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Excitation of surface plasmons enables super-Planckian thermal radiation far beyond the blackbody limit. By patterning a single layer of graphene sheet into ribbons, the closed circular dispersion of graphene plasmons is opened to become hyperbolic, leading to broadband singularities of density of states. Extremely high-k evanescent waves can now couple with hyperbolic graphene plasmons. Consequently, a giant enhancement of the near-field radiative heat flux, by more than one order of magnitude, is demonstrated in this study using rigorous numerical simulations. The findings may open promising pathways for highly efficient thermal management, energy harvesting, and subwavelength thermal imaging. [ABSTRACT FROM AUTHOR]

Spectral and directional control of thermal radiation is a challenging yet important task for a number of applications, including thermophotovoltaic energy conversion, solar energy utilization, space thermal management, and high-efficiency incandescent lamps. A number of structures have been proposed as coherent emission sources, with spectral emission peaks in well-defined directions. The grating structure has been commonly used in which coherent emission is achieved by the excitation of surface polaritons or surface waves. Along with the development of metamaterials, magnetic polaritons have also received much attention lately and have been demonstrated to be useful for controlling the radiative properties of periodic structures. Furthermore, planar structures can also support surface waves or optical resonance for thermal emission control, such as the truncated one-dimensional photonic crystals and asymmetric Fabry–Perot structures. This article provides an overview of theoretical and experimental research activities, performed in the Nanoscale Thermal Radiation Lab at the Georgia Institute of Technology, on tailoring the spectral and directional thermal radiative properties. [ABSTRACT FROM AUTHOR]

The near-field radiative heat transfer for two hyperbolic metamaterials, namely, graphite and vertically aligned carbon nanotubes (CNTs), is investigated. Graphite is a naturally existing uniaxial medium, while CNT arrays can be modeled as an effective anisotropic medium. Different hyperbolic modes can be separately supported by these materials in certain infrared regions, resulting in a strong enhancement in near-field heat transfer. It is predicted that the heat flux between two CNT arrays can exceed that between SiC plates at any vacuum gap distance and is about 10 times higher with a 10 nm gap. [ABSTRACT FROM AUTHOR]

Thermal rectification has recently attracted great attention because it could allow heat to flow in a preferred direction and may have promising applications in thermal management and energy systems. In addition to phonon engineering, photon transport at the near-field regime has been recently proposed to realize thermal rectification between planar structures. In the present study, the thermal rectification effect enabled by near-field thermal radiation between intrinsic silicon and other materials was investigated at various temperatures and vacuum gap distances. Strong thermal rectification between intrinsic Si and doped Si (rectificationR = 2.7) and between intrinsic Si and SiO2(R = 9.9) can be achieved with a 5 nm vacuum gap at temperatures of 1000 and 300 K. Rectification larger than one can be obtained in sub-10 nm vacuum gaps for the former configuration and sub-20 nm gaps for the latter configuration. A thermal rectifier made of gold and intrinsic Si is shown to have a rectification factor around 0.85 with temperatures of 600 and 300 K at a wide range of vacuum gaps from 100 to 500 nm. The physical mechanisms of the rectification effect in the three configurations are elucidated, and each of the proposed thermal rectifiers may have its own advantage for applications dealing with different temperatures and vacuum distances. [ABSTRACT FROM AUTHOR]

It is well known that radiative heat transfer in the near field regime can be greatly enhanced because of photon tunneling, during which the classical radiation theory fails to predict the energy propagation direction. In the present article, we investigate the lateral shifts of energy pathways in near-field thermal radiation between two SiC plates separated by a vacuum gap. The energy streamline (ESL) method can be combined with fluctuational electrodynamics to trace the Poynting vector at a fixed parallel wavevector component β. The spectral energy flux exhibits nearly monochromatic enhancement with the excitation of surface phonon polaritons. In the spectral region important for radiative energy transfer, there exist considerable lateral shifts depending on the values of β. The range of β dominantly contributing to the spectral energy flux is identified for different wavelengths by examining the Poynting vector equation. With the use of ESLs, the effect of coupled surface polaritons on the lateral shift is also investigated. Furthermore, the lateral dimension for the SiC plates to be treated as infinitely extended is estimated based on the lateral shifts in the wavelength region that mostly contributes to the total energy flux. [ABSTRACT FROM AUTHOR]

Fast depleting reserves of conventional energy sources has resulted in an urgent need for increasing energy conversion efficiencies and recycling of waste heat. One of the potential candidates for fulfilling these requirements is thermophotovoltaic (TPV) devices. TPV devices generate electricity from either the complete combustion of different fuels or the waste heat of other energy sources, thereby saving energy. The thermal radiation from the emitter is incident on a TPV cell, which generates electrical currents. Applications of such devices range from hybrid electric vehicles to power sources for microelectronic systems. Absence of any moving parts and versatile fuel usage has made TPV devices very appealing for military and space applications. However, the presently available TPV systems suffer from low conversion efficiency. A viable solution to increase their efficiency is to apply microscale radiation principles in the design of different components to utilize the characteristics of thermal radiation at small distances and in microstructures. In order to have a clear understanding of microscale radiation and its role in TPV devices, several critical issues are reviewed in the present work. Emphasis is given to the development of wavelength-selective emitters and filters and the aspects of microscale heat transfer as applied to TPV systems. Recent progress, along with challenges and opportunities for future development of TPV systems are also outlined. Copyright © 2006 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

This work addresses the validity of the local effective medium theory (EMT) in predicting the near-field radiative heat transfer between multilayered metamaterials, separated by a vacuum gap. Doped silicon and germanium are used to form the metallodielectric superlattice. Different configurations are considered by setting the layers adjacent to the vacuum spacer as metal-metal (MM), metal-dielectric (MD), or dielectric-dielectric (DD) (where M refers to metallic doped silicon and D refers to dielectric germanium). The calculation is based on fluctuational electrodynamics using the Green's function formulation. The cutoff wave vectors for surface plasmon polaritons (SPPs) and hyperbolic modes are evaluated. Combining the Bloch theory with the cutoff wave vector, the application condition of EMT in predicting near-field radiative heat transfer is presented quantitatively and is verified by exact calculations based on the multilayer formulation. [ABSTRACT FROM AUTHOR]

This paper describes a theoretical investigation of near-field radiative heat transfer between doped silicon surfaces separated by a vacuum gap. An improved dielectric function model for heavily doped silicon is employed. The effects of doping level, polarization, and vacuum gap width on the spectral and total radiative transfer are studied based on the fluctuational electrodynamics. It is observed that increasing the doping concentration does not necessarily enhance the energy transfer in the near-field. The energy streamline method is used to model the lateral shift of the energy pathway, which is the trace of the Poynting vectors in the vacuum gap. The local density of states near the emitter is calculated with and without the receiver. The results from this study can help improve the understanding of near-field radiation for applications such as thermophotovoltaic energy conversion, nanoscale thermal imaging, and nanothermal manufacturing. [ABSTRACT FROM AUTHOR]