Phonon and heat transport control using pillar-based phononic crystals

Abstract

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.

Coherent Thermal Conduction in Silicon Nanowires with Periodic Wings

“…The impact of local resonances can be seen as flat bands around 6, 9, and 17 GHz, close to resonant frequencies of the wings. To distinguish these two phenomena more clearly, the color of the dispersion branches shows the center of elastic energy ( ξ ) in the y axis, calculated as:ξ=1n/2+d∫VF|y|dV∫VFdV where F is elastic energy density and the integral is evaluated over the volume ( V ) of a unit cell [21,22]⁠⁠. Thus, the lighter branches contained regular non-localized modes (Figure 1c), whereas dark flatter branches contained the modes localized in the wings (Figure 1d).…”

Anchor Loss Reduction of Lamb Wave Resonator by Pillar-Based Phononic Crystal

“…We investigated the propagation of an elastic wave in the PnC formed by a 1-μm-thickness Pt pillar on a thin AlN membrane and discovered that a square unit cell with lattice constant a = 4 μm creates wide bandgaps with a relatively small filling fraction. This pillar-based structure has the following advantages over the holey or fractal structure: (1) the generation of desired large complete bandgaps; (2) the guarantee of the structure strength for the remaining of the origin structure geometry and no etching or hollowing to undermine the structural integrity; (3) a further available design factor (pillar height) to adjust the bandgap; and (4) the possibility to create phononic bandgaps formed by both Bragg interference and local resonances simultaneously [ 24 , 28 ]. Figure 2 a illustrates the basic unit and the detailed geometrical dimensions of the pillar-based PnCs.…”

Thermal Conductivity Reduction in a Nanophononic Metamaterial versus a Nanophononic Crystal: A Review and Comparative Analysis

“…Nanophononic metamaterials have also been studied using finite‐element (FE) analysis as commonly done for macroscale acoustic or elastic metamaterials . However, while linear continuous modeling may provide some insight on the dynamics of local resonances, nonlinear atomic‐scale models are necessary for accurate qualitative and quantitative capturing of the effects of the three key NPM mechanisms responsible for the thermal conductivity reduction.…”