Revealing Oxygen Mixing Mechanisms at Surfaces

Discovery of the chemical mechanisms that mix oxygen into the surface layers of an oxide. Free oxygen and metal atoms on the surface were found to enable these reactions. 

Significance and Impact

The interaction of oxygen with surfaces is of key importance in energy and environmental applications. Understanding these chemical mechanisms impacts thin film synthesis, corrosion, and heterogeneous catalysis.

Research Details

• Epitaxial Fe2O3 and Cr2O3 films and heterostructures were deposited by molecular beam epitaxy
• Intermixing of 18O tracer layers was quantitatively measured by atom probe tomography
• Key intermixing mechanisms were identified using atomistic simulations and informed a model of intermixing during growth

Citation

Adatom-driven oxygen intermixing during the deposition of oxide thin films by molecular beam epitaxy
T. Kaspar, P. Hatton, K. Yano, S. Taylor, S. Spurgeon, B. Uberuaga, D. Schreiber
Nano Letters (2022). Accepted.

Microstructural Dependence of Vacancy Formation in Iron-Oxide Thin Films

Positron annihilation spectroscopy was used to
characterize point defects and measure their depth and size distributions in three different metal/oxide thin films

Significance and Impact

• Initial film morphology leads to very different defect content.
• Physical vapor deposition conditions, namely temperature and bias strictly define oxide morphology.

Research Details

• Two oxide films grown via physical vapor deposition at 600 oC and room temperature exhibited dense-epitactic and columnar-polycrystalline, microstructures respectively. A third oxide film grown in open air at 600 oC exhibited an eqiuaxed, porous morphology.
• Cubic maghemite and magnetite phases in each oxide film were identified via grazing incidence X-ray diffraction.
• Positron annihilation spectroscopy showed a range of average positron lifetimes from 0.23 ns in the high temperature, vapor deposited film, 0.35 ns in the room temperature-grown film, and 0.31 ns in the thermally grown oxide.

Citation

Microstructural Dependence of Vacancy Formation in Iron-Oxide Thin Films
B. Derby, S. Mills, S. Agarwal, J. Valdez, J. Baldwin, M. Schneider, A. Minor, B. Uberuaga, F. Selim, N. Li
Applied Surface Science (2022). DOI: 10.1016/j.apsusc.2022.152844

The Effect of Chromium on Mass Transport Along Ni Grain Boundaries

Molecular dynamics (MD) simulations show that the addition of Cr has a small impact on the rate of defect transport along grain boundaries (GBs) in Ni, tending to reduce mobility in most cases.

Significance and Impact

The addition of Cr alone does not drive the high rates of transport seen during the dealloying of grain boundaries in Ni-Cr alloys exposed to molten salts, indicating that other explanations must be searched for.

Research Details

• Four very different grain boundaries were alloyed with different amounts of Cr and transport due to both interstitials and vacancies were simulated with MD.
• At tilt GBs, Cr significantly reduces interstitial transport along the tilt axis, except for special cases in which Cr is well aligned.
• At twist GBs, interstitials and vacancies migrate at similar rates and Cr slows both mechanisms slightly.

Citation

The effect of Cr alloying on defect migration at Ni grain boundaries
B. P. Uberuaga, P. Simonnin, K. M. Rosso, D. K. Schreiber, and M. Asta
Journal of Materials Science (2022). DOI: 10.1007/s10853-021-06590-x

Radiation Drives Oxygen Through Protective Oxides

Irradiation dramatically accelerates oxygen movement through Cr2O3 by up to 5 orders of magnitude at reactor-relevant temperatures.

Significance and Impact

Nanoscale Cr2O3 films on corrosion-resistant alloys inhibit degradation by slowing atomic transport. Our work shows that irradiation reduces this protectiveness by increasing O mobility under combined irradiation and corrosion conditions in nuclear reactors.

Research Details

• An atomically precise model Cr2O3 film was deposited by molecular beam epitaxy (MBE) with an embedded 18O tracer layer to measure diffusion
• Films were irradiated with Ar+ at reactor-relevant temperatures (350–500°C), and the 18O diffusion was quantified using atom probe tomography (APT)
• A chemical rate theory model was developed to interpret the dose and temperature dependence of radiation-enhanced anion diffusion

Citation

Radiation Enhanced Anion Diffusion in Chromia
K. Yano, A. Kohnert, T. Kaspar, S. Taylor, S. Spurgeon, H. Kim, Y. Wang, B. P. Uberuaga, and D. K. Schreiber
Journal of Physical Chemistry C (2021). DOI: 10.1021/acs.jpcc.1c08705

In situ Monitoring of Corrosion under Irradiation (PIXE-ICE)

Proton-induced X-ray emission (PIXE) spectroscopy provides an in situ diagnostic to continuously monitor the thickness of samples simultaneously corroded and irradiated in the irradiation-corrosion experiment (ICE) III.

Significance and Impact

In situ quantification of irradiation-corrosion experiments is rare; PIXE monitors the dissolution of pure Fe under coupled irradiation and corrosion and gives detailed insight into irradiation-corrosion kinetics upon validation.

Research Details

• Protons induce damage in the sample (pure Fe) and are stopped in the corrosive medium (lead-bismuth eutectic; LBE), simultaneously producing countable X-rays in both.
• The ratio of LBE-Fe X-rays changes over time as corrosion progresses. Here, the ratio increases as Fe is dissolved and LBE penetrates the sample.
• Thickness estimates for ICE are based on X-ray ratios from a standard with known Fe foil thicknesses.

Citation

Continuous Monitoring of Pure Fe Corrosion in Pure Lead-Bismuth Eutectic Under Irradiation with Proton-Induced X-Ray Emission Spectroscopy
F. Schmidt, M. Chancey, H. Kim, Y. Wang, and P. Hosemann
JOM (2021). DOI: 10.1007/s11837-021-04954-x

Oligomeric solvation of Cr in molten fluorides

Employed ab-initio molecular dynamics simulations to quantify the nature of the solvation structure of Cr corrosion products in molten fluoride salts, including the role of chemistry-dependent medium-range oligomeric order in the solvent salts, and solute charge state.

Significance and Impact

This work establishes new principles for understanding corrosion product solvation in molten fluorides. It challenges views of solvation that rely only on dissociated fluorine ions and instead emphasizes the role of medium-range oligomeric order, which varies strongly with salt chemistry, and has substantial implication on dissolution mechanisms in molten salt corrosion.

Research Details

• Density functional theory-driven molecular dynamics was performed on three different molten fluoride solvents: 2KF-NaF, 2LiF-BeF2, and 3LiF-AlF3
• We analyzed both the short- and medium-range order of these salts using radial distribution functions, cage-correlation functions, and oligomer size distributions
• We added Cr0, Cr+2, and Cr+3 to each salt to determine how Cr incorporates into the existing salt
  structure
• Found that Cr can be solvated not only by dissociated F-, but also by oligomers of host associates
• Derived evidence of direct bonding between Cr0 and Be/Al, which suggests Cr0 dissolution may be
  possible

Citation

Ab-initio simulation studies of chromium solvation in molten fluoride salts
N. Winner, H. Williams, R. Scarlat, M. Asta
Journal of Molecular Liquids 335, 116351 (2021). DOI: 10.1016/j.molliq.2021.116351

Dissolution of Oxide Films in Molten Fluoride Salt

Pre-existing oxide films on structural materials are unstable when exposed to molten salt, dissolving and ultimately reacting to form new oxides at microstructural features.

Significance and Impact

This work helps elucidate the corrosion mechanism of metals and their oxides in molten fluoride salt and provides new insight into the chemical purity of the salt and its lifetime in nuclear reactor environments.

Research Details

• The corrosion behavior of oxide films grown on 316L stainless steel in molten FLiNaK salt at 700 °C was studied using electrochemical impedance spectroscopy, and oxide dissolution rate was determined.
• After the oxide film first dissolves, Cr and Fe are further dissolved from the alloy into the salt, inducing intergranular cracking of the steel.
• The dissolution of the oxide forms O2− impurities in the salt, which reacts with steel and form FeCr2O4 precipitates at GB.

Citation

Electrochemical study of the dissolution of oxide films grown on type 316L stainless steel in molten fluoride salt
J. Qiu, D.D. Macdonald, R. Schoell, J. Han, S. Mastromarino, J.R. Scully, D. Kaoumi, P. Hosemann
Corrosion Science 186, 109457 (2021). DOI: 10.1016/j.corsci.2021.109457

Positron annihilation spectroscopy of defects in nuclear and irradiated materials

The paper represents the first review article on PAS of nuclear and irradiated materials, explaining PAS in a context relevant to nuclear and material scientists and discusses in depth PAS contributions in understanding radiation damage mechanisms since the 1970’s. It offers a vision of new positron capabilities for in-situ measurements and studies of materials under extreme conditions.

Significance and Impact

This review will help guide the nuclear materials community on the application and interpretation of PAS in irradiated materials and advanced defect studies in emerging new materials for advanced nuclear technology. Understanding radiation damage mechanisms in these materials cannot be accomplished without the implementation of PAS thanks to its unique sensitivity (1 ppm) and atomic scale resolution of defects.

Research Details

• The principles of PAS defects, its techniques, and data analysis were explained.
• The important milestones in PAS studies of neutron, ion and He irradiated materials, and defect kinetics and annealing, were featured.
• The most recent results that expanded PAS capability to full quantification of radiation induced defects in terms of size, density and depth distribution were discussed.
• Development of in-situ PAS capability was addressed. Design ideas for in-situ PAS during irradiation, annealing, and corrosion were suggested and their potential impact was demonstrated.
• The potential and important role that PAS can play in the development of small modular reactors was discussed.

Citation

Positron annihilation spectroscopy of defects in nuclear and irradiated materials - a review
F.A. Selim
Materials Characterization 174, 110952 (2021). DOI: 10.1016/j.matchar.2021.110952

Interplay between defect transport and spins

We reveal a coupling between defect migration and magnetic spins in antiferromagnetic oxides – migration creates misaligned spins which change the migration barrier for the defect.

Significance and Impact

We reveal a coupling between defect migration and magnetic spins in antiferromagnetic oxides – migration creates misaligned spins which change the migration barrier for the defect.

Research Details

• Density functional theory calculations were performed to determine the migration energy of cation interstitials in both magnetic and non-magnetic corundum-structured oxides.
• Two limits were considered for Fe2O3 – an important oxide in energy materials and in the corrosion of iron-based materials – an antiferromagnetic structure in which spins are antialiigned and a ferromagnetic structure in which they are aligned.
• The migrating interstitial leads to misaligned spins, each of which costs about 0.5 eV. This causes a direct increase in the migration barrier of the interstitial by about 1 eV compared to the ferromagnetic case.

Citation

Interplay between defect transport and cation spin frustration in conundrum-structured oxides
A. Banerjee, A. A. Kohnert, E. F. Holby, B. P. Uberuaga
Physical Review Materials 5, 034410 (2021). DOI: 10.1103/PhysRevMaterials.5.034410

Effects of Radiation-Induced Defects on Corrosion

Some irradiation-corrosion (IC) phenomena (radiolysis, irradiation-assisted stress corrosion cracking) have been studied extensively, but little work has targeted the role of radiation damage in the material itself on corrosion.

Significance and Impact

Experimental evidence indicates that radiation-induced defects in the metal can impact corrosion behavior, yet quantitative observations and mechanistic explanations are often lacking. Understanding the similarities and differences of these interactions in different corrosive medium – material couples is vital for alloy development, coolant chemistry control, and, ultimately, reactor safety, especially for Gen-IV reactor concepts.

Research Details

• We summarized the state of knowledge about IC interactions between several reactor-relevant materials (steels and nickel- and zirconium-based alloys) and different coolants (water, heavy liquid metal, and molten fluoride).
• An extensive body of literature discusses IC effects in water, specifically in zirconium alloys. Lessons learned in experimental design and interpretation of results can be applied to Gen-IV coolants as well.
• Effects of defects in the metal are often overshadowed by more obvious effects, such as radiolysis, so careful experimental design is required.

Citation

Effects of Radiation-Induced Defects on Corrosion
F. Schmidt, P. Hosemann, R.O. Scarlat, D.K. Schreiber, J.R. Scully, B.P. Uberuaga
Annual Review of Materials Research 51 (2021). DOI: 10.1146/annurev-matsci-080819-123403

A New Radiation Damage Mechanism from Combined PAS and TEM Analysis

Pre-existing voids interact with collision cascades to modify damage creation. Small vacancy clusters are created while pre-existing voids shrink, indicating a direct interaction between the voids and collision cascades.

Significance and Impact

Energy-moderated positron beams can probe the depth dependence of radiation damage produced by ion beams, allowing for non-destructive analysis of damage and paving the way for new in situ studies and fundamental examination of defect production in defect-free films.

Research Details

• Highly defective Fe films grown and irradiated at LANL were interrogated using both positrons (BGSU, HZDR) and TEM (NCSU, PNNL).
• Positron annihilation spectroscopy and TEM (transmission electron microscopy) revealed complementary aspects of defect evolution with dose.
• Combined, the two techniques reveal a new mechanism for how collision cascades interact with pre-existing damage.

Citation

A New Mechanism for Void-Cascade Interaction from Non-destructive Depth-resolved Atomic-scale Measurements of Ion Irradiation-induced Defects in Fe
S. Agarwal, M. O. Liedke, A. C. L. Jones, E. M. Reed, A. A. Kohnert, B. P. Uberuaga, Y. Q. Wang, J. Cooper, D. Kaoumi, N. Li, R. Auguste, P. Hosemann, L. Capolungo, D. J. Edwards, M. Butterling, E. Hirschmann, A. Wagner, F. A. Selim
Science Advances 6, eaba8437 (2020). DOI: 10.1126/sciadv.aba8437

Measurement and simulation of vacancy formation in 2 MeV self-irradiated pure Fe

In-situ positron spectroscopy critical for capturing the transient populations of fast-moving defects induced by radiation damage

Positrons are sensitive to individual point defects, allowing for the measurement of different monovacancy concentrations from different irradiation dose rates.

(above) In-situ versus ex-situ measurement of vacancy concentration using positrons. Ex-situ experiments only capture larger, extended defects as the smaller defects disappear too quickly to measure ex-situ.

(above) 3D representation of the overlap between beam intensities from ions (red) and positrons (blue) after implantation in Fe. The positron beam intensity profile should be narrow and fit within the ion beam profile so the positrons sample the maximum damage region induced by the ion beam.

Significance and Impact

By simulating simultaneous radiation damage and positron spectroscopy, we can reveal properties of the smallest radiation-induced defects, leading to greater understanding of degradation mechanisms in nuclear materials.

Research Details

• Simulations reveal massive changes in monovacancy concentration with ion irradiation dose rate.
• Positron annihilation spectroscopy simulations reveal even small changes in monovacancy concentrations are detectable with positrons. Standard rate theory for radiation damage supports the conclusion that radiation-damage induced vacancies can be distinguished from thermally induced vacancies.
• The promise of the in-situ technique is to directly observe the smallest vacancy-type point defects in the damage cascade in a way not previously possible via ex-situ measurements.

Citation

Measurement and simulation of vacancy formation in 2 MeV self-irradiated pure Fe
R. Auguste, M. O. Liedke, F. A. Selim, B. P. Uberuaga, A. Wagner, P. Hosemann
Journal of Materials 72, 2436 (2020). DOI: 10.1007/s11837-020-04116-5

A critical assessment of the thermodynamics of vacancy formation in Fe2O3 using hybrid density functional theory

Exchange-correlation (XC) dependent vacancy thermodynamics in Fe2O3

Accuracy of DFT calculations are strongly dependent on the “flavor” of the XC functionals, with the choice of XC functional affecting defect energy levels and defect formation energies; hybrid functionals are necessary for realistic properties.

(above) HSE06 computed formation energies of iron (top left) and oxygen (top right) vacancies under O-rich conditions and Fe rich conditions as a function of the Fermi energy and charge state of the vacancy.  Under O-rich conditions, the formation energies of the two defects are similar, but under Fe-rich conditions, the disparity between the two is very large.

Significance and Impact

Vacancy properties in Fe2O3 are important for understanding the formation of rust/iron oxide, which is a major degradation process in structural materials, leading to significant economic impact each year.

Research Details

• DFT calculations are carried out to calculate the formation energy of oxygen and iron vacancies within hematite as a function of the Fermi energy over a range of charge states.
• In addition to HSE06, other XC functionals, such as GGA+U, SCAN are considered to critically asses the vacancy thermodynamics.
• Consistent with experiment, HSE06 prediction indicates that that Fe2O3 is relatively easily reduced but not oxidized.
• Effect of two different Hubbard-U and 7 different charge states for Fe and 5 different charge states for O are explored. The impact of cell size on the defect formation energy and relaxed defect structure is also studied

Citation

Critical Assessment of the Thermodynamics of Vacancy Formation in Fe2O3 Using Hybrid Density Functional Theory
A. Banerjee, A. A. Kohnert, E. F. Holby, B. P. Uberuaga
The Journal of Physical Chemistry C 124, 23988 (2020). DOI: 10.1021/acs.jpcc.0c07522 

Hydrogen induced p- and n-type conductivity in an ultra-wide band gap oxide

Hydrogen doping induces both n and p type conductivity in Ga2O3

n-type conductivity in Ga2O3 is over 9 orders of magnitude when high amount of hydrogen is incorporated, eventually switching to p-type conductivity as the hydrogen content is reduced.

Significance and Impact

By demonstrating for the first time a bipolar doping mechanism, the potential applications in optoelectronics and high-power electronics in which Ga2O3 might be used expands greatly.

Research Details

• Conductivity experiments reveal massive changes in conductivity with H doping.
• Positron annihilation spectroscopy reveals that the H is filling Ga vacancies. Density functional theory supports the conclusion that these vacancies can hold up to 4 H atoms, changing the net charge state of the defect.
• Thermally stimulated luminescence shows the creation of shallow acceptors and shallow donors as the vacancies are filled with 2 hydrogen and 4 hydrogen, respectively

Citation

Chemical manipulation of hydrogen induced high p-type and n-type conductivity in Ga2O3
M. M. Islam, M. O. Liedke, D. Winarski, M. Butterling, A. Wagner, P. Hosemann, Y. Wang, B. P. Uberuaga, F. A. Selim.
Scientific Reports 10, 6134 (2020). DOI: 10.1038/s41598-020-62948-2

Electrical Properties of Thermal Oxide Scales on pure Iron in Liquid Lead-Bismuth Eutectic (LBE)

Impedance response of oxide scales in liquid LBE

The impedance response of oxidized iron in a liquid LBE environment, a potential reactor coolant material, is sensitive to the integrity, thickness and defect density of the oxide scales.

(right) A cartoon shows the effect of oxidation temperature and time on oxide scales on pure Fe and the impedance behavior in liquid LBE. The resistance of the oxide film increases with both increasing oxidizing temperature and oxidation time, as both lead to a thicker film with fewer defects. In particular, the thicker films are less porous, leading to less ability of the film to short-circuit the EIS signal.

Significance and Impact

Electrochemical Impedance Spectroscopy (EIS) is an important in-situ probe into the properties of evolving oxide scales. However, EIS has not been widely applied to liquid metals. This work shows that EIS can be used to understand scale formation in liquid metals, but the interpretation is complex, depending on multiple factors beyond just the thickness of the oxide film.

Research Details

• The electrical properties of the pre-oxidized iron in liquid LBE were evaluated by EIS.
• With increasing the oxidation temperature, the resistance of the oxide film increases, due to the formation of a thicker scale and fewer defects over time.
• The impedance behavior of these oxide scales as measured in LBE is very different than when measured in more standard aqueous environments. Work is ongoing to understand the origin of this difference.

Citation

Electrical Properties of thick Oxide scales on pure iron in liquid lead-bismuth eutectic
J. Qiu, J. Han, R. Schoell, M. Popovic, E. Ghanbari, D. Kaoumi, J. R. Scully, D. D. Macdonald, P. Hosemann
Corrosion Science 176, 109052 (2020). DOI: 10.1016/j.corsci.2020.109052

A pathway to synthesizing single-crystal
Fe and FeCr films

Microstructure of single-crystal Fe and FeCr films depend on PVD conditions

The microstructure of single-crystal Fe and FeCr thin films, model systems for structural materials used in nuclear reactor components, is highly sensitive to the deposition temperature, substrate bias, and Cr content.

Significance and Impact

The synthesis and control over the morphology of model Fe-based alloys is vital for understanding the fundamental mechanisms that govern coupled irradiation and corrosion material response. This study significantly advances the understanding of how adding Cr solutes impacts the growth mechanisms governing Fe films during vapor deposition.

Research Details

• Fe and FeCr thin films were deposited using physical vapor deposition under a matrix of substrate temperature and bias.
• The film surface, chemistry, and microstructure were characterized using advanced electron microscopy.
• It was found that when Cr is alloyed with Fe, higher substrate temperature and bias are required for single-crystal films.
• Accelerated molecular dynamics simulations revealed that adding Cr to a growing Fe film impedes surface transport during the growth.

Citation

A pathway to synthesizing single-crystal Fe and FeCr films
B. Derby, J. Cooper, T. Lach, E. Martinez, H. Kim, J.K. Baldwin, D. Kaoumi, D.J. Edwards, D.K. Schreiber, B.P. Uberuaga, N. Li
Surface and Coatings Technology 403, 126346 (2020). DOI: 10.1016/j.surfcoat.2020.126346