Aiming at the strong capacitive impedance of piezoelectric stack actuators, the principle to improve the dynamic performance of piezoelectric stack actuators through increasing the peak values of the output current and output power of power amplifiers are explored. Based on the error-amplified principle, the method that enlarges the output voltage of the dynamic power amplifiers through using a high-voltage operational amplifier in series with the power booster section, as well as the method that improves the peak values of the output current and output power through paralleling multiple power booster units utilizing the class AB quasi-complementary symmetry power amplifier circuits, are proposed and analyzed. Utilizing the proposed principle and method, a high-voltage and high-power amplifier for driving piezoelectric stack actuators is developed, simulated, and tested. The research results indicate that the developed power amplifier not only can break through the limit that each single power booster unit can only achieve the power output <125 W, but also can disperse the current and power averagely among the power booster units in parallel, which are beneficial to realizing the high power output with good static and dynamic performance and enhancing the reliability of the power amplifier.

We report a room temperature study of the direct band gap photoluminescence of tensile-strained Ge/Si0.13Ge0.87 multiple quantum wells grown on Si-based germanium virtual substrates by ultrahigh vacuum chemical vapor deposition. Blueshifts of the luminescence peak energy from the Ge quantum wells in comparison with the Ge virtual substrate are in good agreement with the theoretical prediction when we attribute the luminescence from the quantum well to the cΓ1-HH1 direct band transition. The reduction in direct band gap in the tensile strained Ge epilayer and the quantum confinement effect in the Ge/Si0.13Ge0.87 quantum wells are directly demonstrated by room temperature photoluminescence.

An electrochemical method was introduced to prepare graphene/polyurethane foams with gradient graphene distribution, and this composite shows obvious anisotropic reflection of electromagnetic waves.