@gameover,
I recall the SiC heater graphs having a negative R curve that flattened out at a high temperature. Perhaps that is engineered with grain sizes and geometry?
I have no idea about the rest of the materials. but I would think there would be some sort of limit. (Maybe there isn't)
Something to consider when experimenting with this stuff.
There are some neat videos (several were posted on the forum a year ago maybe) of glass being turned into a melting-glowing conductor with 120 V.