Abstract:Phonons can display both wave-like and particle-like behaviour during thermal transport. While thermal transport in silicon nanomeshes has been previously interpreted by phonon wave effects due to interference with periodic structures, as well as phonon particle effects including backscattering, the dominant mechanism responsible for thermal conductivity reductions below classical predictions still remains unclear. Here we isolate the wave-related coherence effects by comparing periodic and aperiodic nanomeshe… Show more
“…Compared to the straight beam with w = 75 nm, the tortuous beam with s = 395 nm has 130 additional scattering corner sites. Following diffuse scattering from the boundary within the structures, these phonons scatter in the opposite direction of the dominant heat flow, resulting in a reduction of MFP 22 .…”
Section: Resultsmentioning
confidence: 99%
“…All of the measurements are performed under vacuum to minimize convective heat losses from the beams, and the heat loss through radiation is considered negligible compared to the heat conduction through the samples at all temperatures in the study 31 . The thermal interfacial resistances between the sample and the platforms are negligible since the devices are monolithically fabricated, which minimizes contact resistances 13, 22, 32 . The uncertainty is primarily due to the measurement of the sample dimensions, and the uncertainty propagation is available in Supplementary Information.Figure 2Scanning electron microscope (SEM) images of the measurement structure with false-colored serpentine heater/thermometers.…”
Here we study single-crystalline silicon nanobeams having 470 nm width and 80 nm thickness cross section, where we produce tortuous thermal paths (i.e. labyrinths) by introducing slits to control the impact of the unobstructed “line-of-sight” (LOS) between the heat source and heat sink. The labyrinths range from straight nanobeams with a complete LOS along the entire length to nanobeams in which the LOS ranges from partially to entirely blocked by introducing slits, s = 95, 195, 245, 295 and 395 nm. The measured thermal conductivity of the samples decreases monotonically from ~47 W m−1 K−1 for straight beam to ~31 W m−1 K−1 for slit width of 395 nm. A model prediction through a combination of the Boltzmann transport equation and ab initio calculations shows an excellent agreement with the experimental data to within ~8%. The model prediction for the most tortuous path (s = 395 nm) is reduced by ~14% compared to a straight beam of equivalent cross section. This study suggests that LOS is an important metric for characterizing and interpreting phonon propagation in nanostructures.
“…Compared to the straight beam with w = 75 nm, the tortuous beam with s = 395 nm has 130 additional scattering corner sites. Following diffuse scattering from the boundary within the structures, these phonons scatter in the opposite direction of the dominant heat flow, resulting in a reduction of MFP 22 .…”
Section: Resultsmentioning
confidence: 99%
“…All of the measurements are performed under vacuum to minimize convective heat losses from the beams, and the heat loss through radiation is considered negligible compared to the heat conduction through the samples at all temperatures in the study 31 . The thermal interfacial resistances between the sample and the platforms are negligible since the devices are monolithically fabricated, which minimizes contact resistances 13, 22, 32 . The uncertainty is primarily due to the measurement of the sample dimensions, and the uncertainty propagation is available in Supplementary Information.Figure 2Scanning electron microscope (SEM) images of the measurement structure with false-colored serpentine heater/thermometers.…”
Here we study single-crystalline silicon nanobeams having 470 nm width and 80 nm thickness cross section, where we produce tortuous thermal paths (i.e. labyrinths) by introducing slits to control the impact of the unobstructed “line-of-sight” (LOS) between the heat source and heat sink. The labyrinths range from straight nanobeams with a complete LOS along the entire length to nanobeams in which the LOS ranges from partially to entirely blocked by introducing slits, s = 95, 195, 245, 295 and 395 nm. The measured thermal conductivity of the samples decreases monotonically from ~47 W m−1 K−1 for straight beam to ~31 W m−1 K−1 for slit width of 395 nm. A model prediction through a combination of the Boltzmann transport equation and ab initio calculations shows an excellent agreement with the experimental data to within ~8%. The model prediction for the most tortuous path (s = 395 nm) is reduced by ~14% compared to a straight beam of equivalent cross section. This study suggests that LOS is an important metric for characterizing and interpreting phonon propagation in nanostructures.
“…In that case, what is constant is the surface roughness (Δrms). Here we use a constant p, and the rationale behind studies which use constant Δrms versus constant p 31,48,60,61,62,63,64 is that the microscopic details of phonon scattering at interfaces are poorly understood anyway. 64 However, either way gives vary similar results without any qualitative or quantitative differences in disordered structures.…”
Hierarchical material nanostructuring is considered to be a very promising direction for high performance thermoelectric materials. In this work we investigate thermal transport in hierarchically nanostructured silicon. We consider the combined presence of nanocrystallinity and nanopores, arranged under both ordered and randomized positions and sizes, by solving the Boltzmann transport equation using the Monte Carlo method. We show that nanocrystalline boundaries degrade the thermal conductivity more drastically when the average grain size becomes smaller than the average phonon mean-free-path. The introduction of pores degrades the thermal conductivity even further. Its effect, however, is significantly more severe when the pore sizes and positions are randomized, as randomization results in regions of higher porosity along the phonon transport direction, which introduce significant thermal resistance. We show that randomization acts as a large increase in the overall effective porosity. Using our simulations, we show that existing compact nanocrystalline and nanoporous theoretical models describe thermal conductivity accurately under uniform nanostructured conditions, but overestimate it in randomized geometries. We propose extensions to these models that accurately predict the thermal conductivity of randomized nanoporous materials based solely on a few geometrical features. Finally, we show that the new compact models introduced can be used within Matthiessen's rule to combine scattering from different geometrical features within ~10% accuracy. . 3.7 Wm −1 K −1 for an average pore size of ~ 30 nm and grain sizes between 50 and 80 nm. 21 By reducing both pore and grain sizes, however, Basu et al. reported κ = 1.2 Wm −1 K −1 at 40% porosity in p-type silicon. 22 A recent work in SiGe nanomeshes, reported ultralow κ of 0.55 ± 0.10 Wm −1 K −1 for SiGe nanocrystalline nanoporous structures, a value well below the amorphous limit. 23 A significant amount of work can be found in the literature attempting to clarify these experimental observations. However, theoretical investigations of thermal conductivity in highly/hierarchically disordered nanostructures (which include not only crystalline boundaries, but also pores of random sizes placed at random positions) are very limited. Understanding the qualitative and quantitative details of such geometries on the thermal conductivity would allow the design of more efficient thermoelectrics and heat management materials in general. In this work, we solve the Boltzmann transport equation for phonons in disordered Si nanostructures using the Monte Carlo (MC) method. Monte Carlo, which can capture the details of geometry with relative accuracy, is widely employed to understand phonon transport in various nanostructures such as nanowires, 24,25,26 thin films, 27,28 nanoporous materials, 29,30,31,32,33 polycrystalline materials, 10,15,34,35,36 nanocomposites, 37,38 corrugated structures, 39,40,41,42 silicon-on-insulator devices, 43 etc. We consider geometries that include grai...
“…[77] A limiting factor is that the size and the spacing of the pores in experiments is usually one order of magnitude larger than that studied in atomistic simulations and the κ p reduction is not as drastic. [78,79,80] In np-Si κ p reduction originates from modified phonon dispersion relations, and the geometry and size of the pores can be designed to get κ p < 1 Wm −1 K −1 at reasonably low porosity (30%). [81] Similarly to what observed for nanostructures, a layer of amorphous material at the pore surface enhances the reduction of κ p .…”
Section: Nanoporous Silicon and Sige Heterostructuresmentioning
Thermoelectric devices convert temperature gradients into electrical power and vice versa, thus enabling energy scavenging from waste heat, sensing and cooling.Yet, many of these attractive applications are hindered by the limited efficiency of thermoelectric materials, especially in the low temperature regime. This review provides a summary of the recent advances in the design of new efficient silicon-based thermoelectric materials through nanostructuring, alloying and chemical optimization, emphasizing the contribution from theory and atomistic modeling.Thermoelectric (TE) devices can be used to scavenge thermal energy, and can be combined to photovoltaics to enhance energy conversion efficiency. Peltier coolers will play an increasingly important role in active heat management at the small scale, for example in electronic devices and sensors. However, the largescale implementation of TE technology stems from improving the efficiency and the processing costs of materials. The conversion efficiency of TE materials is determined by their dimensionless figure of merit,
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