R. Checchetto

Catalytic effect on hydrogen desorption in Nb-doped microcrystalline MgH2

Mg and Nb-doped Mg films were deposited by rf magnetron sputtering. Morphological and structural analysis were performed by scanning electron microscopy and x-ray diffraction. The desorption kinetics has been investigated by using a Sievert-type apparatus. The overall activation energy and the reaction order controlling desorption are (141±5)kJmol−1H and n≈4 for Mg and (51±5)kJmol−1H and n≈1 for Nb-doped (5at.%) Mg. It is suggested that Nb atoms dispersed in the MgH2 grains catalyzes the dissociation of the hydride phase and that the rate limiting step in the H2 desorption is given by the H atomic migration through interconnected transformed domains of h‐Mg.

Structural evolution of Fe-Al multilayer thin films for different annealing temperatures

The phase formation during thermal annealing of Fe/Al multilayer thin films prepared by electron-beam evaporation, with an overall atomic concentration ratio of Fe:Al = 1:1, has been studied by Rutherford backscattering spectrometry (RBS), x-ray diffraction spectroscopy (XRD), and conversion-electron Mossbauer spectroscopy (CEMS). At the annealing temperature of 473 K some degree of atomic mixing between Fe and Al layers is revealed only by CEMS. At 573 K a large degree of atomic mixing is indicated also by RBS, leading to the nucleation and growth of the B2 FeAl intermetallic phase, as detected by means of XRD and CEMS. At 673 K all Fe atoms have reacted and the multilayer film is transformed into a defective B2 phase. Annealing at higher temperature increases the structural order of the B2 phase. We suggest that the observed phase formation occurs in three stages: (1) formation of a thin intermixed layer between Fe and Al in the as-deposited sample; (2) Al migration into the initial intermixed layer; (3) B2 phase growth at the interface between the intermixed layer and the Fe layer.

Hydrogen kinetics in magnesium hydride: On different catalytic effects of niobium

The hydrogen desorption kinetics from pure and Nb-doped MgH2 samples was studied as function of the Nb concentration (6×10−4<[Nb∕Mg]<5×10−2). 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 1at.%, the rate limiting step is hydrogen diffusion.

Deuterium storage in nanocrystalline magnesium thin films

Nanocrystalline magnesium deuteride thin films with the β-MgD2 structure were prepared by vacuum evaporation of hexagonal magnesium (h-Mg) samples and thermal annealing in 0.15 MPa D2 atmosphere at 373 K. Thermal desorption spectroscopy analysis indicated that the rate-limiting step in the deuterium desorption was given by the thermal decomposition of the deuteride phase. The activation energy Δg of the β-MgD2→h-Mg+D2 reaction scaled from 1.13±0.03 eV in 650-nm-thick films to 1.01±0.02 eV in 75-nm-thick films most likely as consequence of different stress and defect level. Positron annihilation spectroscopy analysis of the thin-film samples submitted to deuterium absorption and desorption cycles reveal the presence of a high concentration of void-like defects in the h-Mg layers after the very first decomposition of the β-MgD2 phase, the presence of these open volume defects reduces the D2 absorption capacity of the h-Mg thin film.

Nb clusters formation in Nb-doped magnesium hydride

Extended x-ray absorption fine structure spectroscopy, x-ray diffraction and transmission electron microscopy were used to analyze the Nb coordination and clustering in Nb-doped (5 at. %) h-Mg film samples deposited by rf magnetron sputtering. Results show that the catalytic effect of the Nb doping in the H2 absorption and desorption kinetics is connected with the formation of Nb nanoclusters dispersed in the host matrix. The H2 desorption from β-MgH2 is favored by local elastic stresses produced by β-NbH0.89 clusters on the MgH2 matrix that reduces the stability of the hydride phase and by preferential paths in the nanocomposite hydride.

Analysis of the hydrogen permeation properties of TiN-TiC bilayers deposited on martensitic stainless steel

Abstract The efficiency of TiN-TiC bilayer coatings, deposited by ion-beam-assisted deposition on martensitic steel, as a hydrogen permeation barrier was investigated by a gas phase method; the hydrogen permeability in the TiN-TiC bilayers is very low, at least 104 times lower than in the steel substrate in the temperature interval 470–570 K. Possible physical mechanisms, responsible for the reduced permeability of the ceramic bilayers, are discussed. In particular, from our experimental results, it can be concluded that chemisorption and/or hydrogen jumping from surface sites to the first subsurface atomic layer represents the hydrogen permeation limiting process.

Hard coating adhesion on ion implanted polymer surfaces

Abstract Modern plastics are of great importance in many practical applications and their performance can be enhanced by surface modification to improve their hardness, wear and chemical resistance. Metallic coatings, in particular hard Cr, have been successfully deposited by various techniques; unfortunately, the low polymer surface tension opposed to the high intrinsic stresses of the coatings often gave adhesion problems. The conventional pre-treatment of polymers in view of metallization is based on hazardous and pollutive agents. We used a combination of ion implantation and vapor deposition (performed in the same chamber) allowing for the production of well adherent coatings. N + ions were implanted at medium–low doses on polycarbonate substrates. Following ion irradiation, chromium films were deposited by evaporation. The implanted substrates were characterized with respect to their structure by Raman spectroscopy, wettability and nanohardness. The mechanical properties of the coatings were examined as a function of the ion beam treatment. The coatings were characterized with respect to morphology, scratching resistance as well as nanohardness. It was observed that, without the ion pre-treatment, the coatings were poorly adherent. Due to the high level of stresses developed in the Cr layers, the coatings on unimplanted samples appeared broken, as expected. On the contrary, the adhesion of the coatings was appreciably better for the pre-implanted specimens. This was certainly due to the superior mechanical properties exhibited by the implanted polymers as well as to enhanced wettability induced by energetic ion bombardment. Scratch tests showed an optimized tribological behavior for the ion implanted/coated polymer surfaces.

Titanium thin film deposition in a deuterium atmosphere

Abstract Titanium thin films have been deposited on Si(100) wafers by electron beam evaporation in the 50–450 °C temperature range. Some samples were deposited in high vacuum conditions while others in a deuterium gas atmosphere with the aim to study the effect of the D 2 gas on the crystallographic orientation of the deposited layers. Compositional studies of the titanium films have been carried out by elastic recoil detection analysis and Rutherford backscattering spectroscopy, while structural analysis was performed by X-ray diffraction. It was found that Ti films deposited in a high vacuum present the (002) (minimum surface energy) preferred orientation. Titanium film deposited in a deuterium atmosphere at low temperatures present high concentration of adsorbed deuterium and show the (100) (maximum surface energy) preferred orientation; when deposited at high temperatures samples present very low deuterium concentrations and show again the (002) preferred orientation. A possible explanation of the above results is that the surface mobility of Ti ad-atoms is limited by D 2 molecule adsorption, owing to the surface site occupancy.

Structural evolution of Fe-Al multilayers submitted to thermal annealing

In this paper, we present an experimental study on the structural evolution of B2 type FeAl multilayers submitted to thermal annealing. Multilayer samples have been obtained by evaporating 15 couples of 10 nm Fe and 14 nm Al layers on (100)-oriented Si wafers in an UHV chamber. After deposition, the samples have been heat treated at temperatures ranging from 375 to 775 K. Some samples were also annealed in air to promote the formation of a surface oxide layer. Different analytical techniques were used in order to achieve a complete characterisation of the deposited layers.

Aluminum and iron surface modification by deuterium ion implantation and thermal desorption process

Abstract Fe and Al thin films were implanted at room temperature with 20 keV D 2 ions at 3×10 16 ions/cm 2 fluence. The correlation between the effusion of the implanted deuterium and the evolution of the surface morphology was studied by thermal desorption spectroscopy (TDS) and scanning electron microscopy (SEM). Deuterium effusion from Fe thin films occurs at ∼430 K and is controlled by the D 2 surface recombination process. The deuterium effusion profile from Al thin films appears more structured. SEM analysis reveals blisters in the as-implanted samples and the increase of their number density and size up to a temperature of ∼500 K. TDS analysis indicates effusion of parts of the implanted deuterium in the 500–600 K temperature range: deuterium is released from the blisters as indicated by the decrease of their number density and size observed by SEM in this temperature interval. The major part of deuterium effuses at ∼630 K and the effusion process is controlled by the migration of deuterium through the surface oxide layer: the effusion process has an activation energy of 1.8±0.1 eV which is very close to the activation energy for deuterium diffusion in γ-Al 2 O 3 . The deuterium effusion kinetics from Fe–Al multilayers is nearly the same as in Fe thin film samples.