Abstract
Two series of Fe and Al double substituted MnSiγ chimney ladders with a nominal valence electron count, VEC=14 per transition metal were prepared (γ=1.75). Simultaneous replacement of Mn with Fe and Si with Al yielded the Mn1−xFexSi1.75−xAlx series while the second Mn1−xFexSi1.75–1.75xAl2x series follows the pseudo-binary between MnSi1.75 and FeAl2. Scanning electron microscopy and elemental mapping revealed that ~60% of the nominal Al content ends up in the product with the remainder lost to sublimation, and that up to 7% Al can be substituted in the main group sublattice. Profile analysis of X-ray powder diffraction data revealed gradual changes in the cell metrics, consistent with the simultaneous substitution of Fe and Al in a fixed ratio. All samples are p-type with VEC≈13.95 from the structural data and ~1×1021 holes cm−3 from variable temperature Seebeck measurements. The substituted samples have lower electrical resistivities (ρ300K=2–5mΩcm) due to an improved microstructure. This leads to increased thermoelectric power factors (largest S2/ρ=1.95mWm−1 K−2) compared to MnSiγ. The thermal conductivity for the Mn0.95Fe0.05Si1.66Al0.1 sample is 2.7Wm−1K−1 between 300 and 800K, and is comparable to literature data for the parent material.
Highlights
Thermoelectric energy converters can be used to increase the efficiency of any heat generating process
X-ray powder diffraction and microscopy imaging confirm that Fe and Al double substitution is an effective route to increase the amount of Al substitution in the MnSiγ chimney ladder structure
EDX elemental analysis suggests that about 60% of the nominal Al content is found in the final product while the remainder is lost due to sublimation
Summary
Thermoelectric energy converters can be used to increase the efficiency of any heat generating process. Over the past decades a large number of material systems have been investigated for thermoelectric power generation, including lead tellurides, skutterudites, half-Heuslers and silicon-germanium alloys [2,3,4,5,6]. Another group of materials under investigation are intermetallic compounds with the Nowotny chimney-ladder (NCL) structure. These form for a variety of combinations of group 4–9 transition metals and group 13–14 main group elements with VEC E14 [7,8,9,10]. When the VEC is close to 14, the Fermi energy is located in the middle of a narrow band gap and the chimney ladder compounds exhibit semiconducting properties
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