Abstract
The detrimental effects of grain boundaries have long been considered responsible for the low critical current densities in high temperature superconductors. In this paper, we apply the quantitative approach used to identify the cause of the ‘weak-link’ grain boundary behaviour in YBa2Cu3O7 (Wang et al 2017 Supercond. Sci Technol. 30 104001), to the Bi2Sr2CaCu2O8 and Bi2Sr2Ca2Cu3O10 materials that we have fabricated. Magnetic and transport measurements are used to characterise the grain and grain boundary properties of micro- and nanocrystalline materials. Magnetisation measurements on all nanocrystalline materials show non-Bean-like behaviour and are consistent with surface pinning. Bi2Sr2CaCu2O8: our microcrystalline material has very low grain boundary resistivity which is similar to that of the grains such that (assuming a grain boundary thickness (d) of 1 nm) equivalent to an areal resistivity of The transport values are consistent with well-connected grains and very weak grain boundary pinning. However, unlike low temperature superconductors (LTS) in which decreasing grain size increases the pinning along the grain boundary channels, any increase in pinning produced by making the grains in our Bi2Sr2CaCu2O8 materials nanocrystalline was completely offset by a decrease in the depairing current density of the grain boundaries caused by their high resistivity. We suggest a different approach to increasing from that used in LTS materials, namely incorporating additional strong grain and grain boundary pinning sites in microcrystalline materials to produce high values. Bi2Sr2Ca2Cu3O10: both our micro- and nanocrystalline samples have of at least 103. This causes strong suppression of across the grain boundaries, which explains the low transport values we find experimentally. Our calculations show that low in untextured polycrystalline Bi2Sr2Ca2Cu3O10 material is to be expected and the significant effort in the community in texturing samples and removing grain boundaries altogether is well-founded.
Highlights
IntroductionHigh critical current density (Jc) in superconducting materials is usually the most important technological figure of merit
In polycrystalline low temperature superconductors (LTS), such large forces were often achieved by reducing the grain size
The most interesting sample we have studied is polycrystalline Bi2Sr2CaCu2O8 with grain sizes on the order of microns
Summary
High critical current density (Jc) in superconducting materials is usually the most important technological figure of merit. In HTS, it is well established that large angle grain boundaries can cause Jc to drop by several orders of magnitude, depending on the misorientation angle [5,6,7] This has become known as the ‘weak-link problem’, a problem so severe that the most important commercial HTS materials, YBa2Cu3O7−x (YBCO) and Bi2Sr2Ca2Cu3O10, are produced in pseudo single-crystal tape and wire form. These forms require use of complex and expensive fabrication processes in order to minimise grain misalignment and eliminate high angle grain boundaries. Weak-links were uniquely identified by low Jc, it is important to identify why Jc is low – whether the grain boundary depairing current density (JDN) is low or if there is low pinning (JPN)
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