Hierarchical Engineering of Microstructural Lengthscales in Fe-Si Based Alloys to Minimize Thermal Conductivity for Thermoelectric Applications

Semiconducting β-FeSi2 is a candidate thermoelectric material whose constituents are abundant and eco-friendly, but significant improvements in thermoelectric properties are needed before it becomes viable in applications. has been shown in the literature as an effective method to reduce thermal conductivity. This dissertation research investigated processing methods to nanostructure a two-phase mixture of β with Si1-xGex lamellae, via control of eutectic solidification and eutectoid decomposition. The consequent microstructures were characterized, and related to the thermal conductivity via the independent thermal scattering contributions of heterointerfaces.

The binary Fe-Si system was first investigated as a means to understand the eutectoid decomposition (α-FeSi2 → β-FeSi2 + Si). Low temperature aging of binary samples produced cooperatively-grown Si lamellae, which decomposed into nanowires and spheroids upon further aging. The growth velocity (v) and interlamellar spacing (λ) of pearlitic colonies obeys a relation of the type vλ^n = f(T). This bounds the activation energy for the diffusion, although the exact mechanism could not be specified from the data. We also found that the eutectoid Si are polycrystalline with interfaces primarily being twin boundaries on the {111} planes.

We then alloyed small quantities of Ge to the system (now a ternary Fe-Si-Ge) to enhance thermoelectric properties and widen the design space. Eutectic solidification (L → α-FeSi2 + Si1-zGez) was used to create meso-scale lamellae. Eutectic morphology and Ge disposition amongst the product phases can be controlled through the solidification rate. By increasing the rate from ~10^2 ˚C/s to ~10^6 ˚C/s, eutectic lengthscales can be reduced by over two orders of magnitude, while the Ge concentration in the lamellae and matrix roughly doubles. Subsequent aging produced eutectoid decomposition (α-FeSi2 → β-FeSi2 + Si1-yGey) where the additional diamond cubic product is interleaved with the eutectic lamellae, creating a hierarchical structure.

Preliminary work exploring ultra-rapid solidification rates, ~10^9 ˚C/s, was performed via pulse laser melting (PLM) with a high-power UV laser. Although there are numerous processing challenges to overcome before routine synthesis is possible, PLM samples show potential for developing an optimized microstructure for maximal thermal scattering.

The Si-rich region of the Fe-Si-Ge ternary phase diagram, which was previously unexplored, has been mapped through Rietveld analysis and differential scanning calorimetry. Compositional ranges for α-FeSi2 + ε-FeSi + Si1-zGez and β-FeSi2 + ε-FeSi + Si1-yGey three-phase regions were experimentally determined for both 1000 ºC and 900 ºC sections. Although our composition resolution was not fine enough for differential scanning calorimetry to resolve liquidus surface features, we were able to estimate the position of the ternary eutectic point and the ε-FeSi/SiGe cotectic line.

For ternary alloy specimens, melt-spinning and low temperature aging significantly lowered thermal conductivity, which decreased from 22.8 W m-1 K-1 down to 8.3 W m-1 K-1. We analyzed the thermal conductivity in terms of a series thermal resistance model, via Matthiessen’s rule, and showed that Ge composition has a significant effect on phonon scattering at the β-FeSi2/Si1-xGex heterointerface. By increasing Ge concentration from 0 to 30 at%, the thermal boundary conductance is reduced by an order of magnitude.