Kristian Fossheim

Enhanced flux pinning in Bi2Sr2CaCu2O8+x superconductor with embedded carbon nanotubes

Abstract This paper describes a new approach towards enhanced flux pinning in high-temperature superconductors, and with a potential for scale-up to bulk application. It is shown that carbon nanotubes can be embedded in Bi 2 Sr 2 CaCu 2 O 8+ x superconductor, thus displacing the superconducting material from a cylindrical volume suitable for strong pinning of vortex lines along its length. The best-case superconductor-nanotube composite thus prepared was found to surpass the pure ceramic superconducting matrix in terms of pinning properties. Although optimum conditions were yet not reached the intragrain critical current density was estimated to have been increased by about one order of magnitude.

Microstructure and flux distribution in both pure and carbon-nanotube-embedded Bi2Sr2CaCu2O8+δ superconductors

Abstract In order to improve pinning properties of bulk Bi 2 Sr 2 CaCu 2 O 8+ δ (Bi-2212) materials, samples of both pure Bi-2212 and Bi-2212 with carbon nanotubes embedded (CNTE Bi-2212) have been prepared by partial-melting processing. The preparation conditions are chosen so that a significant fraction of carbon nanotubes can be successfully embedded in the material, as indicated by thermogravimetric analysis. The microstructure and composition of non-superconducting second phases are found to be different in these two types of samples. By means of magneto-optical (MO) imaging, flux distributions in both types of samples are investigated up to T =77 K. The MO investigation reveals the propagation of a flux front in both pure and CNTE Bi-2212, showing that there is a strong coupling between grains (clusters) which enables the flow of inter-granular currents. This grain coupling persists in our field range of ±180 mT. In bulk non-textured ceramic high- T c superconductors, the flux fronts caused by currents flowing through the entire sample are observed for the first time. Intra-granular current densities are obtained from the images as a function of grain size. The MO investigations have revealed the differences in the current densities caused by the presence of carbon nanotubes, showing that the carbon nanotubes are indeed functioning like columnar defects produced by heavy-ion irradiation. The increase of the flux penetration field is also a manifestation of the increase of the transport current density in the CNTE Bi-2212. The superconducting properties in our samples are very well reproducible.

Bi2Sr2CaCu2Ox bulk superconductor with MgO particles embedded

Abstract MgO nano-size particles in various concentrations were added to the Bi-2212 superconductor precursor before partial melting. The resulting ceramic superconductors had a transition sharpness and volume fraction which increased with MgO addition up to 9 wt.%. Compositional variations in the superconducting matrix with varying MgO addition were found, with possible important consequences for the pinning mechanism, since compositional variations are expected to be accompanied by variations in Tc. By TEM it was directly documented that MgO particles could be embedded within the superconducting Bi-2212 grains. Hence wetting occurs. Thus, the requirements for MgO to play an effective role in pinning are satisfied. It is suggested that two distinct pinning mechanisms, both related to MgO addition, are likely to be present. Magneto-optic images clearly confirm the improvement of pinning, and screening, with increasing addition of MgO to Bi-2212.

Evidence for pinning by (Sr, Ca)2CuOy particles in partial-melting processed bulk Bi2Sr2CaCu2O8+δ ceramics

Abstract Flux distributions of partial-melting processed Bi 2 Sr 2 CaCu 2 O 8+ δ ceramics are obtained using magneto-optic imaging. In remanent states ( μ 0 H a =0 T), large amounts of trapped flux are observed along (Sr,Ca) 2 CuO y particles embedded in the Bi 2 Sr 2 CaCu 2 O 8+ δ matrix. Despite the relatively large size of these particles (up to 30 μ m), the pinning effect is similar to that of Y 2 BaCuO 5 particles in melt-processed YBa 2 Cu 3 O 7− δ . Furthermore, we discuss how the pinning capability of non-superconducting particles of different sizes and densities will show up in magneto-optic images.

Elastic and specific heat critical properties of La1.85Sr0.15CuO4

Abstract Using a continuous wave resonant ultrasonic technique we have made elastic measurements on single crystalline La 1.85 Sr 0.15 CuO 4 . Our measurements of c 33 show a sharp minimum at T c of the superconducting transition. The variation of the elastic constant is thermodynamically related to both the heat capacity and thermal expansion. A comparison of our measurements of the peak in heat capacity and the minimum of the elastic constant shows strong similarities, both for the zero field and field (|| c ) measurements. Analysis of the data also appear to be consistent with 3D-XY scaling. By variation of the ultrasonic frequency, we observe that the softening near T c is frequency dependent, and decreasing with increasing frequency. This behavior is to be expected when probing the critical dynamics of spacial fluctuation of the order parameter, for example spontaneously generated vortex loops in the vicinity of T c .

Comparison of fully-reacted and partly-reacted Bi2Sr2CaCu2Oy precursor powders

Abstract The properties of commercially-made fully-reacted and partly-reacted Bi-2212 precursor powders and their influence on the microstructure and critical current density (J c ) of fully-processed Bi-2212/Ag tape samples have been investigated. The results indicate that with the optimal processing conditions, at 4.2K in a field ofup to 15T, both the transport J c and the magnetization J c in the samples fabricated from the partly-reacted powder are about one order of magnitude higher than those in the samples fabricated from the fully-reacted powder. It is also shown that nearly all grains of the fully-reacted powder are superconducting Bi-2212 phase; the grains of the partly-reacted powder are composed of mainly Bi-2212 phase with a similar perovskite structure to that of the superconducting Bi-2212 phase, but not fully converted into superconducting phase.

Excitation field dependence, frequency dependence and anisotropy of the irreversibility line in MPMG YBa2Cu3O7 with Y2BaCuO5 inclusions

Most applications of superconducting materials require high critical current density. Melt-powder-melt-grown (MPMG) YBa2Cu3O7 with Y2BaCuO5 inclusions were found to have the highest critical current in bulk to date. The authors report a systematic study of the irreversible behaviour in the MPMG material. The irreversibility line was measured in external fields up to 8 T using an AC susceptibility technique. Large anisotropy of the irreversibility line for different orientations of the field was observed. The frequency dependence was found to be logarithmic in the range 10 to 105 Hz. In order to study the excitation field dependence in detail, they applied AC fields from 0.01 G up to 30 G.

Strong diamagnetic response and specific heat anomaly aboveTcin underdopedLa2−xSrxCuO4

By measuring ac susceptibility using a very low amplitude of the ac field

Step-like oscillations in the resistance of two weakly linked aluminum rings

(l1\mathrm{mG})

Increased flux pinning in both pure and carbon-nanotube-embedded Bi-2212 superconductors

it is shown that underdoped samples of