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Search Results: 1 - 10 of 582453 matches for " D. A. Cardwell "
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A new seeding technique for the reliable fabrication of large, SmBCO single grains containing silver using top seeded melt growth
Y-H Shi,A R Dennis,D A Cardwell
Physics , 2014, DOI: 10.1088/0953-2048/28/3/035014
Abstract: Silver (Ag) is an established additive for improving the mechanical properties of single grain, (RE)BCO bulk superconductors (where RE = Sm, Gd and Y). The presence of Ag in the (RE)BCO bulk composition, however, typically reduces the melting temperature of the single crystal seed in the top seeded melt growth (TSMG) process, which complicates significantly the controlled nucleation and subsequent epitaxial growth of a single grain, which is essential for high field engineering applications. The reduced reliability of the seeding process in the presence of Ag is particularly acute for the SmBCO system, since the melting temperature of SmBCO is very close to that of the generic NdBCO(MgO) seed. SmBCO has the highest superconducting transition temperature, Tc, and exhibits the most pronounced "peak" effect at higher magnetic field of all materials in the family of (RE)BCO bulk superconductors and, therefore, has the greatest potential for use in practical applications (compared to GdBCO and YBCO, in particular). Development of an effective seeding process, therefore, is one of the major challenges of the TSMG process for the growth of large, high quantity single grain superconductors. In this paper, we report a novel technique that involves introducing a buffer layer between the seed crystal and the precursor pellet, primarily to inhibit the diffusion of Ag from the green body to the seed during melt processing in order to prevent the melting of the seed. The success rate of the seeding process using this technique is 100% for relatively small batch samples. The superconducting properties, Tc, Jc and trapped fields, of the single grains fabricated using the buffers are reported and the micro-structures in the vicinity of the buffer of single grains fabricated by the modified technique are analysed.
Temporal Transcriptional Regulation of IL-10-Induced Anti-Inflammatory Genes in LPS-Triggered Macrophages  [PDF]
Amanda F. Dillow, Leah N. Cardwell, Tyler J. Smith, Brad D. Groppe, Brian A. Peterson, Maxwell A. Sickman, Brian K. Weaver
Open Journal of Immunology (OJI) , 2014, DOI: 10.4236/oji.2014.43013
Abstract:

Interleukin-10 (IL-10) mediates an anti-inflammatory response that is executed through the expression of IL-10-induced genes. Certain IL-10-induced genes, exemplified by TNIP3, are induced by IL-10 only in conjunction with a pro-inflammatory signal. We sought to characterize the mechanism whereby IL-10 and Toll-like receptor signals synergized to induce expression of genes like TNIP3 inmacrophages. Stimulation with IL-10 and lipopolysaccharide (LPS) synergistically induced an increase in the transcription rate of TNIP3, while having no effect on its mRNA stability. This transcriptional mechanism proved to be generalizable to 14 other genes that also were synergistically induced by IL-10 and LPS in monocytes/macrophages. Although all of the genes had this synergistic transcriptional regulation in common, they could be divided into three subsets based on their differential requirements for de novo protein synthesis and kinetics of expression: namely, primary response genes, early secondary response genes, and late secondary response genes. This coordinated and temporal pattern of transcriptional regulation in response to IL-10 and LPS was conserved in both human and mouse monocytes/macrophages, and it was associated with differential dependencies on PI3K and JNK signaling pathways. These results underscore the complex nature of the IL-10-induced transcriptional response that occurs specifically in LPS-triggered macrophages.

Magneto-thermal phenomena in bulk high temperature superconductors subjected to applied AC magnetic fields
P Vanderbemden,P Laurent,J-F Fagnard,M Ausloos,N Hari Babu,D A Cardwell
Physics , 2010, DOI: 10.1088/0953-2048/23/7/075006
Abstract: In the present work we study, both theoretically and experimentally, the temperature increase in a bulk high-temperature superconductor subjected to applied AC magnetic fields of large amplitude. We calculate analytically the equilibrium temperatures of the bulk sample as a function of the experimental parameters using a simple critical-state model for an infinitely long type-II superconducting slab or cylinder. The results show the existence of a limit heat transfer coefficient (AUlim) separating two thermal regimes with different characteristics. The theoretical analysis predicts a "forbidden" temperature window within which the temperature of the superconductor can never stabilize when the heat transfer coefficient is small. In addition, we determine an analytical expression of two threshold fields Htr1 and Htr2 characterizing the importance of magneto-thermal effects and show that a thermal runaway always occurs when the field amplitude is larger than Htr2. The theoretical predictions of the temperature evolution of the bulk sample during a self-heating process agree well with the experimental data. The simple analytical study presented in this paper enables order of magnitude thermal effects to be estimated for simple superconductor geometries under applied AC magnetic fields and can be used to predict the influence of experimental parameters on the self-heating characteristics of bulk type-II superconductors.
Evidence for high inter-granular current flow in single-phase polycrystalline MgB2 superconductor
K. Kawano,J. S. Abell,M. Kambara,N Hari Babu,D. A. Cardwell
Physics , 2001, DOI: 10.1063/1.1405807
Abstract: The distribution of magnetic field in single-phase polycrystalline bulk MgB2 has been measured using a Magneto-Optical (MO) technique for an external magnetic field applied perpendicular to the sample surface. The MO studies indicate that an inter-granular current network is readily established in this material and the current is not limited by weak-linked grain boundaries. The grain boundaries are observed to resist preferential magnetic field penetration, with the inter-grain mechanism dominating the current flow in the sample at temperatures up to 30K. The results provide clear evidence that the intra-granular current flow is isotropic. A critical current density of ~10^4 Acm-2 was estimated at 30K in a field of 150mT from the MO measurements. These results provide further evidence of the considerable potential for MgB2 for engineering applications.
The spin anisotropy of the magnetic excitations in the normal and superconducting states of optimally doped YBa2Cu3O6.9 studied by polarized neutron spectroscopy
N. S. Headings,S. M. Hayden,J. Kulda,N. Hari Babu,D. A. Cardwell
Physics , 2011, DOI: 10.1103/PhysRevB.84.104513
Abstract: We use inelastic neutron scattering with spin polarization analysis to study the magnetic excitations in the normal and superconducting states of YBa2Cu3O6.9. Polarization analysis allows us to determine the spin polarization of the magnetic excitations and to separate them from phonon scattering. In the normal state, we find unambiguous evidence of magnetic excitations over the 10-60meV range of the experiment with little polarization dependence to the excitations. In the superconducting state, the magnetic response is enhanced near the "resonance energy" and above. At lower energies, 10
Remagnetization of bulk high-temperature superconductors subjected to crossed and rotating magnetic fields
P Vanderbemden,Z Hong,T A Coombs,M Ausloos,N Hari Babu,D A Cardwell,A M Campbell
Physics , 2007, DOI: 10.1088/0953-2048/20/9/S10
Abstract: Bulk melt-processed Y-Ba-Cu-O (YBCO) has significant potential for a variety of high field permanent magnet-like applications, such as the rotor of a brushless motor. When used in rotating devices of this kind, however, the YBCO can be subjected to both transient and alternating magnetic fields that are not parallel to the direction of magnetization and which have a detrimental effect on the trapped field. These effects may lead to a long-term decay of the magnetization of the bulk sample. In the present work, we analyze both experimentally and numerically the remagnetization process of a melt-processed YBCO single domain that has been partially demagnetized by a magnetic field applied orthogonal to the initial direction of trapped flux. Magnetic torque measurements are used as a tool to probe changes in the remanent magnetization during various sequences of applied field. The application of a small magnetic field between the transverse cycles parallel to the direction of original magnetization results in partial remagnetization of the sample. Rotating the applied field, however, is found to be much more efficient at remagnetizing the bulk material than applying a magnetizing field pulse of the same amplitude. The principal features of the experimental data can be reproduced qualitatively using a two-dimensional finite-element numerical model based on an E-J power law. Finally, the remagnetization process is shown to result from the complex modification of current distribution within the cross-section of the bulk sample.
An AC susceptometer for the characterization of large, bulk superconducting samples
P Laurent,J F Fagnard,B Vanderheyden,N Hari Babu,D A Cardwell,M Ausloos,P Vanderbemden
Physics , 2008, DOI: 10.1088/0957-0233/19/8/085705
Abstract: The main purpose of this work was to design, develop and construct a simple, low-cost AC susceptometer to measure large, bulk superconducting samples (up to 32 mm in diameter) in the temperature range 78-120 K. The design incorporates a double heating system that enables a high heating rate (25 K/hour) while maintaining a small temperature gradient (< 0.2 K) across the sample. The apparatus can be calibrated precisely using a copper coil connected in series with the primary coil. The system has been used successfully to measure the temperature dependence of the AC magnetic properties of entire RE-Ba-Cu-O [(RE)BCO] bulk superconducting domains. A typical AC susceptibility measurement run from 78 K to 95 K takes about 2 hours, with excellent temperature resolution (temperature step ~ 4 mK) around the critical temperature, in particular.
Neutron Irradiation of MgB2 Bulk Superconductors
M. Eisterer,M. Zehetmayer,S. Toenies,H. W. Weber,M. Kambara,N. Hari Babu,D. A. Cardwell,L. R. Greenwood
Physics , 2001, DOI: 10.1088/0953-2048/15/2/101
Abstract: Sintered samples of MgB2 were irradiated in a fission reactor. Defects in the bulk microstructure are produced during this process mainly by the 10B(n,a)7Li reaction while collisions of fast neutrons with the lattice atoms induce much less damage. Self-shielding effects turn out to be very important and lead to a highly inhomogeneous defect distribution in the irradiated samples. The resulting disorder enhances the normal state resistivity and the upper critical field. The irreversibility line shifts to higher fields at low temperatures and the measured critical current densities increase following irradiation.
Behavior of bulk high-temperature superconductors of finite thickness subjected to crossed magnetic fields
Ph. Vanderbemden,Z. Hong,T. A. Coombs,S. Denis,M. Ausloos,J. Schwartz,I. B. Rutel,N. Hari Babu,D. A. Cardwell,A. M. Campbell
Physics , 2007, DOI: 10.1103/PhysRevB.75.174515
Abstract: Crossed magnetic field effects on bulk high-temperature superconductors have been studied both experimentally and numerically. The sample geometry investigated involves finite-size effects along both (crossed) magnetic field directions. The experiments were carried out on bulk melt-processed Y-Ba-Cu-O (YBCO) single domains that had been pre-magnetized with the applied field parallel to their shortest direction (i.e. the c-axis) and then subjected to several cycles of the application of a transverse magnetic field parallel to the sample ab plane. The magnetic properties were measured using orthogonal pick-up coils, a Hall probe placed against the sample surface and Magneto-Optical Imaging (MOI). We show that all principal features of the experimental data can be reproduced qualitatively using a two-dimensional finite-element numerical model based on an E-J power law and in which the current density flows perpendicularly to the plane within which the two components of magnetic field are varied. The results of this study suggest that the suppression of the magnetic moment under the action of a transverse field can be predicted successfully by ignoring the existence of flux-free configurations or flux-cutting effects. These investigations show that the observed decay in magnetization results from the intricate modification of current distribution within the sample cross-section. It is also shown that the model does not predict any saturation of the magnetic induction, even after a large number (~ 100) of transverse field cycles. These features are shown to be consistent with the experimental data.
Growth of Strongly Biaxially Aligned MgB2 Thin Films on Sapphire by Post-annealing of Amorphous Precursors
A. Berenov,Z. Lockman,X. Qi,Y. Bugoslavsky,L. F. Cohen,M. -H. Jo,N. A. Stelmashenko,V. N. Tsaneva,M. Kambara,N. Hari Babu,D. A. Cardwell,M. G. Blamire,J. L. MacManus-Driscoll
Physics , 2001, DOI: 10.1063/1.1424070
Abstract: MgB2 thin films were cold-grown on sapphire substrates by pulsed laser deposition (PLD), followed by post-annealing in mixed, reducing gas, Mg-rich, Zr gettered, environments. The films had Tcs in the range 29 K to 34 K, Jcs (20K, H=0) in the range 30 kA/cm2 to 300 kA/cm2, and irreversibility fields at 20 K of 4 T to 6.2 T. An inverse correlation was found between Tc and irreversibility field. The films had grain sizes of 0.1-1 micron and a strong biaxial alignment was observed in the 950C annealed film.
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