I'm doing research for an upcoming project that involves accelerating a small polyproylene ball using electricity to heat and ablate a substance, creating pressure behind the projectile. Currently, I'm planning to use a material such as tungsten or aluminum in a very thin film or wire to arc and consequently heat and vaporize the other substance. My inquiry is what material with a given amount of energy in the form of heat, will create the highest pressure? Obviously water has been used for a long time for these types of applications in, for example steam engines and nuclear power plants. It seems to me, though, that water has a high heat of vaporization when compared to other substances, in other words, it takes a larger amount of energy to vaporize a given amount of water. Is this important in this application? Or, would it be more important to have a substance that has a higher vapor pressure at the temperatures that will be experiences inside the chamber, which will probably be in the tens of thousands of degrees F. Any advice will be appreciated.