Small is Different: Melting and Freezing of Small Particles
We’re investigating the melting and freezing behavior of small metal particles (or clusters) containing <1000 atoms. In this size regime, the thermodynamic properties are different from the bulk material, and depend on the number of atoms. Adding or subtracting a single atom can make a huge difference.
It is only recently that experimental methods have been developed to study the melting and freezing of such small objects. We perform calorimetry measurements on unsupported (i.e. levitated) size-selected particles. Figure 2 shows the heat capacity measured for Al53 as a function of temperature. The peak in the heat capacity at around 620K is due to the latent heat, and we take the center of this peak to be the melting temperature. For bulk aluminum the peak in the heat capacity is <1K wide, and the melting point is 934K. The peak for Al53 indicates that it melts over a 200K temperature range. Within this range, each Al53 cluster rapidly switches back and forth between being entirely solid and entirely liquid. The area under the peak is the latent heat, which for Al53 is about 20% of the bulk value.
Figure 3 shows the variation in the melting temperatures for aluminum clusters with 30-80 atoms. There are enormous size-dependent fluctuations, with some clusters melting at close to the bulk melting point (934K) while for the larger clusters the melting temperature is much lower.
Current and future research topics
Super-cooling. Cooling a liquid below its thermodynamic freezing point is called super-cooling. This phenomenon occurs because it is difficult to form the small solid crystallite needed to initiate freezing. It should be possible to super-cool some small particles by hundreds of degrees.
Impurities. According to simulations, a single atom can raise or depress the melting point of a small particle suggesting the possibility of tuning the melting temperature by changing the impurity atom.
Liquid Clusters. Their small size means that the atoms in liquid clusters are confined. We’re interested in examining the effect that this confinement has on the properties of liquid clusters, and in particular whether there are “special” liquid clusters with properties different from their neighbors.
This research is supported by the National Science Foundation. |
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