Effect of Si/C Ratio on Thermoelectric Properties of β-FeSi2 Mechanically Alloyed with (Si+C) Additions

In order to improve the thermoelectric performance of β-phase, the thermoelectric properties of hot-pressed n-type Fe 0.98 Co 0.02 Si 2 and p-type Fe 0.92 Mn 0.08 Si 2 mechanically alloyed with 4mass% of (Si + C) powders at various Si/C molar ratios (Si/C = 0.5 ∼ 2) have been investigated. Both the n-type and p-type mechanically alloyed and hot-pressed samples are composed of mostly the β-phase with a dispersion of a small amount of metallic e-phase particles. The amount of the s-phase decreases with increasing Si/C ratio in samples containing 4 mass%(Si+C). The e-phase disappears at Si/C = 1.5 for the n-type and 1.75 for p-type FeSi 2 . Many fine a-SiC particles around 20 nm form in the samples mechanically alloyed for 20h and hot-pressed at 1173K for 1 h. The thermoelectric power increases with increasing Si/C ratio due to the increase in the amount of the semiconducting β-phase by the reaction of the metallic s-phase with some part of Si, which was added to form SiC. The thermoelectric power exhibits a maximum at Si/C = 1.5 in n-type FeSi 2 containing 4 mass%(Si + C) and Si/C = 1.75 for p-type. Although the electrical resistivity is increased by the addition of (Si + C), the Si/C ratio dependence of the electrical resistivity is not drastic for both n-type and p-type FeSi 2 . The addition of (Si + C) markedly decreases the thermal conductivity of both n-type and p-type FeSi 2 due to the dispersion of fine α-SiC particles. The maximum figure of merit values appear at Si/C = 1.5 in n-type FeSi 2 samples containing 4 mass%(Si + C) and Si/C = 1.75 for p-type.