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Effect of Sintering Temperature on the Rolled Copper-Sheathed Monofilament MgB2 Superconducting Wire S. Herbirowo1, A. Imaduddin1, N. Sofyan2 and A. H. Yuwono2 1 Research Center for Metallurgy and Materials – Indonesian Institute of Sciences (P2MM – LIPI), Kawasan Puspiptek, Serpong, Tangerang Selatan 15314,Indonesia 2 Department of Metallurgical and Materials Engineering, Faculty of Engineering Universitas Indonesia, Depok 16424, Indonesia * Corresponding author, e-mail: [email protected] Abstract. The manufacture of low critical temperature (Tc) MgB2 superconductor wire fabricated by powder-in-tube (PIT) is a multi-step process. The main difficulty is that the value of Tc superconductor wire is determined by various factors different for each step. The objective of this study is to analyze the effect of selected sintering parameters on the properties of final rolled material. The fabrication process of 2 m rolled-copper sheath monofilament superconductor MgB2 wire using mechanical deformation process including rolling and drawing has been carried out. Starting powders, containing nominal composition of MgB2, were inserted into Cu tube and rolled until it reached a diameter of 2 mm. The process of rolling of copper tube was subsequently repeated in order to obtain 3 samples and then they were heattreated at 800°C respectively. We evaluated the doping effect of two types of multiwall carbon nanotube (CNT) with different aspect ratios on MgB2/Cu monofilament wires. Surface morphology was analyzed with SEM, crystal structure of MgB2 with XRD, whereas temperature dependence resistivity with four-point probe. According to XRD results, MgB2 phase were detected in all samples. SEM-EDX images showed porosity and dispersed all elements along cross sectional surface of the samples. The sample with low heating temperature showed more pores than that of high heating temperature. However, the pores increased as the CNTs doped increased of up to 5 wt% make it hard to identify the critical temperature. The sample with heating time of 30 minutes showed a critical temperature of 42 K. Keyword: MgB2, superconducting wire, copper-sheath, critical temperature, critical temperatures INTRODUCTION Recently, many researchers have focused their studies on the effect of chemical substitution on the structure and properties of MgB2. As one of the few successful substitutions, C substitution at B site has been carried out by several groups [1, 2]. The results on C solubility and the effect of C doping on Tc reported so far vary significantly due to the precursor materials, fabrication techniques and processing condition used. Earlier studies have shown that C does substituted at B site, causing decrease in the ‘a’ lattice parameter but ‘c’ lattice parameter remains unchanged [3,4]. One of the methods to prepare MgB2 superconducting wires is by the Powder-In-Tube (PIT) method, whereas the sheath materials used for this purpose are Ag, Cu and Fe. it has been found that, in term of the core composition and microstructure, the Cu sheathed has purer MgB2 phase compared to that of other sheathed materials with only a few micro cracks regardless of the heat treatment conditions . Further, annealing process is known to have an important role in improving the TC value and the stability of MgB2 phase . This study was conducted to determine the optimal annealing process for Cu/MgB2 superconducting wire. The interaction between the sheath materials and the MgB2 core is studied and discussed. EXPERIMENTAL METHOD Specimens were prepared by weighing a stoichiometric amount of raw materials, i.e. MgB2 (Aldrich, ~100 mesh, > 99%) at 0, 5, and 10 wt.% carbon nanotubes (CNT, Cheap Tubes Inc., ~ 20-40 nm). Once that was done, the raw material of MgB2 powders and CNT were mixed and ground. Grinding was done by using agate mortar for 1 hour. In this event, the powder mixtures prepared through the grinding were then poured into copper tube having an outside diameter of 6 mm and an inside diameter of 4 mm. The tube containing a mixture of MgB2 and CNT was then rolled and drawn, repeatedly until the diameter became 2 mm . These fabricated wires were sintered at 800C for 30 min under high purity argon (Ar) gas. The heating rate was 5C/min. After sintering, the wires were furnace-cooled to room temperature. A pure MgB2/Cu wire was also fabricated for comparison by applying the same processing. Characterizations were performed by using X-ray diffraction (XRD, SmartLab Rigaku) with Cu Kα (λ =1.5418 Å) at 2ϴ = 10 – 80°) to see the crystal structure, scanning electron microscope equipped with EDX (SEM/EDX, JEOL 6390A) to see the surface morphology and composition distribution, and resistivity against temperature by using a cryogenic magnet (Teslatron PT, Oxford) employing a four-point probe method to see the nature of superconductivity. RESULTS AND DISCUSSION Figure 1 shows the representative XRD pattern of short and long CNT doped MgB2/Cu wires with various composition of 0; 5; 10%wt after sintering temperature at 800C for 30 min. XRD measurements were performed on the ground MgB2 core samples. As can be seen in this figure, the diffraction patterns of both CNTs showed a broad peak, indicating an amorphous- like phase. Due to their intrinsic nature, the features of XRD patterns of CNTs were close to those of graphite. These phases were not clearly observed in the XRD patterns of CNT doped MgB2/Cu after sintering. 10% CNT 5% CNT 0% CNT Figure 1. Representative powder X-Ray diffraction patterns of MgB2/Cu CNTs composite wire sampels However, small amount of un-reacted CNT could exist in the MgB2 core, because the CNT peak overlaps the MgB2 (101) peak at 2θ ≈ 42.5. We observed that all samples sintered at CNT composition 5wt% seem to have well developed MgB2 with small amounts of MgO. It can be observed that the XRD patterns of the CNT doped /Cu wire are almost independent of the sintering temperature. The peaks in the pattern can be well indexed by MgB2 and CNTs. The CNTs can be detected by XRD in the form of graphitic carbon. As higher density of CNTs were mixed in the sample, the intensity of (101) peak of carbon became stronger, indicating the presence of CNTs. The crystal lattice parameters of 10 wt% CNTs admixed MgB2 have been found to be a = 0.30850 nm, c = 0.5230 nm and also for carbon (graphite) phase have been found to be a =2.4700 nm, c = 6.7900 nm. We propose that due to the presence of CNTs and their catalytic influence, the contraction of B–B plane in MgB2 takes place and the lattice parameter ‘a’ becomes smaller than that of MgB2, while the lattice parameter ‘c’ remains nearly constant. The decrement in the lattice parameter ‘a’ implies that carbon atoms present as impurities in CNTs . The variation of resistance with temperature of MgB2–CNTs composites with varying amount of CNTs has been measured by standard four-probe method and is shown in Fig. 2. The critical temperature of the as synthesized MgB2–CNTs composites span a range ~27 to 40K. The critical temperature of MgB2–CNTs samples increases as the CNTs content increases. Samples show metal like behavior up to the CNTs content 10 wt.% above 40K and semiconducting behavior when CNTs content exceeds 5wt%. Figure 2. shows the dependence of the critical temperature (Tc) on the CNTs content. It can be seen that Tc of the samples increases as CNTs content increases. The increase Tc, based on the known results can be taken to be due to substitution of C in the honeycomb-net plane of boron. Since carbon is unlikely to come from a very stable configuration like CNT, it seems that the carbon, which is present as impurities in the CNT sample (there was no purifications done for the CNT sample synthesized by us), gets incorporated in the boron network. This is in consistent with the recent reports on carbon doped MgB2. The increase of resistivity results due to a relative high resistivity of CNTs (the CNTs employed by us have semiconducting characteristics) compared to MgB2[9,10]. This is thought to occur because the sintering temperature is too high so that the MgB2 compound inside the superconducting wire become unstable. Moreover, according to the resistivity above TC temperature, at samples heated with 800°C, the curve of the resistivity is not linear. This is expected because of the diminishing of the conductivity. It is estimated that Cu layer becomes thinner and lead oxide reaction which cause the wires also have semiconductor properties. Fig. 2. Resistance vs Temperature behavior of of MgB2/Cu wt% CNTs composite samples (= 0, 5, and 10 wt% CNT) Inset shows the variation of critical temperature with different of CNTs Figure 3 shows the EDX mapping images of 10 wt.% CNT-doped MgB2 wires. The images show the cross-section image of wire and the distribution of B, Mg and C elements inside the wire. The EDX results revealed that Mg element dispersed homogenously inside the wire. The distributions of B and C elements show similar pattern indicating CNT homogenously mixed with MgB2. When concentration of CNTs becomes 10 at.%, the SEM of this sample as compared to the pure sample show that the CNTs is dispersed into MgB2 for the 10 at.% admixed samples (Fig. 3b). Fig 3. Analysis of EDX mapping surface of 0 wt.% CNT-doped MgB2 Fig. 4 shows the EDX mapping images of 10 wt.% CNT-doped MgB2 wires. The images show the cross-section image of wire and the distribution of B, Mg and C elements inside the wire. The EDX results revealed that Mg element dispersed homogenously inside the wire. The distributions of B and C elements show similar pattern indicating CNT homogenously mixed with MgB2. CONCLUSION Carbon nanotubes-doped MgB2 superconducting wires have been successfully made by mixing the materials, inserted and sheathed in Cu tube and rolled/ drawn repeatedly into wires having a diameter of 2 mm. Based on XRD results, there are MgO and C (graphite) phases found in 10 wt% samples. It indicates the oxidation process within powder-in-tube method. By using EDX mapping method, carbon (graphite) element was completely dispersed inside the tubes. The addition of CNT did not seem to have a significant influence on the crystal structure of MgB2. The temperature dependence of resistivity results showed that the critical temperatures were detected and keep shifting toward high in addition of 10wt% CNT. ACKNOWLEDGEMENT The authors would like to thanks Universitas Indonesia through Hibah Publikasi Internasional Terindeks untuk Tugas Akhir Mahasiswa Tahun Anggaran 2016 No. 2104/UN2.R12/HKP. 05.00/2016. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. W. Mickelson, John Cumings, W. Q. Han, and A. Zettl, Phys. Rev. B 65 52505 (2002). R. H. T. Wilke, S. L. Bud'ko, P. C. Canfield, D. K. Finnemore Raymond, J. Suplinskas S. T. Hannahs, Phys. Rev. Lett. 92 217003 (2004). 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