Improving critical current density of Nb₃Sn by optimizing pinning potential of grain boundary and grain size

Abstract

Nb3Sn superconductor is of significant interest for applications in constructing high-field magnets beyond the limit of NbTi. However, its critical current density decreases rapidly at high magnetic fields ( > 12 T) and the state-of-the-art level of Nb3Sn superconductors still cannot meet the requirements of the planned future accelerator magnets. The primary flux pinning centers for Nb3Sn wire mainly arise from the grain boundaries (GBs). In the present paper, we theoretically investigate, through time-dependent Ginzburg-Landau theory and with graphics-processing unit parallel technique, the vortex pinning and the critical current density in large-scale polycrystalline Nb3Sn superconductor by varying the pinning potential of GB and grain size at various magnetic fields. Unlike the conventional dot-like pinning systems, it is found that the critical current is not a monotonous function by suppressing the superconductivity of the GBs. The optimal pinning potential of GB for maximum critical current density strongly depends on magnetic fields. Furthermore, we find that the critical current density can be significantly enhanced by reducing grain size at low magnetic fields, while increase of critical current density cannot always be observed at high magnetic fields. Actually, critical current density even decreases by reducing grain size, which depends on the superconductivity of GBs. The findings in the paper provide theoretical foundations to achieve further improvement of Nb3Sn with optimizing the flux pinning.

Numerical simulation of mechanical behaviors and intergranular fracture of polycrystalline Nb3Sn and superconducting filaments

“…Grain boundaries have their own properties and act as microstructural discontinuities, separating the grains. Grain boundary structures are of particular importance because most technological superconductors are polycrystalline and the grain boundary defects strongly affect magnetic flux distribution and limit J c [44,45]. The superconductivity group of Durham University has constructed a framework for analytical and computational the critical current of two-dimensional (2D) and three-dimensional (3D) polycrystalline systems with grain boundaries based on the time-dependent Ginzburg-Landau theory [46][47][48][49][50].…”

Role of grain boundary networks in vortex motion in superconducting films

“…[26] The investigations on random GB networks with complex orientations which can model polycrystalline materials also play a crucial role in gaining a deeper knowledge about grain boundary mechanism on vortex motion. [27,28] In the present work, the GB networks are generated in the coordinates of the nuclei of crystals by using Voronoi tessellation. In order to study the vortex dynamics in superconducting films, the GB is assumed to be composed of uniformly distributed special sites and an attraction well formed by the local electric fields.…”