arXiv subjects

Mathieu Francoeur

Publications and source records attributed to Mathieu Francoeur.

At least 19 recordsLinked to original sources

arXiv2026

Radiation conduction in polaritonic nanowires

Phonon polaritons have attracted increasing interest as a means to offset the reduction in phonon thermal conductivity in nanostructures caused by enhanced boundary scattering. However, the interpretation of the limited experimental data on phonon polariton-mediated conduction is hindered by the lack of comprehensive full-wave models for predicting thermal electromagnetic transport in solid-state systems. Here, the radiative thermal conductivity in the diffusive regime along SiO2 nanowires near room temperature is predicted using the fluctuational electrodynamics-based discrete system Green's function method. At 400 K, for nanowire diameters of 66 nm and 132 nm, modest radiative conductivities of 0.0264 W/m-K and 0.0165 W/m-K, respectively, are obtained, with contributions dominated by surface phonon polaritons. In contrast, the kinetic theory combined with nanowire dispersion relations derived from Maxwell's equations predicts conductivities that can be nearly two orders of magnitude larger than those obtained from fluctuational electrodynamics, and are largely dominated by bulk phonon polaritons propagating within the nanowire volume. Improved agreement between the kinetic theory and fluctuational electrodynamics for the total radiative conductivity can be achieved by introducing an effective mean free path that accounts for the reduced mean free path of bulk phonon polaritons outside the Reststrahlen spectral bands of SiO2. However, even with this correction, the kinetic theory fails to accurately capture the spectral distribution of radiative conductivity. This work establishes a solid foundation for the development of phonon polariton-based systems for thermal management in micro/nanoelectronic devices.

cond-mat.mes-hall↗

arXiv2025

Near-field radiative heat transfer in the dual nanoscale regime between polaritonic membranes

The enhancement and attenuation of near-field radiative heat transfer between polaritonic SiC, SiN and SiO2 subwavelength membranes is analyzed. Fluctuational electrodynamics simulations combined with a modal analysis show that all membranes support corner and edge modes, which can induce a large 5.1-fold enhancement for SiC and a 2.1-fold attenuation for SiO2 of the heat transfer coefficient with respect to that between infinite surfaces. The enhancement or attenuation is directly related to material losses which reduce the density of available electromagnetic states between the membranes.

cond-mat.mes-hall↗

arXiv2024

Generalized many-body approach for near-field radiative heat transfer between nonspherical dipoles

A generalized fluctuational electrodynamics-based many-body approach for calculating near-field radiative heat transfer (NFRHT) between nonspherical dipoles is proposed. The geometric parameters of nonspherical dipoles are implemented in the definition of the self-term of the free-space Green's function. Dipole polarizability is defined a posteriori from the free-space Green's function solution such that polarizability calculation is an optional post-processing step rather than a required input. Both strong and weak forms of the generalized many-body approach are presented. It is shown that the approximate weak form is less computationally expensive but is only applicable to small particles characterized by size parameters less than ~0.24. The generalized many-body method is compared against an analytical solution for NFRHT between two spheroidal dipoles. A good agreement is obtained, and the small discrepancies are ascribed to differences in approximations for multiple reflections. The generalized many-body method is then applied to analyze the near-field spectral conductance between two SiC ellipsoidal dipoles. Results reveal that changes in the orientation of one of the ellipsoidal dipoles lead to active tuning of localized surface phonon resonance by up to three orders of magnitude. Finally, the spectral radiative thermal conductivity of a metamaterial composed of 1000 SiO2 ellipsoidal particles is studied. The metamaterial displays anisotropic radiative thermal conductivity, with resonance values differing by up to a factor of 2.8 between different directions. The generalized many-body model of NFRHT presented in this paper may be used to develop particle-based metamaterials with novel, engineered radiative thermal properties.

physics.app-ph↗

arXiv2023

Large enhancement of near-field radiative heat transfer in the dual nanoscale regime enabled by electromagnetic corner and edge modes

It is well established that near-field radiative heat transfer (NFRHT) can exceed Planck's blackbody limit1 by orders of magnitude owing to the tunneling of evanescent electromagnetic frustrated and surface modes2-4, as has been demonstrated experimentally for NFRHT between two large parallel surfaces5-7 and between two subwavelength membranes8,9. However, while nanostructures can also sustain a much richer variety of localized electromagnetic modes at their corners and edges,10,11 the contributions of such additional modes to further enhancing NFRHT remain unexplored. Here, for the first time, we demonstrate both theoretically and experimentally a new physical mechanism of NFRHT mediated by these corner and edge modes, and show it can dominate the NFRHT in the "dual nanoscale regime" in which both the thickness of the emitter and receiver, and their gap spacing, are much smaller than the thermal photon wavelengths. For two coplanar 20 nm thick SiC membranes separated by a 100 nm vacuum gap, the NFRHT coefficient at room temperature is both predicted and measured to be 830 W/m2K, which is 5.5 times larger than that for two infinite SiC surfaces separated by the same gap, and 1400 times larger than the corresponding blackbody limit accounting for the geometric view factor between the emitter and receiver. This enhancement is dominated by the electromagnetic corner and edge modes which account for 81% of the NFRHT between these SiC membranes. These findings are important for future NFRHT applications in thermal management and energy conversion.

physics.optics↗

arXiv2023

Impacts of thermal aging and associated heat losses on the performance of a Pyromark 2500-coated concentrated solar power central receiver

Pyromark 2500 is a widely used coating for concentrated solar power central receiver systems due to its high absorptivity, ease in application, and relatively low cost. Pyromark's performance is quantified by its figure of merit (FOM), which relates the coating's heat losses to its solar-to-thermal conversion efficiency. After long-term exposure to high temperatures (>750{\deg}C) and irradiance levels, Pyromark's absorptivity and FOM decrease. The aim of this research is to evaluate changes in Pyromark's absorptivity, heat losses, and FOM as a function of thermal aging. This work also compares the most common FOM expression, which neglects convection losses, to an FOM that includes all heat losses experienced by a central receiver. Isothermal aging experiments are conducted on Pyromark-coated Inconel 600 substrates at 750{\deg}C. The spectral, hemispherical absorptivity of the samples is measured at room temperature with a spectrophotometer and input into a finite element analysis model that includes radiation and convection boundary conditions. The heat flux and temperature output by the model are used to determine the heat losses and FOM of the Pyromark samples. After 151 h of thermal aging, the sample with the thinnest Pyromark coat maintains the most stable total, hemispherical absorptivity. Conversely, the total, hemispherical absorptivity of the sample with the thickest Pyromark coat drops by a maximum of 1.73%, and the corresponding maximum drop in FOM is 1.90% when windy conditions (which are expected around central receivers) are assumed. In windy conditions, convection losses constitute between 21% and 24% of the samples' total heat loss; thus, the most common FOM expression in the literature overestimates the samples' FOM by ~4.40%. An analysis of the samples' heat losses indicates that reflection losses exceed emission losses when the absorptivity declines significantly.

physics.app-ph↗

arXiv2023

Near-field thermal emission from metasurfaces constructed of SiC ellipsoidal particles

We model near-field thermal emission from metasurfaces structured as two-dimensional arrays of ellipsoidal SiC particles. The modeling approach is developed from fluctuational electrodynamics and is applicable to systems of ellipsoidal particles within the dipole limit. In all simulations, the radial lengths of particles are restricted to the range of 10 to 100 nm, and interparticle spacing is constrained to at least three times the particle characteristic length. The orientation and dimensions of constituent ellipsoidal particles are varied to tune localized surface phonon resonances and control the near-field energy density above metasurfaces. Results show that particle orientation can be used to regulate the relative magnitude of resonances in the energy density and particle dimensions may be changed to adjust the frequency of these resonances within the Reststrahlen band. Metasurfaces constructed from particles with randomized dimensions display comparatively broadband thermal emission rather than the three distinct resonances seen in metasurfaces made with ellipsoidal particles of equivalent dimensions. When the interparticle spacing in a metasurface exceeds about three times the particle characteristic length, the spectral energy density above the metasurface is dominated by individual particle self-interaction and can be approximated as a linear combination of single-particle spectra. When interparticle spacing is at the lower limit of three times the characteristic length, however, multiparticle interaction effects increase, and the spectral energy density above a metasurface deviates from that of single particles. This work provides guidance for designing all-dielectric, particle-based metasurfaces with desired near-field thermal emission spectra, such as thermal switches.

physics.app-ph↗

arXiv2022

Orientation effects on near-field radiative heat transfer between complex-shaped dielectric particles

The effect of orientation on near-field radiative heat transfer between two complex-shaped superellipsoid particles of SiO2 is presented. The particles under study are 50 nm in radius and of variable concavity. Orientation is characterized by the degree of rotational symmetry in the two-particle systems, and the radiative conductance is calculated using the discrete system Green's function approach to account for all electromagnetic interactions. Results reveal that the total conductance in some orientations can be up to twice that of other orientations when particles are at center-of-mass separation distances of 110 nm. Orientation effects are not significantly correlated with system rotational symmetries but are strongly correlated with the minimum vacuum gap distance between particles. As such, orientation effects on near-field radiative heat transfer are a consequence of particle topology, with more extreme topologies leading to a continuation of orientation effects at larger particle center-of-mass separation distances. The concave superellipsoid particles display significant orientation effects up to a center-of-mass separation distance approximately equal to 3.9 times the particle radius, while the convex superellipsoid particles display significant orientation effects up to a center-of-mass separation distance approximately equal to 3.2 times the particle radius. In contrast to previous anisotropic, spheroidal dipole studies, these results of complex-shaped superellipsoid particles suggest that orientation effects become negligible when heat transfer is a volumetric process for all orientations. This work is essential for understanding radiative transport between particles that have non-regular geometries or that may have geometrical defects or abnormalities.

physics.app-ph↗

arXiv2022

Near-field radiative heat transfer between irregularly shaped dielectric particles modeled with the discrete system Green's function method

Near-field radiative heat transfer (NFRHT) between irregularly shaped dielectric particles made of SiO2 and morphology characterized by Gaussian random spheres is studied. Particles are modeled using the discrete system Green's function (DSGF) approach, which is a volume integral numerical method based on fluctuational electrodynamics. This method is applicable to finite, three-dimensional objects, and all system interactions are defined independent of thermal excitation by a generalized system Green's function. The DSGF method is deemed suitable to model NFRHT between irregularly shaped particles after verification against the analytical solution for chains of two and three SiO2 spheres. The NFRHT results reveal that geometric irregularity in particles leads to a reduction of the total conductance from that of comparable perfect spheres at vacuum separation distances smaller than the particle size, a regime in which NFRHT is a surface phenomenon. At vacuum separation distances larger than the particle size, NFRHT becomes a volumetric process, and the total conductance between irregularly shaped particles converges to that of comparable perfect spheres. Spectral analysis reveals, however, that particle irregularity leads to damping and broadening of resonances at all separation distances, thereby highlighting the importance of the DSGF method for spectral engineering in the near field. The reduced spectral coherence when particle size is larger than the vacuum separation distance is attributed to coupling of surface phonon-polaritons within the randomly generated, distorted particle features. For particle size smaller than the vacuum separation distance, resonance broadening and damping is linked with the multiple localized surface phonon modes supported by the composite spherical harmonic morphologies of the Gaussian random spheres.

physics.app-ph↗

arXiv2021

First-principles calculations of phonon transport across a vacuum gap

Phonon transport across a vacuum gap separating intrinsic silicon crystals is predicted via the atomistic Green's function method combined with first-principles calculations of all interatomic force constants. The overlap of electron wave functions in the vacuum gap generates weak covalent interaction between the silicon surfaces, thus creating a pathway for phonons. Phonon transport, dominated by acoustic modes, exceeds near-field radiation for vacuum gaps smaller than ~ 1 nm. The first-principles-based approach proposed in this work is critical to accurately quantify the contribution of phonon transport to heat transfer in the extreme near field.

cond-mat.mes-hall↗

arXiv2021

Extreme Near-Field Heat Transfer Between Gold Surfaces

Extreme near-field heat transfer between metallic surfaces is a subject of debate as the state-of-the-art theory and experiments are in disagreement on the energy carriers driving heat transport. In an effort to elucidate the physics of extreme near-field heat transfer between metallic surfaces, this Letter presents a comprehensive model combining radiation, acoustic phonon and electron transport across sub-10-nm vacuum gaps. The results obtained for gold surfaces show that in the absence of bias voltage, acoustic phonon transport is dominant for vacuum gaps smaller than ~2 nm. The application of a bias voltage significantly affects the dominant energy carriers as it increases the phonon contribution mediated by the long-range Coulomb force and the electron contribution due to a lower potential barrier. For a bias voltage of 0.6 V, acoustic phonon transport becomes dominant at a vacuum gap of 5 nm, whereas electron tunneling dominates at sub-1-nm vacuum gaps. The comparison of the theory against experimental data from the literature suggests that well-controlled measurements between metallic surfaces are needed to quantify the contributions of acoustic phonon and electron as a function of the bias voltage.

cond-mat.mes-hall↗

arXiv2020

Revisiting the Figure of Merit of Concentrated Solar Power Receivers

The figure of merit (FOM) is a widely used metric to characterize the performance of concentrated solar power (CSP) receivers by comparing the amount of solar thermal energy retained by the receiver to the incident concentrated solar radiation. However, the FOM is a strong function of the concentration factor and receiver temperature, thus direct comparison of FOM values measured under disparate operating conditions is inappropriate. To remedy this problem, the present study proposes a new metric called the receiver effectiveness calculated by normalizing the actual FOM with its theoretical maximum. The receiver effectiveness can be employed for comparing receiver performances regardless of their operating conditions, and can be treated as more-like the second law efficiency of thermodynamics. In addition, a theoretical limit of the CSP plant efficiency is also examined by combining the maximum FOM and the Carnot efficiency for different concentration factors and receiver temperatures. The calculated maximum CSP plant efficiency clearly indicate that optimizing FOM does not always lead to a better CSP plant performance. Along with the FOM, the proposed receiver effectiveness and maximum CSP system efficiency should be considered as complementary metrics to evaluate the performance of the CSP system.

physics.app-ph↗

arXiv2020

A three-dimensional thermal model of the human cochlea for magnetic cochlear implant surgery

In traditional cochlear implant surgery, physical trauma may occur during electrode array insertion. Magnetic guidance of the electrode array has been proposed to mitigate this medical complication. After insertion, the guiding magnet attached to the tip of the electrode array must be detached via a heating process and removed. This heating process may, however, cause thermal trauma within the cochlea. In this study, a validated three-dimensional finite element heat transfer model of the human cochlea is applied to perform an intracochlear thermal analysis necessary to ensure the safety of the magnet removal phase. Specifically, the maximum safe input power density to detach the magnet is determined as a function of the boundary conditions, heating duration, cochlea size, implant electrode array radius and insertion depth, magnet size, and cochlear fluid. A dimensional analysis and numerical simulations reveal that the maximum safe input power density increases with increasing cochlea size and the radius of the electrode array, whereas it decreases with increasing electrode array insertion depth and magnet size. The best cochlear fluids from the thermal perspective are perilymph and a soap solution. Even for the worst case scenario in which the cochlear walls are assumed to be adiabatic except at the round window, the maximum safe input power density is larger than that required to melt 1 $\rm{mm^3}$ of paraffin bonding the magnet to the implant electrode array. By combining the outcome of this work with other aspects of the design of the magnetic insertion process, namely the magnetic guidance procedure and medical requirements, it will be possible to implement a thermally safe patient-specific surgical procedure.

physics.app-ph↗

arXiv2020

Experimental validation of a three-dimensional heat transfer model within the scala tympani with application to magnetic cochlear implant surgery

Magnetic guidance of cochlear implants is a promising technique to reduce the risk of physical trauma during surgery. In this approach, a magnet attached to the tip of the implant electrode array is guided within the scala tympani using a magnetic field. After surgery, the magnet must be detached from the implant electrode array via localized heating and removed from the scala tympani which may cause thermal trauma. Objectives: The objective of this work is to experimentally validate a three-dimensional (3D) heat transfer model of the scala tympani which will enable accurate predictions of the maximum safe input power to avoid localized hyperthermia when detaching the magnet from the implant electrode array. Methods: Experiments are designed using a rigorous scale analysis and performed by measuring transient temperatures in a 3D-printed scala tympani phantom subjected to a sudden change in its thermal environment and localized heating via a small heat source. Results: The measured and predicted temperatures are in good agreement with an error less than 6$\%$. Conclusions: The validated 3D heat transfer model of the scala tympani is finally applied to evaluate the maximum safe input power to avoid localized hyperthermia when detaching the magnet. For the most conservative case where all boundaries except the insertion opening are adiabatic, the power required to release the magnet attached to the implant electrode array by 1 mm$^3$ of paraffin is approximately half of the predicted maximum safe input power. Significance: This work will enable the design of a thermally safe magnetic cochlear implant surgery procedure.

physics.app-ph↗

arXiv2020

Near-field radiative heat transfer between dissimilar materials mediated by coupled surface phonon- and plasmon-polaritons

Near-field radiative heat transfer (NFRHT) between dissimilar materials supporting surface polaritons in the infrared is of critical importance for applications such as photonic thermal rectification and near-field thermophotovoltaics. Here, we measure NFRHT between millimetersize surfaces made of 6H-SiC and doped Si, respectively supporting surface phonon-polaritons (SPhPs) and surface plasmon-polaritons (SPPs) in the infrared, separated by a 150-nm-thick vacuum gap spacing maintained via SiO2 nanopillars. For purpose of comparison, measurements are also performed between two doped Si surfaces. The measured radiative flux is in good agreement with theoretical predictions based on fluctuational electrodynamics. A flux enhancement beyond the blackbody limit of ~ 8.2 is obtained for the SiC-Si sample, which is smaller than the enhancement for the Si-Si sample (~ 12.5) owing to the spectral mismatch of the SiC and Si light lines, and SPhP and SPP resonances. However, due to lower losses in SiC than Si and weaker SPhP-SPP coupling than SPP coupling, the near-field enhancement for the SiC-Si sample exhibits a more pronounced monochromatic behavior with a resonant flux that is ~ 5 times larger than the resonant flux for the Si-Si sample. This work demonstrates that it is possible to modulate NFRHT via surface polariton coupling, and will accelerate the development of energy conversion and thermal management devices capitalizing on the near-field effects of thermal radiation between dissimilar materials.

physics.app-ph↗

arXiv2020

Spatial correlation of the thermally generated electromagnetic field in layered media

A general formulation for the cross-spectral density tensor enabling calculation of the spatial correlation of the thermally generated electromagnetic field in layered media is derived. The formulation is based on fluctuational electrodynamics, and is thus applicable in the near and far field of heat sources. The resulting cross-spectral density tensor is written in terms of a single integration over the parallel wavevector, as the angular integrations leading to numerical instability are evaluated analytically. Using this formulation, the spatial correlation length in the near field of a film made of silicon carbide (SiC) supporting surface phonon-polaritons (SPhPs) in the infrared is analyzed. It is shown that the spatial correlation length of a SiC heat source suspended in vacuum decreases substantially by decreasing its thickness owing to SPhP coupling. In the limit of a 10-nm-thick SiC film, the spatial correlation length is similar to that of a blackbody. The results also reveal that it is possible to control the spatial coherence of a thin SiC heat source via dielectric and metallic substrates, respectively allowing and preventing SPhP coupling. This suggests that active modulation of thermal emission via thin films supporting surface polaritons in the infrared is possible by using a phase change material substrate such as vanadium dioxide.

cond-mat.mes-hall↗

arXiv2020

Design of an indium arsenide cell for near-field thermophotovoltaic devices

An indium arsenide photovoltaic cell with gold front contacts is designed for use in a near-field thermophotovoltaic (NF-TPV) device consisting of millimeter-size surfaces separated by a nanosize vacuum gap. The device operates with a doped silicon radiator maintained at a temperature of 800 K. The architecture of the photovoltaic cell, including the emitter and base thicknesses, the doping level of the base, and the front contact grid parameters, are optimized for maximizing NF-TPV power output. This is accomplished by solving radiation and charge transport in the cell via fluctuational electrodynamics and the minority charge carrier continuity equations, in addition to accounting for the shading losses due to the front contacts and additional series resistance losses introduced by the front contacts and the substrate. The results reveal that these additional loss mechanisms negatively affect NF-TPV performance in a non-negligible manner, and that the maximum power output is a trade-off between shading losses and series resistance losses introduced by the front contacts. For instance, when the cell is optimized for a 1 x 1 mm2 device operating at a vacuum gap of 100 nm, the losses introduced by the front contacts reduce the maximum power output by a factor of ~ 2.5 compared to the idealized case when no front contact grid is present. If the optimized grid for the 1 x 1 mm2 device is scaled up for a 5 x 5 mm2 device, the maximum power output is only increased by a factor of ~ 1.08 with respect to the 1 x 1 mm2 case despite an increase of the surface area by a factor of 25. This work demonstrates that the photovoltaic cell in a NF-TPV device must be designed not only for a specific radiator temperature, but also for specific gap thickness and device surface area.

physics.app-ph↗

arXiv2019

Heat transfer analysis in an uncoiled model of the cochlea during magnetic cochlear implant surgery

Magnetic cochlear implant surgery requires removal of a magnet via a heating process after implant insertion, which may cause thermal trauma within the ear. Intra-cochlear heat transfer analysis is required to ensure that the magnet removal phase is thermally safe. The objective of this work is to determine the safe range of input power density to detach the magnet without causing thermal trauma in the ear, and to analyze the effectiveness of natural convection with respect to conduction for removing the excess heat. A finite element model of an uncoiled cochlea, which is verified and validated, is applied to determine the range of maximum safe input power density to detach a 1-mm-long, 0.5-mm-diameter cylindrical magnet from the cochlear implant electrode array tip. It is shown that heat dissipation in the cochlea is primarily mediated by conduction through the electrode array. The electrode array simultaneously reduces natural convection due to the no-slip boundary condition on its surface and increases axial conduction in the cochlea. It is concluded that natural convection heat transfer in a cochlea during robotic cochlear implant surgery can be neglected. It is found that thermal trauma is avoided by applying a power density from $2.265 \times 10^7$ W/m$^3$ for 114 s to $6.6\times10^7$ W/m$^3$ for 9 s resulting in a maximum temperature increase of 6$^\circ$C on the magnet boundary.

physics.med-ph↗

arXiv2019

Thermal radiation in systems of many dipoles

Systems of many nanoparticles or volume-discretized bodies exhibit collective radiative properties that could be used for enhanced, guided, or tunable thermal radiation. These are commonly treated as assemblies of point dipoles with interactions described by Maxwell's equations and thermal fluctuations correlated by the fluctuation-dissipation theorem. Here, we unify different theoretical descriptions of these systems and provide a complete derivation of many-dipole thermal radiation, showing that the correct use of the fluctuation-dissipation theorem depends on the definitions of fluctuating and induced dipole moments. We formulate a method to calculate the diffusive radiative thermal conductivity of arbitrary collections of nanoparticles; this allows the comparison of thermal radiation to other heat transfer modes and across different material systems. We calculate the radiative thermal conductivity of ordered and disordered arrays of SiC and SiO2 nanoparticles and show that thermal radiation can significantly contribute to thermal transport in these systems. We validate our calculations by comparison to the exact solution for a one-dimensional particle chain, and we demonstrate that the dipolar approximation significantly underpredicts the exact results at separation distances less than the particle radius.

cond-mat.mes-hall↗