Electrical and thermal transport properties of superconducting materials relevant for applications

Abstract

Superconducting materials are used in several applications among which the large-scale ones take advantage of the high energy transport capabilities of superconductors due to the low losses in electrical conduction. From a practical point of view, it is necessary to achieve the highest possible current density values without dissipation, thus the critical current density Jc has to be increased. Indeed, once the biasing current density increases above the critical value, the transition of the material to the normal state, referred also as a quench event, can be observed. Often this transition is not gradual. Thus, an important feature to consider is the voltage stability under current biasing in the dissipative regime. In particular, if the Flux-Flow Instability (FFI) phenomenon is triggered, the transition in the current-voltage (I-V) results in a sudden voltage jump. Electrical and thermal stability is strictly demanding in order to avoid quench phenomena or to adopt protection strategies. In a quench phenomenon, once a normal zone has started to grow, it will continue to expand under the combined actions of heat conduction and ohmic heating at a constant velocity referred as normal zone propagation velocity (NZPV). Thus, NZPV evaluation plays a key role in the protection strategies against quench-induced damages. NZPV depends mainly on electrical and thermal properties of the various components of the technical superconductor, which is made in composite form, containing both superconducting, conducting and insulating materials. Heat transfer through the conductor is the main channel for removing heat form the superconducting medium and, consequently, preventing a quench. In this work it is reported the study of the electrical transport properties of superconducting materials relevant for applications. In particular, it has been focused on pinning properties and voltage stability of superconducting materials and on electrical and thermal characterization of technical superconductors. Electrical characterization of Fe(Se,Te) thin films grown on CaF2 substrate has been performed in order to investigate the pinning properties and the voltage stability aspects in view of tapes manufacturing. Electrical and thermal properties of Bi-2212 technical superconductors have been investigated, in order to obtain information about the key factors for the quench analysis, i.e. electrical and thermal conductances. Regardless of the superconducting material, an accurate study based on avoiding quench by means of a deep investigation of electrical and thermal transport properties becomes relevant for applications. [edited by author]