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

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Abstract
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.