Effects of Cu addition to n-type β-FeSi2/Si composite on the Si precipitation and its thermoelectric properties

Abstract

We investigated the influence of Cu addition to Co-doped β-FeSi2/Si thermoelectric material. We expected the addition of Cu to accelerate the eutectoid decomposition of α-Fe2Si5 phase resulting in a finer distribution of Si secondary phase. We added 1 mass% and 2 mass% of Cu followed by the annealing process in various conditions. We obtained a significant decrease of Si size, reaching less than 100 nm for composites with 2 mass% Cu, annealed at 650 °C-2 h. Within the same amount of Cu, Si size was clearly increased after annealed at 800 °C-4 h, suggesting that the phase transition is accelerated with the existence of Cu. The thermal conductivity value was greatly reduced for sample with 2 mass% Cu, compared with the experimental value of single β-FeSi2 and calculated value from the rule of mixture. This proves that the fine distribution of Si help suppress thermal conductivity despite the high value of the Si phase itself. However, the excessive amount of Cu (2 mass%) degenerated the electrical properties of β-FeSi2/Si. Nonetheless, the sample with 1 mass% Cu annealed at 800 °C for 4 h showed the highest ZT value of 0.1, indicating that it is essential to keep the balance of Cu amount and annealing conditions towards TE performance enhancement.