Study of the sintering and oxidation behavior to the HfB2/SiC system. Synthesis, reactivity and chemical-physical characterization.

International audienceThe first aim of our work consists in synthesizing Ultra High Temperature HfB2/SiC ceramics, used as structural materials in aeronautic and aerospace areas, by Spark Plasma Sintering (SPS). This process is used to obtain fully dense materials (>99% relative density) with fine grains, at lower sintering temperature and shorter dwell time than conventional techniques. Sintered materials are then oxidized in a solar furnace and characterized in order to better understand the oxidation mechanisms

High temperature oxidation of carbides UHTC in air

International audienceUltra-High Temperature Ceramics having melting points above 3500 K and high thermal conductivities are envisaged to elaborate future receivers of concentration solar power plants. The high pressure and solar temperature reactor (Réacteur Hautes Pression et Température Solaire, REHPTS) at the focus of the Odeillo 5 kW solar furnace was used to investigate the oxidation of three carbides (ZrC/MoSi 2 , ZrC/TaSi 2 , HfC/MoSi 2) that could be candidate. The concentration of the additives (TaSi 2 or MoSi 2) was 20 v.% in each composition. Each sample was oxidized in static air (P = 87 kPa) during 20 minutes at 1800, 2000 and 2200 K. Experiments were filmed using a video camera and the gaseous phase was analyzed by mass spectrometry. Various characterizations have shown that the natures of the carbide and additive would affect the composition of the oxide layer and therefore the high-temperature behavior of the sample

Oxidation Behavior of HfB2-SiC and ZrB2-SiC Ultra-High Temperature Ceramics in different air atmospheres

International audienceUltra-High Temperature Ceramics, in particular diborides of the IVb group, are promising materials for extreme environments, more specifically for thermal protection systems of hypersonic vehicles during their atmospheric reentry at temperatures higher than 1800 °C. The main objective of this work is to study the oxidation behavior in air of (Hf or Zr)B2-SiC composites under severe conditions. The first stage consists in elaborating fully-dense (Hf or Zr)B2-SiC ceramics, from 0 to 30 vol.% SiC, with similar and controlled microstructures using Spark Plasma Sintering to obtain materials with fine grains, high relative density (> 99 %) at lower temperatures and shorter dwell times. An optimization of sintering parameters has been carried out for every composition. The second step consists in understanding the oxidation mechanisms of both composites. To this end, these materials have been oxidized at several temperatures using concentrated solar energy, including the oxidation in atomic oxygen. The mechanisms have been highlighted through the study of the oxidized layers by combining XRD, SEM and Raman spectroscopy with imaging and by the monitoring of oxidation kinetics. This work was helped by a first thermodynamic approach of both systems through the modeling of ternary diagrams

Multilayer multifunctional advanced coatings for receivers of concentrated solar power plants

International audienceThe extending market of concentrated solar power plants requires high-temperature materials for solar surface receivers that would ideally heat an air coolant beyond 1300 K. This work presents investigation on high-temperature alloys with ceramic coatings (AlN or SiC/AlN stacking) to combine the properties of the substrate (creep resistance, machinability) and of the coating (slow oxidation kinetics, high solar absorptivity). The first results showed that high temperature oxidation resistance and optical properties of metallic alloys were improved by the different coatings. However, the fast thermal shocks led to high stress levels not compatible due to the differences in thermal expansion coefficients

DNA Fingerprinting of Pearls to Determine Their Origins

We report the first successful extraction of oyster DNA from a pearl and use it to identify the source oyster species for the three major pearl-producing oyster species Pinctada margaritifera, P. maxima and P. radiata. Both mitochondrial and nuclear gene fragments could be PCR-amplified and sequenced. A polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) assay in the internal transcribed spacer (ITS) region was developed and used to identify 18 pearls of unknown origin. A micro-drilling technique was developed to obtain small amounts of DNA while maintaining the commercial value of the pearls. This DNA fingerprinting method could be used to document the source of historic pearls and will provide more transparency for traders and consumers within the pearl industry

Behavior at High Temperature of Metallic Liners (Ta, Nb) Used in the Sandwich Cladding Material of the GFR

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Oxydation et propriétés thermo-radiatives d’UHTCs en conditions extrêmes (espace et énergie)

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Thermal Decomposition of Pyrite FeS₂ under Reducing Conditions

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High Temperature Oxidation of SiC under Helium with Low Oxygen Partial Pressure

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