Sample and length-dependent variability of 77 and 4.2 K properties in nominally identical RE123 coated conductors

We present a broad study by multiple techniques of the critical current and critical current density of a small but representative set of nominally identical commercial RE123 (REBa2Cu3O7-delta, RE = rare Earth, here Y and Gd) coated conductors (CC) recently fabricated by SuperPower Inc. to the same nominal high pinning specification with BaZrO3 and RE2O3 nanoprecipitate pinning centers. With high-field low-temperature applications to magnet technology in mind, we address the nature of their tape-to-tape variations and length-wise I-c inhomogeneities by measurements on a scale of about 2 cm rather than the 5 m scale normally supplied by the vendor and address the question of whether these variations have their origin in cross-sectional or in vortex pinning variations. Our principal method has been a continuous measurement transport critical current tool (YateStar) that applies about 0.5 T perpendicular and parallel to the tape at 77 K, thus allowing variations of c-axis and ab-plane properties to be clearly distinguished in the temperature and field regime where strong pinning defects are obvious. We also find such infield measurements at 77 K to be more valuable in predicting 4.2 K, high-field properties than self-field, 77 K properties because the pinning centers controlling 77 K performance play a decisive role in introducing point defects that also add strongly to J(c) at 4.2 K. We find that the dominant source of I-c variation is due to pinning center fluctuations that control J(c), rather than to production defects that locally reduce the active cross-section. Given the 5-10 nm scale of these pinning centers, it appears that the route to greater I-c homogeneity is through more stringent control of the REBCO growth conditions in these Zr-doped coated conductors.