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Critical current density in the MgB2 nanoparticles prepared under autogenic pressure at elevated temperature
Chemical Physics Letters 433 (2006) 115–119 www.elsevier.com/locate/cplett
Critical current density in the MgB2 nanoparticles prepared under autogenic pressure at elevated temperature Vilas Ganpat Pol a, Swati Vilas Pol a, Israel Felner b, Aharon Gedanken a
a,*
Department of Chemistry and Kanbar Laboratory for Nanomaterials, Bar-Ilan University Center for Advanced Materials and Nanotechnology, Ramat-Gan 52900, Israel b Racah Institute of Physics, Hebrew University, Jerusalem 91904, Israel Received 9 August 2006; in final form 27 October 2006 Available online 12 November 2006
Abstract This Letter reports on obtaining MgB2 nanoparticles revealing an intra-granular enhanced critical current density (JC P 108 A/cm2) for the undoped sample. A rapid, single-stage, high yield, reproducible, straightforward, scalable, and competent approach at 750 C is described herein for the synthesis of superconducting MgB2 nanoparticles. The magnetization studies indicated clearly the existence of bulk superconductivity in MgB2 nanoparticles at TC 38 K. Electron spin resonance and microwave absorption study substantiated the paramagnetic nature of the MgB2 nanoparticles in the normal state. The plausible reaction mechanism leading to the formation of MgB2 is explained. 2006 Elsevier B.V. All rights reserved.
1. Introduction The discovery of low cost, high-temperature superconductivity in MgB2 (TC = 39 K) has triggered intense interest in the scientific community [1]. MgB2 adopts a simple hexagonal AlB2-type crystal structure with interleaved two-dimensional boron and magnesium layers [2]. The effect of a boron isotope on MgB2 indicated that this material is a conventional phonon-mediated BCS (Bardeen– Cooper–Schrieffer) superconductor [3]. Until now, MgB2 has been synthesized in various forms: polycrystalline [1], thin films [4], wires [5], and tapes [6]. Mechanical alloying is a well-established method for preparing meta-stable amorphous, quasi-crystalline, and nanocrystalline MgB2 materials [7]. The mechanical alloying of Mg and B powders at ambient temperatures, followed by hot pressing, produced 100 nm high density, nanocrystalline samples with improved pinning [8]. However, the nanocrystalline MgB2 preparation is time consuming and difficult because *
Corresponding author. Fax: +972 3 535 1250. E-mail address: [email protected] (A. Gedanken).
0009-2614/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2006.11.021
of the large difference in vapor pressure between B and Mg, as well as magnesium’s high susceptibility to oxidation. Recently, Ma et al. described the considerably enhanced critical current densities at 4.2 K [9] in MgB2 tapes doped by 5 wt% nanocarbon. They reported high JC values of 1.8 · 104 A/cm2 at 10 T and 2.8 · 103 A/cm2 at 14 T for the samples sintered at 750 C. Wang et al. also demonstrated the enhancement of critical current density in MgB2 by doping it with nano-SiC, Si, or C doping (<10%) and compared [10] the critical current density with the undoped sample. They accounted both magnetic and transport JC properties as high as 2 · 104 A/cm2 in 8 T at 5 K, one or two orders of magnitude higher than the undoped MgB2. Electron-spin resonance (ESR) is known to be a very sensitive method for probing paramagnetic centers and microwave absorption in both conventional and high-TC superconductors [11]. Recently, a comparative ESR study of nano size and micron size MgB2 specimens verified the presence of intense conduction electron spin resonance (CESR) in the normal state, with a relatively high spin relaxation rate [12] and a Pauli susceptibility of xs = 2.5 · 105 emu/mol.
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In this Letter, we report on obtaining MgB2 nanoparticles revealing the intra-granular critical current density (JC P 108 A/cm2) for the undoped sample. The bulk synthesis of the superconducting MgB2 nanoparticles was conducted under autogenic pressure at elevated temperature (RAPET) under inert atmosphere. A rapid, single-stage, high yield, reproducible, straightforward and competent approach at 750 C is described herein for the synthesis of MgB2 nanoparticles that can be scaled up for bulk production. The magnetization studies indicated clearly the existence of bulk superconductivity in MgB2 nanoparticles at TC 38 K. Electron spin resonance and microwave absorption study substantiated the paramagnetic nature of the MgB2 nanoparticles in the normal state. 2. Experimental Stoichiometric commercial crystalline Mg (diameter 15– 30 lm, purity 99.9%, Acros) and amorphous B powders (<5 lm, purity >96% (1% Mg and oxygen impurities), Alfa Aesar) were weighed and mixed homogeneously with a mortar and pestle. To avoid reactivity with the inner walls of the stainless steel reactor (Swagelok), the precursors were placed in a 15 mm diameter quartz tube that was introduced into a 4 mL Swagelok union at room temperature, inside a nitrogen-filled glove box. The Swagelok reactor was closed tightly with the other plug [13] and kept inside the 2.5 in. diameter quartz tube under a slow flow of nitrogen to prevent the surface oxidation of the Swagelok during the heat treatment. The temperature of the tube furnace was raised to 750 C at a rate of 25 C/min and maintained at 750 C for 3 h. The Swagelok-reactor heated at 750 C was gradually cooled (6 h) to room temperature, opened, and a black powder was obtained. For 1 g of reactants, 850 mg product was obtained; therefore the yield is 85%. The black material was characterized by morphological, compositional, structural, magnetic and surface area measurements without further treatment. An analogous reaction carried out at 700 C yielded a multi-phase product. On the other hand, a reaction at 800 C resulted in MgB2 with a slight increase in the particle diameters. Earlier, such a one-stage, efficient, economic, and simple RAPET technique was employed for the fabrication of various fascinating nanostructures [14,15]. The phase purity of the obtained MgB2 sample was characterized by a Bruker AXS D* Advance X-ray Powder diffractometer (XRD) with Cu Ka radiation (k = 1.5418 ˚ ). The oxygen contamination in the obtained MgB2 nanoA powder was measured by an oxygen analyzer (Eager 200). Dc magnetic measurements were performed in a commercial (Quantum Design) super-conducting quantum interference device (SQUID) magnetometer. A surface area analyzer (Micromeritics (Gemini 2375)) was used to measure the surface area of the as-prepared MgB2 nanoparticles. The morphology and energy dispersive X-ray analysis (EDX) of the materials were investigated by high resolution scanning electron microscopy (HR-SEM, JSM,
7000 F). The exact particle diameter and morphology of MgB2 nanoparticles are observed by HR-TEM [JEOL3011] operated at 200 kV accelerating voltage. The room temperature electron paramagnetic resonance (Bruker EPR spectrometer-ER083 CS) signal is measured for MgB2 nanoparticles. The g-factor and the ESR intensity are calibrated using a standard sample supplied by the Bruker Co. 3. Results and discussion Fig. 1 presents the wide angle XRD pattern of a MgB2 nanoparticle sample prepared at 750 C under inert atmosphere inside the Swagelok reactor. The diffraction lines can be indexed to a hexagonal (space group: P622) MgB2 ˚. phase, with lattice constants a = 3.083, and c = 3.521 A The peak intensities and positions match well with PDF No. 74-982. A very small shoulders belong to MgO (indicated by *) is noticed at 2H = 43 and 62.50 due to the partial oxidation of Mg. A SEM image of a MgB2 sample prepared at 750 C (shown in Fig. 2) indicates that the particle diameters are in the range of 40–80 nm. EDX analysis confirms the presence of B, C, and Mg. The reason for the small B signal (at 0.185 keV) is low energy. It is well known that the
Fig. 1. The powder XRD pattern of the MgB2 sample produced at 750 C.
Fig. 2. Scanning electron micrograph of: (a) a MgB2 sample prepared at 750 C via RAPET and (b) EDS analysis of MgB2 sample.
V.G. Pol et al. / Chemical Physics Letters 433 (2006) 115–119
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Fig. 3. The TEM of: (a) MgB2 nanoparticles and (b) a single MgB2 nanoparticle at high resolution and its ED pattern (inset).
quantitative determination of the low atomic weight atoms is lacking in the EDX method. The carbon signal originates from the supported conducting carbon tape. No other impurities are detected in the EDX analysis. The measured BET surface area of the as-prepared MgB2 nanoparticles is 13.9 m2/g with a pore diameter of 0.96 cc/g. The TEM (Fig. 3a) of the MgB2 nanoparticles contains spherical and rectangular nanoparticles with smooth surfaces. Diameters between 40 nm and 80 nm are observed for the MgB2 particles. Some of the particles are sintered to each other to form aggregates. Fig. 3b demonstrates the HR-TEM image of the edge of a single particle, which provides further evidence for the crystallinity, as well the identification of the product as MgB2 particles. Furthermore, it illustrates the perfect arrangement of the atomic layers with an interlayer spacing of 0.256 nm between the (1 0 0) lattice planes. This value is very close to the literature (0.267 nm) magnitude for the hexagonal lattice of the MgB2. The structure of the particle appears to be perfect as there is no evidence of dislocations or defects. The inset illustrates the identified diffraction pattern for crystalline MgB2 nanoparticles. In Fig. 4a an intense electron paramagnetic resonance signal is presented for MgB2 nanoparticles at room temperature. The g-factor is a dimensionless constant and is equal to 2.002319 for an unbound electron [16]. For MgB2 nanoparticles, the measured peak-to-peak separation (DHpp) value is 54G and the g-value is 1.993. Here the g-value is less than that of the unbound electron. The g factor of MgB2 nanopowder has already been determined from the Lorentz fit of the CESR (conduction electron spin resonance) line. The g = 2.0015 at 45 K decreases slightly to g = 1.998 at RT [17]. The calculated g value is in good agreement with the CESR study of MgB2, showing a small deviation from the free-electron g value.
Zero-field-cooled (ZFC) and field cooled (FC) magnetic studies of MgB2 nanoparticles are presented in Fig. 4b. The shielding fraction deduced from the ZFC branch is higher than 100% (typical for nanometer size particles), indicating the superconducting bulk nature of the material. The size distribution of the nanoparticles (40– 80 nm) results in a distribution in the lower critical field (HC1). Therefore, the ZFC branch is not a straight line,
Fig. 4. (a) Room temperature electron paramagnetic resonance signal obtained for MgB2 nanoparticles and (b) magnetic study of MgB2 nanoparticles performed at 15.4 Oe.
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Fig. 5. (a) Isothermal magnetic hysteresis loops for a MgB2 sample measured at various temperatures and (b) critical current density JC at H = 0 and 1 T, plotted at various temperatures for MgB2 nanoparticles.
as would be expected for a bulk superconductor measured under such a low applied field. The M(H) plots measured at various temperatures for a MgB2 sample are presented in Fig. 5a. It can be seen that the initial slopes of the curves exceed the theoretical value of the shielding fraction, and that the lines deviate from linearity at very low fields due to the size distribution discussed above. The critical current density (JC) determined by the well known Bean model [18]: J C ¼ 30 Dm=d; where Dm is the width of the M(H) loop at a given field and temperature, and d is the average grain size of the sample. The values deduced for the upper limit of the size distribution (80 nm) from Fig. 5a are exhibited in Fig. 5b, which presents the temperature field dependence of critical current density (JC) at various applied fields. The critical current density (JC) values are P108 A/cm2, as presented in Fig. 5b (these are the lower limit calculated values). This value is taken at around 10 K, because this
will be the temperature that closed cycle He refrigerators will eventually work if the superconductivity of MgB2 will reach an industrial stage. Fig. 5b shows a linear dependence of JC on the temperature for both H = 0 T and H = 1 T. These numbers exceed the critical values obtained for the other C/SiC doped and undoped nano-size MgB2 materials reported in the literature [9,10]. We take into account the fact the values reported in Refs. [9,10] are for bulk samples, whereas our represents the current which flows within the grains. Our achieved intra-grain critical current density values are significantly high as compared to bulk values reported in the literature. This is due to the following probable reasons. (1) High purity [19] or nominal purity [20] B powder can also achieve enhanced critical current density in MgB2 material. (2) The reaction between amorphous B and crystalline Mg under autogenic pressure leads to the fabrication of MgB2 nanoparticles with minor oxidation (<3 wt%). (3) Particle size distribution (40–80 nm) allowed the formation of dense MgB2 material. (4) The MgB2 particles are well connected (necked with each other) during RAPET reaction, minimizing the gain boundaries (TEM image in Fig. 3a). On the other hand, since Ref. [10] revealed that doping the MgB2 enhances the critical current density, it may be that some of the unreacted amorphous B acts as a pinning center, enhancing the critical current density. The exact reason for the high JC is currently under further investigation. The mechanism of the reaction leading to the formation of MgB2 can be explained as follows. Previously, nanoMgB2 samples were prepared by the solid state reaction [21] between boron powder (<1 lm) and nanometeric magnesium powder (<40 nm) at 800 C, under inert atmosphere. The authors claimed that nanosized Mg grains and a relatively low temperature are the two parameter requirements for obtaining nano MgB2 particles. The present reaction is quite different; it is carried out between micrometer size Mg (diameter 15–30 lm) and amorphous B (micron size) powders, yielding nanosized (<80 nm) MgB2 particles under autogenic pressure at 750 C within 3 h. The high-pressure (2 GPa) synthesis of a bulk superconductive MgB2 material at 800–900 C is also reported [22]. In the current reaction, the micrometer diameter Mg powder might have melted at 650 C (MP = 648 C) and diffused to the amorphous B powder to fabricate the nanosized MgB2 particles at 750 C. In one of our control experiments in which we tried to react crystalline B powder with crystalline Mg under identical conditions, no reaction occurred. This result can be interpreted as being due to the higher reactivity of the amorphous boron as compared with the crystalline B. The higher reactivity of amorphous boron is perhaps related to the higher concentration of dangling electrons available for the amorphous state. This phenomenon also initiated the formation of nanosize nuclei, since the reaction is also carried out under autogenic pressure of the reactants at 750 C.
V.G. Pol et al. / Chemical Physics Letters 433 (2006) 115–119
4. Conclusions The bulk synthesis of paramagnetic, superconducting MgB2 nanoparticles via reaction under autogenic pressure at elevated temperature (750 C) in an inert atmosphere is reported herein. The superconductivity at 38 K in MgB2 nanoparticles is confirmed by magnetization studies. The obtained enhanced JC indicates the effectiveness of the present rapid, one-step, high yield, reproducible, straightforward, and competent RAPET approach for the synthesis of MgB2 nanoparticles that can be applied for large scale production. Acknowledgements V.G. Pol and S.V. Pol are thankful to Bar-Ilan University for the financial assistance. This work is partially supported by the Ministry of the National Infrastrucure of Israel (Grant No. 039-7141) . References [1] J. Nagamatsu, N. Nakagawa, Y.Z. Murakana, J. Akimitsu, Nature 410 (2001) 63. [2] H.J. Choi, D. Roundy, H. Sun, M.L. Cohen, S.G. Louie, Nature 428 (2002) 758. [3] S.L. Budko, G. Lapertot, C. Petrovic, C.E. Cunningham, N. Anderson, P.C. Canfeld, Phys. Rev. Lett. 86 (2001) 1877.
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[4] C.B. Eom, M.K. Lee, J.H. Chol, L.J. Belenky, Nature 411 (2001) 558. [5] S. Jin, H. Mavoori, C. Bower, R.B. Van Dover, Nature 411 (2001) 563. [6] H. Kumakura, A. Matsumoto, H. Fujii, K. Togano, Appl. Phys. Lett. 79 (2001) 2435. [7] L. Schultz, J. Eckert, in: H. Beck, H.J. Gunterodt (Eds.), Glassy Metals III, Topics in Applied Physics, 72, 1994, p. 69. [8] A. Gmbel, J. Eckert, G. Fuchs, K. Nenkov, K.H. Muller, L. Schultz, Appl. Phys. Lett. 80 (2002) 2725. [9] Y. Ma, X. Zhang, G. Nishijima, K. Watanabe, S. Awaji, Xuedong Bai, Appl. Phys. Lett. 88 (2006) 72502. [10] X.L. Wang et al., Physica C 63 (2004) 408. [11] K.W. Blazey, in: J.G. Bednorz, K.A. Muller (Eds.), Earlier and Recent Aspects of Superconductivity, Springer Series in Solid State Sciences, vol. 90, Springer-Verlag, Berlin, 1990, p. 262. [12] V. Likodimos, M. Pissas, Phys. Rev. B 65 (2002) 172507. [13] S.V. Pol, V.G. Pol, A. Gedanken, Chem. Eur. J. 10 (2004) 4467. [14] V.G. Pol, S.V. Pol, B. Markovsky, J.M. Calderon-Moreno, A. Gedanken, Chem. Mater. 18 (2006) 1512. [15] S.V. Pol, V.G. Pol, A. Gedanken, Adv. Mater. 18 (2006) 2023. [16] Willard, Merrit, Dean. Instrumental Methods of Analysis. fifth ed. 1974, p. 236. [17] F. Simon et al., Phys. Rev. Lett. 87 (2001) 7002. [18] C.P. Bean, Rev. Mod. Phys. 36 (1964) 31. [19] S.K. Chen, K.A. Yates, M.G. Blamire, J.L. MacManus-Driscoll, Supercond. Sci. Technol. 18 (2005) 1473. [20] W. Ha¨ßler et al., Supercond. Sci. Technol. 19 (2006) 512. [21] C. Chen, Z. Zhou, X. Li, J. Xu, Yu-hao Wang, Z. Gao, Q. Feng, Solid State Commun. 131 (2004) 275. [22] T.A. Prikhna, W. Gawalek, Ya.M. Savchuk, V.E. Moshchil, N.V. Sergienko, A.B. Surzhenko, M. Wendt, S.N. Dub, Physica C 386 (2003) 565.
Critical current density in the MgB2 nanoparticles prepared under autogenic pressure at elevated temperature Vilas Ganpat Pol a, Swati Vilas Pol a, Israel Felner b, Aharon Gedanken a
a,*
Department of Chemistry and Kanbar Laboratory for Nanomaterials, Bar-Ilan University Center for Advanced Materials and Nanotechnology, Ramat-Gan 52900, Israel b Racah Institute of Physics, Hebrew University, Jerusalem 91904, Israel Received 9 August 2006; in final form 27 October 2006 Available online 12 November 2006
Abstract This Letter reports on obtaining MgB2 nanoparticles revealing an intra-granular enhanced critical current density (JC P 108 A/cm2) for the undoped sample. A rapid, single-stage, high yield, reproducible, straightforward, scalable, and competent approach at 750 C is described herein for the synthesis of superconducting MgB2 nanoparticles. The magnetization studies indicated clearly the existence of bulk superconductivity in MgB2 nanoparticles at TC 38 K. Electron spin resonance and microwave absorption study substantiated the paramagnetic nature of the MgB2 nanoparticles in the normal state. The plausible reaction mechanism leading to the formation of MgB2 is explained. 2006 Elsevier B.V. All rights reserved.
1. Introduction The discovery of low cost, high-temperature superconductivity in MgB2 (TC = 39 K) has triggered intense interest in the scientific community [1]. MgB2 adopts a simple hexagonal AlB2-type crystal structure with interleaved two-dimensional boron and magnesium layers [2]. The effect of a boron isotope on MgB2 indicated that this material is a conventional phonon-mediated BCS (Bardeen– Cooper–Schrieffer) superconductor [3]. Until now, MgB2 has been synthesized in various forms: polycrystalline [1], thin films [4], wires [5], and tapes [6]. Mechanical alloying is a well-established method for preparing meta-stable amorphous, quasi-crystalline, and nanocrystalline MgB2 materials [7]. The mechanical alloying of Mg and B powders at ambient temperatures, followed by hot pressing, produced 100 nm high density, nanocrystalline samples with improved pinning [8]. However, the nanocrystalline MgB2 preparation is time consuming and difficult because *
Corresponding author. Fax: +972 3 535 1250. E-mail address: [email protected] (A. Gedanken).
0009-2614/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2006.11.021
of the large difference in vapor pressure between B and Mg, as well as magnesium’s high susceptibility to oxidation. Recently, Ma et al. described the considerably enhanced critical current densities at 4.2 K [9] in MgB2 tapes doped by 5 wt% nanocarbon. They reported high JC values of 1.8 · 104 A/cm2 at 10 T and 2.8 · 103 A/cm2 at 14 T for the samples sintered at 750 C. Wang et al. also demonstrated the enhancement of critical current density in MgB2 by doping it with nano-SiC, Si, or C doping (<10%) and compared [10] the critical current density with the undoped sample. They accounted both magnetic and transport JC properties as high as 2 · 104 A/cm2 in 8 T at 5 K, one or two orders of magnitude higher than the undoped MgB2. Electron-spin resonance (ESR) is known to be a very sensitive method for probing paramagnetic centers and microwave absorption in both conventional and high-TC superconductors [11]. Recently, a comparative ESR study of nano size and micron size MgB2 specimens verified the presence of intense conduction electron spin resonance (CESR) in the normal state, with a relatively high spin relaxation rate [12] and a Pauli susceptibility of xs = 2.5 · 105 emu/mol.
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In this Letter, we report on obtaining MgB2 nanoparticles revealing the intra-granular critical current density (JC P 108 A/cm2) for the undoped sample. The bulk synthesis of the superconducting MgB2 nanoparticles was conducted under autogenic pressure at elevated temperature (RAPET) under inert atmosphere. A rapid, single-stage, high yield, reproducible, straightforward and competent approach at 750 C is described herein for the synthesis of MgB2 nanoparticles that can be scaled up for bulk production. The magnetization studies indicated clearly the existence of bulk superconductivity in MgB2 nanoparticles at TC 38 K. Electron spin resonance and microwave absorption study substantiated the paramagnetic nature of the MgB2 nanoparticles in the normal state. 2. Experimental Stoichiometric commercial crystalline Mg (diameter 15– 30 lm, purity 99.9%, Acros) and amorphous B powders (<5 lm, purity >96% (1% Mg and oxygen impurities), Alfa Aesar) were weighed and mixed homogeneously with a mortar and pestle. To avoid reactivity with the inner walls of the stainless steel reactor (Swagelok), the precursors were placed in a 15 mm diameter quartz tube that was introduced into a 4 mL Swagelok union at room temperature, inside a nitrogen-filled glove box. The Swagelok reactor was closed tightly with the other plug [13] and kept inside the 2.5 in. diameter quartz tube under a slow flow of nitrogen to prevent the surface oxidation of the Swagelok during the heat treatment. The temperature of the tube furnace was raised to 750 C at a rate of 25 C/min and maintained at 750 C for 3 h. The Swagelok-reactor heated at 750 C was gradually cooled (6 h) to room temperature, opened, and a black powder was obtained. For 1 g of reactants, 850 mg product was obtained; therefore the yield is 85%. The black material was characterized by morphological, compositional, structural, magnetic and surface area measurements without further treatment. An analogous reaction carried out at 700 C yielded a multi-phase product. On the other hand, a reaction at 800 C resulted in MgB2 with a slight increase in the particle diameters. Earlier, such a one-stage, efficient, economic, and simple RAPET technique was employed for the fabrication of various fascinating nanostructures [14,15]. The phase purity of the obtained MgB2 sample was characterized by a Bruker AXS D* Advance X-ray Powder diffractometer (XRD) with Cu Ka radiation (k = 1.5418 ˚ ). The oxygen contamination in the obtained MgB2 nanoA powder was measured by an oxygen analyzer (Eager 200). Dc magnetic measurements were performed in a commercial (Quantum Design) super-conducting quantum interference device (SQUID) magnetometer. A surface area analyzer (Micromeritics (Gemini 2375)) was used to measure the surface area of the as-prepared MgB2 nanoparticles. The morphology and energy dispersive X-ray analysis (EDX) of the materials were investigated by high resolution scanning electron microscopy (HR-SEM, JSM,
7000 F). The exact particle diameter and morphology of MgB2 nanoparticles are observed by HR-TEM [JEOL3011] operated at 200 kV accelerating voltage. The room temperature electron paramagnetic resonance (Bruker EPR spectrometer-ER083 CS) signal is measured for MgB2 nanoparticles. The g-factor and the ESR intensity are calibrated using a standard sample supplied by the Bruker Co. 3. Results and discussion Fig. 1 presents the wide angle XRD pattern of a MgB2 nanoparticle sample prepared at 750 C under inert atmosphere inside the Swagelok reactor. The diffraction lines can be indexed to a hexagonal (space group: P622) MgB2 ˚. phase, with lattice constants a = 3.083, and c = 3.521 A The peak intensities and positions match well with PDF No. 74-982. A very small shoulders belong to MgO (indicated by *) is noticed at 2H = 43 and 62.50 due to the partial oxidation of Mg. A SEM image of a MgB2 sample prepared at 750 C (shown in Fig. 2) indicates that the particle diameters are in the range of 40–80 nm. EDX analysis confirms the presence of B, C, and Mg. The reason for the small B signal (at 0.185 keV) is low energy. It is well known that the
Fig. 1. The powder XRD pattern of the MgB2 sample produced at 750 C.
Fig. 2. Scanning electron micrograph of: (a) a MgB2 sample prepared at 750 C via RAPET and (b) EDS analysis of MgB2 sample.
V.G. Pol et al. / Chemical Physics Letters 433 (2006) 115–119
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Fig. 3. The TEM of: (a) MgB2 nanoparticles and (b) a single MgB2 nanoparticle at high resolution and its ED pattern (inset).
quantitative determination of the low atomic weight atoms is lacking in the EDX method. The carbon signal originates from the supported conducting carbon tape. No other impurities are detected in the EDX analysis. The measured BET surface area of the as-prepared MgB2 nanoparticles is 13.9 m2/g with a pore diameter of 0.96 cc/g. The TEM (Fig. 3a) of the MgB2 nanoparticles contains spherical and rectangular nanoparticles with smooth surfaces. Diameters between 40 nm and 80 nm are observed for the MgB2 particles. Some of the particles are sintered to each other to form aggregates. Fig. 3b demonstrates the HR-TEM image of the edge of a single particle, which provides further evidence for the crystallinity, as well the identification of the product as MgB2 particles. Furthermore, it illustrates the perfect arrangement of the atomic layers with an interlayer spacing of 0.256 nm between the (1 0 0) lattice planes. This value is very close to the literature (0.267 nm) magnitude for the hexagonal lattice of the MgB2. The structure of the particle appears to be perfect as there is no evidence of dislocations or defects. The inset illustrates the identified diffraction pattern for crystalline MgB2 nanoparticles. In Fig. 4a an intense electron paramagnetic resonance signal is presented for MgB2 nanoparticles at room temperature. The g-factor is a dimensionless constant and is equal to 2.002319 for an unbound electron [16]. For MgB2 nanoparticles, the measured peak-to-peak separation (DHpp) value is 54G and the g-value is 1.993. Here the g-value is less than that of the unbound electron. The g factor of MgB2 nanopowder has already been determined from the Lorentz fit of the CESR (conduction electron spin resonance) line. The g = 2.0015 at 45 K decreases slightly to g = 1.998 at RT [17]. The calculated g value is in good agreement with the CESR study of MgB2, showing a small deviation from the free-electron g value.
Zero-field-cooled (ZFC) and field cooled (FC) magnetic studies of MgB2 nanoparticles are presented in Fig. 4b. The shielding fraction deduced from the ZFC branch is higher than 100% (typical for nanometer size particles), indicating the superconducting bulk nature of the material. The size distribution of the nanoparticles (40– 80 nm) results in a distribution in the lower critical field (HC1). Therefore, the ZFC branch is not a straight line,
Fig. 4. (a) Room temperature electron paramagnetic resonance signal obtained for MgB2 nanoparticles and (b) magnetic study of MgB2 nanoparticles performed at 15.4 Oe.
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Fig. 5. (a) Isothermal magnetic hysteresis loops for a MgB2 sample measured at various temperatures and (b) critical current density JC at H = 0 and 1 T, plotted at various temperatures for MgB2 nanoparticles.
as would be expected for a bulk superconductor measured under such a low applied field. The M(H) plots measured at various temperatures for a MgB2 sample are presented in Fig. 5a. It can be seen that the initial slopes of the curves exceed the theoretical value of the shielding fraction, and that the lines deviate from linearity at very low fields due to the size distribution discussed above. The critical current density (JC) determined by the well known Bean model [18]: J C ¼ 30 Dm=d; where Dm is the width of the M(H) loop at a given field and temperature, and d is the average grain size of the sample. The values deduced for the upper limit of the size distribution (80 nm) from Fig. 5a are exhibited in Fig. 5b, which presents the temperature field dependence of critical current density (JC) at various applied fields. The critical current density (JC) values are P108 A/cm2, as presented in Fig. 5b (these are the lower limit calculated values). This value is taken at around 10 K, because this
will be the temperature that closed cycle He refrigerators will eventually work if the superconductivity of MgB2 will reach an industrial stage. Fig. 5b shows a linear dependence of JC on the temperature for both H = 0 T and H = 1 T. These numbers exceed the critical values obtained for the other C/SiC doped and undoped nano-size MgB2 materials reported in the literature [9,10]. We take into account the fact the values reported in Refs. [9,10] are for bulk samples, whereas our represents the current which flows within the grains. Our achieved intra-grain critical current density values are significantly high as compared to bulk values reported in the literature. This is due to the following probable reasons. (1) High purity [19] or nominal purity [20] B powder can also achieve enhanced critical current density in MgB2 material. (2) The reaction between amorphous B and crystalline Mg under autogenic pressure leads to the fabrication of MgB2 nanoparticles with minor oxidation (<3 wt%). (3) Particle size distribution (40–80 nm) allowed the formation of dense MgB2 material. (4) The MgB2 particles are well connected (necked with each other) during RAPET reaction, minimizing the gain boundaries (TEM image in Fig. 3a). On the other hand, since Ref. [10] revealed that doping the MgB2 enhances the critical current density, it may be that some of the unreacted amorphous B acts as a pinning center, enhancing the critical current density. The exact reason for the high JC is currently under further investigation. The mechanism of the reaction leading to the formation of MgB2 can be explained as follows. Previously, nanoMgB2 samples were prepared by the solid state reaction [21] between boron powder (<1 lm) and nanometeric magnesium powder (<40 nm) at 800 C, under inert atmosphere. The authors claimed that nanosized Mg grains and a relatively low temperature are the two parameter requirements for obtaining nano MgB2 particles. The present reaction is quite different; it is carried out between micrometer size Mg (diameter 15–30 lm) and amorphous B (micron size) powders, yielding nanosized (<80 nm) MgB2 particles under autogenic pressure at 750 C within 3 h. The high-pressure (2 GPa) synthesis of a bulk superconductive MgB2 material at 800–900 C is also reported [22]. In the current reaction, the micrometer diameter Mg powder might have melted at 650 C (MP = 648 C) and diffused to the amorphous B powder to fabricate the nanosized MgB2 particles at 750 C. In one of our control experiments in which we tried to react crystalline B powder with crystalline Mg under identical conditions, no reaction occurred. This result can be interpreted as being due to the higher reactivity of the amorphous boron as compared with the crystalline B. The higher reactivity of amorphous boron is perhaps related to the higher concentration of dangling electrons available for the amorphous state. This phenomenon also initiated the formation of nanosize nuclei, since the reaction is also carried out under autogenic pressure of the reactants at 750 C.
V.G. Pol et al. / Chemical Physics Letters 433 (2006) 115–119
4. Conclusions The bulk synthesis of paramagnetic, superconducting MgB2 nanoparticles via reaction under autogenic pressure at elevated temperature (750 C) in an inert atmosphere is reported herein. The superconductivity at 38 K in MgB2 nanoparticles is confirmed by magnetization studies. The obtained enhanced JC indicates the effectiveness of the present rapid, one-step, high yield, reproducible, straightforward, and competent RAPET approach for the synthesis of MgB2 nanoparticles that can be applied for large scale production. Acknowledgements V.G. Pol and S.V. Pol are thankful to Bar-Ilan University for the financial assistance. This work is partially supported by the Ministry of the National Infrastrucure of Israel (Grant No. 039-7141) . References [1] J. Nagamatsu, N. Nakagawa, Y.Z. Murakana, J. Akimitsu, Nature 410 (2001) 63. [2] H.J. Choi, D. Roundy, H. Sun, M.L. Cohen, S.G. Louie, Nature 428 (2002) 758. [3] S.L. Budko, G. Lapertot, C. Petrovic, C.E. Cunningham, N. Anderson, P.C. Canfeld, Phys. Rev. Lett. 86 (2001) 1877.
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