High critical current densities found for thick Nb12Al3Ge superconducting deposits made by high-rate sputtering

Superconducting critical current densities up to 5.2 × 105 A/cm2 at 75 kOe and 4.2°K, and up to 4.4 × 105 A/cm2 at 100 kOe were measured for a 66-μm-thick Nb12Al3Ge deposit made at a rate of 1 μm/min by high-rate sputtering. Current-density-vs-transverse-magnetic-field curves were either linear or nearly linear, and the current density dropped only 33% over the field range investigated (50–100 kOe). The sample that carried the highest current densities was deposited at 20°C and heat treated one day at 750°C. Current densities for another sample from the same deposit, but heat treated 20 days at 750°C, were about 14% lower than for the sample heat treated one day. The deposits were body-centered-cubic (bcc) when deposited at 20°C. Heat treatment at 750°C produced a single metastable A-15 phase containing grains of 350-Å diameter; the small grain size presumably accounted for the high current density. Current densities measured with the field perpendicular to the deposit plane were only about 14% lower than with the field parallel to the deposit plane, which indicated the A-15 phase grains formed by heat treatment at 750°C were nearly equiaxed. The high current density results and the availability of high-rate sputtering equipment encourage fabrication of practical quantities of sputter-deposited Nb–Al–Ge superconducting products. Estimated fabrication rates and costs were reasonable.