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

概要

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Due to a strong quantum confinement effect, both the thermal conductivity $\alpha$ 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 $(\alpha C)^{1/2}$ of nc-PS, since the theoretical acoustic output is inversely proportional to the $(\alpha C)^{1/2}$ 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 $(\alpha C)^{1/2}$. The obtained experimental results indicate that the acoustic output deteriorates with a gradual increase in $(\alpha C)^{1/2}$ 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.
2007-04-30