A. Morawski

X-ray single-crystal structure refinement of the 129 K superconductor HgxPb1−xBa2Ca3Cu4O10+δ

Abstract Crystals of Hg 0.8 Pb 0.2 Ba 2 Ca 3 Cu 4 O 10.14 were measured on an X-ray four-circle single-crystal diffractometer and the crystal structure was refined to R = 0.023. The space group is P4/mmm and the lattice parameters are a = 3.8530(5) A , and c = 18.968(3) A . Lead is incorporated at the Hg site, but it is shifted slightly from the origin in the z = 0 plane. A model of Pb 4+ clusters in the basal plane is proposed, which is in agreement with recent TEM results. A reduction of electron density at the Hg site is attributed to the substitution by Cu or CO 3 groups. Stacking faults of 1223 or 1245 material are manifested in the Fourier maps, and it was possible to refine the amount to 3.8%. However, these stacking faults induce additional electron density at the O(4) site ( 1 2 , 1 2 0 ), and cause an overestimation of the excess oxygen content, if it is not corrected carefully. The bondlengths are distances are in agreement with those of other members of the Hg-12( n −1) n series, and show systematic changes with increasing number n of CuO 2 layers. The effect of stacking faults on the diffraction of all members of the Hg-12( n −1) n family is discussed, and a method for their determination is given.

Arsenic antisite defects as the main electron traps in plastically deformed GaAs

It is found from DLTS measurements that plastic deformation of GaAs single crystal creates a new kind of electron traps with an activation energy of 0.37 eV, and gives rise to an increase in the concentration of main electron traps with an energy of 0.80 eV. By comparing the concentrations of the main electron traps before and after deformation with analogous concentrations of AsGa paramagnetic centers, found by EPR experiments, it is concluded that the centers observed in both cases are of the same origin. A nonstandard feature of the main traps is discovered: linear dependence of the DLTS-peak amplitude on the logarithm of the filling-pulse duration time. This feature can be explained in terms of the barrier-limited capture rate, assuming the traps are arranged in rows.

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 influence of HIP on the homogeneity, Jc, Birr, Tc and Fp in MgB2 wires

Unreacted MgB2 wires were made at Hyper Tech Research, USA by a continuous tube forming and filling method using mixtures of Mg and B with and without SiC powder additions. All of the wires underwent hot isostatic pressure (HIP) treatment at the Institute of High Pressure. The first part of the wire was annealed at a pressure of 1 GPa, and the second part was annealed at 0.1 MPa. In this work, we show the influence of high pressure on critical current density (Jc), pinning force (Fp), critical temperature (Tc), irreversible magnetic fields (Birr) and the Fp scaling and microstructure of MgB2 wires. The results obtained indicate that after annealing at high pressure, the MgB2 wires show increases of Jc and Fp in high magnetic fields (8 T–12 T); in SiC doped MgB2 wires, Fpmax shifts to higher magnetic fields. We also compared the Jc of the doped and undoped MgB2 wires (without HIP and with HIP). The scanning electron microscope (SEM) results show that HIP increases the density of MgB2 material and improves its uniformity.

Hot isostatic pressing of multifilamentary MgB2 wires in solid state media for large scale application

Multifilamentary MgB2 wires were annealed under the high pressure of low-friction, solid state media such as BN and graphite powders. The idea was to replace the hot isostatic pressing in argon medium, which is beneficial to wires transport properties but is difficult for application on an industrial scale. The experimental results show that annealing in BN or graphite under 200 MPa leads to enhancement of the critical current, similar to that observed earlier for high pressure argon annealing. An unwanted effect of the process is the flattening of the round wires, which after the treatment yields an elliptical cross section with b to a ratio of around 85 ± 5%. Comparison of critical currents and microstructure of samples synthesized with different parameters are presented.

Superconducting Properties Comparison of SiC Doped Multifilamentary

Wires with 6 filaments of cores surrounded by a Nb barrier, and Cu, Monel and Glidcop sheath, drawn by Hyper Tech Inc. to diameters of 0.83 mm, have been treated in a Hot Isostatic Pressure process under high Ar pressures. The pressure of the HIP varied in different processes in order to find the optimal sintering conditions to obtain the highest at high magnetic fields. Also a stainless steel sheathed MgB2 wire has been used for comparison. The effect of sintering under high pressures on transport properties of those wires was measured by means of critical current and pinning forces Fp. The samples of 70 and 20 mm length were measured in parallel and perpendicular magnetic fields, respectively. High magnetic fields, of up to 14 T, were obtained in a Bitter magnet. The microstructure of annealed samples was investigated by SEM analysis.

{\rm MgB}_{2}

Abstract Crystals of Hg0.5Pb0.5Ba2Ca4Cu5O12+δ were measured on an X-ray four-circle single-crystal diffractometer and the crystal structure was refined to R = 0.033. The onset of the superconducting transition of the crystals ranges from 101 K-115 K. The space group is P4/mmm and lattice parameters of the five-layer material are a = 3.8529(3) A, and c = 22.172(2) A . Hg is partially substituted for Pb and heavy atoms are shifted away from the origin by ∼0.38 A . Stacking faults of 1234 or 1256 material are manifest in the Fourier maps, and the amount was refined to 4.4%. These stacking faults induce an additional electron density at the ideal O(4) site and mask a possible oxygen occupancy at ( 1 2 , 1 2 , 0 ). However, electron density is observed at a distance of 0.6 A from ( 1 2 , 1 2 , 0 ), suggesting a shift of the excess oxygen atom from its ideal position. Bondlengths and distances show systematic changes with increasing number n of CuO2 layers. Changes occur mainly in the rock-salt type layer, whereas the CaCuO2 units remain stable over the whole H-12(n−1)n series.

Wires of Various Sheaths (Cu, Monel, Glidcop) After High Pressure HIP Treatment

Abstract A comparison of fast-neutron irradiation effects on the irreversibility line (IL) and the intragrain critical current densities ( J c ) in the 90 K and the 60 K phase of YBa 2 Cu 3 O 7- x (123) and in YBa 2 Cu 4 O 8 (124) ceramics is presented. The 124 compound shows a transition into the reversible regime at distinctly lower temperatures and fields than the 90 K phase of 123. However, the IL of 124 lies above the one of the 60 K phase. Upon irradiation we observe a pronounced shift of the position of the IL to higher fields and temperatures for the 60 K phase of 123 and for the 124 compound. The IL remains almost unchanged in the 90 K phase of 123 at the relatively low fluences employed so far. We also observe an increase of J c after irradiation. The values of the J c enhancement as well as their temperature and field dependence vary considerably among these compounds.

X-ray single crystal structure analysis of the 1245 type superconductor Hg0.5Pb0.5Ba2Ca4Cu5O12+δ

Unreacted MgB2 wires fabricated from SiC-doped precursor material by Hyper Tech Research, Inc. have been used to make small-diameter (14 mm) superconducting coils. All coils made of 500 mm length wires were subjected to hot isostatic pressure (HIP) treatment. The critical current density (Jc) parameters of coils were compared to straight samples characteristics. Both types of samples have been measured in perpendicular magnetic field configuration for Jc and pinning force density (Fp) evaluation. No significant Jc difference between the long wires on coils and straight wires was found. These results suggest that the critical current (Ic) for coils can be determined for straight samples (25 mm). SEM analysis indicated that a small diameter of the coil does not influence the structure of in situ MgB2 wire. (Some figures may appear in colour only in the online journal)

Comparison of fast-neutron irradiation effects in YBa2Cu3O7-x (123) and YBa2Cu4O8 (124) ceramics

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.