The concept of numerically solving the rate theory equations was used in determining hydrogen ret... more The concept of numerically solving the rate theory equations was used in determining hydrogen retention to carbon, oxygen and argon impurities in bulk tungsten. In the simulations, a hydrogen pulse with low energy and low fluence was subjected to tungsten with varying impurity concentrations. Retention and release of hydrogen atoms from impurities and intrinsic high-energy traps was monitored during and after the pulse. At 500 K the detrapping of hydrogen and diffusion to the W surface was found to take place in long timescales after the pulse shutdown.
The diffusion of hydrogen in tungsten is studied as a function of temperature, hydrogen concentra... more The diffusion of hydrogen in tungsten is studied as a function of temperature, hydrogen concentration and pressure using Molecular Dynamics technique. A new analysis method to determine diffusion coefficients that accounts for the random oscillation of atoms around the equilibrium position is presented. The results indicate that the hydrogen migration barrier of 0.25 eV should be used instead of the presently recommended value of 0.39 eV. This conclusion is supported by both experiments and density functional theory calculations. Moreover, the migration volume at the saddle point for H in W is found to be positive: ∆V m ≈ 0.488Å^3 , leading to a decrease in the diffusivity at high pressures. At high H concentrations, a dramatic reduction in the diffusion coefficient is observed, due to site blocking and the repulsive H-H interaction. The results of this study indicates that high flux hydrogen irradiation leads to much higher H concentrations in tungsten than expected.
The diffusion of monovacancies in tungsten is studied computationally over a wide temperature ran... more The diffusion of monovacancies in tungsten is studied computationally over a wide temperature range from 1300 K until the melting point of the material. Our modelling is based on Molecular Dynamics technique and Density Functional Theory. The monovacancy migration barriers are calculated using nudged elastic band method for nearest and next-nearest neighbour monovacancy jumps. The diffusion pre-exponential factor for monovacancy diffusion is found to be two to three orders of magnitude higher than commonly used in computational studies, resulting in attempt frequency of the order 10^15 Hz. Multiple nearest neighbour jumps of monovacancy are found to play an important role in the contribution to the total diffusion coefficient, especially at temperatures above 2/3 of T m , resulting in an upward curvature of the Arrhenius diagram. The probabilities for different nearest neighbour jumps for monovacancy in W are calculated at different temperatures.
Particle irradiation produces defects which trap hydrogen isotopes and impurities in nuclear reac... more Particle irradiation produces defects which trap hydrogen isotopes and impurities in nuclear reactor materials. However, a comprehensive understanding of the basic mechanisms, and the final outcome of this process is still lacking. Here the evolution of defects, hydrogen, and impurities in tungsten during and after deuterium irradiation is simulated by solving rate theory equations. The results are in excellent agreement with irradiation experiments. Our results show that hydrogen is mainly trapped in tungsten monovacancies, and trapping in larger vacancy clusters increase with increasing implantation energy. The slow hydrogen desorption observed in experiments after irradiation, was found to be mainly due to detrapping of the weakly bound sixth hydrogen from monovacancies. Impurities are shown to play a significant role in decreasing Frenkel pair annihilation during irradiation, by trapping self-interstitial atoms. Moreover, we conclude that the formed impurity self-interstitial atom complexes could be the nucleation site for formation of large interstitial type dislocation loops observed experimentally.
First-principles calculations were used in determining the binding and trapping properties of hyd... more First-principles calculations were used in determining the binding and trapping properties of hydrogen to point defects in tungsten. Hydrogen zero-point vibrations were taken into account. It was concluded that the monovacancy can hold up to five hydrogen atoms at room temperature. The hydrogen was found to distort the self-interstitial atom configuration geometry. The interaction of hydrogen with the transmutation reaction impurities Re and Os were studied. It was found that the substitutional Re and Os have a negligible effect on the hydrogen trapping whereas the interstitial Os may increase the hydrogen inventory in tungsten.
The first-principles calculations were used to study the hydrogen energetics on the 100 tungsten ... more The first-principles calculations were used to study the hydrogen energetics on the 100 tungsten 2 2R45° surface. Two equilibrium sites for H at the surface are identified, with a low migration barrier from the energetically clearly higher long bridge site to the short bridge site. At low coverages, the majority of H surface diffusion events take place via the short bridge sites. The energetics for H penetration from the surface to the solute site in the bulk was defined, showing that the bulk H diffusion via neighboring tetrahedral sites takes place at depths beyond the second subsurface layer.
A reactive interatomic bond-order potential for bcc tungsten is presented. Special attention in t... more A reactive interatomic bond-order potential for bcc tungsten is presented. Special attention in the potential development was given for obtaining accurate formation and migration energies for point defects, making the potential useful in atomic scale simulations of point and extended defects. The potential was used to calculate binding energies and trapping distances for vacancies in vacancy clusters and the recombination radius for self-interstitial atom and monovacancy.
First principle calculations were used to study the hydrogen migration properties in bulk bcc
tun... more First principle calculations were used to study the hydrogen migration properties in bulk bcc tungsten. Hydrogen has low solubility in tungsten and occupies the tetrahedral interstitial site with an energy difference of 0.38 eV compared to the octahedral interstitial site. The hydrogen diffusion coefficient was evaluated using the harmonic transition state theory and was found to agree with the experimental results at temperatures above 1500 K. The height of the migration barrier between two adjacent tetrahedral sites was found to be 0.21 eV, which is lower than the value 0.39 eV obtained for the migration barrier from degassing measurements in the temperature range between 1100 and 2400 K. The tunneling correction to the diffusion rate provides much better agreement with the experimental result at 29 K than the extrapolated experimental D from high temperature measurements.
We report in situ electron microscopy observations of the plastic deformation of individual nanom... more We report in situ electron microscopy observations of the plastic deformation of individual nanometer-sized Au, Pt, W, and Mo crystals. Specifically designed graphitic cages that contract under electron irradiation are used as nanoscopic deformation cells. The correlation with atomistic simulations shows that the observed slow plastic deformation is due to dislocation activity. Our results also provide evidence that the vacancy concentration in a nanoscale system can be smaller than in the bulk material, an effect which has not been studied experimentally before.
We would like to discuss the role that 1 MeV tritons produced in deuterium–deuterium fusion react... more We would like to discuss the role that 1 MeV tritons produced in deuterium–deuterium fusion reactions might play in a long-pulse or steady-state fusion reactor. Albeit a small minority in quantity compared to the fuel tritium, the fusion tritons have significantly longer penetration length in materials and can have detrimental consequences for the integrity of the components. Because deeply deposited atoms are not easily removed from the plasma-facing components, the fusion tritium inventory in a steady-state device is expected to be limited only by decay. Furthermore, unlike fuel tritium, it is not evenly distributed on the plasma-facing components. We conclude that, of the materials considered here, tungsten appears better than carbon or beryllium in this respect. Nonetheless, 1 MeV tritons from deuterium fusion should not be neglected when making material choices for ITER and, especially, for future fusion reactors. In particular, studies on the bulk effects of deeply penetrated tritium in tungsten are urgently needed if metal-wall reactors are considered for the future. This is an interdisciplinary problem needing the attention of material scientists and plasma physicists.
Deuterium retention in the implantation-induced defects in polycrystalline tungsten has been stud... more Deuterium retention in the implantation-induced defects in polycrystalline tungsten has been studied. Deuterium was implanted with different energies and concentrations of retained D were analysed with secondary ion mass spectrometry and nuclear reaction analysis. Annealings were carried out at four predetermined temperatures corresponding to four different defect types that can trap deuterium. A quantitative number of each defect type produced by 5, 15 and 30 keV D implantation with a dose of 5.8 × 10^16 cm^{−2} was obtained.
Tungsten has been proposed for first wall material in thermonuclear reactors, where its behaviour... more Tungsten has been proposed for first wall material in thermonuclear reactors, where its behaviour in the presence of hydrogen containing plasma irradiation at elevated temperatures is of key interest. Deuterium induced defects in polycrystalline tungsten have been studied. Deuterium was implanted into tungsten samples and retained D-concentrations were analyzed with nuclear reaction analysis and secondary ion mass spectrometry. We observed four different defect types that trap deuterium with release temperatures of 455, 560, 663 and 801 K. Total number of each defect type produced by 5.8 x 10^16 cm^{-2} 30-keV D implantation at room temperature was obtained to be 0.260, 0.156, 0.082 and 0.056 traps cm^{-2}/implanted D atom.
Flaking of carbon film was studied by depositing hydrogen doped carbon films on tungsten and moly... more Flaking of carbon film was studied by depositing hydrogen doped carbon films on tungsten and molybdenum substrates. Topographies of the carbon coatings were analyzed with scanning electron microscopy and three-dimension profilometer. Secondary ion mass spectrometry and Raman spectra analysis was used to determine the hydrogen concentration and the number of' sp^3 bonds in the films, respectively. Carbon film properties varied from tetrahedral amorphous carbon (ta-C) to hard hydrogenated amorphous carbon (a-C:H). Carbon coatings hold large residual stresses, which are released during or after deposition. Typical stress relief patterns were observed. In most samples buckling started at some dislocation or at the edge of the film and adhesion energy was deduced from the sizes of the stress relief patterns. It was observed, that carbon films were more adherent to tungsten than to molybdenum and that codeposited hydrogen had a significant effect to flaking propagation in the carbon films.
A Monte Carlo method using ion packets is introduced. The method presented is optimal for the low... more A Monte Carlo method using ion packets is introduced. The method presented is optimal for the low probability scattering simulations of multiple and plural backscattering effects. Non-Rutherford cross sections are easily implemented in the framework of this model.
The atomic layer deposition (ALD) technique has been used to deposit different types of hafnium a... more The atomic layer deposition (ALD) technique has been used to deposit different types of hafnium and zirconium silicates. The technique allows controlling the material thickness and quality due to atomic/molecular layer-by-layer growth mechanism. The films were deposited on 200 mm Si(1 0 0) substrates. Both thickness and Hf/Si or Zr/Si ratio were varied. Rutherford backscattering spectrometry and time-of-flight elastic recoil detection analysis were used to determine film composition and impurities distribution. Thickness and refractive index of the coatings were measured by spectroscopic ellipsometry. Our measurements showed the presence of photoluminescence in the Hf-silicate films.
Six Joint European Torus (JET) divertor tiles were coated with tungsten marker stripes for erosio... more Six Joint European Torus (JET) divertor tiles were coated with tungsten marker stripes for erosion/deposition studies. The average thickness was 3.4 um and the film was uniform within 5% across and within 10% along the stripe. Small amounts of impurities C, Fe, Ni and Cr were found and the in-plane stress was 965 MPa. Resistance to high heat loads was tested in the Neutral Beam Test-bed at JET where a coated carbon fibre composite block was exposed to a total of 113 pulses with a maximum peak power density of 15.7 MW m^{-2} and a maximum surface temperature of 1280 C. No visible changes as a result of thermal cycling nor sputtering of the film by deuterium could be detected.
The interstitial to substitutional nitrogen atoms as a function of concentration in GaNAs were de... more The interstitial to substitutional nitrogen atoms as a function of concentration in GaNAs were determined by nuclear reaction analysis utilizing the 14 N(d,p) 15 N and 14 N(d,) 12 C reactions using ion channeling technique. The GaNAs films with mean nitrogen concentration between 0.3% and 3%, measured with secondary ion mass spectrometry and time-of-flight elastic recoil detection analysis, were grown using gas-source molecular-beam epitaxy. The fraction of nitrogen atoms occupying substitutional sites was observed to increase linearly with increasing nitrogen amount, while the concentration of interstitial nitrogen was nearly constant at 2x10^19 cm^{-3} throughout the concentration region. Annealing at 750 °C decreases the concentration of interstitial nitrogen.
The concept of numerically solving the rate theory equations was used in determining hydrogen ret... more The concept of numerically solving the rate theory equations was used in determining hydrogen retention to carbon, oxygen and argon impurities in bulk tungsten. In the simulations, a hydrogen pulse with low energy and low fluence was subjected to tungsten with varying impurity concentrations. Retention and release of hydrogen atoms from impurities and intrinsic high-energy traps was monitored during and after the pulse. At 500 K the detrapping of hydrogen and diffusion to the W surface was found to take place in long timescales after the pulse shutdown.
The diffusion of hydrogen in tungsten is studied as a function of temperature, hydrogen concentra... more The diffusion of hydrogen in tungsten is studied as a function of temperature, hydrogen concentration and pressure using Molecular Dynamics technique. A new analysis method to determine diffusion coefficients that accounts for the random oscillation of atoms around the equilibrium position is presented. The results indicate that the hydrogen migration barrier of 0.25 eV should be used instead of the presently recommended value of 0.39 eV. This conclusion is supported by both experiments and density functional theory calculations. Moreover, the migration volume at the saddle point for H in W is found to be positive: ∆V m ≈ 0.488Å^3 , leading to a decrease in the diffusivity at high pressures. At high H concentrations, a dramatic reduction in the diffusion coefficient is observed, due to site blocking and the repulsive H-H interaction. The results of this study indicates that high flux hydrogen irradiation leads to much higher H concentrations in tungsten than expected.
The diffusion of monovacancies in tungsten is studied computationally over a wide temperature ran... more The diffusion of monovacancies in tungsten is studied computationally over a wide temperature range from 1300 K until the melting point of the material. Our modelling is based on Molecular Dynamics technique and Density Functional Theory. The monovacancy migration barriers are calculated using nudged elastic band method for nearest and next-nearest neighbour monovacancy jumps. The diffusion pre-exponential factor for monovacancy diffusion is found to be two to three orders of magnitude higher than commonly used in computational studies, resulting in attempt frequency of the order 10^15 Hz. Multiple nearest neighbour jumps of monovacancy are found to play an important role in the contribution to the total diffusion coefficient, especially at temperatures above 2/3 of T m , resulting in an upward curvature of the Arrhenius diagram. The probabilities for different nearest neighbour jumps for monovacancy in W are calculated at different temperatures.
Particle irradiation produces defects which trap hydrogen isotopes and impurities in nuclear reac... more Particle irradiation produces defects which trap hydrogen isotopes and impurities in nuclear reactor materials. However, a comprehensive understanding of the basic mechanisms, and the final outcome of this process is still lacking. Here the evolution of defects, hydrogen, and impurities in tungsten during and after deuterium irradiation is simulated by solving rate theory equations. The results are in excellent agreement with irradiation experiments. Our results show that hydrogen is mainly trapped in tungsten monovacancies, and trapping in larger vacancy clusters increase with increasing implantation energy. The slow hydrogen desorption observed in experiments after irradiation, was found to be mainly due to detrapping of the weakly bound sixth hydrogen from monovacancies. Impurities are shown to play a significant role in decreasing Frenkel pair annihilation during irradiation, by trapping self-interstitial atoms. Moreover, we conclude that the formed impurity self-interstitial atom complexes could be the nucleation site for formation of large interstitial type dislocation loops observed experimentally.
First-principles calculations were used in determining the binding and trapping properties of hyd... more First-principles calculations were used in determining the binding and trapping properties of hydrogen to point defects in tungsten. Hydrogen zero-point vibrations were taken into account. It was concluded that the monovacancy can hold up to five hydrogen atoms at room temperature. The hydrogen was found to distort the self-interstitial atom configuration geometry. The interaction of hydrogen with the transmutation reaction impurities Re and Os were studied. It was found that the substitutional Re and Os have a negligible effect on the hydrogen trapping whereas the interstitial Os may increase the hydrogen inventory in tungsten.
The first-principles calculations were used to study the hydrogen energetics on the 100 tungsten ... more The first-principles calculations were used to study the hydrogen energetics on the 100 tungsten 2 2R45° surface. Two equilibrium sites for H at the surface are identified, with a low migration barrier from the energetically clearly higher long bridge site to the short bridge site. At low coverages, the majority of H surface diffusion events take place via the short bridge sites. The energetics for H penetration from the surface to the solute site in the bulk was defined, showing that the bulk H diffusion via neighboring tetrahedral sites takes place at depths beyond the second subsurface layer.
A reactive interatomic bond-order potential for bcc tungsten is presented. Special attention in t... more A reactive interatomic bond-order potential for bcc tungsten is presented. Special attention in the potential development was given for obtaining accurate formation and migration energies for point defects, making the potential useful in atomic scale simulations of point and extended defects. The potential was used to calculate binding energies and trapping distances for vacancies in vacancy clusters and the recombination radius for self-interstitial atom and monovacancy.
First principle calculations were used to study the hydrogen migration properties in bulk bcc
tun... more First principle calculations were used to study the hydrogen migration properties in bulk bcc tungsten. Hydrogen has low solubility in tungsten and occupies the tetrahedral interstitial site with an energy difference of 0.38 eV compared to the octahedral interstitial site. The hydrogen diffusion coefficient was evaluated using the harmonic transition state theory and was found to agree with the experimental results at temperatures above 1500 K. The height of the migration barrier between two adjacent tetrahedral sites was found to be 0.21 eV, which is lower than the value 0.39 eV obtained for the migration barrier from degassing measurements in the temperature range between 1100 and 2400 K. The tunneling correction to the diffusion rate provides much better agreement with the experimental result at 29 K than the extrapolated experimental D from high temperature measurements.
We report in situ electron microscopy observations of the plastic deformation of individual nanom... more We report in situ electron microscopy observations of the plastic deformation of individual nanometer-sized Au, Pt, W, and Mo crystals. Specifically designed graphitic cages that contract under electron irradiation are used as nanoscopic deformation cells. The correlation with atomistic simulations shows that the observed slow plastic deformation is due to dislocation activity. Our results also provide evidence that the vacancy concentration in a nanoscale system can be smaller than in the bulk material, an effect which has not been studied experimentally before.
We would like to discuss the role that 1 MeV tritons produced in deuterium–deuterium fusion react... more We would like to discuss the role that 1 MeV tritons produced in deuterium–deuterium fusion reactions might play in a long-pulse or steady-state fusion reactor. Albeit a small minority in quantity compared to the fuel tritium, the fusion tritons have significantly longer penetration length in materials and can have detrimental consequences for the integrity of the components. Because deeply deposited atoms are not easily removed from the plasma-facing components, the fusion tritium inventory in a steady-state device is expected to be limited only by decay. Furthermore, unlike fuel tritium, it is not evenly distributed on the plasma-facing components. We conclude that, of the materials considered here, tungsten appears better than carbon or beryllium in this respect. Nonetheless, 1 MeV tritons from deuterium fusion should not be neglected when making material choices for ITER and, especially, for future fusion reactors. In particular, studies on the bulk effects of deeply penetrated tritium in tungsten are urgently needed if metal-wall reactors are considered for the future. This is an interdisciplinary problem needing the attention of material scientists and plasma physicists.
Deuterium retention in the implantation-induced defects in polycrystalline tungsten has been stud... more Deuterium retention in the implantation-induced defects in polycrystalline tungsten has been studied. Deuterium was implanted with different energies and concentrations of retained D were analysed with secondary ion mass spectrometry and nuclear reaction analysis. Annealings were carried out at four predetermined temperatures corresponding to four different defect types that can trap deuterium. A quantitative number of each defect type produced by 5, 15 and 30 keV D implantation with a dose of 5.8 × 10^16 cm^{−2} was obtained.
Tungsten has been proposed for first wall material in thermonuclear reactors, where its behaviour... more Tungsten has been proposed for first wall material in thermonuclear reactors, where its behaviour in the presence of hydrogen containing plasma irradiation at elevated temperatures is of key interest. Deuterium induced defects in polycrystalline tungsten have been studied. Deuterium was implanted into tungsten samples and retained D-concentrations were analyzed with nuclear reaction analysis and secondary ion mass spectrometry. We observed four different defect types that trap deuterium with release temperatures of 455, 560, 663 and 801 K. Total number of each defect type produced by 5.8 x 10^16 cm^{-2} 30-keV D implantation at room temperature was obtained to be 0.260, 0.156, 0.082 and 0.056 traps cm^{-2}/implanted D atom.
Flaking of carbon film was studied by depositing hydrogen doped carbon films on tungsten and moly... more Flaking of carbon film was studied by depositing hydrogen doped carbon films on tungsten and molybdenum substrates. Topographies of the carbon coatings were analyzed with scanning electron microscopy and three-dimension profilometer. Secondary ion mass spectrometry and Raman spectra analysis was used to determine the hydrogen concentration and the number of' sp^3 bonds in the films, respectively. Carbon film properties varied from tetrahedral amorphous carbon (ta-C) to hard hydrogenated amorphous carbon (a-C:H). Carbon coatings hold large residual stresses, which are released during or after deposition. Typical stress relief patterns were observed. In most samples buckling started at some dislocation or at the edge of the film and adhesion energy was deduced from the sizes of the stress relief patterns. It was observed, that carbon films were more adherent to tungsten than to molybdenum and that codeposited hydrogen had a significant effect to flaking propagation in the carbon films.
A Monte Carlo method using ion packets is introduced. The method presented is optimal for the low... more A Monte Carlo method using ion packets is introduced. The method presented is optimal for the low probability scattering simulations of multiple and plural backscattering effects. Non-Rutherford cross sections are easily implemented in the framework of this model.
The atomic layer deposition (ALD) technique has been used to deposit different types of hafnium a... more The atomic layer deposition (ALD) technique has been used to deposit different types of hafnium and zirconium silicates. The technique allows controlling the material thickness and quality due to atomic/molecular layer-by-layer growth mechanism. The films were deposited on 200 mm Si(1 0 0) substrates. Both thickness and Hf/Si or Zr/Si ratio were varied. Rutherford backscattering spectrometry and time-of-flight elastic recoil detection analysis were used to determine film composition and impurities distribution. Thickness and refractive index of the coatings were measured by spectroscopic ellipsometry. Our measurements showed the presence of photoluminescence in the Hf-silicate films.
Six Joint European Torus (JET) divertor tiles were coated with tungsten marker stripes for erosio... more Six Joint European Torus (JET) divertor tiles were coated with tungsten marker stripes for erosion/deposition studies. The average thickness was 3.4 um and the film was uniform within 5% across and within 10% along the stripe. Small amounts of impurities C, Fe, Ni and Cr were found and the in-plane stress was 965 MPa. Resistance to high heat loads was tested in the Neutral Beam Test-bed at JET where a coated carbon fibre composite block was exposed to a total of 113 pulses with a maximum peak power density of 15.7 MW m^{-2} and a maximum surface temperature of 1280 C. No visible changes as a result of thermal cycling nor sputtering of the film by deuterium could be detected.
The interstitial to substitutional nitrogen atoms as a function of concentration in GaNAs were de... more The interstitial to substitutional nitrogen atoms as a function of concentration in GaNAs were determined by nuclear reaction analysis utilizing the 14 N(d,p) 15 N and 14 N(d,) 12 C reactions using ion channeling technique. The GaNAs films with mean nitrogen concentration between 0.3% and 3%, measured with secondary ion mass spectrometry and time-of-flight elastic recoil detection analysis, were grown using gas-source molecular-beam epitaxy. The fraction of nitrogen atoms occupying substitutional sites was observed to increase linearly with increasing nitrogen amount, while the concentration of interstitial nitrogen was nearly constant at 2x10^19 cm^{-3} throughout the concentration region. Annealing at 750 °C decreases the concentration of interstitial nitrogen.
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Papers by T. Ahlgren
tungsten. Hydrogen has low solubility in tungsten and occupies the tetrahedral interstitial site with
an energy difference of 0.38 eV compared to the octahedral interstitial site. The hydrogen diffusion
coefficient was evaluated using the harmonic transition state theory and was found to agree with the
experimental results at temperatures above 1500 K. The height of the migration barrier between two
adjacent tetrahedral sites was found to be 0.21 eV, which is lower than the value 0.39 eV obtained
for the migration barrier from degassing measurements in the temperature range between 1100 and
2400 K. The tunneling correction to the diffusion rate provides much better agreement with the
experimental result at 29 K than the extrapolated experimental D from high temperature
measurements.
tungsten. Hydrogen has low solubility in tungsten and occupies the tetrahedral interstitial site with
an energy difference of 0.38 eV compared to the octahedral interstitial site. The hydrogen diffusion
coefficient was evaluated using the harmonic transition state theory and was found to agree with the
experimental results at temperatures above 1500 K. The height of the migration barrier between two
adjacent tetrahedral sites was found to be 0.21 eV, which is lower than the value 0.39 eV obtained
for the migration barrier from degassing measurements in the temperature range between 1100 and
2400 K. The tunneling correction to the diffusion rate provides much better agreement with the
experimental result at 29 K than the extrapolated experimental D from high temperature
measurements.