Nanocrystalline superconducting islands of Nb3Ge in a resistive grain boundary matrix with a bulk critical temperature of 12 K

We report on nanocrystalline Nb3Ge (A15) produced by hot isostatic pressing and annealing of milled material with a bulk critical temperature and bulk upper critical field of ∼12 K and ∼15 T respectively. Optimum macroscopic bulk critical values were only produced in a narrow window of processing parameters: mechanically alloyed Nb0.75 + Ge0.25 milled for 6 or 20 h was processed in a hot isostatic press for 5 h operating at 2000 bar and 600 °C followed by annealing at 700 °C for 18 h. XRD data suggest that the crystalline components of both the optimum and non-optimum materials all have high Ge content, characteristic of Nb3Ge material with a TC of 15-17 K. In a small number of samples, very small zero-field screening signals were found at ∼17 K that collapsed if a magnetic field of 0.5 T was applied. In contrast, high bulk-screening superconducting critical temperatures were accompanied by high bulk upper critical fields. We found that completely isolated nanocrystalline grains of Nb3Ge would produce signals below the sensitivity of our commercial AC susceptometer and conclude that the nanocrystalline Nb3Ge materials produced in this work with the highest superconducting critical properties consist of very weakly coupled islands of nanocrystalline material with critical temperatures up to ∼17 K in a resistive grain boundary matrix which strongly couples the islands at ∼12 K. We do not attribute the record critical bulk values obtained for Nb3Ge, fabricated using solid-state processing alone, to improving the bulk crystalline properties but to improving the grain boundaries. Optimized nanocrystalline metallic superconductors produced by milling, HIP'ing and annealing have similarities with optimized high temperature and pnictide superconductors where improving and understanding the properties of the grain boundaries are key for improving their high field properties.