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Near-field radiative heat transfer (NFRHT) between multilayer graphene/hBN heterostructures has been demonstrated to exceed the blackbody limit due to the coupling mechanism of surface plasmon polaritons and hyperbolic phonon polaritons, opening the door to applications in thermal management, thermophotovoltaics, and nanoscale metrology. Recent studies have shown that adding vacuum layers within multilayer structures can effectively promote surface modes and thus enhance NFRHT. However, the influence of vacuum layers on NFRHT between multilayer graphene/hBN heterostructures has not been investigated. Moreover, the influence of vacuum layers on coupled resonance modes excited in multilayer structures is worth discussing. In this work, we study the NFRHT based on multilayer graphene/vacuum/hBN/vacuum structures. The results show that as the gap distance increases from 20 nm to 100 nm, the NFRHT of three-cell and six-cell configurations is enhanced, while that of unit-cell configuration is suppressed. The potential mechanism can be identified as the excitation of surface plasmon-phonon polaritons (SPPPs) and hyperbolic plasmon-phonon polaritons (HPPPs) in multilayer structures. The enhancement factor of the six-cell configurations is up to 4.82 when the gap distance is 80 nm. Moreover, the influences of the chemical potential of graphene and the layer thickness on the NFRHT are discussed. The interesting results in this work indicate the perspectives for future near-field research involving coupling of SPPPs and HPPPs, and shed new light on high-performance devices introducing vacuum layers based on near-field radiative heat transfer. [ABSTRACT FROM AUTHOR]

Near-field thermophotovoltaic (NTPV) devices comprising a SiC-hBN-graphene emitter and a graphene-InSb cell with gratings are designed to enhance the performance of the NTPV systems. Fluctuational electrodynamics and rigorous coupled-wave analysis are employed to calculate radiative heat transfer fluxes. It is found that the NTPV systems with two graphene ribbons perform better due to the graphene strong coupling effects. The effects of graphene chemical potential are discussed. It is demonstrated that near-field radiative heat transfer of thermophotovoltaic devices is enhanced by the coupling of surface plasmon polaritons, surface phonon polaritons, hyperbolic phonon polaritons, and magnetic polaritons caused by the graphene strong coupling effects. Rabi splitting frequency of different polaritons is calculated to quantify the mutual interaction of graphene strong coupling effects. Finally, the effects of cell grating filling ratio are investigated. The excitation of magnetic polaritons is affected by the graphene ribbon and the cell filling ratio. This investigation provides a new explanation of the enhancement mechanism of graphene-assisted thermophotovoltaic systems and a novel approach for improving the output power of the near-field thermophotovoltaic system. [ABSTRACT FROM AUTHOR]

Near-field radiative heat transfer has recently attracted increasing interests for its applications in energy technologies, such as thermophotovoltaics. Existing works, however, are restricted to time-independent systems. Here, we explore near-field radiative heat transfer between two bodies under time modulation by developing a rigorous fluctuational electrodynamics formalism. We demonstrate that time modulation can result in the enhancement, suppression, elimination, or reversal of radiative heat flow between the two bodies, and can be used to create a radiative thermal diode with an infinite contrast ratio, as well as a near-field radiative heat engine that pumps heat from the cold to the hot bodies. The formalism reveals a fundamental symmetry relation in the radiative heat transfer coefficients that underlies these effects. Our results indicate the significant capabilities of time modulation for managing nanoscale radiative heat flow. [ABSTRACT FROM AUTHOR]

Thermophotovoltaic (TPV) energy converters hold substantial potential in converting thermal radiation from high-temperature emitters into electrical energy through photovoltaic (PV) cells, offering applications ranging from solar energy harvesting to waste heat recovery. Near-field TPV (NF-TPV) devices, focused on enhancing power output density (POD), exhibit unique potential by harnessing photon tunneling. However, this potential can be mitigated by additional losses arising from high photocurrent densities and corresponding scalability issues. This study comprehensively investigates the effectiveness of multi-junction-based NF-TPV devices, accounting for additional losses. We propose two approximative expressions to quantify the impact of additional losses and characterize current density-voltage curves. Verification against rigorously optimized results establishes a criterion for effective performance. Our method provides precise POD estimations even for devices with 10 or more subcells, facilitating performance analysis across parameters like vacuum gap distance, cell width, emitter temperature, and the number of subcells compared to far-field counterparts. This research outlines a roadmap for the scalable design of NF-TPV devices, emphasizing the role of multi-junction PV cells. The analytical framework we developed will provide vital insights for future high-performance TPV devices. [ABSTRACT FROM AUTHOR]

Thermophotovoltaic (TPV) energy converters hold substantial potential in converting thermal radiation from high-temperature emitters into electrical energy through photovoltaic (PV) cells, offering applications ranging from solar energy harvesting to waste heat recovery. Near-field TPV (NF-TPV) devices, focused on enhancing power output density (POD), exhibit unique potential by harnessing photon tunneling. However, this potential can be mitigated by additional losses arising from high photocurrent densities and corresponding scalability issues. This study comprehensively investigates the effectiveness of multi-junction-based NF-TPV devices, accounting for additional losses. We propose two approximative expressions to quantify the impact of additional losses and characterize current density-voltage curves. Verification against rigorously optimized results establishes a criterion for effective performance. Our method provides precise POD estimations even for devices with 10 or more subcells, facilitating performance analysis across parameters like vacuum gap distance, cell width, emitter temperature, and the number of subcells compared to far-field counterparts. This research outlines a roadmap for the scalable design of NF-TPV devices, emphasizing the role of multi-junction PV cells. The analytical framework we developed will provide vital insights for future high-performance TPV devices.

Efficient thermal management has become crucial in micro and nano devices. This research presents a near-field thermal splitter, a pivotal device for managing thermal energy at distances less than the thermal characteristic wavelength. The thermal splitter, composed of one source and two drains, utilizes the anisotropic properties of α-phase molybdenum trioxide (α-MoO3) nanoparticles. By rotating the source, the direction of heat flux can be precisely controlled. When employing α-MoO3 for both source and drains, the system can achieve a thermal splitting effect ranging from 1 % to 99 %. To one’s interest, the system maintains considerable splitting effect even with drains made of other materials supporting optical modes in the reststrahlen band. The near-field thermal splitter shows robust performance despite variations in distances between the three terminals and source rotation angles. This research provides an effective approach for managing complex heat flux networks, potentially enhancing applications in energy harvesting and advanced thermal management systems.

Both the nonreciprocal surface modes in Weyl semimetal (WSM) with a large anomalous Hall effect and the nonreciprocal photon occupation number on a graphene surface induced by the drift current provide a promising way to manipulate the nonreciprocal near-field energy transfer. Interestingly, the interactions between nonreciprocities are highly important for research in (thermal) photonics but remain challenging. In this study, we theoretically investigated the near-field radiative heat flux transfer between a graphene heterostructure supported by a magnetic WSM and a twist-Weyl semimetal (T-WSM). The nonreciprocal surface mode could be changed by the separation space between two Weyl nodes and the twist angle. Notably, we found that in the absence of a temperature difference between two parallel plates, nonequilibrium fluctuations caused by drift currents led to the transfer of near-field radiative heat flux. Furthermore, these nonreciprocal surface modes interacted with the nonreciprocal photon occupation number in graphene to achieve flexible manipulation of the near-field heat flux size and direction. Additionally, graphene adjustable flux in the case of a temperature difference between the two plates was also discussed. Our scheme can provide a reference for near-field heat flux regulation in nonequilibrium systems.

The near-field effect can be used to improve the output power of the near-field thermophotovoltaic device (NTPV). The near-field radiative heat transfer in the near-field thermophotovoltaic device can be enhanced by the excitation of hyperbolic modes and the coupling of surface plasmon polaritons. In this study, we design a two-body near-field thermophotovoltaic system based on hyperbolic metamaterial. The multilayer structure on the emitter is composed of Ga-doped ZnO (GZO) and hafnium dioxide (HfO2). The gratings are on the InAs photovoltaic cell. Fluctuational electrodynamics and rigorous coupled-wave method are employed to calculate radiative heat transfer. It is found that the NTPV system with multiple microstructures performs better than the NTPV system just with single micro-structures. This NTPV system performs better in a wider vacuum gap. The output power and efficiency are enhanced by the GZO-HfO2 surface plasmon polaritons in multilayer structure. The gratings can monitor the spectral heat flux to match the cell band gap to enhance the performance of the near-field thermophotovoltaic system. This investigation provides a novel approach for improving the output power of a two-body near-field thermophotovoltaic system. [ABSTRACT FROM AUTHOR]

Coupling of surface plasmon polaritons (SPPs) supported by graphene and hyperbolic phonon polaritons (HPPs) supported by hyperbolic materials (HMs) could effectively promote photon tunneling, and hence the radiative heat transfer. In this work, we investigate the polariton hybridization phenomena on near-field radiative heat transfer (NFRHT) in multilayer heterostructures, which consist of periodic graphene/α-MoO3 cells. Numerical results show that increasing the graphene/α-MoO3 cells can effectively enhance the NFRHT when the vacuum gap is less than 50 nm, but suppresses the enhanced performance with larger gap distance. This depends on the coupling of SPPs and HPPs in the periodic structure, which is analyzed by the energy transmission coefficients distributed in the wavevector space. The influence of the thickness of the α-MoO3 film and the chemical potential of graphene on the NFRHT is investigated. The findings in this work may guide designing high-performance near-field energy transfer and conversion devices based on coupling polaritons.

The energy conversion performance of thermophotovoltaic (TPV) systems can be improved when the vacuum gap between the emitter and the TPV cell is at the near-field owing to the photon tunneling of evanescent waves. Among them, the back-gapped-reflector TPV systems have gained interest as a method of improving their conversion efficiency by optimizing the spectral absorption of TPV cells. In this work, we introduce an alternative concept for the back-gapped-reflector TPV systems, namely the medium-bridge near-field TPV system, by building a medium bridge between the metal reflector and the TPV cell using SU8 nanofilm. The SU8 medium-bridge achieves a noticeable improvement in output performance by increasing the spectral absorption of the InAs cell and reducing parasitic absorption losses of the Au substrate. The results indicate that, as a consequence of the improved effect of the medium-bridge, the output power density and efficiency of this system exceed those of the conventional TPV system (which lacks a medium-bridge) by 26.4% and 36.5%, respectively. Moreover, we systematically analyze the modulation of medium-bridge thicknesses and cell thickness on output performance and clarify how both affect energy losses of this near-field TPV system. Our work offers a strategy to improve the energy conversion performance of the near-field TPV system, opening new opportunities for developing near-field energy conversion. [ABSTRACT FROM AUTHOR]

Near-field radiative heat transfer has attracted increasing attention in recent years in the development and manufacturing of thermal devices analogous to the building blocks of current microelectronics. In this work, we study theoretically a near-field thermal transistor operating at room temperature. The source and drain were assumed as indium antimonide (InSb) plates, whereas the gate as a Weyl semimetal (WSM). Numerical results computed using the fluctuational electrodynamics framework indicate that the modulation and/or amplification of the heat flux in the considered transistor can be achieved by modifying the gate temperature, and by the action of an external magnetic field upon the system. Results obtained in this work make the proposed near-field thermal transistor a suitable candidate for the contactless devices for the heat flux control and thermal management at nanoscale. [ABSTRACT FROM AUTHOR]

Featured Application: The obtained knowledge can be useful for understanding and optimizing various technologies of nanostructured materials when fast melting processes take place. Interfacial thermal conductance (ITC) affects heat transfer in many physical phenomena and is an important parameter for various technologies. The article considers the influence of various mesoscopic effects on the ITC, such as the heat transfer through the gas gap, near-field radiative heat transfer, and changes in the wetting behavior during melting. Various contributions to the ITC of the liquid-solid interfaces in the processes of fast pre-melting and melting of metal microparticles are studied. The effective distance between materials in contact is a key parameter for determining ITC. This distance changes significantly during phase transformations of materials. An unusual gradual change in ITC recently observed during pre-melting below the melting point of some metals is discussed. The pre-melting process does not occur on the surface but is a volumetric change in the microstructure of the materials. This change in the microstructure during the pre-melting determines the magnitude of the dispersion forces, the effective distance, and the near-field thermal conductance. The knowledge gained can be useful for understanding and optimizing various technological processes, such as laser additive manufacturing. [ABSTRACT FROM AUTHOR]

Coupling of surface plasmon polaritons (SPPs) supported by graphene and hyperbolic phonon polaritons (HPPs) supported by hyperbolic materials (HMs) could effectively promote photon tunneling, and hence the radiative heat transfer. In this work, we investigate the polariton hybridization phenomena on near-field radiative heat transfer (NFRHT) in multilayer heterostructures, which consist of periodic graphene/α-MoO3 cells. Numerical results show that increasing the graphene/α-MoO3 cells can effectively enhance the NFRHT when the vacuum gap is less than 50 nm, but suppresses the enhanced performance with larger gap distance. This depends on the coupling of SPPs and HPPs in the periodic structure, which is analyzed by the energy transmission coefficients distributed in the wavevector space. The influence of the thickness of the α-MoO3 film and the chemical potential of graphene on the NFRHT is investigated. The findings in this work may guide designing high-performance near-field energy transfer and conversion devices based on coupling polaritons. [ABSTRACT FROM AUTHOR]

Two-dimensional (2D) materials and their vertically stacked heterostructures have attracted much attention due to their novel optical properties and strong light-matter interactions in the infrared. Here, we present a theoretical study of the near-field thermal radiation of 2D vdW heterostructures vertically stacked of graphene and monolayer polar material (2D hBN as an example). An asymmetric Fano line shape is observed in its near-field thermal radiation spectrum, which is attributed to the interference between the narrowband discrete state (the phonon polaritons in 2D hBN) and a broadband continuum state (the plasmons in graphene), as verified by the coupled oscillator model. In addition, we show that 2D van der Waals heterostructures can achieve nearly the same high radiative heat flux as graphene but with markedly different spectral distributions, especially at high chemical potentials. By tuning the chemical potential of graphene, we can actively control the radiative heat flux of 2D van der Waals heterostructures and manipulate the radiative spectrum, such as the transition from Fano resonance to electromagnetic-induced transparency (EIT). Our results reveal the rich physics and demonstrate the potential of 2D vdW heterostructures for applications in nanoscale thermal management and energy conversion. [ABSTRACT FROM AUTHOR]

Thermophotovoltaics (TPVs), a heat recovery technique, is faced with low efficiency and power density. It has been proven that graphene helps add new functionalities to optical components and improve their performance for heat transfer. In this work, I study Near-field radiative heat transfer in TPVs based on a composite nanostructure composed of Indium Tin Oxide (ITO) sheet and a narrow bandgap photovoltaic cell made from Indium Arsenide (InAs). I introduce a new way to calculate nonradiative recombination (NR) and compare the performance with and without the NR being considered. By comparing graphene modulated on the emitter (G-E), on the receiver (G-R), and on both the emitter and the receiver (G-ER), I find the G-ER case can achieve the highest current density. However, constrained by the bandgap energy of the cell, this case is far lower than the G-E case when it comes to efficiency. After applying variant particle swarm optimization (VPSO) and dynamic optimization, the model is optimized up to 43.63% efficiency and 11 W/cm2 electric power at a 10 nm vacuum gap with a temperature difference of 600 K. Compared with before optimization, the improvement is 8.97% and 7.2 W/cm2, respectively. By analyzing the emission spectrum and the transmission coefficient, I find that after optimization the system can achieve higher emissivity above the bandgap frequency, thus achieving more efficient conversion of light to electricity. In addition, I analyze the influence of temperature difference by varying it from 300 K to 900 K, indicating the optimized model at a 900 K temperature difference can achieve 49.04% efficiency and 52 W/cm2 electric power. By comparing the results with related works, this work can achieve higher conversion efficiency and electric power after the optimization of relevant parameters. My work provides a method to manipulate the near-field TPV system with the use of a graphene-based emitter and promises to provide references in TPV systems that use low bandgap energy cells. [ABSTRACT FROM AUTHOR]

Generative adversarial networks (GANs) are one of the most robust and versatile techniques in the field of generative artificial intelligence. In this work, we report on an application of GANs in the domain of synthetic spectral data generation for data-scarce radiative heat transfer applications, an area where their use has not been previously reported. We demonstrate the proposed approach by applying it to an illustrative problem within the realm of near-field radiative heat transfer involving a multilayered hyperbolic metamaterial. We find that a successful generation of spectral data requires two modifications to conventional GANs: (i) the introduction of Wasserstein GANs (WGANs) to avoid mode collapse, and, (ii) the conditioning of WGANs to obtain accurate labels for the generated data. We show that a simple feed-forward neural network (FFNN), when augmented with data generated by a CWGAN, enhances significantly its performance under conditions of limited data availability. In addition, we show that CWGANs can act as a surrogate model with improved performance in the low-data regime with respect to simple FFNNs. Overall, this work contributes to highlight the potential of generative machine learning algorithms in scientific applications beyond image generation and optimization.

The increasing demand for versatile and high-quality near-field radiative heat transfer (NFRHT) has created a critical need for a design approach that can handle numerous candidate structures. In this work, we employ and develop an adaptive hybrid Bayesian optimization (AHBO) algorithm to design the high-quality quasi-monochromatic NFRHT. The candidate materials include hexagonal boron nitride, silicon carbide, and doped silicon. The high-quality quasi-monochromatic NFRHT is optimized over 1.0 × 108 candidate structures to maximize the evaluation factor. It is worth noting that only 2.6% of the candidate structures needed to be calculated to identify the optimal structure. The optimal structure of quasi-monochromatic NFRHT is an aperiodic multilayer metamaterial that differs from conventional periodic multilayer structures. Moreover, we investigate the robustness and mechanisms of the optimal quasi-monochromatic NFRHT with respect to the vacuum gap distance and the temperature difference between the emitter and receiver. In addition, the high-quality multi-peak NFRHT is designed using the AHBO algorithm by improving the definition of the evaluation factor. The results demonstrate that the AHBO algorithm is efficient in designing high-quality quasi-monochromatic and multi-peak NFRHT, and it can be further expanded to other structural designs in the field of energy conversion. [ABSTRACT FROM AUTHOR]

• MD and first-principles calculations derived parameters for thermal radiation. • Simulations revealed temperature variations due to microthermal sources. • Interface structures improved the continuity and stability of heat diffusion. • Phonon-polaritons stabilized thermal transport in micro- and nanoscale gaps. • Nonlinear effects emerged due to high-energy-level heat source values. The thermal transmission properties of polymer-based composites, commonly used in satellites, are crucial for high-precision thermal management applications. The study constructed the structures of polyimide polymers and polyimide/graphene interfaces based on molecular dynamics and fluctuation dissipation theorem. Optical parameters were determined through first-principles calculations, and the thermal transport from a microthermal source was simulated using Reverse Non-Equilibrium Molecular Dynamics (RNEMD) and the modified four-dimensional transfer matrix method (M4D-TMM). Simulations of polymers, interface structures, and interfacial thermal radiation models investigated the transmission dynamics of microthermal sources during heat transport. Findings showed that the differences in the temperature fields formed within the polymer from microthermal heat sources were less than 20 K for heat source inputs ranging from 0.002 W/m² to 20 GW/m². With a microthermal source, the heat diffusion was extensive, the heat transport was continuous, and the temperature field remained stable. At the level of the heat source up to the intermolecular energy level, the temperature field distribution showed significant instability, leading to nonlinear effects. Within the interface structure, the continuity of heat diffusion increased as the heat source intensified. The interface structure demonstrated enhanced temperature sensitivity and improved stability, with minimal fluctuations of only 0.59 %. The increase in heat flux perpendicular to the interface structure was ascribed to near-field radiative heat transfer. As the temperature difference decreased to less than 0.1 K, the influence of evanescent waves on radiative heat flux nearly disappeared. Phonon polaritons maintained temperature stability during thermal transport through micro- and nanoscale gaps. Integrating thermal conduction principles in polyimide polymers and interface structures with near-field thermal radiation provides a theoretical framework for high-precision thermal management in gravitational wave detection. [ABSTRACT FROM AUTHOR]

• The near-field radiative heat transfer between BP/α-MoO 3 heterostructures is studied. • The anisotropic polariton hybridization generates topological reconfiguration and transition. • The heat flux between multilayer heterostructures is further enhanced. • This work provides guidance for designing near-field thermal radiation devices. The effect of polariton hybridization in different materials have been proved to enhance near-field radiative heat transfer (NFRHT). However, the hybrid effect between anisotropic polaritons has not been studied yet. Here, we investigate the NFRHT between two black phosphorus (BP)/α-MoO 3 heterostructures. Compared with the individual BP and α-MoO 3 configurations, the NFRHT between heterostructures is significantly enhanced and can reach twice that of individual BP sheets. We find that the polariton hybridization in heterostructure exhibits complementary and competitive mechanisms, which leads to significant topological reconfiguration of hybrid polaritons, and modulates the intensity of photon tunneling. With the increase of α-MoO 3 film thickness, we observe that the topological structure of hybrid polaritons will undergo a transition of quasi-elliptic, quasi-hyperbolic and multiple resonance states. Moreover, we analyze the NFRHT between multilayer heterostructures, as the periodic cell number increases, the hybrid polaritons of multilayer heterostructures also exhibit obvious topological transitions, resulting in nearly twice the enhancement of NFRHT. Our work reveals the abundant phenomena of photon tunneling reconfiguration caused by anisotropic polariton hybridization and the accompanying enhancement of NFRHT, which may benefit the applications of near-field thermal management. [ABSTRACT FROM AUTHOR]

• Experimentally studied near-field radiative heat transfer between· large-area SiO 2 films on Si substrate. • Near-field radiative heat flux achieved by incorporating SiO 2 film surpasses substrate-only system by up to an order. • Demonstrated the effectiveness of polar dielectric films in enhancing near-field radiative heat transfer. Utilizing polar dielectric films can improve the performance of near-field thermophotovoltaic system. To confirm the enhancement of near-field radiative heat transfer (NFRHT) achieved by large-area polar dielectric films, we built an experimental setup and observe NFRHT between two 250 nm-thick SiO 2 films on intrinsic Si substrate with an area 5 × 5 mm2. The two films are supported by SiO 2 nano-spacers with a height of 302 nm. Temperature difference between the two films varies from 13 K to 42 K at room temperature. The observed near-field radiative heat flux closely matches the theoretical values, which surpasses substrate-only system by up to 9.87 times and could reach up to 7.45 times that of blackbody radiation due to excitation of the surface photon polaritons modes. These results demonstrate the effectiveness of polar dielectric films in enhancing NFRHT. [Display omitted] [ABSTRACT FROM AUTHOR]

• A prototype of electric field-effect thermal transistor (EFETT) and the derived thermal logic gates are proposed. • The EFETT works on modulation of near-field heat flux between the source and drain semiconductors by gate voltage. • Demonstrated that arbitrary thermal logic gates can be built based on the EFETT conveniently. • The performance of EFETTs and thermal logic gates are analyzed and characterized quantitatively. A prototype of electric field-effect thermal transistor (EFETT) and the derived thermal logic gates are proposed, using electric potentials as inputs and temperature as outputs. The EFETT works on the modulation of near-field thermal photons transferred between the source and the drain semiconductors by the electric field-effect induced by gate voltage, thus altering the heat current and terminal temperature. It is demonstrated that arbitrary thermal logic gates can be built based on the EFETT conveniently. The characteristics of the EFETTs and the realized thermal logic gates are analyzed quantitatively. The proposed prototype opens a new avenue for the design of thermal transistors and thermal circuits which may motivate their applications for thermal information processing and thermal management. [ABSTRACT FROM AUTHOR]

Near-field radiative heat transfer (NFRHT) can exceed the blackbody limit by several orders of magnitude owing to the tunneling evanescent waves. Exploiting this near-field enhancement holds significant potential for emerging technologies. It has been suggested that coupled polaritons can give rise to orders of magnitude enhancement of NFRHT. However, a thorough experimental verification of this phenomenon is still missing. Here this work experimentally shows that NFRHT mediated by coupled polaritons in millimeter-size graphene/SiC/SiO 2 composite devices in planar plate configuration can realize about 302.8 ±  35.2-fold enhancement with respect to the blackbody limit at a gap distance of 87  ±  0.8 nm. The radiative thermal conductance and effective gap heat transfer coefficient can reach unprecedented values of 0.136 WK -1 and 5440 Wm -2 K -1 . Additionally, a scattering-type scanning near-field optical measurement, in conjunction with full-wave numerical simulations, provides further evidence for the coupled polaritonic characteristics of the devices. Notably, this work experimentally demonstrates dynamic regulation of NFRHT can be achieved by modulating the bias voltage, leading to an ultrahigh dynamic range of ≈4.115. This work ambiguously elucidates the important role of coupled polaritons in NFRHT, paving the way for the manipulation of nanoscale heat transport, energy conversion, and thermal computing via the strong coupling effect., (© 2024 Wiley‐VCH GmbH.)

Interfacial thermal conductance (ITC) affects heat transfer in many physical phenomena and is an important parameter for various technologies. The article considers the influence of various mesoscopic effects on the ITC, such as the heat transfer through the gas gap, near-field radiative heat transfer, and changes in the wetting behavior during melting. Various contributions to the ITC of the liquid-solid interfaces in the processes of fast pre-melting and melting of metal microparticles are studied. The effective distance between materials in contact is a key parameter for determining ITC. This distance changes significantly during phase transformations of materials. An unusual gradual change in ITC recently observed during pre-melting below the melting point of some metals is discussed. The pre-melting process does not occur on the surface but is a volumetric change in the microstructure of the materials. This change in the microstructure during the pre-melting determines the magnitude of the dispersion forces, the effective distance, and the near-field thermal conductance. The knowledge gained can be useful for understanding and optimizing various technological processes, such as laser additive manufacturing.

The discrete system Green's function (DSGF) method is a fluctuational electrodynamics-based numerical framework for predicting near-field radiative heat transfer (NFRHT) between three-dimensional thermal sources of arbitrary number, shape, size, and material. In the DSGF method, thermal sources are discretized into cubic subvolumes along a cubic lattice, and the system Green's functions between all subvolumes are obtained by solving a system of linear equations. From the system Green's functions, quantities of interest in heat transfer such as the power dissipated and the thermal conductance are calculated. The objective of this paper is to provide a user guide of the DSGF solver publicly available on GitHub. The basics of the DSGF method are first reviewed, followed by a detailed description of the DSGF solver implemented in MATLAB and C. The C implementation is parallelized and includes an iterative procedure which is not available in the MATLAB version. Example problems of NFRHT between two dipoles, two spheres, two cubes, and two membranes that can be used for verification are provided. • The DSGF is a fluctuational electrodynamics-based method for simulating NFRHT. • MATLAB and C implementations of the DSGF solver available on GitHub are described. • NFRHT simulations with the DSGF solver are limited by computational memory. • Memory usage in the C implementation is mitigated via an iterative procedure. [ABSTRACT FROM AUTHOR]

Near-field radiative heat transfer (NFRHT) in point-dipole nanoparticle networks is complicated due to the multiple scattering of thermally excited electromagnetic wave (namely, many-body interaction, MBI). The MBI regime is analyzed using the many-body radiative heat transfer theory at the particle scale for networks of a few nanoparticles. Effect of MBI on radiative heat diffusion in networks of a large number of nanoparticles is analyzed using the normal-diffusion radiative heat transfer theory at the continuum scale. An influencing factor ψ is defined to numerically figure out the border of the different many-body interaction regimes. The whole space near the two nanoparticles can be divided into four zones, non-MBI zone, enhancement zone, inhibition zone and forbidden zone, respectively. Enhancement zone is relatively smaller than the inhibition zone, so many particles can lie in the inhibiting zone that the inhibition effect of many-body interaction on NFRHT in nanoparticle networks is common in literature. Analysis on the radiative thermal energy confirms that multiple scattering caused by the inserted scatterer accounts for the enhancement and inhibition of NFRHT. By arranging the nanoparticle network in aspect of structures and optical properties, the MBI can be used to modulate radiative heat diffusion characterized by the radiative effective thermal conductivity (k eff) over a wide range, from inhibition (over 55% reduction) to amplification (30 times of magnitude). To achieve a notable MBI, it is necessary to introduce particles that have resonances well-matched with those of the particles of interest, irrespective of their match with the Planckian window. This work may help for the understanding of the thermal radiation in nanoparticle networks. • A many-body interaction regime map for NFRHT in the three-nanoparticle system is proposed qualitatively and quantitatively. • A good match between the particle resonance and the Planckian window can guarantee a high effective thermal conductivity (ETC). • Placement of particles in the enhancement zone can increase the ETC by over 30 times, while placement in the inhibition zone can have 55% reduction. [ABSTRACT FROM AUTHOR]

The manipulation of near-field radiative heat transfer (NFRHT) through complex photon tunneling in many-body systems has garnered significant attention, particularly for advancing thermal management in micro- and nanodevices. In this work, we theoretically investigate the enhancement and modulation of NFRHT within a graphene/Bi 2 Se 3 -based three-body configuration. Compared to a two-body configuration, the three-body modulator enhances the near-field radiative heat flux (NFRHF) by 2.77, primarily due to the enhancement of surface plasmon polaritons facilitated by graphene. Moreover, we analyze how NFRHF is modulated by varying the vacuum gap between the emitter and the intermediate, adjusting the thickness of Bi 2 Se 3 films in emitter and intermediate, as well as manipulating the chemical potentials of graphene. Results indicate that by tuning the chemical potentials of graphene, thermal regulation can be enhanced up to a ratio of 2.55. This enhancement is attributed to the excitation of SPPs, which are concentrated in a narrower wave vector space and higher angular frequency range as the chemical potential of graphene increases. These findings have implications for controlling thermal radiation applications utilizing graphene and Bi 2 Se 3 heterostructures. This work demonstrates the superiority of the three-body system in enhancing NFRHT and contributes new insights into the potential applications of Bi 2 Se 3 in NFRHT. [ABSTRACT FROM AUTHOR]

• Spiny particle near-field radiative heat transfer is studied by the TDDA method. • Spines affect near-field radiative heat transfer between particles. • Hollows and gaps affect near-field radiative heat transfer between spiny particles. The spine-like structure appears on some smooth organisms and has a high light absorption cross-section. Inspired by the spiny structure in the nature, this study establishes models of solid and hollow spiny particles. The effects of spine cross-section size, uniform and nonuniform spine distribution, hollow size, gap, and spine dislocation on near-field radiative heat transfer of spiny particles are investigated based on the thermal discrete dipole approximations method. The spectral radiative conductance and volumetric net power distribution are obtained. The results indicate that the coupling of localized surface phonons between two particles is affected by spine cross-section size and spine distribution. Near-field radiative heat transfer of spiny particles is enhanced maximumly when the frequencies are slightly lower than the resonance frequency of smooth particles. The peak of the spectral conductance is red-shifted and decreases with the increase of hollow size. The uniformity of volumetric power absorbed distribution increases and the variation of the relative spectral conductance decreases with increasing gap. The influence of two particles' spine dislocation on near-field radiative heat transfer and volumetric power absorbed distribution is not obvious. [ABSTRACT FROM AUTHOR]

Two-dimensional (2D) materials and their vertically stacked heterostructures have attracted much attention due to their novel optical properties and strong light-matter interactions in the infrared. Here, we present a theoretical study of the near-field thermal radiation of 2D vdW heterostructures vertically stacked of graphene and monolayer polar material (2D hBN as an example). An asymmetric Fano line shape is observed in its near-field thermal radiation spectrum, which is attributed to the interference between the narrowband discrete state (the phonon polaritons in 2D hBN) and a broadband continuum state (the plasmons in graphene), as verified by the coupled oscillator model. In addition, we show that 2D van der Waals heterostructures can achieve nearly the same high radiative heat flux as graphene but with markedly different spectral distributions, especially at high chemical potentials. By tuning the chemical potential of graphene, we can actively control the radiative heat flux of 2D van der Waals heterostructures and manipulate the radiative spectrum, such as the transition from Fano resonance to electromagnetic-induced transparency (EIT). Our results reveal the rich physics and demonstrate the potential of 2D vdW heterostructures for applications in nanoscale thermal management and energy conversion.

We theoretically investigate a three-body near-field radiative heat transfer (NFRHT) system, which consists of three separated parallel plates with different temperatures and certain vacuum spacing. The emitter (body1) and the receiver (body3) are both composed of a monolayer graphene and a VO2 film covered by semi-infinite bulk SiO2. The intermediate (body2) has a composite graphene/VO2/graphene heterostructure. The steady temperature of body2 can be determined based on the fact that the near-field radiative heat flow (NFRHF) emitted by body1 is equal to that received by body3 at thermal steady state. NFRHF can be highly tuned by changing the vacuum gap and the chemical potential of graphene in the three-body system. This paper can provide theoretical guidance for experimental and practical applications of near-field thermal radiation, and are significant for the applications of NFRHT structures containing intermediates in the field of temperature control.

Thermophotovoltaics (TPVs), a heat recovery technique, is faced with low efficiency and power density. It has been proven that graphene helps add new functionalities to optical components and improve their performance for heat transfer. In this work, I study Near-field radiative heat transfer in TPVs based on a composite nanostructure composed of Indium Tin Oxide (ITO) sheet and a narrow bandgap photovoltaic cell made from Indium Arsenide (InAs). I introduce a new way to calculate nonradiative recombination (NR) and compare the performance with and without the NR being considered. By comparing graphene modulated on the emitter (G-E), on the receiver (G-R), and on both the emitter and the receiver (G-ER), I find the G-ER case can achieve the highest current density. However, constrained by the bandgap energy of the cell, this case is far lower than the G-E case when it comes to efficiency. After applying variant particle swarm optimization (VPSO) and dynamic optimization, the model is optimized up to 43.63% efficiency and 11 W/cm2 electric power at a 10 nm vacuum gap with a temperature difference of 600 K. Compared with before optimization, the improvement is 8.97% and 7.2 W/cm2, respectively. By analyzing the emission spectrum and the transmission coefficient, I find that after optimization the system can achieve higher emissivity above the bandgap frequency, thus achieving more efficient conversion of light to electricity. In addition, I analyze the influence of temperature difference by varying it from 300 K to 900 K, indicating the optimized model at a 900 K temperature difference can achieve 49.04% efficiency and 52 W/cm2 electric power. By comparing the results with related works, this work can achieve higher conversion efficiency and electric power after the optimization of relevant parameters. My work provides a method to manipulate the near-field TPV system with the use of a graphene-based emitter and promises to provide references in TPV systems that use low bandgap energy cells.

Dynamic modulation of near-field radiative heat transfer (NFRHT) requires changing the optical responses of the object, which is expected to be realized with the help of the strain engineering. Based on the electric dipole approximation and the reflection Green's function, in this work, we investigate the effect of the hBN film under uniform in-plane strain on the NFRHT between two graphene/SiC core-shell (GSCS) nanoparticles. By changing the magnitude of the strain on the film, the hyperbolic phonon polaritons (HPPs) can be dynamically tuned to match and deviate from the localized surface resonance (LSR), thus significantly modulating the NFRHT between the nanoparticles. When the two GSCS nanoparticles are placed above a strained hBN film, an amplification of the maximum radiative heat flux by about 6 orders of magnitude can be achieved. In addition, after the introduction of graphene to form graphene/hBN heterostructure, the radiative heat flux can be further enhanced with the help of the hybrid polaritons. Especially at a suitable chemical potential and strain, the NFRHT can be enhanced by 4 more orders of magnitude compared to the bare hBN film. This work provides a new insight into efficient and dynamic thermal management among nanoparticles. • Dynamic modulation of NFRHT between two graphene/SiC core-shell nanoparticles induced by strained hBN is investigated. • The HPPs of the substrate can be coupled and decoupled with the LSR of the nanoparticles under the effect of strain. • The modulation contrast can reach almost 5000 when the optimal strain is applied to the hBN film. • The hybrid polaritons supported by graphene/hBN heterostructure help to maintain higher radiative heat flux. [ABSTRACT FROM AUTHOR]

• NFRHT between two multilayered graphene/porous SiC terminals is studied. • Multiple polaritons and their coupling are capable of enhancing near-field flux. • Heat flux is a combination result of the multiple polaritons and the medium loss. • An upper flux enhancement limitation exists for increasing the layer number. • Multiple polaritons-mediated NFRHT mechanisms and regulations are presented. Near-field radiative heat transfer (NFRHT) between two separated terminals can enhance the heat flux determined by blackbody theory by several orders of magnitude. In this work, we study the NFRHT between two terminals made of periodic multilayered graphene/porous silicon carbide (SiC), specifically focusing on the multiple surface polariton effects. The two terminals are parallel and separated by a vacuum gap. The porous SiC-supported surface phonon polaritons (SPhPs) and hyperbolic phonon polaritons (HPhPs), and their coupling with the graphene-supported surface plasmon polaritons (SPPs) leads to apparent enhancement of the radiative heat flux between the gapped terminals. The underlying mechanism of multiple polaritons for amplifying the near-field heat flux is revealed through the energy transmission coefficients for different scenarios. Results show that the heat flux between the multilayered terminals is a combination result of the multiple surface polaritons and the medium loss. When the vacuum gap is set as d = 10 nm, the heat flux between the 50-layered terminals is 1.48 times that between the single-layered terminals. We demonstrate that the NFRHT can be dynamically controlled and optimized by appropriately engineering the number of layers, the chemical potential of graphene, and the porosity and thickness of the porous SiC sheet. This work paves an alternative controllable structure for strengthening and regulating NFRHT and thus has potential applications in micro-nanoscale thermal management. [ABSTRACT FROM AUTHOR]

• Investigation of near-field radiative heat transfer between moving two-dimensional anisotropic surfaces. • Derivation of near-field thermal radiation formulas between moving anisotropic surfaces based on the theory of fluctuating dissipative electrodynamics theory combined with Lorentz transformation. • Exploring the coupling of rotation angles with moving 2D anisotropic surface models. • Exploring the energy transfer coefficient between anisotropic moving surfaces as affected by parameters such as separation distance and electron density. • Provides a new idea for the active regulation of near-field radiative heat transfer. Radiation heat transfer from two-dimensional anisotropic materials has promising applications in nanodevices, such as non-contact thermal modulators, thermal lithography and thermophotovoltaics. Moving surface is expected to provide more possibilities for the modulation of near-field thermal radiation due to its special properties such as supporting hyperbolic propagation and non-reciprocity. In this study, the near-field radiative heat transfer between moving anisotropic surfaces is investigated in the framework of the fluctuation-dissipation electrodynamics theory combined with the Lorentz transformation. It is found that the movement and rotation of the surface lead to the asymmetry of energy transfer in the wave vector space, which affects the energy transfer coefficient and hence the radiation heat flux. The effect of velocity, separation distance, rotation angle and electron density on the energy transfer coefficient and radiative heat flux between moving anisotropic surfaces is revealed. It is found that the heat flux decreases as the velocity increases, the sensitivity of the heat flux variation with velocity decreases as the velocity increases. This study provides a new idea for the active regulation of near-field radiative heat transfer. [ABSTRACT FROM AUTHOR]

• Heat flux direction control via α-MoO 3 rotation; • Near-perfect thermal splitting with anisotropic materials; • Broadened the scope of application for near-field thermal splitter systems; • Splitters maintain performance under various conditions. Efficient thermal management has become crucial in micro and nano devices. This research presents a near-field thermal splitter, a pivotal device for managing thermal energy at distances less than the thermal characteristic wavelength. The thermal splitter, composed of one source and two drains, utilizes the anisotropic properties of α -phase molybdenum trioxide (α -MoO 3) nanoparticles. By rotating the source, the direction of heat flux can be precisely controlled. When employing α -MoO 3 for both source and drains, the system can achieve a thermal splitting effect ranging from 1 % to 99 %. To one's interest, the system maintains considerable splitting effect even with drains made of other materials supporting optical modes in the reststrahlen band. The near-field thermal splitter shows robust performance despite variations in distances between the three terminals and source rotation angles. This research provides an effective approach for managing complex heat flux networks, potentially enhancing applications in energy harvesting and advanced thermal management systems. [ABSTRACT FROM AUTHOR]

The highly tunable surface magneto-plasmon polaritons supported by magneto-optical materials provide reliable means for active adjusting of near-field radiative heat transfer (NFRHT). In our study, we proposed an anisotropic system consisting of two graphene-covered magneto-optical gratings, and theoretically investigate the NFRHT between them controlled by a weak magnetic field. We find that an external magnetic field open a broad-spectrum photo tunneling channel and modify the energy distribution of surface electromagnetic modes in the wave-vector space. The near-field heat flux between two magneto-optical gratings can achieve a remarkable maximum enhancement of 6 times at the magnetic field H = 0.9 T compared to that without a magnetic field. Owing to the coupling between highly tunable surface magneto-plasmon polaritons of graphene and surface modes of InSb, the regulation ratio of the heat flux exceeds 10 from 0.5 T to 0.9 T. In addition, the effects of the separation distance, chemical potential of graphene and structural configuration of two gratings are also investigated. We look forward to the proposed system being used as a thermal switch, providing convenience for tuning thermal radiation and improving the efficiency of energy-utilization devices. [ABSTRACT FROM AUTHOR]

• We have proposed a data-driven machine learning workflow for designing multilayer hyperbolic metamaterials composed of α-MoO 3 , achieving high modulation contrast in near-field thermal radiation. • By combining the multilayer perceptron and bayesian optimization techniques, we optimize the rotation angle, layer thickness, and gap distance of the multilayer metamaterials, resulting in a 97 % improvement in thermal modulation contrast compared to previous single layer structures. • The large thermal modulation contrast is attributed to the controlled alignment and misalignment of hyperbolic plasmon polaritons and hyperbolic surface phonon polaritons in each layer, achieved by manipulating the rotation angles. The thermal modulator based on the near-field radiative heat transfer has wide applications in thermoelectric diodes, thermoelectric transistors, and thermal storage. However, the design of optimal near-field thermal radiation structure is a complex and challenging problem due to the tremendous number of degrees of freedom. In this work, we have proposed a data-driven machine learning workflow to efficiently design multilayer hyperbolic metamaterials composed of α-MoO 3 for near-field thermal radiative modulator with high modulation contrast. By combining the multilayer perceptron and Bayesian optimization, the rotation angle, layer thickness and gap distance of the multilayer metamaterials are optimized to achieve a maximum thermal modulation contrast ratio of 6.29. This represents a 97 % improvement compared to previous single layer structure. The large thermal modulation contrast is mainly attributed to the alignment and misalignment of hyperbolic plasmon polaritons and hyperbolic surface phonon polaritons of each layer controlled by the rotation. The results provide a promising way for accelerating the designing and manipulating of near-field radiative heat transfer by anisotropic hyperbolic materials through the data-driven style. [Display omitted] [ABSTRACT FROM AUTHOR]

Hyperbolic volume and surface phonon polaritons have been studied extensively for enhancing the near-field radiative heat transfer (NFRHT) between hyperbolic materials. Hyperbolic volume phonon polaritons (HVPPs) describe propagating electromagnetic waves in hyperbolic materials while evanescent waves are required for excitation of hyperbolic surface phonon polaritons (HSPPs). Therefore, the dispersion relations of HVPPs and HSPPs are distinct. Here we study the interaction of HVPPs and HSPPs within the context of NFRHT between hyperbolic materials. We find that the dispersion curves of HVPPs and HSPPs in an ultrathin hyperbolic slab can connect smoothly. Particularly, we find that the topology of HVPPs can be convex and flat, rather than concave, and can be controlled by tuning the thickness of the hyperbolic slab, which has not been reported in published literature. We believe our findings presented here may help to deepen our understanding on the interaction between HVPPs and HSPPs, as well as the knowledge on the topology of HVPPs in hyperbolic materials. [ABSTRACT FROM AUTHOR]

Near-field radiative heat transfer has recently attracted increasing interests for its applications in energy technologies, such as thermophotovoltaics. Existing works, however, are restricted to time-independent systems. Here, we explore near-field radiative heat transfer between two bodies under time modulation by developing a rigorous fluctuational electrodynamics formalism. We demonstrate that time modulation can result in the enhancement, suppression, elimination, or reversal of radiative heat flow between the two bodies, and can be used to create a radiative thermal diode with an infinite contrast ratio, as well as a near-field radiative heat engine that pumps heat from the cold to the hot bodies. The formalism reveals a fundamental symmetry relation in the radiative heat transfer coefficients that underlies these effects. Our results indicate the significant capabilities of time modulation for managing nanoscale radiative heat flow., Competing Interests: Competing interests statement:The authors declare no competing interest.

Increasing near-field radiative heat transfer between two bodies separated by a vacuum gap is crucial for enhancing the power density in radiative energy transport and conversion devices. However, the largest radiative heat transfer coefficient between two realistic materials at room temperature is limited to around 2000 W/(m 2 ·K) for a gap of 100 nm. Here, analogous to conventional plate-fin heat exchangers based on convection, we introduce the concept of a nanophotonic heat exchanger, which enhances near-field radiative heat transfer using two bodies with interpenetrating gratings. Our calculations, based on rigorous fluctuational electrodynamics, show that the radiative heat transfer coefficient between the bodies separated by a 100 nm gap can significantly exceed 2000 W/(m 2 ·K) by increasing the aspect ratios of the gratings. We develop a semianalytical heat transfer model that agrees well with the rigorous calculations for design optimization. Our work opens new opportunities for enhancing near-field radiative heat transfer between any materials.

Nanoscale radiative thermal transport between a pair of metamaterial gratings is studied within this work. The effective medium theory (EMT), a traditional method to calculate the near-field radiative heat transfer (NFRHT) between nanograting structures, does not account for the surface pattern effects of nanostructures. Here, we introduce the effective approximation NFRHT method that considers the effects of surface patterns on the NFRHT. Meanwhile, we calculate the heat flux between a pair of silica (SiO2) nanogratings with various separation distances, lateral displacements, and grating heights with respect to one another. Numerical calculations show that when compared with the EMT method, here the effective approximation method is more suitable for analyzing the NFRHT between a pair of relatively displaced nanogratings. Furthermore, it is demonstrated that compared with the result based on the EMT method, it is possible to realize an inverse heat flux trend with respect to the nanograting height between nanogratings without modifying the vacuum gap calculated by this effective approximation NFRHT method, which verifies that the NFRHT between the side faces of gratings greatly affects the NFRHT between a pair of nanogratings. By taking advantage of this effective approximation NFRHT method, the NFRHT in complex micro/nano-electromechanical devices can be accurately predicted and analyzed.

• Spatial distributions of stationary temperatures in periodic-alternating many-body systems is investigated. • Temperature distributions exhibit an alternating spatial pattern and split into two distinct components. • A dual-channel photon heat exchange through a long-range coupling of electromagnetic modes is responsible for the splitting of the temperature distribution. • The thermal relaxation of the temperatures is verified for potential applications in dynamical situations. We investigate the radiative heat transfer and spatial distributions of stationary temperatures in periodic many-body systems composed of alternating slabs of two different materials. We show that temperature distributions exhibit an alternating spatial pattern and split into two distinct components, with each component corresponding to one of the two materials. Spatial temperature variations following the periodicity of the structure can be attributed to a dual-channel photon heat exchange through a long-range coupling of electromagnetic modes supported by bodies of the same material. We also analyze the thermal relaxation of the temperatures in the system to verify potential applications in dynamical situations. The results reveal that tunable nonmonotonic temperature variations can be also designed and utilized at a transient mode. The dual-channel mechanism to control temperature distributions proposed in the present work may pave new avenues for prospective applications in nano devices, especially for thermal photon-driven logic circuitry and thermal management. [ABSTRACT FROM AUTHOR]

• Analyzed the effect of MBI on the heat transfer between two nanoparticles induced by many particles standing in proximity. • The MBI on NFRHT experiences a radical change from inhibition to enhancement when increasing the proximate particle size. • The proximate MBI accounts for a smooth and non-oscillated twisting angle dependence of NFRHT when twisting the proximate particle ensemble. Near-field radiative heat transfer (NFRHT) has received growing attention because of its high intensity far beyond the Planck's black-body limit. Insertion of a third object in proximity of the two particles can significantly influence and manipulate its NFRHT. However, for the system composed of many particles, the effect of many-body interaction (MBI) on NFRHT between arbitrary two particles is still not well understood. In this work, the MBI is studied for two particles with three typical proximate ensembles: particle chain, plane and grating. With the increasing of proximate particle size, the MBI on NFRHT will experience a radical change from inhibition to enhancement. The polarizability of the proximate particle increases with particle radius, which enhances the interaction between the proximate particles and the main particle, and then results in enhancement of NFRHT between the main particles. When twisting the proximate particle ensemble, the proximate MBI accounts for a smooth and non-oscillated twisting angle dependence of NFRHT, and it is different from the oscillation phenomenon of NFRHT for particle gratings. This work deepens the understanding of NFRHT in dense particulate systems. [ABSTRACT FROM AUTHOR]

• We investigate radiative heat transfer among three graphene/SiC core-shell nanoparticles. • The excitation of multiple photon tunneling modes are analyzed. • A near-field thermal modulator with a thermal modulation ratio >40 is realized. • Tuning Fermi level of graphene shells enables the heat splitting ratio to exceed 99. We theoretically demonstrate the modulation and splitting of radiative heat flux in a three-body system consisting of graphene/SiC core-shell (GSCS) nanoparticles on the basis of their highly tunable polarizabilities, which play a dominant role in the coupling of localized surface resonance. The excitation of multiple photon tunneling modes enables the radiative heat transfer to be controlled in different scenarios. Using the dependence of photon tunneling on the Fermi level and radius of the intermediate GSCS nanoparticle, such a many-body system not only holds the potential to be a near-field thermal modulator with a thermal modulation ratio greater than 40, but also allows the heat splitting effect with the ratio over 99 when the intermediate particle becomes the hot source. This work may facilitate dynamical thermal management in a many-body system, and can also benefit the plasmonics community in the near future. [ABSTRACT FROM AUTHOR]

Highlights • Derived formulas for near-field radiative heat transfer in a chain of spheres. • Calculated inter-sphere and sphere-environment radiative heat transfer. • Developed model for experiment measuring radiative transfer between two spheres. • Emphasized implications of sphere-environment interaction in near-field experiments. Abstract In this work, we present expressions for radiative heat transfer between pairs of spheres in a linear chain and between individual spheres and their environment. The expressions are valid for coated spheres of arbitrary size, spacing, and isotropic optical properties. The spheres may be small and closely-spaced, which violates the assumptions foundational to classical radiative transfer. We validate our results against existing formulations of radiative heat transfer, namely the thermal discrete dipole and boundary element methods. Our results have important implications for the modeling and interpretation of near-field radiative heat transfer experiments between spherical bodies. [ABSTRACT FROM AUTHOR]

• Graphene is very promising for large radiative heat flux and modulation ratio. • The heat flux and modulation ratio for graphene drop in the presence of a substrate. • The dielectric substrates BaF 2 , KCl, RbCl and RbBr have the largest heat flux and modulation ratio. • Metals suppress heat flux and modulation ratio of near-field radiative heat transfer of graphene significantly. • The heat flux and modulation ratio for graphene are inversely proportional to the separation gap. A promising material for applications in near-field radiative heat transfer is graphene, due to large heat flux values and the capability to actively control the heat flux using various mechanisms such as applying a bias voltage. Theoretical studies have shown that a heat transfer coefficient as large as ∼1500 W/m2K can be modulated with a ratio of ∼100 for two graphene sheets at 300 K, free-standing in vacuum, with a gap of 100 nm. However, it is not clear how the heat transfer coefficient and the modulation ratio are affected in the presence of a substrate which needs to be used in practice. We study the effect of the presence of substrates on near-field radiative heat flux among two single-layer sheets of graphene. We determine the complex dielectric function that corresponds to maximal heat flux for an emitted electromagnetic mode. We compute the heat flux and modulation ratio for 41 different substrates selected from common dielectric, metallic and semiconducting materials and show that the heat flux and the modulation ratio both reduce in the presence of a substrate. The dielectric substrates such as BaF 2 , KCl, NaF, RbCl and RbBr have the largest modulation ratio and heat flux among the selected substrates. The low-frequency and high-frequency dielectric functions of these substrates are closest to the free space, and their dispersion occurs at low frequencies far from the spectral location of the surface plasmon polaritons of graphene. For low and high graphene sheet temperatures of 300 K and 400 K, respectively, and a separation gap of 100 nm, the largest heat coefficient (= 1436 W/m2K) is obtained for BaF 2 with a modulation ratio of 20.3. The largest modulation ratio at the same temperatures and gap size is 22.5, which is obtained for NaF with a heat transfer coefficient of 1406 W/m2K. [ABSTRACT FROM AUTHOR]