Thermoelectric performance of n-type Mg₂Ge

Area of Science:

• Materials Science
• Solid State Physics
• Nanotechnology

Background:

• Magnesium-based thermoelectric materials (Mg₂X, X = Si, Ge, Sn) are attractive due to their abundance, low toxicity, and good thermoelectric potential.
• Synthesizing high-purity Mg₂X compounds is challenging due to magnesium's reactivity and high vapor pressure.

Purpose of the Study:

• To fabricate high-purity, single-phase n-type Mg₂Ge.
• To investigate the effect of bismuth doping on the thermoelectric properties of Mg₂Ge.
• To understand the mechanism behind improved thermoelectric performance.

Main Methods:

• One-step reaction of magnesium hydride (MgH₂) and elemental germanium (Ge).
• Spark plasma sintering (SPS) to consolidate the material and minimize oxidation.
• Characterization of material purity, phase composition, and thermoelectric properties.

Main Results:

• High-purity, single-phase n-type Mg₂Ge was successfully synthesized.
• Bismuth exhibited limited solubility in Mg₂Ge, forming Mg₂Bi₃ precipitates.
• Bismuth doping increased electrical conductivity up to its solubility limit.
• Mg₂Bi₃ precipitates at grain boundaries significantly scattered phonons, reducing lattice thermal conductivity by ~50%.
• The maximum figure of merit (zT) for n-type Mg₂Ge was increased to 0.32.

Conclusions:

• The synthesis method using MgH₂ and SPS is effective for producing high-purity Mg₂Ge.
• Bismuth doping, via Mg₂Bi₃ precipitates, is a viable strategy to enhance the thermoelectric performance of Mg₂Ge.
• The observed improvement in zT is primarily attributed to reduced lattice thermal conductivity caused by phonon scattering at precipitates.