G. Grasso

Large transport critical currents in unsintered MgB2 superconducting tapes

The powder-in-tube process has been employed to fabricate tape-like conductors with a strong metallic sheath and based on the newly discovered MgB2 superconducting phase. Long superconducting tapes have been prepared by packing reacted MgB2 powders inside pure Ag, Cu, and Ni tubes which are then cold worked by drawing and rolling. Such tapes have shown transport properties as good as bulk MgB2 samples sintered in high pressure and high temperature conditions. At 4.2 K, the highest critical current density of 105 A/cm2 has been achieved on nickel-sheathed single-filament conductors. A direct correlation between the sheath tensile strength and the critical current of the unsintered tape has been observed.

Distribution of the transport critical current density in Ag sheathed (Bi, Pb)2Sr2Ca2Cu3Ox tapes produced by rolling

Abstract By using a strip-cutting technique, we directly measured the lateral distribution of the transport critical current density of Ag sheathed (Bi, Pb) 2 Sr 2 Ca 2 Cu 3 O x tapes. For various long tapes prepared by the rolling technique with a critical current density of 23 000 A/cm 2 at 77 K and 0 T, the local critical current density of longitudinal 0.2 mm wide strips was reproducibly found to vary from 18 000 A/cm 2 at the center to 46 000 A/cm 2 at the sides of the filament. In all cases, a symmetrical behavior of j c at both sides of the central axis was observed. The value of the transport j c of 46 000 A/cm 2 at 77 K and 0 T in the external strips of Bi(2223) tapes produced by rolling processes comes quite close to the highest reproducible values reported so far on short pressed tapes. The field dependence of the critical current densities of the individual strips does not depend on their position inside the tape. Thus the observed difference in j c between central and external strips is not primarily due to a variation of the degree of texturing. A higher degree of compression and a lower amount of secondary phases were found for the external strips with the highest critical current density.

Deformation-induced texture in cold-rolled Ag sheathed bi(2223) tapes

Abstract We have found that the Bi(2212) grains in unreacted Ag/Bi(2223) tapes at the end of the cold-rolling process show already a high degree of texture. X-ray diffraction analysis and current transport measurements at 77 K in magnetic fields up to 0.5 T have been performed before and after reaction on tapes with thicknesses in the range from 75 to 286 μm (filament thickness between 30 and 115 μm). The critical current density j c increases for smaller tape thicknesses, a maximum being found for a tape thickness of about 90 μm, below which sausaging effects are observed. The mean misalignment angles of Bi(2223) grains in these tapes have been determined by angle-dependent transport critical curent data at 77 K. We have found that the mamin reason for the enhancem,ent of j c for thin tapes in due to the higher average texture of the Bi(2223) grains when compared to thick tapes. This is a direct consequence of the high degree of texture of Bi(2212) grains after the cold-rolling process, which has been observed here on 90 μm thick tapes with j c (77 K, 0 T) reaching between 25 and 30 kA/cm 2 .

Pressed and cold rolled Ag-sheathed Bi(2223) tapes: A comparison

Abstract High critical current densities in monofilamentary Ag-sheathed tapes of Bi(2223) have been obtained on tapes produced by different deformation techniques. For cold rolled tapes of > 1 m in length, promising critical currents of j c (77 K, 0 T)=18500 A cm -2 have been measured. These tapes have a relatively weak dependence of j c vs. B their corresponding value j c (77 K, 1 T) being 3550 A cm -2 . The present results on cold rolled tapes are compared to those of typical short uniaxially pressed tapes of j c (77 K, 0 T)=26000 A cm -2 and j c (77 K, 1 T)=5200 A cm -2 . This comparison reveals both for 4.2 and 77 K a weaker field dependence of j c for the rolled tapes with respect to the pressed ones. At zero field, the rolled tapes exhibit lower j c values, but reach higher j c values at high fields, the crossover occuring at 3 T for 77 K and at 14 T for 4.2 K. The j c (4.2 K, 15 T) value for rolled Bi(2223) tapes is 4.7×10 4 A cm -2 . The rolled tapes also exhibit a stronger anisotropy at 4.2 K, where the ratio j c⊥ / j c‖ at 10 T is 2.2 compared to 1.5 for pressed tapes. Scanning electron microscopy and X-ray diffraction analysis are extensively used to investigate the microstructure of the Bi(2223) phase in the two samples.

Hot rolling of silver-sheated Bi (2223) tapes

Abstract Silver-sheated Bi(2223) tapes have been prepared using a newly developed hot-roll apparatus where the rolls can be heated. up to 850°C. At 77 K and 0 T, a critical current density of 18 000 A/cm 2 has been measured on tapes after hot rolling at 800°C. Microstructural analyses and X-ray diffraction have been performed. The field dependence and the anisotropy of the critical current were investigated at 77 K. The hot-rolled tapes show a higher density and better grain alignment compared to cold-rolled tapes. The critical current density at 77 K and 1 T is 3500 A/cm 2 , i.e., it decreases by a factor of 5 will respect to the value in zero field. This is a considerable improvement compared to the published data on cold-rolled tapes where this factor is almost 7 for our reference tape and between 8 and 12 in the published data.

Improvement of Magnetic Field Behavior of Ex-Situ Processed Magnesium Diboride Tapes

MgB2 tapes have been synthesized through the ex-situ powder-in-tube (PIT) method. This technique involves the cold working of tubes of various metals-in this case Ni-previously filled by suitably reacted MgB powders, followed by proper heat treatments. In particular, it allows the use of different starting powders and the control over their properties. We studied the influence of the starting powders on the superconducting properties of the final conductors, and we tried to improve their behavior in magnetic field by mastering their grain size and by inserting appropriate doping. In order to improve the pinning properties of the tapes, the granulometry has been varied through ball-milling of MgB, and different dopants-such as carbon or SiC nanoparticles-have been introduced either on the precursors or on MgB. Critical current measurements on the tapes are presented, both magnetic from the SQUID and transport in high magnetic field. In particular, IV characteristics have been measured up to 13 Tesla at GHMFL (Grenoble) in order to extract the critical current in two directions, i.e. with the field perpendicular and parallel to the tape surface.

Stability considerations of multifilamentary MgB2 tape

The risk of overheating arises if the stability of an MgB2 coil is lost. This is mainly due to the low thermal conductivity of the matrix metal, which is typically iron or nickel. Due to a strong chemical reaction, copper can be used in contact with MgB2 only in specific cases: in situ preparation route and low formation temperature. However, recent developments in the manufacture of MgB2 conductors have resulted in stabilized MgB2 tapes. In any case, the stability considerations of MgB2 conductors are of great importance. We studied computationally the stability of an MgB2/Ni/Fe/Cu tape manufactured by Columbus Superconductors. First, the effective material properties of the tape were computed. Based on these material properties, we computed the basic quench characteristics (minimum propagation zone, minimum quench energy and normal zone propagation velocities) for the tape at 15, 20 and 25 K. The tape unit cell model was used in computations. The computed stability results were compared with the measured ones of a monofilament MgB2/Cu/Ni tape and commercial LTS and HTS. According to the results, the basic quench characteristics computed fit between the ones of LTS and HTS materials.

New Bi-based high-Tc superconducting phases obtained by low-temperature fluorination

Abstract A new superconducting phase, Bi 2 Sr 2 Ca 2 Cu 3 O 8 F 4 , was obtained by fluorination of standard Bi(2223) at moderate temperatures (250–300°C), using as fluorine source ammonium hydrogen difluoride, NH 4 HF 2 . The presence of fluorine in this new phase was confirmed by energy-dispersive X-ray analysis. X-ray and neutron diffraction experiments furthermore showed that fluorine atoms replace oxygen atoms in the BiO layers in the ratio 2:1. The additional anions form a square-mesh layer between neighboring BiF layers. The incorporation of fluorine increases the crystallographic c -axis parameter by ∼1.8 A ( a =5.409 A, b =5.407 A, c =38.792 A). The CuO 2 layers remain undistorted but the distance from the Cu atom to the apical oxygen atom of the square pyramids is decreased to 2.27 A, with respect to 2.42 A in the original phase. The superconducting transition temperature, determined from magnetic susceptibility measurements, was found to be 75 K. When applied to Bi(2212), the same fluorination process produced the new phase Bi 2 Sr 2 CaCu 2 O 6 F 4 with similar structural features.

TEM study of the (Bi,Pb)2Sr2Ca2Cu3Ox phase formation in (Bi,Pb)2Sr2Ca2Cu3Ox silver-sheathed tapes

The phase formation of (Bi,Pb)2-Sr2-Ca2Cu3Ox (denoted as Bi/Pb(2223)) in Bi/Pb(2223) silver-sheathed tapes has been investigated using transmission electron microscopy (TEM). In partly reacted Bi2Sr2CaCu2Ox (Bi(2212)) grains, spatial transitions from Bi(2212) layers to Bi/Pb(2223) layers along the basal plane are observed, but they occur very rarely. These transitions cause considerable lattice distortions. Amorphous regions, which correspond to a liquid phase present during the heat treatment, have slightly higher Bi and Pb contents than the adjacent Bi(2212) grains. An analysis of stacking sequences demonstrates that the majority of the layers is surrounded by layers of the same type and that the conversion rate of the Bi(2212)-to-Bi/Pb(2223) transformation decreases rapidly. These observations support the idea that the Bi/Pb(2223) phase primarily forms via a nucleation-and-growth process, which is orientation-controlled by the unreacted Bi(2212) grains serving as a substrate for the growing Bi/Pb(2223) phase.

Thermal conductivity of a BSCCO(2223) c-oriented tape: a discussion on the origin of the peak

Abstract Thermal conductivity and electrical resistivity measurements have been performed on a highly c -oriented (BiPb) 2 Sr 2 Ca 2 Cu 3 O 10 tape in the range from 20 to 220 K. We analyse the experimental data in order to investigate the origin of the thermal conductivity maximum below T c . Three different cases are considered: (1) the electron contribution to the thermal conductivity, K e , decreases as described by the Bardeen-Rickayzen-Tewordt model, while the phonon contribution, K ph , is responsible for the peak (phonon approach); (2) K e , described by means of the Kadanoff-Martin formalism, shows a peak while K ph is a smooth decreasing function of decreasing temperature (electron approach); (3) both contributions present a peak (phonon + electron approach). The models we use involve some fundamental parameters, such as the amplitude of the energy gap, the Debye temperature, the mean dimension of the grains and the residual electron-impurity scatterint rate. The best fit is obtained by the phonon + electron approach, but the phonon approach yields a more correct value for the energy gap. Finally, within the employed framework, we can exclude that only the electron peak by itself might account for our experimental data.