Analysis of the strain dependence of the superconducting critical properties of single-crystal and polycrystalline Nb₃Sn

Abstract

The aim of this article is to present a three-dimensional (3D) simulation based on Voronoi tessellation, used to assess the strain dependence of the superconducting critical properties of inhomogeneous polycrystalline Nb3Sn at the microstructural level. To support this objective, we first present a polycrystalline model with equiaxed grains used to analyze the relationship between the polycrystalline structure and the strain response. We analyze the Young’s moduli of Nb3Sn at different temperatures with random and preferential polycrystalline orientations, and the relation between the Young’s modulus of polycrystalline Nb3Sn and spontaneous tetragonal transformation is discussed. The results indicate that the elastic properties (Young’s modulus, stress/strain field) of polycrystalline Nb3Sn are mainly associated with ambient temperature and phase transition and are affected by the grain orientation. Meanwhile, an estimation-based model for the superconducting critical properties of single-crystal Nb3Sn is proposed. This model is based on changes in the electron density of states and phonon frequency induced by strain. The results calculated by the estimation-based model are consistent with the experimental data and the values calculated according to first principles. The change in the strain-induced superconducting properties of polycrystalline Nb3Sn is analyzed by combining the numerically simulated strain field of the polycrystal with the estimation-based model of single-crystal Nb3Sn. Compared to ideal single-crystal Nb3Sn, the superconducting critical temperature of the polycrystalline Nb3Sn superconductor has a higher strain sensitivity, which is related to the non-uniformity of the strain field of the polycrystal.

Development of a prototype superconducting radio-frequency cavity for conduction-cooled accelerators

“…Similar results were measured in a 1.5 GHz Cu/Nb/Nb 3 Sn single-cell cavity [5]. The superconducting properties of Nb 3 Sn are very sensitive to strain [65][66][67] and additional stress in the film occurs after depositing the thick Cu outer layer, because of the differential thermal contraction between Cu and the Nb substrate. Since the residual stresses in the film after formation on the Nb substrate are unknown, it is difficult to estimate the absolute strain value in the film after deposition of the Cu layer and cool-down to 4 K. If one neglects any initial residual stress, the maximum strain in the film from Fig.…”

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

“…From figures 6(a)-(c) and 7(a)-(c), at the elastic deformation stage, due to the high stress concentration at the grain boundaries caused by the elastic anisotropy of the crystal, some grain boundaries have been damaged to varying degrees (SDEG > 0). These results seem to explain why the axial tensile strain sensitivity of the superconducting critical properties calculated by numerical simulation [51] and ab initio method [15] under the elastic assumption are lower than the experimental measurements. The reason is that the grain boundary damage in the elastic stage is not considered.…”

“…Within this framework, the flux pinning effect can be visualized and quantified, and the in-field J c can be simulated. On the microscopic scale mechanics, the isotropic mechanical constitutive modelling of Nb 3 Sn no longer applies and the elastic anisotropy of single crystals must be considered [51,52]. During the deformation of polycrystalline materials [53,54], the elastic anisotropy of single crystals and the deformation constraint caused by adjacent grains with different crystallographic orientations induce large stress near the grain boundaries.…”

“…Therefore, microstructural heterogeneities and the anisotropic nature of grains have a remarkable impact on the breakage of Nb 3 Sn. In our previous paper [51], the effect of the inhomogeneous strain on superconducting critical properties of polycrystalline Nb 3 Sn during elastic deformation was investigated by using 3D finite element (FE) polycrystalline microstructure models. Moreover, since the intergranular fracture strength of polycrystalline Nb 3 Sn is much lower than the fracture strength of single grains [55], the grain boundaries become the crack initiation site and fracture path of the microcracks.…”

Coexistence of superconductivity and charge density wave instability in A15-Nb3Sn

“…experimental result 33,50 and other first-principles calculations. 13,61 With increasing pressure, a reduction of l accompanied by a decrease in the electronic DOS near E F is reported. The calculated phonon dispersion curves, total density of phonon states, Eliashberg spectral function a 2 F, and frequency-dependent integrated EPC l of Nb 3 Sn under 0 GPa are presented in Fig.…”