Thermoelectric properties of p- and n-type FeSi₂ prepared by spray drying, compaction and sintering technique

Abstract

FeSi₂ alloys doped with Mn and Co (p-type Fe_0.926Mn_0.074Si₂, n-type Fe_0.980Co_0.020Si₂) were prepared by vacuum induction melting. The ingots were pulverized in a jet-mill, and the powders were granulated by spray drying method using aqueous polyvinyl alcohol (PVA) binder in order to investigate the possibility of production on a large scale. The powders granulated exhibited excellent characteristics of flowability, leading to the smooth feeding into a die cavity in compacting. The powders obtained were compacted at a pressure of 98 MPa. It was debindered at 723 K for 1 h and sintered at 1423 K for 5 h in a hydrogen atmosphere, so that the residual carbon and oxygen contents in a sintered body approached to those in the starting powder. Subsequently it was annealed at 1113 K for 100 h in an argon atmosphere, to produce the semiconducting β-FeSi₂ phase. The thermoelectric figures of merit (Z) for optimum p- and n-type FeSi₂ are 1.75 × 10⁻⁴ (K⁻¹) and 2.0 × 10⁻⁴ (K⁻¹) at about 900 K, respectively, which agree roughly with those obtained by Tani and Kido for FeSi₂ materials prepared by the spark plasma sintering method. It indicates that the spray drying method leading to the production on a large scale is available for the fabrication of the thermoelectric FeSi₂ materials.