Superconducting Wire Performance through Oxygen

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Superior MgB2 Superconducting Wire Performance through Oxygen-Free Pyrene Additive

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Applied Physics Express 5 (2012) 013101 DOI: 10.1143/APEX.5.013101

Superior MgB2 Superconducting Wire Performance through Oxygen-Free Pyrene Additive Minoru Maeda, Jung Ho Kim1 , Yoon-Uk Heo2 , Se Kyun Kwon2 , Hiroaki Kumakura3 , Seyong Choi4 , Yoshitake Nakayama, Yoshiki Takano, and Shi Xue Dou1 Department of Physics, College of Science and Technology, Nihon University, Chiyoda, Tokyo 101-8308, Japan 1 Institute for Superconducting and Electronic Materials, University of Wollongong, North Wollongong, New South Wales 2500, Australia 2 Graduate Institute of Ferrous Technology, Pohang University of Science Technology, Pohang, Gyeongbuk 790-784, Republic of Korea 3 Superconducting Materials Center, National Institute for Materials Science, Tsukuba, Ibaraki 305-0047, Japan 4 Busan Center, Korea Basic Science Institute, Busan 609-735, Republic of Korea Received October 24, 2011; accepted November 25, 2011; published online December 21, 2011 There is great potential to further enhance the critical current density (Jc ) performance of MgB2 wires. Herein, MgB2 wires were fabricated by using oxygen-free pyrene as dopant, and field and temperature dependences of Jc were systematically evaluated. In addition, a strong correlation among sintering temperature, structural parameters, microstructure, and superconducting performance was established. High lattice distortion was obtained by using pyrene as an additive as well as by using a low-temperature sintering process. This arises from vacancy formation, as confirmed by first-principles calculations. The microscopic defects introduce impurity scattering, resulting in the enhancement of the irreversibility field and Jc . # 2012 The Japan Society of Applied Physics

he MgB2 superconductor is a seriously possible replacement for the conventional Nb–Ti superconductor because of its high critical transition temperature of 39 K, lack of weak links, low cost of raw materials, and simple binary composition.1) For real applications, however, further enhancements of the low-field critical current density (Jc ) and irreversibility field (Birr ) in the MgB2 wire have to be considered. Specifically, using carbon-based compounds as additives in MgB2 is currently the most effective route to achieve this.2) Even though enhancement of the low-field Jc has been reported for MgB2 with malic acid additive through a chemical solution route,3,4) which is advantages in terms of both cost and performance compared to other inorganic carbon doping methods,3,4) critical current capability up to the performance of conventional Nb–Ti is required. In the literature, the Jc of the current Nb–Ti wire reaches 105 A cm 2 at 4.2 K and 6 T.5) So far, an inherent problem of pure and carbon doped MgB2 wires is the low mass and volume densities that can be obtained due to its porous nature.2) To overcome this problem, a low-temperature sintering process would be useful below the melting point of magnesium (650 C), since the magnesium is easy to evaporate above its melting temperature, where lower mass density is caused. What is a more serious problem is that residual carbon from the dopant and secondary phases could act as a current-limiting factor in the carbon-doped samples.4) Even in MgB2 with malic acid additive, the residues were found to be existed at the grain boundaries.4) According to our recent work, pyrene, as a polycyclic aromatic hydrocarbon, enables more homogeneous mixing to suppress the aggregation of the residual carbon at the grain boundaries.6) Pyrene in the role of dopant has two major advantages, namely, (1) no oxygen content (C16 H10 ) and (2) low decomposition temperature (145–148 C). However, despite the novel capability, little is known about the transport Jc performance at helium cryogen-free temperatures, which is the most important criterion for industrial applications. In this study, therefore, we report field and temperature dependences of MgB2 wires with pyrene additive. We establish a strong correlation between sintering temperature, structural

T

E-mail address: [email protected]

parameters, microstructure, and superconducting performances. MgB2 /Fe monofilament wires with 10 wt % pyrene additive were prepared by the in situ powder-in-tube method. Pyrene (C16 H10 , 98% 325mesh), magnesium (Mg, 99% 325mesh), and boron (B, 99% 1 m) powders were used as starting materials. The powder mixture was packed into an iron (Fe) sheath 10.0 mm in outer diameter and 8.0 mm in inner diameter. The composite was drawn into a wire 1.42 mm in diameter. The wire was cut into lengths of 35 mm and then sintered under different conditions, ranging from 600 to 800 C and from 30 min to 4 h. X-ray powder diffraction (XRD) measurements were performed to identify the phase composition and extract the structural parameters from Rietveld refinement. Scanning electron microscopy (SEM) observation was employed to assess the morphology and crystal grain size. Transport critical current up to 400 A was measured by using the standard four-probe method with a criterion of 1 V cm 1 . The temperature dependence was also investigated by using a variable temperature insert (VTI) system. The irreversibility field (Birr ) was defined at a Jc criterion of 100 A cm 2 , as obtained from a linear extrapolation of the field dependence of the critical current density. The field and temperature dependence of Jc for MgB2 wires with 10 wt % pyrene, which were sintered either at 600 C for 4 h or at 800 C for 30 min, are shown in Fig. 1. For comparison, an undoped MgB2 wire was also sintered at 650 C for 30 min, which are the best conditions for in-field Jc under our experimental conditions, and the results are plotted as dashed lines in the figure. It is noteworthy that critical current density values at 15, 20, and 25 K are still above 103 A cm 2 , even in fields up to 10, 7, and 4.5 T, respectively, as can be seen in Fig. 1(a). Interestingly, the in-field Jc was found to be considerably decreased by increasing the sintering temperature [Fig. 1(b)]. The increased sintering temperature leads to structural differences in the MgB2 , as illustrated in Table I. Lattice parameters were refined using the RIETAN-2000 program.7) In the case of bare MgB2 , the a- and c-lattice parameters are known to be 3.083–3.086 and 3.521 A, 8,9) respectively. However, the a-lattice parameter was de through pyrene addition. The lattice creased to 3.0695 A

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# 2012 The Japan Society of Applied Physics

Appl. Phys. Express 5 (2012) 013101

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Table I. Refinement results on XRD patterns for MgB2 wire with 10 wt % pyrene additive sintered under different sintering conditions. The values of goodness of fit, s, for all the samples are below 1.27.

Sintering conditions

600 C, 4 h

620 C, 1 h

650 C, 0.5 h

700 C, 0.5 h

800 C, 0.5 h

3.0725 (5) 3.5231 (5)

3.0715 (5) 3.5226 (5)

3.0709 (4) 3.5230 (4)

3.0695 (4) 3.5210 (4)

3.0707 (4) 3.5206 (3)

X

0.21 (1)

0.27 (2)

0.21 (1)

0.21 (1)

0.18 (2)

Y

0.52 (23)

0.37 (28)

0.30 (16)

0.26 (22)

0.20 (16)

Lattice parameters (A) a c Peak broadening coefficients

Weight fractions (%) MgB2

81.5

78.3

80.5

80.8

82.3

MgO

18.5

21.7

19.5

19.2

17.7

(a)

Fig. 2. Cross-sectional SEM images of MgB2 wire with 10 wt % pyrene additive sintered at (a) 600 C for 4 h and (b) 800 C for 30 min. The red circles indicate voids, which are larger for the higher sintering temperature.

(b)

Fig. 1. Magnetic field dependence of transport Jc at 4.2, 10, 15, 20, 25, and 30 K for MgB2 wire with 10 wt % pyrene additive sintered at (a) 600 C for 4 h and (b) 800 C for 30 min. The in-field Jc curves at 4.2 and 20 K for undoped MgB2 wire sintered at 650 C for 30 min are indicated by dashed lines.

spacing was also reduced by increasing the sintering temperature. Quite interestingly, the refined analysis shows a slight increase in the c-lattice parameter for sintering temperature below 650 C, as shown in Table I. Even though expansion and contraction along the c-axis direction hardly occur compared with those along the a-axis direction in the closed-packed hexagonal MgB2 structure, this result may be related to vacancy formation caused by carbon substitution into B sites.4) In order to examine in detail carbon substitution effects on the lattice parameters and vacancy formation behavior, we performed first-principles calculations using the projector-augmented wave method within the generalized gradient approximation for the exchange–correlation interaction.10,11) Supercell geometry in size of 8 formula units was constructed for simulations. It was found that a single carbon substitution into a boron site decreases the a-axis lattice parameter of the basal plane by 0.73%, while the c-axis parameter remains almost equal to the original value

without any change. As a consequence, the lattice volume is contracted 1.43% by carbon substitution. To examine the tendency towards vacancy formation at boron layer, we treated two cases, with and without carbon substitution into a boron site. The calculated vacancy formation energy is 0.37 eV lower with carbon substitution than without carbon. This fact makes it clear that more vacancies are introduced by carbon doping in MgB2 , which is in agreement with experimental observations.4) The actual C substitution level in Mg(B1 x Cx )2 can be inferred from the lattice parameters by comparison with the lattice parameters of single crystal12) and the x of the pyrene doped wires is 0:034{0:039. Vacancies formed at the boron layers due to carbon substitution may cause shrinkage of the a-axis lattice parameter and a slight change of the c-lattice parameter, resulting in structural disorder. Table I also shows the peak broadening coefficients X and Y determined from Rietveld refinement, which are parameters depending on the full-width at half-maximum (FWHM) of XRD peaks.13,14) The X and Y coefficients are determined by the crystallite/subgrain size broadening and the microstrain broadening of FWHM, respectively.13,14) The values of the X coefficients were in a narrow range of 0.18 to 0.27. The average crystallite/subgrain sizes estimated from the coefficients14) were slightly changed, for example, approximately 38 and 44 nm for the pyrene-doped wires sintered at 600 C for 4 h and at 800 C for 30 min, respectively. This result was also indirectly confirmed by SEM observation, as can be seen in Fig. 2. The corresponding samples exhibited more or less the difference. In contrast, the other peak broadening coefficient listed in Table I was found to have a strong correlation with sintering temperature. Specifically,

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at 600 C for 4 h. Our findings provide key atomistic-level insights that pave the way for the Jc enhancement. In summary, we investigated the transport critical current performance of MgB2 wire with 10 wt % pyrene additive over a wide temperature range from 4.2 to 30 K under a high magnetic field up to 15 T, and also evaluated the sintering effects on the structural and superconducting properties. The MgB2 wire with 10 wt % pyrene additive sintered at 600 C showed a decrease in the a-lattice parameter and a slight increase in the c-lattice parameter compared with the undoped wires. Both types of change in the lattice parameters caused extensive structural disorder and improved Birr . Owing to these effects, therefore, pyrene additive as dopant can considerably improve the in-field Jc of MgB2 wires, even over a wide temperature range, where cryogenic losses are tolerable.

Fig. 3. Temperature dependence of Birr for MgB2 wire with 10 wt % pyrene additive sintered at 600 C for 4 h and 800 C for 30 min.

the value of the Y coefficient is two times higher for the pyrene-doped wire sintered at 600 C than for that sintered at 800 C, even though allowance is made for a relatively high margin of error. The increased structural disorder caused by lattice strain can introduce microscopic defects as impurity scattering, shorten the mean free paths, and decrease the superconducting coherence length, resulting in the improvement of the upper critical field (Bc2 ), Birr , and Jc . It is well known that the phase composition and effective cross-sectional area for supercurrent flow, as well as the structural disorder, also have an influence on Jc .2,15) As can be seen in Fig. 2, some voids are indicated by red circles. It is to be noted that voids formed at 600 C seem to be smaller than those formed at 800 C. However, contrary to this, the weight fraction of the secondary phase, MgO, is not sensitive to sintering temperature, as indicated in Table I. This is because Mg is already oxidized during powder mixing and mechanical deformation. The temperature dependence of Birr for the MgB2 wire with 10 wt % pyrene additive is shown in Fig. 3. It was found that the Birr values are increased at lower sintering temperature. The irreversibility field can be theoretically enhanced either by increasing the upper critical field (Bc2 ) or by decreasing the anisotropy parameter ().2) As a result, the enhancement of Bc2 caused by lattice disorder contributes to an increase in the in-field Jc , as can be seen Fig. 1. That is, larger Birr resulted in a considerable increase in the in-field Jc of the MgB2 wire with 10 wt % pyrene additive sintered

Acknowledgments This work was supported by the Australian Research Council (FT110100170) and Hyper Tech Research Inc., OH, USA. This study was also supported by the Japan Society for the Promotion of Science (JSPS) under the Grant-in-Aid program for JSPS fellows and Grant-in-Aid for Research Activity Start-up (Grant No. 23860050).

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