57 research outputs found
The critical current density of advanced internal-Mg-diffusion-processed MgB2 wires
Recent advances in MgB2 conductors are leading to a new level of performance.
Based on the use of proper powders, proper chemistry, and an architecture which
incorporates internal Mg diffusion (IMD), a dense MgB2 structure with not only
a high critical current density Jc, but also a high engineering critical
current density, Je, can be obtained. In this paper, a series of these advanced
(or second - generation, "2G") conductors has been prepared. Scanning electron
microscopy and associated energy dispersive X-ray spectroscopy were applied to
characterize the microstructures and compositions of the wires, and a dense
MgB2 layer structure was observed. The best layer Jc for our sample is 1.07x105
A/cm2 at 10 T, 4.2 K, and our best Je is seen to be 1.67x104 A/cm2 at 10 T, 4.2
K. Optimization of the transport properties of these advanced wires is
discussed in terms of B-powder choice, area fraction, and the MgB2 layer growth
mechanism.Comment: 13 pages, 3 tables, 7 figures (or 8 pp in published version
Critical Current Density and n-values of MgB2 Strands over a Wide Range of Temperatures and Fields
Transport measurements of critical current density, Jct, in monocore
powder-in-tube MgB2 strands have been carried out at temperatures, T, of from
4.2 K to 40 K, and in transverse fields, B, of up to 14 T. Processing methods
used were conventional continuous-tube-forming-filling (CTFF) and
internal-magnesium-diffusion (IMD). Strands with several powder compositions
were measured, including binary (undoped) MgB2, 2% carbon doped MgB2, and 3%
carbon doped MgB2. Magnetization loops (M-B) were also measured, and magnetic
critical current density, Jcm, values extracted from them. The transport,
Jct(B) and magnetic, Jcm(B), critical current densities were compared. Also
studied was the influence of doping on the resistively measured irreversibility
field, Birr and upper critical field Bc2. Critical current densities, Jct, and
n-values were extracted from transport measurements and were found to be
universally related (for all B and T) according to n \propto Jctm in which m =
0.52 \pm 0.11. Likewise n was found to be related to B according to n \propto
B-p with a T-dependent p in the range of about 0.08~0.21. Further analysis of
the field (B) and temperature (T) dependencies of n-value resulted in an
expression that enabled n(B,T), for all B and T, to be estimated for a given
strand based on the results of transport Jct(B) measurements made at one
arbitrarily chosen temperature.Comment: 34 pages, 7 figure
Pressure-induced electronic phase separation of magnetism and superconductivity in CrAs
The recent discovery of pressure induced superconductivity in the binary
helimagnet CrAs has attracted much attention. How superconductivity emerges
from the magnetic state and what is the mechanism of the superconducting
pairing are two important issues which need to be resolved. In the present
work, the suppression of magnetism and the occurrence of superconductivity in
CrAs as a function of pressure () were studied by means of muon spin
rotation. The magnetism remains bulk up to ~kbar while its volume
fraction gradually decreases with increasing pressure until it vanishes at
7~kbar. At 3.5 kbar superconductivity abruptly appears with its
maximum ~K which decreases upon increasing the pressure. In the
intermediate pressure region (~kbar) the
superconducting and the magnetic volume fractions are spatially phase separated
and compete for phase volume. Our results indicate that the less conductive
magnetic phase provides additional carriers (doping) to the superconducting
parts of the CrAs sample thus leading to an increase of the transition
temperature () and of the superfluid density (). A scaling of
with as well as the phase separation between magnetism and
superconductivity point to a conventional mechanism of the Cooper-pairing in
CrAs.Comment: 9 pages, 8 figure
Drawing induced texture and the evolution of superconductive properties with heat treatment time in powder-in-tube in-situ processed MgB2 strands
Monocore powder-in-tube MgB2 strands were cold-drawn and heat-treated at 600C
and 700C for times of up to 71 hours and structure-property relationships
examined. Drawing-induced elongation of the Mg particles led, after HT, to a
textured macrostructure consisting of elongated polycrystalline MgB2 fibers
separated by elongated pores. The superconducting Tc, Jc and Fp were correlated
with the macrostructure and grain size. Grain size increased with HT time at
both 600C and 700C. Jc and hence Fp decreased monotonically but not linearly
with grain size. Overall, it was observed that at 700C, the MgB2 reaction was
more or less complete after as little as 30 min; at 600C, full reaction
completion did not occur until 71 h. into the HT. Transport, Jct(B) was
measured in a perpendicular applied field, and the magnetic critical current
densities, Jcm\bot(B) and Jcm{\phi}(B), were measured in perpendicular and
parallel (axial) applied fields, respectively. Particularly noticeable was the
premature dropoff of Jcm\bot(B) at fields well below the irreversibility field
of Jct(B). This effect is attributed to the fibrous macrostructure and its
accompanying anisotropic connectivity. Magnetic measurements with the field
directed along the strand axis yielded a critical density, Jcm\bot(B), for
current flowing transversely to the strand axis that was less than and dropped
off more rapidly than Jct(B). In the conventional magnetic measurement, the
loop currents that support the magnetization are restricted by the lower of
Jct(B) and Jcm{\phi} (B). In the present case the latter, leading to the
premature dropoff of the measured Jcm(B) compared to Jct(B) with increasing
field. This result is supported by Kramer plots of the Jcm{\phi} (B) and Jct(B)
data which lead to an irreversibility field for transverse current that is very
much less than the usual transport-measured longitudinal one, Birr,t.Comment: 41 pages, 14 figure
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