Tuning oxygen vacancy and growth step for the high performance of Nd1+xBa2−xCu3Oy bulk cryomagnets
Abstract
The Nd1+xBa2−xCu3Oy superconductor is considered to be one of the most promising materials in the REBaCu3Oy family owing to its excellent properties of high critical transition temperature (Tc) and high current density. However, the substitution of Nd on the Ba site occurs easily with this cation non-stoichiometric oxide, particularly in the ambient air, during the melt growth process, leading to a severe deterioration in the superconducting performance. Here, we combined thermodynamic with kinetic methods, namely the composition-control melt-growth (CCMG) and cooling-rate-control melt-growth (CRCMG) method, for the synthesis. We used Ba-rich precursors in the air-processed CCMG, which underwent an optimal cooling step (CRCMG) for increasing the chemical potential of Ba and tuning the oxygen vacancy during the growth. As a result, the Nd1+xBa2−xCu3Oy bulk could reach a high Tc of 95 K in air. In the peritectic reaction, it was found that the higher cooling rate reduces the absorption of oxygen during the formation of Nd1+xBa2−xCu3Oy, resulting in the reduction of the whole cation valence, and therefore, a low x value in Nd1+xBa2−xCu3Oy was gained. In addition, considering their brittle nature and discontinuous growth trend, Nd1+xBa2−xCu3Oy superconductor bulks were optimized for the growth step in a gradual acceleration cooling rate mode. As a result, the cryomagnet (16 mm in diameter) with a high Tc of 95 K and a trapped field of 0.60 T, superior to the previously reported air-processed ones, was successfully obtained. This success in the control of the cation stoichiometry and growth mode opens a promising pathway for the preparation of more functional oxides.