John Niska

Improvement of critical current density in the Bi-Pb-Sr-Ca-Cu-O system through hot isostatic pressing

Abstract The effects of hot isostatic pressing (HIPing) on densification and the superconducting properties of the Bi-Pb-Sr-Ca-O system were investigated. A relative density up to 95% and a critical current density greater than 1100 A/cm 2 at 77 K were achieved through HIPing at 650°C for 2 h under 200 MPa argon. Under these conditions, the product gave a value of J c four times that without HIPing. T o was unaffected by HIPing for samples encapsulated with a combination of glass and silver tubes. However, T o was suppressed from 103 K to 86 K for samples encapsulated with stainless steel and silver tubes while J c increased from 260 A/ cm 2 to 1086 A/cm 2 during HIPing, indicating that the weak links were significantly improved. A new minor phase, having a composition of Bi/Pb/Sr/Ca/Cu=0.58/2.8/3.0/2.1/1.1, was observed in HIPed samples but its effect on J c is not clear.

Formation of the 124-phase superconductor YBa2Cu4O8 by retaining oxygen in a reaction HIP sintering process

The ceramic superconductor YBa2Cu4O8 has been produced by high temperature sintering of a mixture of CuO and YBa2Cu3O7 in a glass capsule under high hydrostatic argon pressure. The resulting highly dense material is investigated by X-ray diffraction, optical and electron microscopy, resistance measurements and hardness measurements, and shown to be a homogeneous High transition temperature superconductor.

Electrical transport properties of dense bulk YBa2Cu4O8 produced by hot isostatic pressing

Abstract Highly dense sintered YBa 2 Cu 4 O 8 has been produced by hot isostatic pressing (HIP). The electrical resistivity ϱ of this material has been measured as a function of temperature T and pressure ϱ in the range 40–650 K and 0–0.7 GPa. Both the temperature dependence and the pressure dependence of ϱ are found to be well described by a model based on the standard Bloch-Gruneisen theory. It is pointed out that ϱ is liner in T only under isobaric conditions, while ϱ is strongly nonlinear in all high- T c superconductors under isochoric (constant volume) conditions. The critical current density of the material is 900 A/cm 2 at 4 K, while the resistivity is 630 μΩ cm at 294 K.

High critical currents in Bi-2223 phase bars☆

Abstract The critical current I c and the critical current density J c of Bi-2223 phase bars have been found to depend on the mass density of the bars. Bars densified using a cold isostatic pressing (CIPing) process have been obtained with transport critical currents of > 150 A with J c levels up to ≈ 10 3 A cm −2 at 77 K in the absence of an applied magnetic field. Bending or kinking of the grains may promote densification during CIPing of a sintered bar.

Measurements of the self-field limitations of the critical current in Bi-2223 superconductive bars

Abstract The problem of relatively low Ic values in large bulk specimens of the Bi-2223 superconductor has been investigated at 77K. Superconductive bars of the Bi-2223 phase with varying geometries have been prepared and tested with critical currents reaching several hundreds of amperes. The best specimens show a good correlation between the self-field generated and the critical current flowing in specimen. A self-field of about 7 A/mm appears to be a design limitation for the use of this material in the bulk form. Minimization of the self-field by the proper choice of bar geometry can be used to get high Ic and Jc specimens.

Straightened voltage effect in high‐Tc superconductors

A new effect called ‘‘ac‐current straightening’’ has been observed in ceramic (Bi,Pb)‐2223 slabs carrying ac current Idc+Iac cos(ωt). The current‐voltage (I‐V) characteristics of the ceramic were measured at 77 K at frequencies ranging from 50 to 20 000 Hz. A spectrum analyzer showed a series of high harmonics in the voltage signal as well as a constant voltage drop. The full set of experimental data has been explained theoretically using the Bean–Kim critical state model with a magnetic field dependent critical current jc(H)=jc(0)/(1+H/H0). A low transport ac current gives a voltage linearly proportional to the frequency and quadratically proportional to the ac‐current amplitude Iac. It consists of odd harmonics only. If a bias dc current is switched on, then even harmonics and a dc‐voltage drop appear. Their amplitudes are proportional to the small parameter Iac/cH0 and depend on the Idc/Iac ratio.

High-temperature X-ray diffraction of the PbBi-2223 phase superconductor in different atmospheres

Measuring of the lattice expansion of the PbBi-2223 superconductor using a modified Philips 1710 system high temperature x-ray diffractometer and three different atmospheres: a vacuum, helium, and air. The results show that the lattice expansion is sensitive to the oxygen partial pressure.

Evidence for unpinned magnetic flux vortices above theIctransition in the 2223-phase high-Tcsuperconductor

A similarity in the dc voltage-current (V-I) curves for both direct and alternating transport currents is used to propose that unpinned flux vortices are generated above theIctransition for dc transport currents, when Abrikosov flux vortices begin to penetrate the superconductor. Two methods can be used to give a dc voltage drop for an ac transport current: (1) if there is a slight dc offset voltage in the ac current which favors vortex loop collapse as in a traditional dcIctest, or (2) if an asymmetric transverse magnetic field is present which favors vortex loop collapse for current in one direction over the reverse direction.

Hot Isostatic Pressing of High Temperature Superconducting Ceramics

Large specimens of ceramic superconductors are difficult to solid-state sinter to full density, while maintaining superconductivity, without the high temperatures and pressures found in a hot isost...

The Production and Properties of Highly Dense Yba2Cu4O8 Prepared by a Reaction Hip Sintering Process

A series of HIP runs have been made in which YBa2Cu3O7 (123) and CuO have been reacted to form the YBa2Cu4O8 phase (124). The reaction kinetics are examined and a densification model proposed for this sintering process. Densification is slow below 700 °C, but nearly fully dense materials are possible above about 900 0C with a HIP pressure of 200 MPa. The 124 phase is stable even under high pressure conditions, which makes it more suitable than the 123 phase for HIP densification. The material has a low fracture toughness typical of ceramic materials, which makes elimination of flaws important to obtaining high fracture strengths.