N. Clayton

Transport properties and exponential n-values of Fe/MgB2 tapes with various MgB2 particle sizes

Fe/MgB2 tapes have been prepared starting with pre-reacted binary MgB2 powders. As shown by resistive and inductive measurements, the reduction of particle size to a few microns by ball milling has little influence on Bc2, while the superconducting properties of the individual MgB2 grains are essentially unchanged. Reducing the particle size causes an enhancement of Birr from 14 to 16 T, while Jc has considerably increased at high fields, its slope Jc(B) being reduced. At 4.2 K, values of 5.3 × 104 and 1.2 × 103 A/cm2 were measured at 3.5 and 10 T, respectively, suggesting a dominant role of the conditions at the grain interfaces. A systematic variation of these conditions at the interfaces is undertaken in order to determine the limit of transport properties for Fe/MgB2 tapes. The addition of 5% Mg to MgB2 powder was found to affect neither Jc nor Bc2. For the tapes with the highest Jc values, very high exponential n factors were measured: n=148, 89 and 17 at 3.5, 5 and 10 T, respectively and measurements of critical current versus applied strain have been performed. The mechanism leading to high transport critical current densities of filamentary Fe/MgB2 tapes based on MgB2 particles is discussed.

Modulation-free phase in heavily Pb-doped (Bi,Pb)2212 crystals

Abstract We report the complete disappearance of the structural modulation in heavily lead-doped Bi 2− x Pb x Sr 2 CaCu 2 O 8+ δ crystals observed by transmission electron microscopy. Crystals with a nominal lead content of x =0.8, corresponding to an effective lead content of x =0.39, yield the non-modulated phase. The superconducting properties of this modulation-free phase (β phase) have been studied and compared to those of undoped crystals displaying the modulated phase (α phase). Magnetisation measurements reveal that the irreversibility field H irr ( T ) and relaxation rates are strongly improved within the β phase. Measurements of the lower critical field, H c1 , show that the anisotropy factor, ϵ , is considerably reduced in the modulation-free crystals. This is the signature of stronger coupling between CuO 2 layers which in turn deeply influences the effectiveness of the pinning. These measurements explain the enhanced pinning properties in moderately Pb-doped crystals in which the α phase and β phase coexist. The enhanced pinning is not only due to the α/β interfaces, which act as effective pinning centers: the emergence of modulation-free domains, characterized by a strongly reduced anisotropy, also significantly contribute to this effect.

Third harmonic susceptibility and pinning properties of Fe/MgB/sub 2/ tapes

We report a study of the pinning properties of Fe/MgB/sub 2/ tapes performed by means of ac and dc magnetic measurements. In particular, the third harmonic response of the ac susceptibility has been measured as a function of the dc magnetic field up to 9 T, at different temperatures, for various frequencies and amplitudes of the ac field. The irreversibility line has been determined using the third harmonic onset criterion for samples made with different ball milled MgB/sub 2/ powders. The study of the harmonic response allows to probe the nature of the vortex pinning and to analyze its influence on the enhancement of the irreversibility field in strongly ball-milled samples. We have investigated the dynamic regimes governing the vortex motion in the tapes by comparing the experimental curves with numerical simulations of the nonlinear diffusion equation for the magnetic field. Furthermore, the pinning mechanism governing the electrical transport properties has been determined by studying the magnetic field dependence of the volume pinning force F/sub P/=J/sub c//spl times/B, calculated by extracting the critical current density J/sub c/(B,T) from magnetic hysteresis loops. Scaling of the F/sub P/(B) curves at different temperatures allows us to establish that the pinning centres are confined to the grain boundaries.

Spray pyrolysis of YBa/sub 2/Cu/sub 3/O/sub 7-x/ coated conductors on (100) SrTiO/sub 3/ substrate

YBa/sub 2/Cu/sub 3/O/sub 7-x/ films have been deposited using spray pyrolysis on (100) SrTiO/sub 3/ substrate. An extended study on the optimization of the composition, carrying gas flow, deposition temperature as well as annealing time was performed. It was found that these parameters are strongly correlated and affect the morphology, orientation and superconducting properties of the YBCO films. As grown films already exhibit J/sub c/ values of 10/sup 4/ A/cm/sup 2/, thus indicating that the correct structure is already formed during deposition. It was found that the temperature gradient in the pyrolysis zone played a fundamental role during the deposition. After modification of this heat gradient by adding a metallic susceptor, a noticeable improvement in the properties of the films was obtained. An in-plane orientation FWHM value of 0.70/spl deg/ was measured from the (102) pole figure. The films exhibited a T/sub c/ onset of 91 K (/spl Delta/T/sub c/=1.5 K), while the J/sub c/ value at 77 K in self-field approached 10/sup 5/ A/cm/sup 2/. Based on an assumed typical intragranular critical current density, we found a 40/spl times/40 /spl mu/m area in our best film where the assumed critical current density is well above 10/sup 6/ A/cm/sup 2/.

Spray Pyrolysis of

YBa/sub 2/Cu/sub 3/O/sub 7-x/ films have been deposited using spray pyrolysis on (100) SrTiO/sub 3/ substrate. An extended study on the optimization of the composition, carrying gas flow, deposition temperature as well as annealing time was performed. It was found that these parameters are strongly correlated and affect the morphology, orientation and superconducting properties of the YBCO films. As grown films already exhibit J/sub c/ values of 10/sup 4/ A/cm/sup 2/, thus indicating that the correct structure is already formed during deposition. It was found that the temperature gradient in the pyrolysis zone played a fundamental role during the deposition. After modification of this heat gradient by adding a metallic susceptor, a noticeable improvement in the properties of the films was obtained. An in-plane orientation FWHM value of 0.70/spl deg/ was measured from the (102) pole figure. The films exhibited a T/sub c/ onset of 91 K (/spl Delta/T/sub c/=1.5 K), while the J/sub c/ value at 77 K in self-field approached 10/sup 5/ A/cm/sup 2/. Based on an assumed typical intragranular critical current density, we found a 40/spl times/40 /spl mu/m area in our best film where the assumed critical current density is well above 10/sup 6/ A/cm/sup 2/.

rm YBa_2rm Cu_3rm O_7-rm x

YBa/sub 2/Cu/sub 3/O/sub 7-x/ films have been deposited using spray pyrolysis on (100) SrTiO/sub 3/ substrate. An extended study on the optimization of the composition, carrying gas flow, deposition temperature as well as annealing time was performed. It was found that these parameters are strongly correlated and affect the morphology, orientation and superconducting properties of the YBCO films. As grown films already exhibit J/sub c/ values of 10/sup 4/ A/cm/sup 2/, thus indicating that the correct structure is already formed during deposition. It was found that the temperature gradient in the pyrolysis zone played a fundamental role during the deposition. After modification of this heat gradient by adding a metallic susceptor, a noticeable improvement in the properties of the films was obtained. An in-plane orientation FWHM value of 0.70/spl deg/ was measured from the (102) pole figure. The films exhibited a T/sub c/ onset of 91 K (/spl Delta/T/sub c/=1.5 K), while the J/sub c/ value at 77 K in self-field approached 10/sup 5/ A/cm/sup 2/. Based on an assumed typical intragranular critical current density, we found a 40/spl times/40 /spl mu/m area in our best film where the assumed critical current density is well above 10/sup 6/ A/cm/sup 2/.