9,032 research outputs found
MgB2 tunnel junctions with native or thermal oxide barriers
MgB2 tunnel junctions (MgB2/barrier/MgB2) were fabricated using a native
oxide grown on the bottom MgB2 film as the tunnel barrier. Such barriers
therefore survive the deposition of the second electrode at 300oC, even over
junction areas of ~1 mm2. Studies of such junctions, and those of the type
MgB2/native or thermal oxide/metal (Pb, Au, or Ag) show that tunnel barriers
grown on MgB2 exhibit a wide range of barrier heights and widths.Comment: 9 pages, 3 figure
Phase formation of polycrystalline MgB2 at low temperature using nanometer Mg powder
The MgB2 superconductor synthesized in a flowing argon atmosphere using
nanometer magnesium powder as the raw materials, denoted as Nano-MgB2, has been
studied by the technique of in-situ high temperature resistance measurement
(HT-RT measurement). The MgB2 phase is identified to form within the
temperature range of 430 to 490 C, which is much lower than that with the MgB2
sample fabricated in the same gas environment using the micron-sized magnesium
powder, denoted as Micro-MgB2, reported previously. The sample density of the
Nano-MgB2 reaches 1.7 g/cm3 with a crystal porosity structure less than a
micrometer, as determined by the scanning electron microscope (SEM) images,
while the Micro-MgB2 has a much more porous structure with corresponding
density of 1.0 g/cm3. This indicates that the Mg raw particle size, besides the
sintering temperature, is a crucial factor for the formation of high density
MgB2 sample, even at the temperature much lower than that of the Mg melting,
650 C. The X-ray diffraction (XRD) pattern shows a good MgB2 phase with small
amount of MgO and Mg and the transition temperature, TC, of the Nano-MgB2 was
determined as 39 K by the temperature dependent magnetization measurement
(M-T), indicating the existence of a good superconducting property.Comment: 10 pages, 4 figure, Solid State Communicatio
Magnetic field processing to enhance critical current densities of MgB2 superconductors
Magnetic field of up to 12 T was applied during the sintering process of pure
MgB2 and carbon nanotube (CNT) doped MgB2 wires. We have demonstrated that
magnetic field processing results in grain refinement, homogeneity and
significant enhancement in Jc(H) and Hirr. The Jc of pure MgB2 wire increased
by up to a factor of 3 to 4 and CNT doped MgB2 by up to an order of magnitude
in high field region respectively, compared to that of the non-field processed
samples. Hirr for CNT doped sample reached 7.7 T at 20 K. Magnetic field
processing reduces the resistivity in CNT doped MgB2, straightens the entangled
CNT and improves the adherence between CNTs and MgB2 matrix. No crystalline
alignment of MgB2 was observed. This method can be easily scalable for a
continuous production and represents a new milestone in the development of MgB2
superconductors and related systems
Doping Effect of Nano-Diamond on Superconductivity and Flux Pinning in MgB2
Doping effect of diamond nanoparticles on the superconducting properties of
MgB2 bulk material has been studied. It is found that the superconducting
transition temperature Tc of MgB2 is suppressed by the diamond-doping, however,
the irreversibility field Hirr and the critical current density Jc are
systematically enhanced. Microstructural analysis shows that the diamond-doped
MgB2 superconductor consists of tightly-packed MgB2 nano-grains (~50-100 nm)
with highly-dispersed and uniformly-distributed diamond nanoparticles (~10-20
nm) inside the grains. High density of dislocations and diamond nanoparticles
may take the responsibility for the enhanced flux pinning in the diamond-doped
MgB2.Comment: 16 pages, 6 figure
Carbon doping of superconducting magnesium diboride
We present details of synthesis optimization and physical properties of
nearly single phase carbon doped MgB2 with a nominal stoichiometry of
Mg(B{0.8}C{0.2})2 synthesized from magnesium and boron carbide (B4C) as
starting materials. The superconducting transition temperature is ~ 22 K (~ 17
K lower than in pure MgB2). The temperature dependence of the upper critical
field is steeper than in pure MgB2 with Hc2(10K) ~ 9 T. Temperature dependent
specific heat data taken in different applied magnetic fields suggest that the
two-gap nature of superconductivity is still preserved for carbon doped MgB2
even with such a heavily suppressed transition temperature. In addition, the
anisotropy ratio of the upper critical field for T/Tc ~ 2/3 is gamma ~ 2. This
value is distinct from 1 (isotropic) and also distinct from 6 (the value found
for pure MgB2).Comment: 11 pages, 13 figures, submitted to Physica
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