MgB2/Fe wire samples were prepared by using ball-milled 96% boron (B) powder with strong semi-crystalline phase. We observed samples that contained ball-milled 96% B in comparison with one made from as-supplied commercial 96% B, with the results showing a significant enhancement in the high field transport critical current density (Jct) due to small grain size and better reactivity. However, the inter-grain connectivity became worse, which could lead to poor Jct in low field and an increased level of disorder

Kim, Jung Ho

Rindfleisch, Matthew

Tomsic, Michael

Xu, Xun

Zhang, Yun

Zhao, Yue

English

Research Online

University of Wollongong Research Online Faculty of Engineering - Papers (Archive) Faculty of Engineering and Information Sciences 1-1-2009 Superconducting properties of MgB2 wire using ball milled low purity boron Xun Xu University of Wollongong, xun@uow.edu.au Jung Ho Kim University of Wollongong, jhk@uow.edu.au Yun Zhang University of Wollongong, yunz@uow.edu.au Yue Zhao University of Wollongong, yue_zhao@uow.edu.au Matthew Rindfleisch Nuclear Hyper Tech Research, Inc See next page for additional authors Follow this and additional works at: https://ro.uow.edu.au/engpapers  Part of the Engineering Commons https://ro.uow.edu.au/engpapers/1124 Recommended Citation Xu, Xun; Kim, Jung Ho; Zhang, Yun; Zhao, Yue; Rindfleisch, Matthew; and Tomsic, Michael: Superconducting properties of MgB2 wire using ball milled low purity boron 2009, 2714-2717. https://ro.uow.edu.au/engpapers/1124 Research Online is the open access institutional repository for the University of Wollongong. For further information contact the UOW Library: research-pubs@uow.edu.au Authors Xun Xu, Jung Ho Kim, Yun Zhang, Yue Zhao, Matthew Rindfleisch, and Michael Tomsic This journal article is available at Research Online: https://ro.uow.edu.au/engpapers/1124    E   Superconducting Properties of MgB2 Wire Using Ball-Milled Low Purity Boron  Xun Xu, Jung Ho Kim, Yun Zhang, Yue Zhao, Matthew Rindfleisch, and Michael Tomsic    Abstract—MgB2 Fe wire  samples were  prepared by  using ball-milled 96% boron (B) powder with strong semi-crystalline phase. We observed samples that contained ball-milled 96% B in comparison with one made from as-supplied commercial 96% B, with the results showing a significant enhancement in the high field transport critical current density ( Ct ) due to small grain size and better reactivity. However, the inter-grain connectivity became worse, which could lead to poor   Ct  in low field and an increased level of disorder.  Index Terms—Ball milling, magnetic critical current density, MgB2    wire, oxygen.   I.  INTRODUCTION  NORMOUS research efforts have been directed at in the past seven years since its discovery in 2001 [1], focusing on materials performance properties, wire conductor development, and coil demonstrations. Unfortunately, however, the critical current density        of un-doped           is drastically decreased with an increasing external field. This is because of its poor flux pinning properties, when compared to high temper- ature superconductors (HTS) [2]. In polycrystalline wire and tape samples, significant break- throughs in the improvement of critical current density        , irreversibility field          , and upper critical field           were achieved through chemical doping. Such doping is effective in improving the            characteristics of           , especially in the high field region. However, the connectivity between the grains might be further improved. In un-doped polycrystalline            samples, however, grain boundary pinning seems to play the dominant role. So, the pre- cursor powders are very important for the properties of the final material.             grain size is strongly influenced by the par- ticle size of the precursor powders, especially of the boron (B) powder [3]–[6]. Mechanical alloying of the precursor powders reduces the grain size and improves the critical current [7]–[9].   Manuscript received August 26, 2008. First published June 30, 2009; current version published July 15, 2009. This work was supported in part by the Aus- tralian Research Council, in part by Hyper Tech Research Inc., OH, USA, in part by Alphatech International Ltd., NZ, in part by Tangshan Weihao Magne- sium Powder Ltd., and in part by the University of Wollongong. X. Xu, J. H. Kim, Y. Zhang and Y. Zhao are with the Institute for Supercon- ducting and Electronic Materials, University of Wollongong, Wollongong, NSW 2522, Australia (e-mail: xx702@uow.edu.au; jhk@uow.edu.au; yz268@uow. edu.au; yue@uow.edu.au). M. Rindfleisch and M. Tomsic are with Hyper Tech Research, Inc., Columbus, OH 43212 USA (e-mail: mrindfleisch@hypertechresearch.com; mtomsic@hy- pertechresearch.com). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Our group focused on the phase transformation and super- conducting properties of              bulk samples created from ball-milled boron (B) powder. As the first step, we studied the effects of ball milling, using different media such as acetone, ethanol, and toluene, on the microstructures and   of the resulting             samples. Using toluene processing led to enhancements in the magnetic critical current density           in high field [10]. Recently, it has been shown that the reactivity of low-cost 96% B powder can be improved by using ball-mill processing, leading to enhanced magnetic critical current den- sity compared to the original 96% B powder [11]. However, these bulk sample results just raised the possibility of using low grade 96% B powder, since the performance of wire made from ball-milled low purity boron is much more important for industrial applications. In this work, we evaluated the superconducting properties of made from low-grade 96% commercial B powder with a strong semi-crystalline phase. The particle size of B, the transport critical current density         , and the microstructures of wire using the ball-milled B are presented in com- parison with a reference sample made under the same sintering conditions.  II.  EXPERIMENTAL METHODS  monofilament wires were prepared by an in situ reaction process and the powder-in-tube method. B powder (Tangshan WeiHao, China) with strong semi-crystalline phase (Boron 96%, Magnesium 1.79%,             Insoluble 0.93%, Moisture 0.17%, etc.) was processed by ball milling, with toluene as the ball-milling medium. The ball-milling process was carried out for 12 hrs at a rotation speed of 160 rpm. The powder to ball ratio was 1: 16 in a planetary ball-mill with an agate jar and balls 5 mm and 10 mm in size. The powders were then dried in a vacuum oven to evaporate the toluene. For our experiments, two kinds of B powders were prepared, with and without ball milling: these are denoted by BP96 and P96, respectively. Here, the ball-milled boron is denoted by the initial B. The boron powder particle size and distribution were determined by a JL-1166 Laser Particle Sizer. We also prepared a reference             sample using 99% amorphous B for comparison. Magnesium (99%, 325 mesh) and the different boron powders with the nominal atomic ratio of                       to ball-milled boron were mixed through grinding and were put into Fe tubes with a length of 140 mm, an outer diameter (O.D) of 10 mm, and an inner diameter (I.D) of 8 mm. The packing process was car- ried out in air. Both ends of the tubes were sealed with aluminum pieces, and then the tubes were drawn to a wire with a diameter          Fig. 1.  XRD patterns for different boron powders.   of 1.4 mm. Short wire samples (4 cm each) were sealed with Zr foil, then sintered, with a heating rate of 5 in flowing high purity Ar to 700 and a dwell time at the final temper- ature of 30 minutes, followed by furnace cooling to room tem- perature. The volume fraction of the superconducting core in the final wire was approximately 48%. The transport critical current at 4.2 K was measured by the standard DC four-probe resis- tive method with a criterion of 1                in magnetic fields up to 12 T. The magnetization was measured at 5 and 20 K using a Physical Properties Measurement System (PPMS, Quantum Design) in  a time-varying magnetic field  with a 50   sweep rate up to 8.5 T. All the samples had their iron sheaths peeled off for measurements and were rod-shaped with a length of 2.5 mm. The magnetic  was derived from the width of the magnetization loop using Bean’s model [12]. The grain mor- phology and the microstructure of the            with ball-milled boron and without ball-milled boron were studied by scanning electron microscopy (SEM).  III.  RESULTS AND DISCUSSION  A.  Boron Powder With Different Purities  Fig. 1 shows the x-ray diffraction (XRD) patterns for the 96% semi-crystalline boron and 99% amorphous boron powders. The patterns can be indexed according to single phase boron, except for the          peaks (        and 27.8 ), which are indicated with dashed lines for both powders. Furthermore, to examine the phase transformation of using 99% B and 96% B, differential thermal analysis (DTA) was performed, and the results are shown in Fig. 2. The heating rate was 5 under flowing Ar, as with our sintering con- ditions. In both samples, there were two exothermal peaks: The first exothermal peak (a) is due to the reaction between melted and Mg. The   has no melting point, but rather a progressive softening and melting range from 300    to 700 under particular conditions [11]. The crystals begin to break down at 300 , and a series of sub-oxides are produced with partial melting until full fusion is reached at 700 . The main reason for the presence of in the B powder is because B has partially oxidized in air. As for the second exothermal Fig. 2.  Differential thermal analysis for MgB  with different boron powders.   peak (b), it can be attributed to the             phase formation. What is interesting is that the second exothermal peaks of the two samples, (b) and (d), show different behavior. Specifically, the second exothermal peak of the sample using 96% B was slightly shifted to higher temperature, unlike the sample using 99% B. In addition, there was a weak endothermal peak (c) be- fore the exothermal peak related to phase formation (d). This weak peak is related to the melting of Mg at around 650 . Using 96% B with crystalline phase can introduce shifting of the second exothermal peak. We conclude that phase formation of using 96% B with some crystalline B can occur after the Mg has melted. This is because using B with crystalline phase requires more energy, due to poor reactivity between Mg and B [3]. This information was of importance in determining optimal sintering conditions for our samples. We speculate that the ap- propriate sintering temperature is above 650     for           using 96% B, and in particular, above the Mg melting point. Note that the first exothermal peak did not shift to higher temperature. If this peak were related to the solid-solid reaction of           , this would also be shifted because of the different crystallinity.  B.  Selected Media for Ball-Milling For this reason, we tried to modify the semi-crystalline boron via wet milling. As a first step, we studied the effects of  different ball-milling media, such  as  acetone                 , ethanol                 , and toluene              , because these liquid media help to make mixing homogeneous. Fig. 3 shows the magnetic field dependence of  for all samples at 5 and 20 K, including  of the reference sample made from as-supplied 99% B. The  value of the toluene sample was estimated to be                        at 8 T and 5 K. This value is comparable to  those  of chemically doped samples under high field. The  value is much higher than that of the pure reference made without any ball-milling process, by a factor of 20. Using ball-milled B with toluene as the ball-milling medium was a highly effective method to enhance the performance under high field. This is because toluene can prevent the oxida- tion of B powder during ball milling, and the small grain size is effective for enhancing flux pinning at the grain boundaries, which represent effective pinning centers. However, at 20 K, the performance of the toluene sample is slightly lower than       Fig. 3.  The magnetic critical current density (J       ) as a function of field for the MgB   samples [10].   TABLE I PARTICLE  SIZE DISTRIBUTION FOR BORON  POWDERS     that of the reference            sample. This is probably because the ball-milled sample had poor grain connectivity.  C.              Wire Results With the Ball-Milled 96% Boron Table I shows the particle size distributions of (a) P96 and (b) BP96. It can be clearly seen that after 12 hrs ball-milling, the original P96 powder size range changed from 0–3 to 0–0.75 . The median value (D50) is termed the average particle di- ameter. This median particle size was reduced from 0.84 to 0.24      . At the same time, if effects from the shape of the B powder are neglected, the small particles led to a more than twofold increase in the specific surface area value, which should improve the reactivity of BP96 powder under the same solid re- action conditions as the original P96. On the other hand, during the formation of           , if the same particle size of Mg powder and fixed sintering conditions are used, the            particle size is determined by the particle size of the B powder. So, the that is formed from ball-milled B powder (BP96) should have small grain size, which must improve the superconducting prop- erties of the sample. SEM images of           wires made from (a) as-supplied boron (P96) and (b) ball-milled boron powders (BP96), denoted as WPS700 and WBPS700, respectively, are shown in Fig. 4. Here, it is clearly observed that the average grain size of the sample prepared from the ball-milled boron is much smaller than in the sample made from the as-supplied boron. The ball-milled sample also seems to be more consolidated. The transport current              performance of these two sam- ples is shown in Fig. 5. It can be clearly seen that the  of samples prepared from the ball-milled boron showed better per- formance in the field range of 5 to 12 T. This indicates that ball     Fig. 4.  Scanning electron microscope (SEM) images for (top) wire sample pre- pared from as-supplied boron, and (bottom) wire sample prepared from ball- milled boron. All samples were sintered at 700  C for 30 minutes.      Fig. 5.  Transport critical current density (J    ) for MgB   wires as a function of external magnetic field at 4.2 K B99S700 is the reference sample from 99% B.    milling causes reduction of the            grain size, which could act as a source of strong pinning centers due to the increased number of grain boundaries, as mentioned above. However it was concluded that the ball-milling could not help to improve the  at magnetic fields below 5 T. Compared to the mag- netic  under magnetic fields of 5 to 8 T, the transport   advantage is lower. Note that the transport current capacity is the real useful  that flows through the whole of the sample. That is to say, the differences between  and  in may be related to features of the microstructure of the supercon- ducting            core, such as porosity, agglomeration of super- conducting crystals, and fraction of MgO as the main secondary phase. These could have negative effects on the , by acting as obstacles to current flow. Quite interestingly, the effects of these obstacles do not appear in magnetic loop measurements. That is why the magnetic  does not represent the real  of wires [14].      IV.  CONCLUSION  In summary, a study of ball-milling effects on the transport critical current density          of                 wires has been con- ducted. We observed that lattice disorder increased due to the ball milling. It caused both a slight reduction in the transition temperature and degradation of connectivity. It is the main cause for the enhancement of the critical current in the high field region.  ACKNOWLEDGMENT  The authors thank Prof. S. X. Dou and Dr. T. Silver for their helpful discussions, and Dr. J. Horvat for his help with mea- surements at the Institute for Superconducting and Electronic Materials, University of Wollongong.  REFERENCES [1] J. Nagamatsu, N. Nakagawa, T. Muranaka, Y. Zenitani, and J. Akim- itsu, “Superconductivity at 39 K in magnesium diboride,” Nature, vol. 410, pp. 63–64, Mar. 2001. [2] D. C. Larbalestier et al., “Strongly linked current flow in polycrys- talline forms of the superconductor MgB ,” Nature, vol. 410, pp. 186–189, Mar. 2001. [3] S. K. Chen, K. A. Yates, M. G. Blamire, and J. L. MacManus-Driscoll, “Strong influence of boron precursor powder on the critical current den- sity of MgB ,” Supercond. Sci. Technol., vol. 18, pp. 1473–1477, Oct. 2005. [4] W. Häßler et al.,  “ MgB   bulk and tapes prepared by mechanical alloying: Influence of the boron precursor powder,” Supercond. Sci. Technol., vol. 19, pp. 512–520, Apr. 2006. [5] G. Giunchi et al., “Grain size effects on the superconducting properties of high density bulk MgB ,” Supercond. Sci. Technol., vol. 17, pp. S583–S588, Aug. 2004. [6] X. Xu et al., “Effect of boron powder purity on superconducting prop- erties of MgB ,” Supercond. Sci. Technol., vol. 19, pp. 466–469, Mar. 2006. [7] O. Perner et al., “Stoichiometry dependence of superconductivity and microstructure in mechanically alloyed MgB ,” J. Appl. Phys., vol. 97, pp. 056105(1)–056105(3), Feb. 2005. [8] C. Fischer et al., “Preparation of MgB   tapes using a nanocrys- talline partially reacted precursor,” Appl. Phys. Lett., vol. 83, pp. 1803(1)–1803(3), Sep. 2003. [9] H. Fang, S. Padmanabhan, Y. X. Zhou, and K. Salama, “High critical current density in iron-clad MgB   tapes,” Appl. Phys. Lett., vol. 82, pp. 4113(1)–4113(3), Jun. 2003. [10] X. Xu, J. H. Kim, W. K. Yeoh, Y. Zhang, and S. X. Dou, “Improved J  of MgB   superconductor by ball milling using different media,” Supercond. Sci. Technol., vol. 19, pp. L47–L50, Nov. 2006. [11] X. Xu et al., “Phase transformation and superconducting properties of MgB   using ball-milled low purity boron,” J. Appl. Phys., vol. 103, pp. 023912(1)–023912(7), Jan. 2008. [12] C. P. Bean, “Magnetization of high-field superconductors,” Rev. Mod. Phys., vol. 36, pp. 31–39, Jan. 1964. [13] J. H. Kim, S. X. Dou, J. L. Wang, D. Q. Shi, X. Xu, M. S. A. Hos- sain, W. K. Yeoh, S. Choi, and T. Kiyoshi, “The effects of sintering temperature on superconductivity in MgB /Fe wires,” Supercond. Sci. Technol., vol. 20, p. 448, Apr. 2007. [14] J. Horvat, W. K. Yeoh, J. H. Kim, and S. X. Dou, “Transport and mag- netic critical current in superconducting MgB  wires,” Supercond. Sci. Technol., vol. 21, p. 065003, Jan. 2008. 

Superconducting properties of MgB2 wire using ball milled low purity boron

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Superconducting properties of MgB2 wire using ball milled low purity boron

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