Near-field Radiative Heat Flux Research Articles

Study on the flame radiative heat transfer and near-field radiation heat flux predictive model of vehicle fires in a tunnel

As a major form of heat transfer, flame thermal radiation has an important effect on the ignition of adjacent energy, fire spread and environmental damage. Forecasting the flame radiation heat flux can provide theoretical basis for fire hazard assessment, fire extinguishing and salvation. Meanwhile, the distinctive structure of the arc tunnel can result in significant variations in the local heat and mass transfer processes during tunnel fires compared to those in rectangular tunnels, leading to different fire characteristics parameters. In this study, experiments and theoretical analyses were conducted to investigate the flame length scale and near-field radiation heat flux on the lower surface of an arc-shaped ceiling induced by vehicle fires in a tunnel. The findings reveal that the flame length scale in the transverse direction beneath the arc-shaped ceiling decreases with an increasing burner aspect ratio and increases with the increasing heat release rate. A dimensionless model of the flame length scale under the arced ceiling with various burner aspect ratios was proposed based on the amount of remaining unburned fuel after flame impingement and the buoyancy component along the arc-shaped ceiling. Simultaneously, radiative heat fluxes on the lower surface also increase with rising heat release rate. The flame geometry beneath the arc-shaped ceiling is assumed to be a combination of a cuboid and an arc plate model based on the actual flame length scale. Subsequently, the view factor, radiative fraction, and flame surface area are analyzed to predict the radiative heat fluxes on the lower surface. The predictions of the cuboid and arc plate flame model show good agreement with the current experimental data. This work presents a dimensionless correlation of the flame length scale and calculates the view factor between the target and the arc plate to predict the near-field radiative heat flux more accurately. This approach also facilitates the understanding of flame spreading behavior and thermal hazards impacted by arced ceiling.

The potential of hyperbolic films for radiative heat transfer in micro/nanoscale

Thin films exhibit substantial potential in energy management and utilization as the development of micro- and nanofabrication technologies. It is well known that thermal radiation is one of the fundamental ways of energy transfer. However, the potential of hyperbolic films for radiative heat transfer is always ignored. Whether the radiative heat flux between hyperbolic films surpasses that of the bulk materials remains insufficiently explored. In this work, we theoretically investigate the radiative heat transfer between hexagonal boron nitride (hBN) at a separation from 20 ​nm to 2 ​μm. The results show that when the optical axis of hBN is oriented in-plane, the near-field radiative heat flux of hBN with a thickness of 10 ​nm exceeds that of hBN bulk by 47% and exceeds the blackbody limit by two orders of magnitude at a gap distance of 20 ​nm. The physical mechanism is attributed to the volume-confined hyperbolic polaritons can be excited in a higher wavevector space. Conversely, when the gap distance is 600 ​nm, the heat flux between films is considerably lower than that of bulk material. This work opens up potential avenues for developing hyperbolic film-dependent thermal devices and strategies for thermal management.

Enhanced near-field radiative heat transfer between nanostructure emitter and GaSb absorber by surface plasmon polaritons and hyperbolic modes

Near-field radiative heat transfer (NFRHT) can overcome the blackbody radiation limit and holds great promise in radiative energy conversion devices such as near-field thermophotovoltaics (NF-TPV). However, NF-TPV is not yet ready for practical applications at larger scales due to the challenges of maintaining the nanoscale gap and the lack of optimized NFRHT with nanostructure emitters. Here, we measure the enhanced near-field radiative heat flux between GaSb absorbers and nanostructure emitters that can be applied in NF-TPV systems, with a cm2-scale plate–plate structure and a temperature difference of 100 K at a gap down to 200 nm. It is demonstrated that the radiative heat flux between the bulk W emitter and the absorber at a 400 nm gap is about an order of magnitude larger than that in the far field, and the heat flux can be further enhanced by 1.6 times at a 200 nm gap. Moreover, the enhanced NFRHT from the surface plasmon polaritons supported by a indium tin oxide film and the hyperbolic modes supported by a W/SiO2 multilayer is also experimentally verified and theoretically analyzed.

Drift current-induced tunable near-field energy transfer between twist magnetic Weyl semimetals and graphene

Abstract 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.

Near-field radiative heat transfer in three-body system based on topological insulator Bi2Se3

This study investigates the near-field radiative heat flux in a three-body system made of Bi2Se3, a topological insulator that supports hyperbolic phonon polaritons (HPPs) in the terahertz frequency range. We demonstrate that the near-field radiative heat flux in the three-body system made of Bi2Se3 is remarkably enhanced compared to the two-body counterpart. The thickness of the intermediate body or the chemical potential of Bi2Se3 can be used to control the radiative heat flux. Our numerical results reveal that the heat flux initially increases and then decreases with the thickness of the intermediate body, indicating that an optimal heat flux can be obtained by selecting an appropriate chemical potential and intermediate body thickness. Moreover, increasing the chemical potential of Bi2Se3 shifts the hybrid surface plasmon phonon polaritons, which arise from the coupling of surface plasmons and HPPs, toward higher frequencies. These findings have important implications for developing thermal devices such as thermal transistors and offer a promising approach to modulating near-field radiative heat transfer and thermal management.

Near-Field Radiative Heat Transfer between Disordered Multilayer Systems

Near-field radiative heat transfer (NFRHT) research is an important research project after a major breakthrough in nanotechnology. Based on the multilayer structure, we find that due to the existence of inherent losses, the decoupling of hyperbolic modes (HMs) after changing the filling ratio leads to suppression of heat flow near the surface mode resonance frequency. It complements the physical landscape of enhancement of near-field radiative heat transfer by HMs and more surface states supported by multiple surfaces. More importantly, considering the difficulty of accurate preparation at the nanoscale, we introduce the disorder factor to describe the magnitude of the random variation of the layer thickness of the multilayer structure and then explore the effect on heat transfer when the layer thickness is slightly different from the exact value expected. We find that the near-field radiative heat flux decreases gradually as the disorder increases because of interlayer energy localization. However, the reduction in heat transfer does not exceed an order of magnitude, although the disorder is already very large. At the same time, the regulation effect of the disorder on NFRHT is close to that of the same degree of filling ratio, which highlights the importance of disordered systems. This work qualitatively describes the effect of disorder on heat transfer and provides instructive data for the fabrication of NFRHT devices.

Near-field radiative heat transfer between on-substrate graphene sheets

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 BaF2, 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 BaF2 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.

Active control of near-field radiative heat transfer between nanoparticles and slab via the multilayered surface modes

We derive the formula for the near-field radiative heat flux between dipole particles and a multilayered slab composed of the uniaxial medium. Based on that, the multilayered structure consisting of SiC and VO2 materials is applied to actively modulate the radiative heat transfer between nanoparticles and slab. Utilizing the modulation feature of the multilayer, surface modes from the multilayer can be strongly coupled to the particle resonance, greatly enhancing the heat flux between the particle and slab. The phase change feature of VO2 allows the active control of multilayered surface modes by changing the slab's temperature, greatly tuning the heat flux. We address the role of multilayer parameters on the modulation effect. We also demonstrate that the multilayered structure can also actively tune the heat flux between the nanoparticle array and slab.

Near-field radiative heat transfer between two α-quartz plates having hyperbolic and double-negative-permittivity bands

Near-field radiative heat transfer (NFRHT) between two semi-infinite α-quartz plates was theoretically investigated based on the fluctuation-dissipation theorem. By setting the temperatures of the two α-quartz plates respectively equal to 300 K and 299 K, the near-field radiative heat flux (NFRHF) was calculated and compared with the cases of SiC and hexagonal boron nitride (hBN), considering the influence of optic axis (OA) orientation. The results show that the NFRHF between the two α-quartz plates can surpass 3 times that between two SiC plates and 7 times that between two hBN plates at a separation distance of 10 nm. This vastly enhanced NFRHT was attributed to excitation of phonon polaritons in the hyperbolic and double-negative-permittivity (DNP) bands of α-quartz. It is found that excited surface phonon polaritons in DNP bands can be hyperbolic-like or elliptic-like, and the NFRHF in DNP bands contributes more than half of the total NFRHF for α-quartz. Furthermore, the hyperbolic and DNP bands that distribute in a wide frequency range give α-quartz the potential to achieve much higher NFRHF than using other materials such as SiC and hBN at different temperatures. Therefore, our work is helpful to deepen the understanding of surface polaritons in hyperbolic and DNP bands and their effect on NFRHT, which may be beneficial to design of energy transfer and conversion devices based on NFRHT.

Active control of the near-field radiative heat transfer between two metal plates through the external electric field

We calculate the near-field radiative heat transfer (NFRHT) between two charged metallic plates with an external electric field oriented perpendicular to the plates. The near-field radiative heat flux is discussed in detail for gold, silver, and copper plates. The results show that the NFRHT between the copper plates is the largest. Due to the coupling between electrostatic interactions and surface fluctuations, the p-wave heat flux is amplified by several orders of magnitude by the perpendicular electric field. With increasing electric-field strength, the heat flux between the copper plates increases rapidly to a maximum value. Moreover, for a separation between the copper plates of 10–20 nm, the magnitude of the perpendicular electric field corresponding to the maximal heat flux increases exponentially with plate separation. We study how to use the external electric field to control the NFRHT between metal plates, which has a wide range of applications in thermal nano-devices.

Super-Planckian radiative heat transfer between macroscale metallic surfaces due to near-field and thin-film effects

In this study, we demonstrate that the radiative heat transfer between metallic planar surfaces exceeds the blackbody limit by employing the near-field and thin-film effects over macroscale surfaces. Nanosized polystyrene particles were used to create a nanometer gap between aluminum thin films of different thicknesses from 80 nm to 13 nm coated on 5 × 5 mm2 silicon chips, while the vacuum gap spacing is fitted from the near-field measurement with bare silicon samples. The near-field radiative heat flux between 13-nm-thick Al thin films at 215 nm gap distance is measured to be 6.4 times over the blackbody limit and 420 times to the far-field radiative heat transfer between metallic surfaces under a temperature difference of 65 K with the receiver at room temperature. The experimental results are validated by theoretical calculation based on fluctuational electrodynamics, and the heat enhancement is explained by non-resonant electromagnetic coupling within the subwavelength vacuum gap and resonant coupling inside the nanometric Al thin film with s polarized waves. This work will facilitate the applications of near-field radiation in thermal power conversion, radiative refrigeration, and noncontact heat control where metallic materials are involved.

Near-Field Radiative Heat Transfer Between Two α-MoO3 Biaxial Crystals

Abstract The near-field radiative heat transfer (NFRHT) between two semi-infinite α-MoO3 biaxial crystals is investigated numerically based on the fluctuation–dissipation theorem combined with the modified 4 × 4 transfer matrix method in this paper. In the calculations, the near-field radiative heat flux (NFRHF) along each of the crystalline directions of α-MoO3 is obtained by controlling the orientation of the biaxial crystals. The results show that much larger heat flux than that between two semi-infinite hexagonal boron nitride can be achieved in the near-field regime, and the maximum heat flux is along the [001] crystalline direction. The mechanisms for the large radiative heat flux are explained as due to existence of hyperbolic phonon polaritons (HPPs) inside α-MoO3 and excitation of hyperbolic surface phonon polaritons (HSPhPs) at the vacuum/α-MoO3 interfaces. The effect of relative rotation between the emitter and the receiver on the heat flux is also investigated. It is found that the heat flux varies significantly with the relative rotation angle. The modulation contrast can be as large as two when the heat flux is along the [010] direction. We attribute the large modulation contrast mainly to the misalignment of HSPhPs and HPPs between the emitter and the receiver. Hence, the results obtained in this work may provide a promising way for manipulating near-field radiative heat transfer between anisotropic materials.

Near-field thermophotovoltaic energy conversion by excitation of magnetic polariton inside nanometric vacuum gaps with nanostructured Drude emitter and backside reflector

In this work, we study the effect of magnetic polariton (MP) on the conversion performance of a near-field thermophotovoltaic (TPV) system made of a nanostructured Drude grating emitter and a nanometer-thick photovoltaic (PV) cell with a lossless metal as the backside reflector. Near-field radiative heat flux is calculated through scattering matrix theory coupled with rigorous coupled-wave analysis based on fluctuational electrodynamics. It is theoretically shown that MP can be excited inside the nanometric vacuum gap and the 100-nm PV cell to spectrally enhance the heat flux above the cell bandgap. The lossless metal backing, which could practically serve as electrode for charge collection, is employed to reflect photons back into the cell for recycling. Contour plots of energy transmission coefficient and an inductor-capacitor circuit model along with magnetic field distribution are used to verify and understand the MP excitation in the near-field. Effects of PV cell thickness and vacuum gap distances, are systematically investigated on the near-field radiative heat flux, electrical power output, and energy conversion efficiency. It is found that the conversion efficiency increases from 0.7% to 12.2% when the semi-infinite In0.18Ga0.82Sb cell is replaced by an ultra-thin PV cell supported by a backside reflector with the emitter at 1000 K and the cell at 300 K separated by 200 nm gap. The conversion efficiency can be also further improved to 17.6% using a nanostructured Drude grating emitter over a planar emitter. This work would open up a new way to improve the performance of TPV devices with nanostructures with near-field thermal radiation.

Ultrafast Tunable Near-Field Radiative Thermal Modulator Made of Ge3Sb2Te6

We show numerically the phase change material Ge3Sb2Te6 (GST) with special configuration as a heat modulator in the regime of near-field radiative heat transfer (NFRHT). The ability of GST to allow ultrafast reversible switch between two phases endows it great potential in practical modulation application. By designing silicon carbide (SiC) nanoholes (NHs) filled with GST, this configuration could achieve a considerable modulation effect and large near-field radiative heat flux. The underlying mechanism can be explained by the observation that the entire configuration supports either hyperbolic modes or surface phonon polaritons (SPhPs) resonance modes and even the combination of both modes, thereby resulting in the remarkable modulation effect. In addition, the effects of the volume filling factor and graphene coverage are also investigated at the vacuum gap distance of 100 nm. With graphene coverage, the modulation factor can be further improved to as high as 0.72 achieved at the volume filling factor of 0.6 with temperature difference of 20 K. The proposed configuration has the potential to effectively modulate heat in the near-field regime for designing heat modulation applications in the future.

Near-field thermal radiation in a cavity

The near-field radiative heat fluxes in the cavity are numerically investigated in this work. The cavity in this study is formed by surrounding the two doped silicon slabs with the boundary materials. The geometric feature of the cavity in this paper is characterized by the dimensionless distance between the doped silicon slabs. First, the near-field radiative heat fluxes between two isolated doped silicon square slabs are numerically investigated and the edge effects on the near-field radiative heat flux between two isolated doped silicon slabs are illustrated. Then the near-field radiative heat flux exchange in the cavity is computed according to the dimensionless distance. The effects of the material type and temperature of the cavity boundary on the radiative heat fluxes are studied. The radiative heat fluxes between the isolated doped silicon slabs without the boundary are diminished as the increment of the dimensionless distance. Contrarily, the radiative heat fluxes between the doped silicon slabs in the cavity can be impressively enhanced as the increment of the dimensionless distance. By using the appropriate dimensionless distance and boundary material, the radiative heat flux in the cavity can be modified to approximate or exceed the radiative heat flux between two semi-infinite doped silicon slabs. This work can be applied to the thermal management in the nanoscale.

Active Magneto-Optical Control of Near-Field Radiative Heat Transfer between Graphene Sheets

Graphene provides an ideal platform for active modulation of near-field radiative heat transfer. While much research here focuses on tuning the chemical potential by doping or gating, this work proposes using a magnetic field to control near-field heat flux between graphene sheets. The authors predict giant thermal magnetoresistance and negative thermal magnetoresistance of the near-field radiative heat flux for different Fermi energies, and Shubnikov-de-Haas-like oscillations in the spectral heat flux. In a static magnetic field, coupling of excited magnetoplasmon-polariton modes of graphene boosts radiative heat transfer beyond the blackbody limit by several orders of magnitude.

Near-field radiative thermal modulation enabled by static magnetic fields

We propose a thermal modulation structure composed of two identical graphene-coated NaBr plates separated by a nanoscale vacuum gap (d). A static magnetic field perpendicular to the graphene sheet induces TE–TM hydrid surface plasmon polaritons between graphene and vacuum gap (called the hybrid GSPPs) and enhances the coupling of the hybrid surface plasmon polaritons in graphene to the surface phonon polaritons in the reststrahlen band of NaBr (called the coupled SPh–SPPs). The hybrid GSPPs and coupled SPh–SPPs play a key role in the thermal modulation. The heat flux is modulated by changing the static magnetic field, the Fermi level (EF) and carrier mobility (μ) in graphene. When the magnetic field varies from 0 to 30 T, the modulation contrast as high as 3.42, 3.25 and 4.48 is acquired at (d, EF, μ)=(60.64 nm, 0.2 eV, 1000 cm2/V · s), (100 nm, 0.21 eV, 1000 cm2/V · s) and (100 nm, 0.2 eV, 2500 cm2/V · s), respectively. The work opens new routes to magnetically modulate near-field radiative heat flux in graphene-based structures, and provides some guidance for applications in thermal management of nano/microscaled devices.

Graphene-mediated near field thermostat based on three-body photon tunneling

We theoretically demonstrate a near field thermostat based on the mechanism of three-body photon heat tunneling and tunability of graphene. The system consists of three separated parallel plates at different temperatures, with the hot and cold body (located at either side of the system) coated with a monolayer of graphene. As a unique two-dimensional material, graphene owns the tunable plasmonic dispersion relation which can be rapidly modulated by changing its chemical potential in an electronical manner. By changing the chemical potentials of graphene, the near field radiative heat flux (NFRHF) transporting in the system can be tuned accordingly. As a consequence, the equilibrium temperature of the intermediate body can be adjusted from the temperature of the hot body to that of the cold one. The results show that the equilibrium temperature can be adjusted from 326 K to 388 K in SiO2-SiO2-SiO2 configuration, which occupies about sixty percent of the temperature range with respect to the cold and hot body. This phenomenon implies that near field contactless temperature adjustment can be carried out by the proposed configuration. The underlying mechanism is attributed to the varied NFRHF with tunable surface plasmon polaritons of graphene. Moreover, three-body photon heat tunneling can benefit the regulation function of the system in the near field regime and an enhanced region for adjusting temperature is observed. An interesting phenomenon is observed that the mismatch between different materials raises the regulation ability to 82 K in SiO2-SiC-SiO2 configuration. The results obtained in this study could find significant applications in the field of near field thermal management and controlling temperature in the separation of micrometer and millimeter scale, especially paves a new way toward improving the performance of near field thermal transistor based on insulator-metal transition material.

Influence of hBN orientation on the near-field radiative heat transfer between graphene/hBN heterostructures

The influence of the optic axis orientation of hexagonal boron nitride (hBN) on the near-field radiative heat transfer between hBN slabs as well as between graphene/hBN heterostructures is studied. A modified 4 × 4 transfer matrix method is employed to calculate the near-field radiative heat flux (NFRHF) between the media. The numerical results show that the NFRHF will decrease when the optic axis of hBN is tilted off the direction of the energy flow for bare hBN slabs. The reason is that hyperbolic phonon polaritons excited in the hyperbolic bands of type I are largely suppressed for tilted optic axis, though surface phonon polaritons can be excited in the hyperbolic bands. On the contrary, the NFRHF between two graphene/hBN heterostructures is affected by the coupling of SPPs excited at the vacuum/graphene interface with those at the graphene/hBN interface and the formation of a hybrid mode, by which the NFRHF is maximum when the hBN slabs are arranged with strong in-plane anisotropy of the surface. The results obtained in this work may provide a promising way for manipulating near-field radiative heat transfer between anisotropic materials.

Surface plasmon-enhanced near-field thermal rectification in graphene-based structures

We propose a thermal rectification structure composed of InSb and graphene-coated 3C-SiC separated by a nanoscale vacuum gap. To obtain an obvious thermal rectification effect, the permittivities of these materials are all considered to be temperature-dependent. Numerical calculations based on fluctuation electrodynamics reveal that the introduction of graphene into the structure enhances significantly near-field radiative heat flux and thermal rectification efficiency owing to the strong coupling of surface plasmon-polaritons between InSb and graphene. In general, the rectification efficiency above 60% can be maintained for the vacuum gap less than 70 nm. The rectification efficiency exceeding 95% is realized for a vacuum gap of 10 nm and a chemical potential of 0.1 eV. Increasing the emitter’s temperature leads to the drastic increase of the rectification efficiency in a wider temperature range. A lower chemical potential seems more favorable to raising rapidly the rectification efficiency. The above results might be helpful in designing a thermal diode with higher efficiency and wider vacuum gap.