SiC coatings grown by liquid injection chemical vapor deposition using single source metal-organic precursors

SiC coatings have been grown by direct liquid injection of organosilanes in a hot-wall chemical vapor deposition reactor (DLICVD). 1,3-disilabutane (DSB) and polysilaethylene (PSE) were used as single-source precursors. Amorphous and stoichiometric SiC coatings were deposited under low pressure on various substrates in the temperature range of 923–1073 K. Thickness gradients due to the temperature profiles and the precursor depletion were observed along the reactor axis but the thickness uniformity could be improved as a function of the deposition conditions. Growth rates as high as 90 μm·h−1 were obtained using pure precursors. The injection of PSE solutions in toluene significantly reduces the deposition rate due to the decrease of the PSE mole fraction but allows a better control of the growth rates and the microstructure of coatings. They exhibit a smooth surface morphology and a very dense structure. The films grown using pure precursors exhibit an Si:C atomic ratio equal to 1:1 while those using toluene solutions are slightly C-rich (54 at.% C). The presence of solvent vapor in the CVD reactor becomes a source of carbon contamination at deposition temperatures equal to or higher than 1073 K. The influence of the growth conditions is discussed, in particular the presence of toluene vapor

Evidence for a Cr metastable phase as a tracer in DLI-MOCVD chromium hard coatings usable in high temperature environment

Cr deposits are widely used as protective coatings but multifunctional performances are required in harsh environments motivating research on new processes. MOCVD of Cr metal coatings was carried out by direct liquid injection (DLI) of a unique solution containing bis(ethylbenzene)chromium as metal source and thiophenol as inhibitor of carbide formation. A low amount (<6%) of the metastable δ-Cr phase was found embedded in the stable α-Cr phase. The formation of this metastable phase originates from both the low deposition temperature (<723 K) and the use of thiophenol. It was not reported under other CVD conditions. Dense coatings were deposited by implementing a multilayer growth mode. Such coatings exhibit a high nanohardness of about 17 GPa. The δ-Cr metastable phase undergoes an irreversible structural transformation to bcc-Cr above 723 K. The mechanical properties of coatings are not affected by the structural transformation because of the similarity of their crystallographic structures (both cubic), their density very close (a volume contraction of only 0.4% during the transformation) and its low content. This metastable phase is a signature of the DLI-MOCVD process and it can be used as a tracer for Cr coatings operating in high temperature environment without loss of the basic properties

Mandible Behavior in Obstructive Sleep Apnea Patients Under CPAP Treatment

Aim: To investigate whether obstructive sleep apnea (OSA) patients present different behaviors of mandible movements before and under CPAP therapy. Materials and Methodology: In this retrospective study, patients were selected according to inclusion criteria: both the diagnostic polysomnography recording showing an OSA with an apnea-hypopnea index (AHI) greater than 25 (n/h) and the related CPAP therapy control recordings were available, presence of mandible movement and mask pressure signals in the recordings, and tolerance to the applied positive pressure. Statistical analysis on four parameters, namely the apneahypopnea index (AHI), the arousal index (ArI), the average of the mandible lowering during sleep (aLOW), and the average amplitude of the oscillations of the mandible movement signal (aAMPL), was performed on two sets of recordings: OSA and CPAP therapy. Results: Thirty-four patients satisfied the inclusion criteria, thus both OSA and CPAP groups included thirty-four recordings each. Significant difference (p < 0.001) was found in the OSA group compared with the CPAP group when considering either the four parameters or only the two ones related to mandible movements. Conclusions: When an efficient CPAP pressure is applied, the mouth is less open and presents fewer broad sharp closure movements, and oscillating mandible movements are absent or very small.Peer reviewe

Chromium carbide growth at low temperature by a highly efficient DLI-MOCVD process in effluent recycling mode

The effect of direct recycling of effluents on the quality of CrxCy coatings grown by MOCVD using direct liquid injection (DLI) of bis(ethylbenzene)chromium(0) in toluene was investigated. The results are compared with those obtained using non-recycled solutions of precursor. Both types of coatings exhibit the same features. They are amorphous in the temperature range 673–823 K. They exhibit a dense and glassy-like microstructure and a high hardness (> 23 GPa). Analyses at the nanoscale revealed a nanocomposite microstructure consisting of free-C domains embedded in an amorphous Cr7C3 matrix characterized by strong interfaces and leading to an overall composition slightly higher than Cr7C3. The stiffness and strength of these interfaces are mainly due to at least two types of chemical bonds between Cr atoms and free-C: (i) Cr intercalation between graphene sheets and(ii) hexahapto η6-Cr bonding on the external graphene sheets of the free-C domains. The density of these interactions was found increasing by decreasing the concentration of the injected solution, as this occurred using a recycled solution. As a result, “recycled” coatings exhibit a higher nanohardness (29 GPa) than “new” coatings (23 GPa). This work demonstrates that using bis(arene)M(0) precursors, direct recycling of effluents is an efficient route to improve the conversion yield of DLI-MOCVD process making it cost-effective and competitive to produce protective carbide coatings of transition metals which share the same metal zero chemistry

High-temperature oxidation resistance of chromium-based coatings deposited by DLI-MOCVD for enhanced protection of the inner surface of long tubes

For nuclear safety issues, there is an international effort to develop innovative “Enhanced Accident Tolerant Fuels” (EATF) materials. EATF cladding tubes are of particular interest because they constitute the first barrier against radioactive fission species dispersal in case of accidental scenario such as LOCA (LOss of Coolant Accident). Actual nuclear fuel claddings are made from Zr-based alloys and to increase safety margins, both mechanical strength and resistance to high-temperature oxidation have to be improved. Several alternatives using high-temperature oxidation resistant coatings for outer-wall protection have been proposed worldwide but there is currently no solution for the inner-wall protection. In order to resist to high temperature steam environment upon LOCA transients, internal Cr-based coatings deposited by DLI-MOCVD (Direct Liquid Injection of MetalOrganic precursors) were investigated. These hard metallurgical coatings could also be used in high-temperature corrosive environments as those encountered in aeronautics and other industries to protect 3D complex components. Thanks to a suitable chemistry of the liquid Cr precursor, bis(ethylbenzene)chromium, different coatings were deposited including: metal Cr, chromium carbides CrxCy and mixed carbides CrxSizCy. The high-temperature behavior of these Cr-based coatings under oxidizing atmospheres has been studied using several techniques and various oxidation tests including pure steam environment followed by water quenching down to room temperature to be representative of LOCA situations. Amorphous CrxCy coatings showed the most promising properties. For instance compared to uncoated substrate, they shift the catastrophic oxidation towards higher temperatures and delay the complete oxidation of the substrate at 1473K of >2h. The results are discussed in terms of oxidation mechanisms and protection of the fuel claddings inner surface deduced from fine characterizations of the samples before and after oxidation tests

Vers des combustibles encore plus robustes en cas d’accident

La protection de l’intérieur et de l’extérieur des gaines de combustible de réacteurs nucléaires contre leur oxydation à haute température est nécessaire pour garantir leur intégrité en conditions accidentelles. Pour ce faire, le procédé DLI-MOCVD a été développé pour déposer en paroi interne de gaine des revêtements à base de chrome : chrome métallique, carbures de chrome amorphes CrCx et carbures mixtes de chrome CrSiyCx. L’optimisation du procédé de dépôt a été réalisée par le couplage d’expériences et de simulations. Il a aussi été montré que la solution de précurseur organométallique et de solvant utilisée pouvait être directement recyclée en sortie de réacteur, sans étapes de séparation et de purification couteuses, augmentant le potentiel d’industrialisation du procédé. Les propriétés physico-chimiques et structurales des revêtements déposés avec ce procédé ont été caractérisées. L’évaluation de leur résistance à l’oxydation à 1 200 °C a révélé les excellentes performances des revêtements CrCx, puisqu’ils retardent l’oxydation catastrophique des gaines de plus de deux heures pour une épaisseur de 10 μm, permettant ainsi des interventions en conditions accidentelles

Small-scale properties of Class 0 protostars from the CALYPSO IRAM-PdBI survey

Because the formation of protostars is believed to be closely tied to the angular momentum problem of star formation, characterizing the properties of the youngest disks around Class 0 objects is crucial. However, not much is known on the structure of the youngest protostellar envelopes, on the small scales at which disks and multiple systems are observed around more evolved YSOs, due to a lack of comprehensive high angular resolution observations (probing 50 au, disk structures are not observed in most Class 0 protostars from our sample, which can be described by various envelope models reproducing satisfactorily the intensity distribution of the dust emission at all scales from 50 au to 5000 au

Comparative analysis of conjugated alkynyl chromophore-triazacyclononane ligands for sensitized emission of europium and terbium

A series of europium and terbium complexes based on a functionalized triazacyclononane carboxylate or phosphinate macrocyclic ligand is described. The influence of the anionic group, that is, carboxylate, methylphosphinate, or phenylphosphinate, on the photophysical properties was studied and rationalized on the basis of DFT calculated structures. The nature, number, and position of electron-donating or electron-withdrawing aryl substituents were varied systematically within the same phenylethynyl scaffold in order to optimize the brightness of the corresponding europium complexes and investigate their two-photon absorption properties. Finally, the europium complexes were examined in cell-imaging applications, and selected terbium complexes were studied as potential oxygen sensors

Emerging processes for metallurgical coatings and thin films

Innovation in thin-film deposition processes, thermal spraying and cladding technologies mostly rely on evolutions of their previous iteration. Along with other examples, five case studies of emerging elaboration processes for metallurgical coatings are described coupled with their applications. In the frame of the lifetime extension of components exposed to aggressive media or their functionalization, this article depicts all the developments of the detailed processes. Physical vapor deposition (PVD) of coatings with exceptional properties is possible thanks to sources generating highly ionized metallic vapors. The control of the average energy per incident species and particularly metallic ions strongly influences the characteristics of the deposited layer obtained, for example, with HiPIMS (High Power Impulse Magnetron Sputtering). While PVD techniques are mainly directive regarding the growth of the coating, chemical vapor deposition (CVD) processes manage to homogeneously coat complex 3D shapes. The use of specific precursors in DLI–MOCVD (Direct Liquid Injection – MetalOrganic CVD), carefully selected from the whole metalorganic chemistry, allows one to efficiently treat heat-sensitive substrates and broadens their application range. The third detailed example of emerging technology is suspension plasma spraying (SPS). Projection of various solutions containing nanoparticles leads to the growth of unusual morphologies and microstructures and to the generation of porous coatings with multi-scaled porosity. On the other hand, cold-spray uses metallic powders with higher granulometry and does not modify them during the deposition process. As a result, high-purity and dense materials are deposited with properties similar to those of wrought materials. Whereas cold-spray is suitable only for ductile metals, laser cladding can be applied to ceramics, polymers and of course metals. Laser cladding is a key technology for advanced metallurgical engineering and alloy development due to its capability for functionally graded materials production and combinatorial synthesis

Molecular Demonstration of a Pneumocystis Outbreak in Stem Cell Transplant Patients: Evidence for Transmission in the Daycare Center

Pneumocystis jirovecii pneumonia (PCP) is a life-threatening infection in hematology. Although occasionally reported, the role of interhuman transmission of P. jirovecii in PCP, compared to that of reactivation, remains an unresolved question; the recommendation to isolate PCP patients in the hematology ward are not well evidence-based. Following an unexpected increase in the number of febrile pneumonia patients with P. jirovecii DNA detected in respiratory samples in our hematology ward, we explored 12 consecutive patients from November 2015 to May 2016. Genotyping of P jirovecii was performed using microsatellite markers. The frequency of simultaneous occupancy of these 12 patients in the same unit on the same day from 4 months prior to the first diagnosis was recorded. In three patients, the P. jirovecii genotype could not be determined because DNA was insufficient. One rare single genotype (Gt2) was found in four of the other nine, all allogeneic stem cell transplant recipients. The transmission map showed that these 4 patients had multiple opportunities to meet on the same day (median, 6.5; range, 4–10) at the daycare center. It was much less among the eight non-Gt2 patients (median, 1; range, 0–9; P = 0.048). This study, based on modern molecular technics, strongly suggests that interhuman transmission of P. jirovecii between allogeneic stem cell transplant recipients is possible. P. jirovecii DNA detected in respiratory specimens supports that isolation and respiratory precautions be recommended in such cases in the hematology ward