Abstract
Thermoelectric materials of α-Fe0.91Mn0.09Si2.5+x mass%Cu (0≤x≤2) were produced by spark-plasma sintering (SPS) and their thermoelectric properties were investigated. The densification was completed in sixty seconds after the temperature reached 1173 K . Liquid phase sintering most likely occurred during the heating because the addition of copper promoted the densification. The relative density of the sintered specimen was between 88 and 90%. When (α+ε)-phase powder, which corresponds to the β-phase composition, was used as the starting material, the eutectoid reaction (α→β+Si) mainly occurs at 943 K . However, there was no β-phase in the α-phase specimen annealed under the same conditions. It was clear that the ε-phase accelerates the eutectoid reaction. Copper accelerates transformation to the β-phase. The eutectoid reaction was completed within only 300 s at 943 or 973 K, and also occurred at 1003 K but the rate was very low. Compared with the (α+ε)-phase specimen, the power factor of the α-phase specimen was very low, because the electrical resistivity of the α-phase specimen was larger than that of the (α+ε)-phase specimen. The silicon grains dispersed in the matrix should be an obstacle to the electrical conduction. The annealing conditions were controlled so that the coarsening of the silicon particles would improve the electrical resistivity. The electrical resistivity was not improved, though the silicon grains could be coarsened. An excess amount of copper and an excess annealing time had a negative influence on the Seebeck coefficient and the power factor. This phenomenon was also observed in the (α+ε)-phase specimen. Fe0.91Mn0.09Si2.5+20 mass%Cu alloy was produced to analyze the reaction between copper and Fe0.91Mn0.09Si2.5 in detail. It was revealed from EPMA results that the composition of the β-phase was shifted from the nominal composition. The copper-rich phase is considered to a eutectic composition of Cu–30 at%Si, and it has little thermoelectric properties. The optimum composition was Fe0.91Mn0.09Si2.5+0.5%Cu, and the optimum annealing conditions were 300 s at 973 K . The power factor of the optimum specimen was about 74% of that of the optimum (α+ε)-phase specimen.
