Thermal Radiation Spectrum Research Articles

Determination of the temperature of an object irradiated by microwaves using spectral pyrometry

Recently, due to increasing environmental requirements, microwave carbonization method has been widely investigated for the production of activated carbon from biomass waste. Microwave carbonization is a more energy efficient and environmentally friendly method compared to traditional carbonization in thermal furnaces, but to date there are still a number of questions about the reproducibility and temperature stability of this process. During microwave irradiation, the temperature of biomass samples changes continuously and complex physical and chemical processes occur in them. For deeper study of these processes and determination of optimal modes of microwave treatment it is necessary to know the temperature dynamics of biomass samples. For this purpose, the method of spectral pyrometry based on the allocation of sections of the spectra of thermal radiation of samples coinciding with the Planck spectrum was used. In the specified spectral sections these samples are gray emitters. The method is effective for an unknown emission coefficient, continuously varying with changes in the microstructure, chemical composition and phase state of the sample. The sample irradiated by microwaves was a sample of orthophosphoric acid-treated cotton down weighing 1 g. The microwave source was a magnetron-type generator with a maximum output power of 600 W and an operating frequency of 2450 MHz. Thermal spectra of the irradiated sample were recorded by small-size spectrometers of visible (350–760 nm) and near-infrared (650–1050 nm) ranges. The time of irradiation of samples by microwaves was varied in the range of 60–180 s. To process the obtained spectra, the program “Spectral Pyrometry” was used, which reads the recorded spectrum, processes it, plots it in the necessary coordinates and calculates the temperature. Analysis of the obtained results revealed different types of thermal radiation spectra of the irradiated sample - spectra similar to spectra of a completely black body, spectra with different temperature zones of the sample, spectra with atomic lines and molecular bands. The results obtained are useful for the study of microwave influence on various objects, research of processes occurring during carbonization of biomass, as well as for the development of more effective modes of production of activated carbons by microwave method.

Steering Thermal Radiation

Thermal radiation is a physical phenomenon exhibiting dual characteristics of both light and heat. Sunlight, serving as the primary source of energy, emanates as thermal radiation from a high-temperature surface at 5,700 K. It is also responsible for the feeling of warmth experienced when individuals congregate and non-contact measurement of body temperature. Thus, thermal radiation exists everywhere in our daily life. However, in the early 20th century, thermal radiation posed a challenge to physicists. The endeavor to elucidate the spectrum of thermal radiation led to the concept of light as photons, therefore signaling the advent of quantum physics. It is known that thermal radiation uniformly emits, irrespective of its direction and polarization, with the spectrum dictated by Planck’s law. Yet, this commonplace should be modified when thermal radiation encounters the principle of nanophotonics. Herein lies the ability to modulate the intensity of thermal radiation across desired wavelengths, directions, and polarizations. In this article, we will delve into the latest research findings concerning the manipulation of thermal radiation and its promising applications.

Controlling Thermal Radiation in Photonic Quasicrystals Containing Epsilon-Negative Metamaterials

The transfer matrix approach is used to study the optical characteristics of thermal radiation in a one-dimensional photonic crystal (1DPC) with metamaterial. In this method, every layer within the multilayer structure is associated with its specific transfer matrix. Subsequently, it links the incident beam to the next layer from the previous layer. The proposed structure is composed of three types of materials, namely InSb, ZrO2, and Teflon, and one type of epsilon-negative (ENG) metamaterial and is organized in accordance with the laws of sequencing. The semiconductor InSb has the capability to adjust bandgaps by utilizing its thermally responsive permittivity, allowing for tunability with temperature changes, while the metamaterial modifies the bandgaps according to its negative permittivity. Using quasi-periodic shows that, in contrast to employing absolute periodic arrangements, it produces more diverse results in modifying the structure’s band-gaps. Using a new sequence arrangement mixed-quasi-periodic (MQP) structure, which is a combination of two quasi periodic structures, provides more freedom of action for modifying the properties of the medium than periodic arrangements do. The ability to control thermal radiation is crucial in a range of optical applications since it is frequently unpolarized and incoherent in both space and time. These configurations allow for the suppression and emission of thermal radiation in a certain frequency range due to their fundamental nature as photonic band-gaps (PBGs). So, we are able to control the thermal radiation by changing the structure arrangement. Here, the We use an indirect method based on the second Kirchoff law for thermal radiation to investigate the emittance of black bodies based on a well-known transfer matrix technique. We can measure the transmission and reflection coefficients with associated transmittance and reflectance, T and R, respectively. Here, the effects of several parameters, including the input beam’s angle, polarization, and period on tailoring the thermal radiation spectrum of the proposed structure, are studied. The results show that in some frequency bands, thermal radiation exceeded the black body limit. There were also good results in terms of complete stop bands for both TE and TM polarization at different incident angles and frequencies. This study produces encouraging results for the creation of Terahertz (THz) filters and selective thermal emitters. The tunability of our media is a crucial factor that influences the efficiency and function of our desired photonic outcome. Therefore, exploiting MQP sequences or arrangements is a promising strategy, as it allows us to rearrange our media more flexibly than quasi-periodic sequences and thus achieve our optimal result.

Experimental Complex for Spectral Diagnostics of Particle Flow Temperature during Gas-Thermal Spraying

The paper presents an instrumental diagnostic complex developed by the authors. The compact LR1 spectrometer for spectral diagnostics of particle temperature of two-phase flow is an integral part of the complex, as well as the calibration tool with the reference emitter. Additionally, a special emitter consisting of miniature light bulbs attached closely to each other was used in test experiments. This special emitter served as a simulation model of particle thermal radiation of gas-thermal flow used for spraying of functional coatings on technical products. The program-controlled adjustment of current flow through bulbs interconnected in different configurations yields a variety of simulated test histograms of the temperature distribution of the bulbs that correspond to actual particle temperature distribution histograms. The emitter with light bulbs has been used as a simulation model to confirm the validity of the physical and mathematical model of the integral (total) spectrum of particle thermal radiation for the proposed experimental spectrum registration method employing the LR1 spectrometer.

Fano Resonance in Near-Field Thermal Radiation of Two-Dimensional Van der Waals Heterostructures.

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

Density of states effects on emission and scattering of photons in plasmas

Plasma supports electromagnetic waves propagation for frequencies higher than plasma frequency but features dielectric permittivity less than 1. This property leads to photon density of states (DOS) lower than in vacuum and should result in subnatural spectral linewidths, sub-Planckian spectrum of thermal radiation, and sub-Rayleigh scattering as well as in lower inelastic photon scattering including Raman scattering. Lamb shift will be altered as well though the decisive contribution from high-energy modes in this case makes the photon DOS effect rather small since plasma DOS converges with the vacuum one in the limit of infinite frequencies. We emphasize the basic character of all these phenomena though absolute values of corrections in many real experiments may appear to be small as compared to other factors. We found that dissipative losses make possible DOS effects smaller though not vanishing and additionally bring about indefinite growth of DOS in the low-frequency limit.

Spectral splitting solar energy transfer in small-diameter multimode optical fiber based on two-stage concentration

A two-stage concentration system (TSC) is proposed for transfer spectral splitting solar energy via a small-diameter multimode optical fiber. The TSC includes a Fresnel lens that concentrates the energy before passing it through UV700 for spectral splitting. The system employs an optical fiber coupler for the second concentration stage and a single-diameter multimode fiber for energy transfer. We established a mathematical model of TSC, coupled optical fiber transmission, and analyzed it via simulation and experimentation. Simulation results demonstrate that the TSC reduces the spot's energy by 42% of the thermal radiation spectrum, decreases the focal spot radius by 58%, and improves the energy flux by 10.93 times with UV700. Additionally, experimental findings reveal that the TSC reduces the focal spot energy by 40.46% of the thermal radiation spectrum, decreases the fiber end temperature by 59.52%, identifies the optimal coupling distance at 308.75 mm, and yields a coupling efficiency of 60.7%.

Study of radiation power spectra in the 1-D photonic periodic dielectric structure

We explored the thermal power radiation spectrum ρ (ω, T) of electromagnetic radiation in a 1D photonic periodic structure combining Si and Sio2 with truncated Sio2 media and an absorbing substrate. A theoretical model based on a transfer matrix for both normal and oblique incidence angles together with Kirchhoff’s second rule is used to predict the thermal radiation power spectrum. In order to study the radiation spectra, we used the TE and TM modes with truncation parameters and incidence angles in the mid, left, and right gap frequencies, which correspond to the mid, left, and right mid-gap temperatures. Layers nA&nB are A and B’s indices are considered to be constant and controlled in these modes.

Коррекция искажений спектров радиотеплового излучения атмосферы, получаемых по данным радиометра-спектрометра Р22М при быстро меняющихся гидрометеорологических условиях

Коррекция искажений спектров радиотеплового излучения атмосферы, получаемых по данным радиометра-спектрометра Р22М при быстро меняющихся гидрометеорологических условиях

The role of EM wave polarization on radiative heat transfer across a nanoscale gap

This work presents a novel study of radiative heat transfer between closely separated plates based on an extension of Planck’s spectrum of thermal radiations to systems with a steady heat flux. This extension together with electromagnetic wave theory is chosen specifically to avoid the commonly used so-called fluctuation dissipation theory, which is also limited to equilibrium systems. The spectrum of thermal radiation with a heat flux is described by the introduction of an analog of a chemical potential, which creates a bias toward the direction of heat transfer. This is the first comprehensive study of radiative heat transfer based on the generalization of Planck’s spectrum for systems with a heat flux, which eliminates contradictions arising when a heat flux is described in terms of the laws limited to equilibrium systems. The total heat flux is split into fluxes carried by waves with different frequencies, directions of propagation, and polarizations. This simplifies the analysis because due to the stochastic independence, the energy fluxes of such waves are additive, and this also reveals that the heat carrying capacity of radiation with the parallel polarization is significantly higher than that of the perpendicularly polarized radiation. This suggests that the rate of radiative heat transfer may be noticeably increased by the control of the polarization of thermal radiation.

Cutting temperature measurement in turning using fiber-optic multi-spectral radiation thermometry and its application in tool wear status recognition

The cutting temperature is essential for phenomena understanding and quality improvement in metal cutting while its in-situ online measurement is still a challenge. This paper presents a near-infrared fiber-optic multi-spectral method for in-situ online cutting temperature measurement. Using thermal radiation spectrum for temperature measurement, the method optimizes the lower limit of temperature measurement to 150 °C while improving accuracy. The calibration shows that in the range of above 250 °C, the average relative error of temperature measurement is stable below 0.5%. The titanium alloy cutting experiments are carried out. In-situ online measurement of tool temperatures in dry/wet cuttings are realized using the self-developed system. The influence of cutting parameters on cutting temperature is studied, and the real-time response of the temperature measurement system to the cutting state is verified. As for industrial application, the capability of the system in heavy-duty turning is proved by railway wheelsets turning experiments. Tool wear experiments are conducted, and a positive correlation between the cutting temperature and tool wear is revealed. Tool wear status recognition is realized based on cutting temperature by sparse autoencoder and k-means clustering, and a recognition accuracy of 97.3% is achieved. These results indicate promising prospects in cutting mechanism research, machining status monitoring and industrial applications, empowering the advancement of intelligent manufacturing and industry 4.0.

Density of States Effects on Emission and Scattering of Photons in Plasmas

Plasma supports electromagnetic waves propagation for frequencies higher than plasma frequency but features dielectric permittivity less than 1. This property leads to photon density of states (DOS) lower than in vacuum and should result in subnatural spectral linewidths, sub-Planckian spectrum of thermal radiation, and in sub-Rayleigh photon scattering as well as in lower inelastic photon scattering including Raman scattering in molecular and crystalline structures. We emphasize the basic character of these phenomena though absolute values of corrections from this property of plasmas in many real experiments may appear to be small as compared to other factors. We found that dissipative losses when being taken into account make possible DOS effects smaller though not vanishing and additionally bring about indefinite growth of DOS in the low-frequency limit.

Spectrum of Thermal Radiation of Surface Plasmon-Polaritons Diffracted at the Edge of a Plane Metal Face

Spectrum of Thermal Radiation of Surface Plasmon-Polaritons Diffracted at the Edge of a Plane Metal Face

A Simple Model of Radiation from a Magnetized Neutron Star: Accreted Matter and Polar Hotspots

A simple and well known model for thermal radiation spectra from a magnetized neutron star is further studied. The model assumes that the star is internally isothermal and possesses a dipole magnetic field (B≲1014 G) in the outer heat-insulating layer. The heat transport through this layer makes the surface temperature distribution anisotropic; any local surface element is assumed to emit a blackbody (BB) radiation with a local effective temperature. It is shown that this thermal emission is nearly independent of the chemical composition of insulating envelope (at the same taken averaged effective surface temperature). Adding a slight extra heating of magnetic poles allows one to be qualitatively consistent with observations of some isolated neutron stars.

Two-blackbody portraits of radiation from magnetized neutron stars

ABSTRACT We study a simple model describing thermal radiation spectra from magnetized neutron stars. The model assumes that a star is nearly spherical and isothermal inside and possesses dipole magnetic fields (B ≲ 1014 G) near the surface, which make the surface temperature distribution non-uniform. We assume further that any surface element emits a blackbody (BB) spectrum with a local effective temperature. We show that such thermal spectra (including phase-resolved) are accurately approximated by simple equivalent two-BB (2BB) models. We introduce and study phase-space maps of 2BB parameters and show that these maps can be useful for interpreting neutron star observations, in which 2BB spectral fits have been done.

The quantum confinement effect on the spectrum of near-field thermal radiation by quantum dots

The quantum confinement effect on the spectrum of near-field thermal radiation by periodic and random arrays of quantum dots (QDs) is investigated. The local density of states (LDOS) thermally emitted by QD arrays made of three lead chalcogenides, namely, lead sulfide, lead selenide, and lead telluride, is computed at a near-field distance from the arrays. The dielectric function of the QDs is extracted from their absorption spectra by utilizing an optimization technique. The thermal discrete dipole approximation is used for computing the LDOS. It is shown that the peak wavenumber of near-field LDOS emitted by periodic arrays of lead chalcogenide QDs can be significantly modulated (up to 4490 cm−1) by varying the size of the dots. The LDOS is proportional to the imaginary part of the QDs' polarizability, which peaks at the bandgap energy of the QDs. The bandgap energy of the QDs (and thus the LDOS peak) is significantly affected by the quantum confinement effect, which is size dependent. While the magnitude of thermal radiation by random arrays of QDs can be different from the periodic arrays with the same filling factor by up to ±26%, the LDOS spectrum and peak location are the same for both periodic and random arrays. The peak wavenumber of near-field radiative heat transfer between the QD arrays is also strongly affected by quantum confinement in the QDs, and thus, it can be tuned by changing the size of the QDs.

Theory of exciton thermal radiation in semiconducting single-walled carbon nanotubes.

Spectral control of thermal radiation is an essential strategy for highly efficient and functional utilization of thermal radiation energy. Among the various proposed methods, quantum confinement in low-dimensional materials is promising because of its inherent ability to emit narrowband thermal radiation. Here, we theoretically investigate thermal radiation from one-dimensional (1D) semiconductors characterized by the strong quantum correlation effect due to the Coulomb interaction. We derive a simple and useful formula for the emissivity, which is then used to calculate the thermal radiation spectrum of semiconducting single-walled carbon nanotubes as a representative of 1D semiconductors. The calculations show that the exciton state, which is an electron-hole pair mutually bound by the Coulomb interaction, causes enhancement of the radiation spectrum peak and significant narrowing of its linewidth in the near-infrared wavelength range. The theory developed here will be a firm foundation for exciton thermal radiation in 1D semiconductors, which is expected to lead to new energy harvesting technologies.

Optimization of data acquisition algorithm for temperature and emissivity distribution measurement using snapshot hyperspectral imaging systems

Many industrial and scientific applications require precise measurement of temperature distribution over the surface of the inspected objects with a priori unknown emissivity. In these cases, imaging spectroscopy is an effective approach for quantitative thermography. This method is based on the measurement of thermal radiation spectrum in each pixel and its approximation of Planck's law by selecting proper values of temperature and emissivity. Snapshot hyperspectral imaging systems are especially interesting for this task as they provide an arbitrary number, positions and order of spectral bands and, therefore, enable multiple data acquisition modes. In this paper, we show that conventional equidistant location of spectral bands is not optimal in terms of temperature measurement error minimization. More effective approach is aggregation of the spectral bands into two narrow ranges located around particular wavelengths. We prove these statements by the theoretical consideration, mathematical modeling and experimental study on the tungsten plates using acousto-optical hyperspectral imager. Obtained results may be useful for the development and improvement of quantitative thermography systems with optimized data acquisition modes for various applications.

Thermal radiation equilibrium: (nonrelativistic) classical mechanics versus (relativistic) classical electrodynamics

Physics students continue to be taught the erroneous idea that classical physics leads inevitably to energy equipartition, and hence to the Rayleigh–Jeans law for thermal radiation equilibrium. Actually, energy equipartition is appropriate only for nonrelativistic classical mechanics, but has only limited relevance for a relativistic theory such as classical electrodynamics. In this article, we discuss harmonic-oscillator thermal equilibrium from three different perspectives. First, we contrast the thermal equilibrium of nonrelativistic mechanical oscillators (where point collisions are allowed and frequency is irrelevant) with the equilibrium of relativistic radiation modes (where frequency is crucial). The Rayleigh–Jeans law appears from applying a dipole-radiation approximation to impose the nonrelativistic mechanical equilibrium on the radiation spectrum. In this discussion, we note the possibility of zero-point energy for relativistic radiation, which possibility does not arise for nonrelativistic classical-mechanical systems. Second, we turn to a simple electromagnetic model of a harmonic oscillator and show that the oscillator is fully in radiation equilibrium (which involves all radiation multipoles, dipole, quadrupole, etc) with classical electromagnetic zero-point radiation, but is not in equilibrium with the Rayleigh–Jeans spectrum. Finally, we discuss the contrast between the flexibility of nonrelativistic mechanics with its arbitrary potential functions allowing separate scalings for length, time, and energy, with the sharply-controlled behavior of relativistic classical electrodynamics with its single scaling connecting together the scales for length, time, and energy. It is emphasized that within classical physics, energy-sharing, velocity-dependent damping is associated with the low-frequency, nonrelativistic part of the Planck thermal radiation spectrum, whereas acceleration-dependent radiation damping is associated with the high-frequency adiabatically-invariant and Lorentz-invariant part of the spectrum corresponding to zero-point radiation.

Spectral pyrometry of surface and flame

The development of high-speed aircraft requires the study of high-temperature materials and of combustion processes in engine combustion chambers. One of the main parameters in these studies is the temperature, which must be measured up to 3000K. Such high temperatures can be measured only by non-contact methods. One of these methods is spectral pyrometry, which is based on temperature measurement from the energy distribution in thermal radiation spectrum of investigated object. This paper presents the results of spectral pyrometry application to measure the surface temperatures and hydrocarbon flame temperatures.