Numerical modelling of iron-pnictide bulk superconductor magnetization

Abstract

Iron-based superconductors exhibit a number of properties attractive for applications, including low anisotropy, high upper critical magnetic fields (H c2) in excess of 90 T and intrinsic critical current densities above 1 MA cm −2 (0 T, 4.2 K). It was shown recently that bulk iron-pnictide superconducting magnets capable of trapping over 1 T (5 K) and 0.5 T (20 K) can be fabricated with fine-grain polycrystalline Ba 0.6 K 0.4 Fe 2 As 2 (Ba122). These Ba122 magnets were processed by a scalable, versatile and low-cost method using common industrial ceramic processing techniques. In this paper, a standard numerical modelling technique, based on a 2D axisymmetric finite-element model implementing the H-formulation, is used to investigate the magnetisation properties of such iron-pnictide bulk superconductors. Using the measured J c (B, T) characteristics of a small specimen taken from a bulk Ba122 sample, experimentally measured trapped fields are reproduced well for a single bulk, as well as a stack of bulks. Additionally, the influence of the geometric dimensions (thickness and diameter) on the trapped field is analysed, with a view of fabricating larger samples to increase the magnetic field available from such trapped field magnets. It is shown that, with current state-of-the-art superconducting properties, surface trapped fields >2 T could readily be achieved at 5 K (and >1 T at 20 K) with a sample of diameter 50 mm. Finally, an aspect ratio of between 1 and 1.5 for R/H (radius/thickness) would be an appropriate compromise between the accessible, surface trapped field and volume of superconducting material for bulk Ba122 magnets.

Toward Optimization of Multi-Pulse, Pulsed Field Magnetization of Bulk High-Temperature Superconductors

“…The electromagnetic properties are simulated using the 2D axisymmetric H-formulation, implemented in the commercial software package COMSOL Multiphysics 5.2a. This has been used previously by the authors to simulate bulk hightemperature superconductors under various magnetisation conditions [14]- [18], as well as FC magnetisation of MgB2 [19] and iron-pnictide [20] bulks.…”

“…Model #2 (ZFC) model is similar to that presented in [15], [20], where isothermal conditions are assumed because the magnetisation process is slow (uniform applied magnetic field with a ramp rate of 50 mT/s). The finite n value used (20) accounts for flux creep relaxation, resulting in a logarithmic decay of the trapped field after the field is removed.…”

“…Model #2 (ZFC) model is similar to that presented in [15], [20], where isothermal conditions are assumed because the magnetisation process is slow (uniform applied magnetic field with a ramp rate of 50 mT/s). The finite n value used (20) accounts for flux creep relaxation, resulting in a logarithmic decay of the trapped field after the field is removed. There is good consistency between the FC and ZFC(t = 0 min) results, as should be expected [30], and trapped fields of around 80% of this is not an uncommon observation in practical experiments after waiting for flux creep relaxation.…”

“…In this paper, a 2D axisymmetric finite element method based on the H-formulation, previously utilised by the authors for single pulse investigations [14]- [20], is extended to the case of multi-pulse PFM. Firstly, the maximum trapped field capability of a representative bulk sample is estimated using two types of numerical model simulating FC/ZFC magnetisation techniques.…”

An Explanation for Observed Flux Creep in Opposite Direction to Lorentz Force in Partially Magnetized Bulk Superconductors

“…The numerical model used in this paper has been used previously to simulate bulk high-temperature superconducting (HTS) materials under various magnetization conditions [5]- [9], as well as field-cooling (FC) magnetization of MgB 2 [10] and iron-pnictide [11] bulks. The governing equations for the electromagnetic properties of the of the bulk superconductor are derived from Maxwell's equations -namely, Faraday's (1) and Ampere's (2) laws:…”