W. Pachla

The role of MgO content in ex situ MgB2 wires

An experimental study of the effect of MgO content in the MgB2 powder used for ex situ made composite wires was carried out. Two single-core MgB2/Fe/Cu wires were made using commercial MgB2 powders from Alfa Aesar containing different fraction of MgO. Critical temperature and critical currents of as-deformed and heat-treated wires were measured. The differences between the wires are discussed and correlated with the MgO content. It was found that by increasing the amount of MgO, the inter-grain connectivity worsens, but well distributed and low size MgO particles improve flux pinning.

Fabrication of high strength nanostructured aluminium alloys by hydrostatic extrusion

Abstract In the present work, the possibility of obtaining high strength nanostructured aluminium alloys by hydrostatic extrusion has been shown. Billets of 2017 aluminium alloy were subjected to hydrostatic extrusion either immediately after water quenching or after subsequent ageing. The results have shown that processing by hydrostatic extrusion offers a possibility of grain refinement down to the nanometre scale in the age-hardenable aluminium alloy. However, the effectiveness of the process depends on its initial microstructure. The most promising results are obtained if the alloy is deformed in the as-quenched condition. The hydrostatically extruded aluminium alloy exhibits very high tensile strength with reasonable ductility. The stability of mechanical properties over a wide range of temperatures (from cryogenic to well above the ambient) is discussed. The results obtained are compared to similar 2XXX aluminium alloys processed by other severe plastic deformation methods.

Microhardness profiles in BSCCO/Ag composites made by various technological steps

The Vickers microhardness of a BSCCO core in Ag-sheathed composites made by various deforming processes such as drawing, cold and hot hydrostatic-extrusion, rolling, and uniaxial pressing have been measured. Because the hardness of a material, defined as the resistance to indentation, can be simply measured, it is used for density indication in the ceramic core. Complex microhardness profiles in BSCCO core cross-sections have been compared in composites deformed by the mentioned processes. These results give information on how the applied deforming process can effect the ceramic core density as well as its homogeneity and so influence the critical current density and current distribution in BSCCO/Ag superconducting tapes.

Structure, grain connectivity and pinning of as-deformed commercial MgB2 powder in Cu and Fe/Cu sheaths

Single-core MgB2 wires and tapes have been made by the powder-in-tube (PIT) method using commercial MgB2 powder (Alfa Aesar). Composites have been made using the two-axial rolling process in Cu and/or Fe/Cu sheaths. Alternative deformations by wire drawing, rotary swaging and cold isostatic pressing have been applied to PIT wires and tapes. Current–voltage characteristics and transport current densities in the self-field and in the external field were measured. It was found that the grain connectivity of ex situ MgB2 is affected by the applied sheath and the mode of deformation. Two-axial rolling has generated the highest powder density resulting in the best grain connectivity. The highest transport current densities of 8700 A cm−2 and 55 830 A cm−2 were measured for Cu and Fe/Cu sheathed square wires, respectively. Cold isostatic pressing at 1.5 GPa has increased current density and n-exponent, which suggests an improvement in grain connectivity. It was found that the external pressure improves the inter-grain connectivity but decreases the pinning in MgB2 cores.

The effect of fabrication pressure on critical transport current density in press-sinter processing of Bi(2223)Ag tapes

Critical current at 77 K and microhardness measurements were used to study the effect of pressure in uniaxial cold pressing and a press-sinter schedule in Ag-clad Bi(2223) tapes. Three types of tape containing different 2223/2212 phase ratios were prepared by the same powder in tube process and subsequently pressed and sintered in several cycles with rising, constant, and declining pressure. The sintering was performed at constant temperature and time interval in air. The effect of uniaxial cold pressing on critical current density at 77 K is discussed with the aim of finding out its optimal value in every press-sinter state.

The influence of hydrostatic extrusion on the properties of an austenitic stainless steel

The paper presents the results of investigations into the mechanical properties and tribological characteristics of 316 LVM processed by hydrostatic extrusion (HE). The mechanical properties were characterized by microhardness measurements and compression tests. The wear properties were investigated using a pin-on-disc tribometer under dry and lubricated conditions. The friction coefficient was measured as a function of the time of the wear test. The results indicate that the hydrostatic extrusion process significantly improves the mechanical properties and the wear resistance of 316 LVM stainless steel. The results are discussed in terms of the microstructural changes induced during processing by hydrostatic extrusion.

Effects of the high-pressure treatment of ex situ MgB2 superconductors

Single-core MgB2/Cu and MgB2/Fe/Cu wires and tapes have been made by the powder-in-tube method with the use of commercial MgB2 powder (Alfa Aesar) involving hydrostatic extrusion and two-axial rolling processes. High-pressure post-treatments such as cold isostatic pressing, double hydrostatic extrusion and uniaxial pressing were applied. The grain connectivity, Ic(B) curves, pinning in MgB2 grains, upper critical field, deformation homogeneity, compact density and preferable alignment crystallites for the [001] (c-axis) direction were all improved and enhanced by application of various high-pressure treatments for the MgB2 compound and composites. Hydrostatically extruded MgB2 wires have been shown to be better than rolled wires and are very promising for possible application in windings.

Electrical and mechanical properties of Bi-2223/Ag/barrier/Ag composite tapes

Bi-2223/Ag/barrier/Ag single-core tapes with various oxide barrier materials (BaZrO3 , SrCO3 , ZrO2 and Al2 O3 ) have been prepared by PIT. The I -V curves and ac susceptibility measurements have been performed for tapes with identical heat treatment history. Youngs moduli of these tape samples have been evaluated by three-point bending and the structure of barrier powders and BSCCO cores was analysed by SEM and XRD, respectively. It was found that the shape of I -V curves, the current transfer lengths and the mechanical properties of tapes are all affected by the oxide barrier type and the barrier thickness, as well as by its porosity and uniformity. This is because the oxide barrier controls the oxygen diffusion during the tape heat treatment and simultaneously the HTS phase formation kinetics, its purity and content within the superconducting core. On the base of the results obtained for single-core tapes, two kinds of multifilamentary composite were made and tested.

Hot pressing of Bi(Pb)-Sr-Ca-Cu-O superconducting pellets

Dense and 2223 single-phase bulk bars of approximately 0.3 mm2 cross-section were fabricated by uniaxial hot pressing. Effects of preliminary annealing and intermediate grinding on the properties of hot pressed bars are discussed. Optimal structure and physical properties were obtained for preliminary annealing of duration 100-120 h and subsequent hot pressing under a pressure of approximately=25 MPa, and 80 h at a temperature of 850 degrees C for both processes, that resulted in highest contact measured value of Jc=11500 A cm-2 at 77 K and 0 T.

Thermomechanical treatment, structure and transport currents in multicore Bi(2223)/Ag tapes

Abstract A set of 19 filament Bi(2223)/Ag tapes differing in thermomechanical treatment were analyzed structurally (XRD, SEM and TEM) and electrically by transport I c measurement in self field, in external magnetic field up to 1.2 T, and by I c -angular dependence, all at 77 K. The results obtained show that the optimal state for the application of a roll-sinter step is at the 2223/2212 phase ratio near to 1 (50% of 2223). The structural results (2223 phase content, grains morphology and texture) were studied, and they were related to the transport currents. I c (0 T), I c ( B ) and I c ( α ) as well as the n -exponent of current–voltage characteristics and its relation to I c are studied and discussed in connection with the structure induced by the thermomechanical treatment applied.