Strained Lamellar Structures Leading to Improved Thermoelectric Performance in Mg₃Sb_1.5Bi_0.5

Area of Science:

• Materials Science
• Solid-State Physics
• Thermoelectrics

Background:

• Magnesium antimonide-bismuth (Mg3Sb2-xBix) solid-solutions are promising thermoelectric materials for mid-temperature applications.
• Their performance is linked to complex band structures and phonon scattering, but microstructure's role is less understood.

Purpose of the Study:

• To investigate the impact of microstructure, specifically lamellar structures, on the thermoelectric properties of Mg3+xSb1.5Bi0.5.
• To correlate microstructural features with lattice thermal conductivity and overall thermoelectric figure of merit (zT).

Main Methods:

• Synthesis of Mg3+xSb1.5Bi0.5 compositions with varying Mg content (x = 0.2, 0.3, 0.4).
• Temperature-dependent characterization of thermoelectric properties (Seebeck coefficient, electrical conductivity, thermal conductivity).
• Analysis of lattice thermal conductivity using mean free path (MFP) spectrum analysis.
• Microstructural analysis to identify and quantify lamellar structures.

Main Results:

• Compositions with low Mg content (x = 0.2) exhibited reduced lattice thermal conductivity (κL) due to the presence of lamellar structures (200-500 nm).
• These lamellar regions, resulting from Bi/Sb compositional fluctuations, enhanced phonon scattering through size and interfacial strain.
• Electrical properties, including power factor (S²σ) and weighted mobilities, remained largely unaffected by these microstructures.

Conclusions:

• Microstructure modification, specifically introducing lamellar structures, is an effective strategy to reduce lattice thermal conductivity in Mg3Sb2-xBix.
• This approach allows for significant tuning of the thermoelectric figure of merit (zT) by ~25% through preferential phonon scattering.
• Tailoring microstructure offers a new pathway for optimizing thermoelectric performance in these materials.