Spectral Shape of Relaxations in Silica Glass

Precise low-frequency light scattering experiments on silica glass are presented, covering a broad temperature and frequency range (9 GHz < \nu < 2 THz). For the first time the spectral shape of relaxations is observed over more than one decade in frequency. The spectra show a power-law low-frequency wing of the relaxational part of the spectrum with an exponent $\alpha$ proportional to temperature in the range 30 K < T < 200 K. A comparison of our results with those from acoustic attenuation experiments performed at different frequencies shows that this power-law behaviour rather well describes relaxations in silica over 9 orders of magnitude in frequency. These findings can be explained by a model of thermally activated transitions in double well potentials.Comment: 4 pages, 3 figures, submitted to Phys. Rev. Let

Characterization of CrN/CrAlN/Cr2O3 Multilayers Coatings Synthesized by DC Reactive Magnetron Sputtering

The CrN/CrAlN/Cr2O3 multilayer coatings were deposited by reactive magnetron sputtering DC on 90CrMoV8 stainless steel under various oxygen flow rates. The structure and crystalline phases are characterized by the x-ray diffractometer. Through SEM, a dense and coherent is revealed in CrN/CrAlN/Cr2O3 multilayer coatings. The friction and wear behaviors obtained with the ball-on-disc test show that all multilayer films exhibit a good wear resistance, especially the one with an oxygen flow rate of 10 sccm. Nevertheless, in sea water the film without a top layer of Cr2O3 have the lowest coefficient of friction. This behavior is attributed to the interfacial strengthening and the existence of the upper passivation layer Cr2O3. Adding to that, the film obtained under an oxygen flow rate of 10 sccm show the lowest grain size and the maximum hardness and elastic modulus could Respectively, 45 and 417 GPa.Région Bourgogn

Recent trends and challenges in developing boride and carbide-based solar absorbers for concentrated solar power

Concentrating solar power technology has gained increased attention as a source for clean energy. However, because of low power density, it requires large areas and materials with high photothermal conversion efficiency, to harness solar energy. The solar receiver coated with solar selective material plays a critical role in efficiently converting solar energy to useful power. Because of the moderate solar selective nature and excellent thermal properties, significant efforts are currently being invested on the use of ultra-high temperature ceramics as solar selective absorbers. This literature survey presents a critical review of the research and development of early transition metal boride and carbide-based solar selective absorbers, both in bulk and tandem coating formats. In particular, we have emphasized the use of transition metal borides and carbides as intrinsic absorbers as bulk or even exploiting a multilayer design approach to improve the absorptance in the solar spectrum region. One of the key aspects is to introduce a gradient in optical constants (refractive index, extinction coefficient) to tailor high absorptance and low emissivity in tandem absorber coatings, with insightful understanding into microstructure-optical property correlation. The challenges in developing tandem solar absorbers with long-term thermal stability at mid to high temperature window in different working atmosphere are emphasized to spur the development of next generation spectrally-selective solar absorbers

Broadband quasi-omnidirectional sub-wavelength nanoporous antireflecting surfaces on glass substrate for solar energy harvesting applications

A cost effective, facile and scalable method to fabricate the stable broadband antireflective (AR) surface on glass substrates for solar energy applications is still a challenge. In this paper, we have demonstrated a simple and non-lithographic method to fabricate the broadband quasi-omnidirectional AR nanoporous surface on glass substrates by hydrofluoric (HF) acid based vapor phase etching method. Both-sides etched sodalime glass substrate under optimized conditions showed broadband enhanced transmittance with maximum total transmittance of ~97% at 598 nm. The measured transmittance exceeds by ~5.4% as compared to plain glass (91.6%). Field emission scanning electron microscopy results showed that an AR nanoporous surface with graded porosity was formed on sodalime glass substrate after etching. Due to the graded porosity, the fabricated nanoporous surface on sodalime glass substrate showed excellent broadband enhanced transmittance, and exhibited low reflectance <2.8% over a wide range of incidence angles (8–48°). The mechanism of nanostructured surface formation and the effect of etching parameters on transmittance have been discussed in detail. To get more insight, the theoretical transmittance of the optimized sample has been determined by finite difference time domain simulation, which confirms a good agreement of AR property with the experimental results. Furthermore, these AR nanoporous surface showed good adhesion property, excellent thermal and chemical stability, and exhibited outstanding stability against outdoor exposure. These properties signify its strong potential in various solar energy devices

Applications of CrAlN ternary system in wood machining of Medium Density Fibreboard (MDF)

Nowadays, medium density fibreboard (MDF) composite wood is more and more used in the furniture industry to replace bulk wood such as oak, beech, etc. Indeed, this material presents good mechanical properties, is easy to machine, homogeneous, exists in different dimensions (thickness, etc.), is cheaper than bulk wood and finally can be covered by an adhesive decorative coat. Nevertheless, even if this material is homogeneous, it is abrasive enough to tend to the breakdown of the conventional carbide tools employed during its routing process. That is why it is necessary to improve the wear resistance of these tools. One solution is to protect them with a hard coating. The present study deals with the development of ternary CrAlN hard layers obtained by PVD method on carbide tools employed in second transformation of wood. CrAlN coatings have been optimized and then applied on carbide tools in routing of three types of MDF: standard, waterproof and fireproof. The aim of these wood machining tests was first to define the ability to be machined of the three kinds of MDF and second to compare the effectiveness of CrAlN coatings during the routing tests of these materials.ISOROY of St-Dizier (France

Engineering the Microstructure of Silicon Nanowires by Controlling the Shape of the Metal Catalyst and Composition of the Etchant in a Two-Step MACE Process: An In-Depth Analysis of the Growth Mechanism

In this work, slanted, kinked, and straight silicon nanowires (SiNWs) are fabricated on Si(111) and (100) substrates using a facile two-step metal-assisted chemical etching nanofabrication technique. We systematically investigated the effect of crystallography, morphology of Ag catalyst, and composition of etchant on the etch profile of Ag catalyst on Si(111) and (100) substrates. We found that the movement of AgNPs inside the Si is determined by physiochemical events such as Ag/Ag interaction, Ag/Si contact, and diffusion kinetics. Further, from detailed TEM and micro-Raman spectroscopy analyses, we demonstrate that the metal catalyst moves in the crystallographically preferred etching direction (viz., ) only when the interface effect is not predominant. Further, the metal-assisted chemical etching (MACE) system is highly stable at low-concentration plating and etching solutions, but at high concentrations, the system loses its stability and becomes highly random, leading to the movement of Ag catalyst in directions other than ⟨100⟩. In addition, our studies reveal that Ag nanostructures growth on Si(111) and (100) substrates through galvanic displacement is controlled by substrate symmetry and surface bond density. Finally, we demonstrate that by using an optimized balance between the Ag morphology and concentration of the etchant, the angle in slanted SiNWs, kink position in kinked SiNWs, and aspect ratio of straight SiNWs can be controlled judiciously, leading to enhanced optical absorption in the broadband solar spectrum