Research Of Metal Silicide Thermoelectric Materials Prepared By Rapid Solidification And Hot Pressing

Semiconducting p-FeSi2 and MnSi 1.75 based alloys are of great interests due to their potential applications in thermoelectric generators used to recover waste energy from exhaust gas and other infrastructure heat losses in industrial processing plants, and also due to their attractive characteristics such as low cost, environmental friendship, chemical stability at high temperatures.In the present work, P-type higher manganese silicides (HMS) MnSi1.75_x and N-type p-FeSi2 thermoelectric alloys were prepared with levitation melting, rapid solidification (RS) and hot uniaxial pressing (HUP). The microstructures of the materials were analysed with XRD and SEM, and the transport properties were measured. Both theoretical estimation and experimental measurements were done for the thermoelectric power generator module using p-FeSi2 and HMS as the P- and N-leg.State-of-the-art techniques of RS and HUP were used firstly to prepare HMS in the present work. It was shown that the microstructures of the MnSius-x prepared by RS and HUP consist of locally parallelly distributed MnSi thin flakes in the semiconductor MrLjSi7 matrix due to the gwos/'-directional solidification during RS. It was found that with the increase of the silicon content, the electrical conductivities of the materials decrease and both Seebeck coefficients and thermal conductivities increase. The ZT value of MnSi 1 75 is the highest in the MnSii.75-x in the range from room temperature to 600℃. The maximal ZT value of MnSi1.75 is 0.42 at 500℃.It was found that the electrical properties of the MnSi1 75-x samples are improved with the annealing at 800℃ for 8h, because the amount of metal phase MnSi decreases when the MnSi1 75-x are annealed.It was found that doping with Cr was an effective method to improve the electrical properties of MnSius.x. The doping content of Cr has a significant effect to the electrical conductivities and the Seebeck coefficients of the Cr-doped Mn1-xCrxSi1.73. The electrical conductivities increase and the Seebeck coefficients decrease with the increasing doping amount of Cr. The highest power factor of the Mn1xCrjSi1.73 was found for x = 0.02, which is 260tiWnf 'K'1 higher than that of the undoped MnSi].73 at 500℃.The p-phase transformation are almost completed after 20 hours annealing at 800℃ for ail Fei.jCOjSii-yAlj, samples prepared by RS and HUP. The highest power factor of about 1070 K"1 at 500℃ was measured for the sample doped with 5at% Co.The maximum power outputs were calculated and measured for a thermoelectric module. It was show that the power outputs from experiments were much lower than that calculated. Itwas suggested that both electric and thermal resistances at the contact interfaces should be taken into account for the estimation of the power output of a thermoelectric generator module.