Understanding limiting factors in the performance of high Tc superconductors

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A superconductor is a material in which electricity can flow without energy loss. This is unlike ordinary metals which waste energy by getting hot due to the interaction between the flowing current and the material. The phenomenon of superconductivity has fascinated scientists and technologists since its discovery over 90 years ago. Unfortunately superconductors only work at low temperatures and there is always a limit to the maximum amount of electricity that can be transported. This explains why, in spite of repeated predictions, we don't yet see superconductors on electricity pylons and in everyday objects.In the last few years however materials, high-Tc superconductors , have been developed which work at temperatures, which while still a long way below freezing, are practical with existing refrigeration technology. Again there is a catch in that these ceramic materials have a granular structure. The granular structure can be thought of as the materials consisting of many individual crystals (similar to quartz or salt) connected together. The individual crystals are termed grains and the interfaces between them are grain boundaries. Unfortunately electricity does not flow well across grain boundaries and this limits the performance of large samples of high-Tc superconductor.The 'grain-boundary' problem is however being overcome by growing the superconductor on a carefully made strip of metal which aligns the ceramic grains so as to allow the current to flow easily. This means that the limiting factor is now not always the grain boundaries in these materials. We have recently shown that there is a cross-over point where the maximum current stops being limited by the grain boundaries and starts being limited by the individual grains. This depends on temperature, strength of any applied magnetic field and also the direction of any applied magnetic field.The final barrier to more widespread use of superconductors is in essence economic, they need to transport more current, more cheaply than the existing technology. My project seeks to understand how the current in the most promising superconducting material is affected by magnetic fields of various orientations and by the way the material is made. I am also intending to isolate individual grains and pairs of grains from these materials to study them and the interfaces between them. This will allow us to understand how new ways of improving the current carrying capacity work in detail and which of these techniques should be used in what combination to produce the best superconductor for each potential application.