Observation of second sound in graphite over 200 K

Second sound refers to the phenomenon of heat propagation as temperature waves in the phonon hydrodynamic transport regime. We directly observe second sound in graphite at temperatures of over 200 K using a sub-picosecond transient grating technique. The experimentally determined dispersion relation of the thermal-wave velocity increases with decreasing grating period, consistent with first-principles-based solution of the Peierls-Boltzmann transport equation. Through simulation, we reveal this increase as a result of thermal zero sound—the thermal waves due to ballistic phonons. Our experimental findings are well explained with the interplay among three groups of phonons: ballistic, diffusive, and hydrodynamic phonons. Our ab initio calculations further predict a large isotope effect on the properties of thermal waves and the existence of second sound at room temperature in isotopically pure graphite.

Microscale Thermal Energy Transfer Between Thin Films with Vacuum Gap at Interface

Transfer of phonons through a silicon–diamond thin film pair with a nano-size gap at the interface is examined. The thin film pair is thermally disturbed by introducing 301 K at the silicon film left edge while keeping the other edges of the thin films at a low temperature (300 K). The radiative phonon transport equation is solved numerically to quantify the phonon intensity distribution in the combined films. The frequency dependent formulation of phonon transport is incorporated in the transient analysis. The thermal boundary resistance is adopted at the interface in the formulations. The near-field radiative heat transfer is also adopted at the gap interface, as the vacuum gap size falls within the Casimir limit. The predictions of thermal conductivity are validated through the thermocouple data. It is observed that predictions of thermal conductivity are in agreement with the experimental data. The ballistic phonons play a major role in energy transfer through the gap; their contribution is more significant than that of the near-field radiative heat transfer. Enlarging the size of the gap reduces the influence of the ballistic phonons on the energy transfer in the films. Increasing the silicon film thickness alters the energy transfer through the gap; in this case, the equivalent equilibrium temperature difference is increased at the interface.

Термоэлектрический механизм полевой эмиссии из углеродных наноструктур

A model describing the features of field emission from carbon nanomaterials is considered. The model is based on taking into account the effect of electron drag by ballistic phonons in the region of the temperature gradient inside the emission center. The model does not require any additional assumptions about the special energy structure of the emission center. Quantitative estimates of the thermopower coefficient, made on the basis of the emission model, are in good agreement with the experimental results.

Influence of heat source size and film thickness on phonon transport in a two-dimensional thin film

Phonon transport in a two-dimensional thin silicon film is considered and the effect of heat source size and the film thickness on the transport characteristics is examined. Frequency dependent Boltzmann equation is incorporated in the analysis to account for the contribution of the ballistic phonons to the energy transport. Equivalent equilibrium temperature is introduced to assess the thermal resistance during the phonon transport in the film. The numerical scheme with the appropriate boundary conditions is used to predict the transport properties, including the effective thermal conductivity, of the thin film. It is found that the heat source size and the film thickness influence the thermal resistance of the film almost equally. The ballistic phonons reduce the film thermal resistance while suppressing the effective thermal conductivity in the thin film.

Heat Pulse Propagation Along Silicon Nanofilms

Recent advances in nanotechnology create a demand for greater scientific understanding of the transient ballistic phonon transport at the nanoscale. It is believed that ballistic phonons may travel for long distances without destruction, but it is unclear how far they can travel. Here, a numerical model is developed to study phonon transport in silicon nanofilms. It is elucidated how thermal pulses are transmitted in silicon nanofilms by longitudinal, ballistic transverse and dispersive transverse phonons. It is found that both ballistic longitudinal and ballistic transverse phonons are highly dissipative so they can only travel for short distances, while dispersive transverse phonons at lower frequencies are less dissipative and can travel for longer distances. There exists a similarity parameter (Knudsen number) in thin-film heat conduction with different thicknesses.

BALLISTIC PHONON TRANSPORT THROUGH GAUSSIAN ACOUSTIC NANOCAVITIES

We investigate ballistic phonon transport through Gaussian acoustic nanocavities in a semiconductor nanowire at low temperatures. When the transverse widths of acoustic nanocavities takes a Gaussian function, it is found that wide band gap and resonant peaks appear in transmission spectra. The phonon-cavity confined modes exist as the number of the nanocavities is large. The phonon transmission and thermal conductance strongly depend on the number and length of nanocavities. The results suggest that the Gaussian acoustic nanocavities may be useful for controlling thermal conductance artificially and the design of phonon devices to manipulate ballistic phonons in nanophononics.

An extended thermodynamic model of transient heat conduction at sub-continuum scales

A thermodynamic description of transient heat conduction at small length and timescales is proposed. It is based on extended irreversible thermodynamics and the main feature of this formalism is to elevate the heat flux vector to the status of independent variable at the same level as the classical variable, the temperature. The present model assumes the coexistence of two kinds of heat carriers: diffusive and ballistic phonons. The behaviour of the diffusive phonons is governed by a Cattaneo-type equation to take into account the high-frequency phenomena generally present at nanoscales. To include non-local effects that are dominant in nanostructures, it is assumed that the ballistic carriers are obeying a Guyer–Krumhansl relation. The model is applied to the problem of transient heat conduction through a thin nanofilm. The numerical results are compared with those provided by Fourier, Cattaneo and other recent models.