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Register a new userAdsorption of Xe and Ar on Quasicrystalline Al-Ni-Co
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An interaction potential energy between and adsorbate (Xe and Ar) and the 10-fold Al-Ni-Co quasicrystal is computed by summing over all adsorbate-substrate interatomic interactions. The quasicrystal atoms coordinates are obtained from LEED experiments and the Lennard-Jones parameters of Xe-Al, Xe-Ni and Xe-Co are found using semiempirical combining rules. The resulting potential energy function of position is highly corrugated. Monolayer adsorption of Xe and Ar on the quasicrystal surface is investigated in two cases: 1) in the limit of low coverage (Henrys law regime), and 2) at somewhat larger coverage, when interactions between adatoms are considered through the second virial coefficient, C_{AAS}. A comparison with adsorption on a flat surface indicates that the corrugation enhances the effect on Xe-Xe (Ar-Ar) interactions. The theoretical results for the low coverage adsorption regime are compared to experimental (LEED isobar) data.
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Thin film quasicrystal coatings have unique properties such as very high electrical and thermal resistivity and very low surface energy. A nano quasicrystalline thin film of icosahedral Al-Ga-Pd-Mn alloy, has produced by flash evaporation followed by annealing. Attempts will be made to discuss the micromechanisms for the formation of quasicrystalline thin film in Al-Ga-Pd-Mn alloys
Pulsed laser deposition from a Nd:YAG laser was employed in production of hundreds of nanometer thick quasicrystalline Ti-Zr-Ni films on glass substrate. The influence of deposition temperature Ts on the structure, morphology and microstructure of the films across their thickness was investigated. The morphology and microstructure features were evaluated by X-ray diffraction and transmission electron microscopy techniques. The low deposition temperatures were found to produce films with nanometer sized grains embedded in an amorphous matrix. The grains exhibit quasicrystalline order. The higher deposition temperatures lead to films whose structure is not uniform all along the growth direction. The layer in contact with the substrate is a very thin amorphous layer. The main part of the film consists of crystallized columns. The columns have grown from a nano-crystallized layer where the size of crystallites increases with increasing thickness.
The interaction of CO with the Fe3O4(001)-(rt2xrt2)R45{deg} surface was studied using temperature programmed desorption (TPD), scanning tunneling microscopy (STM) and x-ray photoelectron spectroscopy (XPS), the latter both under ultrahigh vacuum (UHV) conditions and in CO pressures up to 1 mbar. In general, the CO-Fe3O4 interaction is found to be weak. The strongest adsorption occurs at surface defects, leading to small TPD peaks at 115 K, 130 K and 190 K. Desorption from the regular surface occurs in two distinct regimes. For coverages up to 2 CO molecules per (rt2xrt2)R45{deg} unit cell, the desorption maximum shows a large shift with increasing coverage, from initially 105 K to 70 K. For coverages between 2 and 4 molecules per (rt2xrt2)R45{deg} unit cell, a much sharper desorption feature emerges at 50 K. Thermodynamic analysis of the TPD data suggests a phase transition from a dilute 2D gas into an ordered overlayer with CO molecules bound to surface Fe3+ sites. XPS data acquired at 45 K in UHV are consistent with physisorption. Some carbon-containing species are observed in the near-ambient-pressure XPS experiments at room temperature, but are attributed to contamination and/or reaction with CO with water from the residual gas. No evidence was found for surface reduction or carburization by CO molecules.
Dislocation velocities and mobilities are studied by Molecular Dynamics simulations for edge and screw dislocations in pure aluminum and nickel, and edge dislocations in Al-2.5%Mg and Al-5.0%Mg random substitutional alloys using EAM potentials. In the pure materials, the velocities of all dislocations are close to linear with the ratio of (applied stress)/(temperature) at low velocities, consistent with phonon drag models and quantitative agreement with experiment is obtained for the mobility in Al. At higher velocities, different behavior is observed. The edge dislocation velocity remains dependent solely on (applied stress)/(temperature) up to approximately 1.0 MPa/K, and approaches a plateau velocity that is lower than the smallest forbidden speed predicted by continuum models. In contrast, above a velocity around half of the smallest continuum wave speed, the screw dislocation damping has a contribution dependent solely on stress with a functional form close to that predicted by a radiation damping model of Eshelby. At the highest applied stresses, there are several regimes of nearly constant (transonic or supersonic) velocity separated by velocity gaps in the vicinity of forbidden velocities; various modes of dislocation disintegration and destabilization were also encountered in this regime. In the alloy systems, there is a temperature- and concentration-dependent pinning regime where the velocity drops sharply below the pure metal velocity. Above the pinning regime but at moderate stresses, the velocity is again linear in (applied stress)/(temperature) but with a lower mobility than in the pure metal.
Laser ablation of Al-Ni alloys and Al films on Ni substrates has been studied by molecular dynamics simulations (MD). The MD method was combined with a two-temperature model to describe the interaction between the laser beam, the electrons and the atoms. The challenge for alloys and mixtures is to find the electronic parameters: electron heat conductivity, electron heat capacity and electron-phonon coupling parameter. The challenge for layered systems is to run simulations of an inhomogeneous system which requires modification of the simulation code. Ablation and laser-induced melting was studied for several Al-Ni compounds. At low fluences above the threshold ordinary ablation behavior occurred while at high fluences the ablation mechanism changed in Al$_3$Ni and AlNi$_3$ from phase explosion to vaporization. Al films of various thicknesses on a Ni substrate have also been simulated. Above threshold, 8 nm Al films are ablated as a whole while 24 nm Al films are only partially removed. Below threshold, alloying with a mixture gradient has been observed in the thin layer system.
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