475 research outputs found
Superconductivity in Dense Wires
becomes superconducting just below 40 K. Whereas porous
polycrystalline samples of can be synthesized from boron powders, in
this letter we demonstrate that dense wires of can be prepared by
exposing boron filaments to vapor. The resulting wires have a diameter of
160 , are better than 80% dense and manifest the full shielding in the superconducting state. Temperature-dependent
resistivity measurements indicate that is a highly conducting metal in
the normal state with = 0.38 -. Using this value, an
electronic mean free path, can be estimated, indicating
that wires are well within the clean limit. , , and
data indicate that manifests comparable or better superconducting
properties in dense wire form than it manifests as a sintered pellet.Comment: Figures' layout fixe
Fluxon dynamics by microwave surface resistance measurements in MgB2
Field-induced variations of the microwave surface resistance, Rs(H), have
been investigated in high-density ceramic MgB2. At low temperatures, several
peculiarities of the Rs(H) curves cannot be justified in the framework of
models reported in the literature. We suggest that they are ascribable to the
unconventional vortex structure in MgB2, related to the presence of two gaps.
On the contrary, the results near Tc can be accounted for by the Coffey and
Clem model, with fluxons moving in the flux-flow regime, provided that the
anisotropy of the upper critical field is taken into due account.Comment: 6 pages, 4 figure
Systematic effects of carbon doping on the superconducting properties of Mg(BC)
The upper critical field, , of Mg(BC) has been
measured in order to probe the maximum magnetic field range for
superconductivity that can be attained by C doping. Carbon doped boron
filaments are prepared by CVD techniques, and then these fibers are then
exposed to Mg vapor to form the superconducting compound. The transition
temperatures are depressed about C and rises at about C. This means that 3.5% C will depress from to and
raise from to . Higher fields are probably
attainable in the region of 5% C to 7% C. These rises in are
accompanied by a rise in resistivity at from about
to about . Given that the samples are polycrystalline wire
segments, the experimentally determined curves represent the upper
manifold associated with
Effects of Neutron Irradiation on Carbon Doped MgB2 Wire Segments
We have studied the evolution of superconducting and normal state properties
of neutron irradiated Mg(BC) wire segments as a function
of post exposure annealing time and temperature. The initial fluence fully
suppressed superconductivity and resulted in an anisotropic expansion of the
unit cell. Superconductivity was restored by post-exposure annealing. The upper
critical field, H(T=0), approximately scales with T starting with an
undamaged T near 37 K and H(T=0) near 32 T. Up to an annealing
temperature of 400 C the recovery of T tends to coincide with a
decrease in the normal state resistivity and a systematic recovery of the
lattice parameters. Above 400 C a decrease in order along the c- direction
coincides with an increase in resistivity, but no apparent change in the
evolution of T and H. To first order, it appears that carbon doping
and neutron damaging effect the superconducting properties of MgB
independently
Penetration Depth and Anisotropy in MgB2
The penetration depth lambda of MgB2 was deduced from both the ac
susceptibility chi and the magnetization M(H) of sorted powders. The good
agreement between the two sets of data without geometric correction for the
grain orientation suggests that MgB2 is an isotropic superconductor.Comment: 9 pages, 5 figures; submitted to Physical Review B (February 28,
2001; revised June 28, 2001); reference list update
Temperature-dependent anisotropy in MgB as inferred from measurements on polycrystals
We present data on temperature-dependent anisotropy of the upper critical
field of MgB obtained from the analysis of measurements on high purity, low
resistivity polycrystals. The anisotropy decreases in a monotonic fashion with
increase of temperature
Experimental observation of high field diamagnetic fluctuations in Niobium
We have performed a magnetic study of a bulk metallic sample of Nb with
critical temperature K. Magnetization versus temperature (M {\it
vs} T) data obtained for fixed magnetic fields above 1 kOe show a
superconducting transition which becomes broader as the field is increased. The
data are interpreted in terms of the diamagnetic lowest Landau level (LLL)
fluctuation theory. The scaling analysis gives values of the superconducting
transition temperature consistent with % . We search for
universal 3D LLL behavior by comparing scaling results for Nb and YBaCuO, but
obtain no evidence for universality.Comment: 5 pages, 6 figures, Accepted for publication in Phys.Rev.
Magnetoresistivity and Complete in
Detailed magneto-transport data on dense wires of are reported for
applied magnetic fields up to 18 T. The temperature and field dependencies of
the electrical resistivity are consistent with behaving like a simple
metal and following a generalized form of Kohler's rule. In addition, given the
generally high values and narrow resistive transition widths associated
with synthesized in this manner, combined with applied magnetic fields
of up to 18 T, an accurate and complete curve could be determined.
This curve agrees well with curves determined from lower field measurements on
sintered pellets and wires of . is linear in over a wide
range of temperature (7 K 32 K) and has an upward curvature for
close to . These features are similar to other high , clean limit,
boron-bearing intermetallics: and .Comment: minor changes in styl
Analysis of Geometrical Relationships and Friction Losses in Small-Diameter Lay-Flat Polyethylene Pipes
[EN] The use of lay-flat polyethylene pipes to irrigate horticultural crops has been receiving widespread attention in the last decade, due to the significant improvements in their hydraulic performance, their potentially high application efficiency, and their limited installation costs. However, even if hydraulic design procedures for conventional microirrigation systems are fairly well established, there is still the need to know how different pipe-wall thicknesses of lay-flat pipes can affect the pipe geometry under different operating pressures as well as the related consequences on friction losses. This paper, after comparing two different procedures (caliper and photographic) to assess the geometry of lay-flat polyethylene pipes under different operating pressures, analyzes the friction losses per unit of pipe length, J, in order to identify and to assess a procedure for their evaluation. Hydrostatic tests, initially carried out on pipes with wall thicknesses of 6, 8, and 10 thousandth of an inch (mil), evidenced that the pipe dimensions measured with both methods are quite similar, despite the generally higher standard deviations characterizing caliper measurements when compared to photographic method. Tests allowed to verify that most of the changes in pipe dimensions occur within a range of pressure from 0 kPa to about 30 kPa, with pipe horizontal width and vertical height quite similar at higher pressures and pipes have a tendency to become circular. Additionally, due to the elasticity of the material, over a certain limit of water pressure, both the pipe dimensions tend to rise, with a trend depending on pipe thickness. According to the experimental data, the relationships between pipe effective diameter and water pressure were then determined for the three considered pipes. Moreover, based on measured friction losses and pipe effective diameters, it was confirmed that the relationship between the Darcy-Weisbach friction factor, f, and the Reynolds number, R, can be described by a power equation in which, by assuming a value of -0.25 for the exponent, it results a coefficient c = 0.285, lower than the theoretical. For the three investigated pipes the errors associated to estimated J were finally evaluated by considering (1) the experimental relationships between friction factor and Reynolds number as well as between pipe diameter and operating pressure (Case A); (2) the same value of c, but pipe effective diameters of 16.20, 16.10, and 15.85 mm corresponding to p = p(lim) (Case B); (3) the standard procedure, with a value of c = 0.302 and the pipe diameter equal to 16.10 mm, as suggested by the manufacturer. The results evidenced that suitable estimations of J need to account for the variations of the pipe effective diameter with water pressure. On the other hand, incorrect values of pipe diameter combined with inexact values of the friction factor generate inaccurate estimations of friction losses, with unavoidable consequences in pipe design. (C) 2015 American Society of Civil Engineers.The research was cofinanced by Universita di Palermo (FFR 2011) and Ministero dell'Istruzione, dell'Universita e della Ricerca (PRIN 2010). All the authors setup the research and discussed the results. V. Alagna and D. Autovino carried out the experimental measurements and G. Provenzano wrote the paper. A special thank to the Committee for International Relations Office (CORI) of University of Palermo to support the research cooperation with the University of Valencia.Provenzano, G.; Alagna, V.; Autovino, D.; Manzano Juarez, J.; Rallo, G. (2016). Analysis of Geometrical Relationships and Friction Losses in Small-Diameter Lay-Flat Polyethylene Pipes. Journal of Irrigation and Drainage Engineering. 142(2):1-9. https://doi.org/10.1061/(ASCE)IR.1943-4774.0000958S19142
Order parameter of MgB_2: a fully gapped superconductor
We have measured the low-temperature specific heat C(T) for polycrystalline
MgB_2 prepared by high pressure synthesis. C(T) below 10 K vanishes
exponentially, which unambiguously indicates a fully opened superconducting
energy gap. However, this gap is found to be too small to account for Tc of
MgB_2. Together with the small specific heat jump DeltaC/gamma_nTc=1.13,
scenarios like anisotropic s-wave or multi-component order parameter are called
for. The magnetic field dependence of gamma(H) is neither linear for a fully
gapped s-wave superconductor nor H^1/2 for nodal order parameter. It seems that
this intriguing behavior of gamma(H) is associated with the intrinsic
electronic properties other than flux pinning.Comment: 7 pages, 5 figures; revised text and figures; references updated,
Phys. Rev. Lett., in pres
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