Synthesis of Al-doped Mg2Si1−xSnx compound using magnesium alloy for thermoelectric application
Graphical abstract
Introduction
Thermoelectric materials have received renewed interest for potential applications in waste heat recovery [1]. The performance of a thermoelectric material at a given temperature T is determined by a dimensionless figure of merit ZT = S2σT/k, where S, σ, k represents the Seebeck coefficient, electrical conductivity and thermal conductivity, respectively. Good thermoelectric materials require a combination of high Seebeck coefficient and electrical conductivity and low thermal conductivity.
Magnesium silicide and its alloyed compounds with Mg2Sn, have been recognized as one family of high-performance thermoelectric materials, with the promising application in converting waste heat into electricity at 500–800 K [2], [3], [4], [5]. As compared to the state-of-the-art lead telluride and skutterrudite based materials [1], [6], Mg2Si(Sn) alloys are non-toxic, contain abundant constituent elements and are light weight. Many attempts have been made to optimize the thermoelectric properties of Mg2(Si, Sn) alloys by tuning the microstructure and doping atoms [7], [8], [9], [10], [11], [12]. The incorporation of Mg2Sn reduces the thermal conductivities of Mg2(Si, Sn) solid solutions greatly with respect to Mg2Si compound, thanks to their similar anti-Fluorite cubic crystalline structure and lattice thermal scattering. In most cases, antimony and bismuth are selected as n-type dopants to enhance electrical conductivity [13], [14], [15]. The ZT value has been reported to be above unity in Bi or Sb doped Mg2Si1−xSnx (x = 0.4–0.6), as a consequence of the increased electrical conductivity and lowered thermal conductivity [2], [3], [13]. Aluminum has been regarded historically as a good dopant for Mg2Si, however, there is little information on the thermoelectric properties of Al-doped Mg2Si1−xSnx materials [16], [17].
Melting and solidification have been used to fabricate the solid-solution compounds from elemental mixtures (i.e. ingot, shot or granule) in an airproof environment [2], [18]. This approach suffers some difficulties when applied to Mg2(Si, Sn): large difference in melting point between Si and Mg and high vaporizing pressure of Mg. Usually, 5% more magnesium than the stoichiometric amount has to be charged to compensate for Mg loss. Recently, solid-state synthesis from the constituent elemental powders has been adopted to avoid the usage of high-temperature melting [13], [19], [20]. This method requires multiple steps of solid state reaction plus pulverization and long term annealing to ensure the compositional and phase homogeneity. Typically, mixed raw materials are cold pressed, placed into a quartz tube in vacuum, and then allowed to react at high temperature [13]. B2O3 flux synthesis has been used to facilitate the process [21], [22].
In our recent study [23], chips of AZ31 magnesium alloy were selected as a magnesium source to produce Al-doped n-type Mg2Si using a solid state reaction. Spark plasma sintering was then applied to consolidate the as-synthesized powders into bulk materials. In the present study we use this approach to fabricate Al-doped n-type Mg2(Si, Sn) compounds in an attempt to raise the ZT by lowering thermal conductivity. Industrial grade Mg alloys were used, and the alloying element of aluminum acts as dopant. Thermoelectric properties of Mg2Si and Mg2Sn are provided as a reference.
Section snippets
Experimental section
Ingots of industrially pure magnesium, AZ31 alloy and tin (99.9 pure) were used as the starting materials. AZ31 magnesium alloy contains approximately 3% Al and 1% Zn in weight besides the dominant Mg. Silicon fine powders (325 mesh, 99% pure) were purchased from Sigma Aldrich Corporation.
Four ingots of Mg–Sn and AZ31-Sn alloys with these compositions: Mg-40wt%Sn, Mg-50wt%Sn, AZ31-40wt%Sn and AZ31-50wt%Sn, were manufactured by co-melting the respective ingots in a heater furnace at 650–800 °C
Results and discussion
Fig. 2 gives the X-ray diffraction patterns for Mg2Si–Mg2Sn powder after the solid state reaction. The patterns of S1 and S2 are the XRD results for Al-doped Mg2Si and undoped Mg2Sn powders. All strong diffraction peaks are indexed to those of the Mg2Si phase (JCPDS 00-034-0458) and Mg2Sn phase (JCPDS 00-007-0274), respectively. Both Mg2Si and Mg2Sn intermetallic compounds have the same face-centered cubic structure with different lattice constants. A weak peak at 38.4° with an asterisk in
Conclusion
Chips of Mg2Sn–Mg alloy could be employed to fabricate Mg2Si1−xSnx thermoelectric compounds through a solid state reaction with silicon fine powders at 700 °C. No grinding or ball milling was employed. The as-synthesized Mg2Si1−xSnx powders contained two phases of the partially formed solid solution. Spark plasma sintering consolidated the powders into bulk materials, but it did not produce the exact single phase of Mg2Si1−xSnx. Melts were observed after the spark plasma sintering. The doping
Acknowledgement
Thanks Hirotaka Nishiate san at AIST for thermal conductivity and Hall measurements.
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