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Direct Pyrolysis Method for Superconducting Crystalline MgB2 Nanowires Renzhi Ma,* Yoshio Bando, Takao Mori, and Dmitri Golberg Advanced Materials Laboratory, National Institute for Materials Science, Namiki 1-1, Tsukuba, Ibaraki 305-0044, Japan Received January 15, 2003. Revised Manuscript Received May 26, 2003
A direct pyrolysis route from MgB2 nanoparticles (100-500 nm) to crystalline MgB2 nanowires (10-20 nm diam) was developed. In this process, MgB2 nanoparticle precursor was prepared through an in situ solid reaction of Mg metal and amorphous boron powder. After directly pyrolyzing the precursor via infrared irradiation at 900 °C in a high-purity argon atmosphere, uniform MgB2 nanowires were produced at the edges of the precursor nanoparticles. Structural characterization and elemental composition analysis identified that the nanowires were made of MgB2, presumably incorporated or alloyed with some oxygen. Superconductivity of the samples was confirmed and it is indicated that the nanowires may have a transition temperature (Tc) comparable with that of bulk MgB2.
Introduction Nagamatsu et al. have recently discovered that MgB2 is a superconductor with a transition temperature (Tc) of 39K,1 which is promising for practical applications in superconducting devices. MgB2 adopts a very simple hexagonal AlB2-type crystal structure (space group P6/ mmm), comprising interleaved two-dimensional boron and magnesium layers.2,3 MgB2 has been synthesized in various forms: bulk (polycrystals), thin films, wires, and tapes, as well as single crystals.4 A remarkable range of one-dimensional (1D) nanostructures can now be synthesized, including nanotubes,5 nanowires,6 and nanobelts.7 These nanostructures are ideal candidates for basic understanding of the behavior of a given matter in downsized geometries and suggest new electronic devices with dimensions smaller than those achievable in traditional lithographic methods. It is therefore of fundamental interest to synthesize MgB2 nanostructures such as nanoparticles and nanowires and to study the effect of dimensionality/ size on superconductivity.8 MgB2 nanoparticles approximately 40-100 nm in size have been synthesized through mechanical alloying.9 Improved superconducting properties were observed in the nanocrystalline samples. Recently, 1D polycrystalline MgB2 nanowires (50-400 nm in diameter) have also * To whom correspondence should be addressed. E-mail:
[email protected]. Fax: (+81) 29-851-6280. (1) Nagamatsu, J.; Nakagawa, N.; Muranaka, T.; Zenitani, Y.; Akimitsu, J. Nature 2001, 410, 63. (2) Jones, M. E.; Marsh, R. E. J. Am. Chem. Soc. 1954, 76, 1434. (3) Choi, H. J.; Roundy, D.; Sun, H.; Cohen, M. L.; Louie, S. G. Nature 2002, 428, 758. (4) Buzea, C.; Yamashita, T. Supercond. Sci. Technol. 2001, 14, R115-R146. (5) Iijima, S. Nature 1991, 354, 56. (6) Morales, A. M.; Lieber, C. M. Science 1996, 273, 1836. (7) Pan, Z. W.; Dai, Z. R.; Wang, Z. L. Science 2001, 291, 1947. (8) Bezryadin, A.; Lau, C. N.; Tinkham, M. Nature 2000, 404, 971. (9) Gmbel, A.; Eckert, J.; Fuchs, G.; Nenkov, K.; Muller, K. H.; Schultz, L. Appl. Phys. Lett. 2002, 80, 2725.
been successfully prepared through a two-step vapor transport and reaction process.10 In this particular method, amorphous boron nanowires were first synthesized and then transformed into polycrystalline MgB2 nanowires after reacting with a Mg vapor. The transition temperature of the polycrystalline nanowires was measured to be ∼33 K. However, because of the unstable and decomposing nature of MgB2 at elevated temperature, the conventional pyrolysis of MgB2 yielded MgO nanostructures.11-13 To fabricate MgB2 nanostructures, a rapid pyrolysis protocol appears necessary to avoid the decomposition and oxidization of MgB2 precursors. In this paper, we report a direct pyrolysis route to prepare crystalline MgB2 nanowires (10-20 nm diam). The nanowires were obtained through the rapid pyrolysis of an MgB2 nanoparticle precursor in an infrared-irradiation-heating furnace. It was characterized that the nanowires were MgB2, presumably incorporated/alloyed with some oxygen. Experimental Section MgB2 was first made using a magnesium metal (99.9%, 325 mesh) and an amorphous boron powder (99.9%, ∼50 nm) through “in situ” solid reaction. The starting powders were well mixed in a molar ratio of 1.5:2. Then, the mixture was charged into a sealed BN crucible and heat treated in a quartz tube in an induction furnace filled with a high-purity Ar gas at 750-780 °C for 2 h, followed by furnace cooling to room temperature. The resultant product was employed as the precursor for the subsequent pyrolysis. The pyrolysis process was carried out in an infrared-irradiation-heating furnace under a high-purity Ar gas flow. Infrared-irradiation heating rapidly increased the target precursor temperature to 900 °C within 5 min and this temperature was held over 40 min. (10) Wu, Y.; Messer, B.; Yang, P. Adv. Mater. 2001, 13, 1487. (11) Brutti, S.; Ciccioli, A.; Balducci, G.; Gogli, G.; Manfrinetti, P.; Palenzona, A. Appl. Phys. Lett. 2002, 80, 2892. (12) Yin, Y.; Zhang, G.; Xia, Y. Adv. Funct. Mater. 2002, 12, 293. (13) Klug, K. L.; Dravid, V. P. Appl. Phys. Lett. 2002, 81, 1687.
10.1021/cm021823l CCC: $25.00 © 2003 American Chemical Society Published on Web 06/28/2003
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Figure 1. MgB2 + Mg nanoparticle precursor synthesized through in situ reaction of Mg and B. (a) SEM image of the precursor consisting of nanoparticles of 100-500 nm. (b) XRD spectrum exhibiting MgB2 and unreacted Mg phases.
Figure 2. XRD display showing that a product mainly consists of MgB2 after rapid pyrolysis in an infrared-irradiation-heating furnace under high-purity Ar atmosphere. The precursor and product were both observed using a JSM500 scanning electron microscope (SEM) operated at 1020 kV. The X-ray diffraction (XRD) patterns were obtained on a RINT2200 X-ray diffractometer with Cu KR radiation. The product was also ultrasonically dispersed in acetone and transferred to a carbon-coated copper grid for transmission electron microscopy (TEM) observations. A field emission JEM3000F high-resolution electron microscope operated at 300 kV equipped with a Gatan-666 electron energy loss spectrometer (EELS) and energy-dispersive X-ray spectrometer (EDS) was employed to perform the microanalysis. Magnetic susceptibility measurements of the samples were carried out at low temperatures using a superconducting quantum interference device (SQUID) magnetometer.
Results and Discussion Figure 1a displays a typical SEM image of the precursor obtained through the reaction of magnesium metal and amorphous boron. The material mainly consists of particles of approximately 100-500-nm grain size. The XRD pattern of the product showed that the as-prepared precursor was composed of MgB2 with some traces of unreacted Mg metal (Figure 1b). Figure 2 depicts an XRD spectrum taken from the product after infrared-irradiation heating. Compared with Figure 1b, it is notable that the diffraction peaks originated from the unreacted Mg almost disappeared. In general, the XRD spectrum can be well indexed as an MgB2 phase. However, several extra low-intensity peaks were emerged in this pattern, which could be ascribed to the traces of MgO. This indicates that the product contains a very little amount of MgO after the
Figure 3. SEM images depicting nanowires grown on the edge of MgB2 particles.
infrared-irradiation pyrolysis. Because of the high volatility of Mg in a broad temperature window and a tendency to form MgO in the presence of oxygen, some MgO may be formed in the product as a result of the reaction of Mg with the extremely small amount of oxygen pollution in Ar gas. Figure 3 depicts SEM images of the product after infrared-irradiation heating. Abundant nanowires could be clearly observed at the edges of MgB2 nanoparticles. These nanowires are tiny, and have uniform diameter (10-20 nm) and length (100-200 nm). It appears that
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Figure 5. SQUID measurements indicating transition temperatures of TC ) 39 and 36.5 K for a nanowire-containing sample and an MgB2+Mg sample, respectively. Note two components of the superconductivity in the nanowire-containing sample. Figure 4. TEM characterizations of MgB2 nanowires. (a) TEM image of a typical nanowire. (inset) ED pattern indexed as an MgB2 phase with superstructures. (b) EDS showing that the nanowire is composed of B, O, and Mg. (c) EELS confirming the existence of B in nanowires. (d) HRTEM image of a nanowire.
the nanowires are indeed grown from the surface of the MgB2 nanoparticles. Figure 4a displays a TEM image of a typical nanowire. Energy-dispersive X-ray spectra (EDS) and electron energy loss spectra (EELS) were taken to identify the elemental composition of the nanowires. The EDS (Figure 4b) measurements identify that the nanowires are composed of B, Mg, and O. The EELS measurements also confirm the existence of B in the nanowires (Figure 4c). The quantitative analyses give an atomic ratio of Mg/B/O of 1.0:2.0:0.3-0.5. This indicates that the nanowires may be composed of MgB2 with some incorporation of oxygen. It is also apparent from Figure 4a that the nanowires are coated with an amorphous layer (2-3 nm thick). We suspect that the detected oxygen content may mainly originate from the surface layers, although a detailed analysis of the oxygen spatial distribution was not possible as the nanowires appeared to be very sensitive to electron-beam irradiation and usually collapsed within 3-5 min during TEM observations. An electron diffraction (ED) pattern of the nanowire is shown as the inset in Figure 4a. As labeled by the asterisks, the primary reflection spots can be indexed to a hexagonal MgB2 (a ) 3.08 Å, c ) 3.52 Å) viewed along the [0001] zone axis. A selected high-resolution TEM (HRTEM) image of a nanowire is shown in Figure 4d, which also clearly displays the hexagonal MgB2 lattice fringes. These data confirm that the nanowires are indeed MgB2-type crystals. Nevertheless, the ED pattern also exhibits abundant extra points, implying the existence of stacking faults and periodic domain superstructures, along with the primary MgB2 reflections. The reason for the formation of the superstructures in the MgB2 nanowires may be oxygen incorporation as revealed by the elemental composition analyses. As described above, the XRD data shown in Figure 2 also
presume the existence of an MgO phase in the product. Although MgB2 and MgO have different crystal symmetries, i.e., P6/mmm and Fm3m, respectively, their stacking sequence of Mg and B (or O) atoms and lattice spacings in certain crystallographic orientations are very similar. Such orientation relationships may imply the possible low-energy epitaxial growth of the two phases and the existence of a good lattice-match at the MgO and MgB2 interfaces, where the interfacial misfit dislocations and the associated strain are not detectable.14,15 It has been found that oxygen can replace boron in the ideal MgB2 structure.16 Some researchers have also observed oxide (MgO) or Mg(B,O) precipitates in a polycrystalline MgB2.17-20 We suggest that there are also some MgO and Mg(B,O) domains or precipitates in the present MgB2 nanowires. In fact, incorporation of oxygen into MgB2 may be beneficial to improving superconductivity, because the oxide sites may act as effective flux pinning centers increasing the critical current dramatically.16 But it is worth noting that the total oxygen content should be controlled in a very limited level. Otherwise, MgO structures will be formed instead of MgB2.11-13 In the current techniques, it appears that the rapid pyrolyzing rate of the precursor realized by infrared induction ensure the formation of MgB2 based crystals. Figure 5 depicts the magnetic susceptibility of the product after infrared-irradiation heating (noted as (14) Zhu, Y.; Wu, L.; Volkov, V.; Li, Q.; Gu, G.; Moodenbaugh, A. R.; Malac, M.; Suenaga, M.; Tranquada, J. Physica C 2001, 356, 239. (15) Li, J. Q.; Li, L.; Zhou, Y. Q.; Ren, Z. A.; Che, G. C.; Zhao, Z. X. Chin. Phys. Lett. 2001, 18, 680. (16) Eom, C. B.; Lee, M. K.; Choi, J. H.; Belenky, L. J.; Song, X.; Cooley, L. D.; Naus, M. T.; Patnaik, S.; Jiang, J.; Rikel, M.; Polyanskii, A.; Gurevich, A.; Cai, X. Y.; Bu, S. D.; Babcock, S. E.; Hellstrom, E. E.; Larbalestier, D. C.; Rogando, N.; Regan, K. A.; Hayward, M. A.; He, T.; Slusky, J. S.; Inumaru, K.; Haas, M. K.; Cava, R. J. Nature 2001, 411, 558. (17) Klie, R. F.; Idrobo, J. C.; Browing, N. D.; Regan, K. A.; Rogado, N. S.; Cava, R. J. Appl. Phys. Lett. 2002, 79, 1837. (18) Klie, R. F.; Idrobo, J. C.; Browing, N. D.; Serquis, A.; Zhu, Y. T.; Liao, X. Z.; Mueller, F. M. Appl. Phys. Lett. 2002, 80, 3970. (19) Liao, X. Z.; Serquis, A. C.; Zhu, Y. T.; Huang, J. Y.; Peterson, D. E.; Mueller, F. M.; Xu, H. F. Appl. Phys. Lett. 2002, 80, 4398. (20) Johsi, J. P.; Sarangi, S.; Sood, A. K.; Pal, D.; Bhat, S. V. Pramana (J. Phys.) 2002, 58, 361.
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“nanowire-containing sample”) as a function of temperature. The Mg-enriched MgB2 nanoparticle precursor (noted as “MgB2+Mg sample”) was also measured as shown for comparison. The arrows in Figure 5 indicate transition temperatures of TC ) 39 and 36.5 K in the nanowire-containing sample and MgB2+Mg sample, respectively. A transition temperature TC ) 36.5 K for the MgB2+Mg sample is different from that reported for a bulk MgB2.1 The slight depression of TC could be due to the nanoparticle morphology or contamination by impurities such as Mg.21,22 For the nanowire-containing samples, there appear to be two components of superconductivity with different transition temperatures: one of nearly 39 K and a lower one as can be seen in the curvature. From the coincidence with the MgB2+Mg sample, the lower-temperature superconductivity component is likely to be of the same origin, namely, reflecting the nanoparticle morphology or contamination effect. Considering that the nanowirecontaining sample has a large amount of MgB2 nanowires on the surface of the nanoparticles, it is assumed here that the MgB2 nanowires exhibit superconductivity with a TC of approximately 39 K, which is comparable
with that of the bulk. This is somewhat different from the previously reported Tc of ∼33 K in polycrystalline MgB2 nanowires which are slightly doped with Si.10 Further studies on samples made of pure MgB2 nanowires, such as those prepared via the present method, on films which have large relative surface areas, should make this fully clear.
(21) Slusky, J. S.; Rogado, N.; Regan, K. A.; Hayward, M. A.; Khalifah, P.; He, T.; Inumaru, K.; Loureiro, S. M.; Haas, M. K.; Zandbergen, H. W.; Cava, R. J. Nature 2001, 410, 343. (22) Jung, C. U.; Kim, H. J.; Park, M. S.; Kim, M. S.; Kim, J. Y.; Du, Z.; Lee, S. I.; Kim, K. H.; Betts, J. B.; Jaime, M.; Lacerda, A. H.; Boebinger, G. S. Physica C 2002, 377, 21.
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Conclusions In summary, we have successfully synthesized crystalline MgB2 nanowires (10-20-nm diam) via a direct rapid pyrolysis method of MgB2 nanoparticles in an infrared-irradiation-heating furnace. The nanowires are basic MgB2 crystals with some oxygen incorporation. The investigations demonstrated that the control of oxygen incorporation was very crucial in synthesizing MgB2 nanostructures. Some experimental parameters, i.e., suitable precursor, high-purity protection gas, and rapid pyrolyzing rate, are necessary to ensure the formation of MgB2-based structures rather than oxides (MgO)11-13 or other ternary phases.23,24 Superconductivity measurements indicated a TC of approximately 39 K for these nanowires, which is comparable with that of bulk MgB2.
(23) Ma, R.; Bando, Y.; Sato, T. Appl. Phys. Lett. 2002, 81, 3467. (24) Ma, R.; Bando, Y.; Golberg, D.; Sato, T. Angew. Chem., Int. Ed. 2003, 42, 1836.
