Effect of Si/C ratio on thermoelectric properties of β-FeSi2 mechanically alloyed with (Si plus C) additions

In order to improve the thermoelectric performance of beta-phase, the thermoelectric properties of hot-pressed n-type Fe0.98Co0.02Si2 and p-type Fe0.92Mn0.08Si2 mechanically alloyed with 4 mass% of (Si + C) powders at various Si/C molar ratios (Si/C = 0.5 similar to 2) have been investigated. Both the n-type and p-type mechanically alloyed and hot-pressed samples are composed of mostly the beta-phase with a dispersion of a small amount of metallic a-phase particles. The amount of the epsilon-phase decreases with increasing Si/C ratio in samples containing 4 mass%(Si + C). The epsilon-phase disappears at Si/C = 1.5 for the n-type and 1.75 for p-type FeSi2. Many fine alpha-SiC particles around 20 nm form in the samples mechanically alloyed for 20 h and hot-pressed at 1173 K for 1 h. The thermoelectric power increases with increasing Si/C ratio due to the increase in the amount of the semiconducting beta-phase by the reaction of the metallic epsilon-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 FeSi2 containing 4 mass% (di + 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 FeSi2. The addition of (Si + C) markedly decreases the thermal conductivity of both n-type and p-type FeSi2 clue to the dispersion of line alpha-SiC particles. The maximum figure of merit values appear at Si/C = 1.5 in n-type FeSi2 samples containing 4 mass% (Si + C) and Si/C = 1.75 for p-type.