Because the materials are mosaics of tiny crystalline grains, and the flow of charge is hindered when it has to pass across the 'grain boundaries' between adjacent grains. Mannhart's group have now overcome this obstacle and increased the critical current in YBCO by almost a factor of ten. The holes are depleted at grain boundaries, which hinders the current. So the researchers found a way to replenish the grain boundaries with holes. One way to add holes to YBCO is to 'dope' it with small amounts of calcium.
Calcium ions can move about more readily at the grain boundaries than within the grains.
So some of the calcium leaves the doped layers and gathers preferentially at the boundaries of the undoped YBCO layers, helping them to transfer holes from one grain to the next. This development, says Paul Grant of the Electric Power Research Institute in Palo Alto, California, might help to usher in a second generation of superconducting wires over 100 times cheaper than the first. Hammerl, G. et al. O 7- d at 77 K from calcium doping of grain boundaries.