arXiv subjects

Bong Jae Lee

Publications and source records attributed to Bong Jae Lee.

14 recordsLinked to original sources

arXiv2024

Maximum plasmon thermal conductivity of a thin metal film

Due to their extremely long propagation lengths compared to the wavelengths, surface plasmon polaritons (SPPs) have been considered as a key in enhancing thermal conductivity in thin metal films. This study explores the conditions at which the plasmon thermal conductivity is maximized, considering the thickness-dependent metal permittivity. We derived the analytical solutions for the plasmon thermal conductivity in both the thin-film and thick-film limits to analyze the effect of the permittivities of metals and substrates. From the analytical solutions of plasmon thermal conductivity, we deduced that the plasmon thermal conductivity is proportional to the electron thermal conductivity based on the Wiedemann-Franz law. Additionally, we analyzed the conditions where the enhancement ratio of the thermal conductivity via SPPs is maximized. Metals with high plasma frequency and low damping coefficient are desirable for achieving the maximum plasmon thermal conductivity as well as the maximum enhancement ratio of thermal conductivity among metals. Significantly, 10-cm-long and 14-nm-thick Al film demonstrates most superior in-plane heat transfer via SPPs, showing a 53.5\% enhancement in thermal conductivity compared to its electron thermal counterpart on a lossless glass substrate.

physics.app-ph↗

arXiv2023

Thermal Radiation at the Nanoscale and Applications

There has been a paradigm shift from the well-known laws of thermal radiation derived over a century ago, valid only when the length scales involved are much larger than the thermal wavelength (around 10 $μ$m at room temperature), to a general framework known as fluctuational electrodynamics that allows calculations of radiative heat transfer for arbitrary sizes and length scales. Near-field radiative heat transfer and thermal emission in systems of sub-wavelength size can exhibit super-Planckian behaviour, i.e. flux rates several orders of magnitude larger than that predicted by the Stefan-Boltzmann (or blackbody) limit. These effects can be combined with novel materials, e.g. low-dimensional or topological systems, to yield even larger modifications and spectral and/or directional selectivity. We introduce briefly the context and the main steps that have led to the current boom of ideas and applications. We then discuss the original and impactful works gathered in the associated Special Topic collection, which provides an overview of the flourishing field of nanoscale thermal radiation.

physics.app-ph↗

arXiv2023

Plasmon thermal conductivity of thin Au and Ag films

We investigated the thermal conductivity of surface plasmon polaritons (SPPs) propagating along thin Au and Ag films on a SiO$_2$ substrate with a Ti adhesive layer. To determine the propagation length and skin depth of SPPs along Au and Ag thin films, we numerically solved the dispersion relation while considering the size effect of the permittivity of metal. Additionally, we derived the spatial distribution of SPPs along the film thickness to analyze the effect of the Ti adhesive layer on the plasmon thermal conductivity of Au and Ag thin films. Our theoretical predictions revealed a decrease of approximately 30\% in plasmon thermal conductivity when considering the size effect of the permittivity of thin metal films. Furthermore, this causes the film thickness at which maximum thermal conductivity occurs to increase by about 30\%. Taking these factors into account, we calculated the optimal thickness of Au and Ag films, along with Ti adhesive layers, on SiO$_2$ substrates to be approximately 20 nm. By fabricating a sample with the optimal thickness of Au and Ag films, we experimentally demonstrated that the plasmon thermal conductivity of Au and Ag films can be as high as about 20\% of their electron contribution. This research will broaden the thermal design applications of ballistic thermal transport by SPPs propagating along thin metal coatings in microelectronics.

physics.app-ph↗

arXiv2023

Bifacial near-field thermophotovoltaic converter with transparent intermediate substrate

Thermophotovoltaic (TPV) converters are capable of generating electrical energy from infrared radiation emitted by an emitter powered by waste heat or solar energy. Key performance metrics for TPV converters are power output density (POD), which represents the electrical energy per unit area of the photovoltaic (PV) cell, and converter efficiency (CE), which indicates the proportion of radiative energy converted into electrical energy. A common method to significantly enhance POD is maintaining a sub-micron vacuum gap between the emitter and PV cell to leverage the near-field thermal radiation. On the other hand, bifacial TPV conversion, operating in the far-field regime, has been proposed to enhance CE by efficiently recycling the sub-bandgap energy radiation. However, bifacial TPV converters face a challenge in cooling the PV cell because the excess heat should be transferred in the lateral direction to side-edge cooling channels. Therefore, careful thermal engineering and management are required when employing near-field thermal radiation effects on bifacial TPV converters. In this study, we propose a bifacial near-field TPV (NF-TPV) converter that incorporates intrinsic Si intermediate layers, aiming to enhance both POD and CE. Si layers cover both sides of the PV cell to play a crucial role in PV cell cooling while addressing surface mode photonic loss in NF-TPV converters. We comprehensively analyze the influences of design parameters for a practical design of the bifacial NF-TPV converter. Our results demonstrate that a single-junction InAs cell can harvest 4.38 W/cm$^2$ of electrical energy with 27.2\% CE from 1000 K graphite emitters at a 100 nm vacuum gap. Despite the challenge in the cooing, our bifacial NF-TPV converter demonstrates 2.4 times larger POD with 2.7\% larger CE compared to conventional NF-TPV converters.

physics.app-ph↗

arXiv2022

Boosting thermal conductivity by surface plasmon polaritons propagating along a thin Ti film

We experimentally demonstrate a boosted in-plane thermal conduction by surface plasmon polaritons (SPPs) propagating along a thin Ti film on a glass substrate. Owing to a lossy nature of metal, SPPs can propagate over centimeter-scale distance even with a supported metal film, and resulting ballistic heat conduction can be quantitatively validated. Further, for a 100-nm-thick Ti film on glass substrate, a significant enhancement of in-plane thermal conductivity compared to bulk value ($\sim 35\%$) is experimentally shown. This study will provide a new avenue to employ SPPs for heat dissipation along a supported thin film, which can be readily applied to mitigate hot-spot issues in microelectronics.

physics.app-ph↗

arXiv2021

Thermophotovoltaic energy conversion in far-to-near-field transition regime

Recent experimental studies on near-field thermophotovoltaic (TPV) energy conversion have mainly focused on enhancing performance via photon tunneling of evanescent waves. In the sub-micron gap, however, there exist peculiar phenomena caused by the interference of propagating waves, which is seldom observed due to the dramatic increase of the radiation by evanescent waves in full spectrum range. Here, we experimentally demonstrate the oscillatory nature of near-field TPV energy conversion in the far-to-near-field transition regime (250-2600 nm), where evanescent and propagating modes are comparable due to the selective spectral response by the PV cell. Noticeably, it was possible to produce the same amount of photocurrent at different vacuum gaps of 870 and 322 nm, which is 10\% larger than the far-field value. Considering the great challenges in maintaining nanoscale vacuum gap in practical devices, this study suggests an alternative approach to the design of a TPV system that will outperform conventional far-field counterparts.

physics.optics↗

arXiv2021

Comprehensive analysis of optimized near-field tandem thermophotovoltaic system

It is well known that performance of a thermophotovoltaic (TPV) device can be enhanced if the vacuum gap between the thermal emitter and the TPV cell becomes nanoscale due to the photon tunneling of evanescent waves. Having multiple bandgaps, multi-junction TPV cells have received attention as an alternative way to improve its performance by selectively absorbing the spectral radiation in each subcell. In this work, we comprehensively analyze the optimized near-field tandem TPV system consisting of the thin-ITO-covered tungsten emitter (at 1500 K) and GaInAsSb/InAs monolithic interconnected tandem TPV cell (at 300 K). We develop a simulation model by coupling the near-field radiation solved by fluctuational electrodynamics and the diffusion-recombination-based charge transport equations. The optimal configuration of the near-field tandem TPV system obtained by the genetic algorithm achieves the electrical power output of 8.41 W/cm$^2$ and the conversion efficiency of 35.6\% at the vacuum gap of 100 nm. We show that two resonance modes (i.e., surface plasmon polaritons supported by the ITO-vacuum interface and the confined waveguide mode in the tandem TPV cell) greatly contribute to the enhanced performance of the optimized system. We also show that the near-field tandem TPV system is superior to the single-cell-based near-field TPV system in both power output and conversion efficiency through loss analysis. Interestingly, the optimization performed with the objective function of the conversion efficiency leads to the current matching condition for the tandem TPV system regardless of the vacuum gap distances.

physics.app-ph↗

arXiv2020

Measurement of effective thermal conductivity of LaNi$_5$ powder packed bed

Effective thermal conductivity of LaNi$_5$ powder packed bed was analyzed with customized guarded hot-plate (GHP) apparatus. Here, GHP was designed for precise measurement of effective thermal conductivity of metal-hydride powders even with small sample amounts (2.12$\times$10$^4$ mm$^3$). Dimensions of sample container and apparatus were determined through two-dimensional (2-D) steady-state heat conduction analysis. Calibration experiment and uncertainty analysis were conducted to validate the accuracy of the GHP. Based on the measurements of the residual thermal conductivity of the LaNi$_5$ packed bed, effect of particle size on contact factor of LaNi$_5$ packed bed was estimated. By applying the Yagi and Kunii (YK) model to the effective thermal conductivity of LaNi$_5$ packed bed, effect of contact factor and gas thermal conductivity on characteristic length of gas film were newly analyzed. Factors of YK model were modified in present work and validated through comparison with experimental data from previous literature.

physics.app-ph↗

arXiv2020

Tailoring the Spectral Absorption Coefficient of a Blended Plasmonic Nanofluid Using a Customized Genetic Algorithm

Recently, plasmonic nanofluids (i.e., a suspension of plasmonic nanoparticles in a base fluid) have been widely employed in direct-absorption solar collectors because the localized surface plasmon supported by plasmonic nanoparticles can greatly improve the direct solar thermal conversion performance. Considering that the surface plasmon resonance frequency of metallic nanoparticles, such as gold, silver, and aluminum, is usually located in the ultraviolet to visible range, the absorption coefficient of a plasmonic nanofluid must be spectrally tuned for full utilization of the solar radiation in a broad spectrum. In the present study, a modern design process in the form of a genetic algorithm (GA) is applied to the tailoring of the spectral absorption coefficient of a plasmonic nanofluid. To do this, the major components of a conventional GA, such as the gene description, fitness function for the evaluation, crossover, and mutation function, are modified to be suitable for the inverse problem of tailoring the spectral absorption coefficient of a plasmonic nanofluid. By applying the customized GA, we obtained an optimal combination for a blended nanofluid with the desired spectral distribution of the absorption coefficient, specifically a uniform distribution, solar-spectrum-like distribution, and a step-function-like distribution. The resulting absorption coefficient of the designed plasmonic nanofluid is in good agreement with the prescribed spectral distribution within about 10\% to 20\% of error when six types of nanoparticles are blended. Finally, we also investigate how the inhomogeneous broadening effect caused by the fabrication uncertainty of the nanoparticles changes their optimal combination.

physics.optics↗

arXiv2019

Surface-plasmon-enhanced near-field radiative heat transfer between planar surfaces with a thin-film plasmonic coupler

In last decade, there have been enormous efforts to experimentally show the near-field enhancement of radiative heat transfer between planar structures. Several recent experiments also have striven to achieve further enhanced heat transfer with the excitation of coupled surface polaritons by introducing nanostructures on both emitter and the receiver; however, these symmetric structures are hardly employed in real-world applications. Here, we demonstrate substantially increased near-field radiative heat transfer between asymmetric structures (i.e., doped Si and SiO$_2$) by using a thin Ti film as a plasmonic coupler. The measured near-field enhancement at vacuum gap of 380 nm is found to be 3.5 times greater than that for the case without the coupler. The enhancement mechanism is thoroughly elucidated for both polarizations and a dimensionless parameter, which can quantify the coupling strength of the surface polaritons at vacuum, is suggested. As a thin film can be readily used in many engineering applications, this study will facilitate the development of the high-performance engineering devices exploiting the near-field thermal radiation.

physics.app-ph↗

arXiv2019

Thermal Conductivity Enhancement by Surface Electromagnetic Waves Propagating along Multilayered Structures with Asymmetric Surrounding Media

Enhancement of thermal conductivity via surface electromagnetic waves (SEWs) supported in nanostructures has recently drawn attention as a remedy for issues raised due to the reduction of thermal conductivity in nanoscale confinement. Among them, multilayered structures on a substrate are prevalent in nano-sized systems, such as electronic nanodevices, meaning that analysis on those structures is indispensable. In this work, three basic multilayered structures are selected and the analytical expressions for SEWs supported in each structure are derived. This analytical approach enables us to figure out which factors are crucial for enhancing SEW thermal conductivity using multilayers. It is also found that the solution can be extended to various materials and provide the guidelines on which configurations are desirable for increasing the thermal conductivity. Furthermore, the analytical solutions reduce the calculation time significantly such that the optimal configuration, which can additionally yield SEW thermal conductivity of 1.27 W/m$\cdot$K corresponding to 90\% of the thermal conductivity of bulk glass, is found with the genetic algorithm. This study thus provides a new method for efficiently managing thermal issues in nano-sized devices.

physics.app-ph↗

arXiv2018

Performance analysis of Schottky-junction-based near-field thermophotovoltaic system

Numerous studies have reported performance enhancement of a thermophotovoltaic (TPV) system when an emitter is separated by nanoscale gaps from a TPV cell. Although a $p$-$n$-junction-based TPV cell has been widely used for the near-field TPV system, a Schottky-junction-based near-field TPV system has drawn attention recently with the advantage of the easy fabrication. However, existing studies mostly focused on the generated photocurrent only in the metal side due to the fact that required energy for the metal-side photocurrent (i.e., Schottky barrier height) is smaller than the bandgap energy. Here, we suggest the precise performance analysis model for the Schottky-junction-based near-field TPV system, including photocurrent generation on the semiconductor side by considering the transport of minority carriers within the semiconductor. It is found that most of the total photocurrent in the Schottky-junction-based near-field TPV system is generated in the semiconductor side. We also demonstrate that further enhancement in the photocurrent generation can be achieved by re-absorbing the usable photon energy in the metal with the help of a backside reflector. The present work will provide a design guideline for the Schottky-junction-based near-field TPV system taking into account three types of photocurrents.

physics.app-ph↗

arXiv2018

Optical measurements of three-dimensional microscopic temperature distributions around gold nanorods excited by surface plasmonics

The measurement and control of the temperature in microscopic systems, which are increasingly required in diverse applications, are fundamentally important. Yet, the measurement of the three-dimensional (3D) temperature distribution in microscopic systems has not been demonstrated. Here, we propose and experimentally demonstrate the measurement of the 3D temperature distribution by exploiting the temperature dependency of the refractive index (RI). Measurement of the RI distribution of water makes it possible to quantitatively obtain its 3D temperature distribution above a glass substrate coated with gold nanorods with sub-micrometer resolution, in a temperature range of 100C and with a sensitivity of 2.88C. The 3D temperature distributions that are obtained enable various thermodynamic properties including the maximum temperature, heat flux, and thermal conductivity to be extracted and analyzed quantitatively.

physics.optics↗

arXiv2018

Performance analysis of a direct-absorption parabolic trough solar collector

A parabolic trough solar collector is a dominant technology for high-temperature industrial applications, but efficient use of a conventional surface-based parabolic trough solar collector (SBPTSC) is limited by its high radiation loss due to the high surface temperature. Recently, direct-absorption parabolic trough solar collector (DAPTSC) using nanofluids has been proposed, and its thermal efficiency has been reported to be 5-10$\%$ higher than the conventional SBPTSC for inlet temperature up to 250$^\circ$C. However, the inner tubes of the receivers of the existing DAPTSCs are all transparent, so the sun rays entering the inner tube can only travel once through the nanofluids. As a result, the optical path length for the sun rays is limited by the inner tube size, which in turn requires high value of the absorption coefficient of nanofluids. Due to the approximately linear relation between the absorption coefficient and the particle concentration, higher absorption coefficient is likely to cause particle agglomeration, leading to detrimental effects on maintaining stable collector performance. In the current study, the transparent DAPTSC is improved by applying a reflective coating on the upper half of the inner tube outer surface, such that the optical path length is doubled compared to the transparent DAPTSC; thus, the absorption coefficient of the nanofluids can be reduced accordingly. The coated DAPTSC is found to have obvious advantage compared to the transparent DAPTSC at absorption coefficient below 0.5 cm$^{-1}$ for a receiver with inner tube diameter of 7 cm. In addition, performance of the transparent DAPTSC, the coated DAPTSC and the SBPTSC with black chrome coating have been compared to explore their advantageous operation conditions, such as inner tube diameter, flow rate, and inlet temperature, with or without a glass envelope for vacuum evacuation.

physics.app-ph↗