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In contrast to established work that uses diffuse surface scattering as the mechanism to lower the in-plane thermal conductivities, we show that the largest reduction of heat conduction in thin films is achieved via specular scattering under cross-plane heat transport. Our results create a new perspective for heat conduction manipulation since smooth surfaces—in contrast to rough surfaces—are shown to be more effective in suppressing thin film heat conduction. [ABSTRACT FROM AUTHOR]

We report a phonon transport study to elucidate nanoscale thermal conduction in silicon-germanium superlattices considering interactions of phonons with multiple structural length scales. Our results clearly demonstrate the need for quantifying the impact of all relevant length variables in superlattices, i.e., the mean free path and wavelength of phonons, the periodicity of the structure, total size of the superlattice, and the length scale of interfacial disorder, to fully understand the heat conduction in superlattices. Our predictions show that thermal conduction can be ballistic travelling across multiple low roughness interfaces of the superlattice even at room temperatures. In contrast to in-plane transport, we find that the strong surface scattering encountered in the cross-plane direction limits the phonon transport to mean-free-paths of less than 1 μm and wavelengths less than 10 nm even in alloyed superlattices of periods up to 50 nm. This strong role of boundaries also manifests itself in the form of thermal conductivity anisotropy in superlattices. We also investigate the impact of the number of periods and total structural size on the thermal conductivity which is critical for accurate experimental reporting of thermal conductivities. [ABSTRACT FROM AUTHOR]

The phonon is the physical particle responsible for the transmission of sound and heat; controlling the properties of phonons in materials could trigger many advances, which are reviewed here. Prepare for the age of phononics In the emerging research area of phononics, control over the mechanical vibrations that transmit sound and heat -- phonons -- plays a central role. Like photons and electrons, phonons can be treated as particles for many purposes, so can be harnessed and manipulated for useful applications. The phonon spectrum covers a wide range of effects, from low- frequency acoustics, to ultrasound and to heat, so that phononic techniques could enable a wide range of applications such as in earth quake protection, acoustics and heat management. In this review, Martin Maldovan discusses several approaches to the control of phonons at different length scales, for example phononic crystals, metamaterials, thermoelectrics and optomechanical devices. Today's digital revolution is underpinned by the high degree of control that can be imposed over electrons in semiconductors; Maldovan argues that precise control over phonons could have similar surprising and exciting consequences. The phonon is the physical particle representing mechanical vibration and is responsible for the transmission of everyday sound and heat. Understanding and controlling the phononic properties of materials provides opportunities to thermally insulate buildings, reduce environmental noise, transform waste heat into electricity and develop earthquake protection. Here I review recent progress and the development of new ideas and devices that make use of phononic properties to control both sound and heat. Advances in sonic and thermal diodes, optomechanical crystals, acoustic and thermal cloaking, hypersonic phononic crystals, thermoelectrics, and thermocrystals herald the next technological revolution in phononics., Author(s): Martin Maldovan [sup.1] [sup.2] Author Affiliations: (1) Department of Materials Science and Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, USA, (2) School of Chemical and [...]

We report an approach to modulate thermal conduction that utilizes phonon coupling in layered nanostructures. While phonon coupling has been used previously to enhance thermal transport of an embedded layer in a tri-layer structure, the impact of coupling on cladding layers has remained unclear. Here, we develop a methodology to quantitatively evaluate the impact of phonon coupling on each layer in a tri-layer structure. We uncover that the underlying phonon-injection mechanism behind thermal conductivity enhancement can also be leveraged to reduce the thermal conductivity of an embedded silicon thin-film below its free-standing value. We evaluate the dependence of resultant thermal conductivity modulations on structural parameters and find that they are critically dependent on layer spacings and interface properties. We also extend the tri-layer transport analysis to bi-layer structures and report how phonon coupling leads to analogous thermal conductivity modulations. The results of this work open new avenues within the rational thermal design by elucidating a new method that can be used to both increase and reduce thermal conductivities and advance the basic understanding of nanoscale thermal transport by incorporating the role of phonon spectral coupling. The prospects of being able to modulate the thermal conductivity can radically change how we control heat flow in electronic, optoelectronic, and thermoelectric materials. [ABSTRACT FROM AUTHOR]

Phonon-surface scattering is the fundamental mechanism behind thermal transport phenomena at the nanoscale. Despite its significance, typical approaches to describe the interaction of phonons with surfaces do not consider all relevant physical quantities involved in the phonon-surface interaction, namely, phonon momentum, incident angle, surface roughness, and correlation length. Here, we predict thermal conduction properties of thin films by considering an accurate description of phonon-surface scattering effects based on the rigorous Beckmann-Kirchhoff scattering theory extended with surface shadowing. We utilize a Boltzmann transport based reduced mean-free-path model for phonon transport in thin-films to predict the wavelength and mean-free-path heat spectra in Si and SiGe films for different surface conditions and show how the thermal energy distribution can be tailored by the surface properties. Using the predicted wavelength spectra, we also introduce a measure to quantify phonon-confinement effects and show an enhanced confinement in Ge alloyed Si thin films. The impact of surface roughness and correlation lengths on thermal conductivities is also studied, and our numerical predictions show excellent agreement with experimental measurements. The results allow to elucidate and quantitatively predict the amount of thermal energy carried by different phonons at the nanoscale, which can be used to design improved optoelectronic and thermoelectric devices. [ABSTRACT FROM AUTHOR]

Recent advances in coordinate transformations of Fick's equation have paved the way for the design of metamaterial devices that can manipulate mass diffusion flux. The control of diffusion paths has a great potential for the design of novel catalytic and separation systems in chemical and biomolecular engineering. In order to explore these new applications, it is necessary to understand mass diffusion in coordinate transformation metamaterial devices. In this work, we present a comprehensive study on the impact of structure and material properties on the resultant physical properties of mass concentrator metamaterial shells. The concentration gradient at the core, the total mass flow rate towards the core, and the disturbance of the external concentration field are systematically examined in order to provide guidelines for the rational design and fabrication of metamaterial mass concentrators. A practical case is also presented where the concentration of oxygen diffusing in a polymeric system is studied. [ABSTRACT FROM AUTHOR]

We study the influence of phonon boundary scattering on the transport of thermal energy in semiconductor nanowires from micro to nano length scales. We use a kinetic theory model based on the Boltzmann transport equation that accurately calculates the reduction of the phonon mean free paths by considering their directional dependence and the fundamental statistical definition of the mean free path. As a result, our model does not use phenomenological formulas to account for the reduction of the phonon mean free paths due to boundary scattering. The transport of thermal energy is also fully divided into that carried by different polarizations by separating phonon group velocities and relaxation times for transverse and longitudinal phonons. We study the correctness of using frequency independent versus frequency dependent models for describing the specularity of the nanowire boundary. We also examine the validity of the assumption that phonons in the semiconductor nanowire maintain their bulk phonon dispersion relations and that modifications to the dispersion relations due to phonon confinement effects can be neglected. The thermal conductivities of silicon nanowires are calculated for different length scales and temperatures and good agreement is obtained with experiments. The theoretical results in this paper can be used to understand and quantitatively predict heat transport in nanowires, which is critical for increasing the efficiency of thermoelectric and electronic devices. [ABSTRACT FROM AUTHOR]

Understanding thermal energy transport in polycrystalline semiconductors is important for the efficiency of electronic devices and thermoelectric materials. In this paper, we study the reduction of the transport of thermal energy in polycrystalline semiconductors generated by the shortening of the phonon mean free paths due to grain boundary scattering. We calculate the reduction of the thermal conductivity in polycrystals, from macro-to-nanograin sizes and different temperatures, by using a theoretical approach based on the kinetic theory of transport processes. The approach involves an exact expression for the reduction of the phonon mean free paths that includes their directional, frequency, and polarization dependence. By comparing the results of our model for the reduced thermal conductivity of the grain against the thermal boundary Kapitza resistance calculated by others, we find that the thermal conductivity of polycrystalline Si and SiC materials is dominated by the reduced thermal conductivity of the grain. We also show that in order to accurately calculate the thermal conductivity, the proportion of heat transported by transverse and longitudinal phonons must be correctly taken into account. By using the model, we study grain boundary scattering effects on the reduction of the thermal conductivity of polycrystalline silicon and silicon carbide. The calculated results are compared with experiments at different temperatures and grain sizes without using free adjustable variables (e.g., defects concentration) or phenomenological formulas to account for the reduced thermal conductivity of the grain. [ABSTRACT FROM AUTHOR]

We study the effects of phonon boundary scattering on the transport of thermal energy in semiconductor thin films across multiple length scales and temperatures. We use a model based on the kinetic theory of transport processes that accurately calculates the reduction of the phonon mean free paths by including the effects of spatial location and propagation direction of phonons. We investigate how the effective phonon mean free paths and the resultant thermal conductivities are reduced by the film length scale and surface roughness. The thermal conductivities of silicon and germanium thin films are calculated for temperatures between 4 K and 500 K and thicknesses from nano to micro and good agreement is obtained with experimental measurements. The theoretical study in this paper helps to understand and quantitatively predict the transport of thermal energy in nanoscale materials, which can be used to improve the efficiency of optoelectronic devices and thermoelectric materials. [ABSTRACT FROM AUTHOR]