The effect of oxygenation temperature on the distribution of Ca in Dy123/0.2Ca ceramics and superconducting properties

The effect of annealing temperature for oxygenation (Toxg) on grain boundary (GB) segregation and the distribution of calcium in the bulk was studied for Dy123/Caz (z = 0, 0.2) ceramics for Toxg = 480 and 550 deg. C, where the first case corresponds to full oxygenation and the second to a little oxygen deficiency. Energy selective x-ray analysis techniques (with an ultra-fine probe) in field emission transmission electron microscopy were used for the characterization of the microstructure. It was found that Ca segregates at GBs in both cases, but the extent of such segregation strongly depends on Toxg. The GB segregation for Toxg = 550 deg. C was much lower than for 480 deg. C. For Toxg = 480 deg. C, the calcium concentration was 1.5-3.5 times higher than in the bulk in 60% of the GBs, and segregation was absent in 40% of the GBs. For Toxg = 550 deg. C, only 25% of the GBs showed Ca segregation, with a concentration 1.2-1.3 times exceeding that in the bulk. In both cases, the width of the segregation profile was about 10 nm. The change in Toxg also affected the distribution of Ca in the bulk. In this way, the changes in the superconducting properties were directly correlated to the microstructure