Understanding limiting factors in the performance of high Tc superconductors

超导体是一种电流可以在其中流动而不损失能量的材料。这与普通金属不同，普通金属由于流动的电流和材料之间的相互作用而变热，从而浪费能量。超导现象自90多年前被发现以来，一直吸引着科学家和技术人员。不幸的是，超导体只能在低温下工作，而且可以传输的最大电量总是有限制的。这就解释了为什么，尽管一再预测，我们仍然没有在电塔和日常物品中看到超导体。然而，在过去的几年里，已经开发出了在温度下工作的材料，高温超导体，尽管温度仍然远低于冰点，但对现有的制冷技术来说是可行的。还有一个问题是，这些陶瓷材料具有颗粒状结构。颗粒结构可以被认为是由许多单独的晶体(类似于石英或盐)连接在一起的材料。单个晶体被称为颗粒，它们之间的界面是晶界。不幸的是，电流不能很好地穿过晶界，这限制了高T_c超导体大样本的性能。然而，通过在精心制作的金属带材上生长超导体，使陶瓷颗粒对准，从而使电流容易流动，可以克服晶界问题。这意味着现在的限制因素并不总是这些材料中的晶界。我们最近已经证明，存在一个交叉点，在该交叉点上，最大电流不再受晶界的限制，而开始受到单个晶界的限制。这取决于温度、任何外加磁场的强度以及任何外加磁场的方向。超导体更广泛使用的最后一个障碍本质上是经济的，它们需要传输更多电流，而且比现有技术更便宜。我的项目试图了解最有希望的超导材料中的电流是如何受到不同方向的磁场和材料制造方式的影响。我还打算从这些材料中分离出单独的颗粒和成对的颗粒，以研究它们和它们之间的界面。这将使我们能够详细地了解提高电流承载能力的新方法是如何工作的，以及这些技术中的哪一种应该以什么组合使用来为每一种潜在的应用产生最佳超导体。

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.