The interaction of KCl with tungsten single crystal surfaces

The interaction of KCl vapor with W single crystal surfaces is studied with the aim of obtaining information on the adsorption of ionically bonded molecules. The atomically smoothest and roughest surfaces, (110) and (111), are used. The adsorbate is characterized by LEED, thermal desorption spectroscopy (TDS) and by photoemission experiments which can be related to known work function data. It is found that the adsorbate is undissociated on W(110) and to a large extent also on W(111) at room temperature and that no ordered structures are formed. Electron bombardment causes KCl dissociation and Cl desorption which is complete with sufficiently high electron doses. Close-packed K monolayers can be formed this way.     

Electron-phonon interaction in polar solids with application to the tungsten bronzes

We investigate the interaction between electrons and lattice vibrations in polar solids taking into account their microscopic dielectric properties and constructing in a semi-phenomenological manner the nonlocal dielectric function, ?_GG'(q). The results of earlier, phenomenological theories, such as the Clausius-Mossotti formula for the macroscopic screening constant and Fröhlich's electron-phonon coupling parameter α are obtained from the RPA dielectric function when the dipole approximation is used to calculate the screening by the strongly localized valence electrons. For polar metals we also consider the screening by the free conduction electrons. We present and discuss the results of the screened electron-phonon interaction as a function of the phonon wavevector q for the sodium tungsten bronzes.     

The interaction between non-parallel {102} crystallographic shear planes in slightly reduced tungsten trioxides

The displacements of non-parallel {102} CS planes in slightly reduced WO₃ observed by electron microscopy were quantified with the theory of elasticity for an isotropic continum. By this calculation, the magnitude of the force between cations in CS planes was estimated. It was found that the principle force involved was an attractive one between cations in the CS plane which was calculated to be $\text{～ 1.2 × 10}{}^{13}\text{(}\text{dyne}\text{cm}{}^2\text{) = 7.8 (}\text{eV}\text{A}\text{?}{}^3\text{)}$. Using this value, the magnitude of the strain energy was also evaluated, and was found to be larger in value than the energy associated with the dissociation of oxygen atoms from WO₃ lattice. This suggests that the strain energy plays a dominant role in the behaviour of CS planes.     

The interaction of helium metastable atoms with a clean and co covered polycrystalline tungsten surface

An investigation has been made of secondary electron ejection resulting from the impact of helium metastable atoms on a clean and CO covered polycrystalline tungsten surface respectively. The ejected election energy distribution from the clean tungsten surface is found to differ from that resulting from the neutralization of slow incident ions. In the present case the distribution is narrow and peaked at approximately 2 eV. When the tungsten surface is exposed to CO it is found that the total yield of secondary electrons increases, but that the energy distribution remains essentially the same. The increase in the total yield following CO exposure is shown to be associated with bonding of the CO in the α-state. A tentative explanation for the de-excitation mechanism is proposed, based on Auger de-excitation of the helium metastable atom.     

Relaxation of tungsten adatoms on tungsten surfaces

The relaxation energy of a tungsten atom on tungsten surfaces is estimated to be about 1 eV. It seems difficult to neglect the variation of that energy in the interaction between adatoms.     

Anisotropic interaction between self-interstitial atoms and 1/2<111> dislocation loops in tungsten

We investigate the interaction between 111 self-interstitial atoms(SIAs) and 1/2111 self-interstitial dislocation loops in tungsten(W) via atomistic simulations. We explore the variation of the anisotropic distribution of binding energies with the shapes and sizes of the 1/2[111] loop and the nonequivalent configurations of 111 SIAs. For an arbitrarily shaped loop, SIA can be more easily trapped in the concave region of the loop than the convex region, which forms a loop whose curvature is closer to that of a circular loop. The direction of SIAs can largely affect the interaction behaviors with the loop. The capture distance of an SIA by the edge of a circular-shaped 1/2[111] loop is clearly elongated along the direction of the SIA; however, it weakly depends on the size of the loop. Then, we analyze the slanted ring-like capture volume of 111 SIAs formed by the circular loop based on their generated anisotropic stress fields. Furthermore, the binding energies obtained from the elastic theory and atomistic simulations are compared. The results provide a reasonable interpretation of the growth mechanism of the loop and the anisotropic interaction that induces irregular-shaped capture volume, affording an insight into the numerical and Object Kinetic Monte Carlo simulations to evaluate the long-term and large-scale microstructural evolution and mechanical properties of W.     

Hydrogen interaction with Al2O3-coated tungsten under plasma irradiation

The effect of amorphous films of Al₂O₃ on deuterium trapping in polycrystalline tungsten under plasma irradiation (150 eV/D) and high fluence (up to 4 × 10²⁴ D/m²) was studied by the methods of nuclear reaction product spectroscopy and thermal desorption spectroscopy. It was determined that the hydrogen permeation blocking properties of aluminium oxide lead to a great increase in deuterium trapped in tungsten. With the complete erosion of the film, this effect continues but with a lower amplitude. Extrapolation of this property to beryllium oxide implies that the presence of beryllium oxide on the surface of the tungsten tiles of the ITER leads to a considerable increase in the tritium inventories accumulated in them.     

De Haas-Shubnikov effect in tungsten

We have observed de Haas—Shubnikov oscillations in a single crystal of tungsten in magnetic fields between 55 and 105 kG. The amplitudes are compared to those predicted by the oscillatory relaxation time mechanism proposed by Pippard, and while the field dependence of the amplitude agrees with the theory, the measured absolute amplitudes are too large by a factor of about three. This discrepancy probably results from the assumption of an isotropic relaxation time.     

Diffusion of tungsten clusters on tungsten (110) surface

Using molecular dynamics simulation and modified analytic embedded-atom method, we have investigated the self-diffusion of clusters on a tungsten (110) surface. As compared to the linear-chain configuration, the close-packed islands for tungsten clusters containing more than nine adatoms have been predicted to be more stable with the relatively lower binding energies. The migration energies show an interesting and oscillating behavior with increasing cluster size. The tetramer, hexamer and octamer have obviously higher migration energies than the others. The different atomic configurations and diffusion mechanisms have been determined during the diffusion processes. It is clear that the dimer-shearing mechanism occurs inside the hexamer, while it occurs at the periphery of heptamer. The successive hopping mechanism of individual atom is of critical importance in the migration of the clusters containing five or fewer adatoms. In addition, the diffusion of a cluster with nine adatoms is achieved through the changes of the cluster shape.     

Electrochromism in tungsten trioxide films obtained by reactive magnetron sputtering of a tungsten target

The results of structural investigations of electrochromic films of WO₃, obtained by reactive magnetron sputtering of a tungsten target, are presented. It is shown that at low temperatures an amorphous film of WO₃ is formed on the substrate. At a substrate temperature greater than 250°C the film that is formed is polycrystalline and has the structure of hydrotungstic bronze H_0.33WO₃. When annealed in air it transfers into the triclinic modification of WO₃. Spectral investigations showed that the electrochromic properties are most pronounced in amorphous films. Films of WO₃, having a polycrystalline structure after annealing in air, practically lose their ability for electrochromic coloration. Institute of High-Current Electronics, Sib. Otd., Russ. Akad. Nauk. Translated from Izvestiya Vysshykh Uchebnykh Zavedenii, Fizika, No. 5, pp. 3–7, May, 1996.     

Softening processes in single-crystal tungsten ribbons

Recovery processes in rolling-deformed (001)[110] tungsten single crystals having various degrees of purity are investigated. It is shown that the dislocation structure formed in the plastic deformation of tungsten single crystals is transformed in subsequent high-temperature anneals to a system of dislocation subboundaries; only polygonization, which preserves the single-crystal structure, takes place in samples heated to a temperature close to the melting point. The formation of subboundaries proceeds in two stages with subsequent transformation of the unstable structure to a configuration having an energy minimum. The decisive factor affecting the polygonization rate is the stacking fault energy; the presence of impurities also has a significant influence. Fiz. Tverd. Tela (St. Petersburg) 40, 498–502 (March 1998)     

Doppler-shifted acoustic cyclotron resonance in tungsten

Doppler-shifted cyclotron resonances have been observed acoustically in high purity tungsten single crystals. The resulting Fermi surface area derivatives agree generally with those obtained from magnetomorphic size effect experiments and with calculations based on an empirical model fitted to de Haas-van Alphen data. Certain non-central orbit zones of the latter model are modified to fit both sets of results. The resulting correction to the hole octahedron sheet is minimal, the modified electron jack has a more spherical shape, and the necks are slightly further away from the zone center. The dimensions of the re-parameterised model agree better with the results obtained from other Fermi surface measurements.     

Accumulation of deuterium in radiation-damaged tungsten

Experimental studies have been performed to determine the effect of high-level radiation damage on the accumulation of deuterium and erosion of tungsten samples exposed to deuterium plasma. Tungsten samples were exposed first to fast helium ions having an energy of 3–4 MeV (providing from one to ten displacements per atom) and then to deuterium plasma up to a dose of 10²⁵ ion/m². The effects of deformation and modification of the surface microstructure have been observed. The concentrations of helium and deuterium have been measured by the methods of elastic nuclear proton backscattering and nuclear recoil detection of helium ions. A high concentration of deuterium in the damaged layer of a tungsten sample has been measured, and helium has been detected in a layer ～5 μm thick. The proposed method shows promise for determining the lifetime of materials used in fusion reactors and measuring the concentration of tritium accumulated in these materials.     

Formation of combustible gases during interaction of tungsten and zirconium with supercritical fluid H2O/CO2

Oxidation of bulk samples of tungsten (923 K) and zirconium (773 and 873 K) by H₂O/CO₂ supercritical fluid (molar ratio [CO₂]/[H₂O] = 0.17–0.26) at a pressure of about 300 atm is investigated. Oxidation produces monoclinic WO₃, monoclinic W₁₉O₅₅, monoclinic ZrO₂, H₂, CO, CH₄, and carbon (on the surface of tungsten oxide). Differences in oxidation mechanisms for tungsten and zirconium are revealed. CO₂ molecules take part in the oxidation of tungsten only after oxide formation in reaction with H₂O. Zirconium is oxidized fully, and oxidation of tungsten terminates in the formation of the oxide layer at the metal surface.     

Trap mutation in He-doped ion-implanted tungsten

In this work, we report on an investigation of the defect complexes InVe₂He _n in tungsten by means of PAC. The release steps InVe₂He _n InV₂He _n-1 + He were identified forn=1t–5. The dissociation energies, in the range of 2.9–4.5 eV, agree perfectly with the values that have been previously determined by means of THDS. The quadrupole frequency is about 122 Mrad/s forn=1,2 and about 101 Mrad/s forn=3,4. These values are smaller than the frequency of the undecorated vacancy (n=0). The vacancy mutates into a divacancy if it is filled with about 10 He atoms, giving rise to a quadrupole frequency of about 218 Mrad/s. This so-called trap mutation ultimately leads to bubble information. We observed at least three different bubble-associated quadrupole interactions.     

Dechanneling of MeV protons in tungsten

Abstract

A systematic investigation of the dechanneling of protons in tungsten has been completed for the <111> axial direction and also for the (110) plane. Measurements at room temperature have been made at energies of 0.5, 2.0, 3.5, 8.0 and 12.0 MeV; at 2.0 MeV, the effect of target temperature has been investigated over the range 77–873°K. In the axial case, the dechanneling rate increases strongly with the vibrational amplitude _ρ, but not as strongly as the ρ² dependence reported by the Catania group (Refs. 4–7) for silicon and germanium. Also, unlike silicon and germanium, the tungsten axial data exhibit practically no energy dependence and so cannot be fitted by the zρ²/E scaling parameter suggested by the Catania group. On the other hand, the planar dechanneling data in tungsten exhibit the predicted (E ^?1)dependence and relatively littie temperature dependence, in good agreement with previous work.     

Giant field amplification in tungsten nanowires

The original RF-sputtering-assisted technique to produce metal wires with tip-curvature radii at the nanometer scale was developed and applied to tungsten. The wire tips were characterized by scanning electron microscopy. Field-emission tests of those wires exhibit excellent performance and reliable processability. Very high field-amplification factors of 18000 were demonstrated. Received: 1 November 2002 / Accepted: 30 November 2002 / Published online: 11 April 2003 RID="*" ID="*"Corresponding author. Fax: +1-972/479-4482, E-mail: alexander.umnov@fla.fujitsu.com