M.F. Carrasco

Electrical field freezing effect on laser floating zone (LFZ)-grown Bi2Sr2Ca2Cu4O11 superconducting fibres

The electrical assisted laser floating zone (EALFZ) solidification process makes the tailoring of fibre microstructures possible. The application of a dc electrical current of during the solidification process of Bi2Sr2Ca2Cu4O11 nominal composition fibres strongly modified phase development, crystal shapes and effective distribution coefficients. Growth conditions with the solidification interface positively polarized deviate the system from metastability, leading to the development of the equilibrium cuprate (SrxCa1?x)14Cu24O41?(14/24) as primary phase dendrites. Compared to the morphology of the SrxCa1?xCuO2?(1/1) primary crystals of the conventional LFZ process, the 14/24 crystals are aligned higher along the fibre axis, with half the thickness and twice the extension. One of the major effects of EALFZ is the control of the effective distribution coefficients, k. At equal values of the fibre pulling rate, R, the copper partition between the liquid and the solid is the most affected, the kCu increasing from 1 to 1.22 due to the ionic drift from the zone melt to the negative polarized feed rod. Bismuth and calcium effective distribution coefficients present the lowest values (kBi = 0.69 and kCa = 1.13) in these conditions, according to the field-modified BPS theory. When the reverse current is applied, the dendritic morphology disappears and a globular structure of completely new phases develops.

Pulling rate and current intensity competition in an electrically assisted laser floating zone

Bi?Sr?Ca?Cu?O (BSCCO) superconducting fibres were grown using the laser floating zone technique assisted by a dc electrical current (electrically assisted laser floating zone?EALFZ process). The simultaneous effect of pulling rate and applied current intensity was evaluated on the premises of the: (i) elemental partition coefficient between solid and molten zone; (ii) phase development; (iii) crystal size and alignment degree of the primary and superconducting phases; (iv) phase transformation during heat treatment and (v) superconducting properties. The highest critical current density value (JC = 2240?A?cm?2) was obtained for the fibres pulled at intermediate pulling rates (75?mm?h?1) grown under a maximum dc electric current intensity (200?mA).

Bi-Sr-Ca-Cu-O superconducting fibres processed by the laser floating zone technique under different electrical current intensities

The effect of the intensity of the direct electrical current on the morphology and properties of superconducting BSCCO fibres grown by the electrically assisted laser floating zone (EALFZ) technique is presented. Stereological analysis and x-ray diffraction (XRD) pole plots reveal textural enhancement and crystal refinement as the main features when the current intensity increases. Analysis with scanning electron microscopy/energy dispersive x-ray spectroscopy also discloses changes in the phase fraction and nature. Improvement of the grain alignment is a result of the intensification of ionic migration in the solid?melt interface along the fibre axis. This leads to a net increase in the length of primary dendrites and a reduction in dendrite thickness and interdendritic spacing. The increase in current intensity also favours the vanishing of the 2201 phase and of the residual melt. The development of the 2223 phase during isothermal annealing is very pronounced for the EALFZ fibres grown under high current intensities due to the refinement of the reactant phases. The current transport capability after isothermal annealing of fibres is improved with application of a low current intensity (50?mA), reflecting a net change of the morphological characteristics. However, when a high current intensity (200?mA) is applied the unfavourable orientation of the newly formed 2223 crystals and the loss of crystallite quality of the 2212 phase due to intergrowth of 2223 structure in the original 2212 structure lead to poor superconducting properties.

The effect of current direction on superconducting properties of BSCCO fibres grown by an electrically assisted laser floating zone process

The application of an electric current of 200 mA through the molten zone of BSCCO superconducting fibres during laser floating zone processing constitutes an upgrade for improving the grain alignment. When a direct electric current (positively polarized fibre) passes through the solidification interface, the solidification conditions approach equilibrium, favouring the development of a higher amount of 2212 and 14/24 stable phases. The new electrically assisted laser floating zone (EALFZ) technique also improves the 2223 phase formation in fibres heat treated at high temperatures (860 °C). However, the 2223 crystals grow perpendicularly to the fibre axis at the interface between the 2212 and 14/24 phases, crossing the crystals of the main phase responsible for the current transport, cancelling the alignment effect. The resultant current density and critical temperature values were Jc77 K = 230 A cm−2 and Tc = 85 K, respectively. When a lower heat treatment temperature was accomplished (820 °C), the 2223 transverse crystals do not develop and a higher current density value of Jc77 K = 510 A cm−2 was achieved, although with a critical temperature of Tc = 90 K.

The Effect of Annealing Temperature on the Transport Properties of BSCCO Fibres Grown by LFZ and EALFZ

Superconducting fibres of Bi-Sr-Ca-Cu-O system were grown by the conventional Laser Floating Zone (LFZ) process and by the new technique of Electrical Assisted Laser Floating Zone (EALFZ). The presence of an electric field during solidification process significantly changes the fibre microstructure with Bi2Sr2Ca2Cu3O10 nominal composition. The higher cristallinity of the asgrown EALFZ fibres, resulting from the current application during solidification, leads to a transformation rate enhancement during subsequent annealing. A strong correlation between the transport properties and the final microstructure of the annealed fibres is observed. The highest critical current density of the EALFZ fibres is obtained after heat treatment at 845°C while for the LFZ ones similar values are obtained only after annealing at 860°C.

Preparation and Properties of New Superconductor Material MgB2

Bulk MgB2 samples were synthesized by hot isostatic pressing under pressures up to 200MPa at 950°C. In these conditions, full densification of samples was obtained (~98% of theoretical density). SEM, EDS and XRD analysis on final dense bodies were used to evaluate samples, and show increasingly better control over the amounts of secondary MgO (down to ~10%) and complete prevention of formation of MgB4 by using simple glass encapsulation techniques and addition of Mg(s) to the capsule. The samples display superconducting properties, including a narrow critical transition in electrical properties (Tc ~36-38K). Magnetic studies were performed, allowing the determination of the superconducting fraction and critical current density Jc of the materials. Contrary to the Tc, the Jc is quite sensitive to the processing and microstructure and values from 0.3 to 0.6x106 A/cm2 are obtained at 10K. The reduction of Jc with the applied magnetic field requires further improvements to reduce weak links.