Heat dissipation in the quasiballistic regime studied using the Boltzmann equation in the spatial frequency domain

Hua, Chengyun; Minnich, Austin J.

doi:10.1103/physrevb.97.014307

Cited by 24 publications

(19 citation statements)

References 27 publications

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“…All three TDTR measurements differ from our results, however, in that they never observe a return toward the diffusive prediction at constant duty cycle, when the heat source period is comparable to the dominant substrate phonon MFPs. The reason for this difference might come from the different experimental temporal frequencies, as suggested by recent work by Hua and Minnich [43].…”

Section: Resultsmentioning

confidence: 97%

Engineering Nanoscale Thermal Transport: Size- and Spacing-Dependent Cooling of Nanostructures

et al. 2019

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Nanoscale thermal transport is becoming ever more technologically important with the development of next-generation nanoelectronics, nanomediated thermal therapies, and high efficiency thermoelectric devices. However, direct experimental measurements of nondiffusive heat flow in nanoscale systems are challenging, and first-principle models of real geometries are not yet computationally feasible. In recent work, we used ultrafast pulses of short-wavelength light to uncover a previously-unobserved regime of nanoscale thermal transport that occurs when the width and separation of heat sources are comparable to the mean free paths of the dominant heat-carrying phonons in the substrate. We now systematically compare thermal transport from gratings of metallic nanolines with different periodicities, on both silicon and fused-silica substrates, to map the entire nanoscale thermal transport landscape -from closely spaced through increasingly isolated to fully isolated heat-transfer regimes. By monitoring the surface profile dynamics with subangstrom sensitivity, we directly measure thermal transport from the nanolines into the substrate. This allows us to quantify for the first time how the nanoline separation significantly impacts thermal transport into the substrate, making it possible to reach efficiencies that are within a factor of 2 of the diffusive (i.e., thin-film) limit. We also show that partially isolated nanolines perform significantly worse, because cooling occurs in a regime that is intermediate between close-packed and fully isolated heat sources. This work thus confirms the surprising prediction that closely spaced nanoscale heat sources can cool more quickly than when far apart. These results show that our predictive model is validated by experiment over a broad parameter space, which is important for benchmarking theories that go beyond the Fourier model of heat diffusion, and for informed design of nanoengineered systems.

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Section: Resultsmentioning

confidence: 97%

Engineering Nanoscale Thermal Transport: Size- and Spacing-Dependent Cooling of Nanostructures

et al. 2019

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“…We demonstrated in Ref. [63] that the spectral distribution of the source term alters the surface temperature response. In other words, even though the first term in Eq.…”

Section: Discussionmentioning

confidence: 99%

Generalized Fourier's law for nondiffusive thermal transport: Theory and experiment

et al. 2019

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Phonon heat conduction over length scales comparable to their mean free paths is a topic of considerable interest for basic science and thermal management technologies. However, debate exists over the appropriate constitutive law that defines thermal conductivity in the nondiffusive regime. Here, we derive a generalized Fourier's law that links the heat flux and temperature fields, valid from ballistic to diffusive regimes and for general geometries, using the Peierls-Boltzmann transport equation within the relaxation time approximation. This generalized Fourier's law predicts that thermal conductivity not only becomes nonlocal at length scales smaller than phonon mean free paths but also requires the inclusion of an inhomogeneous nonlocal source term that has been previously neglected. We provide evidence for the validity of this generalized Fourier's law through direct comparison with time-domain thermoreflectance measurements in the nondiffusive regime without adjustable parameters. Furthermore, we show that interpreting experimental data without the generalized Fourier's law can lead to inaccurate measurement of thermal transport properties.

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“…Non-diffusive heat transport can take place when the heat carrier's mean free path becomes comparable with, or exceeds, the pump spot size or the diffusive heat penetration depth ' ¼ ffiffiffiffiffiffiffiffiffiffiffiffiffiffi K=pCf p . 22,23 Predicting when a departure from diffusive transport occurs can be complicated, as nondiffusive transport may take place anisotropically, 22 according to heater geometry, 23 or depend on the nature of the interface between two materials. 24 Analysis of experimental data in which non-diffusive transport takes place using a model that only considers diffusive heat transport often leads to obtaining thermal conductivities that fall below that of bulk values or frequency-dependent thermal properties.…”

Section: S Kmentioning

confidence: 99%

Measuring the thermal properties of anisotropic materials using beam-offset frequency domain thermoreflectance

Rahman

Shahzadeh

Braeuninger‐Weimer

et al. 2018

Journal of Applied Physics

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Thermoreflectance techniques have become popular to measure the thermal properties of thin films such as thermal conductivity and thermal boundary conductance (TBC). Varying the focused spot sizes of the beams increases the sensitivity to in-plane heat transport, enabling the characterization of thermally anisotropic materials. However, this requires realignment of the optics after each spot size adjustment. Offsetting the probe beam with respect to the pump beam and modulating over a wide range of frequencies (5 kHz to 50 MHz) yield better sensitivity to the thermophysical properties of anisotropic materials without varying the spot sizes. We demonstrate how beam-offset frequency domain thermoreflectance can be used to measure the in-and out-of-plane thermal conductivity as well as the TBC simultaneously from a single data set by working at reduced spot sizes. Lowering the laser spot size allows us to detect signals over a wide range of frequencies and use larger beam offsets, thanks to the increase in the thermoreflectance signal. We measure the anisotropic thermal properties of a range of materials, including single layer Graphene on SiO 2 , which is of interest for novel electronic devices.

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Heat dissipation in the quasiballistic regime studied using the Boltzmann equation in the spatial frequency domain

Cited by 24 publications

References 27 publications

Engineering Nanoscale Thermal Transport: Size- and Spacing-Dependent Cooling of Nanostructures

Engineering Nanoscale Thermal Transport: Size- and Spacing-Dependent Cooling of Nanostructures

Generalized Fourier's law for nondiffusive thermal transport: Theory and experiment

Measuring the thermal properties of anisotropic materials using beam-offset frequency domain thermoreflectance

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