Imaging Opportunities for High-pressure Research
D. Walker
Lamont-Doherty Earth Observatory, Columbia University, Palisades NY 10964
Interesting physical chemistry insight often arises from observations of reaction zones between chemically incompatible substances. Geologic skarns in nature and corrosion in the lab are examples of such spatially-resolved reaction zones. Observation of the growth of new intermediate phases and of the compositional shifts of adjacent phases, are extremely useful for defining phase equilibria and saturation boundaries in an efficient manner. Recognition of the spatial dimension to the reaction information provides considerably more insight than characterization of bulk-mixture reactions alone. In natural rocks from depth, all scales from map pattern, to mesoscopic, to hand-specimen, to microscopic, and to grain boundaries provide features of interest after the reaction has been exhumed to the Earth’s surface. However in the laboratory under extreme conditions such as high pressure, spatial scales are much more limited towards the lowest end of the natural range. The limits are the small physical dimensions of the reaction chamber and the limited times available for mass transfer to generate reaction encrustations physically large enough to be observable. Mother nature is more patient but has a rather limited palette of compositions that can encounter one another in interesting reaction situations - which are generally not observable in real time.
The ability to observe reactions in
progress at very small, spatially-resolved scales at high-pressure would
represent a new opportunity for high-pressure research in the diamond
anvil cell (DAC). The prospect of sub-micron beams of hard X-rays opens
the possibility of in situ characterization of DAC spatial-reaction
experiments on the sub-micron scale which is the appropriate scale for
many DAC reaction phenomena, for instance between micron-sized grains.
Real-time imaging is especially important for unquenchable reactions.
Examples of solubility measurements of non-metals in molten Fe under DAC
conditions by X-ray absorption-contrast imaging illustrate the potential
power of the spatially resolved reaction imaging technique. The examples
also illustrate that huge advances could be made by improvement in spatial
resolution of an order of magnitude better than the current state of the
art at ~1 micron. The ERL prospectus suggests that advances to
considerably under 1-micron beams may be achievable.
If so, then the new limits to progress will be in our ability to stably exploit the raster strategy for micro-imaging and our ability to spatially arrange the initial reactants in an optimal fashion with micro- and nano-fabrication techniques.
An X-ray absorption contrast image is taken through a DAC in which a compressed Fe gasket has a hole loosely stuffed with ~10 micron FeO powder. Spatial resolution of this image approaches a micron.
