Tungsten Composites Research Articles

Tensile deformation behavior of tungsten fibre-reinforced tungsten composite specimens in as-fabricated state

To overcome the inherent brittleness of tungsten, which is a promising candidate for a plasma-facing material in a future fusion device, tungsten fibre-reinforced tungsten composites (Wf/W) have been developed. As a part of the materials characterisation program on Wf/W, we present the results of first tensile tests of as-fabricated Wf/W in this contribution. The results give insight on the ultimate tensile strength properties and reveal the active toughening mechanisms under tension load within the composite. Fibre bridging, fibre necking as well as fibre pull out were observed. This is leading to the typical pseudo ductile behavior of the composite which is characterized by a rising load bearing capability despite multiple matrix cracks accompanied by non catastrophic crack propagation (in the matrix). The description of the mechanical tests is supplemented by detailed microstructural investigations.

Die Technology and Construction for Compacting Gas-Saturated Tungsten and Carbon Powders

Analysis is provided for known and new technology and construction of dies for molding gas-saturated powders of tungsten and carbon powder composites made on the basis of YuUrGU scientist inventions. Gas saturation of tungsten powder is connected with the unusually low bulk density equal to 0.06–0.12 of the density of pore-free tungsten, and small powder particle size within the limits of 1–3 μm. High gas saturation of powder composites based on carbon is determined by the high capacity of carbon for adsorption of gas and liquid vapor molecules at particle surfaces.

Fabrication and current-voltage characteristics of Mo1 − xWxS2/graphene oxide heterojunction diode

Tungsten-substituted molybdenum disulfide (Mo1−xWxS2) shows excellent semiconductor properties related to the adjustable band gap structure. Mo1−xWxS2 is an n-type semiconductor that has high carrier mobility, adjustable electrical characteristics. Graphene, the thinnest two-dimensional material, shows good thermal conductivity and high carrier mobility. The oxygen adsorption effects of graphene permit precise control of its Fermi level and semiconductor properties. We combined Mo1−xWxS2 films with graphene oxide (GO) in this study to fabricate a pn interface and thoroughly research the energy band and electrical properties for a heterojunction diode. According to the results, the Mo1−xWxS2 electron affinity decreases with increasing x value of tungsten (W) composition (x=0.0–1.0, Δx=0.2). The I–V characteristics of the Mo1−xWxS2/GO heterojunction diodes can be accurately tuned by conduction band bowing effect with different W composition of the Mo1−xWxS2 films. By applying our fabricating method, the Mo1−xWxS2 with different W composition provides a new application of semiconductor device. In this study we using Mo1−xWxS2 films and GO to fabricate a heterojunction diode, it shows different electrical properties with different x value of Mo1−xWxS2. The Mo1−xWxS2 electron affinity decreases with increasing of W composition x value (x=0.0–1.0, Δx=0.2). The Mo1−xWxS2/GO heterojunction diode I–V characteristics can be accurately tuned using the conduction band bowing effect with different W compositions. Mo1−xWxS2 with different W compositions using our fabrication method provides new semiconductor device applications.

Effect of Tungsten Addition on the Anti-fouling Property of the Electroless Ni-W-P Deposits

Abstract Ternary Ni-W-P deposits were prepared on the mild steel (1015) substrate using electroless plating by varying sodium tungstate concentration. Surface morphology, microstructure, and the microhardness of electroless Ni-W-P deposits were investigated by scanning electron microscopy (SEM), X-ray diffraction (XRD) and a MH-6 Vickers diamond indenter, respectively. The results demonstrate that the bath concentration plays a significant role in obtaining ternary Ni-W-P deposits containing various composition of phosphorus and tungsten, which decides the surface morphology and the structure. Although nodular formation is a common feature of electroless Ni-W-P deposits, the increase of tungsten content decreases the phosphorus content of these deposits and hence changes the composition of the nanocrystalline phase. The co-deposition of tungsten in the deposits increases the microhardness due to the solid solution strengthening of nickel induced by tungsten in the deposits which resists regional plastic deformation. Moreover, the results of differential scanning calorimetry (DSC) show that Ni-W-P deposits with high tungsten content exhibit higher crystallization temperature. Further fouling experiments indicate that the Ni-W-P deposit surfaces with different tungsten contents inhibit the adhesion of fouling compared with the mild steel surface. The experiments also indicate that the fouling adhesion rate is intimately related to the tungsten content. However, the relationship between the fouling adhesion rate and surface roughness of the ternary Ni-W-P deposits is not found in these experiments.

Investigation of microstructure and irradiation behavior of W–Nb/Ti composites prepared by spark plasma sintering

W–Nb/Ti composites were prepared by machine milling and spark plasma sintering. Field–emission scanning electron microscopy, high–resolution transmission electron microscopy, and X-ray diffraction analyses were used to characterize the samples. Thermal desorption spectroscopy (TDS) was used to measure the deuterium retention after only deuterium irradiation. The deuterium retention in W–4wt%Ti–wt1%Nb was lower than that in pure W (D2+ flux ∼1022 ions/(m2s), ion energy ∼5keV), Nb and Ti will improve the deuterium irradiation resistance of tungsten composite. In addition, transgranular and intergranular fractures were observed on the surface of the W–4wt% Ti–1wt% Nb composite. A Ti(X)W(1−X) solid solution was formed during sintering. The maximum tensile strength (410.53MPa) was detected in the W–4wt%Ti–1wt%Nb composite at 600°C.

Properties of drawn W wire used as high performance fibre in tungsten fibre-reinforced tungsten composite

High strength and creep resistance also at high temperature, combined with a high thermal conductivity and high melting point make tungsten (W) an ideal material for highly loaded areas in future fusion reactors. However, as a typical bcc metal tungsten features an intrinsic brittleness up to very high temperature and is prone to operational embrittlement. Tungsten fibre-reinforced tungsten composite (Wf/W) utilizes extrinsic toughening mechanisms similar to ceramic fibre-reinforced ceramics and therefore overcomes the brittleness problem. The properties of the composite are to a large extend determined by the properties of the drawn tungsten wire used as reinforcement fibres. W wire exhibits a superior strength and shows ductile behaviour with exceptional local plasticity. Beside the typical mechanisms observed for ceramic composites the ductile deformation of the fibres is therefore an additional very effective toughening mechanism. Tension tests were used to investigate this phenomenon in more detail. Results show that there is a region of enhanced localized plastic deformation. The specific energy consumption in this region was estimated and used to suggest optimisation options for Wf/W composites.

Electrode erosion and lifetime performance of a triggered corona-stabilized switch in SF6at a repetition rate of 1 kHZ

This paper describes the work undertaken to investigate the electrode erosion and lifetime performance of an electrically triggered, corona stabilized (TCS) switch. The switch, which had a rod-plane geometry, was filled with SF6 at a pressure of 0.8 bar (1.0bar = 100kPa) absolute and was operated at a pulse repetition frequency (PRF) of 1kHz. The erosion rates of anode and cathode electrode materials such as elkonite (a copper tungsten composite), brass, aluminum and two types of stainless steel was measured, and their surfaces were studied optically, photographed and chemically analyzed. The erosion rate of the anode (rod) electrode materials ranged from 21.39 x 10-6 cm-3C-1 for elkonite to 60.4 x 10-6 cm-3C--1 for brass, whilst a cathode (trigger disc) erosion rate as high as 51.23 x 10-6cm-3C--1 for aluminum was measured. The lifetime of the TCS switch was experimentally determined for each of the electrode material tested, and was found to be inversely proportional to the erosion rate of the rod electrode. The electrode surface morphology and the gas-electrode compound distribution have also been evaluated to address the erosion mechanism in effect. Additionally, the influence of rod electrode surface conditions and gas deterioration has been investigated in respect to the performance and operational behavior of the TCS switch.

Chemically deposited tungsten fibre-reinforced tungsten – The way to a mock-up for divertor applications

The development of advanced materials is essential for sophisticated energy systems like a future fusion reactor. Tungsten fibre-reinforced tungsten composites (Wf/W) utilize extrinsic toughening mechanisms and therefore overcome the intrinsic brittleness of tungsten at low temperature and its sensitivity to operational embrittlement. This material has been successfully produced and tested during the last years and the focus is now put on the technological realisation for the use in plasma facing components of fusion devices. In this contribution, we present a way to utilize Wf/W composites for divertor applications by a fabrication route based on the chemical vapour deposition (CVD) of tungsten. Mock-ups based on the ITER typical design can be realized by the implementation of Wf/W tiles. A concept based on a layered deposition approach allows the production of such tiles in the required geometry. One fibre layer after the other is positioned and ingrown into the W-matrix until the final sample size is reached. Charpy impact tests on these samples showed an increased fracture energy mainly due to the ductile deformation of the tungsten fibres. The use of Wf/W could broaden the operation temperature window of tungsten significantly and mitigate problems of deep cracking occurring typically in cyclic high heat flux loading. Textile techniques are utilized to optimise the tungsten wire positioning and process speed of preform production. A new device dedicated to the chemical deposition of W enhances significantly, the available machine time for processing and optimisation. Modelling shows that good deposition results are achievable by the use of a convectional flow and a directed temperature profile in an infiltration process.

Effects of Composition and Fe on the Liquid Phase Sintered W-Brass Composites

In this study, W-brass with the composition of tungsten (W) ranging from 50 – 70 wt. % and the balance is brass and other samples with 1 wt. % Fe activator added to the above W and brass composition were both liquid phase sintered at 11500C. The effects of composition and Fe activator on the densification, microstructure and hardness were investigated. As a result of Zn evaporation, compacts expansion, porosity and mutual insolubility between W andbrass, the sintered densities of all the samples were lower than their green densities. Though the Fe activated samples equally expanded, their relative sintered densities were higher than the non-activated samples. Optimum densification was obtained in the samples with 40 – 50 wt. % brass). The porosity in the sintered samples ranges from 24.09% - 27.51%. The hardness increases with the increase in the volume fraction of W. The general low hardness values are a direct consequence of low sintered densities.

Toughness and microstructure of tungsten fibre net-reinforced tungsten composite produced by spark plasma sintering

To enhance the toughness of tungsten based on the extrinsic energy dissipation mechanism, tungsten fibre net-reinforced tungsten composites (W/Wf) containing different tungsten fibre net layers were produced by spark plasma sintering method (SPS). The number of tungsten fibre net layers is 2, 4, 6, and 8, corresponding to a fraction of tungsten fibres as 11.35, 23.90, 32.78, and 39.75wt%, respectively. The relative density of all SPSed samples is as high as 96.3–98.1% depending upon the fibre content. Close integration at low fibre content but a few voids at higher fibre content were detected between the fibres and matrix. Tensile tests indicate that the plasticity of W/Wf composites is higher than that of pure tungsten, which can be ascribed to the fibres pullout, interface debonding, and plastic deformation of fibres. In particular, the composite with four layers of tungsten fibre net exhibits the largest plasticity owing to the good bonding between the fibres and matrix.

Chemical synthesis and characterization of poly(3,4-ethylenedioxythiophene)/tungsten composite materials in the presence of ionic liquids

Poly(3,4-ethylenedioxythiophene)/tungsten (PEDOT/W) composites were prepared by an in situ chemical oxidative polymerization of 3,4-ethylenedioxythiophene in different ionic liquids; 1-butyl-3-methylimidazolium tetrafluoroborate (BMIMBF4), 1-butyl-3-methylimidazolium hexafluorophosphate (BMIMPF6), 1-butyl- 3-methylimidazolium bis(trifluoromethylsulfonyl) imide (BMIMTFSI), and 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl) imide (BMPTFSI). These polymer/metal hybrids (PEDOT/W) were subsequently characterized for their structural, crystalline, thermal and morphological properties by Fourier-transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), thermogravimetric analysis (TGA) and scanning electron microscopy (SEM). The significant enhancement in properties can be attributed to the nanoscale particle size and uniform size distribution of PEDOT/W and the synergistic effect between the inorganic nano-W and organic PEDOT material.

Development of tungsten fibre-reinforced tungsten composites towards their use in DEMO—potassium doped tungsten wire

For the next step fusion reactor the use of tungsten is inevitable to suppress erosion and allow operation at elevated temperature and high heat loads. Tungsten fibre-reinforced composites overcome the intrinsic brittleness of tungsten and its susceptibility to operation embrittlement and thus allow its use as a structural as well as an armour material. That this concept works in principle has been shown in recent years. In this contribution we present a development approach towards its use in a future fusion reactor. A multilayer approach is needed addressing all composite constituents and manufacturing steps. A huge potential lies in the optimization of the tungsten wire used as fibre. We discuss this aspect and present studies on potassium doped tungsten wire in detail. This wire, utilized in the illumination industry, could be a replacement for the so far used pure tungsten wire due to its superior high temperature properties. In tensile tests the wire showed high strength and ductility up to an annealing temperature of 2200 K. The results show that the use of doped tungsten wire could increase the allowed fabrication temperature and the overall working temperature of the composite itself.

High Temperature Thermo-mechanical Properties of HfC Reinforced Tungsten Matrix Composites

In order to improve the mechanical properties of tungsten at room and elevated temperature, hafnium carbide (HfC) reinforced tungsten matrix composites were prepared using the spark plasma sintering technique. The effect of HfC content on the compressive strength and flexural strength of the tungsten composites was investigated. Mechanical properties of the composites were also measured at elevated temperatures and their trends, with varying reinforcement volume fraction, were studied. The effect of reinforcement fraction on the thermal properties of the composites was investigated. The thermal conductivity and diffusivity of the composites decreased with increasing temperature and reinforcement volume fraction. An inherently low thermal conductivity of the reinforcement as well as interfacial losses was responsible for lower values of thermal conductivity of the composites. Values of coefficient of thermal expansion of the composites were observed to increase with HfC volume fraction.

Role of interface tailoring by Cu coating carbon nanotubes to optimize Cu–W composites

Abstract

Surfacing With Tungsten-containing Ores

Experimental research has been carried out into AN 26C flux surfacing with flux cored wires made of dust-like carbon and fluorine containing components and tungsten- containing ores. The investigations have revealed the fact that tungsten can be reduced immediately from tungsten-containing oxide materials. The chemical composition of added metal has been determined, as well as slag compositions after surfacing; hardness, wear resistance have been investigated and metallographic tests have been carried out. Thermodynamic calculations of reactions of WO3, as well as W2C and WC carbides reduction to W by carbon and carbon oxide have been made in standard conditions at temperature T = 1500 - 6000 K. The obtained thermodynamic characteristics of reactions have demonstrated that in the course of WO3 reduction tungsten is most likely to be obtained in terms of thermodynamics, subsequently W2C and WC carbides. After these reactions those of W2C and WC carbides obtaining are thermodynamically possible via reduced tungsten and carbon composition.

Effect of reinforcement phase on the mechanical property of tungsten nanocomposite synthesized by spark plasma sintering

Nanostructured tungsten composites were fabricated by spark plasma sintering of nanostructured composite powders. The composite powders, which were synthesized by mechanical milling of tungsten and Ni-based alloy powders, are comprised of alternating layers of tungsten and metallic glass several hundred nanometers in size. The mechanical behavior of the nanostructured W composite is similar to pure tungsten, however, in contrast to monolithic pure tungsten, some macroscopic compressive plasticity accompanies the enhanced maximum strength up to 2.4GPa by introducing reinforcement. We have found that the mechanical properties of the composites strongly depend on the uniformity of the nano-grained tungsten matrix and reinforcement phase distribution.

Simple preparation of tungsten supported carbon nanoreactors for specific applications: Adsorption, catalysis and electrochemical activity

Porous carbon supported tungsten carbide nanoreactors, two sizes (∼25 and 170nm), were designed using economical petroleum pitch residue followed by tungsten (W) doping. X-ray diffractions showed both carbon tungsten composites (CTC-25 and CTC-170) contained tungsten subcarbide (W2C) and monocarbide (WC) as the major and minor crystalline phases, respectively. The present study provides a multiple perspective of carbon tungsten composites (CTCs) for methanol oxidation (as an electrode), adsorption (as an adsorbent) and degradation (as a solid catalyst) of methylene blue (MB). The operational electrodes were designed from both CTCs and used as a catalyst in an electrocatalysis process. The electrocatalysts exhibited high and stable catalytic performance (CTCE-25>CTCE-170) in methanol electro-oxidation. The newly synthesized W-doped carbon nanoreactors were used successfully as an adsorbent for MB and a heterogeneous catalyst for MB oxidation. Ordered CTC-25 and CTC-170 exhibited dynamic MB adsorption within 15min and complete oxidation of MB in 25–40min. A synergetic effect between tungsten carbide and the carbon cage framework was noted.

Recent Progress in Processing of Tungsten Heavy Alloys

Tungsten heavy alloys (WHAs) belong to a group of two-phase composites, based on W-Ni-Cu and W-Ni-Fe alloys. Due to their combinations of high density, strength, and ductility, WHAs are used as radiation shields, vibration dampers, kinetic energy penetrators and heavy-duty electrical contacts. This paper presents recent progresses in processing, microstructure, and mechanical properties of WHAs. Various processing techniques for the fabrication of WHAs such as conventional powder metallurgy (PM), advent of powder injection molding (PIM), high-energy ball milling (MA), microwave sintering (MW), and spark-plasma sintering (SPS) are reviewed for alloys. This review reveals that key factors affecting the performance of WHAs are the microstructural factors such as tungsten and matrix composition, chemistry, shape, size and distributions of tungsten particles in matrix, and interface-bonding strength between the tungsten particle and matrix in addition to processing factors. SPS approach has a better performance than those of others, followed by extrusion process. Moreover, deformation behaviors of WHA penetrator and depleted uranium (DU) Ti alloy impacting at normal incidence both rigid and thick mild steel target are studied and modelled as elastic thermoviscoplastic. Height of the mushroomed region is smaller for α=0.3 and it forms sooner in each penetrator as compared to that for α=0.2.

Specific Features of Mechanism for Dust Production from Tungsten Armor under Action of ELMs

Analysis of comprehensive investigations of divertor armor erosion under action of edge-localized modes (ELMs) using the PEGASUS-3D code has been performed. Different erosion mechanisms for tungsten (W) and carbon fiber composite (CFC) armor materials have been revealed. This difference explains almost 3 orders of magnitude difference in the erosion rates of CFC and W. Investigation of the crack pattern evolution under repetitive ELM-like surface heat load has been done. Special treatment for tungsten armor tiles has been proposed for reduction of dust production rate in ITER operation regime.

Modified sol-gel synthesis of ultradispersed powders of cobalt, nickel, molybdenum, and tungsten oxides and composites based on them

An approach is proposed to the creation of laboratory technology of ultradispersed VI and VIII groups metal oxide powders, as well as composites based on them, intended to provide a broad range of fine-grained ceramics. The study covers the modified sol-gel synthesis of cobalt, nickel, tungsten, and molybdenum oxides powders and composites based on them by MeVIIIMeVIO x -type having a particle size of 10 to 1000 nm. The influence of the reaction mixture composition on the phase composition of such binary systems was shown. The conditions of obtaining of metal oxide powders with certain dispersions were investigated: the molar ratio stabilizer/metal, thermal treatment mode, etc. The effect of various sol stabilizers, hexamethylenetetramine, N,N-dimethyloctylamine, and tetraethylammonium hydroxide, on the dispersity of metal oxide powders was shown. All synthesized powders have been characterized using instrumental methods: X-ray diffraction, transmission electron microscopy, N2 adsorption-desorption, and a laser confocal microscope. The optimal parameters of modified sol-gel synthesis of ultradispersed powders have been determined: the qualitative and quantitative compositions of the reaction mixture, thermal treatment, etc. The developed approach allows to synthesize ultradispersed metal oxide particles with a given size and vary it within 10–130 nm range with a precision of 20–30 nm.