Optimization of properties of Fe1-xCoxSi2+z for energy conversion and sensors

A study on the thermoelectric (TE) properties of a set of Fe_1-xCo_xSi_2+z samples compositions (x=0÷0,08; z=-0,08÷ 0,10) prepared by hot uniaxial pressing (HUP) is presented. The ZT optimum was found for x=0,06÷0,07 and z=-0,04: The optimum x- and z-values are given not only by the related change of electron concentration but also by the band structure sensitivity to the Fe:Si ratio, by Fe/Co-substitution, and probably also Fe,Co/Si substitution, which leads to antisite defects. According to the electron configuration, Co seems to have better ability to enter Si sublattice. The XRD measurements indicate also a simultaneous creation of vacancies V_Fe and antisites Si_Fe. Prolonged ball milling was found to be ineffective with respect to ZT-improvement although it decreases the thermal conductivity. Graphite is nearly indifferent in β-FeSi₂ up to 2 wt.%. It increases markedly the abrasion of milling balls and vessel, which actually means unintentional doping. The sensor applications optimum is found for Fe_0.97Co_0.02Si_2.1 giving high and stable temperature independent Seebeck coefficient convenient for heat flux sensors with linear output characteristics.