The microstructure of Nb₃Sn layers in multifilamentary composites manufactured by bronze technology and the internal-tin (IT) method differing in the strand design has been studied. The IT strands of International Thermonuclear Experimental Reactor (ITER) type and high-J_c type are characterized by both the higher Sn concentration and the higher fraction of equiaxed grains than bronze-route composites. In all the samples under study, the fraction of equiaxed grains correlates with the average concentration of Sn in Nb₃Sn layers. The Ti doping of the bronze matrix in the bronze-processed wires results in the reduction in average grain sizes of Nb₃Sn grains and in a higher fraction of equiaxed grains with a higher Sn concentration, which leads to a higher non-Cu J_c (calculated as a ratio of critical current I_c to a cross-sectional area of a strand without stabilizing Cu), 997 A/mm² at 12 T, 4.2 K, compared to a composite of the same design with artificially Ti-doped Nb filaments. The ITER type IT strand demonstrates a somewhat higher fraction of equiaxed Nb₃Sn grains with much larger average grain sizes (120 nm). The grain coarsening in this IT strand results in a decrease in J_c compared to other ITER type strands studied. The highest values of J_c are ensured in the high-J_c type of IT wires with 7 extended tin sources by the highest fraction (92%) of equiaxed Nb₃Sn grains despite their coarser sizes (92 nm).

The microstructure and thermal stability of multifiber in situ Cu-18Nb composites with a true strain ( e) of 10.2 and 12.5 have been studied by the methods of scanning and transmission electron...

The effect of diffusion annealing on the structure of superconducting layers of the Nb₃Sn phase and the extent of transformation of niobium filaments of multifilamentary Nb/Cu-Sn superconductors obtained using the bronze process has been investigated by scanning and transmission electron microscopy. The samples differed in the titanium concentration and method of alloying. It has been shown that the thickness of layers of the Nb₃Sn phase grows with an enhancement in the titanium concentration in Nb filaments and that it is composites with the alloyed bronze matrix that exhibit the most favorable structure and morphology from the viewpoint of reaching the maximum current-carrying capacity of the conductor in all of the regimes of diffusion annealing under study.

The pinning force density, $F_p$, is one of the main parameters that characterize the resilience of a superconductor to carrying a dissipative-free transport current in an applied magnetic field. Kramer (1973) and Dew-Hughes (1974) proposed a widely used scaling law for this quantity, where one of the parameters is the pinning force density maximum, $F_{p,max}$, which represents the maximal performance of a given superconductor in an applied magnetic field at a given temperature. Since the late 1970s to the present, several research groups have reported experimental data on the dependence of $F_{p,max}$ on the average grain size, $d$, in Nb₃Sn-based conductors. $F_{p,max}\left(d\right)$ datasets were analyzed and a scaling law for the dependence $\left|F_{p,max}\left(d\right)\right|=A\times ln\left(1/d\right)+B$ was proposed. Despite the fact that this scaling law is widely accepted, it has several problems; for instance, according to this law, at $T=4.2 \mathrm{K}$ and $d\ge 650 \mathrm{nm}$, Nb₃Sn should lose its superconductivity, which is in striking contrast to experiments. Here, we reanalyzed the full inventory of publicly available $F_{p,max}\left(d\right)$ data for Nb₃Sn conductors and found that the dependence can be described by the exponential law, in which the characteristic length, $\delta$, varies within a remarkably narrow range of $\delta =175\pm 13 \mathrm{nm}$ for samples fabricated using different technologies. The interpretation of this result is based on the idea that the in-field supercurrent flows within a thin surface layer (thickness of $\delta$) near grain boundary surfaces (similar to London’s law, where the self-field supercurrent flows within a thin surface layer with a thickness of the London penetration depth, $\lambda$, and the surface is a superconductor–vacuum surface). An alternative interpretation is that $\delta$ represents the characteristic length of the exponential decay flux pinning potential from the dominant defects in Nb₃Sn superconductors, which are grain boundaries.