Effects of mechanical alloying and copper addition on thermoelectric properties of n-type and p-type β-FeSi2

The effects of mechanical alloying (MA) process by vibration ball milling and Cu addition on the thermoelectric properties of n-type Fe1-xCoxSi2 and p-type Fe1-xMnxSi2 have been investigated. It has been found by X-ray diffraction analysis that mechanical alloying (MA) even after a short period is very effective to form the beta-phase from the mixture of the alpha-Fe2Si5 and epsilon-FeSi phases during hot-pressing. Both of the hot-pressed n-type and p-type samples are composed of mostly the beta-phase with dispersion of a small amount of epsilon-phase particles. The amount of the epsilon-phase increases with increasing Cu addition. EDX analysis of the n-type and p-type samples has shown that the beta-phase contains no Cu, while the epsilon-phase contains Cu and Co or Mn. Since the Co and Mn contents in the epsilon-phase are higher than in the beta-phase, the increment of the amount of the E-phase decreases the Co and Mn contents in the beta-phase, and significantly changes the thermoelectric power. Cu addition to the n-type Fe1-xCoxSi2 significantly decreases the electrical resistivity, and a small amount of Cu addition increases the thermoelectric power, resulting in the improvement of the performance. It is considered that the decrease in the electrical resistivity results from the dispersion of the metallic epsilon-phase in the semiconducting beta-phase matrix. On the other hand, Cu addition to the p-type Fe1-xMnxSi2 decreases the performance, which results from a significant decrease in the thermoelectric power due to the decrease in the Mn content of the beta-phase as well as the dispersion of the metallic epsilon-phase. The thermal conductivity of the n-type and p-type samples is slightly increased by Cu addition.