Influence of Nanoparticles on the Thermoelectric Properties of Magnesium Silicides

Abstract

A significant fraction of energy is lost during power generation for transportation as a form of waste heat. Approximately over three quarters of waste heat have the temperature range from 200 to 700 ° C. Solid solution materials made of magnesium silicide and magnesium tin have the potential of utilization in thermoelectric (TE) devices in such temperature ranges due to their availability, relatively low density (3.02 g/cm3), non-toxicity compared to classical Te-Pb TE materials and high stability from room temperature (RT) to 600 ° C. The environmentally friendly n-type Mg2 (Si, Sn) thermoelectric solid solution bulk materials were prepared from powder elements via direct melting and current-assisted hot-press sintering. For the direct melting method, raw element magnesium, tin, and silicon powders were homogeneously mixed and put into a stainless steel container. Then, the sample was heated under a vacuum condition at 500–600 ° C for several hours to obtain crystalline Mg2Si, Mg2Sn, and Mg2 Si0.4Sn0.6 ingots respectively. These ingots were ground by using a high energy ball miller to obtain their particles whose sizes are distributed from tens of nanometers to micro-meters. Several doping material with different doping ratio were added to compare doping effect on Mg2 (Si, Sn) solid solutions. The samples were hot-pressed with electrical current (400–600A) in order to create phonon scattering centers such as nano-size particles and lamellar structures for improving the dimensionless figure of merit, ZT value. Samples were cut into slender pieces flash method. Then, ZT value was calculated from ZT = α 2 Tσ /κ and plotted as a function of temperature. The results were also compared with the results obtained from to test electrical conductivity, by using a four-probe method as well as the Seebeck coefficient,. Thermal conductivity measured as a function of temperature from RT to 600 °C by using a the Harman technique. The nano-structure size, size distribution as well as crystallinity were characterized by using transmission electron microscopy (TEM), scanning electron microscopy (SEM), and x-ray diffraction (XRD). The chemical composition was examined by energy-dispersive x-ray spectroscopy (EDS).Copyright © 2012 by ASME