Elsevier

Journal of Alloys and Compounds

Volume 649, 15 November 2015, Pages 1060-1065
Journal of Alloys and Compounds

Synthesis of Al-doped Mg2Si1−xSnx compound using magnesium alloy for thermoelectric application

https://doi.org/10.1016/j.jallcom.2015.07.197Get rights and content

Highlights

  • Aluminium is used as dopant in n-type Mg2(Si,Sn) thermoelectric compounds.

  • Scrap magnesium alloy is used instead of pure raw material, and ball milling is avoided.

  • Dual phases of solid solution lower thermal conductivity.

  • Thermoelectric conversion of one-leg material is demonstrated.

Abstract

Mg2Si1−xSnx thermoelectric compounds were synthesized through a solid-state reaction at 700 °C between chips of Mg2Sn–Mg eutectic alloy and silicon fine powders. The Al dopants were introduced by employing AZ31 magnesium alloy that contains aluminum. The as-synthesized Mg2Si1−xSnx powders were consolidated by spark plasma sintering at 650–700 °C. X-ray diffraction and scanning electron microscopy revealed that the Mg2Si1−xSnx bulk materials were comprised of Si-rich and Sn-rich phases. Due to the complex microstructures, the electrical conductivities of Mg2Si1−xSnx are lower than Mg2Si. As a result, the average power factor of Al0.05Mg2Si0.73Sn0.27 is about 1.5 × 10−3 W/mK2 from room temperature to 850 K, being less than 2.5 × 10−3 W/mK2 for Al0.05Mg2Si. However, the thermal conductivity of Mg2Si1−xSnx was reduced significantly as compared to Al0.05Mg2Si, which enabled the ZT of Al0.05Mg2Si0.73Sn0.27 to be superior to Al0.05Mg2Si. Lastly, the electric power generation from one leg of Al0.05Mg2Si and Al0.05Mg2Si0.73Sn0.27 were evaluated on a newly developed instrument, with the peak output power of 15–20 mW at 300 °C hot-side temperature.

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.

References (26)

  • A.D. LaLonde et al.

    Materialstoday

    (2011)
  • A.U. Khan et al.

    Scr. Mater.

    (2013)
  • M. Ioannou et al.

    J. Solid State Chem.

    (2013)
  • X.K. Hu et al.

    J. Alloys Comp.

    (2014)
  • Q.S. Meng et al.

    J. Alloys Comp.

    (2011)
  • V.K. Zaitsev et al.

    Phys. Rev. B

    (2006)
  • Q. Zhang et al.

    Appl. Phys. Lett.

    (2008)
  • S.K. Bux et al.

    J. Mater. Chem.

    (2011)
  • T. Nemoto et al.

    J. Electron. Mater.

    (2010)
  • J.Q. Guo et al.

    J. Electron. Mater

    (2012)
  • W. Liu et al.

    J. Mater. Chem.

    (2012)
  • Z.L. Du et al.

    J. Electron. Mater

    (2012)
  • L.X. Chen et al.

    J. Mater. Res.

    (2011)
  • Cited by (16)

    • Aluminum as promising electrode for Mg<inf>2</inf>(Si,Sn)-based thermoelectric devices

      2021, Materials Today Energy
      Citation Excerpt :

      Moreover, Ag is well-known as a p-type dopant for Mg2(Si,Sn), it is, therefore, plausible that its diffusion into the n-type material decreases its carrier concentration. On the other hand, Al was predicted and shown to be a rather poor dopant in Mg2(Si,Sn) [62–66], with the data focusing on the Si-rich side of the spectrum. In our case of hypothetical uncontrolled Al-doping, the effect on carrier concentration would, therefore, be expected to be rather weak.

    • Defect-induced room-temperature visible light luminescence in Mg <inf>2</inf> Si:Al films

      2018, Applied Surface Science
      Citation Excerpt :

      Besides, it is known from the calculations of formation energies of the group IIIA impurities (B, Al, Ga, and In), the formation energy of Al in the Mg-site substitution is the lowest [19]. The electronic transport and thermoelectric properties of Al-doped Mg2Si have been reported [16,17,20–23]. However, there have been no reports concerning the effect of Al substitution on the photoelectric properties and Raman scattering and PL of Al-doped Mg2Si.

    • Recent progress in p-type thermoelectric magnesium silicide based solid solutions

      2017, Materials Today Energy
      Citation Excerpt :

      Table 1 lists all (to the best of our knowledge) reported p-type dopants for Mg2X. Al has also been considered as a p-type dopant [70,85], however, mostly n-type is observed experimentally [76,97–101]. Other experimentally investigated dopants include In Ref. [85] and K. However, for the latter one the obtained thermoelectric properties were poor, presumably due to a low carrier concentration [71,82].

    View all citing articles on Scopus
    View full text