The Experts below are selected from a list of 4098 Experts worldwide ranked by ideXlab platform
Brian D. Wirth - One of the best experts on this subject based on the ideXlab platform.
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Theoretical Model of Helium Bubble Growth and Density in Plasma-Facing Metals.
Scientific reports, 2020Co-Authors: Karl D. Hammond, Dimitrios Maroudas, Brian D. WirthAbstract:We present a theoretically-motivated model of Helium Bubble density as a function of volume for high-pressure Helium Bubbles in plasma-facing tungsten. The model is a good match to the empirical correlation we published previously [Hammond et al., Acta Mater. 144, 561-578 (2018)] for small Bubbles, but the current model uses no adjustable parameters. The model is likely applicable to significantly larger Bubbles than the ones examined here, and its assumptions can be extended trivially to other metals and gases. We expect the model to be broadly applicable and useful in coarse-grained models of gas transport in metals.
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Helium Bubble bursting in tungsten
Journal of Applied Physics, 2013Co-Authors: Faiza Sefta, N. Juslin, Brian D. WirthAbstract:Molecular dynamics simulations have been used to systematically study the pressure evolution and bursting behavior of sub-surface Helium Bubbles and the resulting tungsten surface morphology. This study specifically investigates how Bubble shape and size, temperature, tungsten surface orientation, and ligament thickness above the Bubble influence Bubble stability and surface evolution. The tungsten surface is roughened by a combination of adatom “islands,” craters, and pinholes. The present study provides insight into the mechanisms and conditions leading to various tungsten topology changes, which we believe are the initial stages of surface evolution leading to the formation of nanoscale fuzz.
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tungsten surface evolution by Helium Bubble nucleation growth and rupture
Nuclear Fusion, 2013Co-Authors: Faiza Sefta, Karl D. Hammond, Brian D. Wirth, N. JuslinAbstract:Molecular dynamics simulations reveal sub-surface mechanisms likely involved in the initial formation of nanometre-sized ?fuzz? in tungsten exposed to low-energy Helium plasmas. Helium clusters grow to over-pressurized Bubbles as a result of repeated cycles of Helium absorption and Frenkel pair formation. The self-interstitials either reach the surface as isolated adatoms or trap at the Bubble periphery before organizing into prismatic ?1?1?1? dislocation loops. Surface roughening occurs as single adatoms migrate to the surface, prismatic loops glide to the surface to form adatom islands, and ultimately as over-pressurized gas Bubbles burst.
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Molecular dynamics simulations on the effect of sub-surface Helium Bubbles on the sputtering yield of tungsten
Journal of Nuclear Materials, 2013Co-Authors: Faiza Sefta, Karl D. Hammond, N. Juslin, Brian D. WirthAbstract:Abstract Tungsten is being considered for the divertor in ITER and for future nuclear fusion reactors because of its high melting temperature and high thermal conductivity. While the sputtering yield of tungsten is generally low, previous observations of surface modification due to plasma exposure raise questions about the effects of surface morphology and sub-surface Helium Bubble populations on the sputtering behavior. Results of computational molecular dynamics are reported that investigate the influence of sub-surface Helium Bubble distributions on the sputtering yield of tungsten {1 0 0} and {1 1 0} surfaces induced by Helium ion exposure in the range of 300 eV to 1 keV. The specific microstructures and incident ion energies have been coordinated with planned experiments, which will be reported in the future. The calculated sputtering yields are in reasonable agreement with a wide range of experimental data, but do not show any significant variation as a result of the pre-existing Helium Bubbles.
J A Hinks - One of the best experts on this subject based on the ideXlab platform.
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in situ Helium implantation and tem investigation of radiation tolerance to Helium Bubble damage in equiaxed nanocrystalline tungsten and ultrafine tungsten tic alloy
Materials, 2020Co-Authors: Osman El Atwani, Kaan Unal, William Streit Cunningham, Saryu Fensin, J A Hinks, Graeme Greaves, S A MaloyAbstract:The use of ultrafine and nanocrystalline materials is a proposed pathway to mitigate irradiation damage in nuclear fusion components. Here, we examine the radiation tolerance of Helium Bubble formation in 85 nm (average grain size) nanocrystalline-equiaxed-grained tungsten and an ultrafine tungsten-TiC alloy under extreme low energy Helium implantation at 1223 K via in-situ transmission electron microscope (TEM). Helium Bubble damage evolution in terms of number density, size, and total volume contribution to grain matrices has been determined as a function of He+ implantation fluence. The outputs were compared to previously published results on severe plastically deformed (SPD) tungsten implanted under the same conditions. Large Helium Bubbles were formed on the grain boundaries and Helium Bubble damage evolution profiles are shown to differ among the different materials with less overall damage in the nanocrystalline tungsten. Compared to previous works, the results in this work indicate that the nanocrystalline tungsten should possess a fuzz formation threshold more than one order of magnitude higher than coarse-grained tungsten.
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investigating Helium Bubble nucleation and growth through simultaneous in situ cryogenic ion implantation and environmental transmission electron microscopy
Materials, 2019Co-Authors: Caitlin A Taylor, Graeme Greaves, Samuel A Briggs, Anthony Monterrosa, Emily Aradi, Joshua D Sugar, David B Robinson, Khalid Hattar, J A HinksAbstract:Palladium can readily dissociate molecular hydrogen at its surface, and rapidly accept it onto the octahedral sites of its face-centered cubic crystal structure. This can include radioactive tritium. As tritium β-decays with a half-life of 12.3 years, He-3 is generated in the metal lattice, causing significant degradation of the material. Helium Bubble evolution at high concentrations can result in blister formation or exfoliation and must therefore be well understood to predict the longevity of materials that absorb tritium. A hydrogen over-pressure must be applied to palladium hydride to prevent hydrogen from desorbing from the metal, making it difficult to study tritium in palladium by methods that involve vacuum, such as electron microscopy. Recent improvements in in-situ ion implantation Transmission Electron Microscopy (TEM) allow for the direct observation of He Bubble nucleation and growth in materials. In this work, we present results from preliminary experiments using the new ion implantation Environmental TEM (ETEM) at the University of Huddersfield to observe He Bubble nucleation and growth, in-situ, in palladium at cryogenic temperatures in a hydrogen environment. After the initial nucleation phase, Bubble diameter remained constant throughout the implantation, but Bubble density increased with implantation time. β-phase palladium hydride was not observed to form during the experiments, likely indicating that the cryogenic implantation temperature played a dominating role in the Bubble nucleation and growth behavior.
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engineering self organising Helium Bubble lattices in tungsten
Scientific Reports, 2017Co-Authors: Robert Harrison, J A Hinks, Graeme Greaves, S E DonnellyAbstract:The self-organisation of void and gas Bubbles in solids into superlattices is an intriguing nanoscale phenomenon. Despite the discovery of these lattices 45 years ago, the atomistics behind the ordering mechanisms responsible for the formation of these nanostructures are yet to be fully elucidated. Here we report on the direct observation via transmission electron microscopy of the formation of Bubble lattices under He ion bombardment. By careful control of the irradiation conditions, it has been possible to engineer the Bubble size and spacing of the superlattice leading to important conclusions about the significance of vacancy supply in determining the physical characteristics of the system. Furthermore, no Bubble lattice alignment was observed in the directions pointing to a key driving mechanism for the formation of these ordered nanostructures being the two-dimensional diffusion of self-interstitial atoms.
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Helium Bubble formation in ultrafine and nanocrystalline tungsten under different extreme conditions
Journal of Nuclear Materials, 2015Co-Authors: O Elatwani, J A Hinks, Graeme Greaves, Khalid Hattar, S S Harilal, A HassaneinAbstract:Abstract We have investigated the effects of Helium ion irradiation energy and sample temperature on the performance of grain boundaries as Helium sinks in ultrafine grained and nanocrystalline tungsten. Irradiations were performed at displacement and non-displacement energies and at temperatures above and below that required for vacancy migration. Microstructural investigations were performed using Transmission Electron Microscopy (TEM) combined with either in-situ or ex-situ ion irradiation. Under Helium irradiation at an energy which does not cause atomic displacements in tungsten (70 eV), regardless of temperature and thus vacancy migration conditions, Bubbles were uniformly distributed with no preferential Bubble formation on grain boundaries. At energies that can cause displacements, Bubbles were observed to be preferentially formed on the grain boundaries only at high temperatures where vacancy migration occurs. Under these conditions, the decoration of grain boundaries with large facetted Bubbles occurred on nanocrystalline grains with dimensions less than 60 nm. We discuss the importance of vacancy supply and the formation and migration of radiation-induced defects on the performance of grain boundaries as Helium sinks and the resulting irradiation tolerance of ultrafine grained and nanocrystalline tungsten to Bubble formation.
Faiza Sefta - One of the best experts on this subject based on the ideXlab platform.
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Helium Bubble bursting in tungsten
Journal of Applied Physics, 2013Co-Authors: Faiza Sefta, N. Juslin, Brian D. WirthAbstract:Molecular dynamics simulations have been used to systematically study the pressure evolution and bursting behavior of sub-surface Helium Bubbles and the resulting tungsten surface morphology. This study specifically investigates how Bubble shape and size, temperature, tungsten surface orientation, and ligament thickness above the Bubble influence Bubble stability and surface evolution. The tungsten surface is roughened by a combination of adatom “islands,” craters, and pinholes. The present study provides insight into the mechanisms and conditions leading to various tungsten topology changes, which we believe are the initial stages of surface evolution leading to the formation of nanoscale fuzz.
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tungsten surface evolution by Helium Bubble nucleation growth and rupture
Nuclear Fusion, 2013Co-Authors: Faiza Sefta, Karl D. Hammond, Brian D. Wirth, N. JuslinAbstract:Molecular dynamics simulations reveal sub-surface mechanisms likely involved in the initial formation of nanometre-sized ?fuzz? in tungsten exposed to low-energy Helium plasmas. Helium clusters grow to over-pressurized Bubbles as a result of repeated cycles of Helium absorption and Frenkel pair formation. The self-interstitials either reach the surface as isolated adatoms or trap at the Bubble periphery before organizing into prismatic ?1?1?1? dislocation loops. Surface roughening occurs as single adatoms migrate to the surface, prismatic loops glide to the surface to form adatom islands, and ultimately as over-pressurized gas Bubbles burst.
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Molecular dynamics simulations on the effect of sub-surface Helium Bubbles on the sputtering yield of tungsten
Journal of Nuclear Materials, 2013Co-Authors: Faiza Sefta, Karl D. Hammond, N. Juslin, Brian D. WirthAbstract:Abstract Tungsten is being considered for the divertor in ITER and for future nuclear fusion reactors because of its high melting temperature and high thermal conductivity. While the sputtering yield of tungsten is generally low, previous observations of surface modification due to plasma exposure raise questions about the effects of surface morphology and sub-surface Helium Bubble populations on the sputtering behavior. Results of computational molecular dynamics are reported that investigate the influence of sub-surface Helium Bubble distributions on the sputtering yield of tungsten {1 0 0} and {1 1 0} surfaces induced by Helium ion exposure in the range of 300 eV to 1 keV. The specific microstructures and incident ion energies have been coordinated with planned experiments, which will be reported in the future. The calculated sputtering yields are in reasonable agreement with a wide range of experimental data, but do not show any significant variation as a result of the pre-existing Helium Bubbles.
Michael Nastasi - One of the best experts on this subject based on the ideXlab platform.
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Resistance to Helium Bubble Formation in Amorphous SiOC/Crystalline Fe Nanocomposite.
Materials (Basel Switzerland), 2018Co-Authors: Tianyao Wang, Jonathan Gigax, Lin Shao, Michael NastasiAbstract:The management of radiation defects and insoluble He atoms represent key challenges for structural materials in existing fission reactors and advanced reactor systems. To examine how crystalline/amorphous interface, together with the amorphous constituents affects radiation tolerance and He management, we studied Helium Bubble formation in Helium ion implanted amorphous silicon oxycarbide (SiOC) and crystalline Fe composites by transmission electron microscopy (TEM). The SiOC/Fe composites were grown via magnetron sputtering with controlled length scale on a surface oxidized Si (100) substrate. These composites were subjected to 50 keV He+ implantation with ion doses chosen to produce a 5 at% peak He concentration. TEM characterization shows no sign of Helium Bubbles in SiOC layers nor an indication of secondary phase formation after irradiation. Compared to pure Fe films, Helium Bubble density in Fe layers of SiOC/Fe composite is less and it decreases as the amorphous/crystalline SiOC/Fe interface density increases. Our findings suggest that the crystalline/amorphous interface can help to mitigate Helium defect generated during implantation, and therefore enhance the resistance to Helium Bubble formation.
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resistance to Helium Bubble formation in amorphous sioc crystalline fe nanocomposite
Materials, 2018Co-Authors: Tianyao Wang, Jonathan Gigax, Lin Shao, Michael NastasiAbstract:The management of radiation defects and insoluble He atoms represent key challenges for structural materials in existing fission reactors and advanced reactor systems. To examine how crystalline/amorphous interface, together with the amorphous constituents affects radiation tolerance and He management, we studied Helium Bubble formation in Helium ion implanted amorphous silicon oxycarbide (SiOC) and crystalline Fe composites by transmission electron microscopy (TEM). The SiOC/Fe composites were grown via magnetron sputtering with controlled length scale on a surface oxidized Si (100) substrate. These composites were subjected to 50 keV He+ implantation with ion doses chosen to produce a 5 at% peak He concentration. TEM characterization shows no sign of Helium Bubbles in SiOC layers nor an indication of secondary phase formation after irradiation. Compared to pure Fe films, Helium Bubble density in Fe layers of SiOC/Fe composite is less and it decreases as the amorphous/crystalline SiOC/Fe interface density increases. Our findings suggest that the crystalline/amorphous interface can help to mitigate Helium defect generated during implantation, and therefore enhance the resistance to Helium Bubble formation.
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Helium Bubble growth at BCC twist grain boundaries
Journal of Nuclear Materials, 2011Co-Authors: J Hetherly, Michael Nastasi, Enrique Martinez, Alfredo CaroAbstract:Abstract We study the growth of Helium Bubbles in α-Fe at low angle twist grain boundaries and in bulk using molecular dynamics and Metropolis Monte Carlo simulations. We describe the pressures and volumes of the Helium Bubbles and analyze the maximum pressure a Bubble can sustain before emitting interstitial loops. We give a quantitative analysis of how these emitted loops behave differently in the bulk and at the grain boundary.
Graeme Greaves - One of the best experts on this subject based on the ideXlab platform.
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in situ tem investigation of nano scale Helium Bubble evolution in tantalum doped tungsten at 800 c
Journal of Nuclear Materials, 2021Co-Authors: Iuliia Ipatova, Graeme Greaves, Salvador Pachecogutierrez, S C Middleburgh, M J D Rushton, Enrique JimenezmeleroAbstract:Abstract The aim of this work is to probe the Helium induced defect production and accumulation in 40 keV He+ irradiated polycrystalline W and its alternative alloy W-5wt.%Ta using transmission electron microscopy (TEM) combined with in-situ Helium irradiation at 800°С. A maximum damage level of 1 dpa with a maximum He-to-dpa ratio of 5.5 at%/dpa has been reached in this work for both materials, which corresponds to an ion fluence of 7.33 × 1016 He+/cm2. The presence of radiation-induced dislocation loops was not observed at this temperature. The low density of the incipient Bubbles in W has been already detected at 0.004 dpa, which corresponds to a fluence of 3.3 × 1014He+/cm2. The experiments conducted at 800°C have shown that the addition of 5wt.% of tantalum into tungsten may diminish the binding of He ions with vacancies into complexes, which serve as the core of the Bubble, thus hindering Helium Bubble formation below 0.02 dpa and their further growth and population at higher damage levels. By exceeding the damage dose ≥0.3 dpa, a progressive transition from a spherical to a faceted shape of the Bubbles has been observed in W but not in the W-5Ta alloy. At 1 dpa, >80% of the Bubbles in W were of the faceted type with the facet planes of {110}.
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in situ Helium implantation and tem investigation of radiation tolerance to Helium Bubble damage in equiaxed nanocrystalline tungsten and ultrafine tungsten tic alloy
Materials, 2020Co-Authors: Osman El Atwani, Kaan Unal, William Streit Cunningham, Saryu Fensin, J A Hinks, Graeme Greaves, S A MaloyAbstract:The use of ultrafine and nanocrystalline materials is a proposed pathway to mitigate irradiation damage in nuclear fusion components. Here, we examine the radiation tolerance of Helium Bubble formation in 85 nm (average grain size) nanocrystalline-equiaxed-grained tungsten and an ultrafine tungsten-TiC alloy under extreme low energy Helium implantation at 1223 K via in-situ transmission electron microscope (TEM). Helium Bubble damage evolution in terms of number density, size, and total volume contribution to grain matrices has been determined as a function of He+ implantation fluence. The outputs were compared to previously published results on severe plastically deformed (SPD) tungsten implanted under the same conditions. Large Helium Bubbles were formed on the grain boundaries and Helium Bubble damage evolution profiles are shown to differ among the different materials with less overall damage in the nanocrystalline tungsten. Compared to previous works, the results in this work indicate that the nanocrystalline tungsten should possess a fuzz formation threshold more than one order of magnitude higher than coarse-grained tungsten.
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investigating Helium Bubble nucleation and growth through simultaneous in situ cryogenic ion implantation and environmental transmission electron microscopy
Materials, 2019Co-Authors: Caitlin A Taylor, Graeme Greaves, Samuel A Briggs, Anthony Monterrosa, Emily Aradi, Joshua D Sugar, David B Robinson, Khalid Hattar, J A HinksAbstract:Palladium can readily dissociate molecular hydrogen at its surface, and rapidly accept it onto the octahedral sites of its face-centered cubic crystal structure. This can include radioactive tritium. As tritium β-decays with a half-life of 12.3 years, He-3 is generated in the metal lattice, causing significant degradation of the material. Helium Bubble evolution at high concentrations can result in blister formation or exfoliation and must therefore be well understood to predict the longevity of materials that absorb tritium. A hydrogen over-pressure must be applied to palladium hydride to prevent hydrogen from desorbing from the metal, making it difficult to study tritium in palladium by methods that involve vacuum, such as electron microscopy. Recent improvements in in-situ ion implantation Transmission Electron Microscopy (TEM) allow for the direct observation of He Bubble nucleation and growth in materials. In this work, we present results from preliminary experiments using the new ion implantation Environmental TEM (ETEM) at the University of Huddersfield to observe He Bubble nucleation and growth, in-situ, in palladium at cryogenic temperatures in a hydrogen environment. After the initial nucleation phase, Bubble diameter remained constant throughout the implantation, but Bubble density increased with implantation time. β-phase palladium hydride was not observed to form during the experiments, likely indicating that the cryogenic implantation temperature played a dominating role in the Bubble nucleation and growth behavior.
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engineering self organising Helium Bubble lattices in tungsten
Scientific Reports, 2017Co-Authors: Robert Harrison, J A Hinks, Graeme Greaves, S E DonnellyAbstract:The self-organisation of void and gas Bubbles in solids into superlattices is an intriguing nanoscale phenomenon. Despite the discovery of these lattices 45 years ago, the atomistics behind the ordering mechanisms responsible for the formation of these nanostructures are yet to be fully elucidated. Here we report on the direct observation via transmission electron microscopy of the formation of Bubble lattices under He ion bombardment. By careful control of the irradiation conditions, it has been possible to engineer the Bubble size and spacing of the superlattice leading to important conclusions about the significance of vacancy supply in determining the physical characteristics of the system. Furthermore, no Bubble lattice alignment was observed in the directions pointing to a key driving mechanism for the formation of these ordered nanostructures being the two-dimensional diffusion of self-interstitial atoms.
-
Helium Bubble formation in ultrafine and nanocrystalline tungsten under different extreme conditions
Journal of Nuclear Materials, 2015Co-Authors: O Elatwani, J A Hinks, Graeme Greaves, Khalid Hattar, S S Harilal, A HassaneinAbstract:Abstract We have investigated the effects of Helium ion irradiation energy and sample temperature on the performance of grain boundaries as Helium sinks in ultrafine grained and nanocrystalline tungsten. Irradiations were performed at displacement and non-displacement energies and at temperatures above and below that required for vacancy migration. Microstructural investigations were performed using Transmission Electron Microscopy (TEM) combined with either in-situ or ex-situ ion irradiation. Under Helium irradiation at an energy which does not cause atomic displacements in tungsten (70 eV), regardless of temperature and thus vacancy migration conditions, Bubbles were uniformly distributed with no preferential Bubble formation on grain boundaries. At energies that can cause displacements, Bubbles were observed to be preferentially formed on the grain boundaries only at high temperatures where vacancy migration occurs. Under these conditions, the decoration of grain boundaries with large facetted Bubbles occurred on nanocrystalline grains with dimensions less than 60 nm. We discuss the importance of vacancy supply and the formation and migration of radiation-induced defects on the performance of grain boundaries as Helium sinks and the resulting irradiation tolerance of ultrafine grained and nanocrystalline tungsten to Bubble formation.