Dual-Axis STEM Tomography of Dy-doped YBa2Cu3O7-x Coated Superconductors

Abstract

High temperature superconductors (HTS), especially YBa 2 Cu 3 O 7-x (YBCO), are among the most promising materials for electronic and magnetic applications involving electronic devices and wire technologies. The main challenge in optimizing the critical current density of the HTS is the immobilization of the vortices by means of artificial flux pinning centers. Various methods such as neutron [1] and ion irradiation [2] and mechanical or thermal techniques [3] have been used to introduce effective artificial pinning centers. Chemical doping is also a common way of enhancing critical current density giving rise to formation of nanometer-size second phase particles acting as flux pinning centers throughout the superconductor matrix [4]. For effective flux pinning, the size of the pinning centers should be on the order of the coherence length of the superconductor with a uniform size and spatial distribution. Therefore, the size, spatial distribution and shape of the flux pinning centers have a major role in determining the critical current density of superconductor materials.Previous studies showed that the addition of rare earths (Dy, Ho) increases the critical current density for perpendicular magnetic field application [5][6][7]. In this work, the structural effects of the addition of Dy to a YBCO coated conductor have been studied using a Scanning Transmission Electron Microscope (STEM) to ascertain the reasons for the increase in critical current density. The studied sample consists of a 1 µm thick YBCO layer doped with Dy (50% in molar basis) which is grown by metal-organic deposition process on cube textured NiW(5%) alloy substrate buffered with oxide layers with the structure of CeO 2 /YSZ/Y 2 O 3 /NiW. Energy Dispersive X-ray Spectroscopy analysis showed that the added Dy atoms substitute the Y atoms giving rise to a compound (Y 0.5 Dy 0.5 )Ba 2 Cu 3 O 7 as the matrix phase and Y 1.32 Dy 0.66 Cu 2 O 5 as a second phase particles which can be seen in Fig. 1.In order to understand the enhanced flux pinning, STEM tomography experiments were performed to determine the 3-D morphology and distribution of the particles by collecting the high angle scattered electrons with a dark field detector. Utilization of a high angle annular dark field detector (HAADF) in STEM mode minimizes the diffraction effects resulting in a more accurate tomographic reconstruction compared to conventional TEM. Due to minimal contrast between the matrix and particles image processing techniques were applied to improve the tomographic reconstruction. Tomography studies showed that 25% of the material volume is occupied by 71 particles ranging between 13 nm and 135 nm. It was observed the spatial and size distributions of the particles are not uniform. The number density of particles within the regions with small particles is high, whereas it is low in the regions around the large particles. Additionally, the large particles have