Bending strain effect on critical current of Bi-2223 superconductor tapes-report of international round-robin-test

An inter-comparison program (round-robin-test) aimed at the establishment of a standard test method for the bending strain effect on the critical current in Ag alloy-sheathed Bi-2223 tapes was implemented with participation of 12 laboratories around the world. Samples were bent at room temperature using bending devices that were specially designed for this RRT. The critical current was measured at 77 K and in the self-field. After critical current measurement of an unbent sample, bending of the sample and subsequent critical current measurements were done from 0.2% to 1.0% in 0.2% steps of strains on a single sample. Reported data from participants showed that the critical current values start to decrease from 0.4% strain and continue to decrease to 1.0% strain. Moreover, the data scatter starts to increase stepwise at 0.4% strain, being correspondent to the variation in the critical current values with bending strain, and it continues to increase gradually to 1.0% strain. The coefficient of variation values are 2.5%, 5.1% and 6.4% for 0%, 0.4% and 1.0% bending strain, respectively. Results were also compared in terms of the irreversible bending strain. Possible sources of error are discussed

The non-centrosymmetric lamellar phase in blends of ABC triblock and ac diblock copolymers

The phase behaviour of blends of ABC triblock and ac diblock copolymers is examined using self-consistent field theory. Several equilibrium lamellar structures are observed, depending on the volume fraction of the diblocks, phi_2, the monomer interactions, and the degrees of polymerization of the copolymers. For segregations just above the order-disorder transition the triblocks and diblocks mix together to form centrosymmetric lamellae. As the segregation is increased the triblocks and diblocks spatially separate either by macrophase-separating, or by forming a non-centrosymmetric (NCS) phase of alternating layers of triblock and diblock (...ABCcaABCca...). The NCS phase is stable over a narrow region near phi_2=0.4. This region is widest near the critical point on the phase coexistence curve and narrows to terminate at a triple point at higher segregation. Above the triple point there is two-phase coexistence between almost pure triblock and diblock phases. The theoretical phase diagram is consistent with experiments.Comment: 9 pages, 8 figures, submitted to Macromolecule

Domains in Melts of Comb-Coil Diblock Copolymers: Superstrong Segregation Regime

Conditions for the crossover from the strong to the superstrong segregation regime are analyzed for the case of comb-coil diblock copolymers. It is shown that the critical interaction energy between the components required to induce the crossover to the superstrong segregation regime is inversely proportional to mb = 1 + n/m, where n is the degree of polymerization of the side chain and m is the distance between successive grafting points. As a result, the superstrong segregation regime, being rather rare in the case of ordinary block copolymers, has a much better chance to be realized in the case of diblock copolymers with combs grafted to one of the blocks.

MgB₂-based superconductors for fault current limiters

A promising solution of the fault current problem in power systems is the application of fast-operating nonlinear superconducting fault current limiters (SFCLs) with the capability of rapidly increasing their impedance, and thus limiting high fault currents. We report the results of experiments with models of inductive (transformer type) SFCLs based on the ring-shaped bulk MgB2 prepared under high quasihydrostatic pressure (2 GPa) and by hot pressing technique (30 MPa). It was shown that the SFCLs meet the main requirements to fault current limiters: they possess low impedance in the nominal regime of the protected circuit and can fast increase their impedance limiting both the transient and the steady-state fault currents. The study of quenching currents of MgB2 rings (SFCL activation current) and AC losses in the rings shows that the quenching current density and critical current density determined from AC losses can be 10-20 times less than the critical current determined from the magnetization experiments

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

Tensile creep performance of a developmental in-situ reinforced silicon nitride

The evaluation was done between 1300 and 1425 C in ambient air. Minimum creep rate was evaluated vs tensile stress and temperature, and measured tensile creep performances of two different specimen geometries (buttonhead and dogbone - machined from same billet) were compared. This Si nitride exhibited comparable or better creep resistance than other Si nitrides described in the literature. Measured creep response of the material and lifetime were observed to be geometry dependent; the smaller cross-sectioned dogbone samples exhibited faster creep rates and shorter lives, presumably due to faster oxidation-induced damage in this geometry. The tensile creep rates and lifetimes were found to be well represented by the Monkman- Grant relation between 1350 and 1425 C, with some evidence suggesting stratification of the data for the 1300 C tests and a change in dominant failure mode between 1300 and 1350 C. Lastly, values of the temperature-compensated stress exponent and activation energy for tensile creep were found to decrease by 80 and 75% in compression, respectively, illustrating anisotropic creep behavior in this Si nitride

Pinning and trapped field in MgB2- and MT-YBaCuO bulk superconductors manufactured under pressure

The relevant pinning centers of Abrikosov vortices in MgB$_{2}$–based materials are oxygen-enriched Mg-B-O inclusions or nanolayers and inclusions of MgB$_{x}$ (x>4) phases. The high critical current densities, j$_{c}$, of 10$^{6}$ and 10$^{3}$A/cm$^{2}$ at 1 and 8.5 T, respectively, at 20 K can be achieved in polycrystalline materials (prepared at 2 GPa) containing a large amount of admixed oxygen. Besides, oxygen can be incorporated into the MgB$_{2}$ structure in small amounts (MgB$_{1.5}$O$_{0.5}$), which is supported by Auger studies and calculations of the DOS and the binding energy. The j$_{c}$ of melt textured YBa$_{2}$Cu$_{3}$O$_{7-δ }$ (or Y123)-based superconductors (MT-YBaCuO) depends not only on the perfectness of texture and the amount of oxygen in the Y123 structure, but also on the density of twins and micro-cracks formed during the oxygenation (due to shrinking of the c-lattice parameter). The density of twins and microcracks increases with the reduction of the distance between Y$_{2}$BaCuO$_{5}$ (Y211) inclusions in Y123. At 77 K j$_{c}$=8⋅10$^{4}$ A/cm$^{2}$ in self-field and j$_{c}$=10$^{3}$ A/cm$^{2}$ at 10 Т were found in materials oxygenated at 16 MPa for 3 days with a density of twins of 22–35 per μm (thickness of the lamellae: 45-30 nm) and a density of micro-cracks of 200–280 per mm. Pinning can occur at the points of intersection between the Y123 twin planes and the Y211 inclusions. MTYBaCuO at 77 K can trap 1.4 T (38×38×17 mm, oxygenated at 0.1 MPa for 20 days) and 0.8 T (16 mm in diameter and 10 mm thick with 0.45 mm holes oxygenated at 10 MPa for 53 h). The sensitivity of MgB$_{2}$ to magnetic field variations (flux jumps) complicates estimates of the trapped field. At 20 K 1.8 T was found for a block of 30 mm in diameter and a thickness of 7.5 mm and 1.5 T (if the magnetic field was increased at a rate of 0.1 T) for a ring with dimensions 24×18 mm and a thickness of 8 mm

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$_{2}$-based bulk materials (with high level of superconducting characteristics) has AlB$_{2}$ type structure and near MgB$_{1.8-1.68}$O$_{0.2-0.32}$ stoichiometry. The materials demonstrated the critical current densities up to 0.9 – 0.4 MA/cm$^{2}$ jc (at 0 - 1 T, 20 K); up to 15 T B$_{c2}$ (at 22.5 K) and B$_{irr}$ (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 MgB$_{1.75}$O$_{0.25}$; (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$_{2}$ 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$_{c}$, for MgB$_{1.75}$O$_{0.25}$ occurred to be 23.3 K, while for MgB$_{2}$ it was 21.13 K only. Experimental T$_{c}$ of the bulk materials was 35.7-38.2 K