D. De Sousa Meneses

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.

Influence of the network on conductivity in ternary CaO–Al2O3–SiO2 glasses and melts

The electrical conductivity of ternary CaO–Al2O3–SiO2 compositions has been studied both in the solid and the liquid states. Below Tg, calcium migration, described well by an Arrhenius law, is highly influenced by the non-bridging oxygen (NBO) content. A fast enhancement of the conductivity occurs for glasses containing a content of NBO higher than 30–35% which corresponds to a percolation threshold. Above Tg, the temperature dependence of the conductivity is better described by a Vogel–Tammann–Fulcher (VTF) law and the NBO content is no more the pertinent parameter. In the melt state, the network characterised by the silica content, becomes the main parameter that governs the temperature dependence of the conductivity.

Phase transformations of crystalline SiO2 versus dynamic disorder between room temperature and liquid state

The sequence of phase transitions of crystalline silica has been probed by infrared emission spectroscopy. The lattice dynamics, deeply impacted by the low frequency dynamic disorder, exhibit with increasing temperature signs of inhomogeneous broadening of the symmetry allowed normal modes. High frequency supplementary components are also activated. The analysis with a causal Voigt dielectric function model within the framework of hard mode spectroscopy allowed a fine characterization of solid–solid phase transitions. We also report experimental evidence showing that the occurrence of the intermediate ill-defined region above 1300 K is concomitant with the reactivation of the low frequency dynamic disorder; a behavior change that on the way explains the appearance of the negative thermal expansion regime.

Infrared optical properties of α-alumina with the approach to melting: γ-like tetrahedral structure and small polaron conduction

The normal spectral emittance of α-Al2O3 single crystal has been measured from room temperature up to the liquid state and from 20 cm−1 up to 10 000 cm−1, in two polarization configurations. The spectra were fitted with a semi-quantum dielectric function model. AlO4 structure units are revealed within the phonon spectral range more than a hundred degrees below the melting point when heating from the solid state. In parallel, the anomalous increase of emittance observed within the transparency spectral range with the approach to melting appears strongly correlated. Implications on the electronic structure are discussed: the existence of small polaron conduction is suggested which has never been mentioned before.

An X-ray diffractometer coupled with diffuse reflectance infrared Fourier transform spectroscopy and gas chromatography for in situ and in operando characterization: an innovative analytical laboratory instrument

In heterogeneous catalysis, chemical reactions take place at the surface of the material and can be influenced by its structure. To understand better the impact of the material surface and structure on catalytic properties, it is important to characterize them simultaneously. The association of X-ray diffraction and diffuse reflectance infrared Fourier transform spectroscopy, combined in a single dedicated high-temperature and high-pressure reaction cell with an online gas chromatograph, could be the answer to this challenge. For the first time, such an analytical tool has been developed for laboratory applications. The use of this device is illustrated, and it is validated through the in situ study of the thermal decomposition of calcium oxalate.

Interplay between metal nanoparticles and dielectric spacing layers during the growth of Au/Si3N4 multilayers

Competition between ordering and disordering processes during growth of granular Si3N4/[Au(t)/Si3N4]n thin films by vapor deposition is tracked in a simple, unconventional and nondestructive way by probing the infrared response of the dielectric matrix. Si3N4 crystallization occurs as a consequence of the presence of Au nanostructures. The average Si3N4 crystalline order is improved upon increasing the thickness tAu of deposited Au per bilayer. On the contrary, crystalline order is destroyed when the number n of bilayers overcomes a threshold value that varies with t. Additional information provided by conventional measurements (x-ray diffraction, visible absorption spectroscopy) and by transmission electron microscopy observations suggests that the Si3N4 atomic ordering results from seed-induced crystallization on the Au(111) planes. Disordering is tentatively attributed to roughness accumulation upon stacking, whose magnitude is ruled by the roughness of the first deposited Au granular layer.

Raman spectra of crystals

Polarized Raman spectra of crystals (x = 0.1 and 0.7) were studied in the temperature range 10-300 K. Thorough site-symmetry analysis of internal and vibrations combined with factor-group analysis based on hexagonal pseudosymmetry allowed us to make an assignment of the observed modes. The spectra of the x = 0.1 sample are compatible with those of pure including the features of the disorder in the paraelectric phase above 220 K (the broad central peak of the relaxator type and high mode damping) and ordering in the ferroelectric phase. The spectra of the x = 0.7 sample show a much smaller disorder at high temperatures (no broad central mode and lower mode damping) due to the lower ammonium content, but clearly confirm the onset of the dynamic dipolar glass transition (short-range correlations inducing the appearance of long-lived dipolar clusters) near 220 K.

Thermal conductivity measurement of AlN films by fast photothermal method

Aluminum nitride (AlN) films were deposited by reactive direct current Magnetron Sputtering (dcMS) on Si (100) substrates, with different thicknesses, in Ar-N2 gas mixture. The films were characterized by X-ray diffraction (XRD), profilometry, scanning electron microscopy and UV-Visible Ellipsometry. The effect of the thickness on the thermal conductivity of AlN films was investigated using a fast IR pyrometry device. The XRD measurements show that AlN films are texturated along (002) direction. Moreover, X-ray rocking curve measurements indicate that the crystalline quality of the AlN is improved with the increase of film thickness. Optical analyses by IR spectroscopy and UV-Visible Ellipsometry demonstrate a high optical band gap of pure AlN films with semi-transparent behaviour in the IR range (1 to 7 μm). The effective thermal conductivity of the AlN films is strongly dependent on the film thickness. An effective thermal conductivities between (80 ± 05) and (175 ± 15) W.m−1.K−1 were measured for 260 and 8000 nm thick AlN film.

New Brillouin spectroscopy results in CsHSeO4 between room temperature and the superionic transition

Abstract A Brillouin scattering investigation has been performed in CsHSeO4 from room temperature up to 120°C, in a temperature range where a phase transition was reported by some authors below the superionic one at 129°C. For every elastic constant under study, a slight but unambiguous change of slope is found near 50°C. The results are discussed in the light of a recent IR reflectivity study. It is concluded that the Brillouin anomalies corroborate the existence of an additional phase transition between room temperature and the superionic one.

Investigation of elastic properties of KH2AsO4 above room temperature by Brillouin scattering

Abstract The elastic constants C11, C33, C*=(C11+C12+2CE66)/2, CE44, CD44 and CE66 have been investigated from room temperature up to 195°C. Small but unambiguous features are observed for most constants in the vicinity of 125±5°C. They confirm the existence of a transition which has been suggested by several previous studies by other techniques.