I don't know if such a material exists. The problem is SpaceX operates with Deep Cryo fuel (only company that does so as far as I know). They are literally goin

I don't know if such a material exists. The problem is SpaceX operates with Deep Cryo fuel (only company that does so as far as I know). They are literally going to 66K on the LOX and methane. Helium was just temporary as well so once the actually design system is back with SN15 it should be better already. The idea that Starship has is to use the fuel and oxidizer in its gas form. So the methane tank contains liquid methane pressurized by gaseous methane. The benefit of this is that if the engine ingests some of that gaseous methane its less of an issue in terms of engine thrust compared to helium and its possible more recoverable of a situation.