Previously, superconductors had to be cooled with liquid helium, which is much more expensive. But researchers soon found that the new polycrystalline superconductors could support only extremely low current densities at this temperature. This is a result of the large degree of misalignment between the individual crystals in the superconductors. Further studies suggested that oxygen depletion at these large-angle boundaries results in a lack of charge carriers, which leads to the drop in current.
Calcium ions are similar in size to yttrium ions but have a smaller positive charge.
When these calcium ions take the place of yttrium ions in YBaCuO, they act as 'positive holes', charge carriers opposite in polarity to electrons. The group found that the holes diffused preferentially into grain boundaries in the adjacent undoped layers and increased the inter-grain current flow by between three and six times. Contact us .