Potassium-doped Tungsten Research Articles

Incipient plasticity of potassium-doped tungsten under nanoindentation: A comparison between experiments and defect dynamics simulations

The effects of potassium (K) doping on the incipient plasticity of tungsten (W) under nanoindentation were investigated using a combination of experiments and mesoscale defects dynamic simulations. The transmission electron microscopy study reveal that nanometer-sized bubbles were formed through the vaporization of K in specimens prepared by spark plasma sintering. In order to investigate the mechanical properties of the K-doped W specimens, nano-characterization experiments and defect dynamics simulations were conducted, comparing with those in pure W. Nanoindentation tests reveal that the maximum shear yield stress approaches the theoretical strength in annealed pure W, while K-doped W samples exhibit significant yield drop accompanied with stochastic variations. A newly developed mesoscale defect dynamics model to concurrently couple dislocation dynamics with finite element method has been also employed to investigate micro-mechanisms of plasticity under nanoindentation and the effects of K-bubbles on the plastic deformation. The simulations revealed that the localized stress concentration induced by the K-bubbles promoted dislocation nucleation and enhanced plastic deformation, thereby reducing the yield stress, showing good agreement with the experiment.

On the fabrication of ultrafine-grained potassium-doped tungsten: Mechanical milling and spark plasma sintering of K-doped W powder prepared by evaporation-condensation method

Bubble strengthened materials have received lots of attention in recent years, and among them, potassium-doped tungsten (W-K) is extremely desirable for very high-temperature applications such as plasma-facing materials in fusion reactors. However, it is difficult to fabricate ultrafine-grained W-K bulk materials by traditional methods. In this study, a novel preparation method of ultra-fine-grained bubble-strengthened material is presented. Unlike traditional routes, W-K mixed powder was prepared by evaporating and condensing pure K on the surface of nanoscale W powder. Two W-K mixing methods using this evaporation-condensation route were developed. Mechanical milling process combined with regular SPS sintering were carried out on the W-K mixed powder, which successfully manufactured ultrafine-grained (0.9 μm) W-K bulk material with a high density (96.4%). This study can provide important reference for the fabrication of other bubble-strengthened materials.

Numerical study and experimental validation of heat transfer capacity of flat-type divertor for fusion reactor

The divertor must simultaneously withstand unprecedented high heat fluxes of up to 20 MW/m2 and high-energy neutron irradiation of 14-MeV during fusion reactor operation. Accordingly, it is necessary to simultaneously meet the excellent heat dissipation capacity of the divertor and maintain good material performance in harsh neutron irradiation environments. The flat-type divertor demonstrates better heat transfer performance compared to monoblock divertor. Nevertheless, under the condition of a heat flux of 20 MW/m2, the heat transfer and thermal fatigue performance of flat-type divertor using advanced materials are still unidentified. In this study, we conducted a thorough analysis of the heat transfer capabilities and thermal fatigue characteristics of potassium-doped tungsten (KW)/Cu/Oxide dispersion strengthened copper (ODS-Cu)/reduced activation ferritic/martensitic (RAFM) divertor mockup adopts hypervaportron (HV) structure by numerical simulations combined with experiments. The numerical results indicate that the flat-type KW/Cu/ODS-Cu/RAFM divertor mockup expresses excellent heat transfer capacity. The contact area between the edge of the fins and the bottom surface of the ODS-Cu heat sink induces a significant increase in water flow velocity to ∼15 m/s. The peak temperature of loaded KW surface is only ∼985 °C under the flow rate of 5 t/h and inlet temperature of 20 °C. During the high heat flux tests, the prepared flat-type divertor mockup successful endured 1000 cycles of 20 MW/m2 with the peak temperature of 883 °C, and the surface temperature experienced a fluctuation of 2.4 % during the thermal fatigue tests. This study can provide a strong data reference and technical support for the development of fusion reactors, and is of great significance in advancing the commercialization of fusion energy.

DBTT and recrystallization behavior analyses for rolled and forged potassium-doped tungsten alloys

Tungsten-potassium (WK) alloys are regarded as one of the candidates for plasma-facing materials (PFMs) intended for the divertor and first wall of future fusion reactors. Determining the range of the ductile-to-brittle transition temperature (DBTT) and the recrystallization temperature (RCT) for the WK alloy is critical as it is closely related to the safe operational temperature window of the fusion reactor. Variable temperature tensile tests were conducted on rolled and high energy rate forging (HERF) WK alloy specimens to elucidate their DBTT. Additionally, gradient high-temperature isochronal annealing was performed on the HERF samples, coupled with Vickers hardness characterization, to ascertain their distinct RCT for various orientation planes within the same alloy. Subsequently, the DBTT of the as-rolled WK alloy is determined to be approximately between 100 °C and 150 °C, while that of the HERF WK alloy is found to be around lower than 200 °C. Furthermore, the RCT of the ND plane of the HERF WK alloy is approximately 1600 °C, whereas that of the RD counterpart exceeded 1700 °C. The internal mechanisms underlying these phenomena are comprehensively analyzed in this study. This work has identified the safe service temperature window for future fusion reactors with this kind of advanced WK alloy and provided a new perspective for determining the RCT of many other W-based materials.

Effects of non-isothermal annealing on the microstructure of pure and potassium-doped tungsten sheets

Within this study, the effects of non-isothermal annealing procedures, especially different heating rates, were studied for cold rolled pure and potassium doped tungsten sheets. The sheets had a thickness of 0.2 mm. The microstructure was studied via etching of polished cross sections as well as electron backscatter diffraction. The potassium-bubble distribution in the doped tungsten sheets was investigated using scanning electron microscopy.Regarding the microstructure, the doped tungsten sheets presented an elongated, layered microstructure, that was considerably affected by the heating rate. A decrease of the heating rate from 14,400 K/min to 20 K/min resulted in a grain size shift in rolling direction from 70 to 513 μm. On the other hand, pure tungsten sheets showed no significant dependence on the heating rate. Moreover, additional isothermal annealing periods at 2400 °C did generally not affect the microstructure of doped tungsten sheets, which in turn led to a significant increase of the grain size in pure tungsten. This difference was attributed to the grain growth mechanisms, normal grain growth for pure and secondary recrystallization for K-doped tungsten.However, the potassium bubble structure, being responsible for the microstructural differences, was in turn only affected by the maximum annealing temperature. The heating rate differences led to no significant changes in the bubble size distribution. Furthermore, additional isothermal annealing at 2400 °C led to comparably minor changes in the average bubble diameter.

Comparison of thermal shock resistance capabilities of the rotary swaged pure tungsten, potassium-doped tungsten, and W-La2O3 alloys

Comparison of thermal shock resistance capabilities of the rotary swaged pure tungsten, potassium-doped tungsten, and W-La2O3 alloys

Synthesis, Gas Sensing and Photocatalytic Properties of Manganese-Doped and Potassium-Doped Hexagonal Tungsten Trioxide Nanowires

On account of its unique and variable lattice structure and stoichiometric ratio, tungsten oxide is suitable for material modification; for example, doping is expected to improve its catalytic properties. However, most of the doping experiments are conducted by hydrothermal or multi-step synthesis, which is not only time-consuming but also prone to solvent contamination, having little room for mass production. Here, without a catalyst, we report the formation of high-crystallinity manganese-doped and potassium-doped tungsten oxide nanowires through chemical vapor deposition with interesting characterization, photocatalytic, and gas sensing properties. The structure and composition of the nanowires were characterized by transmission electron microscopy and energy-dispersive spectroscopy, while the morphology and chemical valence were characterized by scanning electron microscopy and X-ray photoelectron spectroscopy. Electrical measurements show that the single nanowires doped with manganese and potassium had resistivities of 1.81 × 10−5 Ω·m and 1.93 × 10−5 Ω·m, respectively. The doping contributed to the phase transition from monoclinic to metastable hexagonal for the tungsten oxide nanowires, the structure of which is known for its hexagonal electron channels. The hexagonal structure provided efficient charge transfer and enhanced the catalytic efficiency of the tungsten oxide nanowires, resulting in a catalytic efficiency of 98.5% for the manganese-doped tungsten oxide nanowires and 97.73% for the potassium-doped tungsten oxide nanowires after four hours of degradation of methylene blue. Additionally, the gas sensing response for 20 ppm of ethanol showed a positive dependence of doping with the manganese-doped and potassium-doped responses being 14.4% and 29.7%, respectively, higher than the pure response at 250 °C. The manganese-doped and potassium-doped tungsten oxide nanowires are attractive candidates in gas sensing, photocatalytic, and energy storage applications, including water splitting, photochromism, and rechargeable batteries.

In-situ analysis on defect evolution in potassium-doped tungsten during synergistic He+ and H2+ dual-beam irradiation

Potassium-doped tungsten (KW), as an advanced candidate for first wall material, was developed to mitigate intrinsic embrittlement of bulk W. The addition of gaseous potassium bubbles (KBs) was known to act as effective barrier to the motion of irradiation damage and grain boundaries (GBs). The irradiation response to the He/H synergistic effect and the role of KBs on damage defects were two key issues when evaluating the performance of KW in future fusion reactors. In this work, KW was in-situ irradiated by 30 keV He+ and H2+ with He/H ratios of 2.5, 0.5, and 0.1 at 823 K. The synergy of He, H, and damage defects were discussed. The number density and the average size of irradiation-induced loops and bubbles nearby KBs, nearby GBs, and within the matrix was quantified. Increasing the ratio of H to He enhanced the nucleation sites of loops and promoted the accumulation rates of bubbles. The formation of a loop denuded zone nearby a KB was related to the KB size. The difference between the loop denuded zones nearby KBs and GBs was found. The estimated sink strengths of KBs and GBs were in line with our experimental results.

Superior strength-ductility synergy of high potassium-doped tungsten rods with large swaging deformation

The high ductile-to-brittle transition temperature (DBTT) is a key factor limiting the use of tungsten (W) - based materials in future nuclear fusion reactors. To overcome this challenge, and to obtain W alloys with improved mechanical properties without sacrificing substantial thermal conductivity, a large-sized W rod with a total weight of 42 kg and with 96 ppm doped potassium (K) was fabricated through a powder metallurgy mass production route. Then high-temperature swaging processes were conducted to produce a rod with large plastic deformation of 94.7% (log. Strain of 2.95). The swaged rod showed good low-temperature ductility and high strength simultaneously. At 50 °C, the AKS-W rod fractured in brittle cleavage mode. The fracture changed to a mixture of brittle and ductile at 100 °C, indicating a DBTT of 50 °C - 100 °C. This value was lower than many other reported in pure W and K-doped alloys. The swaged W rod exhibited a high strength of larger than 1.0 GPa up to 100 °C, with good conductivity at low temperatures. The maximum total elongation of 30.24% was obtained at 200 °C. The superior strength-ductility synergy of AKS-W rod was observed to be attributed to its unique microstructure and K bubbles morphology, which evolved during severe hot plastic deformation.

Effect of potassium doping and exposure temperature on the deuterium behavior in large-scale potassium-doped tungsten

In order to meet the needs of future fusion power plants in engineering application, a newly large-volume potassium-doped tungsten (WK) plate with a thickness of 15 mm was prepared by powder metallurgy and hot rolling technology. Pure tungsten (PW) as a comparison material was prepared using the same preparation process. To figure out the effect of K doping and exposure temperature on D retention and blistering morphology in WK and PW, the high-flux (∼1022 m−2 s−1) and low energy (∼ 50 eV) D plasma exposure with different temperatures (423, 473, 523 and 573 K) was performed. The results show that there is serious surface blistering in both PW and WK at all exposure temperatures and PW forms D blisters larger in size but smaller in number compared with WK. The D retention in PW is significantly lower than that in WK at all exposure temperatures. Moreover, the evolution of D blisters and retention in WK is more sensitive to exposure temperature compared to PW. The effects of K doping and exposure temperature on blistering morphology and D retention were analyzed and discussed in detail.

Manufacturing and testing of flat-type divertor mockup with advanced materials

During reactor operation, the divertor must withstand unprecedented simultaneous high heat fluxes and high-energy neutron irradiation. The extremely severe service environment of the divertor imposes a huge challenge to the bonding quality of divertor joints, i.e., the joints must withstand thermal, mechanical and neutron loads, as well as cyclic mode of operation. In this paper, potassium-doped tungsten (KW) is selected as the plasma facing material (PFM), oxygen-free copper (OFC) as the interlayer, oxide dispersion strengthened copper (ODS-Cu) alloy as the heat sink material, and reduced activation ferritic/martensitic (RAFM) steel as the structural material. In this study, a vacuum brazing technology is proposed and optimized to bond Cu and ODS-Cu alloy with the silver-free brazing material CuSnTi. The most appropriate brazing parameters are a brazing temperature of 940 °C and a holding time of 15 min. High-quality bonding interfaces have been successfully obtained by vacuum brazing technology, and the average shear strength of the as-obtained KW/Cu and ODS-Cu alloy joints is ∼268 MPa. And a fabrication route for manufacturing the flat-type divertor target based on brazing technology is set. For evaluating the reliability of the fabrication technologies under the reactor relevant condition, the high heat flux test at 20 MW/m2 for the as-manufactured flat-type KW/Cu/ODS-Cu/RAFM mockup is carried out by using the Electron-beam Material testing Scenario (EMS-60) with water cooling. This paper reports the improved vacuum brazing technology to connect Cu to ODS-Cu alloy and summarizes the production route, high heat flux (HHF) test, the pre and post non-destructive examination, and the surface results of the flat-type KW/Cu/ODS-Cu/RAFM mockup after the HHF test. The test results demonstrate that the mockup manufactured according to the fabrication route still have structural and interfacial integrity under cyclic high heat loads.

Comparison of K-doped and pure cold-rolled tungsten sheets: Microstructure restoration in different temperature regimes

For tungsten in divertor parts of future fusion reactors, a fine-grained microstructure is preferred to reduce its brittle-to-ductile transition temperature and minimize cracking events due to cyclic thermal and mechanical loading. In our ongoing study on tungsten sheets with different degree of deformation by warm- and cold-rolling, the potential of potassium-doping to stabilize the microstructure at high operation temperatures is assessed. Successful production of technically pure and equivalently rolled potassium-doped tungsten sheets up to very high logarithmic strains of 4.6 was already shown in the past with in-depth analysis of the evolution of microstructure and mechanical properties after rolling steps. Our current study investigates the microstructure changes in the same material batch by recovery, recrystallization and grain growth in temperature regimes between 600 °C and 2400 °C. Annealing studies with subsequent microhardness and SEM analysis reveal increased retardation of recrystallization in potassium-doped tungsten with higher rolling strain, but abnormal grain growth at high temperatures is also increased. However, potassium-doped tungsten with low rolling strain shows promising results with much less grain growth than its pure tungsten counterpart.

Rate-controlling deformation mechanisms in drawn tungsten wires

ABSTRACT Undeformed tungsten suffers from a brittleness that makes it unsuitable for applications at low temperatures. Cold-worked tungsten materials such as drawn wires or rolled plates can however show considerable ductility even at low temperatures. The reason for this behaviour is so far not understood. We investigated a series of potassium-doped tungsten wires that were subsequently drawn from one sintered ingot, making them chemically identical. Hence, the properties of the wires could be studied without the influence of different impurity levels. Using transient mechanical tests, namely repeated stress relaxation experiments and strain-rate jump tests, the effective activation volumes and strain-rate sensitivities m of the wires were determined at room-temperature. Based on the obtained results, it is deduced that the motion of screw dislocations by formation and dissociation of kink-pairs is controlling the rate of plastic deformation in all wires that show plasticity at room temperature. It is hence concluded that the ductility of drawn tungsten wires at low temperatures is not due to a change in the rate-controlling deformation mechanisms, but should be a consequence of the microstructural and textural changes during wire drawing.

Decreased surface blistering and deuterium retention in potassium-doped tungsten exposed to deuterium plasma following ion irradiation

A large-size potassium-doped tungsten (KW) plate with a thickness of 15 mm was fabricated via powder metallurgy technology and hot rolling. In order to appraise the irradiation resistance of KW, the surface deuterium (D) blistering and D retention were studied on Fe11+ pre-damaged (0, 0.05 and 0.5 dpa) KW and pure tungsten (PW), which were exposed to ∼60 eV and ∼5 × 1021 m−2 s−1 D plasmas at 500 K at a fluence of ∼5.76 × 1025 m−2. The results indicate that the KW alloy can better inhibit the generation of vacancy defects after Fe11+ ion damage compared with PW because K bubbles can restrain the migration of W self-interstitial atoms and the accumulation of vacancies caused during Fe11+ ion irradiation. The Fe11+ ion pre-damage can relieve the surface blistering and D retention of PW and KW at the same time, and the KW has a better effect of inhibiting D retention, while it does not show a significant advantage in inhibiting surface blistering compared with PW. In addition, the causes of the discrepancy in total D retention and the surface morphology evolution of PW and KW are discussed in detail.

Enhanced photoluminescence of potassium-doped tungsten oxide by acetone exposure.

Studies of optical properties of doped nanocrystals of tungsten trioxide can elucidate new information about the material. A novel molecule-enhanced photoluminescence (PL) of potassium-doped tungsten trioxide (K x WO) was explored in the presence of different gases to understand charge transfer between molecules and K x WO on the properties of the material. We performed Raman spectroscopy and PL experiments in the presence of gaseous acetone or ethanol mixed with other gases (N2 and O2). PL at 630 nm from K x WO was observed and further enhanced when the sample was continuously irradiated with a 532 nm CW laser in acetone. A mechanism of strong emission of the PL induced by the charge transfer between the acetone and the K x WO is proposed.

Effect of potassium bubbles on the thermal shock fatigue behavior of large-volume potassium-doped tungsten alloy

A newly developed large-volume potassium-doped tungsten (W–K) plate with a thickness of 15 mm and a weight of 25 kg by powder metallurgy plus hot rolling was prepared to meet the requirements of the International Thermonuclear Experimental Reactor (ITER) in engineering application. In order to clarify the effect of K doping on the thermal shock performance of W–K alloy, transient thermal shock tests with a single-pulse duration of 1 ms for 100 shots at room temperature were performed. The absorbed power density is set to 0.33, 0.44, 0.55 and 0.66 GW m−2, respectively. Furthermore, the microstructure, Vickers micro-hardness before and after the transient thermal shock, thermal conductivity and relative density were also characterized. The results indicate that the cracking threshold of rolled W–K is 0.44–0.55 GW m−2, which possesses a better transient thermal shock resistance compared with the most of advanced W-based materials. This is mainly because K doping can significantly improve the high-temperature stability and mechanical properties of W material without reducing its thermal conductivity. In particular, K bubbles can also effectively inhibit the formation and propagation of cracks during thermal shock. Moreover, the cracking mechanism of rolled W–K alloy is also discussed in detail. This study is helpful for building a trusted ITER database on advanced W-based materials that provides useful references for the selection of future plasma-facing materials.

Effect of Fe11+ ion combined with helium and deuterium plasmas irradiation on the transient thermal shock behaviors of pure and potassium-doped tungsten

The large-scale warm-rolled pure tungsten (PW) and hot-rolled potassium-doped tungsten (W-K) plates were prepared by powder metallurgy technology to meet the demands of International Thermonuclear Experimental Reactor (ITER) in engineering application. In order to evaluate the performance of plasma-facing materials (PFM) in a more realistic service environment as much as possible, the influence of combined irradiation effects of high-energy iron (Fe) ion, deuterium (D) and helium (He) plasmas on the transient thermal shock behavior in the PW and W-K was investigated. The results indicate that the PW can generate more vacancy- and dislocation-type defects than that of W-K after irradiation. The cracking thresholds for un-irradiated PW and W-K are >0.66 and 0.44–0.55 GW/m2, respectively. However, the cracking threshold for both samples decreased to 0.22–0.33 GW/m2 after combined irradiation, indicating that the combined irradiation can significantly reduce the thermal shock resistance of PW and W-K. In addition, according to the statistical results of the crack density and width on the surface of irradiated samples, irradiated W-K owns better thermal shock resistance than irradiated PW under the same thermal shock condition. These results were also verified by the determination of residual stresses. Besides, the relationship between the evolution of irradiation-induced defects and K bubbles is also discussed in detail. These studies can provide an important reference for the identification of future PFM and numerical simulation under combined irradiation plus high thermal load.

Chemical Vapor Deposition-Fabricated Manganese-Doped and Potassium-Doped Hexagonal Tungsten Trioxide Nanowires with Enhanced Gas Sensing and Photocatalytic Properties.

Owing to its unique and variable lattice structure and stoichiometric ratio, tungsten oxide is suitable for material modification; for example, doping is expected to improve its catalytic properties. However, most of the doping experiments are conducted by hydrothermal or multi-step synthesis, which is not only time-consuming but also prone to solvent contamination, having little room for mass production. Here, without a catalyst, we report the formation of high-crystallinity manganese-doped and potassium-doped tungsten oxide nanowires through chemical vapor deposition (CVD) with interesting characterization, photocatalytic, and gas sensing properties. The structure and composition of the nanowires were characterized by transmission electron microscopy (TEM) and energy-dispersive spectroscopy (EDS), respectively, while the morphology and chemical valence were characterized by scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS), respectively. Electrical measurements showed that the single nanowires doped with manganese and potassium had resistivities of 1.81 × 0−5 Ω·m and 1.93 × 10−5 Ω·m, respectively. The doping contributed to the phase transition from monoclinic to metastable hexagonal for the tungsten oxide nanowires, the structure of which is known for its hexagonal electron channels. The hexagonal structure provided efficient charge transfer and enhanced the catalytic efficiency of the tungsten oxide nanowires, resulting in a catalytic efficiency of 98.5% for the manganese-doped tungsten oxide nanowires and 97.73% for the potassium-doped tungsten oxide nanowires after four hours of degradation of methylene blue. Additionally, the gas sensing response for 20 ppm of ethanol showed a positive dependence of doping with the manganese-doped and potassium-doped responses being 14.4% and 29.7%, respectively, higher than the pure response at 250 °C. The manganese-doped and potassium-doped tungsten oxide nanowires are attractive candidates in gas sensing, photocatalytic, and energy storage applications, including water splitting, photochromism, and rechargeable batteries.

Irradiation hardening behaviors of large-scale hot rolling potassium-doped tungsten alloy under synergistic irradiations of Fe11+ ion combined with deuterium and helium plasmas

In this study, a 25 kg potassium doped tungsten (KW) plate dispersed K bubbles was prepared by hot rolling firstly. In order to evaluate the mechanical properties of KW after irradiation, the synergistic irradiation of Fe11+ ion, He and D plasmas were performed on KW and pure tungsten (PW). 3.5-MeV Fe11+ ion irradiation was used to simulate displacement damage induced by neutrons. The total fluence and irradiation temperature were 5.31 × 1014 m−2 for 0.5 displacements per atom (dpa) damage and 400 K, respectively. Positron annihilation Doppler broadening spectrometry (PA-DBS) technique was applied to measure the type of defect in both undamaged and damaged PW & KW samples. The total ion fluence helium (He) and deuterium (D) were kept constant at 1 × 1025 He m−2 and 5 × 1024 D m−2, separately. Subsequently, the hardness value of PW and KW received from CSM nanoindentation technique was analyzed and compared. The results show that the increase of vacancy-type defects in KW is restrained after irradiation compared with PW. For un-irradiated PW and KW samples, there are almost the same pop-in events, which indicate that PW and KW a similar plastic deformation at room temperature. The KW shows a better irradiation hardening resistance than PW under the same irradiation condition. Besides, the evolution of irradiation hardening as well as their relationship with K bubbles were discussed in detail.

Large-scale potassium-doped tungsten alloy with superior recrystallization resistance, ductility and strength induced by potassium bubbles

Potassium-doped tungsten (KW) plate with a weight of 25 kg was prepared by hot rolling. Subsequently, part of the KW suffered annealing at 1600, 1800 °C for 0.5 h. For the rolled state, K bubble showed the size and number density of 71 nm, 7.6×1018 m−3 in the grain interior and 98 nm, 1.9×1019m−3 at the grain boundaries. To figure out the influence of K bubbles on recrystallization resistance and mechanical properties, microstructure and tensile performance were examined on the stress relieved and high-temperature annealed KW. The results indicated that the KW displayed excellent recrystallization resistance as the K bubble strings inhibited the transverse movement of grain boundaries. Tensile tests at 100, 150, 200, 250, 300, 500 and 700 °C showed the DBTT was 50–100 °C for the stress relived KW and about 200 °C for the high-temperature annealed KW. Moreover, the KW plate exhibited large ductility as K bubbles in the grain interior can act as the source to emit dislocation and barrier to dislocation movement thereby lead to lots of mobile dislocation. Besides, the high number density of nano-sized K bubbles also improved the KW strength, and the strength contribution of K bubbles calculated by the Orowan mechanism was 245.6 MP at room temperature.