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(Bi,Pb)-2223 phase formation in Ag clad tapes
Materials Science and Engineering B58 (1999) 206 – 214
(Bi,Pb)-2223 phase formation in Ag clad tapes A. Trautner, D. Go¨hring, P. Haug, B. Sailer, A. Ehmann, W. Wischert, S. Kemmler-Sack * Institut fu¨r Anorganische Chemie der Uni6ersita¨t Tu¨bingen, Auf der Morgenstelle 18, D-72076 Tu¨bingen, Germany Received 6 June 1998; accepted 23 November 1998
Abstract The most promising material for fabricating long-length conductors is (Bi,Pb)-2223 produced by the powder-in-tube process in Ag clad tapes. Intensive studies have shown that the critical current density of fully processed tapes is affected by a large variety of processing parameters, but the nature of the (Bi,Pb)-2223 phase formation is still controversial. In this study certain steps of the 2223 phase formation were investigated at different stages of sintering under the influence of various temperature/time schedules of thermomechanical processing and the important role of Pb is shown. The main effects of Pb in the phase forming process are: (i) Acceleration of the 2223 formation via a rapid generation of (Bi,Pb)-2212; (ii) introduction of grain growth and texture; (iii) increase of the temperature window for the 2223 formation; (iv) decrease of reaction temperature; (v) adjustment of the hole concentration of the 2223 phase; (vi) operation as buffer for charge carriers via the temperature dependent equilibrium between Pb4 + and Pb2 + ; and (vii) creation of Pb containing pinning centres. © 1999 Elsevier Science S.A. All rights reserved. Keywords: High temperature superconductor; (Bi,Pb)-2223 Phase formation; Powder-in-tube-process; Critical current density
1. Introduction One of the most interesting industrial application of high-Tc superconductors at 77 K is in form of longlength conductors for transmission cables, high field magnets, motors and generators. The most promising material is (Bi,Pb)-2223 produced in form of Ag clad tapes by using the powder-in-tube (PIT) method [1,2]. Despite the complexity of the multinary system Ag– Bi –Pb–Sr–Ca–Cu – O [3] substantial progress was achieved in increasing the critical current density of Ag-sheathed (Bi,Pb)-2223 tapes. The highest transport critical current densities ( jc) of about 105 A cm − 2 at 77 K/0 T were reported so far for short pressed tapes made by the Ag-wire-in-tube method [4] demonstrating the capacity of the (Bi,Pb)-2223 material. It is desired to reach similar values for long industrial tapes. Actually, the obtained jc values are a limiting factor in the practical application. The best values for rolled short lengths are situated in the range of 5.8×104 A cm − 2 * Corresponding author. Tel.: +49-7071-2972439; fax: +49-7071296918. E-mail address: [email protected] (S. Kemmler-Sack)
(77 K/0 T [5]). Several important reasons for a limitation of jc in (Bi,Pb)-2223 tapes have been recognized: (i) inhomogeneity of the precursor; (ii) phase formation conditions and phase diagram; (iii) inhomogeneity of the deformation process during tape production; (iv) microstructure and grain boundaries; and (v) anisotropy of the critical current density [2]. One of the keys for an increase of the critical current density will be the understanding of the formation mechanism for (Bi,Pb)-2223 in Ag clad tapes. Although numerous studies have been published, the nature of (Bi,Pb)-2223 formation is still controversial [2,4,6–9]. One of the main difficulties in comparing the results of the different authors consist in large variations of the phase assemblage and chemical composition of the multiphase precursor as well as of the employed gas atmosphere (air or reduced oxygen content). The aim of the present study is to follow certain steps of the (Bi,Pb)-phase formation during the PIT process in Ag clad tapes with the intention to develop an improved understanding of the parameters influencing the current carrying capacity of (Bi,Pb)-2223 conductors.
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2. Experimental The employed precursor powder has the nominal cation ratio Bi:Pb:Sr:Ca:Cu=1.8:0.4:2:2.1:3. It was prepared via a solution of nitrates in a spray dry machine and fired at about 800°C for several hours to decompose the nitrates. Ag sheathed monocore and multifilamentary (Bi,Pb)-2223 tapes having 55 filaments were prepared by the standard PIT-technique via bundling, drawing and rolling [10]. The monocore tape was rolled to a cross section : 2.6 × 0.17 mm, with a typical core diameter approximately 99 mm. The cross section for the 55 filamentary tape is : 5 × 0.2 mm. The green tape was then cut into 3 cm long sections for thermomechanical processing. Between each sintering step, the individual sections were uniaxially pressed at 1.2 GPa. The processed tapes were characterized by XRD (CuKa radiation, Philips powder diffractometer, Au standard) after peeling the Ag clad from the surface of the tape. The 2223 content was evaluated via the peak intensity I according to 2223[%]= I[0010(2223)]/ {I[0010(2223)]+ I[008(2212)] +I[006(2201)]} ×100. The average oxygen content of the ceramic was determined after separation from the Ag clad via redoxtitration by using solutions of (NH4)2Fe(SO4)2 and Ce(SO4)2. Critical current density measurements were performed at each stage of the PIT process at 77 K in self field using the 1 mV cm − 1 criterion. The dc magnetic susceptibility was measured with a SQUID magnetometer (Quantum Design). Microstructure examinations of PIT conductors were performed on selected samples using a scanning electron microscope.
3. Results and discussion
3.1. Processes during the first sintering step During the process of drawing, bundling and rolling the precursor inside the green tape is transformed into some ill crystallized materials, consisting mainly of the 2212 phase and a mixture of plumbates of type Ca2PbO4/3321 (Fig. 1a). However, an important part of the oxidic precursor remains amorphous and cannot be detected by XRD. So, the overall intensity of the X-ray diffraction pattern is very low. Astonishingly, after a short heating time of only 20 min at 790°C/air certain diffraction peaks start to intensify (Fig. 1b) being identified as mainly (001) reflections of the 2212 phase (see below). From the decrease of the group of plumbate reflections in Fig. 1b we deduce the incorporation of Pb from the crystalline precursor phases into the crystalline 2212 phase as one reason for this fast crystal growth. However, in addition to the crystalline compounds the large amount of amorphous material must be simultaneously considered. As can be seen from the overall
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increase of the intensity in the XRD pattern the amorphous phases have started to transform into crystalline material, thus working as a feed for the growth of the 2212 phase. So, in agreement with the observation of several authors for powder mixtures [2,11,12] as well as for tapes [13] the first stage of (Bi,Pb)-2223 formation consists in the formation of (Bi,Pb)-2212. However, this process cannot only be described by a simple transfer of Pb from the crystalline plumbates of the precursor material into the simultaneously crystalline Bi-2212 phase, because a large supply of the precursor is present in amorphous form. So, as important second process an incorporation of Pb from these amorphous materials into crystalline 2212 must be considered. As third process the feeding of a 2212 grain growth from the amorphous material takes place with crystalline (Bi,Pb)-2212 as seed crystals. The simultaneous presence of these three processes explains the observed rapid grain growth of the 2212 phase (see below). Note that formation of (Bi,Pb)-2212 starts at a relatively low temperature during the first sintering step. The presently employed reaction temperature of 790°C is much lower than 839°C as reported for this process in monofilamentary Ag sheathed tapes in the literature [2]. Furthermore, it is very likely that this process will start even earlier, but presumably with reduced velocity. So, the employed heating ramp within the first step of thermomechanical processing will be of enormous importance for the entire phase forming process and not only the final manner of cooling down after the last furnace period (cf. Section 3.2). From the stronger increase of the (001) reflections of the 2212 phase relative to the (h0l) peaks in Fig. 1b it follows additionally, the development of a texture with the c-axis lying normal to the Ag interface. This effect is more pronounced for sintering of the green tape at higher temperatures. Fig. 1c gives the XRD pattern for
Fig. 1. Section of the XRD pattern from the surface of the core of monocore tapes (a) green tape with twofold intensity, (b) after heating at 790°C/20 min in air and (c) 850°C/20 min in air. The main reflections of the 2212 phase (subcell) are indicated together with that of the plumbates Ca2PbO4 and 3321.
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an example at 850°C/air/20 min. In this case the (001) peaks of the 2212 phase are dominant, indicating a c-axis alignment of the 2212 grains and the development of a texture. This observation is easily explained by the assumption that the remaining 2212 crystals in the green tape have adopted a preferential orientation during the mechanical processing and operate as seed crystals for the growth of textured (Bi,Pb)-2212. Note that at the 850°C stage the plumbate peaks have practically vanished in the XRD pattern, thus indicating a high growth rate of (Bi,Pb)-2212. Very instructive is the investigation of the overall oxygen content of the ceramic core of the tape, scratched out from the Ag clad after the first sintering step at 790 or 850°C, respectively. Keeping in mind that the oxidation state of Pb in the plumbate phases is + 4, whereas in the 2212 phase it is considered as + 2, one expects a decrease of the total oxygen content via the incorporation of Pb from the plumbate phases into 2212. The experimental values of the oxygen content of 10.85 for the green tape, 10.85 after 790°C/air/20 min and 10.48 after 850°C/air/20 min indicate that the incorporation of Pb is no simple reaction. At inferior temperatures the total oxygen content is conserved, indicating that Pb enters the 2212 lattice in the + 4 state. Since Pb4 + (r = 0.775 × 10 − 1 nm; CN 6 [14]) is much smaller than Pb2 + (r = 1.19 × 10 − 1 nm) a substitution into smaller lattice sites is more likely than in the extended rocksalt-like layers. Indeed, in case of (Bi,Pb)2223 a substitution of Pb for Ca has been found by neutron powder diffraction [15]. However, at a reaction temperature of 850°C a significant loss of oxygen is observed, which can be interpreted via an incorporation of Pb2 + into the 2212 lattice. So, the formation of (Bi,Pb)-2212 is a complex temperature dependent process, starting with the incorporation of Pb4 + at inferior temperatures ( : 790°C/air), where Pb4 + is still stable. Finally, at higher temperatures (:850°C/air) Pb2 + is superior in its stability to Pb4 + and will substitute in the rocksalt-like layers. Note that at this stage of the reaction the average hole concentration per Cu ion of 0.2 has nearly obtained the ideal value around 0.25 holes per Cu ion for the development of superconductivity. From earlier investigations it is known that in air the substitution of Pb for Bi is accompanied with a change of the lattice parameters and the development of an orthorhombic distortion, whereas in Ar atmosphere materials with higher Pb content and a monoclinic distorted lattice are formed [16]. These results were later on confirmed by other authors [12,17,18]. According to TEM investigations in combination with EDX analysis [19] the process of transformation of Bi-2212 into (Bi,Pb)-2212 is not the result of a simple atomic substitution since the wavelength of the modulation depends on the Bi/Pb ratio. So, the true crystal structure is
unique for distinct levels of substitution, but the corresponding individual Bi/Pb ratios are yet unknown. For a relatively high Pb content of x around 0.6 (of the system Bi2-xPbxSr2CaCu2Oz) the structure becomes finally unmodulated. Note that only in the unmodulated region the formation of a real solid solution is expected. However, this case is unimportant for a practical application in Ag clad tapes since the Pb content of the precursor is situated at values not higher than x: 0.4. Moreover, the Bi/Pb ratio does not only influence the ideal crystal structure of the 2212 phase but also its real structure. The typical effect of a high Pb content is a drastical reduction of the number of stacking faults and defects [19]. This finding may be of importance for the generation of the pinning centres. Within the first sintering step the generation of (Bi,Pb)-2223 starts soon after the formation of (Bi,Pb)2212. Since the determination of the 2223 content by XRD via peak intensity gives only a value near the Ag interface without lateral resolution a more complete information is obtainable for monocore tapes by analyzing via XRD the 2223 content at the surface in comparison to the corresponding value of the entire ceramic, scratched out from the Ag clad. Our experimental data for several monocore tapes indicate uniformely the preferred formation of 2223 at the Ag interface. This observed lateral 2223 distribution is in agreement with the results of Hellstrom et al. [20] as well as with the final lateral jc distribution in monofilament tapes [2]. In further experiments we studied the correlation between the temperature of the first sintering step, the amount of 2223 and the critical current, Ic, for several monocore and 55-filamentary tapes with very similar results (Fig. 2). All reactions were performed in reduced oxygen partial pressure of 0.08 bar (N2/O2(8%)). In comparison with air reactions this reduction leads to slightly inferior reaction parameters due to a lowering of all melting temperatures. Fig. 2a gives as example the data for a monocore and Fig. 2b for a 55 filamentary tape after a heat treatment of 10 h at various temperatures. In both cases the 2223 content starts to increase up to about 822/823°C, reaching maximum values between 70 and 80%. In case of Ic a maximum was found in a similar region. A further increase in temperature depresses the 2223 content as well as Ic because the 2223 phase decomposes at higher temperatures. The complete data set of Fig. 2 suggest that an optimal temperature for the 2223 phase forming process exists.
3.2. Processes during the second and third sintering step Several second and third heat treatments were started with monocore and 55 filamentary tapes resulting from
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Fig. 2. Influence of heating temperature on 2223 content and critical current density after 10 h in N2/O2(8%) for (a) a monocore and (b) a 55 filamentary tape.
various temperatures applied during the first sintering step (cf. Section 3.1). All tapes were sequentially pressed and thermally processed. Some results were summarized in Fig. 3 for a three step sintering in N2/O2(8%) at 808, 814 or 820°C of some monocore tapes from Fig. 2a. In contrast with the results of the first sintering step (cf Section 3.1) the data indicate that the correlation between 2223 content and Ic is no longer valid for all employed temperatures. In case of a three step firing at the relatively low temperature of 808°C both values still increases simultaneously, but remain comparably low. For the sequence of 814°C both parameters again correlate and a steep increase of both values is observed. However, by choosing the ‘best’ temperature range from the first sintering step around 820°C a still higher growth rate of 2223 is observed during the second sintering step, but Ic does not follow this development and remains comparably low even after the third sintering step. Most probably, the liquid phase was already nearly consumed after the second sintering step and cracks, introduced by pressing before
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the final sintering step could no longer heal. From a comparison of the entire 814 and 820°C series it follows that the 2223 content increases in both cases by about 20% between the first and second step. This development continues in the 814°C case, obtaining after the third step a similar 2223 amount as in the 820°C case after the second step. However, in the former case Ic is nearly doubled. Two results can be deduced from this series of experiments: (i) the application of firing conditions which develop an optimal 2223 content after the first sintering step is unsuitable for obtaining finally high critical currents, (the validity of this correlation was observed in all our experiments with monocore and 55 filamentary tapes in reduced oxygen atmosphere as well as in air); and (ii) in case of a three step treatment the 2223 growth rate between the first and second step should be moderate as well. So, the 2223 content and the Ic values do not correlate on either step of the tape sintering process, because for large growth rates the intergrain properties as well as the morphology of the crystals are incompletely developed. However, not only the 2223 distribution and the intergrain properties are affected by the employed temperature program but also the intragrain properties. As example Fig. 4 indicates the x versus T (4a) and M versus H (4b) data for monocore tapes after the third sintering step at 808 or 814°C, respectively. From a comparison of the x versus T traces of the 808 and 814°C material it follows that the ZFC trace proceeds more sluggish in the 808°C case. This finding indicates fluctuations in the composition of the superconducting material. So, the knowledge of the real chemical com-
Fig. 3. Influence of the heating temperature for three heating steps in N2/O2(8%) on the 2223 content (%; indicated in numbers) and critical current for monocore tapes (first sintering step according to Fig. 2a).
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Fig. 4. (a) x vs. T and (b) M vs. H for Ag clad monocore tapes after the third firing step at ( ) 808 and () 814°C.
position of the 2223 grains together with the distribution of the elements over the different lattice sites should be desired for the understanding of the phase forming process, whereas a simple correlation between 2223 amount and superconducting properties is not conclusive. However, informations about the chemical composition and the atomic distribution within the 2223 structure are yet incomplete due to the complex crystal structure together with the large number of the five different cations. Moreover, the presence of vacancies as well as the substitution of one cation on different lattice sites must be considered. The latter case has been described above for the couple Pb4 + /Pb2 + (cf. Section 3.1). It is generally accepted that Pb stabilizes the 2223 phase without specifying its location and average oxidation state. In contrast, the preparation of Pb free 2223 is difficult due to a very narrow temperature window [2,21,22], in spite of the fact that the first trace of 110 K superconductivity was observed in a Bi-cuprate material without Pb [23]. Similarly, to the (Bi,Pb)-2212 system [16,24] the crystal structure of (Bi,Pb)-2223 depends on the Bi/Pb-ratio. This is valid for the substruc-
ture as well as for the true, modulated structure: With increasing Pb content an orthorhombic distortion of the subcell develops [16] and the wavelength of the modulation correlates with the Bi/Pb-ratio [19]. From M versus H traces taken between 5 and 77 K for several tapes after different sintering steps at various temperatures it follows that the intergrain properties are strongly affected by the entire set of parameters. Examples for the most interesting 77 K case are given in Fig. 4b. The M(H) traces indicate that the width of the hysteresis loop 2Mr is larger for the 814°C material than for the 808°C sample. According to the Bean model jcm 3Mr/(2rxV) [25] the critical current density jcm has nearly doubled in the 814°C material at 77 K/0 T. Additionally, the position of the positive maximum of the hysteresis loop is shifted in the 814°C case more strongly to positive values of H than for the 808°C sample. According to loc.cit. [26] this shift indicates an improvement of the intergrain connectivity in the material from 814°C. Several SEM investigations of the surface of Ag tapes after various furnace periods indicate uniformely that the grain growth is maximal in the first step of the 2223 formation. This observation is confirmed by in situ measurements of texture and phase development in (Bi,Pb)2223 Ag tapes [8]. The grain growth starts via the (Bi,Pb)-2212 phase (cf Section 3.1). SEM micrographs of the surface of the ceramic after the first sintering step (an example is given in Fig. 5) indicate the formation of large extended plates. In this case the 2223/2212 ratio still amounts to about 50:50 and jc as low as : 1.5 kA cm − 2 (77 K, self field). Note that a texture begins to develop, but remains still incomplete. After two compression steps and the third sintering period the 2223 amount increases to above 80% with jc : 20 kA cm − 2 (77 K, self field). Moreover, the texture has improved by the intermediate compression steps. However, the original large crystal plates were destroyed during compression and do not recover within the process of 2223 formation in the subsequent furnace periods, because the 2223 formation is not accompanied by a pronounced grain growth [27]. So, the grain growth and the 2223 phase formation are two independent processes. In situ measurements with hard X-rays on completely encased Ag tapes give identical results [8]. Consequently, the grain morphology is determined within the first furnace period and this step will be decisive for the following 2223 forming processes. As consequence of the multiphase nature of the precursor local fluctuations in the chemical composition exists and the transformation of 2212 into 2223 in Ag tapes needs very long sintering times to reach equilibrium conditions. So, in reality the transformation remains always incomplete. The process of 2223 formation stops at a level of around 90% 2223, though
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unreacted secondary phases are still present, but unsuitable for a progressive 2223 growth due to long diffusion paths. A more complete 2223 formation is expected from highly homogeneous precursor material. It is well known that excessive amounts of mechanical work can be detrimental after a substantial amount of the precursor powder has been converted to (Bi,Pb)-2223. However, there is although another mechanism, which is detrimental to a tape with large amounts of 2223. This process starts, when the employed, originally optimized temperature/time schedule for the third sintering step exceeds a certain reaction time. At this stage the 2223 phase starts to decompose and Ic begins simultaneously to decline, as
Fig. 6. Influence of the sintering time of a Ag clad tape on Ic (77 K/0 T). The 2223 content (%) is indicated in numbers.
Fig. 5. SEM images of the surface of the ceramic core from an Ag tape after the (a) first and (b) third furnace period.
was observed for various of our monocore and 55 filamentary tapes. Fig. 6 shows as example the results for several samples of the same 55 filamentary tape after different times for the third sintering step at 823°C in N2/O2(8%). It is obvious that the decrease of Ic correlates with the decline of the 2223 content. As main decomposition product a well textured 2212 phase was identified by XRD, but the remainder of the 2223 2212+ 0011 decomposition reaction in form of one or more Ca/Cu/O phase(s) was only detected via XRD after the longest sintering step of twelve days in form of one small peak from Ca2CuO3. Thus, at a first stage of the decomposition the Ca/Cu/O phase(s) must segregate in practically amorphous form between the textured 2212 plates, adopting finally a crystalline form after long sintering times. It should be mentioned that the once decomposed 2223 phase recovers only very sluggishly during additional sintering steps. We performed several corresponding experiments for tapes with a partially decomposed 2223 phase as consequence of a short melting process via a temperature pulse around 840– 860°C. To avoid any decomposition of the 2223 phase during the final sintering step a depression of the firing temperature is suitable as has been observed for 2223 pellets some years ago [28]. In case of Ag tapes slow cooling [29,30] as well as heating at inferior temperatures [31] was recommended. However, it must be simultaneously taken in mind, that precipitation of 2212 through a 2212 2223 2212 conversion may be advantageous via the generation of pinning centres [32]. Actually, it is not clear yet whether 2212 or the associated secondary phases such as 3321 [33] contribute to the pinning. It cannot be excluded that amorphous phases may represent pinning centres as well. Such a case was observed recently for (Bi,Pb)2223 tapes with Ag/Cu alloy sheaths doped with Ti [34].
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3.3. The (Bi,Pb) -2223 phase formation inside Ag-clad tapes The formation of the (Bi,Pb)-2223 phase and its decomposition inside Ag-clad tapes is influenced by a variety of processing parameters and a definitive answer about all processes inside Ag tapes cannot yet be given. In the following we will discuss some findings of this study with respect to the 2223 phase formation in Ag tapes.
3.3.1. Adjustment of the oxygen content It is well known that high-Tc superconductors need a certain hole concentration, obtained via doping of carriers from the charge reservoir in the rocksalt-like layers into the CuO2 sheets. To adjust the hole concentration within the ceramic core of the Ag clad tape every portion of the filament should adopt the correct carrier concentration. This process is realized via the intercalation or delivery of oxygen. As has been demonstrated in Section 3.1 the oxygen content of the ceramic is influenced by the heat treatment and diminishes with increasing temperature. In the 850°C case an oxygen diffusion from the oxide core to the Ag sheath takes place. The oxygen diffusion through Ag is complex. The diffusion coefficient for 830°C is about 4 ×10 − 6 cm2 s − 1. However, only in a very early stage of the first heat treatment of some minutes the interface Ag/ceramic is open for the oxygen diffusion. The diffusion will be rapidly hampered by the growth of extended textured (Bi,Pb)-cuprate plates close to the Ag sheath (cf. Section 3.1 and Section 3.3.2). The crystals consist at the beginning of (Bi,Pb)-2212, later on a transformation to 2223 takes place. The growth of (Bi,Pb)-2223 close to the Ag was observed by several authors (cf. Section 3.1). Due to the intimate contact between the Ag sheath and the preferentially in the (a,b) direction growing plates of (Bi,Pb)-2212 and/or of (Bi,Pb)-2223 the interface will be covered by textured plates. This material will now determine the oxygen exchange with the exterior. The oxygen diffusion in (Bi,Pb)-cuprates is anisotropic and much faster in the (a,b) plane than in c-direction. In single crystals of 2201 and 2212 the oxygen diffusion coefficients at 830°C for the c-direction are situated between 10 − 10 and 10 − 11 cm2 s − 1 [35]. The value for polycrystalline 2212 at 830°C is about 10 − 9 cm2 s − 1. The diffusion coefficients of 2223 are unknown, but due to the similarities in the crystal structure of layered (Bi,Pb)-cuprates the values are assumed to be situated in the same range. So, the oxygen transport through the interface Ag/ceramic is progressively slowed down by the 2212/2223 grain growth. Due to the strong correlation between partial pressure of oxygen and the temperature of the 2223 phase formation the generation of 2223 will be retarded in the interior of a filament if the starting oxygen
content is higher than necessary or vice versa. Especially, the former case may result in a swelling of the ceramic due to a sudden release of the excess oxygen from the ceramic, which is unable to diffuse with sufficient velocity through the interface. As a consequence the 2223 forming temperature increases as in the cases described in Section 3.3.2 and Section 3.3.3. However, the application of the correct temperature for the interior portion will deteriorate the 2223 phase of the interface and depress the critical current density. So, if the process of 2223 formation includes a step of oxygen delivery the phase forming process must be stopped before the transformation of the precursor into 2223 is finished, unless a 2223 decomposition starts at the interface.
3.3.2. Influence of Ag It is known that the 2223 structure does not incorporate Ag. However, the temperature of the 2223 phase forming process is depressed in presence of Ag by about 10–20°C due to a dissolution of smaller quantities of Ag in the liquid phase. The liquid will transport some Ag into the core of the ceramic and explain the presence of Ag after the thermomechanical processing in the interior portion of the ceramic within a filament [36]. However, due to the high Ag concentration at the Ag interface a concentration gradient of silver will be present from the outer to the internal portion of the ceramic layer. Furthermore, the melting temperature of the liquid is similarly influenced and will be lower at the Ag interface. Consequently, melting will start at this region and will sluggishly propagate within the core of the ceramic (see below). As has been demonstrated by several experiments the 2223 formation is maximum at the Ag interface (cf. Section 3.1). As supplementary influence on dissolution of Ag in the liquid phase the rapid formation of (Bi,Pb)-2212 at the Ag interface must be considered (cf. Section 3.1). The formation of the extended 2212 plates will hamper or even interrupt the contact between Ag interface and melt. So, the subsequent delivery of Ag to the melt is limited, intensifying the decent of the Ag concentration from the outer to the interior portion of the ceramic. Due to the increase of the melting temperature of the liquid phase in the interior portion of the ceramic (as consequence of the concentration gradient of Ag) its generation needs increasing temperatures, unless the transformation 2212 2223 cannot take place. These opposing effects of decreasing Ag concentration and increasing 2223 phase forming temperature throughout the ceramic from the outer to the inner portion make the application of one suitable 2223 forming temperature difficult, because 2223 will decompose under the influence of too high temperatures (cf Section 3.2). Moreover, due to the different growth history of 2223 at the interface (lower temperature) and in the interior
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portion (higher temperature) its sensitivity against heat will increase for the outer portion of the 2223 phase. Furthermore, this sensitivity will be maintained for all sintering steps. Consequently, every increase in temperature will affect the 2223 crystals at the interface more seriously than in the core of the ceramic.
3.3.3. Endothermic effects during 2223 formation DTA measurements performed on Ag sheathed tapes with a 2223-precursor by several authors [2,37,38] indicate two main endothermic peaks. The peak at lower temperature reflects the formation of a liquid phase, whereas at higher temperatures a reaction between two or more of the secondary phases starts. Note that both temperatures are dependent on the surrounding oxygen partial pressure, the Ag content of the melt (for values 210%) and the Bi/Pb ratio. Due to the excellent thermal conductivity of the Ag sheath, the deficiency in temperature during the 2223 formation is easily compensated for the ceramic near the interface to the Ag clad. However, this is not valid for the internal portion. In this case the loss of heat must be compensated by a transport of heat from the exterior via the outer ceramic layers. Due to the comparably inferior thermal conductivity of the cuprate phases a temperature gradient develops with the result of a retarded 2223 formation in the central portion of the oxide layer. However, as mentioned in Sections 3.3.1 and 3.3.2 it is not suitable to compensate the loss of heat in the interior by the application of a higher reaction temperature to the tape, due to the sensitivity of the external 2223 portion against an increase in temperature. It is expected that the differences between the outer and the internal portion of a ceramic layer within a filament will diminish by reducing its diameter. 4. Conclusions In the process of 2223 phase formation during thermomechanical processing of Ag clad tapes the presence of Pb is indispensable. Its beneficial role can be described by several topics: The Pb content (i) accelerates the 2223 formation via a rapid generation of (Bi,Pb)2212; (ii) introduces grain growth and texture; (iii) increases the temperature window of the 2223 formation, thus making the 2223 phase suitable for a practical application; (iv) decrease the reaction temperature; (v) adjusts the hole concentration of the 2223 phase via the temperature dependent equilibrium between Pb4 + and Pb2 + ; (vi) works as buffer for charge carriers in supplement to the charge reservoir in the rocksalt-like layers; and (vii) may finally create pinning centres via 2212 as decomposition product of (Bi,Pb)-2223, Pbcontaining phases as e.g. 3321 or fine inclusions of amorphous material.
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It is expected that the production of industrial multifilamentary (Bi,Pb)-2223 tapes with high critical current densities will improve by adjusting the phase forming process throughout the entire ceramic section of the filament via the oxygen content created over the Pb4 + /Pb2 + equilibrium of (Bi,Pb)-2212 and/or several Pb4 + containing secondary phases as e.g. Ca2PbO4 or 3321.
Acknowledgements The authors are indebted to the Bundesministerium fu¨r Bildung, Wissenschaft, Forschung and Technologie and the Siemens AG (FKZ 13N6481) for financial support. This work was funded by the Verband der Chemischen Industrie. We wish to thank Professor Dr R.P. Huebener and T. Nissel for the SEM micrographs.
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[20] W. Zhang, E.E. Hellstrom, M. Polak, Supercond. Sci. Technol. 9 (1996) 971. [21] S. Lo¨sch, H. Budin, O. Eibl, M. Hartmann, T. Rentschler, M. Rygula, S. Kemmler-Sack, R.P. Huebener, Physica C177 (1991) 271. [22] P.V. Sastry, A.R. West, J. Mat. Chem. 4 (1994) 647. [23] H. Maeda, Y. Tanaka, M. Fukotomi, T.A. Sano, Jpn. J. Appl. Phys. 27 (1988) 209. [24] T. Rentschler, S. Kemmler-Sack, P. Kessler, H. Lichte, Physica C219 (1994) 167. [25] S. Elschner, S. Gauss, Supercond. Sci. Technol. 5 (1992) 300. [26] M.R. Koblischka, L. Pust, A. Galkin, P. Nalevka, Appl. Phys. Lett. 70 (1997) 514. [27] T. Rentschler, S. Lo¨sch, P. Zoller, C. Schulz, E. Raff, T. Fries, S. Kemmler-Sack, J. Alloys Compd. 195 (1993) 43. [28] S. Lo¨sch, H.-W. Neumuller, W. Wischert, S. Kemmler-Sack, Phys. Stat. Sol. (A) 126 (1991) 221. [29] J.A. Parell, D. Larbalestier, G.N. Riley Jr., Q. Li, R.D. Parella, M. Teplitsky, Appl. Phys. Lett. 69 (1996) 2915.
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(Bi,Pb)-2223 phase formation in Ag clad tapes A. Trautner, D. Go¨hring, P. Haug, B. Sailer, A. Ehmann, W. Wischert, S. Kemmler-Sack * Institut fu¨r Anorganische Chemie der Uni6ersita¨t Tu¨bingen, Auf der Morgenstelle 18, D-72076 Tu¨bingen, Germany Received 6 June 1998; accepted 23 November 1998
Abstract The most promising material for fabricating long-length conductors is (Bi,Pb)-2223 produced by the powder-in-tube process in Ag clad tapes. Intensive studies have shown that the critical current density of fully processed tapes is affected by a large variety of processing parameters, but the nature of the (Bi,Pb)-2223 phase formation is still controversial. In this study certain steps of the 2223 phase formation were investigated at different stages of sintering under the influence of various temperature/time schedules of thermomechanical processing and the important role of Pb is shown. The main effects of Pb in the phase forming process are: (i) Acceleration of the 2223 formation via a rapid generation of (Bi,Pb)-2212; (ii) introduction of grain growth and texture; (iii) increase of the temperature window for the 2223 formation; (iv) decrease of reaction temperature; (v) adjustment of the hole concentration of the 2223 phase; (vi) operation as buffer for charge carriers via the temperature dependent equilibrium between Pb4 + and Pb2 + ; and (vii) creation of Pb containing pinning centres. © 1999 Elsevier Science S.A. All rights reserved. Keywords: High temperature superconductor; (Bi,Pb)-2223 Phase formation; Powder-in-tube-process; Critical current density
1. Introduction One of the most interesting industrial application of high-Tc superconductors at 77 K is in form of longlength conductors for transmission cables, high field magnets, motors and generators. The most promising material is (Bi,Pb)-2223 produced in form of Ag clad tapes by using the powder-in-tube (PIT) method [1,2]. Despite the complexity of the multinary system Ag– Bi –Pb–Sr–Ca–Cu – O [3] substantial progress was achieved in increasing the critical current density of Ag-sheathed (Bi,Pb)-2223 tapes. The highest transport critical current densities ( jc) of about 105 A cm − 2 at 77 K/0 T were reported so far for short pressed tapes made by the Ag-wire-in-tube method [4] demonstrating the capacity of the (Bi,Pb)-2223 material. It is desired to reach similar values for long industrial tapes. Actually, the obtained jc values are a limiting factor in the practical application. The best values for rolled short lengths are situated in the range of 5.8×104 A cm − 2 * Corresponding author. Tel.: +49-7071-2972439; fax: +49-7071296918. E-mail address: [email protected] (S. Kemmler-Sack)
(77 K/0 T [5]). Several important reasons for a limitation of jc in (Bi,Pb)-2223 tapes have been recognized: (i) inhomogeneity of the precursor; (ii) phase formation conditions and phase diagram; (iii) inhomogeneity of the deformation process during tape production; (iv) microstructure and grain boundaries; and (v) anisotropy of the critical current density [2]. One of the keys for an increase of the critical current density will be the understanding of the formation mechanism for (Bi,Pb)-2223 in Ag clad tapes. Although numerous studies have been published, the nature of (Bi,Pb)-2223 formation is still controversial [2,4,6–9]. One of the main difficulties in comparing the results of the different authors consist in large variations of the phase assemblage and chemical composition of the multiphase precursor as well as of the employed gas atmosphere (air or reduced oxygen content). The aim of the present study is to follow certain steps of the (Bi,Pb)-phase formation during the PIT process in Ag clad tapes with the intention to develop an improved understanding of the parameters influencing the current carrying capacity of (Bi,Pb)-2223 conductors.
0921-5107/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 1 - 5 1 0 7 ( 9 8 ) 0 0 4 3 4 - 6
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2. Experimental The employed precursor powder has the nominal cation ratio Bi:Pb:Sr:Ca:Cu=1.8:0.4:2:2.1:3. It was prepared via a solution of nitrates in a spray dry machine and fired at about 800°C for several hours to decompose the nitrates. Ag sheathed monocore and multifilamentary (Bi,Pb)-2223 tapes having 55 filaments were prepared by the standard PIT-technique via bundling, drawing and rolling [10]. The monocore tape was rolled to a cross section : 2.6 × 0.17 mm, with a typical core diameter approximately 99 mm. The cross section for the 55 filamentary tape is : 5 × 0.2 mm. The green tape was then cut into 3 cm long sections for thermomechanical processing. Between each sintering step, the individual sections were uniaxially pressed at 1.2 GPa. The processed tapes were characterized by XRD (CuKa radiation, Philips powder diffractometer, Au standard) after peeling the Ag clad from the surface of the tape. The 2223 content was evaluated via the peak intensity I according to 2223[%]= I[0010(2223)]/ {I[0010(2223)]+ I[008(2212)] +I[006(2201)]} ×100. The average oxygen content of the ceramic was determined after separation from the Ag clad via redoxtitration by using solutions of (NH4)2Fe(SO4)2 and Ce(SO4)2. Critical current density measurements were performed at each stage of the PIT process at 77 K in self field using the 1 mV cm − 1 criterion. The dc magnetic susceptibility was measured with a SQUID magnetometer (Quantum Design). Microstructure examinations of PIT conductors were performed on selected samples using a scanning electron microscope.
3. Results and discussion
3.1. Processes during the first sintering step During the process of drawing, bundling and rolling the precursor inside the green tape is transformed into some ill crystallized materials, consisting mainly of the 2212 phase and a mixture of plumbates of type Ca2PbO4/3321 (Fig. 1a). However, an important part of the oxidic precursor remains amorphous and cannot be detected by XRD. So, the overall intensity of the X-ray diffraction pattern is very low. Astonishingly, after a short heating time of only 20 min at 790°C/air certain diffraction peaks start to intensify (Fig. 1b) being identified as mainly (001) reflections of the 2212 phase (see below). From the decrease of the group of plumbate reflections in Fig. 1b we deduce the incorporation of Pb from the crystalline precursor phases into the crystalline 2212 phase as one reason for this fast crystal growth. However, in addition to the crystalline compounds the large amount of amorphous material must be simultaneously considered. As can be seen from the overall
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increase of the intensity in the XRD pattern the amorphous phases have started to transform into crystalline material, thus working as a feed for the growth of the 2212 phase. So, in agreement with the observation of several authors for powder mixtures [2,11,12] as well as for tapes [13] the first stage of (Bi,Pb)-2223 formation consists in the formation of (Bi,Pb)-2212. However, this process cannot only be described by a simple transfer of Pb from the crystalline plumbates of the precursor material into the simultaneously crystalline Bi-2212 phase, because a large supply of the precursor is present in amorphous form. So, as important second process an incorporation of Pb from these amorphous materials into crystalline 2212 must be considered. As third process the feeding of a 2212 grain growth from the amorphous material takes place with crystalline (Bi,Pb)-2212 as seed crystals. The simultaneous presence of these three processes explains the observed rapid grain growth of the 2212 phase (see below). Note that formation of (Bi,Pb)-2212 starts at a relatively low temperature during the first sintering step. The presently employed reaction temperature of 790°C is much lower than 839°C as reported for this process in monofilamentary Ag sheathed tapes in the literature [2]. Furthermore, it is very likely that this process will start even earlier, but presumably with reduced velocity. So, the employed heating ramp within the first step of thermomechanical processing will be of enormous importance for the entire phase forming process and not only the final manner of cooling down after the last furnace period (cf. Section 3.2). From the stronger increase of the (001) reflections of the 2212 phase relative to the (h0l) peaks in Fig. 1b it follows additionally, the development of a texture with the c-axis lying normal to the Ag interface. This effect is more pronounced for sintering of the green tape at higher temperatures. Fig. 1c gives the XRD pattern for
Fig. 1. Section of the XRD pattern from the surface of the core of monocore tapes (a) green tape with twofold intensity, (b) after heating at 790°C/20 min in air and (c) 850°C/20 min in air. The main reflections of the 2212 phase (subcell) are indicated together with that of the plumbates Ca2PbO4 and 3321.
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an example at 850°C/air/20 min. In this case the (001) peaks of the 2212 phase are dominant, indicating a c-axis alignment of the 2212 grains and the development of a texture. This observation is easily explained by the assumption that the remaining 2212 crystals in the green tape have adopted a preferential orientation during the mechanical processing and operate as seed crystals for the growth of textured (Bi,Pb)-2212. Note that at the 850°C stage the plumbate peaks have practically vanished in the XRD pattern, thus indicating a high growth rate of (Bi,Pb)-2212. Very instructive is the investigation of the overall oxygen content of the ceramic core of the tape, scratched out from the Ag clad after the first sintering step at 790 or 850°C, respectively. Keeping in mind that the oxidation state of Pb in the plumbate phases is + 4, whereas in the 2212 phase it is considered as + 2, one expects a decrease of the total oxygen content via the incorporation of Pb from the plumbate phases into 2212. The experimental values of the oxygen content of 10.85 for the green tape, 10.85 after 790°C/air/20 min and 10.48 after 850°C/air/20 min indicate that the incorporation of Pb is no simple reaction. At inferior temperatures the total oxygen content is conserved, indicating that Pb enters the 2212 lattice in the + 4 state. Since Pb4 + (r = 0.775 × 10 − 1 nm; CN 6 [14]) is much smaller than Pb2 + (r = 1.19 × 10 − 1 nm) a substitution into smaller lattice sites is more likely than in the extended rocksalt-like layers. Indeed, in case of (Bi,Pb)2223 a substitution of Pb for Ca has been found by neutron powder diffraction [15]. However, at a reaction temperature of 850°C a significant loss of oxygen is observed, which can be interpreted via an incorporation of Pb2 + into the 2212 lattice. So, the formation of (Bi,Pb)-2212 is a complex temperature dependent process, starting with the incorporation of Pb4 + at inferior temperatures ( : 790°C/air), where Pb4 + is still stable. Finally, at higher temperatures (:850°C/air) Pb2 + is superior in its stability to Pb4 + and will substitute in the rocksalt-like layers. Note that at this stage of the reaction the average hole concentration per Cu ion of 0.2 has nearly obtained the ideal value around 0.25 holes per Cu ion for the development of superconductivity. From earlier investigations it is known that in air the substitution of Pb for Bi is accompanied with a change of the lattice parameters and the development of an orthorhombic distortion, whereas in Ar atmosphere materials with higher Pb content and a monoclinic distorted lattice are formed [16]. These results were later on confirmed by other authors [12,17,18]. According to TEM investigations in combination with EDX analysis [19] the process of transformation of Bi-2212 into (Bi,Pb)-2212 is not the result of a simple atomic substitution since the wavelength of the modulation depends on the Bi/Pb ratio. So, the true crystal structure is
unique for distinct levels of substitution, but the corresponding individual Bi/Pb ratios are yet unknown. For a relatively high Pb content of x around 0.6 (of the system Bi2-xPbxSr2CaCu2Oz) the structure becomes finally unmodulated. Note that only in the unmodulated region the formation of a real solid solution is expected. However, this case is unimportant for a practical application in Ag clad tapes since the Pb content of the precursor is situated at values not higher than x: 0.4. Moreover, the Bi/Pb ratio does not only influence the ideal crystal structure of the 2212 phase but also its real structure. The typical effect of a high Pb content is a drastical reduction of the number of stacking faults and defects [19]. This finding may be of importance for the generation of the pinning centres. Within the first sintering step the generation of (Bi,Pb)-2223 starts soon after the formation of (Bi,Pb)2212. Since the determination of the 2223 content by XRD via peak intensity gives only a value near the Ag interface without lateral resolution a more complete information is obtainable for monocore tapes by analyzing via XRD the 2223 content at the surface in comparison to the corresponding value of the entire ceramic, scratched out from the Ag clad. Our experimental data for several monocore tapes indicate uniformely the preferred formation of 2223 at the Ag interface. This observed lateral 2223 distribution is in agreement with the results of Hellstrom et al. [20] as well as with the final lateral jc distribution in monofilament tapes [2]. In further experiments we studied the correlation between the temperature of the first sintering step, the amount of 2223 and the critical current, Ic, for several monocore and 55-filamentary tapes with very similar results (Fig. 2). All reactions were performed in reduced oxygen partial pressure of 0.08 bar (N2/O2(8%)). In comparison with air reactions this reduction leads to slightly inferior reaction parameters due to a lowering of all melting temperatures. Fig. 2a gives as example the data for a monocore and Fig. 2b for a 55 filamentary tape after a heat treatment of 10 h at various temperatures. In both cases the 2223 content starts to increase up to about 822/823°C, reaching maximum values between 70 and 80%. In case of Ic a maximum was found in a similar region. A further increase in temperature depresses the 2223 content as well as Ic because the 2223 phase decomposes at higher temperatures. The complete data set of Fig. 2 suggest that an optimal temperature for the 2223 phase forming process exists.
3.2. Processes during the second and third sintering step Several second and third heat treatments were started with monocore and 55 filamentary tapes resulting from
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Fig. 2. Influence of heating temperature on 2223 content and critical current density after 10 h in N2/O2(8%) for (a) a monocore and (b) a 55 filamentary tape.
various temperatures applied during the first sintering step (cf. Section 3.1). All tapes were sequentially pressed and thermally processed. Some results were summarized in Fig. 3 for a three step sintering in N2/O2(8%) at 808, 814 or 820°C of some monocore tapes from Fig. 2a. In contrast with the results of the first sintering step (cf Section 3.1) the data indicate that the correlation between 2223 content and Ic is no longer valid for all employed temperatures. In case of a three step firing at the relatively low temperature of 808°C both values still increases simultaneously, but remain comparably low. For the sequence of 814°C both parameters again correlate and a steep increase of both values is observed. However, by choosing the ‘best’ temperature range from the first sintering step around 820°C a still higher growth rate of 2223 is observed during the second sintering step, but Ic does not follow this development and remains comparably low even after the third sintering step. Most probably, the liquid phase was already nearly consumed after the second sintering step and cracks, introduced by pressing before
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the final sintering step could no longer heal. From a comparison of the entire 814 and 820°C series it follows that the 2223 content increases in both cases by about 20% between the first and second step. This development continues in the 814°C case, obtaining after the third step a similar 2223 amount as in the 820°C case after the second step. However, in the former case Ic is nearly doubled. Two results can be deduced from this series of experiments: (i) the application of firing conditions which develop an optimal 2223 content after the first sintering step is unsuitable for obtaining finally high critical currents, (the validity of this correlation was observed in all our experiments with monocore and 55 filamentary tapes in reduced oxygen atmosphere as well as in air); and (ii) in case of a three step treatment the 2223 growth rate between the first and second step should be moderate as well. So, the 2223 content and the Ic values do not correlate on either step of the tape sintering process, because for large growth rates the intergrain properties as well as the morphology of the crystals are incompletely developed. However, not only the 2223 distribution and the intergrain properties are affected by the employed temperature program but also the intragrain properties. As example Fig. 4 indicates the x versus T (4a) and M versus H (4b) data for monocore tapes after the third sintering step at 808 or 814°C, respectively. From a comparison of the x versus T traces of the 808 and 814°C material it follows that the ZFC trace proceeds more sluggish in the 808°C case. This finding indicates fluctuations in the composition of the superconducting material. So, the knowledge of the real chemical com-
Fig. 3. Influence of the heating temperature for three heating steps in N2/O2(8%) on the 2223 content (%; indicated in numbers) and critical current for monocore tapes (first sintering step according to Fig. 2a).
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Fig. 4. (a) x vs. T and (b) M vs. H for Ag clad monocore tapes after the third firing step at ( ) 808 and () 814°C.
position of the 2223 grains together with the distribution of the elements over the different lattice sites should be desired for the understanding of the phase forming process, whereas a simple correlation between 2223 amount and superconducting properties is not conclusive. However, informations about the chemical composition and the atomic distribution within the 2223 structure are yet incomplete due to the complex crystal structure together with the large number of the five different cations. Moreover, the presence of vacancies as well as the substitution of one cation on different lattice sites must be considered. The latter case has been described above for the couple Pb4 + /Pb2 + (cf. Section 3.1). It is generally accepted that Pb stabilizes the 2223 phase without specifying its location and average oxidation state. In contrast, the preparation of Pb free 2223 is difficult due to a very narrow temperature window [2,21,22], in spite of the fact that the first trace of 110 K superconductivity was observed in a Bi-cuprate material without Pb [23]. Similarly, to the (Bi,Pb)-2212 system [16,24] the crystal structure of (Bi,Pb)-2223 depends on the Bi/Pb-ratio. This is valid for the substruc-
ture as well as for the true, modulated structure: With increasing Pb content an orthorhombic distortion of the subcell develops [16] and the wavelength of the modulation correlates with the Bi/Pb-ratio [19]. From M versus H traces taken between 5 and 77 K for several tapes after different sintering steps at various temperatures it follows that the intergrain properties are strongly affected by the entire set of parameters. Examples for the most interesting 77 K case are given in Fig. 4b. The M(H) traces indicate that the width of the hysteresis loop 2Mr is larger for the 814°C material than for the 808°C sample. According to the Bean model jcm 3Mr/(2rxV) [25] the critical current density jcm has nearly doubled in the 814°C material at 77 K/0 T. Additionally, the position of the positive maximum of the hysteresis loop is shifted in the 814°C case more strongly to positive values of H than for the 808°C sample. According to loc.cit. [26] this shift indicates an improvement of the intergrain connectivity in the material from 814°C. Several SEM investigations of the surface of Ag tapes after various furnace periods indicate uniformely that the grain growth is maximal in the first step of the 2223 formation. This observation is confirmed by in situ measurements of texture and phase development in (Bi,Pb)2223 Ag tapes [8]. The grain growth starts via the (Bi,Pb)-2212 phase (cf Section 3.1). SEM micrographs of the surface of the ceramic after the first sintering step (an example is given in Fig. 5) indicate the formation of large extended plates. In this case the 2223/2212 ratio still amounts to about 50:50 and jc as low as : 1.5 kA cm − 2 (77 K, self field). Note that a texture begins to develop, but remains still incomplete. After two compression steps and the third sintering period the 2223 amount increases to above 80% with jc : 20 kA cm − 2 (77 K, self field). Moreover, the texture has improved by the intermediate compression steps. However, the original large crystal plates were destroyed during compression and do not recover within the process of 2223 formation in the subsequent furnace periods, because the 2223 formation is not accompanied by a pronounced grain growth [27]. So, the grain growth and the 2223 phase formation are two independent processes. In situ measurements with hard X-rays on completely encased Ag tapes give identical results [8]. Consequently, the grain morphology is determined within the first furnace period and this step will be decisive for the following 2223 forming processes. As consequence of the multiphase nature of the precursor local fluctuations in the chemical composition exists and the transformation of 2212 into 2223 in Ag tapes needs very long sintering times to reach equilibrium conditions. So, in reality the transformation remains always incomplete. The process of 2223 formation stops at a level of around 90% 2223, though
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unreacted secondary phases are still present, but unsuitable for a progressive 2223 growth due to long diffusion paths. A more complete 2223 formation is expected from highly homogeneous precursor material. It is well known that excessive amounts of mechanical work can be detrimental after a substantial amount of the precursor powder has been converted to (Bi,Pb)-2223. However, there is although another mechanism, which is detrimental to a tape with large amounts of 2223. This process starts, when the employed, originally optimized temperature/time schedule for the third sintering step exceeds a certain reaction time. At this stage the 2223 phase starts to decompose and Ic begins simultaneously to decline, as
Fig. 6. Influence of the sintering time of a Ag clad tape on Ic (77 K/0 T). The 2223 content (%) is indicated in numbers.
Fig. 5. SEM images of the surface of the ceramic core from an Ag tape after the (a) first and (b) third furnace period.
was observed for various of our monocore and 55 filamentary tapes. Fig. 6 shows as example the results for several samples of the same 55 filamentary tape after different times for the third sintering step at 823°C in N2/O2(8%). It is obvious that the decrease of Ic correlates with the decline of the 2223 content. As main decomposition product a well textured 2212 phase was identified by XRD, but the remainder of the 2223 2212+ 0011 decomposition reaction in form of one or more Ca/Cu/O phase(s) was only detected via XRD after the longest sintering step of twelve days in form of one small peak from Ca2CuO3. Thus, at a first stage of the decomposition the Ca/Cu/O phase(s) must segregate in practically amorphous form between the textured 2212 plates, adopting finally a crystalline form after long sintering times. It should be mentioned that the once decomposed 2223 phase recovers only very sluggishly during additional sintering steps. We performed several corresponding experiments for tapes with a partially decomposed 2223 phase as consequence of a short melting process via a temperature pulse around 840– 860°C. To avoid any decomposition of the 2223 phase during the final sintering step a depression of the firing temperature is suitable as has been observed for 2223 pellets some years ago [28]. In case of Ag tapes slow cooling [29,30] as well as heating at inferior temperatures [31] was recommended. However, it must be simultaneously taken in mind, that precipitation of 2212 through a 2212 2223 2212 conversion may be advantageous via the generation of pinning centres [32]. Actually, it is not clear yet whether 2212 or the associated secondary phases such as 3321 [33] contribute to the pinning. It cannot be excluded that amorphous phases may represent pinning centres as well. Such a case was observed recently for (Bi,Pb)2223 tapes with Ag/Cu alloy sheaths doped with Ti [34].
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3.3. The (Bi,Pb) -2223 phase formation inside Ag-clad tapes The formation of the (Bi,Pb)-2223 phase and its decomposition inside Ag-clad tapes is influenced by a variety of processing parameters and a definitive answer about all processes inside Ag tapes cannot yet be given. In the following we will discuss some findings of this study with respect to the 2223 phase formation in Ag tapes.
3.3.1. Adjustment of the oxygen content It is well known that high-Tc superconductors need a certain hole concentration, obtained via doping of carriers from the charge reservoir in the rocksalt-like layers into the CuO2 sheets. To adjust the hole concentration within the ceramic core of the Ag clad tape every portion of the filament should adopt the correct carrier concentration. This process is realized via the intercalation or delivery of oxygen. As has been demonstrated in Section 3.1 the oxygen content of the ceramic is influenced by the heat treatment and diminishes with increasing temperature. In the 850°C case an oxygen diffusion from the oxide core to the Ag sheath takes place. The oxygen diffusion through Ag is complex. The diffusion coefficient for 830°C is about 4 ×10 − 6 cm2 s − 1. However, only in a very early stage of the first heat treatment of some minutes the interface Ag/ceramic is open for the oxygen diffusion. The diffusion will be rapidly hampered by the growth of extended textured (Bi,Pb)-cuprate plates close to the Ag sheath (cf. Section 3.1 and Section 3.3.2). The crystals consist at the beginning of (Bi,Pb)-2212, later on a transformation to 2223 takes place. The growth of (Bi,Pb)-2223 close to the Ag was observed by several authors (cf. Section 3.1). Due to the intimate contact between the Ag sheath and the preferentially in the (a,b) direction growing plates of (Bi,Pb)-2212 and/or of (Bi,Pb)-2223 the interface will be covered by textured plates. This material will now determine the oxygen exchange with the exterior. The oxygen diffusion in (Bi,Pb)-cuprates is anisotropic and much faster in the (a,b) plane than in c-direction. In single crystals of 2201 and 2212 the oxygen diffusion coefficients at 830°C for the c-direction are situated between 10 − 10 and 10 − 11 cm2 s − 1 [35]. The value for polycrystalline 2212 at 830°C is about 10 − 9 cm2 s − 1. The diffusion coefficients of 2223 are unknown, but due to the similarities in the crystal structure of layered (Bi,Pb)-cuprates the values are assumed to be situated in the same range. So, the oxygen transport through the interface Ag/ceramic is progressively slowed down by the 2212/2223 grain growth. Due to the strong correlation between partial pressure of oxygen and the temperature of the 2223 phase formation the generation of 2223 will be retarded in the interior of a filament if the starting oxygen
content is higher than necessary or vice versa. Especially, the former case may result in a swelling of the ceramic due to a sudden release of the excess oxygen from the ceramic, which is unable to diffuse with sufficient velocity through the interface. As a consequence the 2223 forming temperature increases as in the cases described in Section 3.3.2 and Section 3.3.3. However, the application of the correct temperature for the interior portion will deteriorate the 2223 phase of the interface and depress the critical current density. So, if the process of 2223 formation includes a step of oxygen delivery the phase forming process must be stopped before the transformation of the precursor into 2223 is finished, unless a 2223 decomposition starts at the interface.
3.3.2. Influence of Ag It is known that the 2223 structure does not incorporate Ag. However, the temperature of the 2223 phase forming process is depressed in presence of Ag by about 10–20°C due to a dissolution of smaller quantities of Ag in the liquid phase. The liquid will transport some Ag into the core of the ceramic and explain the presence of Ag after the thermomechanical processing in the interior portion of the ceramic within a filament [36]. However, due to the high Ag concentration at the Ag interface a concentration gradient of silver will be present from the outer to the internal portion of the ceramic layer. Furthermore, the melting temperature of the liquid is similarly influenced and will be lower at the Ag interface. Consequently, melting will start at this region and will sluggishly propagate within the core of the ceramic (see below). As has been demonstrated by several experiments the 2223 formation is maximum at the Ag interface (cf. Section 3.1). As supplementary influence on dissolution of Ag in the liquid phase the rapid formation of (Bi,Pb)-2212 at the Ag interface must be considered (cf. Section 3.1). The formation of the extended 2212 plates will hamper or even interrupt the contact between Ag interface and melt. So, the subsequent delivery of Ag to the melt is limited, intensifying the decent of the Ag concentration from the outer to the interior portion of the ceramic. Due to the increase of the melting temperature of the liquid phase in the interior portion of the ceramic (as consequence of the concentration gradient of Ag) its generation needs increasing temperatures, unless the transformation 2212 2223 cannot take place. These opposing effects of decreasing Ag concentration and increasing 2223 phase forming temperature throughout the ceramic from the outer to the inner portion make the application of one suitable 2223 forming temperature difficult, because 2223 will decompose under the influence of too high temperatures (cf Section 3.2). Moreover, due to the different growth history of 2223 at the interface (lower temperature) and in the interior
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portion (higher temperature) its sensitivity against heat will increase for the outer portion of the 2223 phase. Furthermore, this sensitivity will be maintained for all sintering steps. Consequently, every increase in temperature will affect the 2223 crystals at the interface more seriously than in the core of the ceramic.
3.3.3. Endothermic effects during 2223 formation DTA measurements performed on Ag sheathed tapes with a 2223-precursor by several authors [2,37,38] indicate two main endothermic peaks. The peak at lower temperature reflects the formation of a liquid phase, whereas at higher temperatures a reaction between two or more of the secondary phases starts. Note that both temperatures are dependent on the surrounding oxygen partial pressure, the Ag content of the melt (for values 210%) and the Bi/Pb ratio. Due to the excellent thermal conductivity of the Ag sheath, the deficiency in temperature during the 2223 formation is easily compensated for the ceramic near the interface to the Ag clad. However, this is not valid for the internal portion. In this case the loss of heat must be compensated by a transport of heat from the exterior via the outer ceramic layers. Due to the comparably inferior thermal conductivity of the cuprate phases a temperature gradient develops with the result of a retarded 2223 formation in the central portion of the oxide layer. However, as mentioned in Sections 3.3.1 and 3.3.2 it is not suitable to compensate the loss of heat in the interior by the application of a higher reaction temperature to the tape, due to the sensitivity of the external 2223 portion against an increase in temperature. It is expected that the differences between the outer and the internal portion of a ceramic layer within a filament will diminish by reducing its diameter. 4. Conclusions In the process of 2223 phase formation during thermomechanical processing of Ag clad tapes the presence of Pb is indispensable. Its beneficial role can be described by several topics: The Pb content (i) accelerates the 2223 formation via a rapid generation of (Bi,Pb)2212; (ii) introduces grain growth and texture; (iii) increases the temperature window of the 2223 formation, thus making the 2223 phase suitable for a practical application; (iv) decrease the reaction temperature; (v) adjusts the hole concentration of the 2223 phase via the temperature dependent equilibrium between Pb4 + and Pb2 + ; (vi) works as buffer for charge carriers in supplement to the charge reservoir in the rocksalt-like layers; and (vii) may finally create pinning centres via 2212 as decomposition product of (Bi,Pb)-2223, Pbcontaining phases as e.g. 3321 or fine inclusions of amorphous material.
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It is expected that the production of industrial multifilamentary (Bi,Pb)-2223 tapes with high critical current densities will improve by adjusting the phase forming process throughout the entire ceramic section of the filament via the oxygen content created over the Pb4 + /Pb2 + equilibrium of (Bi,Pb)-2212 and/or several Pb4 + containing secondary phases as e.g. Ca2PbO4 or 3321.
Acknowledgements The authors are indebted to the Bundesministerium fu¨r Bildung, Wissenschaft, Forschung and Technologie and the Siemens AG (FKZ 13N6481) for financial support. This work was funded by the Verband der Chemischen Industrie. We wish to thank Professor Dr R.P. Huebener and T. Nissel for the SEM micrographs.
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