Microstructure of superconducting ceramics YBa2Cu3O7–x sintered with addition of seed grains

Abstract

The growth of seed grains of YBa 2 Cu 3 O 7-x (Y123) ceramics in fine-grained matrix was studied. For this, two batches of powders were prepared. The first batch consisted only of fine-grained powder with a particle size less than 1 μm and was used to prepare the so-called reference samples. The second batch was a fine-grained powder with addition of 5 wt. % seed grains with sizes 60 -80 μm. Powders were pressed and sintered in the temperature range 925 -1000°C for 5 h. The effect of sintering temperature on density, grain size, structure of the seed-growth zone were investigated. In samples of both types the highest density is observed after sintering at T = 925 °C, and at higher temperatures the density decreases due to a release of oxygen as a result of a partial decomposition of the Y123 phase on grain boundaries. Above T ≈ 960°C the density of the reference samples decreases more strongly than in the samples with seeds. Seeds initiate the growth of anomalously large grains. The strongest growth of large grains occurs at T ≈ 960°C. Above T ≈ 960°C the grain growth is significantly slowed down that is associated with an increase in porosity of the samples. Large grains affect the growth of grains of fine-grained matrix: in the samples with seeds the mean grain size of fine-grained matrix is higher than in the reference samples. A scheme explaining the effect of seed grains on grain growth of fine-grained matrix is proposed.