296 research outputs found
Hydrogen kinetics in magnesium hydride: On different catalytic effects of niobium
No full textThe hydrogen desorption kinetics from pure and Nb-doped MgH2 samples was studied as function of the Nb concentration 610−4Nb/Mg510−2. Structural and kinetics analyses indicate that Nb acts as catalyst both when Nb atoms aggregate forming NbH clusters dispersed in the MgH2 and also when Nb is contained as atomic impurity. It is suggested that the local atomic environment around the Nb atom acts as seed for h-Mg phase nucleation which constitutes the rate limiting step for the hydrogen kinetics while, when the Nb concentration exceeds about 1 at. %, the rate limiting step is hydrogen diffusion
3D statistics from TEM observations of TPFG EEPROM memory cells
No full textCommercial EEPROM devices based on Textured Polysilicon Floating Gate (TPFG) technology were observed by TEM in cross-sectional view. At the polysilicon surfaces where electron tunneling takes place during the WRITE/ERASE cycle many tips where detected, as expected. A statistical model correlating the observed 2D tip distribution with the relevant 3D distribution is proposed.Des dispositifs commerciaux EEPROM fabriqués selon la technologie TPFG (textured polysilicon floating gate) ont été observés en coupe par microscopie électronique à transmission. Comme prévu, de nombreuses pointes sont visibles sur la surface du silicium polycristallin où l'émission tunnel d'électrons se produit lors du cycle ECRITURE/EFFACEMENT. On propose un modèle statistique reliant les distributions 2D observées de pointes avec la distribution 3D réelle
Microstructural Characterization of Thin Films Obtained by Laser Irradiation.
No full textThin films obtained by direct pulsed excimer laser irradiation
and by laser reactive ablation were characterized by scanning
electron microscopy, cross sectional transmission electron
microscopy and grazing angle X-ray diffraction. The results
obtained were interpreted in function of the deposition
parameters such as substrate temperature, pressure of the
ambient atmosphere, number of laser pulses and laser fluence
Fatigue life and microstructure of additive manufactured Ti6Al4V after different finishing processes
No full textFinishing methods of additive manufactured metal parts are becoming a key driver of industrial viability, increasingly with additive processes being challenged in demanding end-product applications. The same scenario stresses the requirements as to fatigue life of parts built by Additive Manufacturing (AsM). The paper addresses fatigue life of Ti6Al4V produced by Powder Bed Fusion in four finishing conditions: as-built, tool machined, after tumbling and after tumbling and subsequent shot-peening. Failure mechanisms at the micro-scale are observed in order to reinforce the mechanical results by identifying the role of different surface morphologies in crack initiation. X-ray diffraction (XRD), scanning electron microscopy (SEM) techniques and microanalysis (EDX) are used to investigate microstructural modifications generated by the different finishing methods. Results show that tumbling alone does not improve fatigue life against the as built condition, whereas tumbling and subsequent shot peening allow matching the fatigue endurance of tool machined specimens. The shot peening process causes surface amorphization and implantation of the peening media turning into subsurface inclusions. Based on the results, an optimized finishing process can be envisaged, consisting in prolonged tumbling up to the removal of a stock allowance at least equal to the powder size, before shot peening
Investigation of the Superconducting Properties of Nb Filmscovered by PECVD a-Si:H Layers for Superconducting Qubit Application
No full textAbstractHydrogenated amorphous silicon (a-Si:H) grown by PECVD has a lower loss tangent (tan _) among conventional dielectrics (such as SiO2 and SiNx) and hence is considered as the best amorphous dielectric material for superconducting qubit application. The incorporation of PECVD a-Si:H into the Nb technology requires attention due to the possible degradation of the superconductivity of the Nb films. Superconducting transition temperature (Tc) and residual resistivity (_0) of 20 nm, 50 nm and 100 nm thick Nb films were measured before and after a-Si:H deposition. The penetration of oxygen and hydrogen inside the Nb films was evaluated from the variation of the lattice parameter obtained by X-ray di_raction. The high process temperature (250_ C) and the presence of energetic hydrogen ions during the a-Si:H layer growth caused a decrease of Tc and increase of _0 through two physical processes: 1) oxygen di_usion from the surface Nb oxides and 2) hydrogen di_usion inside the Nb films. The degradation of Tc was reduced with the increase of the film thickness. Nitridation of Nb films and deposition of a sputtered thin amorphous silicon layer (a-Si) on the Nb films (in both cases made in situ after the Nb film deposition) were investigated a s surface treatments to protect the Nb filmsduring PECVD. It was demonstrated that both methods markedly reduce oxygen and hydrogen di_usion into Nb films during a-Si:H deposition, but the a-Si layer was more e_ective to protect the Nb films
CALORIMETRIC EVALUATION OF THE MICROSTRUCTURE OF Al-Cu-Li-Mg 2091 ALLOY
No full textThe precipitation of δ', T1 and S' phases during ageing at 160 °C after quench, has been studied in a commercial 2091 alloy by using Differential Scanning Calorimetry (DSC) technique, 2091 samples have been solubilized, quenched and aged at 160 °C for times ranging from 10 minutes up to 90 hours ; DSC traces have been then obtained during continuous heating ; for each temper several temperature rise speeds have been used. At short times of ageing (10 min) three phases precipitated during ageing have been detected, causing three endothermic effects of dissolution in the DSC traces. For larger times of annealing the peak at lower temperature tends to disappeare and after 2 hours it can never been detected, whilst another thermal effect at higher temperature becames more pronounced and rises up to 90 hours. The peak temperatures depend on the temperature rise speed (thermally activated peaks), and this effect has been used to determine the apparent activation energies of dissolution according to the Ozawa plot. Transmission Electron Microscopy (TEM) and Selected Area Diffraction Pattern (SADP) have been performed for microstructural characterization of precipitated phases. It has been concluded that for short ageing times (up to 2 hours) three phases are present : the δ' (Al3 Li) T1 (Al2 CuLi) and 5' (Al2 CuMg) ; the T1 precipitate has been always detected on defects (dislocations and grain boundaries). For larger times of annealing the T1 phase tends to redissolve while the δ' and S' precipitates grow up to 90 hours
Influence of laser powder bed fusion process parameters on the properties of CuZn42 components: case study of the laser surface energy density
No full textAlthough additive manufacturing (AM) technologies have been rarely used to produce lead-containing brass, the same AM technologies have never been adopted to produce lead-free brass parts based on the CuZn42 alloy. This study aims to fill the gap, demonstrating the feasibility of lead-free brass alloys by laser powder bed fusion (LPBF) technology and investigating structural and mechanical properties of the produced specimens, focusing attention on the role of surface energy density on material properties. Starting from a raw powder of CuZn42 alloy containing alpha, beta and gamma brass phases, fully dense samples with high hardness values were obtained by LPBF. The structural and mechanical properties of the samples were investigated by scanning electron microscopy (SEM), energy-dispersive microanalysis (EDS), X-ray diffraction (XRD) and density and hardness measurements. Results showed that density, hardness and the relative amount of the brass phases depend on the surface energy density (SED) E-s. The investigated range of SED allowed defining the process window ranging from 2 J/mm(2) to 10 J/mm(2), within which fully dense samples can be obtained. A linear dependence of hardness on density was also found, suggesting that deformation mechanisms are mainly due to the presence of residual pores and internal cavities rather than to microstructural features, such as the relative amount of brass phases and crystallographic defects. All results obtained in this work demonstrated, for the first time, that LPBF is suitable to produce components based on the CuZn42 alloy, and that structural and mechanical properties of the produced parts can be properly designed by controlling SED
Single step plasma deposition of hydrocarbon thin films containing Pt nanoclusters for fuel cells applications
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