THUNDER (THin Unimorph DrivER) actuators are pre-stressed piezoelectric devices developed at NASA LaRC that exhibit enhanced strain capabilities. As a result, they are of interest in a variety of aerospace applications. Characterization of their performance as a function of electric field, temperature and frequency is needed in order to optimize their operation. Towards that end, a number of THUNDER devices were obtained from FACE International Co. with a stainless steel substrate varying in thickness form 1 mil to 20 mils. The various devices were evaluated to determine low-field and high-field displacement as well as the polarization hysteresis loops. The thermal stability of these drivers was evaluated by two different methods. First, the samples were thermally cycled under electric field by systematically increasing the maximum temperature from 25 degree(s)C to 200 degree(s)C while the displacement was being measured. Second, the samples were isothermally ages at 0 degree(s)C, 50 degree(s)C, 100 degree(s)C, and 150 degree(s)C in air, and the isothermal decay of the displacement was measured at room temperature as a function of time.