Plane Of Tungsten Research Articles

Mechanisms of helium irradiation blistering and surface deformation in tungsten

Helium irradiation blistering in tungsten is of intense interest for plasma-facing materials under fusion-relevant conditions. Previous studies have revealed that the surface blistering is closely related to the evolution of subsurface microstructures (e.g., bubbles and cracks), but its dependence on the surface orientation and deformation remains unclear. The present work reports the helium irradiation blistering on the surface planes of tungsten {100}, {110} and {111}. A systematic study of helium irradiation blistering was conducted at room temperature by utilizing a Helium Ion Microscope. The cross-sectional microstructural analyses at different ion fluences confirmed that the blistering deformation starts with the nanocrack nucleation, initiated at the peak in the bubble depth distribution via interbubble fracture, and is followed by the gas buildup in a primary cavity. In addition, the comparison of blistering behaviors between 1-µm-diameter and 2-µm-diameter circular areas revealed that only the blisters formed on tungsten {100} can grow to a larger size, whereas those formed on tungsten {110} or {111} tend to become wrinkled or burst when the irradiated area increases. Combined with the micropillar compressions in a Scanning Electron Microscope, it was also found that the surface deformation is orientation-dependent and associated with the number, symmetry and distribution of effective slip systems in body-centered-cubic transition metals. Our findings provide new evidence and insights into the mechanisms of helium irradiation blistering and surface deformation in tungsten.

System for interpolating work function for a cold-field emitter

Simulating electron emission current density J from a hemispherical cold-field emitter requires work function ϕ and field F over the surface of the emitter. Typically, the surface is approximated by a polygonal tile model with polygons whose normals may point in a variety of directions, representing arbitrary facet planes. As ϕ has only been experimentally determined on a few planes of tungsten, the simulation must infer the work function for these nonstandard directions. This paper presents and demonstrates this interpolation system as tailored to crystals with three or more known crystallographic work functions.

A multi-scale study of the adsorption of lanthanum on the (110) surface of tungsten

In this study, we utilize a multi-scale approach to studying lanthanum adsorption on the (110) plane of tungsten. The energy of the system is described from density functional theory calculations within the framework of the cluster expansion method. It is found that including two-body figures up to the sixth nearest neighbor yielded a reasonable agreement with density functional theory calculations as evidenced by the reported cross validation score. The results indicate that the interaction between the adsorbate atoms in the adlayer is important and cannot be ignored. The parameterized cluster expansion expression is used in a lattice gas Monte Carlo simulation in the grand canonical ensemble at 773 K and the adsorption isotherm is recorded. Implications of the obtained results for the pyroprocessing application are discussed.

Crystallographic orientation relationships and interfaces in laser-processed directionally solidified WC–W2C eutectoid ceramics

Crystallographic orientation relationships and interfaces in directionally solidified WC–W2C eutectoids are investigated by electron microscopy and electron back-scattered diffraction. The WC–W2C eutectoids are prepared by a laser surface processing method, which unidirectionally melts and resolidifies ceramic powder substrates. Lamellar-type microstructures are observed in all samples with preferred nominal growth directions of [\( \overline{1} 2\overline{1} 0 \)]WC//[\( \overline{1} 2\overline{1} 0 \)]W2C along the solidification direction. The majority of interface habit planes are found to be (0001)WC//(0001)W2C. The interfaces are found to be semicoherent, with a misfit Burger’s vector of 1/3[\( 2{\bar{1}} {\bar{1}} 0 \)]. An intermediate layer is identified at the interface and is associated with a change in the stacking sequence of the close-packed (0001) tungsten planes.

A study of uranium adsorption to single-crystal tungsten

In this study we explore the adsorption of uranium to the (110) plane of tungsten. Potential functions were constructed to describe the interaction of adsorbed uranium atoms with the tungsten surface and the lateral interaction between adsorbed uranium atoms. Next, the behavior of the uranium adlayer under different conditions was studied through a Monte Carlo simulation of the grand canonical Hamiltonian in an off-lattice model. Our results are consistent with available studies in the literature. The simulation results indicate that the temperature and dipole–dipole interactions play an important role in governing the adsorption process.

Charge-Induced Second-Harmonic Generation in Bilayer WSe2.

Controlling nonlinear light-matter interaction is important from a fundamental science point of view as well as a basis for future optoelectronic devices. Recent advances in two-dimensional crystals have created opportunities to manipulate nonlinear processes electrically. Here we report a strong second-harmonic generation (SHG) in a 2D WSe2 bilayer crystal caused by a back gate field. This unusual process takes place only when the gate polarity causes charge accumulation rather than depletion. Analysis based on a bond-charge model traces the origin of SHG to the nonuniform field distribution within a single monolayer, caused by the accumulated submonolayer screening charge in the tungsten plane. We name this phenomenon charge-induced SHG (CHISHG), which is fundamentally different from the field- or current-induced SHG. Our findings provide a potentially valuable technique for understanding and noninvasive probing of charge and current distributions in future low dimensional electronic devices.

Comparative Theoretical Study of H2 Eley–Rideal Recombination Dynamics on W(100) and W(110)

Quasiclassical molecular dynamics simulations are performed to study the Eley–Rideal recombination of H2 on two crystallographic planes of tungsten. Potential energy surfaces, based on density functional theory, are used to describe the H+H/W(100, 110) interactions. The calculations are carried out within the single adsorbate limit under normal incidence of the impinging H atoms. The influence of the crystallographic anisotropy on reaction cross sections and energy distribution of the formed molecules is analyzed in detail. Despite some discrepancies in the dynamics of recombination between W(100) and W(110) surfaces, translational, rotational, and vibrational energies of the formed molecules do not depend significantly on surface symmetry. Vibrational distribution of formed H2 molecules are found in good agreement with experiments.

Tungsten foil laminate for structural divertor applications – Tensile test properties of tungsten foil

This paper is the third part of our series on tungsten foil laminates. Within the tungsten laminate project we have succeeded in ductilizing tungsten by synthesizing a tungsten laminate made of tungsten foil, which is ductile. By assembling and joining several layers of tungsten foil, the ductile properties of the foil can be extended to the bulk.The aim of this paper is to present the results of tensile tests on 100-μm-thick tungsten foil in as-received and recrystallized conditions (1h at 2000°C). The results show that the mechanical properties of tungsten foil are anisotropic and can be explained by considering (i) the texture of the tungsten foil of {100}〈011〉, (ii) the anisotropic grain shape (0.5μm×3μm×15μm), (iii) the preferred slip direction of body-centered cubic (bcc) metals, the 〈111〉 direction, as well as the preferred cleavage plane of tungsten at room temperature, the {100} plane. Furthermore, the results give further hints for the sources and mechanism of the extraordinary ductility of tungsten foil such as the ‘foil effect’, which is the dislocation annihilation at the free surface. In particular, the ductile material behavior of tungsten foil in the recrystallized condition seems to benefit from the foil effect and this is how a plastic strain of about 30% in a tensile test at 600°C can be explained.

Experimental observation of parametric X-ray radiation directed along the propagation velocity of relativistic electrons in a tungsten crystal

Parametric X-ray radiation (PXR) due to dynamic diffraction of relativistic electrons is experimentally observed at small angles to the propagation velocity of electrons in a tungsten crystal. The specific features of the experimental method are described, and forward PXR reflections from two crystallographic planes of tungsten are reliably measured.

Adsorption studies of Cr on W ( [formula omitted]) plane by probe hole field emission microscopy

A probe hole field emission microscope has been used to study adsorption of chromium (Cr) on microscopic (1 1 0) plane of tungsten (W) tip. Adsorption is studied by measuring variation in workfunction ( Φ) with Cr deposition ( θ) at two temperatures, 722 and 300 K. For the 722 K tip, Φ decreases linearly by about 0.5 eV till completion of one monolayer and a very small increase is observed thereafter, indicating that the formation of Cr monolayer is by two-dimensional islanding. The average dipole moment associated with Cr–W pair is calculated to be 6.0×10 −31 C m. On a tip held at 300 K, Cr atom population on the (1 1 0) plane is low. Therefore, only a small (0.3 eV) decrease in Φ is observed. Also the ΔΦ– θ behaviour is found to be erratic, indicating crystallite formation on the (1 1 0) plane. These observations are compared with those of Berlowitz and Shinn [Surf. Sci. 209 (1989) 345], who used macroscopic W surfaces to study Cr/W system. The results are discussed in view of the existing literature.

Comparison between experimental and computer simulations of current–voltage ( I– V) characteristics of dielectric-coated photon-stimulated field emitters

For the purpose of simulating photon-stimulated field emission by taking account of three-dimensional aspects, a transfer-matrix formulation of electronic scattering was combined with a Floquet expansion of the wave function for taking account of quanta exchanges between the electrons and the external radiation. With specific techniques to preserve numerical stability, this transfer-matrix formalism is well suited to compute the transmission of the field-emitted/photon-stimulated electrons between two electrodes. This theory is applied to the computation of Fowler–Nordheim curves describing the photon-stimulated field emission of a tungsten plane emitter (described by z⩽0), which supports a nanometric protrusion and a dielectric coating. The extraction bias ranges from 12 to 24 V, for an inter-electrode distance of 4 nm. The electromagnetic radiation has a wavelength of 0.67 μm and a power flux density ranging from 5.96×10 10 to 5.96×10 12 W/m 2. The effects due to the protrusion and the dielectric coating are studied. These theoretical results are compared with the experimental data.

Two-process diffusion of titanium on the (1 1 1) tungsten plane by the current fluctuation method in FEM

Field emission current noise of titanium adsorbed on the (111) tungsten crystal face was investigated. Measurements were carried out for coverages ranging from 0.2 to approximately 2 monolayers in the temperature range 300–1500K. A linear combination of two theoretical functions obtained by Gesley and Swanson for the diffusion model of current fluctuations was fitted to the experimental autocorrelation functions. In such a way two diffusion coefficients and two activation energies for titanium surface diffusion were determined. The theoretical model is applicable to a temperature range of 800–1450K. The energy of each diffusion process decreases with increasing coverage, from a value of 1.77eV (the first process) and 1.42eV (the second process) until the saturation value of about 0.3eV is reached for both diffusion processes. The first process occurs below the phase transition point of titanium (1156K) and the second process occurs above it.

Quantum-mechanical simulations of photon-stimulated field emission by transfer matrices and Green’s functions

For the purpose of simulating photon-stimulated field emission by taking account of three-dimensional aspects, a specific formulation of electronic scattering is developed. This formulation relies on both the transfer-matrix and Green's-functions formalisms. It is combined with a Floquet expansion of the wave function for taking account of quanta exchanges between the electrons and the electromagnetic radiation. With specific techniques to preserve numerical stability, this methodology is well suited to compute the transmission of the field-emitted/photon-stimulated electrons between two electrodes and their propagation to a distant screen. The theory is applied to the computation of photon-stimulated field emission from a tungsten plane emitter (described by $zl~0),$ which supports a 1-nm-long hemispherical protrusion. The extraction bias ranges from 12 to 24 V (the interelectrode distance is 4 nm). The electromagnetic radiation has a wavelength ranging from 0.1 to $10\ensuremath{\mu}$ and a power flux density ranging from $5.96\ifmmode\times\else\texttimes\fi{}{10}^{10}$ to $5.96\ifmmode\times\else\texttimes\fi{}{10}^{12}{\mathrm{W}/\mathrm{m}}^{2}.$ Current-relative-increase curves, total-energy distributions (TED), and Fowler-Nordheim plots are provided. The results point out a resonance in the current enhancement at a radiation wavelength of approximately $0.3\ensuremath{\mu}.$ Taking advantage of this resonance and working at low extraction bias leads to a better sensitivity of the emision current to the radiation power flux density.

Chemical mechanisms of tungsten CVD

A composition of adsorbed layers formed from a gaseous mixture of hydrogen with tungsten hexafluoride on single-crystal tungsten planes has been investigated. The heats and constants of adsorption, taking into account the strong lateral interaction of adsorbed W-F-H components, have been calculated by a simple procedure. The equilibrium composition of adsorbed layers at temperature range 400-1000 K, pressure range 1-100 kPa and for hydrogen to fluoride ratio from 4 to 10 has been presented. It has been shown that adsorbed fluorine and lower-valent tungsten fluorides cover the tungsten surface. The film adsorbed on (100) W has a higher density than one on (110) W and the fluorine and fluoride concentrations are considerably higher on (100) W plane.

Adsorption of nickel on tungsten: a probe-hole field emission microscopy study

In a probe-hole field emission microscopy experiment, submonolayer amounts of high-purity nickel were deposited on to a tungsten tip in clean, ultra high vacuum conditions and observations were made on W(1 1 0), (1 2 1) and (1 1 1) planes. The changes in the work function, Δ φ, and corresponding pre-exponential factors, ln B, have been obtained by ten point Fowler–Nordheim plots. The results show that the change in average work function of nickel covered tungsten surface increases and attains a saturation value of +0.52 eV. This is accompanied by a decrease in the pre-exponential factor. Adsorption of nickel on the W(1 1 0) plane causes a decrease in Δ φ in the lower coverage region and then exhibits an abrupt change to reach a saturation value of +0.5 eV as the coverage increases. This has been attributed to surface reconstruction. In the case of the W(1 2 1) plane Δ φ decreases with the number of doses to a value of −0.45 eV. On the W(1 1 1) plane, the Δ φ increases and saturates to a value of +0.50 eV after a brief initial decrease in the low-coverage region. The changes in the work function are explained in terms of atomic and electronic structure of nickel overlayers on various single-crystal planes of tungsten. A comparison of nickel with iron and cobalt adsorption on tungsten shows that the behaviour of nickel is similar to iron.

The diffusion of oxygen on W(110) revisited

The diffusion of oxygen on the (110) plane of tungsten has been reexamined via the field emission fluctuation method, using a digital correlation apparatus. Measurements were carried out over a coverage range of 0.13⩽O/W⩽0.79. Where overlap exists activation energies agree well with previous work; there is qualitative but not quantitative agreement with prefactors D0. Mean-square fluctuations show very little temperature dependence. The present results show that E rises almost linearly from 14 kcal/mol at O/W⩽0.2 to 23 kcal/mol at O/W=0.4, then shows a slight dip at O/W=0.56 and then rises again to 27 kcal/mol at O/W=0.79. D0=4×10−8 cm2/s at O/W=0.13, and rises by six orders of magnitude to 5×10−2 cm2/s at O/W=0.79. It is postulated that the increases in E and D0 with coverage have their origin in polaronic effects, to wit the participation of substrate atoms in the diffusion process and a progressive stiffening of the substrate by O atoms as coverage increases.

Photofield emission from clean and titanium covered (111) plane of tungsten

Optical transitions are observed as connected with the occurrence of shoulders in the photofield emission characteristics. For the photon energy hυ = 1.96 eV, two optical transitions in the 〈111〉 direction of clean tungsten and three maxima of the surface density of states on the (111) plane are observed. One transition, in the bulk energy band, is connected with the Fermi level and the second one with a minimum in the empty band. They are in good agreement with the calculated electronic structure. Thin titanium films (0.5–1.5 monolayer) cause ‘smoothing up’ of the photofield emission characteristics and shoulders (due to optical transitions) are scarcely noticeable, if at all. Noticeable changes in the photofield characteristics are observed after annealing the emitter at a temperature of about 1600 K. At a temperature of ≈ 2100 K titanium is removed almost completely. Similar results have been obtained for the quantum energy hυ = 2.41 eV. Electron states connected with the titanium layer adsorbed on tungsten (111) face have not been observed.

Adsorption studies of cobalt on tungsten (110), (100) and (111) planes by probe-hole field emission microscopy

The changes in the workfunction, Δφ, and in the Fowler-Nordheim pre-exponential factor, ln B, have been investigated for cobalt adsorption on tungsten (110) (100) and (111) planes by probe-hole field emission microscopy. The workfunction variations have been found to be strongly dependent on the temperature in the range 300–580 K on these planes. Δφ decreases with submonolayer coverages of cobalt on the W(110) and W(100) planes. On the W(111) plane, Δφ initially decreases and then exhibits a jump towards positive values at relatively higher does. This abrupt increase in Δφ and the corresponding zero crossover of ln B have been attributed to surface reconstruction. The results are discussed in terms of the electronic structure and the degree of strain produced in the pseudomorphically grown cobalt layers on various crystallographic planes of tungsten.

Adsorption studies of iron on tungsten by probe-hole field emission microscopy

Probe-hole field emission microscopy has been used to study submonolayer and monolayer amounts of iron on various tungsten planes. The adsorption is found to be insensitive to the substrate deposition temperatures of 300, 600, and 1100 K as seen by the variation of the workfunction and the pre-exponential factor with the adsorbate doses. The workfunction of the adsorbate covered surface reduces in case of the (110), (100), and (112) tungsten planes, while it increases in case of the (111) and (116) planes, becoming coverage independent with a workfunction of 4.6 eV. This has been atributed to surface alloying of iron atoms with tungsten because of the structure of these two planes. The results are discussed in the light of the existing experimental results on metallic adsorbates on tungsten.

A field ion microscope study on vapor deposition of copper on tungsten

Field ion microscopy observation was carried out on a copper thin film that was vapor deposited at a right angle to the tip axis onto one half of a tungsten tip cap. When copper was vapor deposited onto a tip kept at around 55 K, the deposit, which was made up of many fine crystallites and of a few larger crystallites at the interface with the substrate on each of the W{110} planes, was observed only on one half of the tip cap facing the evaporator. In addition, a well-ordered copper monolayer was observed on an area slightly wider than one half of the tip cap. By heating this deposit to 200, 300, or 400 K, recrystallization took place from the interface between the copper deposit and the substrate tungsten, and the thick copper area was reduced backwards to the direction of the evaporator. The recrystallization took place preferentially on some low index planes of tungsten. When copper was vapor deposited onto a tip kept at high temperatures (200, 300 or 400 K), copper crystals nucleated preferentially on W{110} planes and the condensed copper area spread to the shadow side by the forward migration of copper under the influence of impinging of depositing atoms.