Grain Boundary Structure of YBa2Cu3O7-δ High Tc Superconductors
The microstructures of YBa2Cu3O7-.delta. (YBCO) high Tc superconducting materials have been studied using analytical electron microscopy. The effects of processing parameters on the interfacial microstructures have been investigated in both bulk materials and thin films. The observations have been interpreted in terms of the influence of interfacial microstructure on the superconducting transport properties (e.g. Tc and jc). Different methods to control the interfacial microstructure have been investigated. The intergranular microstructures strongly affect the superconducting properties. The structure of grain boundaries which have been introduced in a controlled manner has also been investigated and correlated to the electrical characteristics.
In YBCO films grown on (001) single crystal yttria stabilized-ZrO2 (YSZ) substrates by pulsed laser deposition, a change in the growth process occurred at about 740 °C. Polycrystalline films consisting of c-axis oriented grains interconnected by low-angle and characteristic high-angle grain boundaries were formed at deposition temperatures higher than 740 °C. Films with a single orientation relationship between the film and the substrate were obtained at lower deposition temperatures. An intermediate, polycrystalline, BaZrO3 (BZO) layer formed from a reaction between the film and the substrate. The formation of grain boundaries in the films resulted in an increased microwave surface resistance and a decreased critical current density.
In epitaxial YBCO films grown on -tilt YSZ bicrystal substrates, the film boundaries were generally wavy despite a relatively straight morphology of the substrate boundaries. The waviness of the film boundaries was a result of the combined effects of island growth of YBCO on YSZ substrates and grooving at the substrate boundaries prior to the film deposition. In addition, the grooving introduced tilt components perpendicular to the c-axis to the film boundaries. The different geometries of the film boundary plane may have given rise to variations in the physical properties along the boundaries as observed from measurements of jc as a function of applied magnetic field.
A qualitative analysis of the expected dilatational strain field in the vicinity of an array of dislocations provided a basis for the prediction of the critical current density (jc) across a low angle YBCO grain boundary (GB) as a function of its energy. The introduction of the GB energy allowed the extension of the analysis to high angle GBs using established models which predict the GB energy as a function of misorientation angle. The results were compared to published data for jc across -tilt YBCO GBs for the full range of misorientations, showing a good fit.