Thermal conductivity of Fe-Si alloys prepared by the SHS process

Abstract

FeSi2 is one of the most interesting high-temperature thermoelectric transition materials due to its high transition efficiency, low cost and high oxidation resistance at working temperatures [1]. FeSi2 thermoelectric devices have been prepared conventionally through melting raw metals, crushing, sintering and annealing. In this study, a novel processing technique, i.e. self-propagating high-temperature synthesis (SHS) or combustion synthesis, was used to prepare Fe-Si alloys. The SHS process is superior to other conventional methods in terms of saving energy and processing time. The SHS-products are usually highly pure because at extremely high reaction temperatures the volatile contaminants in the samples mostly vaporize. Owing to the very high cooling rate following SHS reaction, high defect concentrations and non-equilibrium structures generally exist in the SHS-produced materials, rendering the products more reactive and metastable [2]. In this Fe-Si alloy system, direct formation of /3 phase from the melt can be expected during SHS reaction when the cooling rate is high enough. The purpose of this study is to measure the thermal conductivity of Fe-Si alloys synthesized by SHS and to investigate the applicability as a thermoelectric generator material. To obtain /3-FeSi2 by the SHS process, iron (>99%) and silicon (98%) powders were mixed according to the composition shown in Table I, corresponding to /3-FeSi 2 and a little excess Si, by considering the loss of Si during combustion reaction because Si is more volatile than Fe. KNO 3 (>99.9%) was also added to the Fe-Si powder mixture in the weight ratio of KNO3/(Fe + Si) = 0.2 to effectively activate ignition of the mixture by arc discharge at room temperature [3]. The powder mixture was compacted to pellets at 100MPa pressure by a uniaxial press. Washing, crushing and screening processes followed the combustion reaction. The produced powder (<325 mesh) was analysed by Xray diffractometer, and shown to be composed of eFeSi, o~-Fe2Si5 and a trace of unreacted Si. This powder was uniaxially pressed into pellets at 100MPa pressure together with the binder PVA, which was removed by heating at 500 °C for 2 h in air, and the pellets were sintered at 1155 °C for 3 h in Ar. X-ray diffraction analysis showed that the sintered samples consisted of a-Fe2Si5 and e-FeSi. The sintered specimens were annealed at 840 °C for 12 h in Ar to transform these two metallic phases to semiconducting ¢l-FeSi2 phase, and compared with those sintered and annealed under the same conditions using a FeSi 2 commercial powder (CERAC, USA). Fig. 1 shows X-ray diffraction patterns for the annealed samples. The main phase /3-FeSi2 can be observed for all samples and the intensity of the eFeSi peaks decreases with increasing Si content. It is considered that the composition of specimens changed to an Fe-richer type compared with the starting