Interactions between organic and inorganic additives during the degreasing of green compacts for the crystal growth of Ag-added Gd–Ba–Cu–O superconductor bulks

A single-grain RE –Ba–Cu–O ( RE is a rare earth element) bulk superconductor comprises the superconducting RE Ba 2 Cu 3 O 7 − x and non-superconducting RE 2 BaCuO 5 phases. Its trapped field properties depend on the density of the precursor compact, which is prepared by compressing a fine powder and often shows pressing cracks. We attempted to suppress these cracks using an Ag-added Gd–Ba–Cu–O fine powder ground by dry ball milling with stearic acid and granulated with an acrylic resin binder. Although both organic additives must be decomposed and exhausted from the green compact by degreasing, we observed that the oxygen released from Ag 2 O promoted the combustion of stearic acid and damaged the green compact. It was found that this thermal runaway could be prevented by decomposing the Ag 2 O to Ag in advance, which was carried out concomitantly with the calcination of GdBa 2 Cu 3 O 7 − x powder. Thus, we successfully prepared a defect-free and dense green compact, and we then obtained a single-grain Ag-added Gd–Ba–Cu–O bulk with a higher trapped field than the conventional bulk.