142 research outputs found
The influence of structural defects on intra-granular critical currents of bulk MgB2
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.Comment: 4 pages, 4 figures, to be published in IEE Transactions in Applied
Superconductivit
The degradation of MgB2 under ambient environment
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.Comment: 15 pages, 3 figure
Microwave performance of high-density bulk MgB2
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
Artificial pinning centers in MgB2 superconducting bulks
The optimization of MgB2 superconducting properties is extremely important for practical application. The introduction of an effective artificial pinning center in this material can enhance its critical current density efficiently. In this paper, VB2 is described as a material that provides effective pinning in the MgB2 bulks. The simultaneous addition of SiC as a carbon source is also analyzed. The samples were prepared using ball milling under a controlled atmosphere and heat treatment with a continuous argon flux. As a result, the superconducting properties could be optimized due to the effective improvement in the pinning behavior as well as with the carbon doping.Fil: Da Silva, L.B.S.. Universidade de Sao Paulo; BrasilFil: Serquis, Adriana Cristina. Consejo Nacional de Investigaciones Cientificas y Tecnicas. Oficina de Coordinacion Administrativa Ciudad Universitaria. Unidad Ejecutora Instituto de Nanociencia y Nanotecnologia. Unidad Ejecutora Instituto de Nanociencia y Nanotecnologia - Nodo Bariloche | Comision Nacional de Energia Atomica. Unidad Ejecutora Instituto de Nanociencia y Nanotecnologia. Unidad Ejecutora Instituto de Nanociencia y Nanotecnologia - Nodo Bariloche.; ArgentinaFil: Hellstrom, Eric. Florida State University; Estados UnidosFil: Rodrigues, D.. Universidade de Sao Paulo; Brasi
Transport Properties and Exponential n-values of Fe/MgB2 Tapes With Various MgB2 Particle Sizes
Fe/MgB2 tapes have been prepared starting with pre-reacted binary MgB2
powders. As shown by resistive and inductive measurements, the reduction of
particle size to a few microns by ball milling has little influence on Bc2,
while the superconducting properties of the individual MgB2 grains are
essentially unchanged. Reducing the particle size causes an enhancement of Birr
from 14 to 16 T, while Jc has considerably increased at high fields, its slope
Jc(B) being reduced. At 4.2K, values of 5.3*10^4 and 1.2*10^3 A/cm^2 were
measured at 3.5 and 10 T, respectively, suggesting a dominant role of the
conditions at the grain interfaces. A systematic variation of these conditions
at the interfaces is undertaken in order to determine the limit of transport
properties for Fe/MgB2 tapes. The addition of 5% Mg to MgB2 powder was found to
affect neither Jc nor Bc2. For the tapes with the highest Jc values, very high
exponential n factors were measured: n = 148, 89 and 17 at 3.5, 5 and 10T,
respectively and measurements of critical current versus applied strain have
been performed. The mechanism leading to high transport critical current
densities of filamentary Fe/MgB2 tapes based on MgB2 particles is discussed.Comment: Presented at ICMC 2003, 25-28 May 200
Carbon nanotubes effects on the relaxation properties and critical current densities of MgB2 superconductor
Addition of nonsuperconducting phases, such as carbon nanotubes, can modify the superconducting properties of MgB2 samples, improving the critical current density and upper critical field. A full understanding of the flux creep mechanism involved is crucial to the development of superconducting magnets in persistent mode, one of the main thrusts for the development of MgB2 wires. In this paper we present a creep study in bulk MgB2 samples, pure and with different amounts of carbon nanotubes additions. We conclude that the most consistent picture at low temperatures is a single vortex pinning regime, where the correlation length is limited by the grain size. We introduce a novel analysis that allows us to identify the region where the Anderson-Kim model is validFil: Pasquini, Gabriela. Universidad de Buenos Aires. Facultad de Cs.exactas y Naturales; Argentina;Fil: Serquis, Adriana Cristina. Comision Nacional de Energia Atomica. Gerencia del Ărea de EnergĂa Nuclear. Instituto Balseiro; Argentina;Fil: Moreno, Alejandro Javier. Universidad de Buenos Aires. Facultad de Cs.exactas y Naturales; Argentina;Fil: Serrano, German Dario. Comision Nacional de Energia Atomica. Gerencia del Ărea de EnergĂa Nuclear. Instituto Balseiro; Argentina; Consejo Nacional de Investigaciones CientĂficas y TĂ©cnicas. Centro CientĂfico TecnolĂłgico - CONICET - Patagonia Norte; Argentina;Fil: L. Civale. Los Alamos National Laboratory, Estados Unidos de AmĂ©rica
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