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| Titel |
Combined measurement of surface, grain boundary and lattice diffusion coefficients on olivine bi-crystals |
| VerfasserIn |
Katharina Marquardt, Ralf Dohmen, Johannes Wagner |
| Konferenz |
EGU General Assembly 2014
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 16 (2014) |
| Datensatznummer |
250095227
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| Publikation (Nr.) |
 EGU/EGU2014-10677.pdf |
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| Zusammenfassung |
| Diffusion along interface and grain boundaries provides an efficient pathway and may control
chemical transport in rocks as well as their mechanical strength. Besides the significant
relevance of these diffusion processes for various geologic processes, experimental data are
still very limited (e.g., Dohmen & Milke, 2010). Most of these data were measured using
polycrystalline materials and the formalism of LeClaire (1951) to fit integrated concentration
depth profiles. To correctly apply this formalism, certain boundary conditions of the diffusion
problem need to be fulfilled, e.g., surface diffusion is ignored, and furthermore the
lattice diffusion coefficient has to be known from other studies or is an additional
fitting parameter, which produces some ambiguity in the derived grain boundary
diffusion coefficients. We developed an experimental setup where we can measure the
lattice and grain boundary diffusion coefficients simultaneously but independent and
demonstrate the relevance of surface diffusion for typical grain boundary diffusion
experiments.
We performed Mg2SiO4 bicrystal diffusion experiments, where a single grain boundary is
covered by a thin-film of pure Ni2SiO4 acting as diffusant source, produced by pulsed laser
deposition. The investigated grain boundary is a 60° (011)/[100]. This specific grain
boundary configuration was modeled using molecular dynamics for comparison with the
experimental observations in the transmission electron microscope (TEM). Both,
experiment and model are in good agreement regarding the misorientation, whereas there
are still some disagreements regarding the strain fields along the grain boundary
that are of outmost importance for the strengths of the material. The subsequent
diffusion experiments were carried out in the temperature range between 800°
and 1450°C. The inter diffusion profiles were measured using the TEMs energy
dispersive x-ray spectrometer standardized using the Cliff-Lorimer equation and EMPA
measurements.
To evaluate the obtained diffusion profiles we adapted the isolated grain boundary model,
first proposed by Fisher (1951) to match several observations: (i) Anisotropic diffusion in
forsterite, (ii) fast diffusion along the grain boundary, (iii) fast diffusion on the
surface of the sample. The latter process is needed to explain an additional flux
of material from the surface into the grain boundary. Surface and grain boundary
diffusion coefficients are on the order of 10000 times faster than diffusion in the lattice.
Another observation was that in some regions the diffusion profiles in the lattice were
greatly extended. TEM observations suggest here that surface defects (nano-cracks,
ect.) have been present, which apparently enhanced the diffusion through the bulk
lattice.
Dohmen, R., & Milke, R. (2010). Diffusion in Polycrystalline Materials: Grain
Boundaries, Mathematical Models, and Experimental Data. Reviews in Mineralogy and
Geochemistry, 72(1), 921–970.
Fisher, J. C. (1951). Calculations of Diffusion Penetration Curves for Surface and Grain
Boundary Diffusion. Journal of Applied Physics, 22(1), 74–77.
Le Claire, A. D. (1951). Grain boundary diffusion in metals. Philosophical Magazine A,
42(328), 468–474. |
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