Pascal Boulet

Radiative and conductive heat transfer in a nongrey semitransparent medium. Application to fire protection curtains

Abstract This paper deals with heat transfer in nongrey media which scatter, absorb and emit radiation. Considering a two dimensional geometry, radiative and conductive phenomena through the medium have been taken into account. The radiative part of the problem was solved using the discrete ordinate method with classical S n quadratures. The absorption and scattering coefficients involved in the radiative transfer equation (RTE) were obtained from the Mie theory. Conduction inside the medium was linked to the RTE through the energy conservation. Validation of the model has been achieved with several simulation of water spray curtains used as fire protection walls.

AlN films deposited by dc magnetron sputtering and high power impulse magnetron sputtering for SAW applications

In this work, aluminium nitride (AlN) films were deposited on silicon substrates buffered by an epitaxial AlN thin film for surface acoustic wave (SAW) applications. The films were deposited by dc magnetron sputtering (dcMS) and high power impulse magnetron sputtering (HiPIMS) deposition techniques. The structural properties of AlN films were investigated using x-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy and atomic force microscopy. In both cases of films deposited by dcMS and HiPIMS, the XRD results showed that the obtained films are oriented, with full width at half maximum rocking curves of around 1 degrees. Raman spectroscopy revealed higher residual stress relaxation in the AlN epilayers grown by HiPIMS compared to AlN grown by dcMS, highlighted by a blue shift in the E2(high) Raman mode. The SAW measurements indicated an insertion loss of AlN-SAW devices of about 53 and 35 dB for the AlN films deposited by dcMS and HiPIMS respectively. The relation between the structural properties of AlN and the characteristics of AlN-SAW devices were correlated and discussed.

Erbium location into AlN films as probed by spatial resolution experimental techniques

Abstract This paper presents a thorough experimental investigation of erbium-doped aluminium nitride thin films prepared by R.F. magnetron sputtering, coupling Scanning Transmission Electron Microscopy X-ray-mapping imagery, conventional Transmission Electron Microscopy and X-ray diffraction. The study is an attempt of precise localisation of the rare earth atoms inside the films and in the hexagonal wurtzite unit cell. The study shows that AlN:Er x is a solid solution even when x reaches 6xa0at.%, and does not lead to the precipitation of erbium rich phases. The X-ray diffraction measurements completed by simulation show that the main location of erbium in the AlN wurtzite is the metal substitution site on the whole range. They also show that octahedral and tetrahedral sites of the wurtzite do welcome Er ions over the [1.6–6%] range. The XRD deductions allow some interpretations on the theoretical mechanisms of the photoluminescence mechanisms and more specifically on their concentration quenching.

Cation size effect on the thermochromic properties of rare earth cobaltites RECoO3 (RE: La, Nd, Sm)

RECoO3 (RE: rare earth) perovskite thin films have been deposited at room temperature by direct current co-sputtering and subsequent annealing in air at 923u2009K during 1u2009day. The effect of the octahedra tilts on the optical properties has been tracked decreasing the RE cation size. The bending and stretching vibrational modes of the CoO6/2 octahedra give information on the Co–O distances and the Co–O–Co angles which determine the Co–O overlap integral and hence the electric properties of these perovskites. Transmittance measurements in the 1.42–100u2009μm wavelength range show a high transparency at room temperature. When temperature increases, an optical screening effect depending on the RE3+ ionic radii (i.e., Co-O-Co angles) and resulting from the electrical behaviour occurs. At the wavelength of 8u2009μm, the transmittance drops from about 90% at room temperature to 50% at the temperature of 516, 600, and 640u2009K for LaCoO3, NdCoO3, and SmCoO3, respectively.

AlN/IDT/AlN/Sapphire SAW Heterostructure for High-Temperature Applications

Recent studies have evidenced that Pt/AlN/Sapphire surface acoustic wave (SAW) devices are promising for high-temperature high-frequency applications. However, they cannot be used above 700°C in air atmosphere as the Pt interdigital transducers (IDTs) agglomerate and the AlN layer oxidizes in such conditions. In this paper, we explore the possibility to use an AlN protective overlayer to concurrently hinder these phenomena. To do so, AlN/IDT/AlN/Sapphire heterostructures undergo successive annealing steps from 800°C to 1000°C in air atmosphere. The impact of each step on the morphology, microstructure, and phase composition of AlN and Pt films is evaluated using optical microscopy, scanning and transmission electron microscopy (SEM and TEM), X-ray diffraction (XRD), and secondary ion mass spectroscopy (SIMS). Finally, acoustical performance at room temperature of both protected and unprotected SAW devices are compared, as well as the effects of annealing on these performance. These investigations show that the use of an overlayer is one possible solution to strongly hinder the Pt IDTs agglomeration up to 1000°C. Moreover, AlN/IDT/AlN/Sapphire SAW heterostructures show promising performances in terms of stability up to 800°C. At higher temperatures, the oxidation of AlN is more intense and makes it inappropriate to be used as a protective layer.

Local heteroepitaxial growth to promote the selective growth orientation, crystallization and interband transition of sputtered NiO thin films

The room temperature growth of highly oriented sputtered NiO thin films on glass and silicon substrates previously covered with a oriented Cu2O film is reported. The results are compared to those obtained from single layer NiO thin films using the same deposition conditions. Electron microdiffraction analyses indicate that NiO columns are heteroepitaxially grown on the columns of a Cu2O seed layer. The well-matched atomic configurations between the Cu atoms in the {111} planes of Cu2O and the O atoms in the {111} planes of NiO may provide a strong driving force to promote this local heteroepitaxial growth. Such heteroepitaxial growth behavior in the columns can significantly improve crystallization. Moreover, valence electron energy loss spectroscopy has been employed to investigate the interband transition properties of the NiO films, which shows that the interband transition intensity can be tuned by this local heteroepitaxial growth.

On the Influence of the Sample Absorptivity when Studying the Thermal Degradation of Materials

The change in absorptivity during the degradation process of materials is discussed, and its influence as one of the involved parameters in the degradation models is studied. Three materials with very different behaviors are used for the demonstration of its role: a carbon composite material, which is opaque, almost grey, a plywood slab, which is opaque and spectral-dependent and a clear PMMA slab, which is semitransparent. Data are analyzed for virgin and degraded materials at different steps of thermal degradation. It is seen that absorptivity and emissivity often reach high values in the range of 0.90–0.95 with a near-grey behavior after significant thermal aggression, but depending on the materials of interest, some significant evolution may be first observed, especially during the early stages of the degradation. Supplementary inaccuracy can come from the heterogeneity of the incident flux on the slab. As a whole, discrepancies up to 20% can be observed on the absorbed flux depending on the degradation time, mainly because of the spectral variations of the absorption and up to 10% more, depending on the position on the slab. Simple models with a constant and unique value of absorptivity may then lead to inaccuracies in the evaluation of the radiative flux absorption, with possible consequences on the pyrolysis analysis, especially for properties related to the early step of the degradation process, like the time to ignition, for example.

Radiative flux emitted by a burning PMMA slab

The degradation of a PMMA sample has been studied based on experimental results obtained for the radiation emission by a burning slab. Observations of the infrared emission perpendicular to the plate, in the range where the optically thin flame is weakly emitting, indicate a plate temperature close to 680 K which is an indication on the surface temperature during the degradation process. Observations from the side allow a flame characterization without the plate emission superimposition. This is a promising way for evaluating data regarding the flame characteristics: temperature, gaz concentration and soot volumetric fraction.

Role of Cu+ on ZnS:Cu p-type semiconductor films grown by sputtering: influence of substitutional Cu in the structural, optical and electronic properties

In this work, ZnS:Cu thin films were synthetized by RF magnetron co-sputtering using ceramic ZnS and metallic Cu targets. The power applied to Cu target was changed to vary the Cu content in the films while other experimental parameters remain constant. Whatever the copper content, only the hexagonal ZnS phase was detected by X-ray diffraction and the films are oriented along the c-axis. For films with a Cu content higher than 10.6 at%, the a and c lattice parameters of the crystal cell decrease leading to a shrinkage of the ZnS:Cu cell. Changes in the microstructural, optical and electronic properties of the films are also observed. Regarding the optical properties, the energy bandgap determined by optical absorption decreases with the Cu content. Moreover, an absorption in the near infrared region appears probably due to a plasmon resonance coming from an increase of the holes carrier concentration. From electrical measurements it was found that above a copper concentration of 10 at%, the films are conductive and exhibit a p-type behavior. These results were discussed taking into account the substitution of Zn atoms by Cu ones. Finally, electron energy loss spectrometry measurements clearly showed that copper atoms are present in the ZnS lattice with an oxidation state of +I.

Investigations of AlN thin film crystalline properties in a wide temperature range by in situ x-ray diffraction measurements: Correlation with AlN/sapphire-based SAW structure performance

Aluminum nitride on sapphire is a promising substrate for SAW sensors operating at high temperatures and high frequencies. To get a measure of the suitability and temperature stability of such devices, an experimental relationship between the SAW performance and the structural properties of the AlN thin films was investigated in the temperature range between the ambient temperature and 1000°C. The crystalline structure of the AlN films was examined in situ versus temperature by X-ray diffraction. The results reveal that the AlN films remain (002) oriented even at high temperatures. A gradual increase of the tensile stress in the film due to the thermal expansion mismatch with the substrate has been observed. This increase accelerates around 600°C as the AlN film crystalline quality improves. This phenomenon could explain the amelioration in the SAW performance of AlN/sapphire devices observed previously between 600°C and 850°C. At higher temperatures, surface oxidation of the AlN films reduces the SAW performance. The potential of ZnO thin films as protective layers was finally examined.