On the applicability of the critical state model to the description of electromagnetic properties of high-Tc superconductors

Fabrication and testing of long length high-{Tc} composite conductors. Final report

“…is the magnetic induction inside a superconductor. This equation is correct in a low frequency range [3]. Indeed, if the external magnetic field changes so fast that the vortex system has no time to relax to the critical state+ the current density will exceed J, and the system may be characterized by some point on the voltage-current characteristics E ( J > Jc = 0.…”

“…AC The study of the frequency dependence of the ac magnetic susceptibility) or the surface impedance of ceramics [3,77,78] and melt-textured [79] YBCO superconductors reveals that a mistake of the determination of Jc by ac methods is not principal at frequencies below 1 kHz. It is shown how it is possible t o use the frequency measurements for the evaluation of the I -V characteristic of a superconductor.…”

“…Some characteristics of this solutions were following: -CU - 3,05 Cryoprecursor was prepared by vacuum drying in temperature interval from -190°C to +65"C with followed pyrolisis of nitrates at 660"C, 1 hour in air. To form the required phase composition, cryopowder was heat treated at 750-780°C during 4 hours in cleaned 0 2 .…”

Magnetic behavior of granular high-Tc superconductors in the weak-link regime

“…On the other hand, the current density J is given by the average of the intergranular (transport) current density and, according to the BeanÕs critical-state model, its magnitude has a critical value J c ðBÞ. The criticalstate model has been successfully applied to study both static and dynamic magnetic phenomena in granular high-temperature superconductors for a broad range of frequencies and fields [6]. In the majority of the investigations the external magnetic field is assumed to vary only in magnitude.…”

The Low-Field Ac Impedance of YBa2Cu3O7-δSuperconducting Slab

“…At high frequencies ac surface impedance measurements may be affected by the skin effect screening [13]. It is known that for high frequencies there is a skin screening, and the ac magnetic field penetrates the sample volume only to the depth δ n = 2/(µ 0 ωσ n ) (σ n -normal state conductivity) [1].…”