Abstract
The oxidation kinetics and mechanisms of higher manganese silicides (HMS) MnSi1.75, MnSi (1.75-x)Gex, MnSi(1.75-x)Alx (with x = 0.005 and 0.01)were studied and the effects of densification methods and dopant concentration discussed. Oxidation experiments were conducted using thermogravimetry (TGA), while post characterization with X-ray Photoelectron Spectroscopy (XPS) and Scanning Electron Microscope (SEM) showed that spark plasma sintering (SPS) is a better densification method than hot pressing (HP). Except for undoped HMS, HMS doped with 0.5at% Ge had the lowest oxidation rate. Stable formation of a SiO2 protective layer was the main reason for improved oxidation resistance in air in the temperature range 200 °C–500 °C.
Highlights
Stochiometric MnSix alloys [1,2] are among the most promising p-type silicide thermal electric materials
Undoped higher manganese silicides (HMS) (a, b, and c) show more robustness to oxidation relative to doped HMS, with total weight gain averaging between 0.35 % and 1.75 % corresponding to HMS_a and HMS_b, respectively
The microstructure of the compounds revealed that batch c had a high amount of SiC precipitates and a sub stantial amount of MnSi secondary phase, batch b has a high amount of MnSi phase, and less SiC precipitates
Summary
Stochiometric MnSix (with 1.71 ≤ x ≤ 1.75) alloys [1,2] are among the most promising p-type silicide thermal electric materials. The uncoated samples grew a SiO2 layer of 5 μm with a Si depleted region (MnSi) of the same thickness and the same trend was observed by Funahashi et al [21], they had carried out their tests for a short period of 10 h. Likewise, they observed that their uncoated sample showed a decrease in the Seebeck coefficient as well as a decrease in the electronic conductivity both of which implied a lower power factor compared to “as-sintered” samples. Coated samples showed resistance to oxidation and retained the same power factor after heat treatment
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