Showing papers on "Thermal radiation published in 2009"

Wide-angle perfect absorber/thermal emitter in the terahertz regime

Abstract: We show that a perfect absorber/thermal emitter exhibiting an absorption peak of 99.9% can be achieved in metallic nanostructures that can be easily fabricated. The very high absorption is maintained for large angles with a minimal shift in the center frequency and can be tuned throughout the visible and near-infrared regime by scaling the nanostructure dimensions. The stability of the spectral features at high temperatures is tested by simulations using a range of material parameters. Since the beginning of the last century it is known that a perfect thermal emitter follows Planck’s law of blackbody radiation. 1 Realistic structures, however, generally do not follow Planck’s law but exhibit a smaller emission. The properties of these emitters strongly depend on the materials and their shapes. From the absorption spectra of a structure the emission properties can be deduced since Kirchhoff’s law directly relates the absorption with the emissivity. The emission is then determined by multiplying the emissivity with the blackbody radiation spectrum. Using photonic crystals, 2,3 it has been shown that this approach is also valid for periodically structured materials. For a number of applications such as thermophotovoltaic converters, it is necessary to control the spectral properties to achieve, e.g., selective emitters in a narrow frequency band corresponding to the band gap of solar cells. 4 In the case of structured metallic surfaces, the changes in the emission spectra are based on surface waves coupled to the external radiation through the periodic surface. 5,6 Alternatively, microcavity resonances can also be used to create narrow-band thermal radiation. 7 Unfortunately, most of the recent designs 6,8 for perfect absorbers/ emitters only work for one incident angle and one polarization. So, there is a need for wide-angle perfect absorber/ emitter nanostructures. In this Brief Report, we suggest a structure which exhibits a large absorption in the terahertz regime for a wide range of angles with respect to the surface. We show that the absorption characteristics are maintained even if the uncertainties in the estimated changes in the material parameters, due to high temperatures, are considered. The proposed structure can be easily manufactured with today’s planar microfabrication techniques. We also comment on the impact of deviations in the geometrical parameters caused by fabricational tolerances. The small size of the structure, in comparison to the wavelength together with the relatively straightforward fabrication, allows for easy integration into various devices, such as perfect thermal emitters, perfect absorbers, bolometers, and very effective light extraction light-emitting diodes LEDs. The suggested structure is shown in Fig. 1. It consists of a metal back plate black with a thickness larger than 200 nm. This is much larger than the typical skin depth in the terahertz regime and avoids transmission through the structure. In this case the reflection is the only factor limiting the absorption. The thickness of the back plate can be adjusted to the specific needs of the final application, e.g., to obtain good heat transport to sensors or to obtain a better stability. On top of the metal plate a spacer layer of silicon nitride SiN is deposited with a thickness Dt. The structure is terminated by an array of metallic stripes with a rectangular cross section. Their arrangement is described by a lattice constant a and their shape is given by a width Ww and a thickness Wt. In this setup a strong resonance with a large field enhancement in the dielectric spacer layer and in between the stripes can be obtained, as will be shown later. Adjusting the size of the metal stripes on the top, the coupling to this resonance can be tuned and the reflection can be minimized. Due to the scalability of Maxwell’s equations, in principle, the structure can be simulated using dimensionless units by dividing all sizes by the lattice constant and using =a / as frequency. However, the Drude model used to describe the metal requires frequencies in terahertz and therefore the lattice constant must be assigned in the simulation. If a shift in the frequencies of the spectral features by adjusting the lattice constant is intended, a different simulation must be done since changes in the dielectric constant would not be considered. In the simulation frequency-dependent material parameters are required. We calculate those using standard methods and adjust their values to take into consideration the high temperatures. The tungsten parts plate and stripes are described by a Drude model

Physics of solar cells : from basic principles to advanced concepts

Abstract: 1 Problems of the Energy Economy 2 Photons 3 Semiconductors 4 Conversion of Thermal Radiation into Chemical Energy 5 Conversion of Chemical Energy into Electrical Energy 6 Basic Structure of Solar Cells 7 Limitations on Energy Conversion in Solar Cells 8 Concepts for Improving the Efficiency of Solar Cells 9 Prospects for the Future

Review of near-field thermal radiation and its application to energy conversion

Abstract: Understanding energy transfer via near-field thermal radiation is critical for the future advances of nanotechnology. Evanescent waves and photon tunneling are responsible for the near-field energy transfer being several orders of magnitude greater than that between two blackbodies. The enhanced energy transfer may be used for improving the performance of energy conversion devices, developing novel nanofabrication techniques, and imaging nanostructures with higher spatial resolution. Near-field heat transfer can be analyzed using fluctuational electrodynamics. This article reviews the fundamentals of near-field radiation and outlines the recent advances in this field. Important results are presented for near-field energy transfer between parallel plates and between multilayered structures. Application of near-field thermal radiation in near-field thermophotovoltaic devices is also discussed. Copyright © 2009 John Wiley & Sons, Ltd.

Numerical solution of the boundary layer flow over an exponentially stretching sheet with thermal radiation

Abstract: In this paper, the problem of steady laminar two-dimensional boundary layer flow and heat transfer of an incompressible viscous fluid with a presence of thermal radiation over an exponentially stretching sheet is investigated numerically. The governing boundary layer equations are reduced into ordinary differential equations by a similarity transformation. The transformed equations are solved numerically using an implicit finitedifference scheme known as the Keller-box method. The numerical solutions for the wall skin friction coefficient, the heat transfer coefficient, and the velocity and temperature profiles are computed, analyzed and discussed.

Quasi-blackbody component and radiative efficiency of the prompt emission of gamma-ray bursts

Abstract: We perform time-resolved spectroscopy on the prompt emission in gamma-ray bursts (GRBs) and identify a thermal, photospheric component peaking at a temperature of a few hundreds keV. This peak does not necessarily coincide with the broad-band (keV-GeV) power peak. We show that this thermal component exhibits a characteristic temporal behavior. We study a sample of 56 long bursts, all strong enough to allow time-resolved spectroscopy. We analyze the evolution of both the temperature and flux of the thermal component in 49 individual time-resolved pulses, for which the temporal coverage is sufficient, and find that the temperature is nearly constant during the first few seconds, after which it decays as a power law with a sample-averaged index of -0.68. The thermal flux first rises with an averaged power-law index of 0.63 after which it decays with an averaged index of -2. The break times are the same to within errors. We find that the ratio of the observed to the emergent thermal flux typically exhibits a monotonous power-law increase during the entire pulse as well as during complex bursts. Thermal photons carry a significant fraction ({approx}30% to more than 50%) of the prompt emission energy (in the observed 25-1900 keV energymore » band), thereby significantly contributing to the high radiative efficiency. Finally, we show here that the thermal emission can be used to study the properties of the photosphere, hence the physical parameters of the GRB fireball.« less

Solution of near-field thermal radiation in one-dimensional layered media using dyadic Green's functions and the scattering matrix method

Abstract: A general algorithm is introduced for the analysis of near-field radiative heat transfer in one-dimensional multi-layered structures. The method is based on the solution of dyadic Green's functions, where the amplitude of the fields in each layer is calculated via a scattering matrix approach. Several tests are presented where cubic boron nitride is used in the simulations. It is shown that a film emitter thicker than 1 μm provides the same spectral distribution of near-field radiative flux as obtained from a bulk emitter. Further simulations have pointed out that the presence of a body in close proximity to an emitter can alter the near-field spectrum emitted. This algorithm can be employed to study thermal one-dimensional layered media and photonic crystals in the near-field in order to design radiators optimizing the performances of nanoscale-gap thermophotovoltaic power generators.

Scattering approach to Casimir forces and radiative heat transfer for nanostructured surfaces out of thermal equilibrium

Abstract: We develop an exact method for computing Casimir forces and the power of radiative heat transfer between two arbitrary nanostructured surfaces out of thermal equilibrium. The method is based on a generalization of the scattering approach recently used in investigations on the Casimir effect. Analogously to the equilibrium case, we find that also out of thermal equilibrium the shape and composition of the surfaces enter only through their scattering matrices. The expressions derived provide exact results in terms of the scattering matrices of the intervening surfaces.

Breakdown of the Planck blackbody radiation law at nanoscale gaps

Abstract: The Planck theory of blackbody radiation imposes a limit on the maximum radiative transfer between two objects at given temperatures. When the two objects are close enough, near-field effects due to tunneling of evanescent waves lead to enhancement of radiative transfer above the Planck limit. When the objects can support electromagnetic surface polaritons, the enhancement can be a few orders-of-magnitude larger than the blackbody limit. In this paper, we summarize our recent measurements of radiative transfer between two parallel silica surfaces and between a silica microsphere and a flat silica surface that show unambiguous evidence of enhancement of radiative transfer due to near-field effects above the Planck limit.

Maximum energy transfer in near-field thermal radiation at nanometer distances

Abstract: Radiative energy transfer at nanoscale distances can exceed that of blackbody radiation by several orders of magnitude due to photon tunneling and the excitation of surface polaritons. While significant progress has been made recently in understanding near-field thermal radiation, an outstanding question remains as whether there exists an upper limit of near-field radiation for arbitrarily selected material properties at finite separation distances. We investigate the maximum achievable radiative heat flux between two parallel plates separated by a vacuum gap from 0.1 to 100 nm. By assuming a frequency-independent dielectric function and introducing a cutoff parallel wavevector component, we find that the ideal dielectric function for the two media that will maximize the near-field radiative transfer is −1+iδ, where δ is the imaginary part. For vacuum gaps greater than 1 nm, the near-field heat transfer peaks when δ⪡1, while at subnanometer gaps, the peak in the energy transfer shifts toward larger values...

Non-contact medical thermometer with stray radiation shielding

Abstract: A non-contact infrared (IR) thermometer for measuring temperature from the surface of an object includes an IR radiation sensor attached to a heating element and a thermal shield having an interior surface positioned within the sensor's field of view that has a high emissivity. An electronic circuit controlling the heating element maintains the temperatures of the sensor and shield substantially close to an anticipated surface temperature of the object. The IR radiation sensor is further thermally coupled to a reference temperature sensor. An optical system positioned in front of the shield focuses thermal radiation from the object on the surface of the sensor, while the shield prevents stray radiation from reaching the sensor. Signals from the IR and reference sensors are used to calculate the object's surface temperature.

Free Convection MHD Flow with Thermal Radiation from an Impulsively-Started Vertical Plate

Abstract: This paper investigates the combined effects of magnetohydrodynamics and radiation on free convection flow past an impulsively starte d isothermal vertical plate with Rosseland diffusion approximation. The fluid consider ed is a gray, absorbing- emitting radiation but a non-scattering medium, with approximate transformations the boundary layer governing the flow are reduced to non-dimensi onal equations valid in the free convection regime. The dimensionless governing equations are solved by the finite element method.

Thermal radiation effect on unsteady MHD free convection flow past a vertical plate with temperature-dependent viscosity

Abstract: This article investigates the influence of radiation and temperature-dependent viscosity on the problem of unsteady MHD flow and heat transfer of an electrically conducting fluid past an infinite vertical porous plate taking into account the effect of viscous dissipation. The governing equations are converted into a system of nonlinear ordinary differential equations via a local similarity parameter which is taken as a function of time. The resulting system of coupled nonlinear ordinary differential equations is solved numerically using the fourth order Runge-Kutta integration scheme with the shooting method. The numerical results for the velocity and the temperature are displayed graphically showing the effects of various parameters. The results show that increasing the Eckert number and decreasing the viscosity of air leads to a rise in the velocity, while increasing in the magnetic or the radiation parameters is associated with a decrease in the velocity. Also, an increase in the Eckert number leads to an increase in the temperature, whereas an increase in radiation parameter leads to a decrease in the temperature.

Characterization of thermal radiation spectra in 2m pool fires

Abstract: A mid-infrared spectrometer was used to measure the thermal radiation spectra within the turbulent flame brush and vapor dome of a 2 m diameter well-controlled pool fire. Fuels used include ethanol, an ethanol/toluene blend, JP-8, and heptane. These unique data provide insight into the relative contributions of soot and gas species emissions to the overall emission. They further assess the impact of absorption of thermal radiation from within the flame zone and the fuel rich region on the incident flux to the fuel pool. In addition, laboratory-scale experiments investigated the spectrally-resolved absorption of thermal radiation by the liquid fuel in the wavelength range of 1.3–4.8 μm, corresponding to the majority of the expected emitted radiation from the fire. The dominant emission in the ethanol fires was from water and carbon dioxide, the products of combustion; while, emission from soot dominated the thermal radiation spectra from the other three fuels. The overall intensity of the thermal radiation reaching the fuel surface for the three soot producing fuels was reduced due to absorption by cold water, carbon dioxide, fuel vapor (due to the C–H bond stretching), and soot. The transmission of thermal radiation through liquid fuel revealed that a significant fraction (>75% for JP-8 and >90% for ethanol) of the thermal radiation that reaches the fuel surface is absorbed within the first 3 mm. All fuels were particularly opaque in the 3.2–3.6 μm range and some fuels (heptane and JP-8) were relatively transparent at wavelengths less than 1.6 μm and from 1.85 to 2.1 μm, where a significant amount of thermal energy exists. These data sets provide a sound basis for assessing the thermal radiation incident upon the fuel surface and suggest the penetration depth of the incident flux through the liquid pool.

Large-Eddy Simulation of Turbulence-Radiation Interactions in a Turbulent Planar Channel Flow

Abstract: Large-eddy simulation (LES) has been performed for planar turbulent channel flow between two infinite, parallel, stationary plates. The capabilities and limitations of the LES code in predicting correct turbulent velocity and passive temperature field statistics have been established through comparison to direct numerical simulation data from the literature for nonreacting cases. Mixing and chemical reaction (infinitely fast) between a fuel stream and an oxidizer stream have been simulated to generate large composition and temperature fluctuations in the flow; here the composition and temperature do not affect the hydrodynamics (one-way coupling). The radiative transfer equation is solved using a spherical harmonics (P1) method, and radiation properties correspond to a fictitious gray gas with a composition- and temperature-dependent Planck-mean absorption coefficient that mimics that of typical hydrocarbon-air combustion products. Simulations have been performed for different optical thicknesses. In the absence of chemical reactions, radiation significantly modifies the mean temperature profiles, but temperature fluctuations and turbulence-radiation interactions (TRI) are small, consistent with earlier findings. Chemical reaction enhances the composition and temperature fluctuations and, hence, the importance of TRI. Contributions to emission and absorption TRI have been isolated and quantified as a function of optical thickness.

Ultrasmall penetration depth in nanoscale thermal radiation

Abstract: Near-field thermal radiation can significantly exceed that predicted by the Stefan–Boltzmann law, especially when surface polaritons are excited such that the energy transfer is through photon tunneling. The penetration depth, or skin depth, of evanescent waves is usually a few tenths of a wavelength. This letter demonstrates that an extremely small skin depth (on the order of a nanometer) can exist for nanoscale thermal radiation between two plates separated by a vacuum gap, even though the dominant wavelengths are in the infrared. Furthermore, the skin depth is proportional to the separation distance.

A new approach to optimizing pigmented coatings considering both thermal and aesthetic effects

Abstract: Pigmented coatings with high reflectivity against solar irradiation can be used to control unwanted thermal heating that occurs as materials absorb sunlight such as heat in buildings that increases cooling loads. However, these surfaces produce glare that is unpleasant to the eye, and the coatings themselves can damage the appearance of the coated object. We introduce a new optimization method that embraces both thermal and aesthetic requirements. Our proposed coatings maximize the reflectivity of the near infrared (NIR) region to reduce thermal heating, while for aesthetic appeal they also minimize the visible (VIS) reflected energy received by human eyes, especially at wavelengths where eye sensitivity is high. The optimization parameter is defined as the ratio of the total reflected energy in the NIR region to that in the VIS region weighted by human eye sensitivity. Titanium dioxide is used as the pigment, and databases of its radiative properties are constructed using the Mie theory. To compute reflectivity, nongray radiative heat transfer in an anisotropic scattering monosized pigmented layer, with independent scattering, including direct and diffuse solar irradiations, is analyzed using radiation element method by ray emission model (REM2). Colors are calculated and optimization parameter is evaluated by using spectral reflectivity. Finally, the optimum values of particle size, volume fraction of pigment, and coating thickness are obtained.

Net exchange parameterization of thermal infrared radiative transfer in Venus' atmosphere

Abstract: [1] Thermal radiation within Venus atmosphere is analyzed in close details. Prominent features are identified, which are then used to design a parameterization (a highly simplified and yet accurate enough model) to be used in General Circulation Models. The analysis is based on a net exchange formulation, using a set of gaseous and cloud optical data chosen among available referenced data. The accuracy of the proposed parameterization methodology is controlled against Monte Carlo simulations, assuming that the optical data are exact. Then, the accuracy level corresponding to our present optical data choice is discussed by comparison with available observations, concentrating on the most unknown aspects of Venus thermal radiation, namely the deep atmosphere opacity and the cloud composition and structure.

Parametric optimization of dielectric functions for maximizing nanoscale radiative transfer

Abstract: Near-field thermal radiation can be several orders of magnitude higher than that between two black bodies. Previous studies have shown that the energy transfer between two semi-infinite media separated by a nanometre vacuum gap is maximized when the real part of the dielectric function is around −1 due to the excitement of surface waves. Real materials can exhibit such a behaviour only within a very small spectral interval. However, by tuning the different adjustable parameters of the dielectric functions, it is possible to estimate the maximum achievable near-field radiative transfer. In this study, the influence of each parameter in the Drude and the Lorentz models on the nanoscale radiation is investigated. Optimal values are obtained for these parameters that maximize the near-field heat flux, which can be more than an order of magnitude higher than previously calculated values for SiC and doped Si. The effect of temperature on the optimal parameters in the Drude model is also discussed. The results will guide future selection and design of materials for the enhancement in near-field heat transfer.

T-shaped plasmonic array as a narrow-band thermal emitter or biosensor

Abstract: A T-shaped plasmonic array is proposed for application as an effective thermal emitter or biosensor. The reflection and thermal radiation properties of a T-shaped array are investigated theoretically. The angular dependent reflectance spectrum shows a clear resonant dip at 0.36eV for full polar angles. No other significant localized resonant mode is found in the investigated spectral range from 0.12eV to 0.64eV. According to the Kirchhoff’s law, the thermal radiation of the proposed structure can lead to a sharp peak at 3.5µm with low sideband emission. We have also found that the T-shaped structure filled with organic material such as PMMA with different thicknesses (10 nm -50 nm) can lead to significant shift of the resonance wavelength. Thus, the T-shaped structure can also be used as a good sensor for organic materials.