Laser heating at megabar pressures: Melting temperatures of iron and other transition metals
The melting temperature of iron at high pressure is key to deriving the temperature in the Earth’s interior because theboundary between the solid inner core and the liquid outer core at about 3.3 Mbar is due to the melting (or freezing) of an iron-rich alloy. Data at core pressures measured in the laser-heated diamond cell (up to 200 GPa) have been reported over ten years ago (Boehler, 1993). In the last few years the experimental data obtained with this technique have converged, but there is still considerable disagreement between shock data, theory and diamond cell measurements. Thispaper will provide some of the latest dateon the phase diagram of iron and compare its melting curve with some other transition metals. This comparison is useful to understand systematic behaviour in transition metal melting.
What is the melting temperature of iron at 3.3 Mbar? This question has addressed by many researchers over the past 20 years and the answers reach from 2000 to 10000 K. Figure 1 represents the latest solutions obtained from shock experiments, diamond cell experiments and from theory, and clearly indicates the difficulties associated with the question.
The experimental techniques for generating high temperatures in the laser-heated diamond cell, pressure and temperature measurements are described in Chapter 9 in this volume (Boehler, 2005). Melting can be detected in situ for most materials by several methods: 1) by measuring discontinuous changes of the absorption of the laser radiation, 2) from changes in the reflectivity of the sample at a wavelength different from that of the heating laser, and 3) direct visual observation of melting on the sample surface.
Figures & Tables
Extreme conditions and their effects on matter and materials are currently fashionable topics in modern science. Perhaps the fascination derives from the unimaginable dimensions that grab our attention and push the boundaries of our imagination. Imagine the pressures in extremely dense neutron stars where electrons and protons are fused together and atoms collapse to the density of an atomic nucleus; imagine temperatures of thousands of degrees Kelvin at the solar surface, or multimegabar and terapascal pressures deep within the interior of our planets. But even a simple droplet of water represents an extreme environment when it comes into contact with an otherwise stable crystal of rock salt, causing the crystal to dissolve as external conditions are drastically changed. We have an inherent desire to understand these diverse kinds of phenomena in nature, the mechanisms of the material changes involved, as well as the extreme conditions which are becoming increasingly demanded to achieve the extraordinary performance of new engineering materials. This rapidly evolving area of science is necessarily interdisciplinary, as it combines fundamental physics, chemistry and biology with geoplanetary and materials science, in addition to increasingly becoming one of the keys to engineering and technology aimed at process optimisation. Current experimental methods permit materials to be studied at pressures of several megabars, temperatures of tens of thousands of degrees Kelvin, and to achieve magnetic fields of several thousand teslas. Moreover, the rapid surge in computer technology has, in turn, permitted the solution of many previously intractable problems, and now even allows the behaviour of matter to be predicted far beyond the range of conditions currently accessible to experimentation. Previously unknown phenomena such as the formation of new phases, new forms of electronic and magnetic order, melting, atomic and electronic excitation, ionisation or the formation of a plasma state might result from exposing matter to extreme conditions well beyond those which were characteristic of the equilibria at the time of formation. With this volume of EMU Notes in Mineralogy we have endeavoured to provide up-to-date reviews of our understanding of the behaviour of minerals and geomaterials at exterior conditions that are sufficiently extreme to induce changes. In total 18 chapters reflect the diversity of this theme, but also demonstrate how strongly interdisciplinary this domain of modern mineralogy has become, bringing together physicists, chemists and geologists as well as experimentalists and computer scientists. The present volume contains the contributions of the lectures presented at the 7th EMU School, held at the University of Heidelberg from June 19 to June 25, 2005.