Mechanical Alloying of Optimized Mg₂(Si,Sn) Solid Solutions: Understanding Phase Evolution and Tuning Synthesis Parameters for Thermoelectric Applications

Abstract

Mechanical alloying by high energy ball milling is an attractive solid -state technique for synthesizing a diverse range of equilibrium and non-equilibrium phase materials. We have studied the synthesis of n -type thermoelectric Mg 2 Si 0.4 Sn 0.6 solid solution, aiming for a fundamental understanding of the mechanisms underlying this synthesis technique. The investigations on powders by XRD and SEM show that milling leads to welding of Mg and Sn but fracturing of Si. This fractured Si diffuses into the ductile matrix on longer milling times resulting in a phase mixture close to the nominal starting composition after 35h of milling. However, single phase pure material was only achievable after sintering, hence the synthesis of Mg 2 (Si,Sn) is a two -step process. Furthermore, a thorough study on the effect of varying synthesis parameters on the thermoelectric properties was performed. No strong influence of milling time on the thermoelectric properties was observed and just 2h of milling followed by compaction was sufficient to obtain a pellet with optimal thermoelectric properties. Moreover, increasing sinter temperature/time deteriorated carrier concentration hence degrading the electronic properties. Thus, optimized thermoelectric properties were obtained for the powder consolidated at 973K/20minutes. Mg 2 Si 0.4 Sn 0.6 synthesized by mechanical alloying achieved a thermoelectric figure of merit zT max~1 .4.

Synthesis of p-type Mg2Si1-xSnx with x = 0-1 and optimization of the synthesis parameters

“…The optimum milling time highly depends on Si:Sn ratio. Si rich compositions require longer milling time than Sn rich compositions because they have less ductile components [27]. On the other hand, for binary Mg 2 Si, a milling duration of 6 h was found to be sufficient as also reported previously [28,29], presumably because only one phase could be formed.…”

“…The phase purity increases with increasing milling time. Based on a previous report on the same synthesis approach, the formation of compositions close to Mg 2 Sn takes place within the 1 hour for ntype material [27]. Therefore, the milling time is varied between 4 and 20 hours to see the difference in microstructure.…”

High efficiency Mg2(Si,Sn)-based thermoelectric materials: scale-up synthesis, functional homogeneity, and thermal stability

“…This value is in the expected range for the heavily doped n-type Mg 2 (Si,Sn) compounds due to the high carrier concentration in the order of 10 20 cm À3 . 17,23,43 By increasing the temperature, the absolute Seebeck value is increased for all the samples and reached an average maximum value of À200 AE 10 mV K À1 at $773 K.…”

“…The gure of merit of all samples starts from around 0.3 at $300 K and enhances with increasing temperature to around 1.3 at $773 K, that is quite comparable to the as-synthesized sample using gas atomization method and among the highest so far reported efficiency for this class of material. 12,15,17,23,66 This provides an average zT of 0.9 over the whole temperature range which is distinctly benecial from the module fabrication point of view. The gure of merit of all samples is within the accepted uncertainty range of calculation (10-20%).…”

“…9,10 Increasing the electrical conductivity can be achieved via increasing the carrier density through doping with non-isoelectronic elements. So far, for n-type doping, group XV elements such as bismuth (Bi) [11][12][13][14][15][16][17][18] and antimony (Sb), [19][20][21][22][23][24][25] which replace silicon (Si), appear to be the most potent in enhancing the carrier concentration (more than 10 20 cm À3 at 300 K); therefore, increasing the electrical conductivity. 16 Reducing the thermal conductivity is also attainable by reducing the lattice contribution (k ph ) via phonon scattering.…”