Mean free path

About: Mean free path is a research topic. Over the lifetime, 4412 publications have been published within this topic receiving 114418 citations.

Analytical interpretation of nondiffusive phonon transport in thermoreflectance thermal conductivity measurements

Abstract: We derive an analytical solution to the Boltzmann transport equation (BTE) to relate nondiffusive thermal conductivity measurements by thermoreflectance techniques to the bulk thermal conductivity accumulation function, which quantifies cumulative contributions to thermal conductivity from different mean free path energy carriers (here, phonons). Our solution incorporates two experimentally defined length scales: thermal penetration depth and heating laser spot radius. We identify two thermal resistances based on the predicted spatial temperature and heat flux profiles. The first resistance is associated with the interaction between energy carriers and the surface of the solution domain. The second resistance accounts for transport of energy carriers through the solution domain and is affected by the experimentally defined length scales. Comparison of the BTE result with that from conventional heat diffusion theory enables a mapping of mean-free-path-specific contributions to the measured thermal conductivity based on the experimental length scales. In general, the measured thermal conductivity will be influenced by the smaller of the two length scales and the surface properties of the system. The result is used to compare nondiffusive thermal conductivity measurements of silicon with first-principles-based calculations of its thermal conductivity accumulation function.

Implications of fluctuations in the distribution functions of interstellar pick-up ions for the scattering of low rigidity particles

Abstract: The distribution functions of interstellar pick-up hydrogen and helium exhibit fluctuations which are correlated An analysis of these fluctuations reveals that they may be the result of variations in, and provide additional evidence for, large mean free paths for these low rigidity particles An analysis of the properties of the distribution function of pick-up hydrogen reveals that these particles do suffer considerable adiabatic cooling, and that the likely cause for the long mean free path is the inability of the particles to scatter through 90° pitch angle

Heat transport by long mean free path vibrations in amorphous silicon nitride near room temperature

Abstract: We present measurements of thermal transport in 500-nm-thick, 35-$\ensuremath{\mu}$m-wide, and 806-$\ensuremath{\mu}$m-long micromachined suspended silicon nitride (Si-N) bridges over the temperature range of 77 to 325 K. The measured thermal conductivity of Si-N (for material grown by low-pressure chemical vapor deposition in two different furnaces) deviates somewhat from previously reported measurements and also shows surprising dependence on surface variation at these relatively high temperatures. Addition of discontinuous gold films causes the thermal conductance of Si-N bridges to drop through the entire measured temperature range, before rising again when thicker, continuous films are added. Similar effects occur when continuous but very low-thermal-conductivity alumina films are deposited. The reduction in thermal conductance upon modification of the Si-N surface is strong evidence that vibrational excitations with long mean free paths carry significant heat even at these high temperatures. By measuring a series of film thicknesses the surface-scattering effects can be mitigated, and the resulting experimental values of the thermal conductivity of alumina and Au thin films compare very well to known values or to predictions of the Wiedemann-Franz law. We also present a modified model for the phonon mean free path in thin-film geometries, and use it along with atomic force microsope scans to show that a very small population of phonons with mean free path on the order of 1 $\ensuremath{\mu}$m and wavelength much longer than the expected thermal wavelengths carry up to $50%$ of the heat in Si-N at room temperature.

Experimental evidence of very long intrinsic phonon mean free path along the c-axis of graphite

Abstract: We report on experimental studies of the average phonon mean free path in the c-axis direction of graphite Through systematically measuring the cross-plane thermal conductivity of thin graphite flakes with thickness ranging from 24 nm to 714 nm via a differential three omega method, we demonstrate that the average phonon mean free path in the c-axis direction of graphite is around 200 nm at room temperature, much larger than the commonly believed value of just a few nanometers This study provides direct experimental evidence for the recently projected very long phonon mean free path along the c-axis of graphite