bi,Pb(2223) outgrowths.

2.3. Relationship between the orientation of Ag grains and outgrowths With a view to a better understanding of the texturing and the outgrowth formation mechanism, we also analysed the effect of Ag grain orientation on the outgrowth formation. Different Ag grain orientations should result in differences in Bi,Pb(2223)/Ag interface energy. According to the outgrowth formation mechanism previously presented, variations in interface energy should not modify the mean length and mean orientation of outgrowths but only their density since the surface energy is linked to the reaction kinetics at the interface with Ag (formation of Bi,Pb(2223) barrier). Figure 10 shows the mean length and the mean misorientation angle for each {hkl} family defined above. Taking into account the statistical uncertainty, we deduce that these values do not exhibit significant variations. The mean density of outgrowths (figure 11(a)) shows more pronounced differences between the different Ag orientation families.

Finally, we calculated a better description of the ‘quantity’ of outgrowths present in each family: the product of the mean length and the mean density. This value is actually proportional to the mean volume of outgrowths (figure 11(b)). In any case Ag orientation does not significantly influence the formation of the high quality Bi,Pb(2223) layer at the interface and does not generate remarkable variations in the formation of outgrowths. According to this study, the only parameter that affects the outgrowth formation is the Ag thickness. We clearly observed a strong reduction of outgrowth density when using thin Ag foils. Within our experimental conditions (samples prepared without a mechanical deformation process), we know that differences in Bi,Pb(2223) microstructure come from the anneal. Outgrowth nuclei, assumed to be misoriented Bi,Pb(2212) grains in the raw pellet, 825

V Garnier et al

are homogeneously distributed along the sample interface. Therefore, differences in outgrowth density after heat treatment result from slightly different reaction conditions. In addition to Ag/BSCCO interface energy variations due to local Ag orientation, we can mention differences in oxygen diffusion at the very beginning of the anneal (when the textured Bi,Pb(2212) layer forms at the interface). Both parameters could explain the observed variations in outgrowth density.

Acknowledgments

3. Conclusions

[1] Feng Y, High Y E, Larbalestier D C, Sung Y S and Hellstrom E E 1993 Appl. Phys. Lett. 62 1553 [2] Merchant N, Luo J S, Maroni V A, Riley G N and Cater W L 1994 Appl. Phys. Lett. 65 1039 [3] Giannini E, Bellingeri E, Marti F, Dhall´e M, Honkim¨aki V and Fl¨ukiger R 2000 Int. J. Mod. Phys. B 14 2688 [4] Thurston T R, Haldar P, Wang Y L, Suenaga M, Jiraswi N and Wildgruber U 1997 J. Mater. Res. 12 891 [5] Poulsen H F, Frello T, Andersen N H, Bentzon M D and Von Zimmermann M 1998 Physica C 298 265 [6] Fahr T, Trinks H-P, Schneider R and Fischer C 2001 IEEE Trans. Appl. Supercond. 11 3399 [7] Cimberle M R, Ferdeghini C, Grasso G, Guasconi P and Malagoli A 2001 IEEE Trans. Appl. Supercond. 11 2987 [8] Mao C, Zhou L, Wu X and Sun X 1997 Physica C 281 159–75 [9] Grasso G, Perin A and Fl¨ukiger R 1995 Physica C 250 43 [10] Li S, Bredeh¨oft M, Gao W, Liu H K, Chandra T and Dou S X 1998 Supercond. Sci. Technol. 11 1011 [11] Passerini R, Dhall´e M, Giannini E and Fl¨ukiger R 2002 Supercond. Sci. Technol. 15 1203 [12] Rouessac V, Wang J, Provost J and Desgardin G 1996 J. Mater. Sci. 31 3387 [13] Kotrly S and Sucha L 1985 Handbook of Chemical Equilibria in Analytical Chemistry (New York: Wiley) [14] Garnier V, Monot I and Desgardin G 2000 Supercond. Sci. Technol. 13 602–11 [15] Grivel J-C and Fl¨ukiger R 1994 Physica C 229 177–82 [16] Passerini R, Dhalle M, Seeber B and Flukiger R 2002 Supercond. Sci. Technol. 15 1507–11

The BSCCO/Ag interface has been investigated during the heating ramp and the first 35 h of heat treatment. Samples were fabricated without any deformation process, so pre-texturing of precursor Bi,Pb(2212) was avoided. Formation of a ‘one grain’ layer made of Bi,Pb(2212) was observed at the early stages of the reaction process. This layer further transforms into Bi,Pb(2223) which then develops down to a depth of 10 µm with a high quality uniaxial texture indicating a reaction induced texturing. The influence of the silver grain micro-texture on the outgrowth formation was investigated. The mean length and the mean misorientation angle were not found to be linked to the Ag grain orientation. The mean density of outgrowths shows more pronounced variations that could be explained by differences in Ag/BSCCO interface energy or oxygen diffusion. The influence of the Ag thickness on the outgrowth formation was also studied. The mean length and mean angle of the outgrowths do not depend on the thickness of the Ag foils. However, the linear density and ratio of outgrowths located at grain boundaries revealed a strong reduction of outgrowth formation for Ag thickness less than 100 µm.

826

This work was supported by the Swiss National Foundation and the National Centre of Competence in Research (NCCR) on materials with novel electronic properties (MaNEP). Dr N Clayton is gratefully acknowledged for carefully reading the manuscript.

References