9 research outputs found
Influence of mechanical reinforcement of MgB2 wires on the superconducting properties
Monofilamentary MgB2-wires with a mechanical tough Nb/Cu/steel or Fe/steel
sheath were prepared and characterized. The steel content was varied to
investigate the reinforcement effect and the consequences for the
superconducting properties of the wires, which were heat treated to achieve
dense and homogeneous filaments. The use of Nb as first sheath layer, having a
smaller thermal expansion coefficient than MgB2, requires the application of
higher amounts of steel to achieve compressive pre-stress on the filament in
comparison to Fe as first wall material. With raised steel content in the
sheath the critical transport currents show field dependent significant
critical current and irreversibility field degradations. First Ic vs. axial
stress and strain experiments confirmed this observation of pre-stress induced
degradations. Consequences for improved wires and for future applications will
be discussed.Comment: 14 pages, 7 figures, 1 table; Submitted to Physica C; presentation at
ISS-2001, Kobe Japa
Mechanically reinforced MgB2 wires and tapes with high transport currents
Monofilamentary MgB2-wires with a 2- or 3-component sheath containing
mechanical reinforcing stainless steel (SS) were prepared and characterized. In
direct contact to the superconductor Nb, Ta or Fe was used. For a selection of
samples with a Fe and Fe/SS sheath, we investigated the transport critical
current behaviour in magnetic fields changing systematically the geometrical
shape from a round wire to a flat tape. A strong increase of the current
densities in flat tapes was observed and possible reasons for this are
discussed. Reinforcing the sheath in the outer layer with different amounts of
stainless steel leads to a systematic field dependent decrease of the transport
critical current density with increasing steel amount. This is an indication
for a pre-stress induced degradation of the critical currents in MgB2 wires and
first Ic-stress-strain experiments seem to confirm this observation and
interpretation.Comment: 12 pages, 5 figures. Submitted to Physica C (Proceedings of EUCAS
2001
Adiabatic normal zone development in MgB2 superconductors
A-priori knowledge of the normal zone development in MgB/sub 2/ conductors is essential for quench protection of applications. Therefore the normal zone propagation in a monofilament MgB/sub 2//Fe conductor under near-adiabatic conditions at 4.2 K has been measured and simulated. The results show normal zone propagation velocities up to several meters per second. In addition, by including the voltage-current relation into the computational model, the influence of the n-value on the normal zone propagation is determined. The simulations show that lower n-values suppress the normal zone propagation velocity due to lower heat generation in the MgB/sub 2/ filaments
The Effect of Chemical Doping and Hydrostatic Pressure on Tc of Y1-yCayBa2Cu3Ox Single Crystals
We performed susceptibility measurements on Y1-yCayBa2Cu3Ox single crystals
under high He pressure. For each Ca content various samples with different
oxygen contents have been prepared to probe the influence of Ca on Tc(x),
dTc/dp(x) and Tc,max. Starting from the parabolic Tc(nh) behavior we calculated
nh values from Tc and Tc,max for each sample. It is shown that in the overdoped
region dTc/dp can be described by a pressure induced charge transfer with
dnh/dp = 3.7E-3 [1/GPa] and a dTc,max/dp value of 0.8 K/GPa, irrespective of
the Ca content. In the underdoped region additional pressure effects lead to a
peak in dTc/dp at approximately 0.11 holes/CuO2 plane. However, with increasing
Ca content this peak is strongly depressed. This is explained in terms of an
increasing disorder in the CuO chain system due to doping. Deviations in dTc/dp
at very low nh values can be assigned to the ortho II ordering in the CuO chain
system.Comment: 13 pages with 6 figures, accepted for publication in Physica
Pseudogap and precursor superconductivity in underdoped cuprate high temperature superconductors: A far-infrared ellipsometry study
Quench onset and propagation in Cu-stabilized multifilament MgB2 conductors
The quench development of Cu-stabilized metal-sheathed MgB2 conductors has been analysed.Experimentally, energy pulses were deposited into the conductor by passing rectangular current pulses through a graphite-based epoxy heater. The temperature and the electric field profiles around the point heat disturbance that gives rise to a quench, as well as their time evolution, were measured from multiple voltage taps and thermocouples along the conductor. The measurements have been done at self-field under two different conditions: in vacuum and in Ne vapour. The experimental results are in qualitative agreement with the simulated ones, obtained by solving the one-dimensional heat balance equation of the system and using a finite n value characterizing the IâV curves (V âI^n). The temperature and current dependences of the minimum quench energy and the quench propagation velocity are presented. Our results show that the nonlinear power-law current sharing in the normal zone had significant influence on the onset of the quench process and resulted in a marked deviation from the classical quench theory based on the critical state model
Preparation and Characterization of Mg1-xB2 Bulk Samples and Cu/Nb Sheathed Wires with Low Grade Amorphous Boron Powder
MgB2 bulk and wire samples were prepared using cheap, low grade amorphous boron powders. Based on chemical analysis performed on the starting reagents, three nominal stoichiometries were studied. It was found that the structural and superconducting properties of the bulk samples were not affected by the composition, but that residual Mg was left in the wires for the nominal MgB2 composition. In contrast, slightly Mg-deficient compositions were free from residual Mg and exhibited higher critical current densities. The MgB2 phase formation kinetics was not influenced by the variations in the nominal powder compositio