Polycrystalline Tungsten Foil Research Articles

Observation of parametric X-ray radiation in an anomalous diffraction region

A new possibility to expand the energy region of diffraction processes based on the interaction of relativistic charged particles with crystalline structures is presented. Diffracted photons related to parametric X-ray radiation produced by relativistic electrons are detected below the low energy threshold for the X-ray diffraction mechanism in crystalline structures for the first time. The measurements were performed during the interaction of 7 MeV electrons with a textured polycrystalline tungsten foil and a highly oriented pyrolytic graphite crystal. The experiment results are in good agreement with a developed model based on the PXR kinematical theory. The developed experimental approach can be applied to separate the contributions of real and virtual photons to the total diffracted radiation generated during the interaction of relativistic charged particles with crystalline targets.

The nature of the brittle-to-ductile transition of ultra fine grained tungsten (W) foil

The aim of this work is to answer the question of whether an ultra fine grained (UFG) microstructure has an impact on the nature of the brittle-to-ductile transition (BDT) of tungsten. Therefore, four-point bend tests were performed using unnotched 100μm UFG tungsten foil and annealed coarse-grained polycrystalline tungsten foil (2073K for 2h in vacuum) with exactly the same chemical composition and dimensions. The reported results for coarse-grained tungsten foil show a strain-rate dependent BDT that occurs between 350 and 450K. The activation energy of the BDT, EBDT, for the coarse-grained material was deduced to be 2.9 (+2.6/−0.9)eV. The results obtained from UFG tungsten foil indicate a strain-rate independent BDT at relatively low temperatures (≈77K). The strong disparity in the nature of the BDT can be attributed solely to the presence of microstructural differences. The possible mechanism controlling the BDT in tungsten for both tungsten foil conditions will be discussed against this background and in the light of previous studies performed on the nature of the BDT of tungsten reported in pertinent literature.

3D-microscopy of hydrogen in tungsten

Abstract The mapping of hydrogen distributions in 3 dimensions and its correlation with structural features allow further insight into mechanisms of hydrogen trapping in tungsten. We studied hydrogen distributions in 25 μm thick polycrystalline tungsten foils by 3D hydrogen microscopy using a proton–proton-scattering method. Two types of tungsten samples were prepared: (i) at 1200 K annealed foils and using 1.8 MeV implantation energy (ii) at 2000 K annealed foils using 200 eV implantation energy. It has been found that large variations of surface hydrogen contamination occur within different samples. Nevertheless, a statistically significant variation of the hydrogen content across grain boundaries has been observed.

D retention in C- and O-contaminated tungsten during D+ irradiation

Deuterium retention during 10eV and 500eV/D+ irradiations at 300 and 500K was compared for 99.96% pure polycrystalline tungsten (PCW) foils and similar PCW foils contaminated with carbon and oxygen impurities. The contaminated foils showed reduced deuterium retention at 500K compared with pure foil specimens while there was no difference for 300K irradiations. X-ray photoelectron spectroscopy (XPS) and secondary ion mass spectroscopy (SIMS) of the unirradiated specimens showed regions of highly localised, non-homogeneous C and O contamination as well as a broader region of low level contamination of about 1–10% areal coverage. It is hypothesised that the C/O contamination creates a diffusion barrier in the PCW foil preventing deuterium from diffusing from the near surface into the bulk, thus reducing overall D retention at 500K. This finding has implications for tritium retention in ITER; if carbon contamination even at low levels can impede T diffusion into the bulk it would mean reduced T loading in W plasma-facing components.

D retention in W due to 10–500 eV D+ irradiation

The retention of deuterium in polycrystalline tungsten foils was measured at 300 and 500K as a function of D+ fluence and energy over the range 10–500eV/D+. The results show a significant increase in D retention as the energy is raised from 10 to 500eV/D+. The reduction at low energies is attributed to the shallow implantation zone of low-energy ions and an associated increase in surface release during implantation. Thermal desorption spectra obtained for the 10–500eV energy range at a fluence of 1024D+/m2 show desorption peaks at ∼600K and ∼750K when the ion energy is >100eV. Spectra for lower energy irradiations show the gradual development of a ∼950K peak along with the reduction of the 600K peak as ion energy is reduced to 10eV/D+. The change in TDS spectra with energy indicates a different trapping mechanism at lower ion energies.

Investigation of evolved layer structure of Ba-containing emission materials

The emission material in High Intensity Discharge (HID) lamps plays a significant role in lowering the electrode work function and thus the lamps´ operating temperature. Ba-containing rare earth- and alkali earth tungstate materials are commonly used as cathode thermionic emission materials because of the production of high intensity discharge. The goal of this present work is to model a cathode tip surface evolved during sintering and to compare the physical and chemical properties of emission materials currently used in the industry. In order to achieve this goal, we investigated and compared the layer structure of Ba-Ca- and Ba-Y-containing emission materials evolved on polycrystalline tungsten foils. Simultaneously, the tendency of the work function was also monitored as a function of Ba/BaO layer thickness. The Ba coverage of cathode is one of the most important factors during the lifetime of HID lamps. The initial Ba diffusion was also examined. We also proposed a layer model, valuable for the structures occurring during the operation of lamps. The chemical composition of the flat samples was analysed by X-ray Photoelectron Spectroscopy (XPS) and the electron emission properties by Work Function Spectroscopy (WFS).

Influence of defect-impurity complexes on slow positron yield of a tungsten moderator: Positron annihilation, Auger, and SIMS study

Polycrystalline tungsten foil annealed at successively higher temperature up to $\ensuremath{\sim}2300\mathrm{K}$ has been investigated for slow positron moderation yield. LINAC-based intense positron beam lifetime studies have revealed that reemitted slow positron and positronium fractions gradually improve upon high-temperature annealing. So as to correlate the presence of defects and chemical impurities with the improvement in slow positron yield, positron annihilation, Auger electron spectroscopy, and secondary ion mass spectroscopy (SIMS), studies have been carried out on virgin and high-temperature $(\ensuremath{\sim}2300\mathrm{K})$ annealed W foils. The positron beam S parameter shows a large value throughout the sample depth corresponding to the virgin sample, while it is lower for annealed samples. This indicates that, as compared to virgin sample, the annealed W sample has a lower concentration of vacancylike defects. Auger studies revealed that in virgin state the surface is fully contaminated with carbon, while the annealed foil shows prominent W peaks. Corroborative SIMS concentration profiles have indicated that the carbon content is much lower in an annealed sample over a large depth region. From these studies, it is concluded that improvement in slow positron yield upon high-temperature annealing is obtained due to the removal of the surface tungsten-carbide layer as well as carbon-vacancy complexes present throughout the sample depth.

Anomalous inverse absorption features in the far-infrared RAIRS spectra of SnCl4 on thin-film SnO2 surfaces

As part of a study of the chemistry of tin oxide chemical vapor deposition precursors at oxide surfaces, we have utilized the so-called buried metal layer approach to obtain far-IR reflection absorption infrared spectroscopy (RAIRS) spectra for SnCl4 adsorbed at a tin oxide surface supported on a polycrystalline tungsten foil. Two types of surface preparation—namely, sputtering with Ar or O2—have been used to clean the tin oxide surfaces prior to experiment. On O2-sputtered surfaces at 300 K, the spectra are dominated by an inverse-absorption feature in the form of a positive-going band in the spectrum, obtained via the ratio of the sample spectrum to that of the clean surface background. At low temperature, the spectra display both the positive-going inverse-absorption feature and the normal negative-going absorption features common to many RAIRS studies from metallic surfaces. On an Ar-sputtered surface, the inverse absorption band is not observed, but is replaced, at least at 300 K, by an absorption band at the same wave number. Simple models are employed to demonstrate how these features may arise from an analysis of the Fresnel equations for the system, and the data are discussed in terms of the possible models for inverse-absorption features that appear in the literature. It is concluded that the features observed are induced by the presence of the adsorbate, but are characteristic of the underlying oxide substrate.

Re-emission of slow positrons from tungsten at elevated temperatures

A series of measurements have been made on the dependence of the intensity and energy spectrum of slow positrons re-emitted from polycrystalline tungsten foil held at temperatures of up to 500 °C. Irrespective of target history, no sharpening of the energy spectrum of re-emitted positrons was observed as the remoderator temperature was increased. Further, a steady decrease in the apparent re-emitted positron intensity was observed, attributed primarily to the increase in positronium formation, as the temperature of the foil was increased from 20 to 500 °C. It is concluded that maintaining a moderator or remoderator at elevated temperatures in standard positron beam systems has no beneficial consequences for the quality or intensity of the beam produced. Reasons for this are discussed.

Effect of carbon pre-implantation on deuterium retention in tungsten

The effect of pre-implanted carbon on deuterium retention in tungsten was investigated using polycrystalline tungsten foils. Pre-implantation was performed with 1 keV C + ions to fluences of 10 21 C +/m 2 and 10 22 C +/m 2 at 300 and 500 K. Using this newly created W/C structure, a deuterium retention study with 1.5 keV D + 3 (500 eV/D +) ions was performed as a function of incident D + fluence and implantation temperature. For 1 keV C + implantations to 10 21 C +/m 2 at 300 K followed by 1.5 keV D + 3 implantations to <10 23D +/m 2 at 300 and 500 K, the amount of D retained was reduced by an order of magnitude compared with W specimens without a C + pre-implantation. At higher D +fluences, ⩾10 24 D +/ m 2 , the retention values were equivalent to those of D + on pure tungsten. For C + implantations to 10 21 C +/m 2 at 500 K, the amount of D retained tended to follow the behaviour of the pure W case. Carbon pre-implantations to a higher fluence (10 22 C +/m 2) resulted in D retention values higher than in pure W for D + fluences < 10 23 D +/ m 2 . At higher D + fluences, the retention behaviour differed significantly, depending on the D + implantation temperature. A possible mechanism is suggested to explain the observed results.

Positron re-emission from tungsten surfaces

Positron re-emission properties of 50 μm thick polycrystalline tungsten foil and single crystalline tungsten specimen with (110) surface were measured as a function of successive heat treatment with and without oxygen by a pulsed monoenergetic positron beam. The tungsten specimens after in situ annealing with oxygen have significantly higher positron re-emission fraction and lower positronium fraction than those after in-situ annealing without oxygen. Furthermore, re-emission fraction at high temperatures of 800–1000°C with an oxygen partial pressure of 2×10 −7 Torr is significantly higher than that without oxygen, suggesting that surface oxygen inhibits positronium formation. This feature is good for electron-linac based slow positron beamlines.

The UEA positron re-emission microscope: first images

Images have been recorded using the laboratory-based positron re-emission microscope constructed at the University of East Anglia (UEA). The microscope has been designed to operate in reflection geometry with the view to studying the surface characteristics of ‘real’, thick samples, without extensive preparation procedures. The first, deliberately low-magnification, images are of an annealed, electroformed nickel mesh (linewidth 10 μm, spacing 40 μm) placed over a well-annealed polycrystalline tungsten foil, both before and after annealing the nickel mesh. The false-colour images indicate (a) a magnification of about 80 ×, as expected from the objective lens used, (b) a resolution of about 10 μm, (c) contrast between surfaces with different positron work functions. Image signal rates of about 45 s −1 were recorded. Plans to improve the magnification/resolution of the system are discussed.

A re-emitted positron energy spectrometer

The development of an ultra-high vacuum apparatus for re-emitted positron energy spectroscopy is described. Positrons from a 22Na source are moderated by a 6 mu m thick polycrystalline tungsten foil in transmission geometry and transported by electrostatic optics to the sample, which is held at 45 degrees to the incident beam axis. Positrons re-emitted from the sample into a 2 degrees angular cone about the surface normal pass through an entrance slit in a second set of electrostatic lenses and are transported to a concentric hemispherical energy analyser. Re-emitted positron energy spectra are accumulated using a computer-controlled multi-channel scaler whose ramp voltage output drives the hemispherical analyser control unit. Examples of measurements and an evaluation of system performance are presented and future applications of the instrument are discussed.

Energy spectroscopy of positrons re-emitted from polycrystalline tungsten

Energy spectra of positrons re-emitted from the surface of polycrystalline tungsten foil have been measured as a function of incident positron energy E in the range 100 eV-3 keV. The measurements were made using an electrostatic positron-beam apparatus incorporating a hemispherical energy analyser. At high E values the re-emitted spectra tend to a characteristic shape, dominated by a work-function peak at 2.7 eV with a low-energy shoulder comprising approximately 25% of the total spectrum. As E decreases a high-energy tail attributed to epithermal positrons increases steadily in relative intensity and extent. Below E approximately=500 eV the contribution of the low-energy distribution to the spectra increases significantly, and at 100 eV is dominant. These observations are interpreted in terms of elastic and inelastic processes at the tungsten surface.

Deuterium Transport and Trapping in Polycrystalline Tungsten

Deuterium permeation studies for polycrystalline tungsten foil have been conducted to provide data for estimating tritium transport and trapping in tungsten-clad divertors proposed for advanced fus...

A study of laser-ion-deposited carbon films on tungsten by x-ray diffraction, field ion microscopy, and electron spectroscopy

Thin carbon films have been deposited on polycrystalline tungsten foil as well as field ion microscope (FIM) tips by laser-ion deposition in a high-vacuum environment with an ion extraction voltage of −2 kV. Structural characterization of these films has been carried out by using low-angle x-ray diffraction (XRD) and FIM. The low-angle XRD reveals the formation of an interfacial α-W2C phase. The FIM image indicates the formation of the α-W2C phase on the tungsten tip. X-ray photoelectron spectroscopy has been utilized to reveal that the bonding character in the film is sp3. Further, x-ray-excited Auger electron spectroscopy has also supported the diamondlike nature of the films. The results are discussed, and a sequence of layers deposited on tungsten is suggested in view of the structural match.

The Chemical Etch of Tungsten Single Crystals and Foils With XeF2

AbstractThe chemical etching of W(100) single crystals and polycrystalline tungsten foils using a XeF2 molecular beam has been studied with and without simultaneous bombardment by energetic inert ions.The surface fluorine concentration, measured under different XeF2 pressures (10−7 to 10−5 torr) and ion current densities, is found to be always less than a monolayer.Depth profiles of the fluorine layer show that in both single crystal and poly samples, the fluorine is surface localized, although slightly deeper in the W(100).A kinetic model is presented to account for the enhanced etching under ion bombardment.The mechanism proposed is mainly a defect driven enhancement of the etch yield.This model is consistent with both the fluorine concentrations measured in this work as well as the etch product yields reported by Winters.

The application of SIMS to the study of CO adsorption on polycrystalline metal surfaces

Secondary ion mass spectroscopy (SIMS) was used to study the adsorption of carbon monoxide on polycrystalline nickel, copper, iron, palladium and tungsten foils. The results demonstrate the ability of SIMS to distinguish, qualitatively, between molecular and dissociative adsorption. A correlation between SIMS results and those obtained by infra-red spectroscopy for molecular adsorption is also suggested.

Molecular beam investigation of adsorption kinetics on bulk metal targets: Nitrogen on tungsten

An ultra high vacuum molecular beam technique for the study of the kinetics of the interaction of reactive gases with well-defined metal surfaces is described. Well collimated beams of nitrogen molecules, with an accurately measured intensity of 10 12 to 10 13 molecules cm −2 sec −1 over a target area of ~ 4 × 10 −2 cm 2, are obtained using a series of five chambers differentially pumped with a combination of high speed diffusion pumps and strategically deposited titanium getter films. Out-of-beam leakage into the adsorption cell was < 10 8 molecules sec −1. A method is described for obtaining absolute values of sticking probabilities and surface coverages with a high degree of accuracy. For nitrogen on a polycrystalline tungsten foil, sticking probabilities and their coverage profiles were found to be independent of incident molecular beam angle, over a range of 60°, with an initial sticking probability, s 0, at 300 K of 0.61 ± 0.02. The sticking probability profile is monotonie over the range examined, and is theoretically described in terms of a modified form of Kisliuk's precursor state model, with self-consistent parameters. On a linear plot, the sticking probability extrapolates to zero at a coverage of 3.0(± 0.5) × 10 14 molecules cm 2. On the W(111) plane s 0 was found to be 0.08 ± 0.01, and the sticking probability falls linearly with increasing coverage, with a saturation coverage of 1.2(± 0.2) × 10 14 molecules cm −2. It is argued that adsorption sites on the (111) plane are created by the presence of surface vacancies or randomly distributed W atoms on the perfect (111) surface. Within the limits of detection of the molecular beam technique no nitrogen adsorption could be detected on the (110) plane of tungsten, in agreement with previous work.

Temperature Coefficients of the Work Functions of the (110) and (100) Surfaces of Tungsten Single Crystals and of Polycrystalline Tungsten Foil

A modification of the Kelvin contact potential difference technique has been used to measure the temperature coefficient of the work function of three tungsten surfaces over the temperature range from 77°–1300°K. Two of the surfaces were polished and etched single-crystal planes having the (110) and (100) orientations, the third was a sample of polycrystalline foil. The results obtained were foil(11±1)×10−5 eV/∘K(110)(6.3±0.6)×10−5 eV/∘K(100)(9.1±0.4)×10−5 eV/∘Kand these are in qualitative agreement with predictions based on the temperature dependence of the electron smoothing component of the surface double layer.