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Huisheng Peng,
Daoyong Chen,
Jian-Yu Huang,
S B Chikkannanavar,
J Hänisch,
Menka Jain, D E Peterson,
S K Doorn,
Yunfeng Lu,
Y T Zhu,
Q X Jia
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ABSTRACT: We report a new type of carbon material-porous colossal carbon tubes. Compared with carbon nanotubes, colossal carbon tubes have a much bigger size, with a diameter of between 40 and 100 mum and a length in the range of centimeters. Significantly, the walls of the colossal tubes are composed of macroscopic rectangular columnar pores and exhibit an ultralow density comparable to that of carbon nanofoams. The porous walls of colossal tubes also show a highly ordered lamellar structure similar to that of graphite. Furthermore, colossal tubes possess excellent mechanical and electrical properties.
Physical Review Letters 11/2008; 101(14):145501. · 7.37 Impact Factor
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L. Zheng,
X. Zhang,
Q. Li,
S. B. Chikkannanavar,
Y. Li,
Y. Zhao,
X. Liao,
Q. Jia,
S. K. Doorn, D. E. Peterson,
Y. Zhu
Advanced Materials 08/2007; 19(18):2567 - 2570. · 13.88 Impact Factor
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ABSTRACT: Pinning in YBa2Cu3O7−x (YBCO) films is compared with mixed rare-earth (RE) films. The field dependence of critical current density and pinning force plots at 75.5 K are analysed. The work provides a guideline as to how to optimize pinning by inclusion of both random and correlated defects. In summary, Y mixed with larger RE ions appears to be optimum for effective pinning across a wide field regime as well as permitting growth at temperatures low enough for technological use.
Superconductor Science and Technology 01/2006; 19(3):S55. · 2.66 Impact Factor
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Advanced Materials 06/2005; 17(12):1527 - 1531. · 13.88 Impact Factor
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J. L. MacManus-Driscoll,
S. R. Foltyn,
B. Maiorov,
Q. X. Jia,
H. Wang,
A. Serquis,
L. Civale,
Y. Lin,
M. E. Hawley,
M. P. Maley, D. E. Peterson
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ABSTRACT: The influence of rare earth (RE) ion size on critical current density (Jc) in epitaxial films of superconducting REBa2Cu3O7−x was studied, where RE is the mixture of two or three rare earth ions. No systematic dependence of Jc on RE ion size was found. However, strongly enhanced critical current densities (Jc’s) were found for the composition Y2/3Sm1/3Ba2Cu3O7−x (YSmBCO). In ∼ 1‐μm-thick films, Jc’s as high as 4.7×106 A cm−2 (75.5 K, 0 T) and 11×104 A cm−2 (75.5 K, 5 T) were obtained on single crystals and 6×104 A cm−2 (75.5 K, 5 T) on buffered metal. The values are up to a factor of 3 higher than for comparative YBCO samples.
Applied Physics Letters 01/2005; 86(3):032505-032505-3. · 3.84 Impact Factor
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X. Z. Liao,
A. Sharma,
M. Wei,
J. L. MacManus-Driscoll,
W. Branford,
L. F. Cohen,
Y. Bugoslavsky,
Y. T. Zhu, D. E. Peterson,
Y. B. Jiang,
H. F. Xu
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ABSTRACT: Two Sr2FeMoO6 samples, synthesized under different conditions and showing very different magnetotransport properties, were analyzed using transmission electron microscopy. The different magnetoresistive behaviors observed in the two samples coincide with the formation of different amounts of striped areas intergrown in the samples. The striped areas are comprised of a disordered and cation deficient phase, SrFe1−x−yMox−zO3−δ. The sample magnetization and magnetoresistance values decrease significantly with an increased presence of the striped phase. The striped phase was promoted by typical reducing formation conditions for Sr2FeMoO6 in Ar-1% H2 gas.
Journal of Applied Physics. 12/2004; 96(12):7747-7749.
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L X Zheng,
M J O'Connell,
S K Doorn,
X Z Liao,
Y H Zhao,
E A Akhadov,
M A Hoffbauer,
B J Roop,
Q X Jia,
R C Dye, D E Peterson,
S M Huang,
J Liu,
Y T Zhu
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ABSTRACT: Since the discovery of carbon nanotubes in 1991 by Iijima, there has been great interest in creating long, continuous nanotubes for applications where their properties coupled with extended lengths will enable new technology developments. For example, ultralong nanotubes can be spun into fibres that are more than an order of magnitude stronger than any current structural material, allowing revolutionary advances in lightweight, high-strength applications. Long metallic nanotubes will enable new types of micro-electromechanical systems such as micro-electric motors, and can also act as a nanoconducting cable for wiring micro-electronic devices. Here we report the synthesis of 4-cm-long individual single-wall carbon nanotubes (SWNTs) at a high growth rate of 11 microm s(-1) by catalytic chemical vapour deposition. Our results suggest the possibility of growing SWNTs continuously without any apparent length limitation.
Nature Material 11/2004; 3(10):673-6. · 32.84 Impact Factor
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ABSTRACT: There are numerous potential applications for superconducting tapes based on YBa(2)Cu(3)O(7-x) (YBCO) films coated onto metallic substrates. A long-established goal of more than 15 years has been to understand the magnetic-flux pinning mechanisms that allow films to maintain high current densities out to high magnetic fields. In fact, films carry one to two orders of magnitude higher current densities than any other form of the material. For this reason, the idea of further improving pinning has received little attention. Now that commercialization of YBCO-tape conductors is much closer, an important goal for both better performance and lower fabrication costs is to achieve enhanced pinning in a practical way. In this work, we demonstrate a simple and industrially scaleable route that yields a 1.5-5-fold improvement in the in-magnetic-field current densities of conductors that are already of high quality.
Nature Material 08/2004; 3(7):439-43. · 32.84 Impact Factor
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J. L. MacManus-Driscoll,
S. R. Foltyn,
Q. X. Jia,
H. Wang,
A. Serquis,
B. Maiorov,
L. Civale,
Y. Lin,
M. E. Hawley,
M. P. Maley, D. E. Peterson
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ABSTRACT: Enhanced in-field critical current densities (Jcs) have been obtained in epitaxial superconducting (RE1,RE2) Ba2Cu3O7−x ( RE1 = rare‐earth ion 1, and RE2 = rare earth ion 2) films grown on both single crystal and buffered metallic substrates. For a constant average RE ionic radius (equal to that of yttrium), there is a systematic dependence of the in-field Jc on the RE ion size variance, with a small, but nonzero, variance being optimum. Compared to the standard YBa2Cu3O7−x composition, a factor of two improvement in Jc (75.5 K) is reproducibly observed at 0.2 T (∥c) for the composition Dy1/3Ho2/3Ba2Cu3O7−x on both single crystal and buffered-metallic substrates. Angular dependent magnetic field studies and transmission electron microscopy indicate the presence of additional pointlike random defects.
Applied Physics Letters 06/2004; 84(26):5329-5331. · 3.84 Impact Factor
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ABSTRACT: MgB2 samples synthesized by solid-state reaction were found to contain several types of crystalline defects, including Mg deficiency, coherent precipitates, screw dislocations, and oxygen impurity, depending on the synthesis conditions. The Mg deficiency introduced significant strain in the crystalline lattice of MgB2. A high density of screw dislocations lying on (0001) planes with a Burgers vector of partially relieved the lattice strain. The remnant strain in the MgB2 lattice directly determines the critical superconducting temperature, which decreases with strain. A small amount of oxygen dissolved in the MgB2 matrix is not responsible for lowering Tc observed in our samples.
Superconductor Science and Technology 06/2004; 17(8):1026. · 2.66 Impact Factor
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ABSTRACT: Two sets of MgB2 samples doped with up to 5 at. % of Al were prepared in different laboratories using different procedures. Decreases in the a and c lattice parameters were observed with Al doping confirming Al substitution onto the Mg site. The critical temperature (Tc) remained largely unchanged with Al doping. For 1 - 2.5 at.% doping, at 20K the in-field critical current densities (Jc's) were enhanced, particularly at lower fields. At 5K, in-field Jc was markedly improved, e.g. at 5T Jc was enhanced by a factor of 20 for a doping level of 1 at.% Al. The improved Jcs correlate with increased sample resistivity indicative of an increase in the upper critical field, Hc2, through alloying. Comment: 17 pages, 4 figures, to be published in Superconductor Science and Technology
05/2004;
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ABSTRACT: We present the fabrication and test results of Hot-Isostatic-Pressed (HIPed) Powder-in-Tube (PIT) MgB$_2$ coils. The coils properties were measured by transport and magnetization at different applied fields ($H$) and temperatures ($T$). The engineering critical current ($J_e$) value is the largest reported in PIT MgB$_2$ wires or tapes. At 25 K our champion 6-layer coil was able to generate a field of 1 T at self-field ($I_c >$ 220 A, $J_e \sim 2.8 \times 10^4$ A/cm$^2$). At 4 K this coil generated 1.6 T under an applied field of 1.25 T ($I_c \sim350$ A, $J_e \sim 4.5 \times 10^4$ A/cm$^2$). These magnetic fields are high enough for a superconducting transformer or magnet applications such as MRI. A SiC doped MgB$_2$ single layer coil shows a promising improvement at high fields and exhibits $J_c > 10^4$ A/cm$^2$ at 7 T. Comment: 8 pages 3 figures
04/2004;
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ABSTRACT: We present a detailed analysis of the effect of heat treatments on the microstructure, magnetization, and transport properties of MgB2 wires produced by the powder-in-tube method. We have used commercial MgB2 powder with 5 at. % Mg powder added as an additional source of magnesium and stainless steel as sheath material. We measure the dc transport critical current that can be increased or decreased by more than one order of magnitude as compared with the as-drawn wire, depending on the annealing parameters. We correlate the changes in the critical current with changes in the microstructure, as determined from scanning and transmission electron microscopy analysis. We show through magnetization measurements of short annealed wires that inappropriate annealing conditions result in a deterioration of the connectivity due to the loss of Mg and in inhomogeneous weak-link limited current flow, rendering the critical state model inapplicable. We discuss the optimization of the annealing conditions that strongly improve the connectivity by eliminating most of the microcracks present in the unannealed wires, where excess Mg promotes the recrystallization. The loss of Mg during the heat treatment may be precluded by annealing long wire lengths with a high heating rate. © 2003 American Institute of Physics.
Journal of Applied Physics 09/2003; 94(6):4024-4031. · 2.17 Impact Factor
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ABSTRACT: MgB<sub>2</sub> wires were produced by the powder-in-tube method, using commercial MgB<sub>2</sub> powder with 5%at Mg powder added as an additional source of magnesium, and stainless steel as sheath material. Depending on the parameters of the heat treatments, the dc transport critical current can be increased or decreased by more than one order of magnitude as compared with the as-drawn wire. Analysis of the zero-field-cooling dc magnetization demonstrates that these large variations are due to changes in the connectivity of the wires. We discuss the optimization of the annealing conditions in terms of the changes in the microstructure, as determined from TEM and SEM analysis. We show that annealing can strongly improve the connectivity by eliminating most of the micro-cracks present in the un-annealed wires. In contrast, inappropriate annealing conditions result in a deterioration of the connectivity due to the loss of Mg.
IEEE Transactions on Appiled Superconductivity 07/2003; · 1.04 Impact Factor
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ABSTRACT: Bulk MgB<sub>2</sub> samples were prepared under different synthesis conditions and analyzed by scanning and transmission electron microscopy. The critical current densities were determined from the magnetization versus magnetic field curves of bulk and powder-dispersed-in-epoxy samples. Results show that through a slow cooling process, the oxygen dissolved in bulk MgB<sub>2</sub> at high synthesis temperatures can segregate and form nanometer-sized coherent precipitates of Mg(B,O)<sub>2</sub> in the MgB<sub>2</sub> matrix. Magnetization measurements indicate that these precipitates act as effective flux pinning centers and therefore significantly improve the intra-grain critical current density and its field dependence.
IEEE Transactions on Appiled Superconductivity 07/2003; · 1.04 Impact Factor
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ABSTRACT: Bulk MgB2 samples were prepared under different synthesis conditions and analyzed by scanning and transmission electron microscopy. The critical current densities were determined from the magnetization versus magnetic field curves of bulk and powder-dispersed-in-epoxy samples. Results show that through a slow cooling process, the oxygen dissolved in bulk MgB2 at high synthesis temperatures can segregate and form nanometer-sized coherent precipitates of Mg(B,O)2 in the MgB2 matrix. Magnetization measurements indicate that these precipitates act as effective flux pinning centers and therefore significantly improve the intra-grain critical current density and its field dependence.
06/2003;
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ABSTRACT: Transmission electron microscopy was used to probe the compositions of individual Co–Mo bimetal catalyst particles and the morphologies of carbon nanotubes (CNTs) catalyzed by these particles under flowing carbon monoxide at 700 °C. It was found that the composition of the catalyst particle at a CNT tip and the distribution of Co within the particle largely determine the morphology of the CNT. A particle with low Co content (<15 at. %) tends to produce a long CNT, while a particle with very high Co content (>85 at. %) tends to produce onion-like structures. These observations provide insight into the CNT growth mechanisms. © 2003 American Institute of Physics.
Applied Physics Letters 04/2003; 82(16):2694-2696. · 3.84 Impact Factor
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A. Serquis,
L. Civale,
D. L. Hammon,
X. Z. Liao,
J.Y. Coulter,
Y. T. Zhu,
M. Jaime, D. E. Peterson,
F. M. Mueller,
V. F. Nesterenko,
Y Gu
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ABSTRACT: The critical current density (Jc) of hot isostatic pressed (HIPed) MgB2 wires, measured by d.c. transport and magnetization, is compared with that of similar wires annealed at ambient pressure. The HIPed wires have a higher Jc than the annealed wires, especially at high temperatures and magnetic fields, and higher irreversibility field (Hirr). The HIPed wires are promising for applications, with Jc>106 A/cm2 at 5 K and zero field and >104 A/cm2 at 1.5 T and 26.5 K, and Hirr ~ 17 T at 4 K. The improvement is attributed to a high density of structural defects, which are the likely source of vortex pinning. These defects, observed by transmission electron microscopy, include small angle twisting, tilting, and bending boundaries, resulting in the formation of sub-grains within MgB2 crystallites. Comment: 13 pages,3 figures
03/2003;
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ABSTRACT: The microstructures of MgB2 wires prepared by the powder-in-tube technique and subsequent hot isostatic pressing were investigated using transmission electron microscopy. Large amount of crystalline defects including small angle twisting, tilting, and bending boundaries, in which high densities of dislocations reside, were found forming sub-grains within MgB2 grains. It is believed that these defects resulted from particle deformation during the hot isostatic pressing process and are effective flux pinning centers that contribute to the high critical current densities of the wires at high temperatures and at high fields. Comment: 16 pages, 6 figures
03/2003;
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ABSTRACT: MgB2 samples prepared by solid-state reaction were investigated using high-resolution transmission electron microscopy (HREM), X-ray energy-dispersive spectroscopy (EDX), electron energy-loss spectroscopy (EELS), and energy-filtered imaging. Large amounts of coherent precipitates with a size range from about 5 nm up to about 100 nm were found in the MgB2 crystallite matrices. The precipitates are of different shapes including sphere, ellipsoid, and faceted polyhedron depending on the size of the precipitates. EDX and EELS analyses confirm that smaller precipitates contain magnesium, boron and oxygen while larger faceted precipitates contain mainly magnesium and oxygen, implying that the oxygen content increases with precipitate size. HREM and electron diffraction investigations found that the precipitates have the same crystal lattice structure as that of MgB2 but with various composition modulations depending on the composition of the precipitates. The precipitates transform to the MgO phase after long exposure to residual oxygen in flowing Ar gas at high temperatures. The effect of the precipitates in different size ranges on flux pinning is discussed. Comment: 29 pages, 9 Figures, to be published in J. Applied Physics
12/2002;
