Phonon wave-packet scattering and energy dissipation dynamics in carbon nanotube oscillators

Abstract

Friction in carbon nanotube (CNT) oscillators can be explained in terms of the interplay between low frequency mechanical motions and high frequency vibrational modes of the sliding surfaces. We analyze single mode phonon wave packet dynamics of CNT based mechanical oscillators, with cores either stationary or sliding with moderate velocities, and study how various individual phonons travel through the outer CNT, interact with the inner nanostructure, and undergo scattering. Two acoustic modes (longitudinal and transverse) and one optical mode (flexural optical) are found to be responsible for the major portion of friction in these oscillators: the transmission functions display a significant dip in the rather narrow frequency range of 5–15 meV. We also find that the profile of the dip is characteristic of the inner core. In contrast, radial breathing and twisting modes, which are dominant in thermal transport, display ideal transmission at all frequencies. We also observe polarization dependent scattering and find that the scattering dynamics comprises of an oscillating decay of localized energy inside the inner CNT. This work provides a way towards engineering CNT linear oscillators with better tribological properties.

Phononic Origins of Friction in Carbon Nanotube Oscillators

“…LA phonons display significant scattering (down to ∼0.5) in this range. The profile of the dip in the transmission plot at low energy range (5–15 meV) is the characteristic of the inner core geometry and, as suggested in our earlier work, can serve as an identifying signature of the core.…”

“…We found the radial breathing and twisting acoustic modes with high group velocity, which exhibit no coupling at the interface, as the enablers of ultralow friction in CNT oscillators. On the other hand, longitudinal acoustic (LA), transverse acoustic, and first-oder optical modes are chiefly responsible for dissipation in CNT oscillators by causing substantial scattering in the 5–15 meV range . LA phonons have recently been found to enhance the flow of confined water in multiwalled CNTs by causing an oscillatory shear stress at the interface …”

“…As stated above, five phonon modes are considered, and for each four cores geometries (Figure ) are studied. The range of phonon energies for each mode covers the entire spectrum of the phonon dispersion plot …”

Phonon Scattering Dynamics of Thermophoretic Motion in Carbon Nanotube Oscillators

“…The wave packets of BR and TW modes transfer almost no energy in to the inner core and show a consistent ideal transmission throughout the frequency range. 31 TA and FO modes showed a significant scattering in the low energy region however the mechanism of scattering is distinct from that of LA mode. Unlike LA phonon, the mode characteristic of TA and FO phonons involve dominant transverse component of atomic displacements and hence no net axial force on the end atoms of the core.…”

“…Along with LA mode, we studied two other acoustic modes (TA and TW) and two optic modes (BR and FO) and found that the scattering mechanism showed by the LA mode (Figure and Figure ) only can initially generate a net axial force on core. The wave packets of BR and TW modes transfer almost no energy in to the inner core and show a consistent ideal transmission throughout the frequency range . TA and FO modes showed a significant scattering in the low energy region however the mechanism of scattering is distinct from that of LA mode.…”