Quantum confinement effects make nanostructured bismuth a promising new thermoelectric material. The goal of this research is to find a route to nanostructured bismuth using a mechanical technique called high-energy ball milling. To do this, the starting materials are loaded into a sturdy vial along with several balls, and the vial and its contents are shaken vigorously for a number of hours in a Spex 8000 Mixer/Mill. Powder caught between elements of the milling media is readily crushed, and amount of energy transferred during collisions is enough to produce far-from-equilibrium microstructures (e.g. amorphous materials, nanostructured materials )
Ball milling bismuth by itself does not yield nanostructure because it coarsens immediately due to its low melting point. Milling bismuth with a second, inert phase, however, yields some good results. We have milled bismuth with fused quartz as well as with magnesia, and both result in composite powders with bismuth particles on the nano-scale. The morphology of these particles is different in each system due to bismuths ability to wet magnesia and its inability to wet silica.
The thermal behavior of these composites has been probed using differential scanning calorimetry (DSC), showing depressed melting and freezing events relative to pure bismuth. These effects are probably particle-size related, and they provide evidence that the majority of the bismuth in these materials belongs to small particles. Surprisingly, both the depressed thermal behavior and the nanostructure persist after multiple melting/freezing cycles in the DSC.
In addition, these materials can be pressed into conducting pellets. Future work will include characterization of the electronic and thermoelectric properties of these materials.
Post-DSC ball milled Bi-SiO2
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