Abnormal grain growth in sintered YBa2Cu3O7-δ nanocrystalline powders produced by high energy ball milling

The sintering and crystal growth of YBa2Cu3O7-δ (123) nanocrystalline powders produced by high energy ball milling has been studied. After rapid densification, microstructural examination indicates for the first time the occurrence of an abnormal grain growth phenomenon or secondary recrystallization (SR). This phenomenon was not observed during the sintering of unmilled YBa2Cu3O7-δ powders. Optical, TEM and calorimetric techniques were used to characterize the occurrence of SR in this system. Following an incubation period where normal grain growth is observed, large tabular grains progressively appear in the microstructure. Their length and shape factors (s = Length / Width) measured on a cross section can reach values as high as 0.5 mm and 46 respectively after 6 h. at 950°C. The bimodal grain size distribution typical of SR processes evolves toward a single distribution of large grains when the faster growing abnormal grains consume the remaining population of normal grains. The observations indicate that SR in YBa2Cu3O7-δ is the result of a combined effect of milling and the presence of a small amount of transient yttrium deficient liquid that wets some of the grain boundaries during the sintering step. The liquid originates either from an incomplete synthesis of the 123 compound and/or by air contamination of the milled powders. The control of this abnormal grain growth is of interest for the formation of bulk textured large grain microstructure desirable for the attainment of good superconducting properties.