Cyril Cayron

This work generalizes our previous works on fcc-bcc martensitic transformations to the larger family of transformations in the fcc-bcc-hcp system and to fcc-fcc mechanical twinning. The analytical expressions of the atomic displacements and lattice distortions are calculated directly from the orientation relationships without any adjustment of free parameter; the unique assumption is that the atoms are hard-spheres that cannot interpenetrate themselves. The habit planes are predicted on the simple criterion that they are unrotated by the distortion, and the results are compared to experimental observations published in literature. It is shown that shuffle is required for transformations implying the hcp phase because the hcp primitive Bravais lattice contains two atoms, instead of one for the fcc and bcc phases. A simple encoding of the lattice distortions and shuffles permits to attribute a groupoid structure to the transformations in the fcc-hcp-bcc system. The analytical expressions of the intermediate states are given and could be used to calculate activation energies. The martensitic distortion occurs in one-step, without shearing, and its accommodation generates orientation gradients in the parent phase, independently of its glide modes. The concept of reversibility is detailed on the basis of crystallographic and morphological arguments. The possibility to apply this approach to diffusion-limited martensitic transformations is discussed.

A one-step hard-sphere model of fcc-bcc phase transformations is proposed. Bain distortion is probably not the "natural" deformation mechanism. It is also probable that these transformations are not "shear" transformations as usually assumed. One-step deformation matrices are proposed for Bain, Pitsch and KS.

Development of techniques such as atom optical pumping, for atomic clocks and matter-wave interferometers (gyrometer, accelerometer, gravimeter and gradiometer), requires laser diodes with high power and excellent spectral (narrow linewidth) and spatial qualities together with high reliability. We have developed high power Fabry-Perot and distributed Feedback lasers diodes (DFB) emitting at 780nm corresponding to the D2 line of Rubidium. We have realized a Fabry-Perot laser diode, without aluminium in the active region, with a cavity length and a waveguide width of respectively 1mm and 4μm and an AR/HR coating on the facets. We obtain a low threshold current around 40mA and a high slope efficiency of 1W/A at 20°C. We obtain a good beam quality M² of 2 at 200mW. These lasers diodes are very interesting to be inserted in an external cavity. For DFB lasers, we used a second order diffraction grating in a GaInP/GaAsP/GaInP waveguide. We calculated the coupling coefficient KL of 1.5 for a length of 2mm, for a width of 4μm. These lasers show a low threshold current (around 65mA) with a slope efficiency around 0.37W/A. We have obtained at 25°C, 145mA an optical power of 25mW at the D2 line of Rubidium with a side mode suppression ratio around 44dB. By the selfheterodyne method, we measured a low linewidth of our DFB laser at 780nm around 1.25MHz (lorentzian fit).

... 1. Grain Boundary Engineering and Σ3 n Multiple Twinning Cyril CAYRON a a CEA-Grenoble, DRT/LITEN, GEM@Minatec Abstract ... 2. Références [1] M. Kumar, WE King, and AJ Schwartz, Acta Mater. 48, 2081-2091, (2000). [2] VY Gertsman and CH Henager Jr. ...

A nano-particle synthesis technology dedicated to solar cells applications Rita Najjar 1,a , Salim Boutami 1 , Cyril Cayron 1 , Nathalie Baclet 1 , Viviane ... Soc., 27 (1969) 941. [10] P. A Buffat, M. Flüeli, R. Spycher, P. Stadelmann and J. P Borel, Faraday Discuss., 92 (1991) 173. ...

ABSTRACT The local physical properties of polycrystalline semiconducting films drive their performances in a wide variety of optoelectronic devices but are still not completely elucidated. These properties are investigated and correlated on the same region of polycrystalline CdTe films by combining electron backscattered diffraction, μ-Laue x-ray experiments using synchrotron radiation, electron beam-induced current, and cathodoluminescence. The local band bending is revealed at random grain boundaries: its characteristics vary from one grain to another, depending on the nature of grain boundaries and the doping level in the nearby grains, in agreement with the theoretical approach of the double Schottky potential barriers. In contrast, no local band bending occurs at Σ3 growth twins since these extended defects have no dangling bonds in their center. Additionally, the density of unpaired dislocations and the components of the strain and stress tensors are found to be highly nonuniform from one grain to another and within the grains themselves. This reveals that grain-to-grain interactions (i.e., collective effects) occur during the Volmer-Weber-type growth. These findings emphasize the critical importance of grain boundary design engineering. They also highlight how polycrystalline semiconducting films work locally and show the complexity of the local physical processes governing their macroscopic performances in optoelectronic devices.

ABSTRACT La0.8Sr0.2CrO3 (LSC) based Ru catalysts are very active in methane steam reforming. Nevertheless, they can be easily poisoned under water-deficient conditions. Ru can be deposited as metallic ruthenium particles decorating the LSC grains or be inserted as Ru ions in the perovskite structure. Both Ru-promoted LSC catalysts were studied in methane steam reforming under water-deficient conditions and characterized after testing. Catalytic activity tests showed that ruthenium metal species are deactivated under water-deficient atmosphere, while ruthenium species inserted in LSC presented a remarkable stability and catalytic activity where residual steam plays a key role. Very unreactive carbon species responsible for deactivation were detected by temperature-programmed oxidation and transmission electron microscopy over metallic ruthenium species. Such species were not observed when ruthenium species are inserted and stabilized into the LSC structure. La0.8Sr0.2Cr0.98Ru0.02O3 appears therefore as a highly promising anti-coking anode material for Solid Oxide Fuel Cells directly fed with methane or natural gas and operating under water-deficient conditions.

La0.8Sr0.2CrO3 based Ru catalysts were studied as potential new anodic materials for Solid Oxide Fuel Cells directly fed with methane and operating at intermediate temperature under water deficient conditions. Two kinds of materials very close in composition were obtained following two different preparation procedures. Catalyst samples were characterized by physicochemical methods (XRD, SEM, BET and Chemical Analysis) and studied in

The microstructure and mechanical properties of a high volume fraction SiC particle reinforced AlCu4MgAg alloy produced by squeeze pressurised infiltration of dense packed particle preforms were characterised. It was found that the addition of 60 vol.% 12 μm SiC particles eliminates the processing related intrinsic macrosegregation of the matrix alloy, induces coarse precipitates in as-cast matrix, but does not impede