Back when I was a student in Engineering School at UT Austin in the late 1970's, I had a grad student TA running the metallurgy lab I was taking. We talked a lot and I asked him what he was working on for his PhD. He was working on high temperature superconductors. He had developed some that were almost room temperature and some better ones that operated at liquid CO2 temperatures. The only problem with both were that they were so brittle he could not have a piece longer than a couple of centimeters because it would break from it's own weight.
The last time I saw him in the early 80's, he was working for GE and still working on these.