Thermoelectric materials capable of converting waste heat into electricity are critical for sustainable energy solutions. Among these, half-Heusler compounds have emerged as promising candidates for mid-to-high-temperature applications due to their excellent thermal stability, mechanical robustness, and tunable electronic properties. These materials, with compositions such as TiNiSn and ZrCoSb, follow the 18-electron rule, which governs their semiconducting behavior and thermoelectric performance. Their synthesis, nanostructuring, and grain boundary engineering play pivotal roles in optimizing their efficiency for power generation in the 400–800°C range.

The 18-electron rule is fundamental to understanding the electronic structure of half-Heusler alloys. These compounds crystallize in the MgAgAs-type structure with a stoichiometry of XYZ, where X and Y are transition or rare-earth metals, and Z is a main-group element. The rule states that a stable half-Heusler phase forms when the total valence electrons from X, Y, and Z sum to 18 per formula unit. For instance, TiNiSn (Ti: 4, Ni: 10, Sn: 4) satisfies this condition, resulting in a narrow bandgap semiconductor ideal for thermoelectric applications. Deviations from this rule often lead to metallic behavior or secondary phases, degrading thermoelectric performance. The precise control of composition and valence electron count is therefore essential to maintain high Seebeck coefficients and low thermal conductivity.

Synthesis methods for half-Heusler thermoelectric nanomaterials must ensure phase purity and controlled microstructure. Arc melting is a widely used technique due to its ability to produce homogeneous ingots. Elements are melted under an inert atmosphere, followed by annealing to achieve atomic ordering. However, arc melting can lead to coarse grains, necessitating additional processing. Mechanical alloying, a solid-state powder processing technique, offers an alternative by repeatedly fracturing and cold-welding precursor powders in a high-energy ball mill. This method yields nanocrystalline powders with enhanced grain boundary scattering, which reduces thermal conductivity. Subsequent spark plasma sintering consolidates these powders into dense bulk materials while preserving nanoscale features. Other approaches, such as melt spinning or solvothermal synthesis, have also been explored to achieve finer grain sizes and improved thermoelectric properties.

Nanoscale grain boundary engineering is crucial for optimizing the thermoelectric figure of merit (zT) in half-Heusler materials. Grain boundaries act as scattering centers for phonons, reducing lattice thermal conductivity without significantly impairing electronic transport. Introducing nanoscale precipitates or secondary phases further enhances phonon scattering. For example, in TiNiSn-based systems, nanoinclusions of TiC or full-Heusler phases can form during processing, disrupting heat-carrying phonons. Additionally, grain boundary composition and chemistry can be tailored through doping or alloying. Elements like Sb or Hf substituted at the Sn or Zr sites in ZrCoSb modify the electronic structure and grain boundary potential, improving carrier mobility and Seebeck coefficients. Advanced characterization techniques, such as transmission electron microscopy and atom probe tomography, reveal these nanoscale features and their impact on thermoelectric performance.

The thermoelectric properties of half-Heusler nanomaterials are highly temperature-dependent. In the mid-to-high-temperature range (400–800°C), these materials exhibit peak zT values between 0.8 and 1.2, making them competitive with traditional bismuth telluride alloys at lower temperatures. The high-temperature stability of half-Heuslers is attributed to their strong intermetallic bonding and resistance to oxidation. For instance, ZrCoSb maintains its structural integrity up to 800°C, with minimal degradation in electrical conductivity. Doping strategies, such as substituting Nb for Ti in TiNiSn or introducing vacancies at the Y site in XYZ compounds, further enhance power factors by optimizing carrier concentration. The interplay between electronic and thermal transport is carefully balanced to maximize efficiency.

Challenges remain in scaling up the production of half-Heusler thermoelectric nanomaterials while maintaining performance. Process reproducibility, cost-effective raw materials, and long-term durability under thermal cycling are critical considerations. Recent advances in additive manufacturing, such as 3D printing of thermoelectric materials, offer potential solutions by enabling complex geometries and reduced material waste. Furthermore, computational modeling aids in predicting optimal compositions and microstructures, accelerating the discovery of new high-performance half-Heusler systems.

In summary, half-Heusler thermoelectric nanomaterials represent a promising avenue for mid-to-high-temperature power generation. Their 18-electron rule-guided design, combined with advanced synthesis and nanoscale engineering, enables high zT values and thermal stability. Continued research into microstructure control and scalable fabrication methods will be essential for their widespread adoption in energy harvesting applications.