Figure of merit zT mapping of p-Mg2Si1−xSnx with respect to carrier concentration.

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.

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Magnesium silicides can be used for thermoelectric energy conversion, as high values of figure of merit zT were obtained for n-type (1.4 at 500 °C) and p-type (0.55 at 350 °C) materials. This, however, needs to be complemented by low resistive and stable contacting to ensure long-term thermogenerator operation and minimize losses. In this study, we selected Cu and Ni 45 Cu 55 as contacting electrodes for their high electrical conductivity, similar coefficient of thermal expansion (CTE) and good adhesion to Mg 2 (Si,Sn). Both electrodes were joined to Mg 2 Si 0.3 Sn 0.7 pellets by hot pressing in a current-assisted press. Microstructural changes near the interface were analyzed using SEM/EDX analysis, and the specific electrical contact resistance r c was estimated using a travelling potential probe combined with local Seebeck scanning. Good contacting was observed with both electrode materials. Results show low r c with Cu, suitable for application, for both n-type and p-type silicides (< 10 µΩ•cm 2 ), with the occurrence of wide, highly conductive diffusion regions. Ni 45 Cu 55 joining also showed relatively low r c values (~ 30 µΩcm 2 ) for n-and p-type, but had a less inhomogeneous reaction layer. We also performed annealing experiments with Cu-joined samples at 450 °C for one week to investigate the evolution of the contact regions under working temperatures. r c values increased (up to ~ 100 µΩcm 2 ) for annealed n-type samples, but remained low (< 10 µΩcm 2 ) for p-type. Therefore, Cu is a good contacting solution for p-type Mg 2 (Si,Sn), and a potential one for n-type if the diffusion causing contact property degradation can be prevented.