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Search Results: 1 - 10 of 332295 matches for " J. Y. Coulter "
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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.
Angular Dependence of C-Axis Magnetoresistance in Bi-2212 Single Crystals With Columnar Defects
N. Morozov,L. N. Bulaevskii,M. P. Maley,J. Y. Coulter,A. E. Koshelev,T. -W. Li
Physics , 1998, DOI: 10.1103/PhysRevB.60.96
Abstract: We measured the angular dependence of the c-axis magnetoresistance rho_c(B) of Bi-2212 irradiated with heavy ions. At temperatures near 68 K the scaling of rho_c(B) with the c-axis magnetic field component B_perp is broken and the in-plane field, B_parallel, affects rho_c. At this temperature, at a specific field B_cr \approx B_Phi/2, magnetoresistance becomes independent of field orientation. This crossing point allows us to estimate the correlation length L of pancake positions along the c axis. We find L/s is about 100 at B=B_cr, where s is the interlayer spacing. This provides evidence of strong enhancement of pancake alignment in the vortex liquid in crystals with columnar defects.
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.
Large field generation with Hot Isostatically Pressed Powder-in-Tube MgB2 coil at 25 K
A. Serquis,L. Civale,J. Y. Coulter,D. L. Hammon,X. Z. Liao,Y. T. Zhu,D. E. Peterson,F. M. Mueller,V. F. Nesterenko,S. S. Indrakanti
Physics , 2004, DOI: 10.1088/0953-2048/17/10/L01
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.
Synthesis, characterization and ageing of MgB2
A. Serquis,R. Schulze,Y. T. Zhu,J. Y. Coulter,D. E. Peterson,N. O. Moreno,P. G. Pagliuso,S. S. Indrakanti,V. F. Nesterenko,F. M. Mueller
Physics , 2001,
Abstract: We studied the influence of sample preparation and defects in the superconducting properties samples using atomic ratios of Mg:B=1:1 and Mg:B=1:2. Samples were characterized by SEM, and XRD, and the magnetization properties were examined in a SQUID magnetometer. The presence of Mg vacancies was determined by Rietveld analysis. Most of the samples exhibited sharp superconducting transitions with Tcs between 37- 39 K. We found a strong correlation between the crystal strain and the Tc. This strain was related to the presence of Mg vacancies. In addition, results showed that some samples degraded 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. This effect was only present in samples with poor connectivity between grains and smaller grain sizes. The degradation was related to a surface decomposition as observed by X-ray Photoelectron Spectroscopy. No correlation was found between this effect and the presence of Mg vacancies.
The influence of microstructures and crystalline defects on the superconductivity of MgB2
A. Serquis,X. Y. Liao,Y. T. Zhu,J. Y. Coulter,J. Y. Huang,J. O. Willis,D. E. Peterson,F. M. Mueller,N. O. Moreno,J. D. Thompson,S. S. Indrakanti,V. F. Nesterenko
Physics , 2002, DOI: 10.1063/1.1479470
Abstract: This work studies the influence of microstructures and crystalline defects on the superconductivity of MgB2, with the objective to improve its flux pinning. A MgB2 sample pellet that was hot isostatic pressed (HIPed) was found to have significantly increased critical current density (Jc) at high fields than its un-HIPed counterpart. The HIPed sample had a Jc of 10000 A/cm2 in 50000 Oe (5 T) at 5K. This was 20 times higher than that of the un-HIPed sample, and the same as the best Jc reported by other research groups. Microstructures observed in scanning and transmission electron microscopy indicate that the HIP process eliminated porosity present in the MgB2 pellet resulting in an improved intergrain connectivity. Such improvement in intergrain connectivity was believed to prevent the steep Jc drop with magnetic field H that occurred in the un-HIPed MgB2 pellet at H > 45000 Oe (4.5 T) and T = 5 K. The HIP process was also found to disperse the MgO that existed at the grain boundaries of the un-HIPed MgB2 pellet and to generate more dislocations in the HIPed the pellets. These dispersed MgO particles and dislocations improved flux pinning also at H<45000 Oe. The HIPing process was also found to lower the resistivity at room temperature.
Hot isostatic pressing of powder in tube MgB2 wires
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
Physics , 2003, DOI: 10.1063/1.1571231
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.
Angular dependent vortex pinning mechanisms in YBCO coated conductors and thin films
L. Civale,B. Maiorov,A. Serquis,J. O. Willis,J. Y. Coulter,H. Wang,Q. X. Jia,P. N. Arendt,J. L. MacManus-Driscoll,M. P. Maley,S. R. Foltyn
Physics , 2003, DOI: 10.1063/1.1655707
Abstract: We present a comparative study of the angular dependent critical current density in YBa2Cu3O7 films deposited on IBAD MgO and on single crystal MgO and SrTiO3 substrates. We identify three angular regimes where pinning is dominated by different types of correlated and uncorrelated defects. We show that those regimes are present in all cases, indicating that the pinning mechanisms are the same, but their extension and characteristics are sample dependent, reflecting the quantitative differences in texture and defect density. In particular, the more defective nature of the films on IBAD turns into an advantage as it results in stronger vortex pinning, demonstrating that the critical current density of the films on single crystals is not an upper limit for the performance of the IBAD coated conductors.
The T1-T2 study: evolution of aerosol properties downwind of Mexico City
J. C. Doran, J. C. Barnard, W. P. Arnott, R. Cary, R. Coulter, J. D. Fast, E. I. Kassianov, L. Kleinman, N. S. Laulainen, T. Martin, G. Paredes-Miranda, M. S. Pekour, W. J. Shaw, D. F. Smith, S. R. Springston,X.-Y. Yu
Atmospheric Chemistry and Physics (ACP) & Discussions (ACPD) , 2007,
Abstract: As part of a major atmospheric chemistry and aerosol field program carried out in March 2006, a study was conducted in the area to the north and northeast of Mexico City to investigate the evolution of aerosols and their associated optical properties in the first few hours after their emission. The focus of the T1-T2 aerosol study was to investigate changes in the specific absorption αABS (absorption per unit mass, with unit of m2 g 1) of black carbon as it aged and became coated with compounds such as sulfate and organic carbon, evolving from an external to an internal mixture. Such evolution has been reported in previous studies. The T1 site was located just to the north of the Mexico City metropolitan area; the T2 site was situated approximately 35 km farther to the northeast. Nephelometers, particle soot absorption photometers, photoacoustic absorption spectrometers, and organic and elemental carbon analyzers were used to measure the optical properties of the aerosols and the carbon concentrations at each of the sites. Radar wind profilers and radiosonde systems helped to characterize the meteorology and to identify periods when transport from Mexico City over T1 and T2 occurred. Organic and elemental carbon concentrations at T1 showed diurnal cycles reflecting the nocturnal and early morning buildup from nearby sources, while concentrations at T2 appeared to be more affected by transport from Mexico City. Specific absorption during transport periods was lower than during other times, consistent with the likelihood of fresher emissions being found when the winds blew from Mexico City over T1 and T2. The specific absorption at T2 was larger than at T1, which is also consistent with the expectation of more aged particles with encapsulated black carbon being found at the more distant location. In situ measurements of single scattering albedo with an aircraft and a ground station showed general agreement with column-averaged values derived from rotating shadowband radiometer data, although some differences were found that may be related to boundary-layer evolution.
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