The problem was the materials themselves. They are ceramics, and therefore brittle. They don’t form wires easily; lengthy strands tend to break and become highly resistive. They are not three-dimensionally symmetric – the superconductivity has a much higher current capacity in one plane than in others, and grains are likely to be randomly oriented unless deposited with great care. The properties are also heavily dependent on composition and a certain level of defects (missing oxygen atoms, for example). And as type-II superconductors, high currents bring with them magnetic flux vortices that can generate their own resistive forces, unless the vortices are pinned by further defects.
And of course the increasing critical temperatures never reached room temperature, like we all half expected.
However, there have been some small-scale applications. Superconducting quantum interference devices can now be made smaller and operated more inexpensively; some RF applications have been put in place as well. Superconducting power cables have even been tried in real life with a 30 meter cable now supplying some power to Copenhagen.
According to the New York Times article, and SuperPower itself, practical large-scale power cables are just around the corner. We’ll see. Apparently SuperPower has hit on a promising technique; using a long metallic substrate tape, polished very smooth, to lay down layers that produce well-connected superconductor along the length of the tape. The process is still a bit on the slow and expensive side though.
Is there something we should be learning from the difficulties here? We’ve spent hundreds of millions of dollars investigating these high Tc compounds – but are we underfunding the applied science that could actually make them practically useful? It’s a little disturbing to your faith in science when something with such seeming promise goes so far awry…