Achieving Minimal Heat Conductivity by Ballistic Confinement in Phononic Metalattices

Abstract

Controlling the thermal conductivity of semiconductors is of practical interest in optimizing the performance of thermoelectric and phononic devices. The insertion of inclusions of nanometer size in a semiconductor is an effective means of achieving such control; it has been proposed that the thermal conductivity of silicon could be reduced to 1 W/m/K using this approach and that a minimum in the heat conductivity would be reached for some optimal size of the inclusions. Yet the experimental verification of this design rule has been limited. In this work, we address this question by studying the thermal properties of silicon metalattices that consist of a periodic distribution of spherical inclusions with radii from 7 to 30 nm, embedded into silicon. Experimental measurements confirm that the thermal conductivity of silicon metalattices is as low as 1 W/m/K for silica inclusions and that this value can be further reduced to 0.16 W/m/K for silicon metalattices with empty pores. A detailed model of ballistic phonon transport suggests that this thermal conductivity is close to the lowest achievable by tuning the radius and spacing of the periodic inhomogeneities. This study is a significant step in elucidating the scaling laws that dictate ballistic heat transport at the nanoscale in silicon and other semiconductors.

Preparation of Ordered Nanoporous Indium Tin Oxides with Large Crystallites and Individual Control over Their Thermal and Electrical Conductivities

“…The demonstration of such control is very important for a fundamental understanding of the design of thermoelectric materials. The correlation between pore size and thermal conductivity in metals and silicon is known; however, electrical conductivity has not been considered. Previous attempts to prepare composites of carbon nanotubes and polymers with low κ values attempted to separately control thermal and electrical conduction; however, the introduction of polymers, which was to reduce κ, significantly decreased σ …”

Oxide-Free Three-Dimensional Germanium/Silicon Core–Shell Metalattice Made by High-Pressure Confined Chemical Vapor Deposition

“…19,20 Using the vapor-phase counterpart of this technique, high-pressure confined chemical vapor deposition (HPcCVD), we are now infiltrating semiconductors into the nanoscale voids of the silica template. 21,22,23 These inverse structures, defined as metalattices, inherit the void structure of the template to form an interconnected periodic lattice.…”

“…Even with ligand modifications, however, it is challenging to integrate particle interconnectivity, periodicity, and quantum confinement into one system. This problem has motivated other synthetic approaches to 3D interconnected periodic structures at the nanoscale, especially the synthesis of mesoporous structures by hard or soft templating methods. , Recently Liu et al reported an approach to synthesizing completely connected, atomically crystalline three-dimensional metal structures using high-pressure confined chemical fluid deposition and silica nanoparticle colloidal crystal templates. , Using the vapor-phase counterpart of this technique, high-pressure confined chemical vapor deposition (HPcCVD), we are now infiltrating semiconductors into the nanoscale voids of the silica template. ,, These inverse structures, defined as metalattices, inherit the void structure of the template to form an interconnected periodic lattice.…”

Beating the Thermal Conductivity Alloy Limit Using Long-Period Compositionally Graded Si1–xGex Superlattices

“…Designing semiconductors with desired thermal conductivity (κ) is of great interest for both thermal management applications and thermoelectricity. It is already well established that building nanostructures with multiple length scales that scatter phonons at different wavelengths is an excellent strategy to reduce the lattice contribution to the thermal conductivity. − The presence of disorder in alloys is also well known to scatter high-frequency phonons, while superlattices (SLs) take advantage of zone folding and of the acoustic mismatch between the SL components to effectively scatter phonons of medium-to-long wavelengths. In SLs, the crystalline quality, the period, the acoustic mismatch between the layers, the roughness of the interfaces, the chemical mixing at the interfaces, and the total thickness have significant impact on the thermal conductivity .…”