Self‐diffusion is a fundamental physical process that, in solid materials, is intimately correlated with both microstructure and functional properties. Local transport behavior is critical to the performance of functional ionic materials for energy generation and storage, and drives fundamental oxidation, corrosion, and segregation phenomena in materials science, geosciences, and nuclear science. Here, an adaptable approach is presented to precisely characterize self‐diffusion in solids by isotopically enriching component elements at specific locations within an epitaxial film stack, and measuring their redistribution at high spatial resolution in 3D with atom probe tomography. Nanoscale anion diffusivity is quantified in a‐Fe2O3 thin films deposited by molecular beam epitaxy with a thin (10 nm) buried tracer layer highly enriched in 18O. The isotopic sensitivity of the atom probe allows precise measurement of the initial sharp layer interfaces and subsequent redistribution of 18O after annealing. Short‐circuit anion diffusion through 1D and 2D structural defects in Fe2O3 is also directly visualized in 3D. This versatile approach to study precisely tailored thin film samples at high spatial and mass fidelity will facilitate a deeper understanding of atomic‐scale diffusion phenomena.
The variability among prior data for FLiBe is 11% for
the liquid
density and 61% for the thermal expansivity. New liquid density and
thermal expansivity data are collected, with particular attention
to uncertainty quantification. We discuss and quantify bounds for
possible sources of variability in the measurements of liquid density:
salt composition (<0.6% per 1 mol % BeF2), salt contaminants
at 100 s ppm to <1 mol% (2%), Li isotopic composition (2%), sample
isothermal conditions (0.2%), dissolved gases (<0.3%), and evolution
of bubbles with temperature transients – depending on Ar or
He cover gas (0.1 or 0.6% for dilatometry, 1 or 5% for hydrostatic
measurements). To aid in quantifying thermal expansivity sensitivity
to composition, we review and generalize the ideal molar volume prediction
for FLiBe; to improve this model, measurements are needed for the
thermal expansivity of BeF2. We collect new data on the
density of liquid FLiBe using the hydrostatic method and 170 g of
hydrofluorinated FLiBe with less than 0.13 mol % impurities (dominantly
Al, K, Na, Mg, Ca), as determined by ICP-MS. We obtain the following:
ρ
F
L
i
B
e
[
kg
m
3
]
=
2245
(
7
)
−
0.424
(
17
)
normalT
[
C
°
]
;
447
0.25em
to
0.25em
820
normalC
°
0.25em
for
0.25em
33.59
(
5
)
mol
%
BeF
2
,
Li
7
Li
6
(
at
)
=
13.544
(
4
)
,
33.02
0.25em
(
5
)
0.25em
normalg
/
mol
0.25em
FLiBe
.
The dominant sources of uncertainty are
the bobber volume uncertainty (0.15%), the mass measurement uncertainty
(0.2%), and possibly the wetting angle of the salt on the wire (<0.3%).
Occasional noise and <0.2% deviation from linearity may be due
to the formation of gas bubbles on the bobber surface from the temperature-dependence
of gas solubility; repeatable results for heating and cooling runs
provide confidence that bubble effects are well managed in this experimental
setup. These are the first measurements of the liquid density of FLiBe
that report error analysis and that measure the liquid composition
before and after density measurements.
Fast atomic diffusion through structural defects in Fe 2 O 3 is directly imaged for the first time at the sub-nanometer level by atom probe tomography. In article number 2001768 by Tiffany C. Kaspar and co-workers, model epitaxial and defect-rich thin films of Fe 2 O 3 are deposited with molecular beam epitaxy, incorporating a tracer layer of 18 O within the film itself. Atomic-level self-diffusion is then visualized and quantified in 3D by APT, revealing ≈10 4 times faster oxygen diffusion along structural flaws than through the pristine lattice.
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