Superconducting Continuous Graphene Fibers via Calcium Intercalation Yingjun Liu,†,§ Hui Liang,‡,§ Zhen Xu,† Jiabin Xi,† Genfu Chen,‡ Weiwei Gao,† Mianqi Xue,*,‡ and Chao Gao*,† †
MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science and Engineering, Key Laboratory of Adsorption and Separation Materials & Technologies of Zhejiang Province, Zhejiang University, 38 Zheda Road, Hangzhou 310027, People’s Republic of China ‡ Institute of Physics and Beijing National Laboratory for Condensed Matter Physics, Chinese Academy of Sciences, Beijing 100190, People’s Republic of China S Supporting Information *
ABSTRACT: Superconductors are important materials in the field of low-temperature magnet applications and long-distance electrical power transmission systems. Besides metal-based superconducting materials, carbon-based superconductors have attracted considerable attention in recent years. Up to now, five allotropes of carbon, including diamond, graphite, C60, CNTs, and graphene, have been reported to show superconducting behavior. However, most of the carbon-based superconductors are limited to small size and discontinuous phases, which inevitably hinders further application in macroscopic form. Therefore, it raises a question of whether continuously carbon-based superconducting wires could be accessed, which is of vital importance from viewpoints of fundamental research and practical application. Here, inspired by superconducting graphene, we successfully fabricated flexible graphene-based superconducting fibers via a well-established calcium (Ca) intercalation strategy. The resultant Ca-intercalated graphene fiber (Ca-GF) shows a superconducting transition at ∼11 K, which is almost 2 orders of magnitude higher than that of early reported alkali metal intercalated graphite and comparable to that of commercial superconducting NbTi wire. The combination of lightness and easy scalability makes Ca-GF highly promising as a lightweight superconducting wire. KEYWORDS: graphene, wet-spinning, graphene fiber, superconducting wire, carbon-based superconductor, Ca intercalation
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under certain conditions. In 1965, Hannay et al. reported superconducting graphite intercalation compounds (GICs) KC8, RbC8, and CsC8 with superconducting transition temperatures (Tc) of 0.55, 0.15, and 0.11 K, respectively.14 Subsequently, more examples of superconducting GICs emerged, and the origin of their superconductivity was theoretically explored. From the early 1990s, metal-intercalated C60 with a higher Tc of up to 33 K has attracted tremendous attention from chemists and physicists 15−19 until the observation of superconductivity in 4 Å single-wall nanotubes (SWCNTs) embedded in a zeolite matrix.20 Then, the renewed interest in carbon-based superconductors turned to GICs with a higher Tc. In 2005, Emery and Weller almost simultaneously reported an important discovery of superconductivity in the bulk intercalation compound CaC6 with a Tc of 11.5 K.21,22 In addition, some extended work systematically investigated the effect of pressure23,24 and the role of interlayer state in the electronic structure,25 providing valuable insight into the
uperconductors play significant roles in low-temperature magnet applications such as nuclear magnetic resonance (NMR), magnetic resonance imaging (MRI), highenergy particle accelerators, and superconducting quantum interference devices, to name but a few. In an electrical grid system, superconducting wires outperform copper or aluminum in transmitting electrical power over a long distance with zero power dissipation.1−4 Superconducting wires are usually made of superconductors by the classic powder-in-tube (PIT) processing technique5,6 or the direct pen writing method.7 For example, brittle niobium−tin (NbTi),8,9 magnesium diboride (MgB2),10,11 and ceramic cuprate superconductors such as BiSrCaCuO (BSCCO)12,13 are packed into copper or silver tubes, which are sealed, evacuated, and further drawn into a fine wire. However, the poor connection between superconducting powders inevitably decreases the uniformity of the wire. Besides metal-based superconducting materials, carbon-based superconductors are another important branch within the realm of superconductivity. To date, at least five amazing structures of carbon, including diamond, graphite, C60, carbon nanotubes (CNTs), and graphene, have shown superconducting behavior © 2017 American Chemical Society
Received: March 2, 2017 Accepted: March 29, 2017 Published: March 29, 2017 4301
DOI: 10.1021/acsnano.7b01491 ACS Nano 2017, 11, 4301−4306
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ACS Nano superconducting mechanism of GICs. Recently, the superconductivity in graphene samples was achieved by a proximity effect with superconducting contacts26−29 and by chemically doping with alkaline (earth) metals.30−35 However, most carbon-based superconductors are limited to the microsize, which inevitably hinders further application in the macroscopic world. Therefore, it raises the question of whether one could find an approach to build continuous carbon-based superconducting wires, which is of vital importance from the viewpoints of fundamental research and practical application. Undoubtedly, translating the individual carbonaceous superconducting units into macroscopic wires especially with meterlong length is a big challenge. On the other hand, graphene fibers (GFs) could be continuously assembled from individual graphene sheets by high-throughput and scalable wet-spinning technology.36,37 Since 2011, GFs have been proven to possess outstanding flexibility, mechanical properties, and electrical conductivity.36−44 The creation of GFs significantly paved the way for constructing carbonaceous fibers from natural graphite. Recently, Gao’s group made a breakthrough in upgrading the tensile strength of GFs up to 2.2 GPa by a full-scale synergetic defect engineering protocol.45 Furthermore, chemical doping dramatically increased the electrical conductivity of GFs to benchmark metals of 106−107 S m−1.46 Despite all of these efforts, superconducting GFs have not yet been realized. Here, inspired by the ongoing superconducting graphene and efficient doping of GFs, we successfully fabricated flexible and continuous graphene-based superconducting fibers via the wellestablished intercalation strategy. We choose calcium metal as intercalatant to react with neat GFs. The resultant Caintercalated GF (Ca-GF) shows a superconducting transition at ∼11.3 K, which is almost 2 orders of magnitude higher than that of early reported alkali metal intercalated GICs and even comparable to that of microsized intercalation compound CaC6 and commercial superconducting NbTi wire (Tc = 9.7 K). More significantly, we observed zero resistance in macroscopic GFs. Given the advantages of solution-processing and adjustable attributes of the macroscopic GF, it should blossom into a potential lightweight superconducting wire with further elevation of Tc in the future.
Figure 1. Schematic illustration of the fabrication process of a CaGF. The intercalation reaction was performed in a two-temperature-zone quartz tube with T1 > T2.
Figure 2. (a) Photograph of Ca-GFs with a yellow color (scale bar: 2 cm). (b) Schemetic structure of a Ca-GF with Ca intercalated between graphene sheets. (c) Bending a Ca-GF on a transparent substrate, showing good flexibility. (d−f) SEM image of a Ca-GF (d) and corresponding elemental mapping images of C (e) and Ca (f) elements in the Ca-GF. Scale bar in d−f: 5 μm. (g−i) Crosssectional morphology of the Ca-GF (g) and corresponding elemental mapping images of Ca (h) and C (i) elements in the Ca-GF. Scale bar in g−i: 2 μm.
RESULTS AND DISCUSSION Figure 1 schematically illustrates the fabrication process of CaGFs. First, graphene oxide (GO) fibers were prepared by wetspinning of a GO liquid crystal dispersion. After graphitization at high temperatures up to 3000 °C, GO fibers were reduced to neat GFs with high purity sp2-hybridized carbon, which were used as host materials for intercalation reactions, according to our previous reports.46 Subsequently, Ca-GFs were synthesized via a facile vapor transport process,47 as developed by Weller et al.22 Typically, a piece of calcium and a bunch of neat GFs (∼20 mg) were sealed in a vacuumed dumbbell-shaped quartz tube, followed by heating in a furnace at 400−500 °C for one to several weeks. After reaction, the tube turns black as a thin layer of Ca metal coated the inner wall (Figure S1 in the Supporting Information). The color of the GFs evolved from silver gray to yellow (Figure 2a), which can be attributed to the increased free carrier concentration by the intercalatant (Figure 2b) and the shift of the Drude plasma frequency into the visible wavelength.47 The surface morphology of the GF features aligned wrinkles along the axis direction resulting from the wet-spinning process,
and no obvious change occurred after Ca intercalation (Figure S2 and Figure 2d). Energy dispersive spectroscopy (EDS) mapping of the Ca-GF demonstrates the uniform distribution of C and Ca elements on its surface and in its internal microstructure (Figure 2e−i and Figure S3). The GF showed excellent flexibility. Even after 1000 bending−releasing cycles, the resistance barely changed, indicating robust mechanical performance and structural stability (Figure S4). This sets a solid base for further fabrication of flexible superconducting wires (Figure 2c). Raman spectra can provide rich structural information about defects, doping contents, and intercalation states of graphenebased materials. In Figure 3a, the GF exhibits a negligible D peak at 1350 cm−1, which is usually forbidden in perfect graphene, indicating the structural integrity of the graphene unit in the GF. The sharp G peak located at 1580 cm−1 induced by tangential vibration of two neighboring carbon atoms also signifies the large lateral size of the graphene laminate crystals 4302
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Figure 3. Raman spectra (a), XRD patterns (b), and XPS spectra (c) of a GF and Ca-GF.
in the GF. The Raman spectra together with X-ray diffraction (XRD) patterns (Figure 3b) verified the high quality of our asprepared GFs. For Ca-GFs, the intensity of the D peak increased slightly, which may result from the increased defect density produced during the chemical intercalation process. The G peak became broadened and downshifted to 1500 cm−1 (Figure 3a). The shift of the G peak after intercalation is due to the enhanced electron−phonon coupling at high doping level, which is consistent with that of donor-type doped graphene and graphite by potassium and calcuim.48,49 As for the 2D peak, a slight downshift also occurred for the Ca-GF, which was probably caused by the small but non-negligible lattice expansion and weaker coupling induced by the intercalatant.50 The intercalation structure of Ca-GF was confirmed by XRD (Figure 3b). Besides the peak at 26.4° corresponding to the (002) reflection of graphite domains, two more peaks labeled as (003) at 19.8° and (006) at 40° were observed, suggesting the existence of a stage 1 intercalation structure in Ca-GF, similar to the intercalation compound CaC6.51 The appearance of a (002) peak indicates the multiphase structure in Ca-GF. Additionally, the incorporation of Ca was further verified by Xray photoelectron spectroscopy (XPS) data, with the peak at 347 and 438 eV assigned to Ca 2p and Ca 2s, respectively (Figure 3c and Figure S5). We further investigated the temperature dependence of electrical transport characteristics. With the decreasing of the temperature from 298 K at zero magnetic field, the resistance of Ca-GF increased slightly until a sharp drop appeared at ∼11.3 K (Figure 4a). On close inspection of the resistance in the temperature range from 20 to 2 K, impressively, the resistance does reach zero near 4 K, essentially proving the existence of superconductivity in Ca-GF (Figure 4b). The relatively broad superconducting transition range about 7 K is likely due to different levels of doping for individual graphene crystallites in GF, resulting from the unsaturated intercalation structure of Ca-GFs as inferred from the XRD pattern (Figure 3b).52 The Ca content in Ca-GF was about 10%, obtained by weighing the mass change of the GFs after the intercalation process. We have tried to increase the Ca concentration by increasing the reaction time, but it was very difficult to obtain Ca-saturated samples like CaC6, because the complicated multifolding microstructure of GF severely hindered the penetration of Ca into the GF. Additionally, possible oxidation of the samples during the transfer process from the glovebox to the testing equipment may also be responsible for the broad transition. Figure 4c shows the evolution of resistance−temperature curves under different applied magnetic fields. With the increment of the magnetic field, the Tc is gradually shifted toward lower temperature and disappeared eventually at 10 000 Oe. The suppressing of Tc by an external magnetic field implies that the superconductivity of Ca-GF is intrinsic.31,53 Figure 4d
Figure 4. (a, b) Temperature dependence of the electrical resistance of Ca-GF, showing a superconducting transition with Tconset = 11.3 K and Tczero = 4 K. (c) Evolution of electrical resistance from 2 to 20 K at different applied magnetic field. (d) Upper critical field Hc2 as a function of temperature determined from (c).
presents the upper critical field Hc2 as a function of temperature determined from the resistance data in Figure 4c. Hc2(T) is defined at the point of Tconset. The Hc2(T) curve yields a slope of −dHc2/dT|Tc = 0.82 kOe/K, which is fitted well with the Werthamer−Helfand−Hohenberg formula Hc2 = −0.69(dHc2/ dT)|TcTc. Taking Tc = 11 K, the upper critical field at zero temperature is estimated to be 6.2 kOe. This high value is comparable to the reported Hc2 in bulk CaC6 determined from M−H plots.21 Besides zero resistance, the capability of shielding the applied magnetic field is another important characteristic of superconducting materials. To further confirm the superconductivity in Ca-GF, magnetization was measured using a conventional superconducting quantum interference device. Since the density of Ca-GF is rather small, a large amount of sample was needed to get better data. During the process of measurement, samples should remain stable when the sample holder moves up and down. The samples were pressed into a capsule with fibers running in all directions and the magnetic field across the sample from different angles. The magnetization as a function of temperature (Figure 5a,b) was performed in two typical modes of zero field cooling (ZFC) and field cooling (FC). For ZFC measurement, the temperature was first cooled to 1.8 K without applied magnetic field, and then the magnetization was tested under a magnetic field of 10 Oe with increasing the temperature at a rate of 1 K min−1. For FC 4303
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GICs has been more deeply debated. For example, Csányi and co-workers suggested a nonconventional exciton pairing mechanism, 25 while Mazin argued that coupling with intercalant phonons is likely to be the main force for superconductivity based on the first-principles calculations. In addition, superconductivity in CaC6 is due to Ca vibrations and not from electronic excitations.56 Further, Calandra and coworker proposed that the carriers are mostly electrons in the Ca Fermi surface coupled with Ca in-plane and C out-of-plane vibrations.57 With the help of angle-resolved photoelectron spectroscopy at a high-energy resolution, Yang et al. revealed that the interaction between the graphene-derived π* band and the interlayer band contributed to the superconductivity of CaC6.35 Thus, the nature of superconductivity is still not clearly understood up to now. Admittedly, more studies are needed to explore the precise origin of superconductivity in macroscopic fibers from both theoretical and experimental aspects. Figure 5. Temperature dependence of magnetization for Ca-GF. (a) Temperature dependence of the moment (M) for Ca-GF with Tc = 11.1 K obtained from ZFC and FC measurements at a field of 10 Oe. The Tc marked in the image is determined at the point where the magnetic susceptibility decreases. (b) Magnification of ZFC/FC curves around the Tc. (c) M−T curves for Ca-GF under different applied magnetic fields (H) ranging from 50 to 5000 Oe. (d) Field dependence of the magnetization isotherm measured at 2 K.
CONCLUSION In this work, we have fabricated superconducting continuous GFs via a facile Ca intercalation approach. The superconducting transition of Ca-GF occurred at ∼11 K. We observed zero electrical resistance of macroscopic Ca-GF at ∼4 K. Considering the merits of lightness, low-cost, and easy preparation of neat GFs, we believe that the superconducting Ca-GF would be useful in extremely low temperature surroundings in aerospace applications. Our strategy of intercalation also offers a promising avenue for the construction of other types of macroscopic superconducting carbon-based materials such as films and tapes.
measurement, the temperature was cooled to 1.8 K with the applied magnetic field, and then the FC magnetization was carried out at the same heating rate of 1 K min−1. Clearly, a significant decrease in susceptibility was seen at ∼11 K, which was consistent with the resistance diagram (Figure 4a,b). This evolution further testified the superconductivity in Ca-GF. Compared with the superconductivity in Ca-doped graphene laminates, the Ca-GF exhibited a much higher Tc and real zero resistance.54 In the other alkali (earth) metal intercalated graphite KC8, RbC8, CsC8, BaC6, and SrC6, the Tc values are 0.55 K, 0.15 K, 0.11 K, 65 mK, and 1.65 K, respectively,55 but the zero resistance temperatures (Tczero) of these compounds were not reported. The M−T curves for Ca-GF at varied magnetic field (Figure 5c) also demonstrate the same trend as in the case of Figure 4c. An obvious drop in M at 9.74 K even at 1000 Oe was observed, meaning that the superconducting phase could be well maintained at weak field. The increased magnetic field depressed the transition of superconductivity, which is in accordance with most of the other superconductors, implying intrinsic superconductivity of Ca-GF. The field dependence of the magnetization isotherm for Ca-GF (measured at 2 K, as shown in Figure 5d) indicates that Ca-GF is a type-II superconductor with strong vortex pinning. The lower critical field (Hc1) of 30 Oe marked in Figure 5d is defined at the point where the fitted straight line deviates from the initial measured susceptibility points, while Hc1 of the reported bulk CaC6 material can be as high as nearly 500 Oe.21 The diminution of Hc1 may be due to the nonuniformity of Ca doping level and the fiber form of graphene making the magnetic field easier to penetrate the superconducting samples. As to the possible superconducting mechanism, we think the recently developed theoretical studies of CaC6 can be used to explain the superconductivity of Ca-GF, since the structure of our Ca-GF is similar to CaC6 as detected from XRD and Raman data. However, the origin of the superconductivity of
METHODS Preparation of GFs. Neat GFs were prepared following the wetspinning protocol according to our previous works.45 Briefly, graphene oxide dispersion in dimehtylformamide (10 mg mL−1) was extruded through a spinneret with a diameter of 150 μm into the coagulation bath containing ethyl acetate. The as-formed GO fibers were dried during the continuous collecting process. Then batches of GO fibers were thermally annealed at 3000 °C for 30 min under an Ar atmosphere. Finally, neat GFs with a silvery luster were obtained after cooling to room temperature. Preparation of Ca-GFs. The Ca-GFs are fabricated via a wellestablished intercalation approach.22 In a glovebox filled with pure argon, GFs and Ca metal granules were placed into a quartz tube separately. The tube was vacuumed to ∼10−6 mbar for 1 h and melt sealed. The tube was kept at 400−500 °C for one to several weeks. After cooling to room temperature, the tube was opened in the glovebox and Ca-GFs were obtained. Characterization. Raman spectra were recorded by a Renishaw inVia-Reflex Raman microscope at an excitation wavelength of 532 nm. The laser power at the sample was kept below 0.5 mW to avoid a laser heating effect. The scanning electron microscopy (SEM) images and EDS elemental mapping were taken on a Hitachi S4800 fieldemission SEM system. XRD measurements were taken on an X’Pert PRO diffractometer (PANalytical) using Cu Kα1 radiation with an Xray wavelength of 1.5406 Å. XPS measurement was carried out on an ESCALAB 250 photoelectron spectrometer (ThermoFisher Scientific) with Al Kα as X-ray source. The electrical resistance was tested using a four-probe technique on a physical property measurement system (Quantum Design). The four-terminal measurement was performed in a device designed by our laboratory. Ca-GFs were put into a rounded appliance, compressed, and sealed. The diameter of the device is 6 mm, the distance between V+ and V− is 2 mm, and the thickness is less than 1 mm. Magnetization measurements were performed in a commercial SQUID-VSM magnetometer (Quantum Design). 4304
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ASSOCIATED CONTENT S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.7b01491. Experimental photo and some characterizations (PDF)
AUTHOR INFORMATION Corresponding Authors
*E-mail (M. Xue):
[email protected]. *E-mail (C. Gao):
[email protected]. Author Contributions §
Y. Liu and H. Liang contributed equally to this work.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (Nos. 51533008, 21325417, 21622407, 51603183) and MOST National Key Research and Development Plan (2016YFA0200200). REFERENCES (1) Larbalestier, D.; Gurevich, A.; Feldmann, D. M.; Polyanskii, A. High-Tc Superconducting Materials for Electric Power Applications. Nature 2001, 414, 368−377. (2) Tinkham, M. Introduction to Superconductivity; Courier Corporation, 1996. (3) Cyrot, M.; Pavuna, D. Introduction to Superconductivity and HighTc Materials; World Scientific, 1992. (4) Kleiner, R.; Buckel, W. Superconductivity: An Introduction; John Wiley & Sons, 2015. (5) Heine, K.; Tenbrink, J.; Thöner, M. High-Field Critical Current Densities in Bi2Sr2Ca1Cu2O8+x/Ag Wires. Appl. Phys. Lett. 1989, 55, 2441−2443. (6) Palombo, M.; Malagoli, A.; Pani, M.; Bernini, C.; Manfrinetti, P.; Palenzona, A.; Putti, M. Exploring the Feasibility of Fe (Se, Te) Conductors by Ex-situ Powder-in-Tube Method. J. Appl. Phys. 2015, 117, 213903. (7) Xue, M.; Chen, D.; Long, Y.; Wang, P.; Zhao, L.; Chen, G. Direct Pen Writing of High-Tc, Flexible Magnesium Diboride Superconducting Arrays. Adv. Mater. 2015, 27, 3614−3619. (8) Lindenhovius, J. L. H.; Hornsveld, E. M.; Den Ouden, A.; Wessel, W. A. J.; Ten Kate, H. H. J. Powder-in-Tube (PIT) Nb/Sub 3/Sn Conductors for High-Field Magnets. IEEE Trans. Appl. Supercond. 2000, 10, 975−978. (9) Godeke, A.; Den Ouden, A.; Nijhuis, A.; Ten Kate, H. H. J. State of the Art Powder-in-Tube Niobium-Tin Superconductors. Cryogenics 2008, 48, 308−316. (10) Glowacki, B. A.; Majoros, M.; Vickers, M.; Evetts, J. E.; Shi, Y.; McDougall, I. Superconductivity of Powder-in-Tube MgB2 Wires. Supercond. Sci. Technol. 2001, 14, 193−199. (11) Yamada, H.; Hirakawa, M.; Kumakura, H.; Kitaguchi, H. Effect of Aromatic Hydrocarbon Addition on In-situ Powder-in-Tube Processed MgB2 Tapes. Supercond. Sci. Technol. 2006, 19, 175−177. (12) Schwartz, J.; Heuer, J. K.; Goretta, K. C.; Poeppel, R. B.; Guo, J., Jr; Raban, G. W. High Temperature Mechanical Properties and High Strength Sheaths for Powder-in-Tube Tapes. Appl. Supercond. 1994, 2, 271−280. (13) Dai, W.; Marken, K. R.; Hong, S.; Cowey, L.; Timms, K.; McDougall, I. Fabrication of T/Sub c/ Coils from BSCCO 2212 Powder in Tube and Dip Coated Tape. IEEE Trans. Appl. Supercond. 1995, 5, 516−519. (14) Hannay, N. B.; Geballe, T. H.; Matthias, B. T.; Andres, K.; Schmidt, P.; MacNair, D. Superconductivity in Graphitic Compounds. Phys. Rev. Lett. 1965, 14, 225. 4305
DOI: 10.1021/acsnano.7b01491 ACS Nano 2017, 11, 4301−4306
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DOI: 10.1021/acsnano.7b01491 ACS Nano 2017, 11, 4301−4306
