Effects of Thermal Effusivity in Nanocrystalline Porous Silicon on Long-Term Operation of Thermally Induced Ultrasonic Emission

Abstract

Due to a strong quantum confinement effect, both the thermal conductivity and heat capacity per unit volume C of nanocrystalline porous silicon (nc-PS) are extremely lower than those of single-crystalline silicon (c-Si). This high contrast between the thermal properties of nc-PS and c-Si makes it possible to produce an efficient ultrasound emitter device for various sensors, speakers, and actuators. The most important parameter in this device is assumed to be the thermal effusivity ðCÞ 1=2 of nc-PS, since the theoretical acoustic output is inversely proportional to the ðCÞ 1=2 of the nc-PS layer. To confirm this assumption, the output stability during a long-term operation has been studied in relation to a change in ðCÞ 1=2 . The obtained experimental results indicate that the acoustic output deteriorates with a gradual increase in ðCÞ 1=2 due to the oxidation of nc-PS and that an appropriate nanostructure control with a passivated surface against oxidation is the key issue for obtaining a practical operation time.