Yuzuru Miyazaki

Abstract:

Higher Manganese Silicides (HMSs) consist of two tetragonal subsystems (sublattices) of [Mn] and [Si] with an irrational c-axis length ratio γ = c_Mn/c_Si, forming an incommensurate Nowotny chimney-ladder (NCL) structure. The [Mn] subsystem has less displacive modulation of atomic sites and 3d electrons of Mn are responsible for the electronic conduction of this family. In contrast, significant displacive modulation of Si atoms in the [Si] subsystem effectively blocks the propagation of phonons and hence to reduce lattice thermal conductivity. A good thermoelectric (TE) material must have a high Seebeck coefficient S and electrical conductivity σ as well as a low thermal conductivity κ and HMSs can be thus regarded as potential TE materials. Using γ, the structure formula of HMSs is represented as MnSi_γ (γ~ 1.73) (Miyazaki et al, 2008).
To comprehend the electronic structure of such complicated composite crystals and their solid solutions, we adopted the concept of valence electron counts, VEC. Based on the VEC concept, HMSs should exhibit a good p-type TE property as the VEC value is slightly smaller than 14. To further enhance p-type performance, a small amount of hole-doping, by a partial substitution of Cr, V, etc. for Mn, is necessary. In contrast, n-type materials can be synthesised by a partial substitution of Fe or Ru.
The incommensurate nature of the present HMS family causes variety of nano/micro structures which enrich physico-chemical properties of the present silicide system. HMSs had a serious problem to form MnSi (monosilicide) striations during the solidification. We have discovered that the formation of such monosilicide striations originates from the temperature dependence of the γ-value, gradually increases from ~1.72 just below the solidification temperature to ~1.74 at around 800 K, with decreasing temperature. Such a change in γ is unfavorable for the practical use because it would easily cause mechanical cracks during heat cycles. We confirmed that the partial substitution of transition elements, e.g., Cr, Co and V for Mn sites effectively dissipates such striations as well as a moderate hole-doping which raises the TE power factor S²σ almost doubled at 800 K. For the Co and V-solid solutions, the γ-value is around 1.72 and less temperature-dependence and it would be thus ideal for the crack-free HMS-based TE materials.