29 research outputs found
Nanoscale grains, high irreversibility field, and large critical current density as a function of high energy ball milling time in C-doped magnesium diboride
Magnesium diboride (MgB2) powder was mechanically alloyed by high energy ball
milling with C to a composition of Mg(B0.95C0.05)2 and then sintered at 1000 C
in a hot isostatic press. Milling times varied from 1 minute to 3000 minutes.
Full C incorporation required only 30-60 min of milling. Grain size of sintered
samples decreased with increased milling time to less than 30 nm for 20-50 hrs
of milling. Milling had a weak detrimental effect on connectivity. Strong
irreversibility field (H*) increase (from 13.3 T to 17.2 T at 4.2 K) due to
increased milling time was observed and correlated linearly with inverse grain
size (1/d). As a result, high field Jc benefited greatly from lengthy powder
milling. Jc(8 T, 4.2 K) peaked at > 80,000 A/cm2 with 1200 min of milling
compared with only ~ 26,000 A/cm2 for 60 min of milling. This non-compositional
performance increase is attributed to grain refinement of the unsintered powder
by milling, and to the probable suppression of grain growth by milling-induced
MgO nano-dispersions.Comment: 12 pages, 11 figure
Florida Energy Assurance Plan
This spring, Florida held the nation’s first statewide emergency preparedness training and exercises geared specifically to the aftermath of severe geomagnetic events. Funded by the State of Florida Division of Emergency Management (FDEM) via a Department of Energy grant and held in collaboration with Watch House International, Inquesta Corporation, and the Florida Institute of Technology, the 17–19 April 2012 workshop had 99 on-site attendees in an oceanfront hotel in Melbourne, Florida, as well as 16 over live Web streaming. The workshop was the capstone to a three-month season of 21 regional space weather training sessions and workshops serving 386 attendees in total. Participants included emergency managers, law enforcement officers, emergency medicine practitioners, and private industry stakeholders, including representatives from utility and telecommunications companies. The three-day statewide workshop began with one day of education and targeted training, featuring space weather experts in government, academia, and private companies, as well as the regional utility grid reliability organization. The following two days were devoted to a tabletop exercise where participants, divided into groups by area of responsibility, worked through an evolving scenario of space weather–related events, running through their preparedness plans and examining responses
Atmospheric conditions and their effect on ball-milled magnesium diboride
Magnesium diboride bulk pellets were fabricated from pre-reacted MgB2 powder
ball milled with different amounts of exposure to air. Evidence of increased
electron scattering including increased resistivity, depressed Tc, and enhanced
Hc2 of the milled and heat treated samples were observed as a result of
increased contact with air. These and other data were consistent with alloying
with carbon as a result of exposure to air. A less clear trend of decreased
connectivity associated with air exposure was also observed. In making the case
that exposure to air should be considered a doping process, these results may
explain the wide varibability of "undoped" MgB2 properties extant in the
literature.Comment: Work presented at ASC 2006 in Seattl
Florida Energy Assurance Plan
This spring, Florida held the nations first statewide emergency preparedness training and exercises geared specifically to the aftermath of severe geomagnetic events. Funded by the State of Florida Division of Emergency Management (FDEM) via a Department of Energy grant and held in collaboration with Watch House International, Inquesta Corporation, and the Florida Institute of Technology, the 17-19 April 2012 workshop had 99 on-site attendees in an oceanfront hotel in Melbourne, Florida, as well as 16 over live Web streaming. The workshop was the capstone to a three-month season of 21 regional space weather training sessions and workshops serving 386 attendees in total
Effect of PVA doping on flux pinning in Bulk MgB2
The synthesis and characterization of PVA (Poly Vinyl Acetate) doped bulk
MgB2 superconductor is reported here. PVA is used as a Carbon source. PVA
doping effects made two distinguishable contributions: first enhancement of Jc
field performance and second an increase in Hc2 value, both because of carbon
incorporation into MgB2 crystal lattice. The susceptibility measurement reveals
that Tc decreased from 37 to 36 K. Lattice parameter a decreased from 3.085 A
to 3.081 A due to the partial substitution of Carbon at Boron site. PVA doped
sample exhibited the Jc values greater than 10^5 A/cm2 at 5 & 10 K at low
fields; which is almost 3 times higher than the pure one, while at high fields
the Jc is increased by an order of magnitude in comparison to pure MgB2. From
R(T)H measurements we found higher Tc values under magnetic field for doped
sample; indicating an increase in Hc2. Also the magnetization measurements
exhibited a significant enhancement in Hirr value. The improved performance of
PVA doped MgB2 can be attributed to the substitution of carbon at boron site in
parent MgB2 and the resulting impact on the carrier density and impurity
scattering. The improved flux pinning behavior could easily be seen from
reduced flux pinning force plots.Comment: 14 Pages of Text + Figs. To appear in Physica
Improved Hc2 in Bulk-Form Magnesium Diboride by Mechanical Alloying With Carbon
High energy milling of MgB2 pre-reacted powder renders the material largely
amorphous through extreme mechanical deformation and is suitable for
mechanically alloying MgB2 with dopants including carbon. Bulk samples of
milled C and MgB2 powders subjected to hot isostatic pressing and Mg vapor
annealing have achieved critical fields in excess of 32T and critical current
density approaching 10^6 A/cm^2.Comment: 13 pages, 3 figures references updated, typos corrected, numerical
error correcte
Towards the Realization of Higher Connectivity in MgB2 Conductors: In-situ or Sintered Ex-situ?
The two most common types of MgB2 conductor fabrication technique - in-situ
and ex-situ - show increasing conflicts concerning the connectivity, an
effective current-carrying cross-sectional area. An in-situ reaction yields a
strong intergrain coupling with a low packing factor, while an ex-situ process
using pre-reacted MgB2 yields tightly packed grains, however, their coupling is
much weaker. We studied the normal-state resistivity and microstructure of
ex-situ MgB2 bulks synthesized with varied heating conditions under ambient
pressure. The samples heated at moderately high temperatures of ~900{\deg}C for
a long period showed an increased packing factor, a larger intergrain contact
area and a significantly decreased resistivity, all of which indicate the
solid-state self-sintering of MgB2. Consequently the connectivity of the
sintered ex-situ samples exceeded the typical connectivity range 5-15% of the
in-situ samples. Our results show self-sintering develops the superior
connectivity potential of ex-situ MgB2, though its intergrain coupling is not
yet fulfilled, to provide a strong possibility of twice or even much higher
connectivity in optimally sintered ex-situ MgB2 than in in-situ MgB2.Comment: 17 pages, 5 figure
Hollow carbon spheres as an efficient dopant for enhancing critical current density of MgB2 based tapes
A significant enhancement of Jc and Hirr in MgB2 tapes has been achieved by
the in situ powder-in-tube method utilizing hollow carbon spheres (HCS) as
dopants. At 4.2 K, the transport Jc for the 850C sintered samples reached
3.1x10^4, and 1.4x10^4 A/cm^2 at 10 and 12 T, respectively, and were better
than those of optimal nano-SiC doped tapes. Furthermore, the Hirr for doped
sample was raised up to 16.8 T at 10 K due to the carbon substitution effect.
The results demonstrate that HCS is one of the most promising dopants besides
nano-carbon and SiC for the enhancement of current capacity for MgB2 in high
fields.Comment: 14 pages, 5 figure
Role of Carbon in Enhancing the Performance of MgB2 superconductor
The enhancement of the critical current density (Jc(H)) of carbon and
nano-SiC doped MgB2 is presented and compared. The upper critical field (Hc2)
being determined from resistivity under magnetic field experiments is though
improved for both C substitution and nano-SiC addition the same is more
pronounced for the former. In MgB2-xCx carbon is substituted for boron that
induces disorder in the boron network and acts as internal pinning centres. The
optimal Jc(H) values are obtained for x = 0.1 sample . In case of nano-SiC
doped in MgB2, the Jc(H) improves more profoundly and two simultaneous
mechanisms seems responsible to this enhancement. Highly reactive nano-SiC
releases free carbon atom, which gets easily incorporated into the MgB2 lattice
to act as intrinsic pinning centres. Further enhancement is observed for higher
nano-SiC concentrations, where the un-reacted components serve as additional
extrinsic pinning centres.Comment: 17 Pages text + Fig
Limits of the upper critical field in dirty two-gap superconductors
An overview of the theory of the upper critical field in dirty two-gap
superconductors, with a particular emphasis on MgB is given. We focus here
on the maximum which may be achieved by increasing intraband
scattering, and on the limitations imposed by weak interband scattering and
paramagnetic effects. In particular, we discuss recent experiments which have
recently demonstrated ten-fold increase of in dirty carbon-doped films
as compared to single crystals, so that the parallel to the ab
planes may approach the BCS paramagnetic limit, . New effects produced by weak interband scattering in the two-gap
Ginzburg-Landau equations and in ultrathin MgB films are
addressed.Comment: Submitted to the special issue of Physica C on MgB on Dec. 3,
200