Patrick Echegut

A spectroscopic method to measure the spectral emissivity of semi-transparent materials up to high temperature

A spectroscopic method to measure directional spectral emissivity for homogeneous and heterogeneous semi-transparent materials, giving access to a large spectral 10–12 000 cm−1 range and to temperatures lying between 600 and 3000 K is reported. Sample heating is supplied by a carbon dioxide laser and the blackbody flux reference is obtained with a lanthanum chromite furnace. Experimental results obtained with this setup on several dielectric oxides such as silica, alumina, and magnesia are in good agreement with the results obtained by an indirect method based on the Kirchhoff’s laws. A brief overview of the setup abilities and a detailed discussion of the emissivity spectra are also proposed.

Recycled Material for Sensible Heat Based Thermal Energy Storage to be Used in Concentrated Solar Thermal Power Plants

Current technologies of concentrated solar power plants (CSP) are under extensive industrial development but still suffer from lack of adapted thermal energy storage (TES) materials and systems. In the case of extended storage (some hours), thousands of tonnes of materials are concerned leading to high investment cost, high energy and GHG contents and major conflicts of use. In this paper, recycled industrial ceramics made by vitrification of asbestos containing wastes (ACW) are studied as candidates to be used as sensible TES material. The material presents no hazard, no environmental impact, good thermophysical properties (λ= 1.4 W m -1 K -1 ; Cp = 1025 J kg -1 K -1 ; p= 3100 kg m -3 ) and at very low investment cost. Thanks to the vitrification process of the wastes, the obtained ceramics is very stable up to 1200 °C and can be directly manufactured with the desired shape. The vitrified ACW can be used as TES material for all kinds of the CSP processes (from medium up to high concentration levels) with properties in the same range than other available materials but with lower cost and without conflict of use. The proposed approach leads also to sustainable TES allowing a pay back of the energy needed for the initial waste treatment. Furthermore, this new use of the matter can enhance the waste treatment industry instead of land fill disposal.

Contribution of Semi-Quantum Dielectric Function Models to the Analysis of Infrared Spectra

A new approach to modeling using semi-quantum dielectric function models is proposed for the retrieval of the optical functions from infrared spectra. The powerful points of the method are shown throughout the analysis of two semitransparent materials, MgO and KBr. All the results are discussed in light of those obtained with classical techniques. This type of model is able to retrieve, for example, the extinction index in a range that covers at least six orders of magnitude and gives access to highly valuable information about high-order phonon processes.

High emissivity of a rough Pr2NiO4 coating

A rough and thick film of doped praseodymium nickel oxide (Pr2NiO4+δ) has been obtained by combining the spray pyrolysis deposition technique with rapid thermal annealing process. The precursors are nitrate solutions. Spectral emissivity measurements from the far infrared up to the midinfrared region at T=1000 K show the strong black body character of the film. This feature is well suited to the thermal efficiency of an industrial infrared ceramic radiant plate.

Structural, dielectric, and optical properties of yttrium calcium borate glasses

Structural and optical properties of stable glasses in the Y2O3–CaO–B2O3 system, containing the same Y/Ca ratio as the YCa4O(BO3)3 (YCOB) crystal, were determined from Raman and reflectance infrared spectroscopy. Changes in optical functions with composition are associated with an increase in the number of non-bridging oxygen and to calcium/yttrium oxides content. Refractive indexes values (from 1.597 to 1.627 at λ=2 μm) are in good agreement with those of the YCOB crystal, an indication that these glasses are potential candidates for optical applications due to their ease of shaping as large bulk samples or fibers.

High temperature NMR study of lithium sodium sulfate

Abstract A new high temperature NMR probe assembly using CO 2 laser as heating system enables a better description of the high temperature phase transitions of LiNaSo 4 from room temperature to the liquid state (940 K). Linewidth and T 1 relaxation time measurements give new information about the cationic diffusion which begins at least 400 degrees below the superionic transition. At this transition a double resonance mechanism between cations jumps and SO 4 rotations is proposed.

Prediction of the thermal radiative properties of an x-ray μ-tomographied porous silica glass

A Monte Carlo ray tracing procedure is proposed to simulate thermal optical processes in heterogeneous materials. It operates within a detailed 3D image of the material, and it can therefore be used to investigate the relationship between the microstructure, the constituent optical properties, and the macroscopic radiative behavior. The program is applied to porous silica glass. A sample was first characterized by 3D x-ray tomography; then, its normal spectral emittance was calculated and compared with the experimental spectrum measured independently by high-temperature infrared emittance spectroscopy. We conclude with a discussion of the light-scattering mechanisms occurring in the sample.

Temperature dependence of the dielectric permittivity of CaF2, BaF2 and Al2O3: application to the prediction of a temperature-dependent van der Waals surface interaction exerted onto a neighbouring Cs(8P3/2) atom

The temperature behaviour in the range 22-500 °C of the dielectric permittivity in the infrared range is investigated for CaF(2), BaF(2) and Al(2)O(3) through reflectivity measurements. The dielectric permittivity is retrieved by fitting reflectivity spectra with a model taking into account multiphonon contributions. The results extrapolated from the measurements are applied to predict a temperature-dependent atom-surface van der Waals interaction. We specifically consider as the atom of interest Cs(8P(3/2)), the most relevant virtual couplings of which fall in the range of thermal radiation and are located in the vicinity of the reststrahlen band of fluoride materials.

Dispersion Relations and Phase Retrieval in Infrared Reflection Spectra Analysis

A variant of the maximum entropy method (MEM), recently developed by Vartiainen et al., is proposed for infrared reflectivity spectra analysis. The procedure uses a new interpolation algorithm to estimate the so-called error phase that is the key step of the model. As shown by several examples, this approach is a good alternative to the well known Kramers–Kronig phase retrieval relation for the determination of the dielectric function and proved to be particularly efficient in dealing with conductor materials.

High temperature emissivity, reflectivity, and x-ray absorption of BiFeO3

We report on the lattice evolution of BiFeO3 as function of temperature using far infrared emissivity, reflectivity, and x-ray absorption local structure. A power law fit to the lowest frequency soft phonon in the magnetic ordered phase yields an exponent β=0.25 as for a tricritical point. At about 200 K below TN∼640 K it ceases softening as consequence of BiFeO3 metastability. We identified this temperature as corresponding to a crossover transition to an order-disorder regime. Above ∼700 K strong band overlapping, merging, and smearing of modes are consequence of thermal fluctuations and chemical disorder. Vibrational modes show band splits in the ferroelectric phase as emerging from triple degenerated species as from a paraelectric cubic phase above TC∼1090 K. Temperature dependent x-ray absorption near edge structure (XANES) at the Fe K edge shows that lower temperature Fe3+ turns into Fe2+. While this matches the FeO wustite XANES profile, the Bi LIII-edge downshift suggests a high temperature very c...