Thermal Conductivity Spectroscopy Technique to Measure Phonon Mean Free Paths

Abstract

Size effects in heat conduction, which occur when phonon mean free paths (MFPs) are comparable to characteristic lengths, are being extensively explored in many nanoscale systems for energy applications. Knowledge of MFPs is essential to understanding size effects, yet MFPs are largely unknown for most materials. Here, we introduce the first experimental technique which can measure MFP distributions over a wide range of length scales and materials. Using this technique, we measure the MFP distribution of silicon for the first time and obtain good agreement with first-principles calculations.

Anisotropic failure of Fourier theory in time-domain thermoreflectance experiments

“…These TDTR measurements observed a failure of Fourier theory in the through-plane direction because d p o ow 0 , meaning the inplane heat current was negligible. On the other hand, the observation of Minnich et al 4 that L A of Si depends on w 0 below 100 K was related to a failure of Fourier theory in the in-plane direction. That Fourier theory fails anisotropically has direct implications for the interpretations of prior experiments that assumed Fourier theory failed isotropically in three-dimensional heat-transfer problems 1,4,7,24 , see Supplementary Note 1 and Supplementary Figs 1 and 2.…”

“…As a result, no single length scale describes the transition from diffusive to ballistic thermal transport. Instead, as length scales in a thermal transport problem decrease, Fourier theory has diminishing predictive power [3][4][5][6][7] .…”

“…For example, deviations between Fourier theory predictions and experimental data have been reported to depend on (1) the width of the nickel heater lines in nanofabricated Ni/sapphire samples 7 , (2) the diameter of the heating pump laser beam, 2w 0 , in time-domain thermoreflectance (TDTR) measurements of bulk Si between 30 and 100 K (ref. 4), (3) the spatial frequency of heating in thermal grating measurements of 400 nm thick Si membranes at 300 K (ref. 10) and (4) the frequency of periodic heating, f, in TDTR measurements of semiconductor alloys and some types of amorphous silicon between 80 and 420 K (refs 6,15).…”

“…The breakdown of Fourier theory at small length scales has implications for nanoelectronics 8 and nanostructured thermoelectric devices 9 and has received a great deal of recent attention [1][2][3][4][5][6][7][10][11][12][13][14] . For example, deviations between Fourier theory predictions and experimental data have been reported to depend on (1) the width of the nickel heater lines in nanofabricated Ni/sapphire samples 7 , (2) the diameter of the heating pump laser beam, 2w 0 , in time-domain thermoreflectance (TDTR) measurements of bulk Si between 30 and 100 K (ref.…”

Ultrafast acousto-plasmonic control and sensing in complex nanostructures

“…In addition, through coherent control and structure design we can tailor which modes are excited by the plasmonically generated superdiffusive electrons. Plasmonic nanostructures may expand the toolbox of phononics techniques 43,44 at the nanoscale. The presented results open new routes for the development of coherent phonon sources and are a step towards the realization of nanophononic devices capable of probing electronic, thermal and phononic states.…”