Positron Annihilation Lifetime Spectroscopy of Off-Stoichiometric and Doped Gd2Ti2O7

An improved windmill includes a conical rotor unit, a frame for supporting the conical rotor unit and a base for supporting the frame. The conical rotor unit has a conical rotor with a plurality of spaced curved blades disposed thereon and a plurality of spaced slant blades located at an outer rim of the conical rotor. The conical rotor has small wind resistance and may rotate under low speed wind to generate rotor rotation. The frame is rotationable about the base to enable the conical rotor to capture wind from different directions. The conical rotor is heavier than conventional windmill rotor and may serve as a fly wheel to store kinetic energy for the windmill to produce steady rotation under different wind speed for a long period of time.