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Critical current densities of superconducting MgB2 tapes prepared on the base of mechanically alloyed precursors
Physica C 406 (2004) 121–130 www.elsevier.com/locate/physc
Critical current densities of superconducting MgB2 tapes prepared on the base of mechanically alloyed precursors C. Fischer, W. H€ aßler *, C. Rodig, O. Perner, G. Behr, M. Schubert, K. Nenkov, J. Eckert 1, B. Holzapfel, L. Schultz Institut f€ur Metallische Werkstoffe, IFW Dresden, Postfach 27 00 16, D-01171 Dresden, Germany Received 19 December 2003; received in revised form 20 February 2004; accepted 10 March 2004
Abstract Cu- and Fe-cladded monofilamentary tapes and 19-filamentary tapes with Fe matrix have been prepared by the powder-in-tube (PIT) method using mechanically alloyed precursor powders consisting of the constituents Mg, B, and MgB2 . Despite of the low Tc values of about 31 K for monofilamentary tapes with Fe sheath maximum critical current densities (Jc ) of 29 and 6 kA/cm2 at 4.2 K in external magnetic fields of 7.5 and 10 T, respectively, were achieved. The maximum Jc value of 18.5 kA/cm2 at 20 K in a field of 2 T was obtained for these conductors. The heat treatment of the monofilamentary tapes under uniaxial pressure of 0.5 GPa led to enhanced Jc values compared to those of tapes annealed under normal pressure. Up to 27 and 11 kA/cm2 were achieved at 4.2 K in external fields of 8 and 10 T, respectively, for samples annealed under pressure. For the 19-filamentary tape Jc values up to 35 and 9 kA/cm2 in fields of 7.5 and 10 T, respectively, were measured at 4.2 K. Investigation of the microstructure by optical microscopy, SEM/ WDX and XRD have shown that the high Jc values may be mainly due to the remarkably small grain size of the MgB2 phase and defects particularly precipitates of magnesium oxide. Jc of the Cu-cladded tapes is one order of magnitude below that of Fe-cladded ones. The main reason for this discrepancy is the higher porosity of the superconducting cores with Cu sheath compared to that with Fe sheath. At higher sintering temperatures strong interactions between the Cu sheath and constituents of the precursor contribute to the suppression of the superconducting properties of Cusheathed conductors. 2004 Elsevier B.V. All rights reserved. PACS: 74.62.BF; 74.70.Ad Keywords: MgB2 -tapes; Mechanical alloying; Tc ; Jc ; Microstructure
1. Introduction * Corresponding author. Tel.: +49-351-4659-662; fax: +49351-4659-541. E-mail address: [email protected] (W. H€aßler). 1 Present address: TU Darmstadt, FB 11 Material- und Geowissenschaften, Petersenstraße 23, D-64287 Darmstadt, Germany.
Since the discovery of superconductivity at 39 K in intermetallic magnesium diboride [1] several research groups have dealt with the development of a low-cost MgB2 conductor using the ‘‘powderin tube’’ (PIT) technique [2–9]. There are three
0921-4534/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2004.03.242
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methods reported in the literature. One (ex situ PIT technique) involves direct filling of metallic tubes with commercial MgB2 powder and then drawing and rolling into tapes followed by sintering at relative high temperatures (800–1000 C). An alternative approach (in situ PIT technique) is characterized by filling the metallic tubes with elemental Mg and B powders and subsequent drawing and rolling into tapes followed by heat treatment, during which the elements react to form MgB2 . This reaction can be performed at essentially lower temperatures (600–700 C) than that required for sintering of MgB2 . This is beneficial to circumvent or reduce undesirable interactions between Mg and B on one hand and the sheath material on the other hand. Both methods facilitate the preparation of tapes with high critical current densities. For example, critical current densities of 10 kA/cm2 at 7.5 T [9] and 9 kA/cm2 at 8 T [10] have been achieved at 4.2 K by ex situ PIT or in situ PIT technique, respectively, using an undoped precursor. Recently, Dou et al. [10] were able to increase Jc by doping of the Mg + B powder mixture with nanometer sized SiC particles. They achieved record values of about 40 kA/cm2 at 5 K and 30 kA/cm2 at 20 K in external fields of 8 and 4 T, respectively. The third approach of the PIT process characterized by the use of a partially reacted precursor was reported recently [11,12]. Partial reaction of Mg with B was induced by mechanical alloying of Mg + B powder mixtures resulting in a precursor powder, which consisted of the constituents Mg, B and MgB2 with nanocrystalline grain size (about 40–100 nm). Jc values up to 22 and 7 kA/cm2 in external magnetic fields of 7.5 and 10 T, respectively, were achieved at 4.2 K for monofilamentary Fe-sheathed tapes [12]. In this paper results of studies about the microstructure, chemical and phase composition of both Cu- and Fe-sheathed tapes prepared using mechanically alloyed pow-
ders are presented. Furthermore the possibilities to enhance Jc by hot pressing and variation of the chemical precursor composition, respectively, have been tested. The preparation of a 19-filamentary conductor is also described.
2. Experimental The synthesis of the precursor powders by mechanical alloying has been described in detail in recent papers [13–15]. Two precursors of different chemical compositions, indicated in Table 1 were used for the preparation of tapes: precursor ‘‘A’’ and ‘‘B’’ for monofilamentary tapes and precursor ‘‘B’’ for a 19-filamentary tape too. The contents of the elements Mg, B and W within the precursors were determined by inductively-coupled plasma– optical emission spectroscopy (ICP–OES) analysis. Carrier gas hot extraction analysis and the combustion method were used to analyse the contents of oxygen and carbon, respectively. Other metallic impurities like Si, K, Mn and Ca were detected by mass spectroscopy. However, the concentration of each of these elements is <0.05 wt.%. They were probably introduced by the milling process similar like the impurities W and C too. The thermal behaviour of the mechanically alloyed precursor powder was investigated by differential scanning calorimetry (DSC) under argon atmosphere with a heating rate of 20 K/min using a Netzsch DSC404. As the first step of the processing of tapes rods of the precursors were prepared by cold isostatic pressing of the powder under a pressure of 350 MPa. The rods were filled into tubes with 10 mm outer and 6 mm inner diameter, which were deformed by shape rolling, drawing and flat rolling to monofilamentary tapes of about 0.35 mm thickness and 3.5 mm width. For the preparation of a multifilamentary tape with Fe matrix 19 monofilamentary Fe-sheathed wires of 1.5 mm
Table 1 Chemical compositions of the precursors Precursor
Mg (wt.%)
B (wt.%)
O (wt.%)
W (wt.%)
C (wt.%)
A B
50.85 55.68
46.97 42.90
1.32 0.73
0.38 0.49
0.14 0.15
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diameter were bundled in a Fe tube of 10 mm outer and 7 mm inner diameter. This composite was deformed in the same manner as the monofilamentary conductors. After completion of the cold deformation 35 mm long pieces were cut off and placed in quartz containers, which were evacuated and then sealed under 18 kPa Ar of 99.999 vol% purity. The embedded tapes were subjected to heat treatment at temperatures between 770 and 970 K for 3–10 h under normal pressure aiming for the formation of MgB2 and sintering of the superconducting core. A few samples were heat treated under uniaxial pressure of about 0.5 GPa for 10–40 min in Ar-atmosphere using inconel-718 pressing tools. Transport critical currents (Jc ) were determined by the standard four probe method using 1 lV/cm as criterion. The measurements were conducted at 4.2 and 20 K, respectively, in fields up to 14 T applied parallel to the main plane of the tapes. Jc was related to the core area of the tape. The area of the core and the overall area of the tape were determined micro-optically from a transverse cross-section of the tape using a computerized image analyser. Typical values of the filling factor (core area:overall area) of monofilamentary tapes were in the range of 0.31–0.37. The filling factor of the 19-filamentary tape varied in the range of 0.095 and 0.12 along the length of the tape. Scanning electron microscopy (SEM) images of polished cross-sections were obtained by means of Jeol SEM JSM.6400 and SEM GEMINI 1530 (LEO Oberkochen) microscopes. Wavelength-dispersive X-ray microanalyses (WDX) were performed using an electron microprobe and SEM XL 30 (Philips). In order to determine the phase composition, crystal size and internal stresses of the superconducting core of annealed Fe-cladded tapes the Fe sheath was peeled off from one side of the tapes and the free surface of the core was investigated by X-ray diffraction including Rietveld refinement of the resulting X-ray patterns.
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Fig. 1. Transverse cross-sections of an unannealed Fe-cladded monofilamentary tape (a) and 19-filamentary tape with Fe matrix (b) and a longitudinal cross-section of the 19-filamentary tape (c).
mentary tape with Fe matrix, respectively, and the longitudinal cross-section of the 19-filamentary tape before annealing. Fig. 1c reveals the irregularity of the filament thickness due to agglomerates of the precursor powder resulting in the variation of the filling factor along the tape length. In order to estimate the favourable temperature range for the annealing of the tapes with the aim to convert the constituents of the precursor into MgB2 the precursor powder was investigated by differential scanning calorimetry (DSC). From the curve of the precursor (Fig. 2) one can conclude
3. Results and discussion Fig. 1 shows the transverse cross-sections of a Fe-cladded monofilamentary tape and a 19-fila-
Fig. 2. Differential scanning calorimetry (DSC) plot of the asmilled precursor ‘‘B’’ which was measured starting at ambient temperature with a heating rate of 20 K/min.
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that the reactions within the precursor start at about 750 K and are completed at about 900 K. Therefore the tapes were annealed in the temperature range of 770–970 K for different times under normal pressure and under an uniaxial pressure of about 0.5 GPa, respectively. Fig. 3 demonstrates the dependence of the critical current densities on the magnetic field (l0 H ) of Cu- and Fe-cladded monofilamentary tapes, respectively, and of the 19-filamentary tape all annealed under normal pressure. The critical current densities of Fe-sheathed tapes are significantly higher compared to those of Cu-cladded ones. Fig. 4, showing the cross-sections of tapes with different sheath materials, suggests that one reason for the lower Jc values of the Cu-cladded tapes is the higher porosity of the superconducting core compared to that of the Fe-cladded conductors. Obviously, the Fe sheath promotes a stronger MgB2 grain connectivity and probably a more effective transition of the current from one grain to the other than the Cu sheath can provide. This fact may be due to the higher mechanical strength of Fe in comparison to Cu, which leads to the transition of a higher load onto the Fe-cladded cores and, hence, to stronger densification by rolling
Fig. 3. Critical current density at 4.2 K as a function of external magnetic field for Cu/MgB2 and Fe/MgB2 monofilamentary tapes prepared from different precursors ‘‘A’’ and ‘‘B’’, respectively, and annealed under different conditions; results obtained for the 19-filamentary tape are also included (the field was applied parallel to the main plane of the tapes).
Fig. 4. Transverse cross-sections of a Cu/MgB2 monofilamentary tape (a) annealed at 870 K for 3 h and Fe/MgB2 monofilamentary tape (b) annealed at 770 K for 3 h.
prior to the annealing. The higher core density before annealing, in turn, results in a higher density of the core after sintering. Another reason for the lower Jc values of Cu-cladded tapes compared to Fe-cladded ones might be the high diffusivity of copper into the precursor core and the high reactivity of copper with the constituents of the precursor. This leads to the formation of a MgCu2 layer at the sheath/core boundary, and also to diffusion reactions in the interior of the core restricting the formation of the superconducting phase as was shown by SEM/WDX studies (Fig. 5). The Cu-rich MgCu phase forms bright channels within the microstructure of the core, while the dark areas are enriched in B exhibiting a chemical composition close to that of the MgB4 phase. A reaction layer consisting of Fe, Mg and B has also been observed at the core/sheath boundary of Fe-sheathed multifilamentary tapes (Fig. 6). The thickness of this layer of a tape annealed at 870 K
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Fig. 7. WDX linescan crossing the sheath/core boundary of a cross-section of the Fe/MgB2 tape annealed at 970 K for 3 h; the distance between the spots equals 2 lm. Fig. 5. SEM image obtained with secondary electrons of the detail of a cross-section and WDX results from the Cu/MgB2 tape annealed at 970 K for 3 h.
Fig. 6. SEM cross-sectional image obtained with secondary electrons of the Fe/MgB2 19-filamentary tape annealed at 870 K for 3 h.
is about 4 lm, whereas at a annealing temperature of 770 K the thickness is only 0.5 lm. Immediately at the filament/sheath interface of the 870 K-tape (Fig. 6) the concentration of the layer is 27 at.% Mg/28 at.% Fe/ 37 at.% B, in the middle of the reaction layer it is 8 at.% Mg/58 at.% Fe/31 at.% B. However, in contrast to Cu the Fe does not diffuse into the interior of the precursor material as was
proved by WDX-analysis using a microprobe (Fig. 7). It could be shown for the tape annealed at 770 K for 3 h that the Fe content of the core area only a few micrometer away from the sheath/core boundary is <0.1 wt.%. It is seen from Fig. 3 and the data of Table 2 that the higher Mg concentration of the precursor ‘‘B’’ compared to precursor ‘‘A’’ does not result in a significant enhancement of Jc of the tapes in contrast to tapes prepared from commercial MgB2 powders with and without an excess of Mg [16]. Both, Fig. 3 and Table 2 also show that Jc of the multifilamentary tape is higher than those of the monofilamentary ones.
Table 2 Comparison of characteristic Jc values at 4.2 K in fields of 8 and 10 T for tapes prepared from different precursors ‘‘A’’ and ‘‘B’’ Composite
Monofil.Fe/A Monofil.Fe/A Monofil. Fe/A
Monofil. Fe/B Monofil. Fe/B 19-fil. Fe/B
Heat treatment 770 K/3 h 870 K/3 h Hot pressed: 0.5 Gpa 870 K/10 min 770 K/3 h 870 K/3 h 770 K/3 h
Critical current density at 4.2 K (kA/cm2 ) 8T
10 T
17.5 21.5 26.7
6.6 4.7 11.0
20.2 21.5 27.7
6.3 6.2 9.4
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Fig. 8. Critical current density at 4.2 K as a function of the external magnetic field for Fe/MgB2 monofilamentary tapes prepared from precursor ‘‘A’’ and heat treated under pressure of 0.5 GPa; in comparison with tapes annealed under ambient pressure the best curve of the Fe/‘‘A’’ tapes shown in Fig. 3 is included in this figure.
In Fig. 8 the Jc ðl0 H Þ-curves of tapes with precursor ‘‘A’’ annealed under enhanced pressure of 0.5 GPa are shown. To compare these curves with Jc values of the tapes annealed under ambient pressure the best curve of the Fe/‘‘A’’ tapes shown in Fig. 3 is included in Fig. 8. A comparison of the Jc values is also given in Table 2. It is remarkable that despite of the short reaction times the critical current densities of tapes heat treated under pressure at appropriate temperatures exceed those of tapes annealed under ambient pressure. The reason seems to be the excellent density of the cores of the hot pressed conductors. It should be noticed that the cross-section of the sample heat-treated under pressure shows also a reaction layer at the sheath/core boundary in contrast to the sample annealed at the same temperature under normal pressure but for a longer time. This suggests that the reaction kinetics have been altered by pressure. Fig. 9 represents the Jc ðl0 H Þ curve of the Fe/B sample annealed at 770 K for 3 h shown in Fig. 3 and the Jc ðl0 H Þ curve at 20 K for the same sample in comparison with curves of doped and undoped tapes, which are to our knowledge the best values published so far [10,17]. At 4.2 K the Jc values of
Fig. 9. Comparison of critical current densities achieved by our group (this work) for MgB2 tapes with the highest Jc values published so far for doped [10] and undoped tapes [17].
our tapes are exceeded only by those of tapes prepared from a precursor doped with nanometer sized SiC particles [10]. The reason for the poor critical current density of our tape at 20 K is the low critical temperature (Tc onset ¼ 34:8 K) of the precursor ‘‘B’’ and consequently the low Tc onset ¼ 31 K of the tape. An improvement of Tc can probably be achieved by managing the impurities introduced by the high energy milling process. It is well known from the literature that large changes of Tc can be caused by substitution of Mg or B by third elements [18–23]. As the data of Table 1 show, particular attention must be paid to the impurity elements oxygen and carbon. While intragrain precipitations of MgO may act as flux pinning centres [18] the incorporation of O into the MgB2 lattice strongly lowers Tc [18–20]. Also C can substitute B in MgB2 thus lowering Tc [21]. In order to discover the reasons for the excellent Jc values at 4.2 K of our tapes the phase composition and microstructure of the superconducting cores of two tapes exhibiting high Jc values after annealing at 770 K for 3 and 6 h, respectively, and of one tape with Jc of only 100 A/cm2 after annealing at 970 K for 3 h have been investigated. Table 3 shows the results obtained by XRD including Rietveld refinement of the resulting X-ray patterns for two tapes. The data of Table 3 give no explanation of the very different Jc values of both samples. A possible reason will be
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Table 3 Data obtained by XRD for two samples annealed at 770 K for 6 h and 970 K for 3 h, respectively Data
Sample annealed at 770 K for 6 h
Sample annealed at 970 K for 3 h
MgB2 (wt.%) MgO (wt.%) WC (wt.%) Size of coherent scattering domains (nm) Lattice parameter a (nm) Lattice parameter c (nm) Lattice strain
85.4 14.1 0.5 29 0.3083 0.3524 1 · 102
86.1 13.4 0.5 54 0.3082 0.3526 7 · 103
discussed in connection with the microstructure (Fig. 13) below. To investigate the chemical composition on the micrometer scale SEM/WDX analyses were performed on fifteen spots chosen by chance on the cross-section of the core of a sample annealed at 970 K for 3 h. The results presented in Fig. 10 reveal an inhomogeneous distribution of the main chemical elements within the superconducting core. The mean values of the element concentrations were calculated to be 31.0 at.% Mg, 61.2 at.% B and 7.6 at.% O. From the data of Table 1 the chemical composition of the superconducting core of this sample was calculated to be 35.1 at.% Mg, 59.7 at.% B and 5.3 at.% O assuming that all the oxygen reacted with Mg forming MgO. The mole fractions of Mg and O determined by SEM/WDX
exceed the corresponding values calculated from the XRD-data. Therefore it is of interest to search in which form the oxygen determined by SEM/ WDX is incorporated into the microstructure of the conductor core. Fig. 11 shows the boron– oxygen concentration correlation obtained from the SEM/WDX-data of Fig. 10. In this figure are also included the boron–oxygen concentration correlations for a mixture of MgB2 + MgO and for MgB2 in which oxygen is partially substituted for B (MgB2 + MgBO). It is clearly seen that the analysed oxygen/boron ratios are significantly higher than the calculated ones for the MgB2 + MgO mixture. The partial substitution of O for B in MgB2 can contribute to this fact but not fully explain it. Another reason might be the chemisorbtion of oxygen at the surface of the
Fig. 10. SEM/WDX results from 15 spots chosen by chance on the cross-section of the core of a monofilamentary Fe/MgB2 tape annealed at 970 K for 3 h.
Fig. 11. Boron–oxygen concentration correlation derived from the data of Fig. 10.
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cross-section of the sample caused by polishing in ambient atmosphere. The partial substitution of oxygen for boron in MgB2 might be responsible for the low values of Tc onset < 34 K measured on these tapes [12]. Fig. 12 showing a detail of the cross-section of the tape annealed at 970 K for 3 h also reveals the inhomogeneous character of the microstructure of the conductor core. This microstructure consisting of dark areas <200 nm embedded in a bright matrix is typical also for tapes with high Jc values after annealing at 770 K. Of course, the fluctuation of the chemical composition shown in Fig. 10 does not reflect quantitatively the difference of the chemical compositions between the dark and bright areas because the SEM/WDX method yields a resolution of only about 1 lm for the chemical analysis. Fig. 13 shows SEM pictures of cross-sections of the tapes annealed for 3 h at 770 and 970 K, respectively, revealing the crystal morphology of the superconducting phase. The size of the recognizable primary grains is of the same order (<100 nm) as the grains of the mechanically alloyed precursor used as starting material [13]. The primary grains of the tapes are partially sintered together forming dense areas which are of larger dimensions in the tape annealed at the higher temperature. This result is supported by the XRDdata of Table 3. Assuming that the size of the
Fig. 12. SEM-image obtained with secondary electrons showing the cross-sectional detail of a tape annealed at 970 K for 3 h.
Fig. 13. SEM image obtained with secondary electrons taken from cross-sectional details of Fe/MgB2 tapes annealed at 770 K (a) and 970 K (b) for 3 h.
coherent scattering domains is determined by large-angle grain boundaries these XRD-data show that the main crystal size of a tape annealed at 970 K is about twice that of a tape annealed at 770 K. However, the difference in the crystal size can not explain the large difference in critical currents. It should be recalled that the critical current density of the tape annealed at 970 K is of the order of only 100 A/cm2 at 4.2 K in self field, while Jc of the tape annealed at 770 K for 3 h reaches the value of 17.5 kA/cm2 at 4.2 K and 8 T (Table 2). The most significant difference of the microstructures shown in Fig. 13a and b, respectively, is that Fig. 13b exhibits areas which suggest that partial melting of the conductor core occurred
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during annealing at 970 K. This assumption is supported by the DSC-curve in Fig. 2 which shows a thermal effect near 870 K. It is clearly seen in Fig. 13b that the melting and recrystallization process leads to deep cavities and may be also to microcracks which are assumed to be the main reason for the poor critical current density.
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Acknowledgements The authors would like to thank I. B€acher, S. Pichl and H. Wendrock for SEM images and carrying out electron probe microanalyses.
References 4. Conclusions Mechanically alloyed Mg + 2B powder mixtures are appropriate precursors for manufacturing of MgB2 conductors by the PIT method. Fe-sheathed monofilamentary tapes and multifilamentary tapes with Fe matrix prepared by using such precursors revealed Jc values up to 29 and 35 kA/cm2 , respectively, at 4.2 K in an external magnetic field of 7.5 T. These values are exceeded only by the best ones published so far for tapes prepared from a SiC doped precursor [10]. The high density of grain boundaries, which can act as pinning centres within the superconducting phase seems to be the main reason for these excellent Jc values. MgO particles of nanometer size may also contribute to the flux pinning [18–20]. Furthermore, the nature of the inhomogeneity of the superconducting phase shown in Fig. 12 has to be clarified and to what extent this may influence the flux pinning. Fe is more suitable as sheath material than Cu due to the higher reactivity of Cu interacting with the precursor and because of the higher mechanical strength of Fe in comparison to Cu. This results in a higher density of the Fesheathed core upon rolling prior to heat treatment, which leads to an improved connectivity of the superconducting grains of Fe-cladded tapes after annealing. An increase of Tc of the precursor powder is expected to enhance Jc of the tapes at 20 K appreciably. This may be realized by modifying the milling process aiming to avoid or at least to minimize the introduction of harmful impurities into the precursor material. Further systematic variation of the Mg/B concentration ratio of the precursor should also contribute to the improvement of the superconducting properties.
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Critical current densities of superconducting MgB2 tapes prepared on the base of mechanically alloyed precursors C. Fischer, W. H€ aßler *, C. Rodig, O. Perner, G. Behr, M. Schubert, K. Nenkov, J. Eckert 1, B. Holzapfel, L. Schultz Institut f€ur Metallische Werkstoffe, IFW Dresden, Postfach 27 00 16, D-01171 Dresden, Germany Received 19 December 2003; received in revised form 20 February 2004; accepted 10 March 2004
Abstract Cu- and Fe-cladded monofilamentary tapes and 19-filamentary tapes with Fe matrix have been prepared by the powder-in-tube (PIT) method using mechanically alloyed precursor powders consisting of the constituents Mg, B, and MgB2 . Despite of the low Tc values of about 31 K for monofilamentary tapes with Fe sheath maximum critical current densities (Jc ) of 29 and 6 kA/cm2 at 4.2 K in external magnetic fields of 7.5 and 10 T, respectively, were achieved. The maximum Jc value of 18.5 kA/cm2 at 20 K in a field of 2 T was obtained for these conductors. The heat treatment of the monofilamentary tapes under uniaxial pressure of 0.5 GPa led to enhanced Jc values compared to those of tapes annealed under normal pressure. Up to 27 and 11 kA/cm2 were achieved at 4.2 K in external fields of 8 and 10 T, respectively, for samples annealed under pressure. For the 19-filamentary tape Jc values up to 35 and 9 kA/cm2 in fields of 7.5 and 10 T, respectively, were measured at 4.2 K. Investigation of the microstructure by optical microscopy, SEM/ WDX and XRD have shown that the high Jc values may be mainly due to the remarkably small grain size of the MgB2 phase and defects particularly precipitates of magnesium oxide. Jc of the Cu-cladded tapes is one order of magnitude below that of Fe-cladded ones. The main reason for this discrepancy is the higher porosity of the superconducting cores with Cu sheath compared to that with Fe sheath. At higher sintering temperatures strong interactions between the Cu sheath and constituents of the precursor contribute to the suppression of the superconducting properties of Cusheathed conductors. 2004 Elsevier B.V. All rights reserved. PACS: 74.62.BF; 74.70.Ad Keywords: MgB2 -tapes; Mechanical alloying; Tc ; Jc ; Microstructure
1. Introduction * Corresponding author. Tel.: +49-351-4659-662; fax: +49351-4659-541. E-mail address: [email protected] (W. H€aßler). 1 Present address: TU Darmstadt, FB 11 Material- und Geowissenschaften, Petersenstraße 23, D-64287 Darmstadt, Germany.
Since the discovery of superconductivity at 39 K in intermetallic magnesium diboride [1] several research groups have dealt with the development of a low-cost MgB2 conductor using the ‘‘powderin tube’’ (PIT) technique [2–9]. There are three
0921-4534/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2004.03.242
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methods reported in the literature. One (ex situ PIT technique) involves direct filling of metallic tubes with commercial MgB2 powder and then drawing and rolling into tapes followed by sintering at relative high temperatures (800–1000 C). An alternative approach (in situ PIT technique) is characterized by filling the metallic tubes with elemental Mg and B powders and subsequent drawing and rolling into tapes followed by heat treatment, during which the elements react to form MgB2 . This reaction can be performed at essentially lower temperatures (600–700 C) than that required for sintering of MgB2 . This is beneficial to circumvent or reduce undesirable interactions between Mg and B on one hand and the sheath material on the other hand. Both methods facilitate the preparation of tapes with high critical current densities. For example, critical current densities of 10 kA/cm2 at 7.5 T [9] and 9 kA/cm2 at 8 T [10] have been achieved at 4.2 K by ex situ PIT or in situ PIT technique, respectively, using an undoped precursor. Recently, Dou et al. [10] were able to increase Jc by doping of the Mg + B powder mixture with nanometer sized SiC particles. They achieved record values of about 40 kA/cm2 at 5 K and 30 kA/cm2 at 20 K in external fields of 8 and 4 T, respectively. The third approach of the PIT process characterized by the use of a partially reacted precursor was reported recently [11,12]. Partial reaction of Mg with B was induced by mechanical alloying of Mg + B powder mixtures resulting in a precursor powder, which consisted of the constituents Mg, B and MgB2 with nanocrystalline grain size (about 40–100 nm). Jc values up to 22 and 7 kA/cm2 in external magnetic fields of 7.5 and 10 T, respectively, were achieved at 4.2 K for monofilamentary Fe-sheathed tapes [12]. In this paper results of studies about the microstructure, chemical and phase composition of both Cu- and Fe-sheathed tapes prepared using mechanically alloyed pow-
ders are presented. Furthermore the possibilities to enhance Jc by hot pressing and variation of the chemical precursor composition, respectively, have been tested. The preparation of a 19-filamentary conductor is also described.
2. Experimental The synthesis of the precursor powders by mechanical alloying has been described in detail in recent papers [13–15]. Two precursors of different chemical compositions, indicated in Table 1 were used for the preparation of tapes: precursor ‘‘A’’ and ‘‘B’’ for monofilamentary tapes and precursor ‘‘B’’ for a 19-filamentary tape too. The contents of the elements Mg, B and W within the precursors were determined by inductively-coupled plasma– optical emission spectroscopy (ICP–OES) analysis. Carrier gas hot extraction analysis and the combustion method were used to analyse the contents of oxygen and carbon, respectively. Other metallic impurities like Si, K, Mn and Ca were detected by mass spectroscopy. However, the concentration of each of these elements is <0.05 wt.%. They were probably introduced by the milling process similar like the impurities W and C too. The thermal behaviour of the mechanically alloyed precursor powder was investigated by differential scanning calorimetry (DSC) under argon atmosphere with a heating rate of 20 K/min using a Netzsch DSC404. As the first step of the processing of tapes rods of the precursors were prepared by cold isostatic pressing of the powder under a pressure of 350 MPa. The rods were filled into tubes with 10 mm outer and 6 mm inner diameter, which were deformed by shape rolling, drawing and flat rolling to monofilamentary tapes of about 0.35 mm thickness and 3.5 mm width. For the preparation of a multifilamentary tape with Fe matrix 19 monofilamentary Fe-sheathed wires of 1.5 mm
Table 1 Chemical compositions of the precursors Precursor
Mg (wt.%)
B (wt.%)
O (wt.%)
W (wt.%)
C (wt.%)
A B
50.85 55.68
46.97 42.90
1.32 0.73
0.38 0.49
0.14 0.15
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diameter were bundled in a Fe tube of 10 mm outer and 7 mm inner diameter. This composite was deformed in the same manner as the monofilamentary conductors. After completion of the cold deformation 35 mm long pieces were cut off and placed in quartz containers, which were evacuated and then sealed under 18 kPa Ar of 99.999 vol% purity. The embedded tapes were subjected to heat treatment at temperatures between 770 and 970 K for 3–10 h under normal pressure aiming for the formation of MgB2 and sintering of the superconducting core. A few samples were heat treated under uniaxial pressure of about 0.5 GPa for 10–40 min in Ar-atmosphere using inconel-718 pressing tools. Transport critical currents (Jc ) were determined by the standard four probe method using 1 lV/cm as criterion. The measurements were conducted at 4.2 and 20 K, respectively, in fields up to 14 T applied parallel to the main plane of the tapes. Jc was related to the core area of the tape. The area of the core and the overall area of the tape were determined micro-optically from a transverse cross-section of the tape using a computerized image analyser. Typical values of the filling factor (core area:overall area) of monofilamentary tapes were in the range of 0.31–0.37. The filling factor of the 19-filamentary tape varied in the range of 0.095 and 0.12 along the length of the tape. Scanning electron microscopy (SEM) images of polished cross-sections were obtained by means of Jeol SEM JSM.6400 and SEM GEMINI 1530 (LEO Oberkochen) microscopes. Wavelength-dispersive X-ray microanalyses (WDX) were performed using an electron microprobe and SEM XL 30 (Philips). In order to determine the phase composition, crystal size and internal stresses of the superconducting core of annealed Fe-cladded tapes the Fe sheath was peeled off from one side of the tapes and the free surface of the core was investigated by X-ray diffraction including Rietveld refinement of the resulting X-ray patterns.
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Fig. 1. Transverse cross-sections of an unannealed Fe-cladded monofilamentary tape (a) and 19-filamentary tape with Fe matrix (b) and a longitudinal cross-section of the 19-filamentary tape (c).
mentary tape with Fe matrix, respectively, and the longitudinal cross-section of the 19-filamentary tape before annealing. Fig. 1c reveals the irregularity of the filament thickness due to agglomerates of the precursor powder resulting in the variation of the filling factor along the tape length. In order to estimate the favourable temperature range for the annealing of the tapes with the aim to convert the constituents of the precursor into MgB2 the precursor powder was investigated by differential scanning calorimetry (DSC). From the curve of the precursor (Fig. 2) one can conclude
3. Results and discussion Fig. 1 shows the transverse cross-sections of a Fe-cladded monofilamentary tape and a 19-fila-
Fig. 2. Differential scanning calorimetry (DSC) plot of the asmilled precursor ‘‘B’’ which was measured starting at ambient temperature with a heating rate of 20 K/min.
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that the reactions within the precursor start at about 750 K and are completed at about 900 K. Therefore the tapes were annealed in the temperature range of 770–970 K for different times under normal pressure and under an uniaxial pressure of about 0.5 GPa, respectively. Fig. 3 demonstrates the dependence of the critical current densities on the magnetic field (l0 H ) of Cu- and Fe-cladded monofilamentary tapes, respectively, and of the 19-filamentary tape all annealed under normal pressure. The critical current densities of Fe-sheathed tapes are significantly higher compared to those of Cu-cladded ones. Fig. 4, showing the cross-sections of tapes with different sheath materials, suggests that one reason for the lower Jc values of the Cu-cladded tapes is the higher porosity of the superconducting core compared to that of the Fe-cladded conductors. Obviously, the Fe sheath promotes a stronger MgB2 grain connectivity and probably a more effective transition of the current from one grain to the other than the Cu sheath can provide. This fact may be due to the higher mechanical strength of Fe in comparison to Cu, which leads to the transition of a higher load onto the Fe-cladded cores and, hence, to stronger densification by rolling
Fig. 3. Critical current density at 4.2 K as a function of external magnetic field for Cu/MgB2 and Fe/MgB2 monofilamentary tapes prepared from different precursors ‘‘A’’ and ‘‘B’’, respectively, and annealed under different conditions; results obtained for the 19-filamentary tape are also included (the field was applied parallel to the main plane of the tapes).
Fig. 4. Transverse cross-sections of a Cu/MgB2 monofilamentary tape (a) annealed at 870 K for 3 h and Fe/MgB2 monofilamentary tape (b) annealed at 770 K for 3 h.
prior to the annealing. The higher core density before annealing, in turn, results in a higher density of the core after sintering. Another reason for the lower Jc values of Cu-cladded tapes compared to Fe-cladded ones might be the high diffusivity of copper into the precursor core and the high reactivity of copper with the constituents of the precursor. This leads to the formation of a MgCu2 layer at the sheath/core boundary, and also to diffusion reactions in the interior of the core restricting the formation of the superconducting phase as was shown by SEM/WDX studies (Fig. 5). The Cu-rich MgCu phase forms bright channels within the microstructure of the core, while the dark areas are enriched in B exhibiting a chemical composition close to that of the MgB4 phase. A reaction layer consisting of Fe, Mg and B has also been observed at the core/sheath boundary of Fe-sheathed multifilamentary tapes (Fig. 6). The thickness of this layer of a tape annealed at 870 K
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Fig. 7. WDX linescan crossing the sheath/core boundary of a cross-section of the Fe/MgB2 tape annealed at 970 K for 3 h; the distance between the spots equals 2 lm. Fig. 5. SEM image obtained with secondary electrons of the detail of a cross-section and WDX results from the Cu/MgB2 tape annealed at 970 K for 3 h.
Fig. 6. SEM cross-sectional image obtained with secondary electrons of the Fe/MgB2 19-filamentary tape annealed at 870 K for 3 h.
is about 4 lm, whereas at a annealing temperature of 770 K the thickness is only 0.5 lm. Immediately at the filament/sheath interface of the 870 K-tape (Fig. 6) the concentration of the layer is 27 at.% Mg/28 at.% Fe/ 37 at.% B, in the middle of the reaction layer it is 8 at.% Mg/58 at.% Fe/31 at.% B. However, in contrast to Cu the Fe does not diffuse into the interior of the precursor material as was
proved by WDX-analysis using a microprobe (Fig. 7). It could be shown for the tape annealed at 770 K for 3 h that the Fe content of the core area only a few micrometer away from the sheath/core boundary is <0.1 wt.%. It is seen from Fig. 3 and the data of Table 2 that the higher Mg concentration of the precursor ‘‘B’’ compared to precursor ‘‘A’’ does not result in a significant enhancement of Jc of the tapes in contrast to tapes prepared from commercial MgB2 powders with and without an excess of Mg [16]. Both, Fig. 3 and Table 2 also show that Jc of the multifilamentary tape is higher than those of the monofilamentary ones.
Table 2 Comparison of characteristic Jc values at 4.2 K in fields of 8 and 10 T for tapes prepared from different precursors ‘‘A’’ and ‘‘B’’ Composite
Monofil.Fe/A Monofil.Fe/A Monofil. Fe/A
Monofil. Fe/B Monofil. Fe/B 19-fil. Fe/B
Heat treatment 770 K/3 h 870 K/3 h Hot pressed: 0.5 Gpa 870 K/10 min 770 K/3 h 870 K/3 h 770 K/3 h
Critical current density at 4.2 K (kA/cm2 ) 8T
10 T
17.5 21.5 26.7
6.6 4.7 11.0
20.2 21.5 27.7
6.3 6.2 9.4
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Fig. 8. Critical current density at 4.2 K as a function of the external magnetic field for Fe/MgB2 monofilamentary tapes prepared from precursor ‘‘A’’ and heat treated under pressure of 0.5 GPa; in comparison with tapes annealed under ambient pressure the best curve of the Fe/‘‘A’’ tapes shown in Fig. 3 is included in this figure.
In Fig. 8 the Jc ðl0 H Þ-curves of tapes with precursor ‘‘A’’ annealed under enhanced pressure of 0.5 GPa are shown. To compare these curves with Jc values of the tapes annealed under ambient pressure the best curve of the Fe/‘‘A’’ tapes shown in Fig. 3 is included in Fig. 8. A comparison of the Jc values is also given in Table 2. It is remarkable that despite of the short reaction times the critical current densities of tapes heat treated under pressure at appropriate temperatures exceed those of tapes annealed under ambient pressure. The reason seems to be the excellent density of the cores of the hot pressed conductors. It should be noticed that the cross-section of the sample heat-treated under pressure shows also a reaction layer at the sheath/core boundary in contrast to the sample annealed at the same temperature under normal pressure but for a longer time. This suggests that the reaction kinetics have been altered by pressure. Fig. 9 represents the Jc ðl0 H Þ curve of the Fe/B sample annealed at 770 K for 3 h shown in Fig. 3 and the Jc ðl0 H Þ curve at 20 K for the same sample in comparison with curves of doped and undoped tapes, which are to our knowledge the best values published so far [10,17]. At 4.2 K the Jc values of
Fig. 9. Comparison of critical current densities achieved by our group (this work) for MgB2 tapes with the highest Jc values published so far for doped [10] and undoped tapes [17].
our tapes are exceeded only by those of tapes prepared from a precursor doped with nanometer sized SiC particles [10]. The reason for the poor critical current density of our tape at 20 K is the low critical temperature (Tc onset ¼ 34:8 K) of the precursor ‘‘B’’ and consequently the low Tc onset ¼ 31 K of the tape. An improvement of Tc can probably be achieved by managing the impurities introduced by the high energy milling process. It is well known from the literature that large changes of Tc can be caused by substitution of Mg or B by third elements [18–23]. As the data of Table 1 show, particular attention must be paid to the impurity elements oxygen and carbon. While intragrain precipitations of MgO may act as flux pinning centres [18] the incorporation of O into the MgB2 lattice strongly lowers Tc [18–20]. Also C can substitute B in MgB2 thus lowering Tc [21]. In order to discover the reasons for the excellent Jc values at 4.2 K of our tapes the phase composition and microstructure of the superconducting cores of two tapes exhibiting high Jc values after annealing at 770 K for 3 and 6 h, respectively, and of one tape with Jc of only 100 A/cm2 after annealing at 970 K for 3 h have been investigated. Table 3 shows the results obtained by XRD including Rietveld refinement of the resulting X-ray patterns for two tapes. The data of Table 3 give no explanation of the very different Jc values of both samples. A possible reason will be
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Table 3 Data obtained by XRD for two samples annealed at 770 K for 6 h and 970 K for 3 h, respectively Data
Sample annealed at 770 K for 6 h
Sample annealed at 970 K for 3 h
MgB2 (wt.%) MgO (wt.%) WC (wt.%) Size of coherent scattering domains (nm) Lattice parameter a (nm) Lattice parameter c (nm) Lattice strain
85.4 14.1 0.5 29 0.3083 0.3524 1 · 102
86.1 13.4 0.5 54 0.3082 0.3526 7 · 103
discussed in connection with the microstructure (Fig. 13) below. To investigate the chemical composition on the micrometer scale SEM/WDX analyses were performed on fifteen spots chosen by chance on the cross-section of the core of a sample annealed at 970 K for 3 h. The results presented in Fig. 10 reveal an inhomogeneous distribution of the main chemical elements within the superconducting core. The mean values of the element concentrations were calculated to be 31.0 at.% Mg, 61.2 at.% B and 7.6 at.% O. From the data of Table 1 the chemical composition of the superconducting core of this sample was calculated to be 35.1 at.% Mg, 59.7 at.% B and 5.3 at.% O assuming that all the oxygen reacted with Mg forming MgO. The mole fractions of Mg and O determined by SEM/WDX
exceed the corresponding values calculated from the XRD-data. Therefore it is of interest to search in which form the oxygen determined by SEM/ WDX is incorporated into the microstructure of the conductor core. Fig. 11 shows the boron– oxygen concentration correlation obtained from the SEM/WDX-data of Fig. 10. In this figure are also included the boron–oxygen concentration correlations for a mixture of MgB2 + MgO and for MgB2 in which oxygen is partially substituted for B (MgB2 + MgBO). It is clearly seen that the analysed oxygen/boron ratios are significantly higher than the calculated ones for the MgB2 + MgO mixture. The partial substitution of O for B in MgB2 can contribute to this fact but not fully explain it. Another reason might be the chemisorbtion of oxygen at the surface of the
Fig. 10. SEM/WDX results from 15 spots chosen by chance on the cross-section of the core of a monofilamentary Fe/MgB2 tape annealed at 970 K for 3 h.
Fig. 11. Boron–oxygen concentration correlation derived from the data of Fig. 10.
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cross-section of the sample caused by polishing in ambient atmosphere. The partial substitution of oxygen for boron in MgB2 might be responsible for the low values of Tc onset < 34 K measured on these tapes [12]. Fig. 12 showing a detail of the cross-section of the tape annealed at 970 K for 3 h also reveals the inhomogeneous character of the microstructure of the conductor core. This microstructure consisting of dark areas <200 nm embedded in a bright matrix is typical also for tapes with high Jc values after annealing at 770 K. Of course, the fluctuation of the chemical composition shown in Fig. 10 does not reflect quantitatively the difference of the chemical compositions between the dark and bright areas because the SEM/WDX method yields a resolution of only about 1 lm for the chemical analysis. Fig. 13 shows SEM pictures of cross-sections of the tapes annealed for 3 h at 770 and 970 K, respectively, revealing the crystal morphology of the superconducting phase. The size of the recognizable primary grains is of the same order (<100 nm) as the grains of the mechanically alloyed precursor used as starting material [13]. The primary grains of the tapes are partially sintered together forming dense areas which are of larger dimensions in the tape annealed at the higher temperature. This result is supported by the XRDdata of Table 3. Assuming that the size of the
Fig. 12. SEM-image obtained with secondary electrons showing the cross-sectional detail of a tape annealed at 970 K for 3 h.
Fig. 13. SEM image obtained with secondary electrons taken from cross-sectional details of Fe/MgB2 tapes annealed at 770 K (a) and 970 K (b) for 3 h.
coherent scattering domains is determined by large-angle grain boundaries these XRD-data show that the main crystal size of a tape annealed at 970 K is about twice that of a tape annealed at 770 K. However, the difference in the crystal size can not explain the large difference in critical currents. It should be recalled that the critical current density of the tape annealed at 970 K is of the order of only 100 A/cm2 at 4.2 K in self field, while Jc of the tape annealed at 770 K for 3 h reaches the value of 17.5 kA/cm2 at 4.2 K and 8 T (Table 2). The most significant difference of the microstructures shown in Fig. 13a and b, respectively, is that Fig. 13b exhibits areas which suggest that partial melting of the conductor core occurred
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during annealing at 970 K. This assumption is supported by the DSC-curve in Fig. 2 which shows a thermal effect near 870 K. It is clearly seen in Fig. 13b that the melting and recrystallization process leads to deep cavities and may be also to microcracks which are assumed to be the main reason for the poor critical current density.
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Acknowledgements The authors would like to thank I. B€acher, S. Pichl and H. Wendrock for SEM images and carrying out electron probe microanalyses.
References 4. Conclusions Mechanically alloyed Mg + 2B powder mixtures are appropriate precursors for manufacturing of MgB2 conductors by the PIT method. Fe-sheathed monofilamentary tapes and multifilamentary tapes with Fe matrix prepared by using such precursors revealed Jc values up to 29 and 35 kA/cm2 , respectively, at 4.2 K in an external magnetic field of 7.5 T. These values are exceeded only by the best ones published so far for tapes prepared from a SiC doped precursor [10]. The high density of grain boundaries, which can act as pinning centres within the superconducting phase seems to be the main reason for these excellent Jc values. MgO particles of nanometer size may also contribute to the flux pinning [18–20]. Furthermore, the nature of the inhomogeneity of the superconducting phase shown in Fig. 12 has to be clarified and to what extent this may influence the flux pinning. Fe is more suitable as sheath material than Cu due to the higher reactivity of Cu interacting with the precursor and because of the higher mechanical strength of Fe in comparison to Cu. This results in a higher density of the Fesheathed core upon rolling prior to heat treatment, which leads to an improved connectivity of the superconducting grains of Fe-cladded tapes after annealing. An increase of Tc of the precursor powder is expected to enhance Jc of the tapes at 20 K appreciably. This may be realized by modifying the milling process aiming to avoid or at least to minimize the introduction of harmful impurities into the precursor material. Further systematic variation of the Mg/B concentration ratio of the precursor should also contribute to the improvement of the superconducting properties.
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