Inert Gas Atoms Research Articles

Assessment of a powder metallurgical processing route for refractory metal silicide alloys

A powder metallurgical (PM) processing route for the manufacturing of two different refractory metal silicide alloys comprising inert gas atomization of presintered bars, hot isostatic pressing, and hot extrusion (reduction in cross section of 6:1) was established. The mechanical properties between room temperature and 1200 °C of the PM-processed Mo-3Si-1B and Nb-24Ti-20Si-5Cr-3Hf-2Al alloys (in wt pct) were assessed with tensile tests vs a state-of-the-art Ni-base single crystalline alloy (CMSX 4) and a directionally solidified (MASC) niobium-base silicide alloy, respectively. The microstructural characterization of both the hot-isostatically pressed and extruded materials was carried out applying scanning electron microscopy (SEM), energy-dispersive X-ray (EDX), and X-ray diffraction (XRD) analysis. The Mo-Si-B alloy is characterized by an intermetallic matrix surrounding globular Mo particles in the hot isostatic press and a nearly continuous molybdenum solid solution matrix with dispersed intermetallic particles in the hot-extruded condition. Hot extrusion results in a substantial reduction of the DBTT of about 200 °C and tensile strengths superior to CMSX 4 at temperatures above 1000 °C. In the case of the Nb-base silicide alloy, a niobium solid solution surrounding intermetallic particles with Nb5Si3-type structure characterizes the final alloy. In the intermediate temperature range of 500 °C to 816 °C, a strength level equivalent to the directionally solidified MASC alloy was observed.

Suppression of inelastic bound-state resonance effects by the dimensionality of an atom-surface scattering event

We develop a multidimensional coupled channel method suitable for studying the interplay of bound state resonance and phonon assisted scattering of inert gas atoms from solid surfaces in one, two and three dimensions. This enables us to get insight into the features that depend on the dimensionality of inelastic resonant processes typically encountered in low energy He atom scattering from surfaces, in general, and to elaborate on the observability of recently conjectured near threshold resonances in scattering from Einstein phonons, in particular.

A theory of vibrational frequency shifts revisited: application to dimers of LiH with the inert gases He, Ne, Ar and Kr

The objective of this paper is to contribute towards an understanding of the anomalous blue vibrational shifts that have been observed on forming some hydrogen bonds. It is shown that linear complexes of the LiH molecule with an inert gas atom Rg exhibit red or blue shifts of the LiH vibrational frequency depending upon whether Rg is attached to the Li or the H atom. The shifts in the frequency of the Li–H vibration on forming the weakly bound linear complexes Li–H…Rg and H–Li…Rg (Rg = He, Ne, Ar, Kr) were determined by ab initio computations at the MP2/6-311++G(2d, 2p) level of theory. These frequency shifts were found to be in good agreement with predictions from a model based on perturbation theory and involving first and second derivatives U′ and U′′ of the interaction energy with respect to displacement of the Li–H bond length from its equilibrium value in the isolated molecule. Concentration of the Li–H vibrational motion in the light H atom causes U′ and U′′ to be dominated by repulsion in Li–H…Rg and by attraction in H–Li…Rg, producing blue and red shifts, respectively. The bond length changes on complexation are well predicted by U′.

Atom Collision-Induced Resistivity of Carbon Nanotubes

We report the observation of unusually strong and systematic changes in the electron transport in metallic single-walled carbon nanotubes that are undergoing collisions with inert gas atoms or small molecules. At fixed gas temperature and pressure, changes in the resistance and thermopower of thin films are observed that scale as roughly M(1/3), where M is the mass of the colliding gas species (He, Ar, Ne, Kr, Xe, CH4, and N2). Results of molecular dynamics simulations are also presented that show that the maximum deformation of the tube wall upon collision and the total energy transfer between the colliding atom and the nanotube also exhibit a roughly M(1/3) dependence. It appears that the transient deformation (or dent) in the tube wall may provide a previously unknown scattering mechanism needed to explain the atom collision-induced changes in the electrical transport.

Mechanical Properties of Amorphous Al85Ni10La5 Alloy by Indentations

AbstractIndentation techniques have been carried out to study the mechanical behavior of amorphous Al85Ni10La5 alloy powders produced by inert gas atomization. The present work reveals that this amorphous alloy undergoes a three-stage crystallization process in the temperature range of 250°C ~ 390°C, with a glass transition temperature of approximately 259°C. In this study, the influence of devitrification on mechanical response was investigated via indentation of the Al85Ni10La5 alloy powders annealed at various temperatures, for instance, at temperatures well below or close to glass transition (235°C, 245°C, 250°C), and well above glass transition (283°C). Moreover, the microstructure evolution and the formation of nanoscale crystallites were studied using TEM and XRD. The influence of devitrification on the indentation response was characterized, paying particular attention to shear band formation and variations in hardness. The hardening behavior was analyzed on the basis of a rule-of-mixtures approach by treating the partially crystallized alloy as a nanocomposite.

Ionization kinetics in a supersonic xenon plasma flow entering electric field

We have studied xenon plasma moving in a supersonic diffuser in external electric and magnetic fields. The main physical parameters of the plasma (electron temperature and density) were determined using specially developed methods based on the theory of continuous optical emission from inert gas atoms. These experimental data are compared to the results of theoretical calculations. Based on an analysis of the results of spectroscopic measurements, a mechanism of plasma ionization is established which is capable of maintaining a high degree of ionization in the supersonic xenon plasma flow.

Microstructure characterization in dc sputtered a-SiC:H films by inert gas effusion measurements

The effusion of argon, neon and helium as well as of hydrogen was used for microstructure characterization of dc sputtered amorphous silicon carbon (a-SiC:H) alloys deposited with various carbon and hydrogen contents. Inert gas atoms were incorporated into the material by ion implantation. Our results suggest that effusion of implanted inert gas atoms is a useful method for microstructure characterisation of a-Si:C:H films.

A Gaseous Radiochemical Neutron Monitor

Real-time neutron detection (monitoring) is based on the ability of atoms of inert radioactive gases generated by nuclear reactions to easily escape from the crystalline lattice of certain solid substances. An inert radioactive gas produced in a detector ampule is transferred by a carrier gas to a proportional gas-flow counter. The decay rate of the inert gas, which is uniquely related to the neutron-flux density in the ampule, is measured with this flow counter. This method is applied to monitor the neutron flux in the RADEX pulse neutron target driven by the linear proton accelerator of the Moscow Meson Facility at the Russian Academy of Sciences' Institute for Nuclear Research. By simultaneously using several nuclear reactions with different threshold energies of inert-gas production, it is possible to measure neutron spectra.

Atomic mechanism of radiation‐induced erosion of field electron emitters

AbstractThe features of erosion of the surface of field electron emitters bombarded by helium and neon ions were studied. Experiments showed that ion bombardment produced tungsten atom transport, the number of atoms transported being proportional to the ion fluence. Analysis of ion bombardment‐induced atomic‐scale surface roughness showed evidence of a non‐dynamic surface atom displacement for primary ion energies below a threshold for stable Frenkel pair formation and cathode sputtering. A quasi‐static mechanism of surface erosion is considered, according to which the process is related to the displacement of metal atoms into low‐coordinated positions upon evolution of the energy of formation of interstitial inert gas atoms. Copyright © 2004 John Wiley & Sons, Ltd.

Characteristics of Ni-Ti-Zr-Si-Sn Bulk Amorphous HVOF Coating

This study was conducted to observe the influence of oxygen to fuel ratio[O2/H2 ratio] of HVOF(High Velocity Oxygen Fuel) process on the amorphous formation and coating properties of NiTiZrSiSn bulk amorphous alloy. The bulk amorphous feedstock was prepared by inert gas atomization. Amorphous contents were followed by differential scanning calorimetry(DSC). Microstructural characterization was done by scanning electron microscope(SEM) and x-ray diffraction(XRD). Mechanical properties were obtained by Vickers microhardness testing and pin on disk wear testing. A maximum 72% amorphous phase could be obtained with an optimum O2/H2 ratio. High amorphous content resulted in a low friction coefficient and weight loss than high oxide content coating and fully crystallized coating.

Preparation and microstructure of Al-Ni-Y powder by rapid solidification

The Al-Ni-Y alloy powder was prepared by rapid solidification technology of inert gas atomization. The diameter of amorphous powder is less than 12 µm. The effects of atomization gas on cooling velocity, morphology, microstructure and microhardness of powder and fine powder ratio were investigated. The results show that the morphology, microstructure and microhardness of powder and fine powder ratio are affected by cooling velocity changed through atomization gas. The cooling velocity of inert gas atomization is more than 1×104 K/s. The larger the cooling velocity, the finer the powder, and the smoother the surface of powder; the smaller the diameter of powder, the larger the microhardness of powder.

A new spray forming technique for the preparation of aluminium rutile (TiO 2) ex situ particle composite

A new spray forming technique has been developed to synthesise aluminium based metal matrix composites. In the present investigation, Al–2Mg–7TiO 2 composite was successfully spray atomised and co-deposited on a rotating substrate. This paper describes the processing methodology for the fabrication of Al–TiO 2 composites, which involves simultaneous introduction of aluminium melt and rutile particles through concentric tubes, followed by inert gas atomisation and deposition on a rotating copper substrate placed vertically below the atomiser at some predetermined height. Mechanical and physical properties of as-cast and mechanically worked composite materials are discussed. For comparison, Al–2Mg–5TiO 2 composite was also prepared by stir cast method and characterised.

Modeling energy transfer in molecular collisions: Statistical theory versus experiment for highly excited toluene and azulene

The recent development and application of the method of kinetically controlled selective ionization has produced detailed and reliable data on the collisional energy transfer kernel P(E′,E) entering master equation theories of unimolecular reaction rates. Here we test the ability of our partially ergodic collision theory (PECT) to predict the functional form of the observed kernel leaving only one parameter, the first moment of the distribution 〈ΔE〉, to be input from other sources. The data explored here include two reactant molecules, toluene and azulene, in collisions with 18 and 8 medium molecules, respectively, ranging from inert gas atoms to n-heptane. The initial energy of the reactant molecule is varied from 10 000 cm−1 to 49 000 cm−1 and 30 000 cm−1, respectively. The energy transfer efficiency βE is about one-tenth of its ergodic collision limit of unity. The PECT is found to fit the monoexponential form of the kernel determined from the experimental data over a broad range of initial energies E including tail regions of very low probability. A minor but systematic deviation is observed for nearly elastic collisions of large medium molecules. The functional fit is good enough to effectively allow the three parameters of the monoexponential experimental kernel to be replaced by a single parameter representing energy transfer efficiency.

CHARACTERISTICS OF MESOSCOPIC PHASE TRANSITIONS IN TWO-DIMENSIONAL SIMPLE NANOSTRUCTURED MATERIALS

In order to study nanostructured materials, a fundamental framework of the theory and the computer-experimental studies is established. The essential characteristics of the mesoscopic phase transitions and critical phenomena in these materials are evaluated by means of this approach. For nanostructured materials consisting of inert gas atoms, we study mesoscopic phase transitions and critical phenomena by generalizing the renormalization theory and the Metropolis Monte Carlo method. The results obtained by the both methods are reported in two papers: computational results in the present paper and the theoretical results in the paper which follows.

Chinese aerospace titanium melt contract for PSI

A specialist UK atomisation company has won major equipment contract to provide the Beijing Institute of Aeronautical Materials (BIAM) with a cold-crucible inert gas atomisation system. This is a short news story only. Visit www.metal-powder.net for the latest powder metallurgy industry news.

Adsorbate Effects on Structure and Shape of Supported Nanoclusters: A Molecular Dynamics Study

Classical molecular dynamics simulations are used to examine the effects of adsorption of inert-gas atoms (Ar, Xe, and He) at various pressures and temperatures on the shape, structure, and diffusion of a 256-atom platinum nanocluster supported on a graphite substrate. The many-body Sutton Chen potential is used to model the Pt−Pt interactions, and the Lennard-Jones potential is employed for the rest of the pair interactions. Gas adsorption on the metal cluster is monitored as a function of pressure at constant temperature, and the evolution of the structure is analyzed as the pressure is increased and then as the gas phase is gradually removed. It is found that the gas phase substantially alters the vacuum cluster structure, and the changes are mostly irreversible in the time frame of the simulations (2 ns), especially at temperatures well below the cluster melting point, where the low energy structures determined under high pressures are very stable. The dynamic properties are also modified by the gas phase; the cluster diffusion depends on the inert gas pressure, being lower at high gas pressures. Although the contact area cluster−substrate is significantly smaller compared to the vacuum case, the size mismatch is reduced because of the presence of the gas-adsorbed layer that forces the rearrangement of the metal atoms leading to distorted structures which favor the Pt−C interactions.

Gaussian-type function set without prolapse for the Dirac-Fock-Roothaan equation.

A Gaussian-type function (GTF) set without a prolapse (variation collapse) is generated for the Dirac-Fock-Roothaan (DFR) equation. The test atom was mercury. The number of primitive GTFs used is between 7 and 62 (abbreviated as 7-62), 6-62, 6-62, 4-36, 4-36, 3-36, and 3-36 for s(+), p(-), p(+), d(-), d(+), f(-), and f(+) symmetries. The respective exponent parameters were determined with even-tempered manner, which requires the minimum and maximum exponents for the respective symmetries. We prepared several sets of these. The total energy (TE) given by the numerical DF (NDF) is -19648.849250 hartree; one of the present sets with largest number of expansion terms gave -19648.849251 hartree. The error (deltaTE) relative to the NDR TE is quite small. We then applied this set to the inert gas atoms Ne (10), Ar (18), Kr (36), Xe (54), Rn (86), and No (102), and also to Es (99) as the representative of the open shell atoms. The absolute values of deltaTE were at most 2.8 x 10(-6) hartree, showing the potential of this set as a universal set.

Fermi-shuttle acceleration of electrons in ion–matter interaction

Large electron yields, compared to theoretical predictions, have often been observed in the high-energy part of the electron spectra in collisions of energetic ions with atomic, molecular or solid targets. The relative enhancement of the electron emission yield at high energies can be especially strong in ion–solid collisions. In this work, following a brief overview, recent experimental evidences are presented for Fermi-shuttle type accelerating electron scattering sequences in ion–atom collisions. Signatures for double (projectile–target, P–T), triple (projectile–target–projectile, P–T–P) and quadruple (P–T–P–T) scattering sequences have been found in different collision systems. Our new results indicate the presence of even higher-order scattering contributions in the collisions of few keV energy N + ions with inert gas atoms. The observations support that high-energy electrons produced by accelerating scattering sequences may play a significant role in ion–solid collisions.

Luminescence study of silicon nanostructures prepared by ion beam mixing

Phase separation of SiOx during high temperature annealing results in silicon nanoclusters embedded in SiO2 matrix. The preparation method which is fully compatible with Si technologies enables independent control of radiation induced damages and stoichiometry of SiOx. The method uses ion beam mixing of a-Si/SiO2 multilayers by inert gas ions. The nanoclusters show a strong photo- and electroluminescence in the visible and near-infrared region. An excess Si concentration dependent redshift and a luminescence intensity dependence on energy and dose of mixing ions are observed. The role of inert gas atoms accumulated in multilayer film during ion beam mixing is investigated. The obtained results are in good agreement with quantum confinement model.

Gradient expansions in density functional calculations of intermolecular potentials

Methods for calculations of intermolecular potentials following the local density approximation (LDA) have earlier been considered, especially for inert gas atom – inert gas atom interactions using the so-called electron gas density functional (EGDF) model. In this Letter some density gradient corrections are discussed and new results are presented. The general conclusion based on the results in the present report seems to be that the EGDF method gives surprisingly good numerical results in the LDA limit for calculations of intermolecular potentials. Corrections due to density gradients contribute only small changes in the sum of the density functional terms considered for the total energy in the EGDF method, but they contribute important corrections to the kinetic energy and to the exchange energy separately.