Magnesioreduction Synthesis of Co-Doped β-FeSi₂: Mechanism, Microstructure, and Improved Thermoelectric Properties

Abstract

β-FeSi 2 and β-Co 0.07 Fe 0.93 Si 2 thermoelectric silicides were synthesized from Fe 2 O 3 and Si powders using a magnesiothermic process. Detailed study of the reaction mechanism by X-ray diraction reveals that liquid Mg is mandatory to initiate the reduction. After completion of the reaction in relatively short time (10 h at 1173 K), the magnesiosynthesized iron disilicides are characterized as powders with grain sizes ranging from 30 to 400 nm and containing a high concentration of stacking faults quantied for 1 Page 1 of 33 ACS Paragon Plus Environment ACS Applied Energy Materials the rst time using a dedicated renement software. The thermoelectric properties of spark plasma sintered pellets with submicrometric grain sizes, high stacking fault density and residual micro-to nanoporosities are presented and compared to corresponding materials synthesized by conventional arc-melting process. Strong thermal conductivity reduction of % at K has been achieved thanks to the mesostructuration induced by the magnesioreduction. It results in an improved maximum gure-of-merit ZT reaching 0.18 at 773 K for β-Co 0.07 Fe 0.93 Si 2 .

Effect of Nanostructuring on the Thermoelectric Properties of β-FeSi2

“…This last material has been investigated for several decades for its thermoelectric properties [7]. It crystallizes in the orthorhombic structure Cmce (Space group 64) [8] and is a semiconductor that can be both n or p type with an indirect bandgap around 0.8 eV [9][10][11]. Its power factor α 2 σ is negligible when it is pure and its total thermal conductivity λ Tot ranges from 10 to 18 W/mK at 300 K [11,12].…”

“…It crystallizes in the orthorhombic structure Cmce (Space group 64) [8] and is a semiconductor that can be both n or p type with an indirect bandgap around 0.8 eV [9][10][11]. Its power factor α 2 σ is negligible when it is pure and its total thermal conductivity λ Tot ranges from 10 to 18 W/mK at 300 K [11,12]. After alloying with Al or Co, α 2 σ is similar to the best industrial thermoelectric (TE) materials [13] and its maximum figure of merit ZT reaches about 0.2 and 0.4, respectively [7] while it is 1 or greater for the best TE materials [14].…”

Influence of Stoichiometry and Aging at Operating Temperature on Thermoelectric Higher Manganese Silicides

“…The magnesioreduction (MR) synthesis of MnSi x , with x = 1.65, 1.74 and 1.80, starting from MnO (Alfa Aesar, 99 %) and Si (Ventron, 99.9 %) is described in details in previous articles 14,15 . The reaction temperature of 1173 K was reached at a rate of K h −1 and held for 8 h before being cooled down by switching off the furnace.…”

Crystal structure and high temperature X-ray diffraction study of thermoelectric chimney-ladder FeGe (γ ≈ 1.52)

“…based on this material remains limited due to the lack of an n-type counterpart with comparable thermoelectric and mechanical properties [11]. Among the other silicides, the current best options are n-type Co-doped FeSi 2 whose ZT only reaches about 0.2 in the same conditions [12] or Sn-doped Mg 2 Si which 2 A c c e p t e d m a n u s c r i p t has much lower mechanical and chemical resistance properties [13].…”