A ceramic helical rotor expander using a double-ended or tandem herringbone type rotor arrangement with bearing and seal assemblies remote from the hot gas inlets and especially capable of operating at an inlet temperature of above 1100.degree. C. The rotors are solid or hollow and bonded to hollow metal shafts, and mounted in a composite or simple prismatic casing. The rotors, casing and shafts are constructed from low expansivity materials. In the preferred embodiment the rotors are constructed of silicon nitride and the shafts constructed of an molybdenum alloy, with the metal shafts being supported in bearings and secured to synchronizing gears. The rotors and casing may be provided with coolant channels therein, and are constructed to eliminate the problem of end leakages at inlet temperature and pressure, and the need for high temperature bearings and seals.