Sound of carbon nanotube assemblies

Strong thermo- and photoacoustic responses have been detected for aligned arrays of multiwalled carbon nanotube (MWNT) forests and solid drawn MWNT sheets. When heated using alternating current or a near-IR laser modulated in 100-20 000 Hz range, the nanotube assemblies generated loud, audible sound, with higher sound pressure being detected from the MWNT sheets. An evaluation of nonlinear distortions of the thermoacoustic signal revealed a highly peculiar behavior of the third and fourth harmonics produced from forests grown on silicon wafers. The peculiarities were especially pronounced for short forests and can be associated with the heat transfer from the MWNT layer to the substrate. For both types of nanotube assemblies, the acoustic signal's amplitude varied with frequency approximately by the power low f(p). The power factor p was found to be unexpectedly high for short forests probably due to heat loss to the substrate. The observed peculiarities can be used for the characterization of the prepared MWNT assemblies. The dependencies can also be helpful for evaluating the properties of thermal interfaces, in particular, those based on carbon nanotubes.