Quantitative description of a very high critical current density Nb-Ti superconductor during its final optimization strain. II. Flux pinning mechanisms

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Abstract
Flux pinning by α-Ti precipitates has been studied in a carefully made Nb 48 wt. % Ti composite having very high critical current density and a macroscopically rather uniform precipitate array. The pinning was studied as a function of field (0–15 T) and temperature (2.3 K—Tc) for a ratio of the precipitate thickness (tppt) and the coherence length (ξ) which was varied between 10 and 0.1 by a large drawing strain. Surprisingly, the maximum bulk pinning force (Fp) occurred for tppt/2ξ≂0.1 and a precipitate spacing about one-eighth the fluxon spacing, suggesting that the flux pinning occurs at clusters of precipitates. In contrast to earlier studies, strict temperature scaling of Fp was not observed; the peak of the pinning force shifted to lower reduced fields as the temperature increased. The effect was largest for the finest scale microstructure. This nonscaling is interpreted in terms of two pinning mechanisms having different field and temperature dependencies. At low reduced fields, and at temperatures close to Tc, the core interaction (δHc) dominates. At higher reduced fields and low reduced temperatures, pinning due to variations in κ also contributes to Fp. Clusters of very fine α-Ti ribbon precipitates give rise to these variations in Hc and κ. A comparison of the specific pinning force with the core interaction model was made close to Tc, where the δHc pinning mechanism dominates. Good agreement was obtained for all precipitate thicknesses, indicating that the pinning can be quantitatively described by the core interaction and the direct summation models. No significant proximity effect suppression of the pinning force at fine precipitate dimensions was observed.