Effect of initial powder size on the synthesis, microstructure and superconducting properties of Nb₃Sn superconductor

Powder metallurgy process is one of the common methods for synthesizing superconducting phases and preparing superconducting wires, in which the initial particle size of the powder has an important impact on the superconducting properties. However, there are few reports regarding the effect of powder size on the preparation of Nb₃Sn superconductor. Therefore, in this work, based on the composition system of Nb:Sn:Cu = 3:1:5 (at.%), the effect of Nb, Sn and Cu powder size on the synthesis, microstructure and superconducting properties of Nb₃Sn superconductor were systematically investigated by powder metallurgy method. The results showed that the synthesis of Nb₃Sn superconducting phase could be greatly affected by the particle size of Nb powder, whereas Cu and Sn powder had negligible effect. Moreover, the microstructure and grain morphology of bulk samples could also be affected by the powder size of Nb. Specifically, reducing the powder size of Nb could improve the reaction efficiency and promote the formation Nb₃Sn layer with a network structure. However, further reduction of powder size of Nb would lead to a deficiency in Sn and the growth of columnar grains. In addition, the bulk sample with the Nb powder size of about 3 μm exhibited the highest value of critical current density (J_c) and reached 1.2 × 10⁸ A/m² at 12 T, which could be attributed to the equiaxed grain morphology, small average grain size (65 nm) and the flux pinning center possibly from additional surface pinning at the nanopores in Nb₃Sn layer.