Polycrystalline Tungsten Research Articles

Dislocation and amorphous ribbons strengthening in tungsten silicide under high pressure and temperature

AbstractTungsten disilicide (WSi2) is a promising material for high‐temperature applications due to its excellent mechanical properties and superior thermal stability. This study aims to investigate the mechanisms underlying the performance enhancement of polycrystalline WSi2 and tungsten silicide (W‐Si) composites synthesized by high‐temperature and high‐pressure (HPHT) strategies, focusing on their phase composition and microstructure. The results show that high pressure can effectively reduce the synthesis temperature of the tungsten silicide system, with the phase composition and microstructure of the products being dominated by the treatment temperature. The high intergranular strain resulting from the reactive process and the HPHT environment facilitated the formation of high‐density dislocations, including dislocation arrays and networks with multiple orientations, especially near the grain boundaries. Additionally, a 3‐nm thick amorphous ribbons were identified between these boundaries. This microstructure, characterized by high‐density dislocations and amorphous ribbons, confers exceptional mechanical properties and oxidation resistance. The differences in oxidation behavior between powder and bulk WSi2 samples underscore the complex relationship between phase stability and material properties under varying processing conditions. These findings indicate that dislocations and amorphous ribbons can achieve in high‐temperature ceramics with strong covalent bonding, which is essential for the design and synthesis of advanced W‐Si‐based ceramics.

High-resolution emission spectroscopy of W I lines: Comparing near-threshold sputtering of mono- and polycrystalline tungsten by Ar ions

This work presents the first experimental study on the near-threshold sputtering regime for monocrystalline low-index plane tungsten targets investigated using high-resolution emission spectroscopy. We analyzed the line shape emitted by sputtered atoms, which contains information on the angular and velocity distribution functions via Doppler broadening. Specifically, we report changes in the line profile of the resonant W I 498.4 nm transition during plasma exposure of polycrystalline and monocrystalline (100) and (111) tungsten targets at the linear plasma device PSI-2. Biasing the targets from −60 V to −100 V provided low-energy argon ions for near-threshold sputtering. The line shapes, measured along the angle of observation perpendicular to the normal of the sample, were significantly broader for the monocrystalline (100) and (111) compared to that of the polycrystalline target. In particular, the (111) target demonstrates a pronounced heart-shaped profile. The modeling captures this distribution via a ∝ cos(θ)exp(−bθ) function—θ is the polar angle—combined with a parameterized Thompson velocity distribution. Furthermore, comparing the experimental data to molecular dynamics simulations at 100 eV illustrates a reasonable agreement of the angular distribution function with the measurements.

Thermodynamic instability of polycrystalline tungsten exposed to low-energy hydrogen irradiation

The stability of polycrystalline tungsten (W) exposed to low-energy (120 eV) and large-flux (1.5 × 1022 ions/m2⋅s) hydrogen (H) irradiation at low-pressure has been evaluated by using scanning electron microscopy-electron backscatter diffraction, energy dispersive spectroscopy, and conductive atomic force microscopy. Both surface degeneration and formation of ultra-fine W crystal grains have been observed during the annealing in the range of 200–900 °C. The thermodynamic stability of irradiated W is significantly affected by the annealing temperature. Our finding suggests that the serious erosion of polycrystalline W by H irradiation results from the thermodynamic instability of irradiated W while the instability is attributed to the strong diffusion of interstitial W atoms under fusion-relevant irradiation conditions. Our calculation confirms the instability of W atoms close to the H clusters, leading to the formation of self-interstitial W atoms in W crystal.

Technology for monitoring the surface emission inhomogeneity in plasma electronics devices

The article discusses a theoretical and experimental investigation of the reflection of slow electrons from the surfaces of single-crystal and polycrystalline tungsten thermionic cathodes. The findings challenge traditional ideas as they confirm that the effective reflection coefficient, reff, can reach values close to unity contrary to prior belief. The reason for this occurrence has been established, which is the additional reflection of slow electrons from a potential barrier near polycrystalline surfaces. A method has been developed to separately measure electron reflection coefficients at the surfaces of thermionic cathodes and at the potential barrier of electrode spot fields with different work functions. The study reveals that the maximum values of reff are achieved on polycrystalline surfaces. Additionally, the work functions and reflection coefficients rhkl have been determined for the faces of single crystals of (110), (112), (100), (111), and (116) oriented tungsten. The proposed method enables control over cathode emission inhomogeneity and makes it possible to mitigate the negative effects of secondary electron emission by suppressing electric fields near the cathode surface.

Use of molybdenum as a pressure calibrant in a cubic press and sound velocity measurement of tungsten to 5 GPa

ABSTRACT The pressure limit of the conventional cubic press is normally below 6 GPa, and the suitable pressure calibrant of the fixed-point method is scarce. In this study, we developed a new method for pressure determination using an acoustic travel time approach. Based on the reported ultrasonic travel times of polycrystalline molybdenum under high pressure in literature, an acoustic gauge of polycrystalline molybdenum was derived. The shear wave travel times of molybdenum were measured at high pressures and utilized for the pressure calibration in a cubic press. The pressures derived from this new travel time method are in excellent agreement with those from the fixed-point method. Subsequent compressional and shear wave velocities of polycrystalline tungsten were measured up to about 5 GPa at room temperature to further verify this method. This travel time method for pressure calibration is valuable for the sound velocity measurement of other materials in an offline laboratory.

Spinodally reinforced W-Cr fusion armour

Here, we introduce a discontinuous spinodal reinforcement strategy in the novel candidate plasma facing material (PFM) of tungsten-chromium alloys. Thermal ageing of a W-34wt%Cr alloy at 1250 °C causes nano-scale lamellae 200–600 nm to form heterogeneously from grain boundaries, which progressively grow into the matrix fully, then coarsen to 1–2 µm after 100 h. The dual-phase microstructure confers exceptional high temperature compressive strength, maintaining 900 MPa at 1000 °C - double that of polycrystalline tungsten. Further, the chromium alloying promotes a dense oxide scale that confers a 2 orders of magnitude improvement in resistance against oxidation at 1000 °C compared to W, which is an important consideration for PFMs under loss of vacuum accident conditions. The dual-phase W-Cr alloy concept's combination of high strength and oxidation resistance represents a new scalable alternative to tungsten, with wide scope for further alloying and process optimisation.

Revealing the role of oxygen on the defect evolution of electron-irradiated tungsten: A combined experimental and simulation study

The evolution of Frenkel pairs has been studied experimentally and theoretically in tungsten, a Body-Centered Cubic metal. We used positron annihilation spectroscopy to characterize vacancy defects induced by electron irradiation in two sets of polycrystalline tungsten samples at room temperature. Doppler Broadening spectrometry showed that some positrons were trapped at pure single vacancies with a lower concentration than expected. At the same time, positron annihilation lifetime spectroscopy revealed that positrons are annihilated in unexpected states with a lifetime 1.44–1.64 times shorter than that of single vacancy (200 ps), namely unidentified (X) defects. Secondary ions mass spectrometry detected a significant concentration of oxygen in these samples, of the same order of magnitude as electron-induced single vacancy. In addition, Cluster dynamics simulated defect behaviors under experimental conditions, and Two-component density functional theory was used to calculate defect annihilation characteristics that are difficult to obtain in experiments. Finally, by combining the theoretical data, we simulated the positron signals and compared them with the experimental data. This enabled us to elucidate the interactions between oxygen and Frenkel Pairs. The X defects were identified as oxygen-vacancy complexes formed during irradiation, as oxygen is mobile in tungsten at room temperature, and can be trapped in a vacancy, while its binding to self-ion atoms leads to their immobilization thus reducing defect recombination. Therefore, we anticipate oxygen to play an important role in the evolution of tungsten microstructure under irradiation.

Analyzing point defect polarization in tungsten and tungsten carbide under high gamma irradiation for radiation shielding applications

In this study, polycrystalline tungsten and tungsten carbide samples were irradiated with gamma rays at energies of 1.17 MeV and 1.33 MeV, respectively. The irradiation was performed using an “MRX-γ-25 M” radiation camera with an activity of approximately 6.54 Gy/s. Doppler positron spectroscopy (DPS) and positron lifetime spectroscopy (PALS) were utilized to investigate the mechanisms of defect formation in the polycrystalline structure of tungsten samples at different doses of gamma irradiation. In the initial tungsten sample, a significant number of free monovacancy (1 V) clusters were present. An increase in the positron lifetime component τ1 was observed with increasing gamma irradiation, accompanied by a decrease in its intensity. The τ2 value (218 ± 2 ps) suggests the presence of divacancies with a considerable intensity (∼20%). As the gamma dose accumulated, a dynamic evolution of structural defects was evident. The tungsten carbide (WC) samples displayed greater plasticity in response to increasing gamma irradiation doses. Changes in the void volume ratio within the samples were recorded as the gamma dose increased, and movement of these voids towards the surface was observed. Additionally, the operational limits of gamma-irradiated tungsten were assessed, determining a functional threshold of up to 3.378 MGy. This study provides valuable insights into the defect dynamics and structural changes in tungsten and tungsten carbide under gamma irradiation, contributing to the understanding of their behavior in radiation environments.

Atomic sawtooth-like metal films for vdW-layered single-crystal growth

Atomic sawtooth surfaces have emerged as a versatile platform for growth of single-crystal van der Waals layered materials. However, the mechanism governing the formation of single-crystal atomic sawtooth metal (copper or gold) films on hard substrates (tungsten or molybdenum) remains a puzzle. In this study, we aim to elucidate the formation mechanism of atomic sawtooth metal films during melting–solidification process. Utilizing molecular dynamics, we unveil that the solidification of the liquid copper initiates at a high-index tungsten facet with higher interfacial energy. Subsequent tungsten facets follow energetically favourable pathways of forming single-crystal atomic sawtooth copper film during the solidification process near melting temperature. Formation of atomic sawtooth copper film is guaranteed with a film thickness exceeding the grain size of polycrystalline tungsten substrate. We further demonstrate the successful growth of centimeter-scale single-crystal monolayer hexagonal boron nitride films on atomic sawtooth copper films and explore their potential as efficient oxygen barrier.

Pressure and temperature dependent ab-initio quasi-harmonic thermoelastic properties of tungsten

We present the ab-initio temperature and pressure dependent thermoelastic properties of body-centered cubic tungsten. The temperature dependent quasi-harmonic elastic constants (ECs) are computed at several reference volumes including both the phonon and the electronic excitations contribution to the free energy and interpolated at different temperatures and pressures. Good agreement with the experimental ECs on a single crystal at ambient pressure is found. The pressure and temperature dependence of the shear sound velocity measured on polycrystalline tungsten by Qi et al is also in agreement with theory. Some discrepancies are found instead for the compressional velocity at high temperature and this is attributed to the temperature derivative of the bulk modulus, higher in theory than in experiment. These conclusions are reached both by PBE and by PBEsol functionals. The two give elastic properties with a similar pressure and temperature dependence although the latter is closer to experiment at 0 K.

Correlation of Structural Changes and Hydrogen Diffusion in Polycrystalline WO3 Thin Films by Combining In Situ Transmission Measurements and Raman Spectroscopy

AbstractDiffusion in polycrystalline tungsten trioxide (WO3) thin films is studied in a lateral geometry to better understand the impact of hydrogen‐induced structural phase transitions on the diffusion. WO3 thin films are coated with polymethylmethacrylate layer (PMMA). The latter is microstructured in such a way that a narrow stripe‐like gap occurs in the PMMA layer exposing the surface of the WO3 thin film. This stripe serves as the contact to the electrolyte in the intercalation experiment with hydrogen. After intercalation, the lateral diffusion of hydrogen inside WO3 below the PMMA layer can be observed, increasing the analyzable path and time scale by several orders of magnitude compared to the film thickness, thus, significantly improving spatial and temporal resolution of in situ transmission and Raman measurements. Spatially resolved transmission measurements in the wavelength range of 633±55 nm show that the diffusion process is dependent on hydrogen concentration and exhibits two regimes describable by different diffusion coefficients. Time‐resolved Raman spectroscopic measurements at different distances from the electrolyte contact area show that the switching between the two diffusion coefficients occurs at the phase transition from the orthorhombic to the tetragonal phase. The results are further supported by a simulation. The measurement approach is universally applicable for electrochromic films or multilayers.

Tungsten fiber-reinforced tungsten composites and their thermal stability

Tungsten will be used as armor material for plasma-facing components in future fusion reactors, but its propensity to embrittlement by microstructural restoration at high temperatures poses a challenge for its use. Tungsten fiber-reinforced tungsten composites (Wf/W) with drawn tungsten wires embedded in a polycrystalline tungsten matrix remedy the inherent brittleness of tungsten and achieve pseudo-ductile behavior via extrinsic toughening mechanisms. As plasma-facing materials experience high heat fluxes during operation, their thermal stability is important. In Wf/W composites, the restoration processes at high temperatures differ significantly between wires and matrix: initially recrystallization dominates in the wires, as they were plastically deformed during wire drawing, whereas abnormal grain growth occurs in the matrix. Growing grains may obstruct the interface between wire and matrix and deteriorate the otherwise improved fracture properties of Wf/W. An yttria interlayer is introduced to separate wire from matrix, to hinder an interplay between the restoration processes and to impede grains from the wire from growing into the matrix and vice versa. Cylindrical model systems containing a single wire in a chemically vapor-deposited matrix are investigated without any interlayer and with an yttria interlayer of either 1 μm or 3 μm thickness. Isothermal annealing at 1450°C for different times up to 2 weeks, followed by microstructural characterization by means of EBSD are carried out to characterize the microstructural evolution. The role of the interlayer on the microstructural evolution is elucidated to establish if decoupling of the restoration processes is actually achieved.

Damage evolution and plastic deformation mechanism of passivation layer during shear rheological polishing of polycrystalline tungsten

Aiming at the problems of low processing efficiency and uneven material removal during shear rheological polishing (SRP) of polycrystalline tungsten, this paper introduces the Fenton-like reaction between H2O2 and Cu2+ to enhance the chemical effect on material removal. Passivation behavior and removal mechanisms were investigated using electrochemical tests, scanning electron microscopy, atomic force microscopy, and other characterization methods. Results show that the Fenton-like reaction exhibits excellent activity under the combined action of H2O2 and Cu(NO3)2, obtaining a satisfactory material removal rate of 702.22 Å⋅min−1 and a surface roughness Rq of 7.05 nm. The efficacy of the H2O2+Cu(NO3)2 slurry lies in its superior removal and passivation performance, attributed to the generation of hydroxyl radicals through the Fenton-like reaction. These radicals facilitate the formation of a WO3 passivation layer with good corrosion resistance on the surface, while enabling easy removal through mechanical wear. In addition, the plastic deformation mechanism of passivation layer was also studied by nanoscratch tests and transmission electron microscopy. It is found that a pure plastic flow groove can be obtained in the load range of 3–100 mN, accompanied by the formation of asymmetric deformation and pile-ups in the subsurface of the main plastic zone.

Emissive cathode immersed in a plasma: plasma–cathode interactions, operation and stability

Thermionic emission from a polycrystalline tungsten emissive cathode immersed in a magnetized plasma column is investigated experimentally and numerically. Electrical and optical measurements of the cathode temperature show a highly inhomogeneous cathode temperature profile due to plasma–cathode interactions. The spatially and temporally resolved cathode temperature profile provides an in-depth understanding of the thermionic electron current, in excellent agreement with experimental data. The plasma-cathode coupling leads to a sharp and heterogeneous rise in temperature along the cathode, which can eventually lead to unstable cathode operation, with divergent current growth. A detailed thermal modeling accurately reproduces the experimental measurements, and allows to quantify precisely the relative importance of heating and cooling mechanisms in the operation of the cathode immersed in the plasma. Numerical resolution of the resulting integro-differential equation highlights the essential role of heterogeneous ohmic heating and the importance of ion bombardment heating in the emergence of unstable regimes. Detailed thermal modelling enables operating regimes to be predicted in excellent agreement with experimental results.

Space-resolved line shape model for sputtered atoms of finite-size targets

High-resolution emission spectroscopy provides valuable information on the physical sputtering process during plasma-wall interaction. Up to now, analyzing the observed spectral lines during sputtering did not account for the finite size of the targets. It becomes crucial if the size of the target becomes comparable with the distance the sputtered atoms travel before emitting the photons. So, for example, the generally used standard emission model based on an infinite target or the point source approximation breaks for observations using two lines of sight: parallel and perpendicular to the normal of the target. It is impossible to achieve consistent results for energy and angular distribution of sputtered atoms. The new space-resolved emission model for finite-size targets developed in this work removes this gap. It incorporates the space-velocity transformation for the distribution function and includes the finite lifetime of excited states. The model was validated using emission spectra of sputtered atoms from a polycrystalline tungsten sample bombarded by monoenergetic Ar+ with kinetic energies of 100 eV to 140 eV at normal incidence in the linear plasma device PSI-2. Using the new model enables the simultaneous fitting of the line shapes of sputtered tungsten for both observation angles. The optimization process is performed using the standard Thompson distribution by separating the energy-dependent parameter and the angular distribution.

Effects of grain boundaries on the evolution of radiation-induced bubbles in polycrystalline tungsten: A phase-field simulation

Tungsten (W) is widely recognized as a promising material for plasma-facing components in nuclear fusion reactors, and the evolution of bubbles in polycrystalline tungsten significantly impacts its properties. This study developed a phase-field model considering elastic inhomogeneity to investigate the evolution of radiation-induced bubbles. The Gibbs free energy of helium bubbles was derived using the Van der Waals equation. An explicit nucleation algorithm accounting for the position of grain boundaries (GBs) was coupled to the model. Furthermore, the model also considered the inequality between the internal pressure of the bubble and the surface tension. The growth kinetics of a single bubble with elastic heterogeneity were examined, and based on these findings, the evolution of bubbles under irradiation was simulated, yielding results consistent with experimental observations. Furthermore, the influence of GBs on radiation-induced bubbles was investigated using the developed model. The simulation results demonstrated that both GBs, as preferential nucleation regions and as sinks for vacancy absorption, had an impact on bubble nucleation and growth. The superior radiation resistance observed in nanocrystalline tungsten was dominantly attributed to the absorption of vacancies by GBs.

Defective Engineering Tuning the Analog Switching Linearity and Symmetry of Two‐Terminal Artificial Synapse for Neuromorphic Systems

AbstractNeuromorphic computing inspired by memristors has gained considerable attention due to its low power and easy integration. However, state‐of‐the‐art two‐terminal resistive switching memristors based on conductive filament formation suffer from high variability and poor controllability. As a three‐terminal device operated through electrochemistry and dynamic insertion/extraction of ions, the electrochemical ion synapse demonstrates deterministic control of electron conductivity based on ion doping. But, integrating the electrochemical ion synapse into crossbar arrays will pose higher challenges and lower integration density. Herein, inspired by first‐principles calculations, a two‐terminal bidirectional plasticity electrochemical artificial synapse with integrated lithium polymer electrolyte and polycrystalline tungsten oxide layer is reported. The linearity and stability of the device weight update are greatly improved by adjusting the defect concentration of the polycrystalline WO3 layer. Even after 16 000 write‐read events in the air, its performance remained almost unchanged. Moreover, it has an over‐limit protection mechanism under one‐way stimulation that exceeds the normal range. Based on this excellent stability, the authors designed and successfully simulated the “muscle memory” that the programmatical organization of the nervous system leads to proficiency in specific actions.

Gas sensing performance of Tungsten doped V2O5 nanorod thin-films deposited by hot filament CVD combined with DC sputtering

High-quality polycrystalline tungsten doped vanadium pentoxide thin-films were prepared using Hot Filament Chemical Vapor Deposition technique (HFCVD) combined with sputtering by changing target to substrate distance and reaction duration. Scanning electron microscopy and X-ray diffraction revealed that the morphologies and crystalline structures of the films are depending on the deposition parameters such as target to substrate distance and deposition time. Raman spectroscopy is also used to analyse the oxidation states of the vanadium-based films. The deposited nanorod thin-film was exposed to ammonia and ethanol gases to test their gas sensing performance. It exhibits a sensing response of 1.8 and 1.65 at a concentration of 200 ppm ammonia and ethanol and has a recovery time of 44 and 31 s, respectively.

Numerical and experimental investigation of thermal regimes of thermionic cathodes of arc plasma torches

The modelling method based on decoupling the simulation of the cathodic part of the arc (the cathode and the near-cathode non-equilibrium plasma layer) from the simulation of the arc on the whole has been extended to cathodes of arc plasma torches, consisting of an insert with a conical tip, made of pure or doped tungsten, and a surrounding water-cooled copper holder. The method was validated by comparison with the experiment, performed on a 200 A DC arc in atmospheric-pressure argon. Standard work function of polycrystalline tungsten of 4.54 eV was used for modelling of pure-tungsten insert and a good agreement with the experiment was found with respect to both the insert tip shape and the temperature distribution in the tip, recorded in the stable operation mode. There are no unambiguous data on the work function for arc cathodes made of doped tungsten, although in situ measurements of the effective work function of cathodes of high-pressure arc discharges provide useful hints. On the other hand, the experiments reported in this work show that the tip temperatures of inserts made of tungsten doped with of thorium, or lanthanum, or yttrium, recorded during the stable-mode operation at the arc current of 200 A, vary in a rather narrow range 3100–3200 K. This suggests that the work functions of doped tungsten inserts, operated in the stable mode, are close to each other as well. Indeed, the results of modelling with the same value of the work function of 3 eV give a reasonably good agreement with the experiment in all three cases.

Degradation of electrical resistivity of tungsten following shielded neutron irradiation

A major challenge for heat transfer in nuclear materials is to ensure thermal mobility after high amounts of neutron irradiation. Tungsten is widely selected as a heat transfer material in fusion reactors. In metals, thermal conductivity is dominated by electrons’ ability to transfer energy. Neutron irradiation generates point defects, clusters, and solid transmutation (e.g.rhenium and osmium in tungsten), which inhibit electron motion. The purpose of this work is to quantify the irradiation-induced change in electron mobility and deconvolute transmutation and microstructural effects on observed changes to electron mobility. Single and polycrystalline tungsten were fast neutron irradiated in the High Flux Isotope Reactor at Oak Ridge National Laboratory to doses between 0.2 and 0.7 displacements per atom (dpa) and temperatures from 500 °C to 1000 °C. Grain growth was observed in all samples. Microstructure and transmutation were quantified. The geometric orientation of samples with elongated grains has been shown to affect electrical resistivity. A mathematical model was developed and used to deconvolute solid-solution transmutation, grain, and temperature-dependent lattice effects on resistivity. At ∼0.4 dpa at ∼590 °C, the combined resistivity degradation due to voids, vacancies, interstitials, and dislocations is estimated to be greater than the contribution from solid solution Re transmutation, which is greater than the contribution from grain boundaries. At doses of ∼0.7 dpa at ∼750 °C, solid solution Re contributions are greater than all other effects combined. This work establishes a basis to predict the effects of irradiation temperature and transmutation on thermal properties of tungsten and highlights the importance of irradiation temperature.