Figure 1

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Figure 2

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Thermal and Electronic Transport
in Eutectoidally-nanostructured β-FeSi2 with Si Nanorods

Generously supported by the II-VI Foundation
PI: Jerry Floro
Graduate Students: Wade Jensen and Naiming Lu
Project Status: beginning

Goals: We will enhance the thermoelectric properties of the β-FeSi2 system via eutectoid nanostructuring and isoelectronic alloying. The system has the advantage that Fe and Si are inexpensive, plentiful and non-toxic, but the technical thermoelectric properties require significant improvement.

Technical Approach: β-FeSi2 is one of a small group of transition metal silicides that is semiconducting rather than metallic. The Fe-Si phase diagram exhibits a eutectoid reaction: α-FeSi2 → β-FeSi2 + Si. Eutectoid decomposition in this system has been examined in the past as a potential means to decrease thermal conductivity. Through thermal annealing of bulk Fe-Si alloys prepared at or around the eutectoid composition, the size, shape and dispersion of the Si nanorods (and any primary phases formed off-eutectoid) can be manipulated, including on the nanoscale. Isoelectronic alloying also offers significant potential for controlling properties. In principle, Ge should preferentially incorporate into the Si phase (complete solid solubility), while Os has an silicide phase isomorphic to β-FeSi2. Alloy scattering of phonons will be significant, and bandgap engineering is now made possible through alloying.

Key Results: We have readily achieved nanoscale dispersions of Si rods in β-FeSi2 (see Fig. 1) and Patrick Hopkins’ group showed a 2.5x reduction in thermal conductivity for nano- vs. macro-scale inclusions. We are examining the crystallographic orientation relations between the β-FeSi2 matrix and the Si rods using electron backscatter diffraction (EBSD). An initial ternary Fe-Si-Ge melt shows macroscale segregation of Ge-rich bands upon slow solidification (Fig. 2), which frustrates attempts to incorporate significant amounts of Ge into the Si rods. We are examining the ternary equilibria of this system, and will be attempting both rapid solidification and ball milling in order to circumvent massive eutectic segregation affects.