Ag monofilamentary tapes using Hall sensor magnetometry

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Physica C 244 (1995) 115-122

Homogeneity study of Bi-2223/Ag monofilamentarytapes using Hall sensor magnetometry Markku Lahtinen a , . , Jaakko Paasi a, Jyrki Sarkaniemi b, Zhenghe Han c, Torsten Freltoft c a Laboratory of Electricity and Magnetism, Tampere University of Technology, P.O. Box 692, SF-33101 Tampere, Finland b Institute of Materials Science, Tampere University of Technology, P.O. Box 589, SF-33101 Tampere, Finland c NKTResearch Center A/S, DK-2605 Brcndby. Denmark Received 6 December 1994

Abstract

In this paper we report a homogeneity study of Bi-2223/Ag monofilamentary tapes prepared by two different mechanical deformation processes, rolling and semi-continuous pressing. We demonstrate how effects of mechanical deformation and heat treatments on the sample homogeneity and local critical current density can be revealed using Hall sensor magnetometry. Typical features of measured magnetic flux density profiles are interpreted by comparing the profiles to data obtained from resistive critical current measurements, SEM micrographs and numerical calculations based on simplified models of screening current distributions. Our experimental results are in agreement with the concepts that pressing leads to a rather homogeneous transport current path in a Bi-2223/Ag tape and rolling is likely to induce transversal cracks and necking in the superconducting ceramic.

1. Introduction

The potential of bulk high-temperature superconductor (HTS) (Bi,Pb)2Sr2Ca2Cu30 x (Bi-2223) material has been demonstrated at several laboratories in the form of short Bi-2223/Ag composite tapes, where local critical current density (Jc) values well above 50 kA/ cm 2 ( 1 I~V/cm, 77 K, 0 T) have been reported. However, long tapes with optimized critical current (Ic) will be needed, if the tapes are to be used in magnets, electrical machines or AC power distribution applications. To meet this requirement both high Jc and good control over the homogeneity of the transport current path must be achieved over long production lengths. The detrimental effects of macroscopic inhomogeneities on tape performance are emphasized in several * Corresponding author. 0921-4534/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI0921-4534

studies [ 1-4]. Larbalestier et al. [4] pointed out that the problems caused by macroscopic defects must be first overcome before full benefits from the optimization of the structure of Bi-2223 ceramic can be expected. Therefore, in this paper we focus on the study of macroscopic defects in Bi-2223/Ag tapes. The role of more microscopic aspects like grain-to-gain, or colony-to-colony boundaries, and inter- and intragrain pinning is not discussed in detail, although the macroscopic inhomogeneities do affect grain-to-grain connectivity and pinning in the intergrain system. The standard approach in the electromagnetic characterization of Bi-2223/Ag tapes, the conventional resistive four-probe method, does not give explicit information about sample inhomogeneities, but results averaged over the sample length. Therefore it may be difficult to assess the significance of different possible

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features limiting Ic of the sample, e.g. sausaging, cracking, secondary phases, lack of texturing and poor connectivity of grains in Bi-2223 ceramic. Clearly these features are closely interrelated, but it is instructive to know whether the I~ of a tape is limited by an inherently low J¢ of uniform ceramic or by macroscopic defects hindering the transport current flow. Hence additional characterization methods giving more explicit information about the sample homogeneity are required. Recently a few homogeneity studies using novel measuring techniques have been published. Larbalestier et al. [4] cut the ceramic core of a Bi-2223/Ag tape into microslices and measured each microslice resistively. They found large J~ variations across the tape cross-section indicating nonuniform transport current distribution in the tape. Glowacki et al. [ 5 ] measured I~ in both longitudinal and transversal direction in a pressed Bi-2223/Ag tape, and hence by comparing Ic values in the two directions they were able to reveal the presence of longitudinal cracks in the ceramic. In a recent study by Cave et al. [6], the electric potential distribution on the surface of a Bi-2223/Ag tape carrying DC transport current was shown to reflect tape microstructure and possible defects in the superconducting ceramic. Variations of the mechanical hardness in the ceramic core of a Bi-2223/Ag tape have been studied by Yamada et al. [7]. Their results show a larger Vickers microhardness near the Ag sheath than in central regions of the core indicating more dense ceramic and better grain-to-grain connectivity and thus a higher Jc close to the Ag-ceramic interface. Although resistive methods form the basis of the electromagnetic characterization of HTS tapes, supplementary information can be obtained using inductive measuring techniques. Especially in homogeneity studies of bulk HTS materials the use of miniature Hall sensors has shown its advantages [8]. Also the homogeneity of monofilamentary Bi-2223/Ag tapes has been studied using Hall sensors [9,10], but no comparative study of flux-density profiles measured along the length of a tape and the structure of the HTS ceramic has been published so far. In the following we report a Hall sensor magnetometry study of monofilamentary Bi-2223/Ag tapes prepared by different mechanical deformation steps. The objectives of the work are twofold. First, the feasibility of Hall sensor magnetometry in lengthwise homogeneity characterization of monofilamentary tape samples is demonstrated. Second, the

macroscopic inhomogeneities affecting screening current distributions the most in the studied samples are identified. This is accomplished by combining data from Hall sensor magnetometry, resistive Ic measurements, SEM micrographs and numerical results based on simplified models of different kinds of defect structures.

2. Experimental procedures 2.1. Sample preparation Monofilamentary Bi-2223/Ag tapes for the present study have been prepared by the standard oxide powder-in-tube method. The experimental details of the method, starting powder composition, and heat treatment parameters have been published elsewhere [ 2]. Mechanical-deformation processes, rolling and pressing, relevant to our experiments are outlined in the following. The rolling process, directly applicable to long-tape samples, has been carried out in a standard rolling mill. However, the common pressing process has been modified to overcome the problem of a limited sample length. In order to process long tapes, a semicontinuous pressing machine has been developed in NKT Research Center. The semi-continuous pressing is carried out by two stampers which can provide vertical press on the tape in steps. For each step, a special designed mechanics enables the stampers to carry out longitudinal movement for transporting the tape, while the pressure on the tape remains almost constant ensuring a smooth overlapping between adjacent pressing steps. Hereafter in this paper "pressing" will refer to the semi-continuous pressing process described above. In order to compare the homogeneity of tapes prepared by different rolling-annealing and pressingannealing steps, we prepared four different sample sets starting from a once annealed 1.2 m long tape. The long tape was first cut into two pieces of equal length. One piece was rolled and annealed and the other was pressed and annealed. The once-rolled piece was further subdivided into two pieces: one piece was given label A, and the other was rolled and annealed again and given label B. Also the once-pressed tape section was subdivided into two pieces, one piece was labelled C and the other was pressed and annealed for a second time and labelled D. Thus we obtained samples A-D pre-

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Table 1 Summary of properties of sample sets A-D, key for sample preparation steps: a is for annealing, p for pressing, r for rolling Sample set

Sample preparation Average ceramic thickness (p~m) It(A) ( 1 ixV/cm, B e = 0 mT, 77 K)

A

B

C

D

a-r-a 70

a-r-a-r-a 60

a-p-a 90

a-p-a-p-a 70

6.0, 8.1

3.3, 3.8, 5.5, 7.3

3.7, 8.1

5.0, 5.1, 9.5, 10.4

pared using one and two rolling-annealing sequences and one and two pressing-annealing sequences, respectively. The typical dimensions of a tape cross-sections were 2.2 mm × 0.12 mm with Ag sheath. The average ceramic core thickness in each sample was determined from SEM micrographs. The thickness values ranged from 60 pLmto 90 ~m, see Table 1. Samples A-D were cut into appropriate lengths for transport critical current measurements (5 cm), SEM studies and Hall sensor magnetometry (2 cm). The transport critical current measurements were carried out at 77 K, 0 T using the standard four-probe method and a 1 I~V/cm electric-field criterion. In order to study variations of the Ic values in samples with nominally the same preparation parameters, I¢ was determined from 2--4 different sample sections from each sample set A-D, see Table 1. In the following we will refer to a typical sample section from one set or the whole set of samples by the symbols A-D; the meaning will be obvious from the context.

2.2. Hall sensor magnetometry Tape sections from sample sets A-D were attached in turn to a sample holder and immersed in liquid nitrogen, in the center of a copper Helmholtz magnet. The

magnet produced an external magnetic field Be perpendicular to the tape surface, i.e. parallel to the z-axis, Fig. 1. As the external field Be was applied, superconducting screening currents started flowing in the superconducting core of the tape producing a sample field Bs. One component, Bz(x), of the total magnetic flux density B(x)=Be+Bs(x) was measured 0.4 mm above the tape surface using a movable miniature Hall sensor with an active area of 100 I~m x 50 p~m. A magnetic flux density profile was recorded as the sensor was linearly moved either along or across the centerline of the tape. In order to allow sufficient amplification of the small Hall voltage proportional to the sample field z component B~, the contribution of Be had to be subtracted from total Hall voltage UH ~Be. This was accomplished by using a second Hall sensor measuring only Be and a special compensating circuit [ 10], which carried out the subtraction and amplification of the two Hall voltages. Hence the output of the correctly balanced measurement system was directly proportional to B~. As the active area of the Hall sensor was small compared to dimensions of typical tape samples, we were able to measure flux density profiles with a high spatial resolution.

3. Results and discussion

3.1. Computed flux density profiles

Fig. 1. Measurement of a magnetic flux density profile of a monofilamentary tape in an external magnetic field applied perpendicular to the tape surface.

In order to gain insight into the interpretation of measured flux density profiles, numerical modelling of intergranular screening current distributions corresponding to various kinds of sample defects has been carried out. Basic examples of macroscopic defects in the ceramic core of a monofilamentary Bi-2223/Ag

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Fig. 2. Computedfluxdensityprofilesof simplifiedscreeningcurrent distributions in (a) a homogeneoussample, (b) a sample with a crack extendingthrough the ceramic, (c) a sample with a crack extending halfwayin the ceramicthickness, (d) a sample with a large defectextendingover4 mm in length, (e) a sampleincluding a non-superconductingregion. tape were constructed from fiat superconducting bars with rectangular cross-sections. This simplified geometry allowed us to make the numerical study of transversal cracks and non-superconducting secondary phases or voids extending through the width of the tape. In the numerical model the intergranular screening current distribution flowing in each flat, superconducting bar was approximated by a racetrack coil with appropriate dimensions and with an infinitesimally small bore. Hence the field at an arbitrary point due to the racetrack coil system could be found from the BiotSavart law using a commercial software package for magnetic-field computation. Numerical results are presented in Fig. 2, where the left column shows computed flux density profiles B~z(x) and the corresponding screening current distributions are sketched on the right. The parameters resulting in the numerical B~ values shown are as follows: Jc = 5000 A/cm 2, overall dimensions of ceramic 0.05 mm X 2 mm X 22 mm, thickness (t) X width (w) ×length (l), and Hall sensor distance from the ceramic surface 0.4 mm. A model of a homogeneous sample with no hindrances to the flow of screening

currents is shown in Fig. 2(a). The flux profile Bs~(x) is smooth with small maxima near the ends of the tape. A transversal crack extending through the ceramic thickness and the corresponding screening currents are shown in Fig. 2(b). The sample field B~(x) goes to zero at x = 0 mm indicating total blockage of screening currents. Fig. 2(c) is the model of a transversal crack extending halfway into the ceramic. Note that Bs~(x) at the sharp minimum is about half of the overall height at the profile. The model in Fig. 2(d), is not a crack, but a non-superconducting region 0.025 m m X 2 m m X 4 mm. (tXw×l). The calculated flux density profile shows a broad minimum corresponding to this section of tape with reduced value of I¢. The last model, Fig. 2 (e), shows a tape including a non-superconducting region, 0.025 mm x 2 mm x 4 mm, (t x w x l), in the center of the tape. The corresponding flux density profile is very similar to that in Fig. 2(d); there is only a slight change in the shape of the wide minimum of B~. From Figs. 2(d and e) we see, that the position of the defect cannot be determined in the z-direction using the present method. This implies that the inhomogeneous current distribution over the cross-section of the tape cannot be determined from a single lengthwise measured flux density profile; only inhomogeneities along the length are revealed. In addition, we cannot distinguish by measuring the Bs~ component only, whether the current flows only in the grain layers near the Ag sheath or homogeneously through the whole ceramic core. The present model of screening currents strongly simplifies the concepts of inter- and intragranular current systems, Jc(B) dependence and the actual geometry of the ceramic core in a Bi-2223/Ag tape. However, the simplifications made can be justified by the arguments given below. From a previous study [ 10] it is clear that in tapes with Jc -- 10 kA/cm 2 a major contribution to the diamagnetic response at 77 K comes from the intergranular current system and hence the intragranular currents can be neglected. The model used also implies the assumption of constant Jc(B). The significance of this assumption can be assessed by comparing remanence profiles after Be >> Bs and profiles measured in an increasing field Be >> Bs. We found that the features present in the profiles at Be >> B~ were reproduced in the remanence profiles, which allowed us to use either set of profiles in assessing the sample homogeneity. The shape of the ceramic cross-

M. Lahtinen et al. / Physica C 244 (1995) 115-122

section definitely differs from a rectangle in typical samples, but in this study we are only interested in the changes of the shape along the tape length. Therefore a model with a rectangular ceramic cross-section is as good as a model with a more complicated geometry. Modelling of small longitudinal cracks was not attempted, since neither the experimental set-up nor the flow of the transport current is sensitive to such defects in the ceramic. In addition to the homogeneity evaluation, the critical current of a homogeneous sample can be estimated by comparing computed and measured flux density profiles. The I¢ value thus obtained is coupled with an extremely low electric-field criterion, E--- 10 - 9 V / m , due to relaxation of Bs(t). Such a direct estimation procedure for 1¢ is valid, because the contribution of intragrain screening currents to the total sample magnetization is small at 77 K.

3.2. Measured flux density profiles Typical lengthwise remanence flux density profiles of samples A-D measured after Be = 100 mT are shown in Fig. 3. Profile A is rather smoothly varying and has only one broad minimum at x - - 2 mm. The shape of 1.2"~a)

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the minimum suggests a defected area extending over about 2 mm, see Figs. 2(d) and (e). Profile B is of the same magnitude as A, but B has a strongly undulating character with relatively sharp minima and maxima. This behavior corresponds to several localized defects like those in Fig. 2(c). Flux density profile C is not that high as A and has two pronounced minima where the remanence signal goes almost to zero indicating a strong reduction in the local Ic. Profile D is clearly the highest of the profiles shown here, Bu --- 1 mT, and has no clear minima, a shape very similar to that in Fig. 2(a) for an ideally homogeneous sample. The Ic value of the homogeneous sample D can be estimated as suggested in Section 3.1. From the tape dimensions and J~ used in Fig. 2(a) we find I~=J~×tXw=50 A~ mm 2 X 0.05 mm X 2.0 mm = 5 A, which is in reasonable agreement with resistively measured Ic values for sample set D, Table 1. However, one must allow for the large difference in electric-field criteria used in inductive and resistive measurement techniques ( 10-9 V/m v e r s u s 10 - 4 V/m) as well as variations of the sample properties along the length of the tape. One sample section from sample set D was measured both inductively and resistively in order to compare the Ic values obtained by different methods. The experiment showed that inductively measured I¢ = 5 A coupled with E = 10 -9 V / m corresponded approximately to resistively measured I~ = 10 A coupled with E = 10 -4 V / m in sample D. The overall signal levels ofBs~ = 0.4 mT in Figs. 3 (a) and (b) and Bu ~--0.2 mT in Fig. 3 ( c ) correspond to Ic = 2 A and I~ = 1 A, respectively, coupled with E = 10 - 9 V/m. Flux density profiles across the tape were also measured in order to study the integrity of the ceramic across the tape. The measured profiles of all samples A-D were V shaped and symmetrical verifying that screening currents were circulating in the homogeneous parts of a sample as shown in Fig. 2(a); see Ref. [ 10]. The profile data revealed no major defects parallel to the long axis of the tape.

3.3. SEM micrograph data

^

-5

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5

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Fig. 3. Typicalremanenceflux densityprofilesof sample sections fromsamplesets A-D in Figs. (a-d), respectively.

In order to identify differences in the structure of tapes A-D we carried out a systematic SEM study of lengthwise and transverse cross-sections of the same tape sections studied using Hall sensor magnetometry. First, comparing the differences between rolled, A, B,

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Fig. 4. SEM micrograph(lengthwisecross-section) showingone of several transversalcracks foundin sampleB. The disintegrationof the Ag-ceramicinterfaceis due to samplepreparation. and pressed C, D, tapes, we found that in the rolled tapes the sausaging effect was quite pronounced, whereas the pressed tapes had a smooth Ag-ceramic interface. There was also significant difference in the porosity of samples A, B and C, D. Roiled samples A, B had several regions where Bi-2223 ceramic had little or no texture, whereas in pressed samples C, D the texture was rather uniform throughout the sample volume. In twice rolled tape B the most outstanding defects were several transversal cracks initiated at necking regions and extending deep into the ceramic, Fig. 4. Such defects were not observed in samples A, C, D. In the samples studied, we did not find clear evidence for cracks in lengthwise direction in neither rolled nor pressed samples. Secondly, comparing the effect of repeated rolling and pressing we found differences in the achieved enhancement in the density and homogeneity of the ceramic. The second rolling did not essentially improve the density, texture or the stoichiometric homogeneity of the ceramic, but introduced cracks, which were not healed during the subsequent heat treatment. The second pressing did not introduce harmful side effects, but did not Show any remarkable improvement of the tape microstructure either. The most notable difference between samples C and D was, that there were not so many non-stoichiometric Cu and Sr rich regions in sample D as there were in sample C. In general, the cores of pressed tapes appeared to be more dense and homogeneous than the cores of rolled tapes.

Having discussed the experimental data, we must consider whether the presented features are typical of the sample-preparation procedures or typical of short sample sections only. Sample properties in one batch with nominally similar processing parameters varied significantly, which was obvious from the resistively measured I~ values, Table 1, and profile measurements repeated on different sample sections. However, the general features of flux density profiles reported here were the same for all sample sections in the sets A-D and there was a good correlation between resistively and inductively determined Ic values. In addition, Parrel et al. have reported similar results on the structure of Bi-2223 ceramic in rolled and pressed tapes based on data from resistive Ic and microhardness experiments [ 11 ]. Hence we believe that the general conclusions below based on the data are justifiable.

4. Conclusions

As shown in Section 3.1, numerical results confirm that magnetic flux density profiles measured along the length of a monofilamentary tape are sensitive to transversal cracks, non-superconducting regions and necking of the superconducting core. In the case of more complex defect structures than those shown in Fig. 2 scanning of the whole sample surface and measurement of more components of Bs is required, if the details of a defect and its effect on Ic are to be evaluated. The advantages of Hall sensor magnetometry are the contactless nature of the inductive method, the easiness of sample preparation and the extreme sensitivity of a screening current distribution in a tape to inhomogeneities or geometrical irregularities present in the ceramic. Furthermore, in the case of a homogeneous sample it is possible to give an estimate of Ic corresponding to an extremely small electric-field criterion at 77 K by comparing measured and computed flux density profiles. One of the disadvantages of the method is that the profiles do not reveal a nonuniform current density distribution over the cross-section of a tape, e.g. a larger Jc close to the Ag-ceramic interface. In addition, the interpretation of flux density profiles is complicated by the uncertainty of the sample features responsible for the observed irregularities in the measured profiles. In the present work the latter issue was

M. Lahtinen et al. / Physica C 244 (1995) 115-122

further investigated in the detailed SEM study of transversal and longitudinal tape cross-sections. The SEM study revealed clear differences in the structure of the ceramic cores after repeated rolling and pressing cycles. In the twice rolled tape B there was a noticeable sausaging effect and several transversal cracks, Fig. 4, which was in agreement with the powder flow theory by Han and Freltoft [ 3]. Such cracks were not observed in the other three sample sets subject to the same handling procedures during sample characterization. The heights of flux density profiles A, B, Figs. 3(a) and (b), were approximately the same, but profile B had a strongly undulating character demonstrating the presence of localized defects, cracks and sausaging in the ceramic. The data above indicate that the second rolling-annealing step has not essentially affected J~ of the ceramic but has introduced mechanical instabilities in the tape. The effect of repeated rolling on I~ of the tape can be estimated from data in Table 1. The I~ values of samples B are slightly lower than those of samples A. Therefore we believe, that in sample B, I~ is limited by transversal cracks and sausaging, although Jc in the homogeneous parts of sample B may be relatively high. The tapes prepared by pressing-annealing cycles had a uniform texture and smooth Ag-ceramic interface. There were few second phase regions in sample D, which was in contrast to samples A, B, C. We believe that the dense ceramic achievable by the uniaxial pressing process promotes a faster diffusion of atoms during annealing. Hence pressing is likely to result in Bi-2223 material with better stoichiometry than rolling. Although there were no clear differences in SEM micrographs of samples C and D, the flux density profiles had different overall signal levels, Figs. 3(c) and (d), sample D having the higher profile and hence higher I~. The beneficial effect of the second pressingannealing step on I~ was also verified by the resistive I~ data, Table 1. We believe that the high I~ values of samples D are related to two advantageous effects taking place during the second pressing-annealing step: enhancement in the quality of grain-to-grain Josephson junctions and an increase in the effective current-carrying cross-section of the tape due to an increase in the amount of stoichiometric material. Near transversal cracks the transport current has to flow in the Ag sheath, when the local I~ value has been exceeded. This, in turn, strongly affects the resistively

121

measured current-voltage characteristics. The variations in current-voltage characteristics due to macroscopic defects partially explain the large variations of Ic values in samples with nominally the same preparation parameters, see Table 1. Another consequence of the sample inhomogeneity is, that the inductively estimated Ic values in Section 3.2 cannot be used to predict resistively measured Ic values of samples A-C with good accuracy. In the case of an inhomogeneous sample, the inductive-measurement technique leads to unpredictable screening current paths in the sample volume, which essentially complicates the determination of I¢ from inductive-measurement data [ 12]. Sample inhomogeneities and current sharing between the Ag sheath and the ceramic may also contribute to the fact that AC loss measurements on typical Bi-2223/ Ag composites cannot be interpreted well using existing models for AC losses in low-temperature superconductors [ 13 ]. In summary, we have shown by comparison to numerical results, resistive I~ measurements and SEM micrographs, how Hall sensor magnetometry reveals sample inhomogeneities along the length of a Bi-2223/ Ag monofilamentary tape and gives an estimate of I~ of a sample at 77 K. In the samples studied, we found that repeated rolling-annealing treatments of tapes lead to sausaging and transversal cracking of the ceramic core limiting Ic. Repeated pressing-annealing cycles resulted in little change in the sample texture observed in SEM micrographs but enhanced Ic. The enhancement was speculated to be due to almost complete transformation of the starting material into Bi-2223 phase and the improvement of Josephson coupling between adjacent grains during the second pressing-annealing step.

Acknowledgements This work is part of the joint project Nordic Program on Applied Superconductivity (NORPAS). We acknowledge financial support by Foundation of Technology in Finland (ML), the Academy of Finland (JP), and Technology Development Centre of Finland. We thank Peter Kottman for carrying out some of the inductive and resistive measurements.

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