The other big issue is the driving gas. You want something with a really low molecular mass for maximum thermal expansion. Hydrogen gas would be ideal if it didn't have a distressing tendency to diffuse into and through metals. And. You know. Explode in contact with heat and Oxygen.

Helium is half as good (twice the MM) ... but incredibly expensive and almost as hard to contain. Doesn't explode though which is good.

And then you're into N2, O2 and you may as well use air for obvious reasons. At about 14 times the MW of hydrogen gas. :/