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Correlation of superconducting properties and microstructure of MgB2 wires and tapes
Physica C 460–462 (2007) 1409–1410 www.elsevier.com/locate/physc
Correlation of superconducting properties and microstructure of MgB2 wires and tapes B. Birajdar
a,*
, O. Eibl a, V. Braccini b, G. Grasso b, W. Pachla c, M. Herrman d, W. Ha¨ßler
d
a
d
University of Tuebingen, Institute of Applied Physics, Auf der Morgenstelle 10, D-72076 Tu¨bingen, Germany b CNR-INFM LAMIA and Columbus Superconductors, Corso Perrone 24, I-16152 Genova, Italy c Institute of High Pressure Physics, Polish Academy of Sciences, Sokolowska 29/37, 01-142 Warsaw, Poland Leibniz-Institute for Solid State and Materials Research Dresden, P.O. Box 270116, D-01171 Dresden, Germany Available online 14 April 2007
Abstract Within the ‘‘Hipermag’’ project, MgB2 wires and tapes were prepared by the powder in tube method using different precursor powders and processing technologies. Either pre-reacted MgB2 (ex-situ), mixture of MgH2 + 2B (in-situ) or mechanically alloyed (MA) MgB2 is used. In this work superconducting properties of a long length 14-filament tape with standard ex-situ powder, MA tapes, and hydrostatically extruded wires with in-situ MgB2 powder with SiC nanoinclusions, are correlated with their microstructure investigated using advanced electron microscopy techniques. Apart from the conventional diffraction imaging, EDX elemental maps in SEM and TEM and electron spectroscopic imaging (ESI) in TEM have been used to study the MgB2 phase formation and granularity. The critical current densities of the wires and tapes were measured at 4.2 and 20 K for different magnetic fields. Wires and tapes prepared by different technologies show large differences by orders of magnitude in their critical current densities. MgB2 colony formation is found as a universal phenomenon with several grains forming a dense colony. Such colonies are embedded in a matrix of reduced density introducing granularity in MgB2 wires and tapes. Wires and tapes that do not show colony formation contain a large fraction of B-rich phases and their critical current density is limited by the MgB2 phase formation. Ó 2007 Elsevier B.V. All rights reserved. Keywords: MgB2; Superconducting tapes; Phase formation
1. Introduction A variety of techniques have been used to synthesise MgB2 wires and tapes and they show critical current densities (Jc) which vary by orders of magnitudes, reasons for which have not been addressed so far. In this paper Jc of different wires and tapes prepared in the ‘‘Hipermag’’ project have been correlated with their microstructure. The synthesis procedure of sample 1 to 6 is described in Table 1 and the references therein. Samples 3 and 4 are called ‘‘in-situ’’ and were prepared using pre-reacted MgB2 while samples 5 and 6 are called ‘‘ex-situ’’ and were prepared using Mg or MgH2 and B precursor powders. *
Corresponding author. Tel.: +49 7071 2976015; fax: +49 7071 295093. E-mail address: [email protected] (B. Birajdar).
0921-4534/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2007.04.121
Sample 3 is a 1.6 km long, thermally stabilised 14-filament tape [2]. Samples 1 and 2 are prepared by mechanical alloying [1] which is a special variant of the in-situ technique. The Jcs were determined by transport measurements using the 1 lV/cm criteria and the Tc was determined by resistivity measurements. Oxidation and carbon contamination artefacts were avoided by careful SEM and TEM sample preparation [1]. Size of the dense arrangement of MgB2 grains hereafter called as colony and volume fraction of B-rich and O-rich phases was estimated using advanced electron microscopy techniques like EDX elemental mapping in SEM and TEM and electron spectroscopic imaging in TEM as outlined by Eyidi et al. [4,5]. Microstructure analysis of samples 4–6 with SiC nanoinclusions was demanding because of the increased number of elements and phases.
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B. Birajdar et al. / Physica C 460–462 (2007) 1409–1410
Table 1 Tc, annealing temperature (h), colony size (S), oxygen mole fraction, and volume fraction of B-rich phases (f) of samples 1–6 #
Tc (K)
Synthesis technique
Powder
h (°C)
S (lm)
f (%)
O (at.%)
1 2 3 4 5 6
36.5 32.5 36.5 38.7 36.0 35.5
Mechanical alloying [1] Mechanical alloying 14-filament [2] HE + TAR [3] HE + TAR 4-rolling [3]
Mg + 2B (fluka boron) Mg + 2B (strem boron) MgB2 MgB2 + SiC MgH2 + 2B + SiC Mg + 2B + SiC
600 650 980 950 670 650
– 1–35 1–6 1–3 – –
4 <0.5 0 0.8 1.5 6.4
3.75 9.34 14.57 8.89 11.53 15.46
PIT, powder in tube; HE, hydrostatic extrusion; TAR, two-axial rolling.
barrier layers. In in-situ samples 5–6 SiC is retained because of the lower annealing temperature, although some of the SiC reacted with Mg to form a Mg2Si secondary phase. The Jc of samples 5–6 is therefore higher than sample 4. However sample 5 is porous and large volume fraction of B-rich phases is found in the Mg excess matrix of samples 5–6. The excess Mg is oxidised at cracks and voids when exposed to air. The phase formation in the in-situ samples 5–6 is thus incomplete and limits their Jc. 3. Summary and conclusion
Fig. 1. Transport Jc(B) plots of samples 1–6 at 4.2 K. *Measured on similarly prepared monofilamentary tapes at 5 K.
2. Results and discussion The Jc(B) curves of samples 1 to 6 are shown in Fig. 1. The Tc, annealing temperature (h), colony size (S), oxygen mole fraction, and volume fraction of B-rich phases (f ) are given in Table 1 and used to explain the large variations in Jcs. In samples 2–4 oxygen was concentrated in the less dense matrix between the colonies. Mechanically alloyed tapes (samples 1 and 2) show highest critical current densities at high fields. This is because of the good connectivity in sample 1 which is dense on 1–10 lm scale and contains less oxygen. However B-rich phases are found and distinct MgB2 colonies are not formed. The MgB2 phase formation is thus incomplete which limits its Jc. On the other hand in sample 2 MgB2 phase formation is improved but granularity exists yielding a reduced connectivity [6]. The microstructure of 14-filament tapes (sample 3) is similar to that of sample 2, although the volume fraction of large MgB2 colonies is lower and granularity is larger. Moreover the grain size is 500 nm in sample 3 and 50 nm in mechanically alloyed tapes, and the reduced grain boundary pinning explains the rapid decrease in Jc of sample 3 at higher fields. In samples 4–6 SiC nanopowder was added for improving the pinning. The SiC in ex-situ sample 4 was oxidised completely during annealing at 950 °C, forming 50 nm wide oxide layers between the MgB2 grains which act as
Depending on technology, the critical current densities of the investigated MgB2 wires and tapes differ by orders of magnitudes, which can be explained by (i) improper MgB2 phase formation or (ii) granularity, i.e. colony formation, with non-superconducting barrier layers at grain boundaries. Irrespective of the preparation technology, oxygen-free dense regions (colonies) surrounded by less dense, oxygen rich regions are a characteristic feature of the investigated MgB2 wires and tapes. These less dense regions show granularity and limit the Jc specially of exsitu samples. In in-situ samples the Jc is limited by an incomplete MgB2 phase formation. Mechanically alloyed tapes show highest Jc for fields greater than 5.5 T because of better connectivity and enhanced grain-boundary pinning. SiC in in-situ samples is not oxidised but a Mg2Si secondary phase is formed. Acknowledgement Financial support by the European Union under the FP6 project ‘‘HIPERMAG’’ is gratefully acknowledged. References [1] W. Ha¨ßler, B. Birajdar, W. Gruner, M. Herrman, O. Perner, C. Rodig, M. Schubert, B. Holzapfel, O. Eibl, L. Schultz, Supercond. Sci. Technol. 19 (2006) 512. [2] Columbus Superconductors, Corso Perrone 24, 16152 Genova, Italy, http://www.columbus-sc.it. [3] W. Pachla et al., Supercond. Sci. Technology 18 (2005) 552. [4] D. Eyidi et al., Micron 34 (2003) 85. [5] D. Eyidi, O. Eibl, T. Wenzel, K.G. Nickel, S.I. Schlachter, W. Goldacker, Supercond. Sci. Technol. 16 (2003) 778. [6] J. Rowell, Supercond. Sci. Technol. 16 (2003) R17.
Correlation of superconducting properties and microstructure of MgB2 wires and tapes B. Birajdar
a,*
, O. Eibl a, V. Braccini b, G. Grasso b, W. Pachla c, M. Herrman d, W. Ha¨ßler
d
a
d
University of Tuebingen, Institute of Applied Physics, Auf der Morgenstelle 10, D-72076 Tu¨bingen, Germany b CNR-INFM LAMIA and Columbus Superconductors, Corso Perrone 24, I-16152 Genova, Italy c Institute of High Pressure Physics, Polish Academy of Sciences, Sokolowska 29/37, 01-142 Warsaw, Poland Leibniz-Institute for Solid State and Materials Research Dresden, P.O. Box 270116, D-01171 Dresden, Germany Available online 14 April 2007
Abstract Within the ‘‘Hipermag’’ project, MgB2 wires and tapes were prepared by the powder in tube method using different precursor powders and processing technologies. Either pre-reacted MgB2 (ex-situ), mixture of MgH2 + 2B (in-situ) or mechanically alloyed (MA) MgB2 is used. In this work superconducting properties of a long length 14-filament tape with standard ex-situ powder, MA tapes, and hydrostatically extruded wires with in-situ MgB2 powder with SiC nanoinclusions, are correlated with their microstructure investigated using advanced electron microscopy techniques. Apart from the conventional diffraction imaging, EDX elemental maps in SEM and TEM and electron spectroscopic imaging (ESI) in TEM have been used to study the MgB2 phase formation and granularity. The critical current densities of the wires and tapes were measured at 4.2 and 20 K for different magnetic fields. Wires and tapes prepared by different technologies show large differences by orders of magnitude in their critical current densities. MgB2 colony formation is found as a universal phenomenon with several grains forming a dense colony. Such colonies are embedded in a matrix of reduced density introducing granularity in MgB2 wires and tapes. Wires and tapes that do not show colony formation contain a large fraction of B-rich phases and their critical current density is limited by the MgB2 phase formation. Ó 2007 Elsevier B.V. All rights reserved. Keywords: MgB2; Superconducting tapes; Phase formation
1. Introduction A variety of techniques have been used to synthesise MgB2 wires and tapes and they show critical current densities (Jc) which vary by orders of magnitudes, reasons for which have not been addressed so far. In this paper Jc of different wires and tapes prepared in the ‘‘Hipermag’’ project have been correlated with their microstructure. The synthesis procedure of sample 1 to 6 is described in Table 1 and the references therein. Samples 3 and 4 are called ‘‘in-situ’’ and were prepared using pre-reacted MgB2 while samples 5 and 6 are called ‘‘ex-situ’’ and were prepared using Mg or MgH2 and B precursor powders. *
Corresponding author. Tel.: +49 7071 2976015; fax: +49 7071 295093. E-mail address: [email protected] (B. Birajdar).
0921-4534/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2007.04.121
Sample 3 is a 1.6 km long, thermally stabilised 14-filament tape [2]. Samples 1 and 2 are prepared by mechanical alloying [1] which is a special variant of the in-situ technique. The Jcs were determined by transport measurements using the 1 lV/cm criteria and the Tc was determined by resistivity measurements. Oxidation and carbon contamination artefacts were avoided by careful SEM and TEM sample preparation [1]. Size of the dense arrangement of MgB2 grains hereafter called as colony and volume fraction of B-rich and O-rich phases was estimated using advanced electron microscopy techniques like EDX elemental mapping in SEM and TEM and electron spectroscopic imaging in TEM as outlined by Eyidi et al. [4,5]. Microstructure analysis of samples 4–6 with SiC nanoinclusions was demanding because of the increased number of elements and phases.
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B. Birajdar et al. / Physica C 460–462 (2007) 1409–1410
Table 1 Tc, annealing temperature (h), colony size (S), oxygen mole fraction, and volume fraction of B-rich phases (f) of samples 1–6 #
Tc (K)
Synthesis technique
Powder
h (°C)
S (lm)
f (%)
O (at.%)
1 2 3 4 5 6
36.5 32.5 36.5 38.7 36.0 35.5
Mechanical alloying [1] Mechanical alloying 14-filament [2] HE + TAR [3] HE + TAR 4-rolling [3]
Mg + 2B (fluka boron) Mg + 2B (strem boron) MgB2 MgB2 + SiC MgH2 + 2B + SiC Mg + 2B + SiC
600 650 980 950 670 650
– 1–35 1–6 1–3 – –
4 <0.5 0 0.8 1.5 6.4
3.75 9.34 14.57 8.89 11.53 15.46
PIT, powder in tube; HE, hydrostatic extrusion; TAR, two-axial rolling.
barrier layers. In in-situ samples 5–6 SiC is retained because of the lower annealing temperature, although some of the SiC reacted with Mg to form a Mg2Si secondary phase. The Jc of samples 5–6 is therefore higher than sample 4. However sample 5 is porous and large volume fraction of B-rich phases is found in the Mg excess matrix of samples 5–6. The excess Mg is oxidised at cracks and voids when exposed to air. The phase formation in the in-situ samples 5–6 is thus incomplete and limits their Jc. 3. Summary and conclusion
Fig. 1. Transport Jc(B) plots of samples 1–6 at 4.2 K. *Measured on similarly prepared monofilamentary tapes at 5 K.
2. Results and discussion The Jc(B) curves of samples 1 to 6 are shown in Fig. 1. The Tc, annealing temperature (h), colony size (S), oxygen mole fraction, and volume fraction of B-rich phases (f ) are given in Table 1 and used to explain the large variations in Jcs. In samples 2–4 oxygen was concentrated in the less dense matrix between the colonies. Mechanically alloyed tapes (samples 1 and 2) show highest critical current densities at high fields. This is because of the good connectivity in sample 1 which is dense on 1–10 lm scale and contains less oxygen. However B-rich phases are found and distinct MgB2 colonies are not formed. The MgB2 phase formation is thus incomplete which limits its Jc. On the other hand in sample 2 MgB2 phase formation is improved but granularity exists yielding a reduced connectivity [6]. The microstructure of 14-filament tapes (sample 3) is similar to that of sample 2, although the volume fraction of large MgB2 colonies is lower and granularity is larger. Moreover the grain size is 500 nm in sample 3 and 50 nm in mechanically alloyed tapes, and the reduced grain boundary pinning explains the rapid decrease in Jc of sample 3 at higher fields. In samples 4–6 SiC nanopowder was added for improving the pinning. The SiC in ex-situ sample 4 was oxidised completely during annealing at 950 °C, forming 50 nm wide oxide layers between the MgB2 grains which act as
Depending on technology, the critical current densities of the investigated MgB2 wires and tapes differ by orders of magnitudes, which can be explained by (i) improper MgB2 phase formation or (ii) granularity, i.e. colony formation, with non-superconducting barrier layers at grain boundaries. Irrespective of the preparation technology, oxygen-free dense regions (colonies) surrounded by less dense, oxygen rich regions are a characteristic feature of the investigated MgB2 wires and tapes. These less dense regions show granularity and limit the Jc specially of exsitu samples. In in-situ samples the Jc is limited by an incomplete MgB2 phase formation. Mechanically alloyed tapes show highest Jc for fields greater than 5.5 T because of better connectivity and enhanced grain-boundary pinning. SiC in in-situ samples is not oxidised but a Mg2Si secondary phase is formed. Acknowledgement Financial support by the European Union under the FP6 project ‘‘HIPERMAG’’ is gratefully acknowledged. References [1] W. Ha¨ßler, B. Birajdar, W. Gruner, M. Herrman, O. Perner, C. Rodig, M. Schubert, B. Holzapfel, O. Eibl, L. Schultz, Supercond. Sci. Technol. 19 (2006) 512. [2] Columbus Superconductors, Corso Perrone 24, 16152 Genova, Italy, http://www.columbus-sc.it. [3] W. Pachla et al., Supercond. Sci. Technology 18 (2005) 552. [4] D. Eyidi et al., Micron 34 (2003) 85. [5] D. Eyidi, O. Eibl, T. Wenzel, K.G. Nickel, S.I. Schlachter, W. Goldacker, Supercond. Sci. Technol. 16 (2003) 778. [6] J. Rowell, Supercond. Sci. Technol. 16 (2003) R17.