Thermoelectric materials have gained significant attention for their ability to convert waste heat into usable electricity, offering a sustainable approach to energy harvesting. Among these, nanostructured magnesium silicide (Mg₂Si) stands out as a promising candidate due to its earth-abundant composition, non-toxicity, and favorable thermoelectric properties in the mid-temperature range of 500–700°C. This article explores the synthesis, doping strategies, and performance optimization of Mg₂Si-based thermoelectrics, focusing on reactive sintering as a fabrication method and the role of antimony (Sb) and tin (Sn) as n-type dopants.

Mg₂Si crystallizes in an antifluorite cubic structure, exhibiting a narrow bandgap of approximately 0.6–0.8 eV, which is suitable for thermoelectric applications in the mid-temperature range. The material’s performance is quantified by the dimensionless figure of merit, zT, defined as zT = (S²σ/κ)T, where S is the Seebeck coefficient, σ is the electrical conductivity, κ is the thermal conductivity, and T is the absolute temperature. Undoped Mg₂Si typically exhibits p-type conduction due to magnesium vacancies, but n-type behavior can be achieved through strategic doping with elements such as Sb or Sn.

Reactive sintering is a widely adopted synthesis method for Mg₂Si due to its simplicity and scalability. The process involves the direct reaction of magnesium and silicon powders under controlled conditions. A typical procedure includes mixing stoichiometric amounts of Mg and Si powders, followed by cold pressing into pellets. The pellets are then sintered in an inert atmosphere at temperatures between 700–900°C. The exothermic nature of the reaction between Mg and Si necessitates careful temperature control to prevent excessive heat generation, which can lead to inhomogeneities or secondary phases. The resulting material is often porous, but this can be mitigated by optimizing sintering parameters such as heating rate, pressure, and dwell time. Reactive sintering offers the advantage of producing Mg₂Si in a single step without the need for pre-alloying, reducing processing costs.

Doping plays a critical role in enhancing the thermoelectric performance of Mg₂Si. For n-type conduction, Sb and Sn are the most effective dopants. Sb substitutes for Si sites, introducing extra electrons into the conduction band and significantly improving electrical conductivity. Studies have shown that Sb doping levels of 1–3 at.% can reduce resistivity by an order of magnitude while maintaining a high Seebeck coefficient. Sn, on the other hand, substitutes for Mg sites and also contributes electrons, though its solubility in Mg₂Si is lower compared to Sb. Co-doping with Sb and Sn has been explored to fine-tune carrier concentration and mobility, further optimizing the power factor (S²σ). For instance, a composition of Mg₂Si₀.₉₈Sb₀.₀₂ has demonstrated a peak zT of ~0.7 at 600°C, while Sn-doped samples achieve slightly lower values due to limited dopant incorporation.

The thermal conductivity of Mg₂Si is another key parameter influencing zT. The intrinsic lattice thermal conductivity of Mg₂Si is relatively low (~3–5 W/m·K at room temperature) due to the complex crystal structure and strong phonon scattering. Nanostructuring the material through ball milling or spark plasma sintering can further reduce thermal conductivity by introducing grain boundaries and defects that scatter phonons. For example, nanostructured Mg₂Si with grain sizes below 100 nm has shown a 20–30% reduction in lattice thermal conductivity compared to bulk counterparts. This reduction, combined with optimized doping, contributes to higher zT values in the mid-temperature range.

The mid-temperature operation of Mg₂Si makes it suitable for applications such as waste heat recovery from industrial processes and automotive exhaust systems. In these scenarios, temperatures often fall within the 500–700°C range, aligning well with the peak performance window of Sb- or Sn-doped Mg₂Si. The material’s stability at elevated temperatures is another advantage, as it does not decompose or oxidize readily under operating conditions. Long-term stability tests have shown that doped Mg₂Si retains its thermoelectric properties after hundreds of hours of thermal cycling, making it a reliable choice for practical applications.

Despite these advantages, challenges remain in the widespread adoption of Mg₂Si thermoelectrics. One issue is the difficulty in achieving precise control over dopant distribution and concentration, which can lead to inhomogeneous properties. Advanced characterization techniques such as electron probe microanalysis and high-resolution transmission electron microscopy are essential for verifying dopant uniformity. Another challenge is the mechanical brittleness of Mg₂Si, which complicates module fabrication and integration. Ongoing research focuses on composite approaches, where secondary phases or nanostructured reinforcements are introduced to improve mechanical strength without degrading thermoelectric performance.

In summary, nanostructured Mg₂Si represents a viable thermoelectric material for mid-temperature energy conversion, leveraging earth-abundant elements and scalable synthesis methods. Reactive sintering provides a straightforward route to fabrication, while Sb and Sn doping enable n-type conduction with enhanced electrical properties. The combination of optimized doping, nanostructuring, and mid-temperature stability positions Mg₂Si as a competitive candidate for waste heat recovery systems. Future work will likely focus on refining dopant strategies, improving mechanical robustness, and scaling up production to meet industrial demands.