This dissertation describes the investigation of the normal-state properties of high-angle grain boundaries in YBa2Cu3O7-d (YBCO) and Y1-xCaxBa2Cu3O7-d (calcium-doped YBCO). YBCO is a high-temperature superconducting material with a superconducting transition temperature up to 93 K. Grain boundaries are interfaces between two crystals or grains, and severely reduce the attainable currents in practical, polycrystalline material. A grain boundary is characterised by the misorientation angle between the two adjacent crystals, which determines the atomic structure of the interface. The structure of low-angle grain boundaries (misorientation angles < ~7°) is well understood; it consists of a regular array of dislocations. For higher misorientation angles the dislocations merge and form a continuously distorted zone. The structure of these high-angle grain boundaries, and, hence, the mechanism for charge transport across the interface, is less well understood. The current-voltage behaviour of grain boundaries below the transition temperature of the YBCO has been investigated extensively, but less data is available of the resistive behaviour of the grain boundary in the normal state above the transition temperature. The doping of YBCO with calcium is known to decrease its transition temperature, but it can simultaneously improve the charge transport properties of grain boundaries in polycrystalline material. YBCO and calcium-doped YBCO thin films were fabricated on bicrystalline substrates. The grain boundaries had misorientation angles between 18° and 45°. The films were processed in order to obtain microscopic devices that made it possible to determine the resistance of the grain boundary below and above the transition temperature. A measurement system was used with which the voltage across the grain boundary can be measured as a function of applied current between 5 K and room temperature. A detailed model for charge transport by tunnelling across a grain boundary was used to interpret the results of the measurements of grain boundaries. An algorithm based on this model was formulated that made it possible to calculate a shape for the potential barrier at the grain boundary from the temperature dependence of its resistance. The microstructure of the grain boundary was investigated using Transmission Electron Microscopy. It was found that the grain boundary can show considerable deviations from a straight path in line with the substrate grain boundary, but there are also locations where such deviations are not observed. Extensive measurements showed that the resistance of the grain boundary decreases with increasing temperature above the transition temperature, and that resistance and the extent of resistance variation increases with misorientation angle. The resistance below the transition temperature was for certain misorientation angles observed to be independent of temperature at voltages sufficiently high to exclude the influence of the superconducting behaviour of YBCO. A reduction of the oxygen content of YBCO (higher value of d) increases the resistance and the temperature dependence of the resistance. The doping of YBCO with calcium decreases the resistance of the grain boundary below and above the transition temperature. The shape of the potential barrier at the grain boundary was calculated on the basis of most resistance-temperature measurements. The shape and size of potential barriers are used to explain the variation of the grain boundary resistance with misorientation angle, oxygen content and calcium doping percentage. The model shows little validity for grain boundaries with a higher misorientation angle and resistance, which indicates that charge transport across the grain boundary does not necessarily take place according the tunnelling with the assumptions made.

In this work, electrical transport properties and the maximum current carrying capability of YBa2Cu3O7-[delta] thin films and so called quasi-multilayers are investigated. These samples are prepared with pulsed laser deposition on single-crystalline substrates (SrTiO3) as well as on biaxially textured Ni tapes. The critical current density of coated conductors is limited by small-angle grain boundaries in low magnetic fields, but by the intra-grain pinning properties in higher magnetic fields. Accordingly, these investigations are divided into two parts: In the first part, the limitation of the critical current density by grain-boundaries and grain boundary networks is investigated with the main focus on the influence of geometrical factors such as the conductor width or the grain aspect ratio. In the second part, a possible enhancement of the critical current density due to different doping types (atomar doping using Zn and precipitate doping using BaMO3 where M is a transition metal) will be discussed. Here, not only the irreversibility field but also the pinning behaviour in very low magnetic fields is of interest to better understand the pinning mechanism of thin films. / In der vorliegenden Arbeit werden elektrische Transporteigenschaften und die maximale Stromtragfähigkeit von YBa2Cu3O7-[delta]-Dünnschichten und -Schichtsystemen, die mit Hilfe der gepulsten Laserdeposition sowohl auf einkristallinem Substrat, SrTiO3, als auch auf biaxial texturierten Ni-Bändern deponiert wurden, untersucht. Da in kleinen Magnetfeldern Kleinwinkelkorngrenzen die kritische Stromdichte in Bandleitern begrenzen, in höheren jedoch die Pinningeigenschaften der Körner, gliedern sich diese Untersuchungen in zwei Teile: Im ersten wird die Limitierung der kritischen Stromdichte jc durch Korngrenzen und Korngrenzennetzwerke näher untersucht, wobei besonders Geometrieeinflüsse, wie die Leiterbahnbreite oder das Aspektverhältnis der Körner, interessieren. Im zweiten wird eine mögliche Erhöhung der kritischen Stromdichte durch verschiedene Dotierungen (atomare Dotierung: Zn, Ausscheidungsdotierung: BaMO3, M Metall) erörtert. Dabei ist nicht nur das Irreversibilitätsfeld interessant, sondern auch das Pinningverhalten in sehr kleinen Magnetfeldern, da so die Pinningmechanismen in Dünnschichten besser verstanden werden können.