Research Of β-FeSi₂ Based Thermoelectric Materials Prepared By Rapid Solidification

The development in thermoelectric materials were reviewed and discussed. FeSi2-based semiconductors have been studied due to its good performance, low cost and potential application in power generation using waste heat. State-of-the-art techniques such as levitation melting, rapid solidification and hot uniaxial pressing (HUP) were used for sample preparation in the present work. The phase transformation, micro structure and transport properties of the materials have been analysed with XRD, SEM/EDX, Raman spectroscopy as well as thermoelectric characterization.A computer real-time data acquisition device for the measurement of Seebeck coefficients and electrical conductivities from 300K to 1000K were designed and installed in the present work. The numerical treatment of the measuring data was discussed also.It was found experimentally that rapid solidification could speed up the peritectoid phase transformation from the metallic a-Fe2Sis and s-FeSi to semiconducting p-FeSJ2. It was showed that the phase equilibrium could be easily reached during the hot-press for the rapidly solidified powders, especially if the sample was doped with aluminium. The p-phase transformation was completed after 20 or 24 hours annealing at 800癈 for all hot-pressed samples. No remarkable grain growth has been found during the phase transformation annealing. High sintering densities over 83% of theoretical ones have been obtained for the hot-pressed samples.The effects of silicon content in the manganese doped FeSia-based alloys on the microstructures and thermoelectric properties of hot-pressed samples have been investigated. It was found that both number and sizes of the silicon particles are increased and the ratios of electric to thermal conductivity were decreased with the increase of the silicon content. The maximal Seebeck coefficients occur when the Si:Fe mole ratio are between 2.0 and 2.1. It was showed that the primary coarse silicon particles could be solidified during rapid solidification and the thermoelectric properties would be worsened if the Si:Fe mole ratio were larger than 2.3. The present experiments showed that best thermoelectric figure of merit could be obtained for the rapidly solidified and hot-pressed manganese doped iron disilicides if the Si:Fe mole ratio were kept between 2.0 and 2.1.Two new in-situ nitrogenation techniques were used for the rapidly solidified FeSiz-based thermoelectric materials for the first time in the present work, i.e., the nitrogenation of the powders before vacuum hot-press, and the nitrogenation during the hot-press in a pure nitrogen atmosphere. It was found that in-situ nitrogenation is an effectivetechnique to decrease the thermal conductivity of the FeSii-based thermoelectric materials. A significant reduction up to 50% on thermal conductivity and a notable improvement on the thermoelectric figure of merit has been found in the present work. The effect of powder oxidation on the thermoelectric properties was also investigated for the first time. It was found the conducting type of the aluminium doped FeSi2-based semiconductors changes form p-type to n-type and again to p-type with the increase of the oxidation time. A tentative discussion about the mechanism of the effect of both nitrogenation and oxidation was given in the present work.A thermal-voltage reversion phenomenon was found for the first time in the present experiment in FeSii-based thermoelectric materials, i.e., the conducting type changes with measuring temperature.The maximal dimensionless thermoelectric figure of merit of 0.13 at 600癈 has been obtained for the manganese doped FeSi2-based semiconductor, and a large power factor of 465 uW-m"'^1 for the aluminium doped FeSii-based materials has been measured in the present work.