Effects of Y2O3 and Y addition on thermoelectric properties of β-FeSi2 synthesized by mechanical alloying and hot pressing

The n-type Fe0.98Co0.02Si2 compacts with Y2O3 (1-6 mass%) dispersion were synthesized by mechanical alloying with Y2O3 powder and subsequent hot pressing. The effects of Y2O3 addition on the thermoelectric properties of the beta-FeSi2 were investigated. The microstructures of the hot pressed samples were observed by transmission electron microscopy (TEM). The TEM observation showed that the fine Y2O3 particles around 10 nm in size were dispersed in the beta phase matrix by mechanical alloying, resulting in a significant reduction in the thermal conductivity due to enhancing phonon scattering. The Seebeck coefficient was also enhanced by Y2O3 addition especially below 800 K, corresponding to the extrinsic conductive region of the beta-FeSi2. Consequently, the figure of merit was significantly improved by 2 mass% Y2O3 addition. The chemical composition of these samples with Y2O3 addition was examined by the energy dispersive X-ray spectroscopy (EDX). The EDX analysis revealed that the added Y2O3 was partially decomposed and a small amount of Y was dissolved in the beta phase matrix. Based on this fact, the enhancement of the Seebeck coefficient caused by Y2O3 addition is considered to be due to reduction in carrier concentration resulted from this Y solution as a p-type dopant in the beta phase matrix, and the behavior of the Seebeck coefficient was found to be well consistent with that of the Y-doped samples synthesized by Y powder addition.