“…The pioneering works mainly studied how local resonances flatten phonon dispersion and open phononic bandgaps, which can suppress propagation of phonons at gigahertz frequencies. In the past few years, this idea expanded beyond the phononic bandgaps and researchers began dreaming about the suppression of phonons in the terahertz frequency range thus reducing the thermal conductivity of materials [30,32,33,55]. However, the Brillouin light scattering experiments could confirm changes in phonon dispersion only up to a few tens of gigahertz [31,48], which affects only a negligible part of the thermal phonon spectrum at room temperature.…”
Section: Discussionmentioning
confidence: 99%
“…This mode flattening is often described as hybridization between the membrane modes and the resonant frequencies of the pillars. Analyzing the relative height of the mode location (ξ) [30,31], we find that the states corresponding to the flattened regions are localized inside the pillars (green color) [30,32–34]. Such localized states at the resonant frequencies are called local resonances.10.1080/14686996.2018.1542524-F0001Figure 1.Phonon dispersion of a pillar-based PnC (solid lines), a silicon membrane (dashed lines), and resonant frequencies of a pillar (horizontal dash-dotted lines).…”
Section: Physics Of Local Resonances and Phononic Bandgapsmentioning
confidence: 99%
“…Usually, researchers tentatively attribute the thermal conductivity reduction to the impact of local resonances on the phonon dispersion [32–34,55,57,58]. Indeed, the occurrence of the local resonances was demonstrated by different simulation techniques [30,38,55,58], and since the local resonances flatten the dispersion branches (Figure 1), the average group velocity becomes lower [32,33,57]. Hence, this effect can be linked to the thermal conductivity reduction.…”
Section: Simulations Of Heat Conductionmentioning
confidence: 99%
“…Xiong et al [32] demonstrated that nanopillars not only generate local resonances but also scatter the phonons and reduce their mean free path by one order of magnitude. Similarly, Neogi et al [61] concluded that the reduction in thermal conductivity is mainly caused by the diffuse scattering of phonons on nanopillars whereas local resonances play only a minor role.…”
Phononic crystals have been studied for the past decades as a tool to control the propagation of acoustic and mechanical waves. Recently, researchers proposed that nanosized phononic crystals can also control heat conduction and improve the thermoelectric efficiency of silicon by phonon dispersion engineering. In this review, we focus on recent theoretical and experimental advances in phonon and thermal transport engineering using pillar-based phononic crystals. First, we explain the principles of the phonon dispersion engineering and summarize early proof-of-concept experiments. Next, we review recent simulations of thermal transport in pillar-based phononic crystals and seek to uncover the origin of the observed reduction in the thermal conductivity. Finally, we discuss first experimental attempts to observe the predicted thermal conductivity reduction and suggest the directions for future research.
“…The pioneering works mainly studied how local resonances flatten phonon dispersion and open phononic bandgaps, which can suppress propagation of phonons at gigahertz frequencies. In the past few years, this idea expanded beyond the phononic bandgaps and researchers began dreaming about the suppression of phonons in the terahertz frequency range thus reducing the thermal conductivity of materials [30,32,33,55]. However, the Brillouin light scattering experiments could confirm changes in phonon dispersion only up to a few tens of gigahertz [31,48], which affects only a negligible part of the thermal phonon spectrum at room temperature.…”
Section: Discussionmentioning
confidence: 99%
“…This mode flattening is often described as hybridization between the membrane modes and the resonant frequencies of the pillars. Analyzing the relative height of the mode location (ξ) [30,31], we find that the states corresponding to the flattened regions are localized inside the pillars (green color) [30,32–34]. Such localized states at the resonant frequencies are called local resonances.10.1080/14686996.2018.1542524-F0001Figure 1.Phonon dispersion of a pillar-based PnC (solid lines), a silicon membrane (dashed lines), and resonant frequencies of a pillar (horizontal dash-dotted lines).…”
Section: Physics Of Local Resonances and Phononic Bandgapsmentioning
confidence: 99%
“…Usually, researchers tentatively attribute the thermal conductivity reduction to the impact of local resonances on the phonon dispersion [32–34,55,57,58]. Indeed, the occurrence of the local resonances was demonstrated by different simulation techniques [30,38,55,58], and since the local resonances flatten the dispersion branches (Figure 1), the average group velocity becomes lower [32,33,57]. Hence, this effect can be linked to the thermal conductivity reduction.…”
Section: Simulations Of Heat Conductionmentioning
confidence: 99%
“…Xiong et al [32] demonstrated that nanopillars not only generate local resonances but also scatter the phonons and reduce their mean free path by one order of magnitude. Similarly, Neogi et al [61] concluded that the reduction in thermal conductivity is mainly caused by the diffuse scattering of phonons on nanopillars whereas local resonances play only a minor role.…”
Phononic crystals have been studied for the past decades as a tool to control the propagation of acoustic and mechanical waves. Recently, researchers proposed that nanosized phononic crystals can also control heat conduction and improve the thermoelectric efficiency of silicon by phonon dispersion engineering. In this review, we focus on recent theoretical and experimental advances in phonon and thermal transport engineering using pillar-based phononic crystals. First, we explain the principles of the phonon dispersion engineering and summarize early proof-of-concept experiments. Next, we review recent simulations of thermal transport in pillar-based phononic crystals and seek to uncover the origin of the observed reduction in the thermal conductivity. Finally, we discuss first experimental attempts to observe the predicted thermal conductivity reduction and suggest the directions for future research.
“…11 We previously used the spectrally-decomposed MFP method to calculate the non-equilibrium MFPs in low-dimensional systems such as carbon nanotubes, 9 anharmonic chains, 11 and nanowires with resonant scatterers. 12 We demonstrated that the non-equilibrium MFPs transparently describe the ballistic-to-diffusive transition in the length-dependence of thermal conductivity 9,11,12 and reveal the effects of structural modifications such as alloying on frequency-dependent phonon transport. 12 Compared to previous calculations for a-Si, the spectrally-decomposed MFP method has several advantages.…”
Vibrational energy transport in disordered media is of fundamental importance to several fields spanning from sustainable energy to biomedicine to thermal management. In this work, we investigate hybrid ordered/disordered nanocomposites that consist of crystalline membranes decorated by regularly patterned disordered regions formed by ion beam irradiation. The presence of the disordered regions results in reduced thermal conductivity, rendering these systems of interest for use as nanostructured thermoelectrics and thermal device components, yet their vibrational properties are not well understood. Here we establish in detail the mechanism of vibrational transport and the reason underlying the observed reduction. The hybrid systems are found to exhibit glass-crystal duality in vibrational transport. Lattice dynamics reveals substantial hybridization between the localized and delocalized modes, which induces avoided crossings and harmonic broadening in the dispersion. Allen/Feldman theory shows that the hybridization and avoided crossings are the dominant drivers of the reduction. Anharmonic scattering is also enhanced in the patterned nanocomposites, further contributing to the reduction. The systems exhibit features reminiscent of both nanophononic materials and locally resonant nanophononic metamaterials, but operate in a manner distinct to both. These findings indicate that such "patterned disorder" can be a promising strategy to tailor vibrational transport through hybrid nanostructures.Received: ((will be filled in by the editorial staff))Revised: ((will be filled in by the editorial staff))
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