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Experimental research on electric field jump in low magnetic fields: Detection of damage in new ex-situ MgB2 barriers in MgB2 wires
Journal of Alloys and Compounds 647 (2015) 303e309
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Experimental research on electric field jump in low magnetic fields: Detection of damage in new ex-situ MgB2 barriers in MgB2 wires D. Gajda a, *, A. Morawski b, A. Zaleski c, M.S.A. Hossain d, M. Rindfleisch e, T. Cetner b a
International Laboratory of High Magnetic Fields and Low Temperatures, Gajowicka 95, 53-421 Wroclaw, Poland Institute of High Pressure Physics, Polish Academy of Sciences, Sokolowska 29/37, 01-142 Warszawa, Poland c lna 2, 50-422 Wroclaw, Poland Institute of Low Temperature and Structure Research, Polish Academy of Sciences, Oko d Institute for Superconducting and Electronic Materials, AIIM, University of Wollongong, North Wollongong, NSW 2519, Australia e Hyper Tech Research, Inc, 1275 Kinnear Road, Columbus, OH 43212, USA b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 20 April 2015 Received in revised form 11 June 2015 Accepted 14 June 2015 Available online 20 June 2015
We explored the incorporation of field sweep (constant current and rapidly increasing magnetic field) into the four-probe method as a new technique to detect defects in barrier layers in superconducting MgB2 wires. This method allows us to observe jumps in the electric field in low magnetic fields. The scanning electron microscopy results indicate that such a jump originates from cracks in Nb barriers and ex-situ MgB2 barriers. Our research indicates that the field sweep allows us to detect damage to barriers that are made of superconducting materials. This method can be the basis for an industrial method for detecting damages in MgB2 wires. These defects reduce the critical current of MgB2 wire. Detection and removal of these defects will allow us to produce MgB2 wires with ex-situ MgB2 and Nb barriers that will have improved critical current density. Manufacturing of MgB2 wires with new ex-situ MgB2 barriers is a new technological concept. This type of barrier is cheaper and easier to manufacture, leading to cheaper MgB2 wires. Moreover, we show that critical current can be measured by two methods: current sweep (constant magnetic field and quickly increasing current) and field sweep. © 2015 Elsevier B.V. All rights reserved.
Keywords: Damage detection MgB2 wires Critical current
1. Introduction The ex-situ MgB2 barrier was invented in 2006 by A. Morawski of the Institute of High Pressure Research in Warsaw (Poland) and B Głowacki of the Department of Materials Science and Metallurgy, University of Cambridge (United Kingdom) [1]. MgB2 wires with exsitu MgB2 barriers are fabricated by the powder-in-tube (PIT) method. This type of barrier is also cheaper and easier to use in the PIT treatment than Nb, Ti, Fe, and Ta barriers. The energy dispersive X-ray spectroscopy (EDX) and X-ray diffraction (XRD) results of Kario et al. [2] indicate that the ex-situ MgB2 barrier allows us to obtain high-purity in-situ MgB2 material in the core or filaments of the wire. This is because the ex-situ MgB2 barrier reduces the reactions of Mg and the sheaths of the wires. Moreover, the ex-situ MgB2 barrier does not increase the hardness after cold drawing as in the case of an Nb barrier. This might indicate that cold drawing will create less damage in ex-situ MgB2 barriers.
* Corresponding author. E-mail address: [email protected] (D. Gajda). http://dx.doi.org/10.1016/j.jallcom.2015.06.103 0925-8388/© 2015 Elsevier B.V. All rights reserved.
Scanning electron microscope (SEM) studies show that the exsitu MgB2 barrier is uniformly distributed along the wire and creates good contact with the wire sheath and the in-situ MgB2 material because it has low shrinkage [2e5]. Measurements of the critical current for MgB2 wires with ex-situ MgB2 barriers, and iron and copper sheaths show that these wires have very low critical current density (Jc) anisotropy of about 2e4 % (with only a small difference between the Jc in perpendicular and parallel magnetic fields) [4,5]. These advantages demonstrate that this barrier can also be applied in other superconducting wires. Further studies are needed to improve the barrier properties. The research showed by H€ aßler [6], Maeda [7], Susner [8], Adamczyk [9], Dou [10] and Zhou [11] for MgB2 wires with Nb barrier and MgB2 materials indicate that these wires have great potential for application. SEM images of MgB2 wires [12] show that a Nb barrier might be damaged after cold drawing. This damage to the barrier causes the formation of phases such as Cu2Mg [13] and pure B. In addition, it reduces the amount of superconducting material and decreases the critical current density. Eikin presents results on the UeI characteristic for low-temperature superconductor (LTS) wires [14]. These measurements suggest that this method makes it possible to detect
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a fault in the structure of NbTi wires. When the current sweep type is used in MgB2 wires, we cannot identify reasons for the appearance of an electric field. Similar electric fields occur for short wires [15] and NbTi wires with damaged microstructure [14]. Studies indicate that the critical current density Jc on short and long MgB2 wires is similar [16,17]. The Jc in these wires is dependent on parameters of the annealing process. In contrast, Jc in the wires with the damaged microstructure depends on the number and size of defects. Currently, there are no reports of a measurement method that makes it possible to detect damage in Nb and ex-situ MgB2 barriers. The lack of such a method impedes study on increasing Jc in MgB2 wires. In this paper, we present the results obtained with the field sweep method for MgB2 wires with Nb and ex-situ MgB2 barriers. These measurements indicate that the field sweep method allows us to detect damage in Nb and ex-situ MgB2 barriers and will allow us to eliminate these defects in the future.
changeable flux of magnetic field. This theory indicates that the loop in Fig. 1 is generated by the electromotive force. In Fig. 1, for quick increase of the magnetic field, we can determine Ic on the basis of 1 mV/cm because the electric field of 2 mV/cm comes from the electromotive force. The measurements showed that the transition to the normal state in the wires during slow and quick increase of the magnetic field is at the same point (blue line A e Fig. 1). The quick increase in the magnetic field allows measurements of Ic for large values of current and does not destroy the samples. We carried out a study of the critical current for undoped and doped MgB2 wires with Nb, Fe (Hyper Tech Research, Inc) sheaths and ex-situ MgB2 barriers (Institute of High Pressure Physics), NbTi wires (Donetsk Institute for Physic and Engineering, National Academy of Sciences of Ukraine), and the commercial bismuth strontium calcium copper oxide (BSCCO e SuperPower) tape in perpendicular and parallel magnetic fields. These results showed that the value of critical current from both measurement methods (current and field sweep method) is the same.
2. Experiment and sample preparation 3. Experimental results Critical current was measured using the four-probe resistive method at 4.2 K by two different methods: the current sweep type constant magnetic field and increasing current [14,18] - and the field sweep type - constant current and increasing magnetic field. Studies were made for specially selected samples with low critical current value. The length of wires was 20 mm in perpendicular magnetic field and 70 mm for measurements in parallel magnetic field. The critical current (Ic) was determined on the basis of the 1 mV/cm criterion. Critical current measurements were conducted in the International Laboratory of High Magnetic Fields and Low Temperatures in Wroclaw [16]. The research was carried out on MgB2 wires with ex-situ MgB2 barriers obtained from the Institute of High Pressure, Polish Academy of Sciences (PAS), and MgB2 wires with Nb barriers fabricated by Hyper Tech Research. The wires were annealed under pressure [19,20] at the Institute of High Pressure in Warsaw. Parameters of the measured samples are presented in Tables 1 and 2 (with Nb wire in copper also studied for comparison). The magnetic field was generated by a Bitter magnet. Its maximum magnetic field of about 14 T was obtained over a time of 170 s. Measurements of critical temperature were conducted by using a physical properties measurement system (PPMS Model 7100, Quantum Design, AC current e frequency of 15 Hz and 100 mA) with fields ranging from 0 up to 14 T at the International Laboratory of High Magnetic Fields and Low Temperatures. Microstructure analysis was performed on a FEI Nova Nano SEM 230 in the Institute of Low Temperature and Structure Research (ILT&SR), PAS in Wroclaw and with the help of a Carl Zeiss microscope in the Institute of High Pressure (IHP), PAS in Warsaw. The value of the critical current in the current sweep type was determined on the basis of 1 mV/cm. The same criterion was used in the field sweep type. Fig. 1 indicates that a quick increase in the magnetic field from 0 T to 14 T creates a loop of electric field (t ¼ 3 min e t the times in which the perpendicular magnetic field increases from 0 T to 14 T, and from 14 T returns to 0 T). A slow increase of the magnetic field does not create this loop, however. These studies have proved that the electromotive force creates a
SEM studies showed that most of the MgB2 wires have good Nb and ex-situ MgB2 barriers, although we sometimes see damage in Nb and ex situ MgB2 barriers. This damage is created during cold drawing of MgB2 wires. SEM images were collected for samples and at places where there was a jump in the electric field in low magnetic fields. The SEM image in Fig. 2 shows breakage in the Nb barrier for SiC doped MgB2 wires with GlidCop® sheath. This breakage reduces the amount of MgB2 superconducting material and creates intermetallic phases inside and outside of the fiber. SEM images and EDX maps (Fig. 3) of cross-sections of MgB2 wires with ex-situ MgB2 barriers show the places where there this barrier is damaged. In MgB2 wires with ex-situ MgB2 barriers, cold drawing occasionally creates the damage in the ex-situ MgB2 material. This damage in the ex-situ MgB2 material allows diffusion of Cu from the sheath to react with Mg as well as the in-situ MgB2 material. This migration through the ex-situ MgB2 barrier creates phases of Cu2Mg and CuMg2 [13]. These phases are very deleterious, because they reduce the number of connections between grains in the ex-situ MgB2 barrier. A smaller number of connections between grains can significantly reduce the transport critical parameters for the ex-situ MgB2 barrier. This process reduces the amount of MgB2 material in the fiber, and may create a higher amount of boride phase in the fiber. Our studies indicate that most samples have the characteristics shown in Fig. 1. Sometimes, however, we see a jump in the electric field in low magnetic fields (Fig. 4(a) e black curve). We will hereafter call this magnetic field rangeethe transition region (TR). Fig. 4(a) shows the results obtained by the field sweep type and a comparison of MgB2 wires with Nb barrier in a GlidCop® sheath with and without a transition region. The transition region in MgB2 wires with Nb barrier is shown in low magnetic fields from 0.1 T to 1 T (sample A1 e black curve (Fig. 4(a)). This range of magnetic field indicates that it comes from the Nb barrier, because pure Nb has upper critical field close to 1 T [21]. We might see that the transition region decreases the critical magnetic field (BK) from 8.5 T to 7 T
Table 1 Composition and construction of MgB2 wires. Samples identifier
Mg:B additive
Barrier
Mono sheath
Multi sheath
Fill factor (%)
Diameter (mm)
Multi filament
A1 (damaged) A2 B1 (damaged) B2
1.10:2 10%SiC 1.10:2 10%SiC 1:2 1:2
Nb Nb ex situ MgB2 ex situ MgB2
Cu (20%) Cu (20%)
Glidcop (30%) Glidcop (30%) Glidcop (50%) Glidcop(50%)
16.6 16.6 50 50
0.83 0.83 1.2 1.2
6 6 1 1
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Table 2 Hot isostatic pressing (HIP) parameters for MgB2 wires with Nb barrier. Samples identifier
Annealing time (min)
Annealing temperature ( C)
Pressure (MPa)
A1 A2 B1 B2
15 15 15 15
700 700 700 700
0.1 0.1 20 20
Fig. 1. Transport E e B (I ¼ const) curves for sample A2 under the field sweep method; 3 min and 8 min are the times in which the perpendicular magnetic field increases from 0 T to 14 T, and from 14 T returns to 0 T.
Fig. 2. Cross-section of SiC doped MgB2 wire with GlidCop® shield for sample A1 (SEM image).
(20%) and creates a jump in the electric field above 1 T with a value of about 7.5 mV/cm. In Fig. 4(b), we see the results of measurements obtained by using the current sweep type. These results indicate that the transition region does not allow us to determine values of Ic on the basis of the 1 mV/cm criterion. On the contrary, the results in Fig. 4(a) demonstrate that the field sweep type might even make it possible to determine the value of Ic in MgB2 wires with damaged Nb barriers. Furthermore, the results indicate that we may determine the precise value of Ic with the criterion of 1 mV/cm for MgB2 wires without a transition region. Fig. 4(c) presents the transport critical current density (Jct) dependence on the magnetic field (B). These results indicate that the transition region reduces the value of Jct in a MgB2 wire with a GlidCop® sheath by about 55%. In Fig. 5(a), (c) and (d), we see the comparative results for MgB2 with ex situ MgB2 barrier. These results indicate that a jump in the electric field in low magnetic fields was observed above 7.5 A (Fig. 5(a)). This jump in the electric field will be called the transition region, because it also appears in low magnetic fields. We do not see a jump in the electric field in low magnetic fields for low
currents from 0.75 A to 1.5 A (Fig. 5(c)). Fig. 6 shows the results of MgB2 wire with good ex situ MgB2 barrier. Comparing the results in Figs. 5(a) and 6(a) we see that the transition region decreases the critical magnetic field and creates the jump in the electric field above 0.05 Te160 mV/cm (Fig. 5(d)). Calculations show that the resistivity at points C, D, E, and F is similar to the GlidCop® resistivity at liquid helium temperature. The results might indicate that the transition region significantly increases the path penetration of current into the superconducting material. Figs. 5(b) and 6(b) show the measurement results obtained by using the current sweep type. The results in Fig. 5(b) indicate that the transition region does not allow us to determine values of Ic on the basis of the 1 mV/cm criterion, because we observe a very high electric field. The results in Fig. 5(a) show that the field sweep type allows assessment of the approximate value of Ic in MgB2 wires with a transition region. On the other hand, Fig. 6(b) indicates that we might determine the value of Ic for MgB2 wires without a transition region. A low electric field creates a penetrating current into the superconducting material. The results in Fig. 5(b) show that the transition at point B has the same parameters (electric and magnetic fields) as the transition in Fig. 5(a) at point A. These results show that the current sweep type allows us to detect superconducting material, although these results do not enable us to determine the exact value of the critical current. Moreover, Fig. 5(b) shows a lack of magnetoresistance in magnetic field from 0 T to 5 T. This result also indicates that we have superconducting material in MgB2 wires. Fig. 5(e) shows the dependence of the transport critical current density (Jct) on the magnetic field (B). These results demonstrate that the transition region reduces the value of Jct in MgB2 wire with an ex-situ MgB2 barrier by about 25e35%. Transport measurements R ¼ f(T) for the samples with the transition region showed that these samples have a jump in resistance at low temperatures (Fig. 7). In MgB2 wires with Nb barriers we see this jump in resistance at about 9 K (Fig. 7(a)). This temperature is very similar to the critical temperature of Nb. In samples without a transition region we do not see jump resistance at about 9 K (Fig. 7(a)). Furthermore, measurements show that the jump in resistance is not visible in samples with their transition region in magnetic field above 1 T (Fig. 7(a)). This result indicates that this jump can be created by Nb material, because Nb is not a superconductor in magnetic fields above 1 T [21]. The results showed that the transition region degrades the critical temperature in MgB2 wires with Nb barriers by about 2%. In MgB2 wires with ex-situ MgB2 barriers, we also see the resistance jump at 7 K (Fig. 7(b)). These results show that this jump in resistance might significantly reduce the critical parameters (Birr and Jc) of the ex-situ MgB2 barrier. Low critical parameters of the ex-situ MgB2 barrier can be caused by low annealing temperature and short annealing time. This jump is not visible in MgB2 wires with ex-situ MgB2 barriers without the transition region (Fig. 7(c)). The results indicate that the transition region does not reduce the transport critical temperature (Tct) in MgB2 wires with an ex-situ MgB2 barrier (Fig. 7(b) and (c)). Fig. 8(a) shows magnetic measurements of critical temperature (Tcm) show that the measured characteristics of samples with and without transition regions are very similar. Furthermore, the results
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Fig. 3. (a) and (b) SEM images; (c) EDX maps for a cross-section of SiC doped MgB2 wire with ex-situ MgB2 barrier in GlidCop® shielding. Red circles show the damaged areas. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 4. (a) Transport E e B (I ¼ const) curves for samples A1 and A2, (b) Transport E e I (B ¼ const) curves for samples A1 and A2, (c) Field dependence of transport critical current density (Jct e B) for samples A1 and A2eJct comparison of MgB2 wires with and without a transition region. The measurements were performed in perpendicular magnetic field.
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Fig. 5. (a), (c), and (d) Transport E e B (I ¼ const) curves for sample B1, (b) Transport E e I (B ¼ const) curves for sample B1. (e) Transport Jct e B curves for samples B1 and B2 e The Jct comparison is for MgB2 wires with and without a transition region. The measurements were performed in parallel magnetic field.
indicate that the value of the magnetic critical temperature does not change in a sample with a damaged Nb barrier. The hysteresis loops of samples with and without a transition region have the same shape (Fig. 8(b)). We observe only a small decrease in magnetization for the sample with a transition region. This decrease is caused by the presence of less superconducting material in the sample with a damaged Nb barrier. 4. Discussion and conclusions Our research has proved that the transition region decreases the critical current and critical field in MgB2 wires with Nb and ex-situ MgB2 barriers. The results show that the appropriate method for current density characterization is the swept-field type, because it allows us to detect damage in MgB2 wires and allows us to estimate the value of the critical current in MgB2 wires with damaged barriers. Furthermore, it is demonstrated that the current sweep type
cannot indicate whether the electric field creates a penetrating current that passes into the superconducting material or damaged barrier. Our results indicate that the damaged barrier lengthens the path of the penetrating current into the superconducting material. The jump in the electric field in MgB2 wires with Nb barriers appears in magnetic field of 0.6 Te1 T. The magnetic field of 1 T is the upper critical field (Bc2) of Nb. Detection of this type of damage can correctly indicate the influence of annealing time, annealing temperature, pressure on the critical parameters of MgB2 wires and its microstructure. These results will help to make progress in the production of MgB2 wires. The jump electric field in MgB2 wires with ex-situ MgB2 barriers appears in magnetic fields from 0.05 T to 0.25 T. The poor connection between the grains in ex-situ MgB2 material can lead to a low Bc2 magnetic field because ex-situ MgB2 material is annealed at low temperature around, 700 C and moderate pressure. The pressure increases the melting temperature of the ex-situ MgB2. The higher melting temperature decreases
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Fig. 6. (a) Transport E e B (I ¼ const) curves for sample B2, (b) Transport E e I (B ¼ const) curves for sample B2. The measurements were performed in parallel magnetic field.
transport critical parameters. These results show that the jump in the electric field might create movement of vortices in the barrier and penetrating current that goes into the superconducting material. SEM images show that MgB2 wires with Nb barriers have their damage on the outer side of this barrier, because the damaged area originates outside the Nb barrier. In MgB2 wires with ex-situ MgB2 the damage is on the inner side of this barrier, because defects are formed inside the ex-situ MgB2 barrier. These defects in the ex-situ MgB2 barrier arise from pure Mg (from the in-situ MgB2 material)
and Cu (from the GlidCop® sheath). Migration of Mg and Cu can reduce the density of the ex-situ MgB2 barrier. Greater density of the ex-situ MgB2 barrier can be obtained by using smaller grains. Smaller grains allow more connections between the grains. Higher connectivity between the grains will also reduce the migration of Mg and Cu. These results indicate that in MgB2 wires with good barriers, the two superconducting materials create one superconducting material. Our measurements show that the cracking of Nb and ex-situ MgB2 barriers divides these materials into separate
Fig. 7. (a) Transport R e T (IAC ¼ 100 mA) curves for samples A1 and A2 with Nb barrier, Transport R e T (IAC ¼ 100 mA) curves for samples (b) B1 and (c) B2 with ex-situ MgB2 barrier.
D. Gajda et al. / Journal of Alloys and Compounds 647 (2015) 303e309
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Fig. 8. (a) Magnetic T dependence (m e T) (B ¼ 20 mT) curves for samples A1 and A2 with Nb barrier; (b) hysteresis loops (m e B) (T ¼ 4.2 K) for samples A1 and A2 with Nb barrier.
superconducting materials. The results in Fig. 5(c) show that low currents do not cause the jump in electric field. The results may indicate that we have a small layer of ex-situ MgB2 barrier which is not damaged. The measurements show that the transport method based on sweeping the temperature (constant magnetic field and current, and increasing temperature) allows us to detect damage in Nb and ex-situ MgB2 barriers. In contrast, the magnetic method of temperature sweeping (constant magnetic field and increasing temperature) does not allow us to detect defects in the Nb barrier. The lack of jumps in electric field and resistance in MgB2 wires with good barriers is positive, because it allows us to apply these barriers in MgB2 wires. These jumps create heat which leads to loss (Jc). The transport methods (field and temperature sweep type) can be the basis for an industrial method for detecting defects in the barriers of MgB2 wires. Acknowledgments This work was supported by the Institute of High Pressure Physics PAS and International Laboratory of HMF and LT and was also partially supported by Australian Research Council (Grant No. DE130101247) and 2014 UOW-URC grants. References [1] Patents: “Composite electric conductors and method for their manufacture” UK 0706919.8; USA: 60/907590; http://www.wipo.int/pctdb/en/index.jsp [2] A. Kario, A. Morawski, W. H€ aßler, M. Herrmann, C. Rodig, M. Schubert, K. Nenkov, B. Holzapfel, L. Schultz, B.A. Glowacki, S.C. Hopkins, Supercond. Sci. Technol. 23 (2010) 025018.
[3] A. Kario, A. Morawski, B.A. Glowacki, T. Lada, M. Smaga, R. Diduszko, D. Kolesnikov, A.J. Zaleski, A. Kondrat, D. Gajda, Acta Phys. Pol. A 111 (2006) 387. [4] D. Gajda, A.J. Zaleski, A. Morawski, A. Karia, Acta Phys. Pol. A 113 (2008) 371. [5] D. Gajda, A. Morawski, A.J. Zaleski, T. Cetner, Acta Phys. Pol. A 118 (2010) 1059. [6] W. Haßler, M. Herrmann, C. Rodig, M. Schubert, K. Nenkov, B. Holzapfel, Supercond. Sci. Technol. 21 (2008) 062001. [7] M. Maeda, D. Uchiyama, M.S.A. Hossain, Z. Mab, M. Shahabuddin, J.H. Kim, J. Alloys Compd. 636 (2015) 29e33. [8] M.A. Susner, T.W. Daniels, M.D. Sumption, M.A. Rindfleisch, C.J. Thong, E.W. Collins, Supercond. Sci. Technol. 25 (2012) 065002. [9] K. Adamczyk, A. Morawski, T. Cetner, A. Zaleski, D. Gajda, M. Rindfleisch, M. Tomsic, R. Diduszko, A. Presz, IEEE Trans. Appl. Supercond. 22 (2012) 6200204. [10] S.X. Dou, W.K. Yeoh, O. Shcherbakova, D. Wexler, Y. Li, Z.M. Ren, P. Munroe, S.K. Tan, B.A. Glowacki, J.L. MacManus-Driscoll, Adv. Mater. 18 (2006) 785. [11] S. Zhou, A.V. Pan, D. Wexler, S.X. Dou, Adv. Mater. 19 (2007) 1373. [12] T. Cetner, A. Morawski, K. Adamczyk, M. Rindfleisch, M. Tomsic, A. Zaleski, D. Gajda, A. Presz, High Press. Res. 32 (2012) 419. [13] M. Wo zniak, K.L. Juda, S.C. Hopkins, D. Gajda, B.A. Glowacki, Supercond. Sci. Technol. 26 (2013) 105008. [14] J.W. Ekin, Experimental Techniques for Low-temperature Measurements, Oxford University Press, 2006. [15] T. Holubek, M. Dhalle, P. Kovac, Supercond. Sci. Technol. 20 (2007) 123. [16] D. Gajda, A. Morawski, A.J. Zaleski, T. Cetner, M. Małecka, A. Presz, M. Rindfleisch, M. Tomsic, C.J. Thong, P. Surdacki, Supercond. Sci. Technol. 26 (2013) 105008. [17] M.S.A. Hossain, C. Senatore, M.A. Rindfleisch, R. Flukiger, Supercond. Sci. Technol. 24 (2011) 075013. [18] S.A. Keys, D.P. Hampshire, Handbook of Superconducting Material II, Institute of Physics Publishing, Bristol and Philadelphia, 2003. [19] D. Gajda, A. Morawski, A. Zaleski, M. Kurnatowska, T. Cetner, G. Gajda, A. Presz, M. Rindfleisch, M. Tomsic, Supercond. Sci. Technol. 28 (2015) 065003. [20] A. Morawski, T. Łada, K. Przybylski, Phys. C 387 (2003) 143. [21] J.D. McCambridge, The Superconducting Properties of Niobium-titanium Alloy Multilayers, Yale University thesis, 1995. Doctor of Philosophy.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Experimental research on electric field jump in low magnetic fields: Detection of damage in new ex-situ MgB2 barriers in MgB2 wires D. Gajda a, *, A. Morawski b, A. Zaleski c, M.S.A. Hossain d, M. Rindfleisch e, T. Cetner b a
International Laboratory of High Magnetic Fields and Low Temperatures, Gajowicka 95, 53-421 Wroclaw, Poland Institute of High Pressure Physics, Polish Academy of Sciences, Sokolowska 29/37, 01-142 Warszawa, Poland c lna 2, 50-422 Wroclaw, Poland Institute of Low Temperature and Structure Research, Polish Academy of Sciences, Oko d Institute for Superconducting and Electronic Materials, AIIM, University of Wollongong, North Wollongong, NSW 2519, Australia e Hyper Tech Research, Inc, 1275 Kinnear Road, Columbus, OH 43212, USA b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 20 April 2015 Received in revised form 11 June 2015 Accepted 14 June 2015 Available online 20 June 2015
We explored the incorporation of field sweep (constant current and rapidly increasing magnetic field) into the four-probe method as a new technique to detect defects in barrier layers in superconducting MgB2 wires. This method allows us to observe jumps in the electric field in low magnetic fields. The scanning electron microscopy results indicate that such a jump originates from cracks in Nb barriers and ex-situ MgB2 barriers. Our research indicates that the field sweep allows us to detect damage to barriers that are made of superconducting materials. This method can be the basis for an industrial method for detecting damages in MgB2 wires. These defects reduce the critical current of MgB2 wire. Detection and removal of these defects will allow us to produce MgB2 wires with ex-situ MgB2 and Nb barriers that will have improved critical current density. Manufacturing of MgB2 wires with new ex-situ MgB2 barriers is a new technological concept. This type of barrier is cheaper and easier to manufacture, leading to cheaper MgB2 wires. Moreover, we show that critical current can be measured by two methods: current sweep (constant magnetic field and quickly increasing current) and field sweep. © 2015 Elsevier B.V. All rights reserved.
Keywords: Damage detection MgB2 wires Critical current
1. Introduction The ex-situ MgB2 barrier was invented in 2006 by A. Morawski of the Institute of High Pressure Research in Warsaw (Poland) and B Głowacki of the Department of Materials Science and Metallurgy, University of Cambridge (United Kingdom) [1]. MgB2 wires with exsitu MgB2 barriers are fabricated by the powder-in-tube (PIT) method. This type of barrier is also cheaper and easier to use in the PIT treatment than Nb, Ti, Fe, and Ta barriers. The energy dispersive X-ray spectroscopy (EDX) and X-ray diffraction (XRD) results of Kario et al. [2] indicate that the ex-situ MgB2 barrier allows us to obtain high-purity in-situ MgB2 material in the core or filaments of the wire. This is because the ex-situ MgB2 barrier reduces the reactions of Mg and the sheaths of the wires. Moreover, the ex-situ MgB2 barrier does not increase the hardness after cold drawing as in the case of an Nb barrier. This might indicate that cold drawing will create less damage in ex-situ MgB2 barriers.
* Corresponding author. E-mail address: [email protected] (D. Gajda). http://dx.doi.org/10.1016/j.jallcom.2015.06.103 0925-8388/© 2015 Elsevier B.V. All rights reserved.
Scanning electron microscope (SEM) studies show that the exsitu MgB2 barrier is uniformly distributed along the wire and creates good contact with the wire sheath and the in-situ MgB2 material because it has low shrinkage [2e5]. Measurements of the critical current for MgB2 wires with ex-situ MgB2 barriers, and iron and copper sheaths show that these wires have very low critical current density (Jc) anisotropy of about 2e4 % (with only a small difference between the Jc in perpendicular and parallel magnetic fields) [4,5]. These advantages demonstrate that this barrier can also be applied in other superconducting wires. Further studies are needed to improve the barrier properties. The research showed by H€ aßler [6], Maeda [7], Susner [8], Adamczyk [9], Dou [10] and Zhou [11] for MgB2 wires with Nb barrier and MgB2 materials indicate that these wires have great potential for application. SEM images of MgB2 wires [12] show that a Nb barrier might be damaged after cold drawing. This damage to the barrier causes the formation of phases such as Cu2Mg [13] and pure B. In addition, it reduces the amount of superconducting material and decreases the critical current density. Eikin presents results on the UeI characteristic for low-temperature superconductor (LTS) wires [14]. These measurements suggest that this method makes it possible to detect
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a fault in the structure of NbTi wires. When the current sweep type is used in MgB2 wires, we cannot identify reasons for the appearance of an electric field. Similar electric fields occur for short wires [15] and NbTi wires with damaged microstructure [14]. Studies indicate that the critical current density Jc on short and long MgB2 wires is similar [16,17]. The Jc in these wires is dependent on parameters of the annealing process. In contrast, Jc in the wires with the damaged microstructure depends on the number and size of defects. Currently, there are no reports of a measurement method that makes it possible to detect damage in Nb and ex-situ MgB2 barriers. The lack of such a method impedes study on increasing Jc in MgB2 wires. In this paper, we present the results obtained with the field sweep method for MgB2 wires with Nb and ex-situ MgB2 barriers. These measurements indicate that the field sweep method allows us to detect damage in Nb and ex-situ MgB2 barriers and will allow us to eliminate these defects in the future.
changeable flux of magnetic field. This theory indicates that the loop in Fig. 1 is generated by the electromotive force. In Fig. 1, for quick increase of the magnetic field, we can determine Ic on the basis of 1 mV/cm because the electric field of 2 mV/cm comes from the electromotive force. The measurements showed that the transition to the normal state in the wires during slow and quick increase of the magnetic field is at the same point (blue line A e Fig. 1). The quick increase in the magnetic field allows measurements of Ic for large values of current and does not destroy the samples. We carried out a study of the critical current for undoped and doped MgB2 wires with Nb, Fe (Hyper Tech Research, Inc) sheaths and ex-situ MgB2 barriers (Institute of High Pressure Physics), NbTi wires (Donetsk Institute for Physic and Engineering, National Academy of Sciences of Ukraine), and the commercial bismuth strontium calcium copper oxide (BSCCO e SuperPower) tape in perpendicular and parallel magnetic fields. These results showed that the value of critical current from both measurement methods (current and field sweep method) is the same.
2. Experiment and sample preparation 3. Experimental results Critical current was measured using the four-probe resistive method at 4.2 K by two different methods: the current sweep type constant magnetic field and increasing current [14,18] - and the field sweep type - constant current and increasing magnetic field. Studies were made for specially selected samples with low critical current value. The length of wires was 20 mm in perpendicular magnetic field and 70 mm for measurements in parallel magnetic field. The critical current (Ic) was determined on the basis of the 1 mV/cm criterion. Critical current measurements were conducted in the International Laboratory of High Magnetic Fields and Low Temperatures in Wroclaw [16]. The research was carried out on MgB2 wires with ex-situ MgB2 barriers obtained from the Institute of High Pressure, Polish Academy of Sciences (PAS), and MgB2 wires with Nb barriers fabricated by Hyper Tech Research. The wires were annealed under pressure [19,20] at the Institute of High Pressure in Warsaw. Parameters of the measured samples are presented in Tables 1 and 2 (with Nb wire in copper also studied for comparison). The magnetic field was generated by a Bitter magnet. Its maximum magnetic field of about 14 T was obtained over a time of 170 s. Measurements of critical temperature were conducted by using a physical properties measurement system (PPMS Model 7100, Quantum Design, AC current e frequency of 15 Hz and 100 mA) with fields ranging from 0 up to 14 T at the International Laboratory of High Magnetic Fields and Low Temperatures. Microstructure analysis was performed on a FEI Nova Nano SEM 230 in the Institute of Low Temperature and Structure Research (ILT&SR), PAS in Wroclaw and with the help of a Carl Zeiss microscope in the Institute of High Pressure (IHP), PAS in Warsaw. The value of the critical current in the current sweep type was determined on the basis of 1 mV/cm. The same criterion was used in the field sweep type. Fig. 1 indicates that a quick increase in the magnetic field from 0 T to 14 T creates a loop of electric field (t ¼ 3 min e t the times in which the perpendicular magnetic field increases from 0 T to 14 T, and from 14 T returns to 0 T). A slow increase of the magnetic field does not create this loop, however. These studies have proved that the electromotive force creates a
SEM studies showed that most of the MgB2 wires have good Nb and ex-situ MgB2 barriers, although we sometimes see damage in Nb and ex situ MgB2 barriers. This damage is created during cold drawing of MgB2 wires. SEM images were collected for samples and at places where there was a jump in the electric field in low magnetic fields. The SEM image in Fig. 2 shows breakage in the Nb barrier for SiC doped MgB2 wires with GlidCop® sheath. This breakage reduces the amount of MgB2 superconducting material and creates intermetallic phases inside and outside of the fiber. SEM images and EDX maps (Fig. 3) of cross-sections of MgB2 wires with ex-situ MgB2 barriers show the places where there this barrier is damaged. In MgB2 wires with ex-situ MgB2 barriers, cold drawing occasionally creates the damage in the ex-situ MgB2 material. This damage in the ex-situ MgB2 material allows diffusion of Cu from the sheath to react with Mg as well as the in-situ MgB2 material. This migration through the ex-situ MgB2 barrier creates phases of Cu2Mg and CuMg2 [13]. These phases are very deleterious, because they reduce the number of connections between grains in the ex-situ MgB2 barrier. A smaller number of connections between grains can significantly reduce the transport critical parameters for the ex-situ MgB2 barrier. This process reduces the amount of MgB2 material in the fiber, and may create a higher amount of boride phase in the fiber. Our studies indicate that most samples have the characteristics shown in Fig. 1. Sometimes, however, we see a jump in the electric field in low magnetic fields (Fig. 4(a) e black curve). We will hereafter call this magnetic field rangeethe transition region (TR). Fig. 4(a) shows the results obtained by the field sweep type and a comparison of MgB2 wires with Nb barrier in a GlidCop® sheath with and without a transition region. The transition region in MgB2 wires with Nb barrier is shown in low magnetic fields from 0.1 T to 1 T (sample A1 e black curve (Fig. 4(a)). This range of magnetic field indicates that it comes from the Nb barrier, because pure Nb has upper critical field close to 1 T [21]. We might see that the transition region decreases the critical magnetic field (BK) from 8.5 T to 7 T
Table 1 Composition and construction of MgB2 wires. Samples identifier
Mg:B additive
Barrier
Mono sheath
Multi sheath
Fill factor (%)
Diameter (mm)
Multi filament
A1 (damaged) A2 B1 (damaged) B2
1.10:2 10%SiC 1.10:2 10%SiC 1:2 1:2
Nb Nb ex situ MgB2 ex situ MgB2
Cu (20%) Cu (20%)
Glidcop (30%) Glidcop (30%) Glidcop (50%) Glidcop(50%)
16.6 16.6 50 50
0.83 0.83 1.2 1.2
6 6 1 1
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Table 2 Hot isostatic pressing (HIP) parameters for MgB2 wires with Nb barrier. Samples identifier
Annealing time (min)
Annealing temperature ( C)
Pressure (MPa)
A1 A2 B1 B2
15 15 15 15
700 700 700 700
0.1 0.1 20 20
Fig. 1. Transport E e B (I ¼ const) curves for sample A2 under the field sweep method; 3 min and 8 min are the times in which the perpendicular magnetic field increases from 0 T to 14 T, and from 14 T returns to 0 T.
Fig. 2. Cross-section of SiC doped MgB2 wire with GlidCop® shield for sample A1 (SEM image).
(20%) and creates a jump in the electric field above 1 T with a value of about 7.5 mV/cm. In Fig. 4(b), we see the results of measurements obtained by using the current sweep type. These results indicate that the transition region does not allow us to determine values of Ic on the basis of the 1 mV/cm criterion. On the contrary, the results in Fig. 4(a) demonstrate that the field sweep type might even make it possible to determine the value of Ic in MgB2 wires with damaged Nb barriers. Furthermore, the results indicate that we may determine the precise value of Ic with the criterion of 1 mV/cm for MgB2 wires without a transition region. Fig. 4(c) presents the transport critical current density (Jct) dependence on the magnetic field (B). These results indicate that the transition region reduces the value of Jct in a MgB2 wire with a GlidCop® sheath by about 55%. In Fig. 5(a), (c) and (d), we see the comparative results for MgB2 with ex situ MgB2 barrier. These results indicate that a jump in the electric field in low magnetic fields was observed above 7.5 A (Fig. 5(a)). This jump in the electric field will be called the transition region, because it also appears in low magnetic fields. We do not see a jump in the electric field in low magnetic fields for low
currents from 0.75 A to 1.5 A (Fig. 5(c)). Fig. 6 shows the results of MgB2 wire with good ex situ MgB2 barrier. Comparing the results in Figs. 5(a) and 6(a) we see that the transition region decreases the critical magnetic field and creates the jump in the electric field above 0.05 Te160 mV/cm (Fig. 5(d)). Calculations show that the resistivity at points C, D, E, and F is similar to the GlidCop® resistivity at liquid helium temperature. The results might indicate that the transition region significantly increases the path penetration of current into the superconducting material. Figs. 5(b) and 6(b) show the measurement results obtained by using the current sweep type. The results in Fig. 5(b) indicate that the transition region does not allow us to determine values of Ic on the basis of the 1 mV/cm criterion, because we observe a very high electric field. The results in Fig. 5(a) show that the field sweep type allows assessment of the approximate value of Ic in MgB2 wires with a transition region. On the other hand, Fig. 6(b) indicates that we might determine the value of Ic for MgB2 wires without a transition region. A low electric field creates a penetrating current into the superconducting material. The results in Fig. 5(b) show that the transition at point B has the same parameters (electric and magnetic fields) as the transition in Fig. 5(a) at point A. These results show that the current sweep type allows us to detect superconducting material, although these results do not enable us to determine the exact value of the critical current. Moreover, Fig. 5(b) shows a lack of magnetoresistance in magnetic field from 0 T to 5 T. This result also indicates that we have superconducting material in MgB2 wires. Fig. 5(e) shows the dependence of the transport critical current density (Jct) on the magnetic field (B). These results demonstrate that the transition region reduces the value of Jct in MgB2 wire with an ex-situ MgB2 barrier by about 25e35%. Transport measurements R ¼ f(T) for the samples with the transition region showed that these samples have a jump in resistance at low temperatures (Fig. 7). In MgB2 wires with Nb barriers we see this jump in resistance at about 9 K (Fig. 7(a)). This temperature is very similar to the critical temperature of Nb. In samples without a transition region we do not see jump resistance at about 9 K (Fig. 7(a)). Furthermore, measurements show that the jump in resistance is not visible in samples with their transition region in magnetic field above 1 T (Fig. 7(a)). This result indicates that this jump can be created by Nb material, because Nb is not a superconductor in magnetic fields above 1 T [21]. The results showed that the transition region degrades the critical temperature in MgB2 wires with Nb barriers by about 2%. In MgB2 wires with ex-situ MgB2 barriers, we also see the resistance jump at 7 K (Fig. 7(b)). These results show that this jump in resistance might significantly reduce the critical parameters (Birr and Jc) of the ex-situ MgB2 barrier. Low critical parameters of the ex-situ MgB2 barrier can be caused by low annealing temperature and short annealing time. This jump is not visible in MgB2 wires with ex-situ MgB2 barriers without the transition region (Fig. 7(c)). The results indicate that the transition region does not reduce the transport critical temperature (Tct) in MgB2 wires with an ex-situ MgB2 barrier (Fig. 7(b) and (c)). Fig. 8(a) shows magnetic measurements of critical temperature (Tcm) show that the measured characteristics of samples with and without transition regions are very similar. Furthermore, the results
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Fig. 3. (a) and (b) SEM images; (c) EDX maps for a cross-section of SiC doped MgB2 wire with ex-situ MgB2 barrier in GlidCop® shielding. Red circles show the damaged areas. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 4. (a) Transport E e B (I ¼ const) curves for samples A1 and A2, (b) Transport E e I (B ¼ const) curves for samples A1 and A2, (c) Field dependence of transport critical current density (Jct e B) for samples A1 and A2eJct comparison of MgB2 wires with and without a transition region. The measurements were performed in perpendicular magnetic field.
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Fig. 5. (a), (c), and (d) Transport E e B (I ¼ const) curves for sample B1, (b) Transport E e I (B ¼ const) curves for sample B1. (e) Transport Jct e B curves for samples B1 and B2 e The Jct comparison is for MgB2 wires with and without a transition region. The measurements were performed in parallel magnetic field.
indicate that the value of the magnetic critical temperature does not change in a sample with a damaged Nb barrier. The hysteresis loops of samples with and without a transition region have the same shape (Fig. 8(b)). We observe only a small decrease in magnetization for the sample with a transition region. This decrease is caused by the presence of less superconducting material in the sample with a damaged Nb barrier. 4. Discussion and conclusions Our research has proved that the transition region decreases the critical current and critical field in MgB2 wires with Nb and ex-situ MgB2 barriers. The results show that the appropriate method for current density characterization is the swept-field type, because it allows us to detect damage in MgB2 wires and allows us to estimate the value of the critical current in MgB2 wires with damaged barriers. Furthermore, it is demonstrated that the current sweep type
cannot indicate whether the electric field creates a penetrating current that passes into the superconducting material or damaged barrier. Our results indicate that the damaged barrier lengthens the path of the penetrating current into the superconducting material. The jump in the electric field in MgB2 wires with Nb barriers appears in magnetic field of 0.6 Te1 T. The magnetic field of 1 T is the upper critical field (Bc2) of Nb. Detection of this type of damage can correctly indicate the influence of annealing time, annealing temperature, pressure on the critical parameters of MgB2 wires and its microstructure. These results will help to make progress in the production of MgB2 wires. The jump electric field in MgB2 wires with ex-situ MgB2 barriers appears in magnetic fields from 0.05 T to 0.25 T. The poor connection between the grains in ex-situ MgB2 material can lead to a low Bc2 magnetic field because ex-situ MgB2 material is annealed at low temperature around, 700 C and moderate pressure. The pressure increases the melting temperature of the ex-situ MgB2. The higher melting temperature decreases
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Fig. 6. (a) Transport E e B (I ¼ const) curves for sample B2, (b) Transport E e I (B ¼ const) curves for sample B2. The measurements were performed in parallel magnetic field.
transport critical parameters. These results show that the jump in the electric field might create movement of vortices in the barrier and penetrating current that goes into the superconducting material. SEM images show that MgB2 wires with Nb barriers have their damage on the outer side of this barrier, because the damaged area originates outside the Nb barrier. In MgB2 wires with ex-situ MgB2 the damage is on the inner side of this barrier, because defects are formed inside the ex-situ MgB2 barrier. These defects in the ex-situ MgB2 barrier arise from pure Mg (from the in-situ MgB2 material)
and Cu (from the GlidCop® sheath). Migration of Mg and Cu can reduce the density of the ex-situ MgB2 barrier. Greater density of the ex-situ MgB2 barrier can be obtained by using smaller grains. Smaller grains allow more connections between the grains. Higher connectivity between the grains will also reduce the migration of Mg and Cu. These results indicate that in MgB2 wires with good barriers, the two superconducting materials create one superconducting material. Our measurements show that the cracking of Nb and ex-situ MgB2 barriers divides these materials into separate
Fig. 7. (a) Transport R e T (IAC ¼ 100 mA) curves for samples A1 and A2 with Nb barrier, Transport R e T (IAC ¼ 100 mA) curves for samples (b) B1 and (c) B2 with ex-situ MgB2 barrier.
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Fig. 8. (a) Magnetic T dependence (m e T) (B ¼ 20 mT) curves for samples A1 and A2 with Nb barrier; (b) hysteresis loops (m e B) (T ¼ 4.2 K) for samples A1 and A2 with Nb barrier.
superconducting materials. The results in Fig. 5(c) show that low currents do not cause the jump in electric field. The results may indicate that we have a small layer of ex-situ MgB2 barrier which is not damaged. The measurements show that the transport method based on sweeping the temperature (constant magnetic field and current, and increasing temperature) allows us to detect damage in Nb and ex-situ MgB2 barriers. In contrast, the magnetic method of temperature sweeping (constant magnetic field and increasing temperature) does not allow us to detect defects in the Nb barrier. The lack of jumps in electric field and resistance in MgB2 wires with good barriers is positive, because it allows us to apply these barriers in MgB2 wires. These jumps create heat which leads to loss (Jc). The transport methods (field and temperature sweep type) can be the basis for an industrial method for detecting defects in the barriers of MgB2 wires. Acknowledgments This work was supported by the Institute of High Pressure Physics PAS and International Laboratory of HMF and LT and was also partially supported by Australian Research Council (Grant No. DE130101247) and 2014 UOW-URC grants. References [1] Patents: “Composite electric conductors and method for their manufacture” UK 0706919.8; USA: 60/907590; http://www.wipo.int/pctdb/en/index.jsp [2] A. Kario, A. Morawski, W. H€ aßler, M. Herrmann, C. Rodig, M. Schubert, K. Nenkov, B. Holzapfel, L. Schultz, B.A. Glowacki, S.C. Hopkins, Supercond. Sci. Technol. 23 (2010) 025018.
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