Preparation And Transport Properties Of β-FeSi₂ Based Thermoelectric Materials

In the present work, the developments in thermoelectric materials were reviewed and discussed. Semiconducting (3-FeSi2 based thermoelectric alloys have been selected as the materials of this study due to its good performance and low cost. For analyzing the mechanism of p phase transition and investgating the way to improve their thermoelectric properties, the phase transformations, microstuctures and transport properties of FeSi2 and Fe2Sis based alloys prepared by melt-spinning (MS), levetation-melting (LM) and mechanical alloying (MA) were studied by means of XRD analysis, SEM/EDS observations and the measurements of transport properties.Firstly, two apparatuses were designed and installed to measure transport properties and dimensionless figure of merit in the temperature range from 300K to 800K, respectively.Thermalcouple legs were used as voltage lead wires when transport properties were measured. Thus the apparatus was simplified, and the temperature difference and voltage between the two ends of a sample can be simultaneously measured, and therefore the Seebeck coefficient can be more accurately calculated. In the other apparatus, dimesionless figure of merit ZT were measured by Harman's method. Direct and alternating current were applied to induce Seebeck voltage and ohm voltage, respectively, from which ZT can be direcly obtained. Design details and measurement specifics were introduced, the origin of the errors involed possiblly and their reductions were discussed.Secondly, the phase transformations and microstructures of FeSi2 and Fe2Sis based alloys were investigated. Rapidly solidified P~FeSi2 thermoelectric materials were prepared by melt-spinning technique for the first time. Because the rapidly solidified FeSi2 alloys have finer structures and therefore the more reactive interfaces for p phase formation from a-Fe2Si5 and e-FeSi metallic phases during the peritectoid reaction, an annealing duration of only 18 hours at 800癈 was required, which is largly reduced comparing with 144 hours for the conventionally induction-melted alloys. A small account of Cu addition can also significantly decrease the time of p phase transition. It was found for the first time that the Cu addition can significantly enhance the p-FeSi2 formation also when the alloy is synthesied with MA process from the element powders. With only 0.2at.% of Cu addition, the p phase forms after only 100 hours MA, which is only about one fifth of that reported in literatures. For the levitation-melted FeSi2 alloys containing Cu, only an annealing duration of 10 hours was needed to complete the p phase transition and 6 hours for the rapidly-solidified ones. Because only the a phase existed in the as-solidified Fe2Si5 based alloys and it would decompose into p phase and silicon with the eutectoid reaction during annealing, the p formation rate was drastically increased compared with that of the peretectiod reaction. It was found that an annealing of only 4 hours was needed for the p phase to be formed from the levitation-melted Fe2Sis basedalloys with Cu addition. 14 hours were required for the rapidly-solidified FeiSis alloys with the same compositions, due to the decrease of the Cu contents during the re-melting of the alloy. It was found that the mechanically ground powders of FeSii and FejSis based alloys were irregular, which induced the porous sintered bodies. Comparatively, the sintered samples of the rapidly solidified alloys were denser than those of the levitation-melted alloys.Thirdly, the transport properties of FeSi: and FeiSis based alloys were investigated. Carrier scattering by both grain boundaries and excess Si particles have great effects on the transport properties of (3-FeSi2 thermoelectric materials. The Seebeck coefficients and electrical resistivities of the rapidly solidified alloys were higher than those of the levitation-melted alloys because a number of grain boundaries created by rapid solidification process enhance the carrier scattering. However, for the Fe2Sis alloys the effect of dispersed s...