Effect of holmium additions on microstructure in YBa2Cu3O7−δ

The microstructure of Y-Ho-Ba-Cu-O samples with two different levels of Ho, Ho/Y = 0.5 and 1.0, processed by the trifluoroacetate metalorganic deposition process, have been studied by transmission electron microscopy (TEM) techniques, including high-resolution TEM, conventional TEM and fine-probe energy-dispersive spectroscopy. Oxide nanoparticles with varying levels of Ho have been characterized in detail, and related to other microstructural features. At a lower level of Ho, the (1 1 0) coarse twins with the boundary spacing of 30-60 nm form. Further increase in Ho content considerably produces fine microtwins, and increases the density of nanoparticles and microtwins. Ho additions increase the critical current for the magnetic field parallel to the c-axis, and decrease it for the field parallel to the a-b plane, resulting in a decreased anisotropy of critical current with field angle compared with undoped Y-Ba-Cu-O samples processed under similar conditions. These trends are discussed in terms of the detailed microstructural observations.