Effects of niobium concentration on the critical current density and hysteresis loss of powder-metallurgy processed Nb3Sn superconducting wires

We have developed Nb3Sn superconducting wires for AC use fabricated by a powder-metallurgy process. Wires with niobium concentrations from 5 to 22.5% were produced and the effects of niobium concentration on the critical current density (J(c)) and the hysteresis losses of these wires were investigated. Assuming that J(c) is proportional to both the volume fraction of the superconducting filaments and the frequency of contact among superconducting filaments, J(c) should be proportional to the square of the niobium concentration. However, it was proportional in the power of 1.5 for the present wires. J(c) of the central region of high niobium concentration wire was relatively low, resulting from less tin diffusion to the central region due to the blocking by niobium filaments distributed near the surface. This effect may be responsible for reducing the power from 2 to 1.5. The hysteresis loss was proportional to the niobium concentration in the power of 2. This power relation can be explained by the fact that the hysteresis loss is proportional to the product of J(c) and the effective filament diameter, which are proportional to the niobium concentration in the powers of 1.5 and 0.5, respectively.