April 10, 2007
Science Cafe
Last night I had the opportunity to combine two of my favorite things: science and beer. (No, I wasn't drinking in the lab.) The Great Lakes Brewery hosted Cleveland's inaugural "Science Cafe", an open forum for the discussion of scientific issues in an informal setting. Last night's discussion focused on global warming, with Case geology professor Beverly Saylor and Case economist Gary Murphy leading the discussion. The forum kicked off with a short Nova film on the connection between global warming and hurricane intensity, followed by a lively discussion on the scientific and economic issues surrounding greenhouse gas emissions and energy policy.
Science Cafe is scheduled to happen the second Monday of each month in the basement of the Great Lakes brewery. Next month's topic: stem cell research.
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April 05, 2007
Welcome to my blog...
If you happen to have clicked on a link to this blog before, you may have been disappointed to see that it was empty. I set up the blog a few months ago without having a clear idea of what to do with it.
Now I've decided it makes sense to use this blog as a way of providing updates on recent research activity in the experimental geochemistry & mineral physics lab (in the Geological Sciences Department at Case Western Reserve University).
Molly Yunker and I recently published a paper in Earth and Planetary Science Letters entitled "Interdiffusion of solid iron and nickel at high pressure". This paper is a product of Molly's B.S./M.S. thesis at Case. She did a fantastic job and has since moved on to pursue a Ph.D. in science education at the University of Michigan.
Atomic diffusion in iron-nickel alloys is a fundamental process in creep deformation of the inner core (which may be responsible for producing the pronounced seismic anistropy observed in this most remote region of the Earth) and in chemical exchange between the solid inner core and the surrounding liquid alloy of the outer core. The inner core is under immense pressure and temperature (more than three million times the pressure we feel at Earth's surface, and similar to the temperature at the surface of the Sun). We can't (yet) perform experiments to measure diffusion rates at such extreme conditions, and this limitation makes our understanding of kinetic processes in Earth's core somewhat murky. Molly's important contribution was to determine diffusion rates in iron-nickel alloys at pressures up to 23 GPa (235,000 atmospheres) and temperatures up to 1973 K (3092 degrees F) using a multi-anvil device. These experiments extend the pressure range over which diffusion rates have been determined by a factor of six, and thus provide a much better basis for extrapolating diffusion-related properties to inner core conditions. An important finding of this study is that diffusion coefficients over the entire pressure range are well described by a simple function of the homologous temperature (i.e. the ratio of the actual temperature to the melting temperature of the alloy).
Another recent publication in Geochimica et Cosmochimica Acta is the product of a collaboration with Dan Lacks and David Rear (a Ph.D. student) in the Chemical Engineering department at Case. This paper presents the results of molecular dynamics simulations on silicate liquids in the simple MgO-SiO2 system at high pressures. Silicate melts have played a key role in the differentiation of the Earth from its earliest history to the present. To understand the earliest history, in particular, we need to know the density, viscosity and chemical diffusion properties of silicate magmas under a wide range of pressures and temperatures. Molecular dynamics simulations are a useful tool in this regard because they allow all of these properties to be calculated, they are not restricted in pressure (as are experiments!) and they provide important insight into the structural controls on melt properties. Although this initial study is on a very simple system, this system encompasses melts with a very wide range of structures. The simulation results are in good agreement with experimental results, where experimental results exist. An important finding is that liquids in the MgO-SiO2 system become more similar to each other at high pressure, leading their transport properties to converge to similar values and their mixing behavior to become more ideal. For example, a pure SiO2 liquid is calculated to be almost 10,000 times more viscous than MgO liquid at atmospheric pressure, but only about three times as viscous at a pressure of 19 gigapascals.
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