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Search Results: 1 - 10 of 462049 matches for " A. Serquis "
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Glassiness in vortex matter in MgB2 probed by a novel scaling method for creep analysis
G. Pasquini,A. Moreno,A. Serquis,G. Serrano,L. Civale
Physics , 2009,
Abstract: The glassy dynamics is a consequence of the elastic properties of the vortex matter, and in principle may occur in any superconductor. However, whereas a large amount of experimental evidence confirms glassiness in high Tc superconductors (HTS), the applicability of the whole framework developed to describe vortex matter in HTS to other superconducting materials is unclear. In this framework, the particular creep behavior of MgB2, larger than creep of conventional superconductor materials but much lower than HTS one, had precluded a complete understanding of the vortex regimes in this material. In this work we present an experimental study of relaxation processes of pure MgB2 bulk samples measured by DC magnetization technique. We propose a novel self-consistent scaling method to analyze the data and extract the activation energies. The observed experimental behavior can be described in a glassy picture, with a unique critical exponent &mu~1, characteristic of collective creep in the intermediate vortex bundle regime.
Correlated enhancement of Hc2 and Jc in carbon nanotube-doped MgB2
A. Serquis,G. Serrano,S. Moreno,; L. Civale,B. Maiorov,; F. Balakirev,M. Jaime
Physics , 2006, DOI: 10.1088/0953-2048/20/4/L02
Abstract: The use of MgB2 in superconducting applications still awaits for the development of a MgB2-based material where both current-carrying performance and critical magnetic field are optimized simultaneously. We achieved this by doping MgB2 with double-wall carbon nanotubes (DWCNT) as a source of carbon in polycrystalline samples. The optimum nominal DWCNT content for increasing the critical current density, Jc is in the range 2.5-10%at depending on field and temperature. Record values of the upper critical field, Hc2(4K) = 41.9 T (with extrapolated Hc2(0) ~ 44.4 T) are reached in a bulk sample with 10%at DWCNT content. The measured Hc2 vs T in all samples are successfully described using a theoretical model for a two-gap superconductor in the dirty limit first proposed by Gurevich et al.
The degradation of MgB2 under ambient environment
A. Serquis,Y. T. Zhu,D. E. Peterson,F. M. Mueller,R. K. Schulze,V. F. Nesterenko,S. S. Indrakanti
Physics , 2002, DOI: 10.1063/1.1481548
Abstract: The superconductivities of samples prepared by several procedures were found to degrade under ambient environment. The degradation mechanism was studied by measuring the change of surface chemical composition of dense MgB2 pellets (prepared by hot isostatic pressure, HIPed) under atmospheric exposure using X-ray Photoelectron Spectroscopy (XPS). Results showed that samples with poor connectivity between grains and with smaller grain sizes degrade with time when exposed to ambient conditions. In these samples, the Tc did not change with time, but the superconducting transition became broader and the Meissner fraction decreased. In contrast, our well-sintered and the HIPed samples remained stable for several months under ambient condition. The degradation was found to be related to surface decomposition as observed by XPS. We observed the formation of oxidized Mg, primarily in the form of a Mg hydroxide, the increase of C and O contents, and the reduction of B concentration in the surface layer of MgB2 samples.
Effect of lattice strain and defects on the superconductivity of MgB2
A. Serquis,Y. T. Zhu,E. J. Peterson,J. Y. Coulter,D. E. Peterson,F. M. Mueller
Physics , 2001, DOI: 10.1063/1.1428109
Abstract: The influence of lattice strain and Mg vacancies on the superconducting properties of MgB2 samples has been investigated. High quality samples with sharp superconducting transitions were synthesized. The variation in lattice strain and Mg vacancy concentrations were obtained by varying the synthesis conditions. It was found that high strain (~1%) and the presence of Mg vacancies (~ 5 %) resulted in lowering the Tc by only 2 K.
The influence of structural defects on intra-granular critical currents of bulk MgB2
A. Serquis,X. Z. Liao,L. Civale,Y. T. Zhu,J. Y. Coulter,D. E. Peterson,F. M. Mueller
Physics , 2003,
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.
Microwave performance of high-density bulk MgB2
A. T. Findikoglu,A. Serquis,L. Civale,X. Z. Liao,Y. T. Zhu,M. E. Hawley,F. M. Mueller,V. F. Nesterenko,Y. Gu
Physics , 2003, DOI: 10.1063/1.1590739
Abstract: We have performed microwave measurements on superconducting hot-isostatically- pressed (HIPed) bulk MgB2 using a parallel-plate resonator technique. The high density and strength of the HIPed material allowed preparation of samples with mirror-like surfaces for microwave measurements. The microwave surface resistance decreased by about 40% at 20 K when the root-mean-square surface roughness was reduced from 220 nm to 110 nm through surface-polishing and ion-milling. The surface resistance was independent of surface microwave magnetic field at least up to 4 Oe and below 30 K. We attribute this behavior, and the overall low surface resistance (~0.8 mOhms at 10 GHz and 20 K), to the high density of our samples and the absence of weak links between grains.
RF Surface Resistance of a HIPped MgB2 Sample at 21 GHz
T. Tajima,F. L. Krawczyk,J. Liu,D. C. Nguyen,D. L. Schrage,A. Serquis,A. H. Shapiro,V. F. Nesterenko,Y. Gu
Physics , 2003,
Abstract: Magnesium diboride (MgB2) is attractive for RF cavity application for particle accelerators because it might not show an increase of RF surface losses at high magnetic surface fields, a phenomenon that has prevented high-Tc superconducting materials such as YBCO from being used for this application. We have measured the RF surface resistance (Rs) at 21 GHz of a MgB2 sample fabricated using Hot Isostatic Press (HIP) at 200 MPa and 1000 C. The results show that polishing with 0.1-micron diamond lapping film followed by a 1500-psi DI water rinse in a clean room reduced the Rs by a factor of 6.2 at 15 K and it is the lowest compared to other published data. The Rs data near the lowest temperature (~13 K) scatter between 0.6 and 1.3 mOhm. The penetration depth and energy gap were estimated to be 263 nm and 1.9-2.7, respectively, for the polished surface.
Observation of coherent oxide precipitates in polycrystalline MgB2
R. F. Klie,J. C. Idrobo,N. D. Browning,A. C. Serquis,Y. T. Zhu,X. Z. Liao,F. M. Mueller
Physics , 2002, DOI: 10.1063/1.1481239
Abstract: Here we describe the results of an atomic resolution study of oxygen incorporation into bulk MgB2. We find that ~20-100 nm sized precipitates are formed by ordered substitution of oxygen atoms onto boron lattice sites, while the basic bulk MgB2 crystal structure and orientation is preserved. The periodicity of the oxygen ordering is dictated by the oxygen concentration in the precipitates and primarily occurs in the (010) plane. The presence of these precipitates correlates well with an improved critical current density and superconducting transition behavior, implying that they act as pinning centers.
Mg(B,O)2 precipitation in MgB2
X. Z. Liao,A. Serquis,Y. T. Zhu,J. Y. Huang,L. Civale,D. E. Peterson,F. M. Mueller,H. F. Xu
Physics , 2002, DOI: 10.1063/1.1568528
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.
Microstructure and high critical current of powder in tube MgB2
A. Serquis,L. Civale,D. L. Hammon,J. Y. Coulter,X. Z. Liao,Y. T. Zhu,D. E. Peterson,F. M. Mueller
Physics , 2002, DOI: 10.1063/1.1561572
Abstract: We report dc transport and magnetization measurements of Jc in MgB2 wires fabricated by the powder-in-tube method, using commercial MgB2 powder with 5 %at Mg powder added as an additional source of magnesium, and stainless steel as sheath material. By appropriate heat treatments, we have been able to increase Jc by more than one order of magnitude from that of the as-drawn wire. We show that one beneficial effect of the annealing is the elimination of most of the micro-cracks, and we correlate the increase in Jc with the disappearance of the weak-link-type behavior.
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