309 research outputs found
FAST REACTOR SPECTRUM MEASUREMENTS. Quarterly Progress Report for the Period Ending July 31, 1970.
Critical Current Density and Current Transfer Length of Multifilamentary MgB2 Strands of Various Design
In this paper, a series of high performing PIT MgB2
strands have been prepared. Transport voltage-current
measurements were performed to determine the effects of C
doping and strand geometry such as filament numbers. The best
Jc for our samples was 1.0 × 105 A/cm2 at 4.2 K, 7 T, for a strand
using B powder with 3% C addition. The current transfer length
(CTL) was also measured for MgB2 short wires with Nb chemical
barrier and Monel outer sheath. The CTL ranged from 2-12 mm,
and had a correlation with the filament numbers.This work was supported by the U.S. Department of Energy, High Energy Physics grant DE-FG02-95ER40900, and
a DOE SBIR.The in-field critical current densities of a set of in situ
CTFF-type PIT MgB2 strands have been investigated in terms
of C doping level, wire diameter and filament number. The
strand with optimal doping level – 3% C achieved the best Jc
of 1.0 × 105 A/cm2 at 4.2 K, 7 T. By fitting it with the
percolation model, the parameters showed that the Bc2 and the
flux pinning strength was improved and the anisotropy ratio
was reduced. Increases in filament count did not change Jc,
although n-values were higher for the monocore strand.
The current transfer length of MgB2 strands with Nb
chemical barriers and Monel sheaths increased with filament
count. The CTL was less than 3.7 mm for the monocore strand,
and it ranged from 2.2-11.8 mm for the 24- filamentary strand
D2, depending on the applied field
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
Structure and properties of MgB2bulks: Ab-initio simulations compared to experiment
Analysis of XRD patterns by Rietveld refinement has been shown that the main phase of superconducting MgB-based bulk materials (with high level of superconducting characteristics) has AlB type structure and near MgBO stoichiometry. The materials demonstrated the critical current densities up to 0.9 – 0.4 MA/cm jc (at 0 - 1 T, 20 K); up to 15 T B (at 22.5 K) and B (at 18 K). The ab-initio simulation confirmed (1) benefits in binding energy and enthalpy of formation if stoichiometry of the solid solution is near MgBO; (2) energetic advantage in case if impurity oxygen present only in each second boron plane of MgB2 cell while the first boron plane of the same cell stays pristine and location of substituted oxygen atoms in the nearby positions. Besides, the results of ab-initio modeling allow explanation of the tendency towards segregation of O-impurity in MgB structure during synthesis or sintering, and formation of Mg-B-O inclusions or nanolayers (with MgO type of structure) which effect pinning. Calculated transition temperatures, T, for MgBO occurred to be 23.3 K, while for MgB it was 21.13 K only. Experimental T of the bulk materials was 35.7-38.2 K
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
Multifilamentary, in-situ Route, Cu-stabilized MgB2 Strands
Transport critical current densities and n-values were measured at 4.2 K in
fields up to 15 T on 7, 19, and 37-stack multifilamentary MgB2 strands made
using an in-situ route. Some strands included SiC additions (particle size 30
nm), while in others Mg-rich compositions were used. Two basic multifilamentary
variants were measured, the first had Nb filamentary barriers, the second had
Fe filamentary barriers. All samples incorporated stabilizer in the form of Cu
101. Simple, one-step heat treatments were used, with temperatures ranging from
700-800C, and times from 10-30 minutes. Transport critical current densities of
1.75 x 105 A/cm2 were seen at 4.2 K and 5 T in 37 stack strands.Comment: 10 pages, 3 figs, 2 table
How managers can build trust in strategic alliances: a meta-analysis on the central trust-building mechanisms
Trust is an important driver of superior alliance performance. Alliance managers are influential in this regard because trust requires active involvement, commitment and the dedicated support of the key actors involved in the strategic alliance. Despite the importance of trust for explaining alliance performance, little effort has been made to systematically investigate the mechanisms that managers can use to purposefully create trust in strategic alliances. We use Parkhe’s (1998b) theoretical framework to derive nine hypotheses that distinguish between process-based, characteristic-based and institutional-based trust-building mechanisms. Our meta-analysis of 64 empirical studies shows that trust is strongly related to alliance performance. Process-based mechanisms are more important for building trust than characteristic- and institutional-based mechanisms. The effects of prior ties and asset specificity are not as strong as expected and the impact of safeguards on trust is not well understood. Overall, theoretical trust research has outpaced empirical research by far and promising opportunities for future empirical research exist
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