Epitaxial CuInSe2 thin films grown by molecular beam epitaxy and migration enhanced epitaxy

While CuInSe2 chalcopyrite materials are mainly used in their polycrystalline form to prepare thin film solar cells, epitaxial layers have been used for the characterization of defects. Typically, epitaxial layers are grown by metal-organic vapor phase epitaxy or molecular beam epitaxy (MBE). Here we present epitaxial layers grown by migration enhanced epitaxy (MEE) and compare the materials quality to MBE grown layers. CuInSe2 layers were grown on GaAs (001) substrates by co-evaporation of Cu, In, and Se using substrate temperatures of 450 ºC, 530 ºC, and 620 ºC. The layers were characterized by high resolution X-ray diffraction (HR-XRD), high-resolution transmission electron microscopy (HRTEM), Raman spectroscopy, and atomic force microscopy (AFM). HR-XRD and HR-TEM show a better crystalline quality of the MEE grown layers, and Raman scattering measurements confirm single phase CuInSe2. AFM shows the previously observed faceting of the (001) surface into {112} facets with trenches formed along the [110] direction. The surface of MEE-grown samples appears smoother compared to MBE-grown samples, a similar trend is observed with increasing growth temperature.The authors would like to acknowledge the CAPES (CAPES-INL 04/14), CNPq, and FAPEMIG funding agencies for financial support. We acknowledge the collaboration project with IMMCSIC (AIC-B-2011-0806). P.M.P.S. acknowledges financial support from EU through the FP7 Marie Curie IEF 2012 Action No. 327367.info:eu-repo/semantics/publishedVersio

TEM, EDX and EELS investigation of aluminium nitride precipitation in low-carbon steels

Precipitation of aluminium nitride in a pure Fe-Al-N alloy was investigated by TEM and by using EDX and EELS techniques. Observations have been carried out on extraction replicas and thin foils. Two modes of AlN precipitation depending on time and annealing temperature have been established. In the case of 3 h annealing at 650°C, the observed precipitates have been identified as aluminium nitride AlN of cubic (rock salt) structure exhibiting Bain orientation relationship with respect to the iron matrix :[math]However in the case of specimens annealed for 5 h at 700°C, precipitation of hexagonal (wurtzite) AlN and exhibiting following orientation relationship :[math]have been observed.EDX analysis confirmed that nitrides formed in both states of annealing contains 50 % nitrogen in atomic percentage, which confirms that we are in the presence of stoichiometric AlN precipitation. This result was supported by EELS spectroscopy

Cold spray under the banner of thermal spray in the whirlwind of additive manufacturing

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SCC crack growth rate of alloy 82 in PWR primary water conditions - effect of a thermal treatment

International audienceStress corrosion cracking (SCC) of wrought alloy 600 and parent weld metals (alloys 182/82) is a significant cause of failure in the pressurized water reactors (PWR). Only a small number of welds fabricated from Alloy 82 is affected by PWSCC. Most of these welds were not thermally heat treated unlike the industrial practice in France. This paper describes constant load crack growth rate (CGR) tests on alloy 82 with and without post weld heat treatment. Metallurgical examination of alloy 82 was carried out using among others Electron. Backscattering Diffraction and Transmission Electron Microscopy. The heat treatment seems to be highly beneficial by decreasing the CGR. This result can be explained by the effect of thermal treatment on the precipitation in alloy 82

Porous diamond foam with nanometric diamond grains using Bias Enhanced Nucleation on iridium

International audiencePorous diamond foam exhibiting a three-dimensional structure was synthesized by applying prolonged Bias Enhanced Nucleation on iridium buffer layers. The Raman spectrum of this material recorded at 325 nm looks very close to the signature obtained for detonation nanodiamonds. HRTEM investigations well confirm the presence of diamond crystals with nanometric size between 2 and 6 nm. All these diamond nanocrystals form a three-dimensional structure very different from ultrananocrystalline diamond (UNCD) generally embedded in a graphitic matrix. According to its high specific surface area, such a 3D material may have promising applications for sensors

SCC crack initiation of alloy 82 in hydrogenated steam at 400 degrees c

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Creep behaviour at high temperature of a stabilized austenitic steel for Gen-IV nuclear power plants

International audienceASTRID is a fast-reactor prototype for the 4th generation of nuclear power plants. The material used for fuel cladding is a cold-worked austenitic stainless steel called AIM1. This grade was developed to limit irradiation-induced swelling and improve microstructural stability and mechanical properties in normal operating conditions. In case of incidental situations, the cladding might rapidly reach higher temperatures (700-950DC) where its stability might be affected. The microstructural and mechanical behaviour in this temperature range is experimentally addressed in this paper.Isothermal creep tests up to 1000DC under a wide range of stress levels enable to study viscoplastic flow, microstructural evolution under stress and damage/failure processes. In order to evaluate the effect of loading, microstructural characterizations (precipitation, recovery, and recrystallization states) on stress-free thermally-aged samples were also performed and compared with post-mortem examinations of creep specimens.Up to 750DC, AIM1 shows better creep strength than previous generations of 15-15Ti grades. Beyond 750DC, dislocation mobility increases which promotes recovery and recrystallization processes. As a consequence, competition between work hardening due to viscoplastic deformation and softening due to dynamic recovery takes place. At 950DC, viscoplastic flow is strongly affected by recrystallization during creep test, especially in the tertiary stage. Softening due to gradual recrystallization leads to longer tertiary stages and higher ductility during tests under lower stress levels (about 40 MPa applied stress). Ductile fracture predominates at any temperature.Characterizations on thin foils and carbon extractive replicas showed a large variety of precipitates, such as Cr-rich borides, phosphides, and Cr- and Ti-rich carbides

Influence of dislocation glide on the spinodal decomposition of fatigued duplex stainless steels

International audienceThe present work is focused on assessing the influence of dislocation movement on spinodal decomposition through scanning transmission electron microscopy (STEM) in combination with energy dispersive X-ray spectroscopy (EDS) analysis in aged duplex stainless steel (DSS) S32750. Dislocation bands and microbands are the prominent dislocation arrangements observed in fatigue tested aged samples. By EDS measurements it was found that the spinodal decomposition was dissolved inside these dislocations structures. Therefore, the mechanism of microband formation developed in the ferritic phase during cycling seems to be responsible for the demodulation of the spinodal decomposition and cyclic softening of the aged DSS

Influence of the carbon texture of platinum/carbon aerogel electrocatalysts on their behaviour in a Proton Exchange Membrane Fuel Cell cathode

International audienceSeven carbon aerogels (CAs) with different pore size distributions were synthesized and used as a catalyst support in proton exchange membrane fuel cell (PEMFC) cathodes. The cathodes were tested in 50 cm 2 membrane electrode assemblies. The results demonstrate that the CA texture significantly influences performance by impacting gas diffusion and proton transport. Also, the Nafion ® loading must be adapted to the CA texture, since its easier penetration into large pores favors their obstruction and results in higher proton resistance and mass-transport voltage losses. Under fixed experimental conditions (notably Nafion ®/carbon ratio = 1), the best CA support displays high specific surface area and pore volume, a majority of mesopores with a pore size distribution peak around 25-30 nm and with some macropores. The work confirms that the carbon support structure must be controlled to reduce mass-transport voltage losses. Doing so would lead to reduce PEMFC overall cost per kW