Introduction

The electric transport properties of a high temperature superconductor are largely determined by the absence or presence of high-angle grain boundaries and their arrangement within the material. Ultimately, the grain boundary properties are governed by the grain boundary structure and composition at the atomic level. An important goal of electron microscopy investigations is to establish correlations between the electrical transport behavior and the structure and chemical composition of grain boundaries. In this chapter we examine, via the example of YBa2Cu3O7–x (YBCO) grain boundaries, grain boundary structures in high Tc materials and their influence on grain boundary transport properties. The weak-link behavior of high-angle grain boundaries will be discussed in view of results from structural investigations at different length scales ranging from the macroscopic to the mesoscopic and microscopic down to the atomic scale.

Polycrystals of YBa2Cu3O7–x typically can carry critical currents that are two orders of magnitude lower than the critical current densities in corresponding single crystals. This reflects the average reduction of the critical current due to high-angle grain boundaries. Thus, high-angle grain boundaries present a considerable impediment to high current applications of high Tc materials. Conversely, the weak-link nature of grain boundaries can be of considerable value when applied to the design of microelectronic devices, such as superconducting quantum interference devices (SQUIDS). In fact, one of the first commercial applications of high Tc materials was based on the function of grain boundaries as Josephson junctions [10.1].

Following Chaudhari et al. [10.2], who measured the superconducting properties of individual grain boundaries in thin films, Dimos et al. [10.3, 10.4] demonstrated a strong correlation between the critical current densities across the grain boundaries and the grain boundary misorientations for several grain boundary geometries, which included tilt, twist, and general grain boundaries.