Los Alamos National LaboratoryFUTURE: Fundamental Understanding of Transport Under Reactor Extremes
An Energy Frontier Research Center funded by the Department of Energy, Office of Basic Energy Sciences

FUTURE Highlights

Highlights of the science developed by FUTURE.
A New Radiation Damage Mechanism from Combined PAS and TEM Analysis (2020)

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.

(right) A cartoon showing the proposed new radiation damage mechanism as revealed from combined PAS and TEM analysis. PAS revealed that small vacancy clusters are created by the irradiation while TEM showed that the voids that were already there shrunk.

A large density of voids and pores were present in the pristine Fe film. As irradiation with Fe ions created collision cascades in the material, those cascades interacted with the pre-existing voids. The voids absorb interstitials created during the cascade, leaving behind an excess of vacancies and vacancy clusters. The net result is that more small vacancy clusters are created while the larger voids that were already in the material shrink.

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 (2020)

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.

(right) 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.
(right) 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 (2020)

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.

(right) 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 (2020)

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.

(right) A combination of positron annihilation spectroscopy and density functional theory identify the stability of four hydrogen ions occupying a single Ga vacancy in Ga2O3.

(right) Thermally stimulated luminescence and conductivity experiments reveal that, as hydrogen is incorporated into the material, the conductivity changes dramatically and, more surprisingly, shifts from n-type to p-type conductivity.

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) (2020)

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 carton 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