A test facility dipole is being developed at LBNL, targeting a 16 T field in a 144 mm wide aperture. The magnet uses a block design, with two double-pancake coils. In order to minimize motion under ...the large Lorentz forces, the coils are preloaded against a thick aluminum shell and iron yoke using bladder and key technology. It is then crucial to verify that the performance of the magnet is not degraded due to strain induced on the Nb 3 Sn conductor during assembly, cool-down and powering. The critical current of extracted strands was measured in a varying background magnetic field and as a function of the applied longitudinal strain. Finite element analysis was used to extract the strain state inside the superconducting strands during magnet assembly and operation. This strain was then compared to the measurements to evaluate potential reversible and irreversible effects on the magnet performances. The results suggest that the magnet can reach 16 T with sufficient margin, with no irreversible degradation in the high field region.
Mechanical properties of rolling-assisted, biaxially-textured substrates (RABiTS) and substrates for ion-beam assisted deposition (IBAD) coated superconductors are measured at room temperature, 76, ...and 4
K. Yield strength, Young’s modulus, and the proportional limit of elasticity are determined, tabulated and compared. Results obtained are intended to serve as a database of mechanical properties of substrates having the same anneal state and texture as those incorporated in the general class of RE–Ba–Cu–O coated conductor composites (RE
=
rare earth). The RABiTS materials measured are pure Ni, Ni–13at.%Cr, Ni–3at.%W–2at.%Fe, Ni–10at.%Cr–2at.%W, and Ni–5at.%W. The IBAD substrate materials included Inconel 625 and Hastelloy C-276. The Ni alloys are substantially stronger and show higher strains at the proportional limit than those of pure Ni. Substrates fully coated with buffer layers, ≈1
μm of Y–Ba–Cu–O, and 3–5
μm of Ag have similar mechanical properties (at 76
K) as the substrate alone. Somewhat surprisingly, plating an additional 30–40
μm of Cu stabilizer onto high-yield-strength (690
MPa) Hastelloy coated conductors ∼100
μm thick, reduces the overall yield strength of the composite structure by only about 10–12% at 76
K and 12–14% at room temperature; this indicates that the Cu layer, despite its relatively soft nature, contributes significantly to the overall strength of even high-strength coated conductors.
Electromechanical Characterization of Bi-2212 Strands Lu, X F; Cheggour, N; Stauffer, T C ...
IEEE transactions on applied superconductivity,
06/2011, Letnik:
21, Številka:
3
Journal Article, Conference Proceeding
Recenzirano
The uniaxial strain dependence of critical current was measured both in tension and compression in Bi 2 Sr 2 CaCu 2 O 8+x (Bi-2212) high-temperature superconducting round wires. Permanent damage to ...the critical current easily occurred due to strain. To improve the electromechanical properties of Bi-2212 wires, development of stronger sheathing materials is needed. Ideal materials would be not only mechanically strong, but also chemically compatible with Bi-2212 during the final heat treatment. To identify such materials, we measured stress-strain properties of some new Ag alloys and extracted their respective Young's modulus values and yield strength. The database may be useful for development of new Bi-2212 strands for fabricating high-field superconducting magnets above 20 T.
A benchmarking experiment was conducted to compare strain measurement facilities at the National Institute of Standards and Technology (NIST) and the University of Twente. The critical current of a ...bronze-route Nb 3 Sn wire, which was fabricated for the International Thermonuclear Experimental Reactor (ITER), was measured as a function of axial strain and magnetic field in liquid helium at both institutes. NIST used a Walters' spring strain device and University of Twente used a bending beam ("Pacman") apparatus. The ITER bronze-route wire investigated had a very high irreversible strain limit that allowed comparing data over a wide range of applied strain between -1% and +1%. Similarities of the data obtained by use of the two apparatuses were remarkable, despite the many differences in their design and techniques.
The slitting of wide Y-Ba-Cu-O coated-conductor tapes to a width desirable for applications allows for considerable reduction in conductor manufacturing cost. Localized damage induced at the slit ...edges may be tolerated provided that mechanical cracks formed in the ceramic layers do not propagate deeper inside the conductor due to mechanical forces and thermal cycling to which the strand will be subjected in actual applications. In order to evaluate the effect of slitting, we used fatigue cycling under transverse compressive stress. These tests simulate conditions in applications such as rotating machinery and industrial magnets. Conductors measured had a rolling-assisted biaxially textured Ni-W substrate (RABiTS), or a Hastelloy-C substrate with an ion-beam assisted deposition (IBAD) buffer template. Samples were fabricated with or without a Cu protection layer, added either before or after slitting. For all these geometries, the critical current exhibited no significant degradation during fatigue testing up to 150 MPa transverse compressive stress and 20,000 cycles. Nevertheless, these results do not imply that slitting is not deleterious to the conductor performance under other experimental conditions.
A tubular technique for economical production of Nb{sub 3}Sn material with large numbers of subelements is being explored by Supergenics I LLC and Hyper Tech Research Inc. The number of subelements ...was increased to 919 (744 subelements plus 175 Cu filaments) by increasing the size at which restacking is carried out. The product exhibited no fabrication problems and was drawn down and tested at a wire diameter of 0.42 mm, where the subelements are 10 {micro}m in diameter. Recently we increased the subelement number to 1387 (1248 subelements plus 139 Cu filaments), which gives a subelement size of 12 {micro}m in 0.7 mm diameter wires. Heat treatment (HT) of different subelement restacks has been investigated, and the best results of critical current and stability are presented. The strain tolerance of the strands with 192 and 744 subelements was also tested, and the strand with fine subelement size showed a high intrinsic irreversible strain limit.
We review variable-temperature, transport critical-current (I c) measurements made on commercial superconductors over a range of critical currents from less than 0.1 A to about 1 kA. We have ...developed and used a number of systems to make these measurements over the last 15 years. Two exemplary variable-temperature systems with coil sample geometries will be described: a probe that is only variable-temperature and a probe that is variable-temperature and variable-strain. The most significant challenge for these measurements is temperature stability, since large amounts of heat can be generated by the flow of high current through the resistive sample fixture. Therefore, a significant portion of this review is focused on the reduction of temperature errors to less than ±0.05 K in such measurements. A key feature of our system is a pre-regulator that converts a flow of liquid helium to gas and heats the gas to a temperature close to the target sample temperature. The pre-regulator is not in close proximity to the sample and it is controlled independently of the sample temperature. This allows us to independently control the total cooling power, and thereby fine tune the sample cooling power at any sample temperature. The same general temperature-control philosophy is used in all of our variable-temperature systems, but the addition of another variable, such as strain, forces compromises in design and results in some differences in operation and protocol. These aspects are analyzed to assess the extent to which the protocols for our systems might be generalized to other systems at other laboratories. Our approach to variable-temperature measurements is also placed in the general context of measurement-system design, and the perceived advantages and disadvantages of design choices are presented. To verify the accuracy of the variable-temperature measurements, we compared critical-current values obtained on a specimen immersed in liquid helium ("liquid" or I c liq) at 5 K to those measured on the same specimen in flowing helium gas ("gas" or I c gas) at the same temperature. These comparisons indicate the temperature control is effective over the superconducting wire length between the voltage taps, and this condition is valid for all types of sample investigated, including Nb-Ti, Nb3Sn, and MgB2 wires. The liquid/gas comparisons are used to study the variable-temperature measurement protocol that was necessary to obtain the "correct" critical current, which was assumed to be the I c liq. We also calibrated the magnetoresistance effect of resistive thermometers for temperatures from 4 K to 35 K and magnetic fields from 0 T to 16 T. This calibration reduces systematic errors in the variable-temperature data, but it does not affect the liquid/gas comparison since the same thermometers are used in both cases.
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