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Register a new userSmart thermal management with near-field thermal radiation
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Added by
Philippe Ben-Abdallah
Publication date
2021
fields
Physics
and research's language is
English
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When two objects at different temperatures are separated by a vacuum gap they can exchange heat by radiation only. At large separation distances (far-field regime) the amount of transferred heat flux is limited by Stefan-Boltzmanns law (blackbody limit). In contrast, at subwavelength distances (near-field regime) this limit can be exceeded by orders of magnitude thanks to the contributions of evanescent waves. This article reviews the recent progress on the passive and active control of near-field radiative heat exchange in two- and many-body systems.
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Thermal control is of critical importance for normal operation of spacecraft. Given thermal radiation is the only means of heat dissipation in space, an efficient thermal control approach for spacecraft is to coat the radiator with a tunable-emittance skin that can tune its heat dissipation according to various thermal conditions. The existing schemes solely relying on far-field thermal radiation, which are based on mechanical, electrochromic or thermochromic working principles, are difficult to combine the advantages of all-solid-state structure, actively and accurate tuning, and large tuning range of heat flux. In this work, we propose a near-field radiation assisted (NFRA) smart skin for thermal control which can tune the heat rejection accurately and in a large range. It contains a metal-insulator-semiconductor (MIS) structure, where the carrier distribution in the semiconductor layer can be electrically altered. In this way, the near-field heat flux, and ultimately the skin emission power expressed using effective emittance, can be controlled as a function of the applied voltage. The variation range of the effective emittance can exceed 0.7 when adjusting the applied voltage from -10 V to 100 V with our preliminary design. This work opens a new way of smart skin design for active spacecraft thermal control.
We calculate, by means of fluctuational electrodynamics, the thermal emission of an aperture filled by vacuum or a material at temperature T. We show that thermal emission is very different whether the aperture size is large or small compared to the thermal wavelength. Subwavelength apertures filled with vacuum (subwavelength blackbody) have their thermal emission strongly decreased compared to classical blackbodies. A simple expression of their emissivity can be calculated and their total emittance scales as T 8 instead of T 4 for large apertures. Thermal emission of disk of materials with a size comparable to the wavelength is also discussed. It is shown in particular that emissivity of such a disk is increased when the material can support surface waves such as phonon polaritons.
Intense electromagnetic evanescent fields are thermally excited in near fields on material surfaces (at distances smaller than the wavelength of peak thermal radiation). The property of the fields is of strong interest for it is material-specific and is important for understanding a variety of surface-related effects, such as friction forces, Casimir forces, near-field heat transfer, and surface-coupled molecular dynamics. On metal surfaces, relevance of surface plasmon polaritons (SPlPs), coupled to collective motion of conduction electrons, has attracted strong interest, but has not been explicitly clarified up to the present time. Here, using a passive terahertz (THz) near-field microscope with unprecedented high sensitivity, we unveil detailed nature of thermally generated evanescent fields (wavelength:lamda0~14.5micron) on metals at room temperature. Our experimental results unambiguously indicate that the thermal waves are short-wavelength fluctuating electromagnetic fields, from which relevance of SPlPs is ruled out.
It is shown that thermally excited plasmon-polariton modes can strongly mediate, enhance and emph{tune} the near-field radiation transfer between two closely separated graphene sheets. The dependence of near-field heat exchange on doping and electron relaxation time is analyzed in the near infra-red within the framework of fluctuational electrodynamics. The dominant contribution to heat transfer can be controlled to arise from either interband or intraband processes. We predict maximum transfer at low doping and for plasmons in two graphene sheets in resonance, with orders-of-magnitude enhancement (e.g. $10^2$ to $10^3$ for separations between $0.1mu m$ to $10nm$) over the Stefan-Boltzmann law, known as the far field limit. Strong, tunable, near-field transfer offers the promise of an externally controllable thermal switch as well as a novel hybrid graphene-graphene thermoelectric/thermophotovoltaic energy conversion platform.
For an oscillating electric dipole in the shape of a small, solid, uniformly-polarized, spherical particle, we compute the self-field as well as the radiated electromagnetic field in the surrounding free space. The assumed geometry enables us to obtain the exact solution of Maxwells equations as a function of the dipole moment, the sphere radius, and the oscillation frequency. The self field, which is responsible for the radiation resistance, does not introduce acausal or otherwise anomalous behavior into the dynamics of the bound electrical charges that comprise the dipole. Departure from causality, a well-known feature of the dynamical response of a charged particle to an externally applied force, is shown to arise when the charge is examined in isolation, namely in the absence of the restraining force of an equal but opposite charge that is inevitably present in a dipole radiator. Even in this case, the acausal behavior of the (free) charged particle appears to be rooted in the approximations used to arrive at an estimate of the self-force. When the exact expression of the self-force is used, our numerical analysis indicates that the impulse-response of the particle should remain causal.
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