Microstructure and thermoelectrical investigations of an N-type magnesium–silicon–tin alloy Radivoje Vracar, Guillaume Bernard-Granger ⇑ , Christelle Navone, Mathieu Soulier, Mathieu Boidot, Jean Leforestier, Julia Simon Commissariat à l’Energie Atomique et aux Energies Alternatives, DRT/LITEN/DTNM/SERE/LTE, 17, rue des Martyrs, 38054 Grenoble Cedex 9, France article info Article history: Received 6 December 2013 Received in revised form 5 February 2014 Accepted 8 February 2014 Available online 15 February 2014 Keywords: Thermoelectric materials Spark plasma sintering Transmission electron microscopy abstract An N-type Mg 2 Si 0.3875 Sn 0.6 Sb 0.0125 powder, directly prepared by mechanical alloying from the raw ele- ments, has been sintered by spark plasma sintering. TEM observations enabled to detect nano-sized inclusions (diameter around 13 nm, 1.9 Â 10 9 inclusions/mm 2 ) dispersed in the bulk of individual grains constituting the sintered polycrystal (grain size around 450 nm). These inclusions (enriched in Sn and impoverished in Mg) are thought to scatter lattice vibrations, leading to a dimensionless-thermoelectrical figure of merit (ZT) around 0.85 at 500 °C. Ó 2014 Elsevier B.V. All rights reserved. 1. Introduction Thermoelectric materials have the ability to directly convert temperature differences to electric voltage and vice versa. By prop- er doping, N-type (majority carriers are electrons) and P-type (holes are majority carriers) thermoelectric materials are achievable. The dimensionless figure of merit ZT (Z =S 2 r/k where S is the Seebeck coefficient, r the electrical conductivity, k the thermal conductivity; T is the absolute temperature) is characterizing the efficiency of a thermoelectric material. The higher the ZT value, the better the thermoelectric behavior for a given material. N and P-types Mg 2 Si x Sn 1Àx alloys, with x varying from 0.2 to 0.8, have been deeply investigated for thermoelectrical applications in the 300–600 °C temperature range [1–10]. Nonetheless, processing methods to elaborate such materials of interest are complex, with numerous subsequent steps, and time consuming [1–10]. To prepare the raw powders he investigated, Liu was using a two-step solid state reaction method [2,7–10]. Commercial high- purity powders of silicon, tin and antimony were weighted in stoi- chiometric amounts and powdered magnesium was added with an excess of several percent over its stoichiometric amount in order to compensate its evaporation at high temperature. The constituents were hand-ground in an agate mortar in a glove box, cold pressed and sealed in quartz tubes under vacuum for the first step of the solid state reaction at 600–700 °C. In order to promote the solid solution formation and to increase the homogeneity of the prod- ucts, the resulting pellets were ground to fine powders in the glove box, cold-pressed again into cylinders and then sealed in quartz tubes in vacuum for the second stage solid state reaction at 700 °C. Finally, after the solid state reaction was completed, the compacts were ground into fine powders in the glove box and con- solidated into dense bulk materials using spark plasma sintering (SPS). For an N-type Mg 2.14 Si 0.39 Sn 0.60 Sb 0.009 composition (anti- mony being the dopant), a ZT peak value of 1.3 is obtained between 465 and 510 °C [7]. Zaitsev manufactured polycrystalline ingots of Mg 2 Si 1Àx Sn x so- lid solution by direct melting of the components in boron crucibles using high-frequency heating [3,4]. Long-time annealing was used for homogenization of the samples. Concentration of current carri- ers was controlled by antimony additions. A ZT peak value of 1.1 is measured for temperatures ranging from 430 to 530 °C for an N-type material having the Mg 2 Si 0.4 Sn 0.6 target composition with 3.7 Â 10 20 carriers/cm 3 [4]. Zhang [5] used a similar process than the one developed by Zait- sev to prepare the materials he investigated. The obtained ingots were ball milled and hot-pressed to obtain dense samples. Using also antimony as a dopant, he obtained a ZT peak value of 1.1–1.2 at 530 °C for an N-type Mg 2 Si 0.3925 Sn 0.6 Sb 0.0075 formulation. In the present communication we are reporting about the microstructure/thermoelectrical properties of a Mg 2 Si x Sn 1Àx sam- ple that was sintered by SPS from a raw powder synthesized by mechanical alloying, a method thought to be simpler than the ones used up to now and presented above. http://dx.doi.org/10.1016/j.jallcom.2014.02.040 0925-8388/Ó 2014 Elsevier B.V. All rights reserved. ⇑ Corresponding author. Tel.: +33 438 78 22 77. E-mail address: guillaume.bernard-granger@cea.fr (G. Bernard-Granger). Journal of Alloys and Compounds 598 (2014) 272–277 Contents lists available at ScienceDirect Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom