ScienceWeek

MATERIALS SCIENCE: AN ATOM-LEVEL VIEW OF MELTING

The following points are made by B.J. Siwick et al (Science 2003 302:1382):

1) Solid-liquid phase transitions are an everyday occurrence. As one of the state variables (such as temperature or pressure) approaches a phase transition point for the melting of a solid, there are equilibrium fluctuations that lead to density changes commensurate with the new phase. Fluctuations important to a collective phase transition under these conditions occur over distributed time and length scales in which the atomic details are washed out.

2) By using short-pulsed lasers to deposit heat at a rate faster than the thermal expansion rate, it is possible to prepare extreme states of solid matter at temperatures well above the normal melting point (referred to as the "strongly driven limit"). Under such conditions, the atomic configuration of the entire excited material volume can be modified on the ultrafast time scale, and the melting transition of the prepared state can be seen as a simple model for transition state processes in general.

3) An atomistic view of such a process requires that the atomic configuration of the material be observed as it passes from the solid to the liquid state. Ideally, one would like to be able to fully resolve the relative atomic motions during the melting process. The information accessible through time-resolved diffraction experiments, where the observable is intimately connected with the atomic structure of the material (1), can approach such a description. The first experiment along these lines used electron diffraction combined with rapid laser heating, but lacked sufficient temporal resolution (20 to 100 ps) and structural sensitivity for an atomic-level perspective on the process (2). Important applications of time-resolved electron diffraction have recently provided atomic-level structural details of reactive intermediates in the gas phase occurring on a similar time scale (3-5). Approaches based on laser-driven x-ray plasma sources have provided improved temporal resolution (200 to 500 fs) but to date have an insufficient signal-to-noise ratio (structural sensitivity) to adequately resolve the atomic details.

4) In summary: The authors report they used 600-femtosecond electron pulses to study the structural evolution of aluminum as it underwent an ultrafast laser-induced solid-liquid phase transition. Real-time observations showed the loss of long-range order that was present in the crystalline phase and the emergence of the liquid structure where only short-range atomic correlations were present; this transition occurred in 3.5 picoseconds for thin-film aluminum with an excitation fluence of 70 millijoules per square centimeter. The sensitivity and time resolution were sufficient to capture the time-dependent pair correlation function as the system evolved from the solid to the liquid state. The authors suggest these observations provide an atomic-level description of the melting process, in which the dynamics are best understood as a thermal phase transition under strongly driven conditions.

References (abridged):

1. A. Guinier, X-Ray Diffraction in Crystals, Imperfect Crystals and Amorphous Bodies (Dover, New York, 1994)

2. S. Williamson, G. Mourou, J. C. M. Li, Phys. Rev. Lett. 52, 2364 (1984)

3. V. A. Lobastov et al., J. Phys. Chem. A 105, 11159 (2001)

4. H. Ihee et al., Science 291, 458 (2001)

5. R. C. Dudek, P. M. Weber, J. Phys. Chem. A 105, 4167 (2001)

Science http://www.sciencemag.org

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ON THE PROCESSES THAT TRIGGER MELTING

The following points are made by Robert W. Cahn (Nature 2001 413:582):

1) Melting has been a focus of physical theory for almost a century, the problem to understand how and why a crystalline solid melts, and what determines the temperature at which this occurs. Many theoretical criteria for melting have been proposed, of which two stand out, those by Frederick Lindemann (1886-1957) and Max Born (1882-1970).

2) Lindemann proposed that melting is caused by vibrational instability in the crystal lattice. Born, on the other hand, proposed that a "rigidity catastrophe" occurs, the catastrophe determining the melting temperature within the bulk crystal: the crystal no longer has sufficient rigidity to withstand melting, so this process is often called "mechanical melting".

3) These two distinct theories have each accumulated an extensive literature. It has been established experimentally that melting begins preferentially at a surface, and that superheating a crystal beyond the melting point set by the surface melting requires this process to be impeded. For example, coating the surface with a metallic layer that has a higher melting point can suppress surface melting and retain the solid phase to bulk temperatures well above the equilibrium melting point.

4) Z-H. Jin et al (Phys. Rev. Lett. 2001 87:055703) have reported a molecular dynamics simulation that demonstrates that as a crystal is heated, melting is triggered by instabilities governed simultaneously by the Lindemann and Born criteria.

Nature http://www.nature.com/nature

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