Phase Formation In Films Research Articles

Influence of the substrate choice on the L1 phase formation of post-annealed Pt/Fe and Pt/Ag/Fe thin films

Pt/Fe and Pt/Ag/Fe layered films were deposited by DC magnetron sputtering on MgO(001), SrTiO3(001), and Al2O3(0001) single crystalline substrates at room temperature. The films were post-annealed between 623 K and 1173 K for 30 s in flowing N2 atmosphere. The onset of the L10-FePt phase formation in films deposited on MgO(001) and SrTiO3(001) substrates was observed after annealing between 773 and 873 K, while chemical L10 ordering sets in for Pt/Fe bilayers on Al2O3(0001) at lower temperatures accompanied by strong (001)-texture. It is concluded that elastic stress, arising from the difference in thermal expansion coefficients between film and substrate, promotes ordering and texture formation.

Structured and Nanoparticle Assembled Co−B Thin Films Prepared by Pulsed Laser Deposition: A Very Efficient Catalyst for Hydrogen Production

Amorphous Co−B-based catalyst powder, produced by chemical reduction of cobalt salts, was used as the target material for Co−B thin film catalyst preparation through pulsed laser deposition (PLD). A comparative kinetic analysis of the sodium borohydride (NaBH4) hydrolysis by using Co−B catalyst added to the hydride solution as powder or as thin film was carried out. Both forms of catalyst (powder and film) were heat-treated at 623 K for 2 h under various atmospheric conditions (in vacuum or by using Ar, H2, and O2 gases) in order to study their effects on H2 generation rate. Surface morphology of the catalyst was studied using scanning electron microscopy (SEM) and atomic force microscopy (AFM), while compositional and bond formation analysis were carried out using X-photoelectron (XPS) and Fourier transform infrared spectroscopy (FT-IR), respectively. Structural characterization of catalysts was performed using the X-ray diffraction (XRD) technique. It was observed that nanoparticles produced during laser ablation process act as active centers in the catalyst films, producing significantly higher rate (about 6 times) of H2 generation than the corresponding Co−B powder. No significant changes were observed for Co−B powder treated in an inert atmosphere (vacuum and Ar) while it caused structural changes in Co−B films. Co2B phase formation in films makes them more efficient catalysts with 28% increase in rate of H2 generation as compared to untreated film. Heat treatment in an oxygen atmosphere causes complete inactivation of powder catalyst, while film still showed excellent catalytic activity with just a longer induction time. The AFM and SEM analysis of the heat-treated films did not show drastic change in surface morphology, indicating that changes in catalytic activity of the films were possibly connected to structural modification and formation of boron oxide on the catalyst surface. We report that by using suitable thin film Co−B catalyst the maximum H2 generation rate of about 5000 mL/(min g of catalyst) can be achieved. This can generate about 0.9 kW (0.7 V) for proton exchange membrane fuel cells (PEMFC), a critical requirement for portable devices.

Reactive Phase Formation in Thin Films: Evolution of Grain Structure

AbstractIt is a well known fact that the properties and performance of polycrystalline materials, including polycrystalline thin films, are strongly affected by the grain structure. Therefore, in treating reactive phase formation in these films, it is (or it will inevitably be) necessary to quantify the grain structure of reactant and product phases and its evolution during the course of the reaction. Theoretical models and the conventional view of thin film reactions, however, have been largely extensions, to small and finite dimensions, of theories and descriptions developed for bulk diffusion couples. These models and descriptions primarily focus on the growth stage and to a much lesser extent on the nucleation stage. Consequently, these models and descriptions are not able to treat the development of product phase grain structure. Recent calorimetric investigations of several thin film systems demonstrate the importance of nucleation kinetics (and hence nucleation barriers) in product phase formation and provide quantitative measures of the thermodynamics and kinetics of formation of the product phases, thereby allowing some degree of comparison with reaction models. Furthermore, microstructural investigations of thin-film reactions demonstrate the non-planarity of the growth front and highlight the role of reactant-phase grain boundaries. In this paper, a summary of these experimental studies and recent theoretical treatments, which combine nucleation and growth in an integrated manner, is presented, with particular emphasis on the Nb/Al system. These experiments and models lead to a new view of reactive phase formation and grain structure evolution as one in which the latter is an integral part of the former. Based on this view, directions for future research are discussed.

Phase formation in films of the As 2Se 3Yb system

Films on the condensation plane were obtained by evaporation of glassy As 2Se 3 and polycrystalline Yb from two furnaces 10 cm apart. The phase formation before and after annealing was investigated by high energy electron diffraction. Films of YbAs 2Se 4, YbAs 4Se 7 and YbAsSe 3 have been obtained. Their crystallographic data and the obtaining regimes of each mentioned phase were established. It was discovered that the obtained phases are formed in the film state at much lower temperatures than those in the bulk samples.