Results 1 - 10 of 24
Results 1 - 10 of 24. Search took: 0.016 seconds
|Sort by: date | relevance|
[en] The development of specialized information-security measures in automatic production control systems is considered. The problem is formulated; the mathematical operations are presented; and the operational algorithms of such components are described.
[en] We present a broad study by multiple techniques of the critical current and critical current density of a small but representative set of nominally identical commercial RE123 (REBa_2Cu_3O_7_−_δ, RE = rare Earth, here Y and Gd) coated conductors (CC) recently fabricated by SuperPower Inc. to the same nominal high pinning specification with BaZrO_3 and RE_2O_3 nanoprecipitate pinning centers. With high-field low-temperature applications to magnet technology in mind, we address the nature of their tape-to-tape variations and length-wise I _c inhomogeneities by measurements on a scale of about 2 cm rather than the 5 m scale normally supplied by the vendor and address the question of whether these variations have their origin in cross-sectional or in vortex pinning variations. Our principal method has been a continuous measurement transport critical current tool (YateStar) that applies about 0.5 T perpendicular and parallel to the tape at 77 K, thus allowing variations of c-axis and ab-plane properties to be clearly distinguished in the temperature and field regime where strong pinning defects are obvious. We also find such in-field measurements at 77 K to be more valuable in predicting 4.2 K, high-field properties than self-field, 77 K properties because the pinning centers controlling 77 K performance play a decisive role in introducing point defects that also add strongly to J _c at 4.2 K. We find that the dominant source of I _c variation is due to pinning center fluctuations that control J _c, rather than to production defects that locally reduce the active cross-section. Given the 5–10 nm scale of these pinning centers, it appears that the route to greater I _c homogeneity is through more stringent control of the REBCO growth conditions in these Zr-doped coated conductors. (paper)
[en] BaFe2As2 (Ba-122) and Ba0.6K0.4Fe2As2 (K-doped Ba-122) powders were successfully synthesized from the elements using a reaction method that incorporates a mechanochemical reaction using high-impact ball milling. Mechanically activated, self-sustaining reactions (MSRs) were observed while milling the elements together to form these compounds. After the MSR, the Ba-122 phase had formed, the powder had an average grain size <1 μm, and the material was effectively mixed. X-ray diffraction confirmed Ba-122 was the primary phase present after milling. Heat treatment of the K-doped MSR powder at high temperature (1120 ° C) and pressure yielded dense samples with high phase purity, but only granular current flow could be visualized by magneto-optical imaging. In contrast, a short, low temperature (600 ° C) heat treatment at ambient pressure resulted in global current flow throughout the bulk sample even though the density was lower and impurity phases were more prevalent. An optimized heat treatment involving a two-step, low temperature (600 ° C) heat treatment of the MSR powder produced bulk material with very high critical current density above 0.1 MA cm−2 at 4.2 K and self-field (SF). (paper)
[en] Nb3Sn strands for high-current, high-field magnets must be cabled before reaction while the conductor is still composed of ductile components. Even though still in the ductile, deformable state, significant damage can occur in this step, which expresses itself by inhomogeneous A15 formation, Sn leakage or even worse effects during later reaction. In this study, we simulate cabling damage by rolling recent high performance powder-in-tube (PIT) and internal tin (IT) strands in controlled increments, applying standard Nb3Sn reaction heat treatments, and then examining the local changes using magneto-optical imaging (MOI), scanning electron microscopy (SEM) and confocal laser scanning microscopy (CLSM). These combined characterizations allow any local damage to the filament architecture to be made clear. MOI directly reveals the local variation of superconductivity while CLSM is extremely sensitive in revealing Sn leakage beyond the diffusion barrier into the stabilizing Cu. These techniques reveal a markedly different response to deformation by the PIT and IT strands. The study demonstrates that these tools can provide a local, thorough, and detailed view of how strands degrade and thus complement more complex extracted strand studies.
[en] Significant performance degradation of superconducting RF (radio frequency) niobium cavities in high RF field is strongly associated with the breakdown of superconductivity on localized multi-scale surface defects lying within the 40 nm penetration depth. These defects may be on the nanometer scale, like grain boundaries and dislocations or even at the much larger scale of surface roughness and welding pits. By combining multiple superconducting characterization techniques including magneto-optical (MO) imaging and direct transport measurement with non-contact characterization of the surface topology using scanning confocal microscopy, we were able to show clear evidence of suppression of surface superconductivity at chemically treated RF-quality niobium. We found that pinning of vortices along GBs is weaker than pinning of vortices in the grains, which may indicate suppressed superfluid density on GBs. We also directly measured the local magnetic characteristics of BCP-treated Nb sample surface using a micro-Hall sensor in order to further understanding of the effect of surface topological features on the breakdown of superconducting state in RF mode.
[en] Biaxially-textured YBa2Cu3O7-δ (YBCO) coated conductors are a central thrust of current efforts to fabricate high-critical-current-density (Jc) wire for large-scale applications of high-Tc superconductivity. Encouragingly high Jc values are obtained in laboratory-length samples. However, magneto-optic imaging indicates that the supercurrent path is percolative even in the best materials. Thus, it seems clear that the full potential current carrying capability of these composites has not been reached. This paper describes coupled magneto-optic imaging and microstructural studies of RABiTSTM type coated conductors that indicate (1) the important roles of other microstructural features in addition to grain boundaries and (2) the need to explore the roles of the oxide buffer layer microstructure in optimizing the composite. (orig.)
[en] SmFeAsO1-xFx was irradiated in a fission reactor by a fast (E>0.1 MeV) neutron fluence of 4 x 1021 m-2. The introduced defects increased the normal state resistivity due to a reduction in the mean free path of the charge carriers. This leads to an enhancement of the upper critical field at low temperatures. The critical current density within the grains, Jc, increases upon irradiation. The second maximum in the field dependence of Jc disappears and the critical current density becomes a monotonically decreasing function of the applied magnetic field.
[en] The transport characteristics and quantum oscillations of magnetoresistance have been studied in n-(001)GaAs/AlxGa1-xAs heterostructures at liquid helium temperatures and uniaxial compressions up to 3.5 kbar. Under such loading conditions, the density of two-dimensional (2D) electrons at the heterointerface is determined primarily by a piezoelectric field induced in the  direction. The screening of this field leads to an increase in the 2D electron density at the heterointerface under compression along the  axis and to a decrease in this density, under compression along the [1 1-bar 0] axis. The formation and subsequent redistribution of a compensating charge at the heterointerface are impeded, which leads to the development of relaxation processes in the stressed system at sufficiently high pressures. The pressure dependences of the anisotropic mobility and the effective cyclotron mass of 2D electrons show that their energy spectrum remains isotropic and the dispersion obeys a parabolic law in the pressure range studied
[en] Using an arc physical vapor deposition process, we have produced nanostructured Mo–Si–Al–Ti–Ni–N coatings with a multilayer architecture formed by Mo2N, AlN–Si3N4, and TiN–Ni and a crystallite size on the order of 6–10 nm. We have studied the physicomechanical properties of the coatings and their functional characteristics: wear resistance, adhesion to their substrates, and heat resistance. According to high-temperature (550°C) wear testing and air oxidation (600°C) results, the coatings studied here are wearand heat-resistant under appropriate temperature conditions. Their properties are compared to those of Mo–Si–Al–N coatings.