Thermoelectric materials capable of operating in high-temperature regimes (500–900°C) are critical for waste heat recovery and power generation in industrial processes, automotive exhaust systems, and aerospace applications. Among these, skutterudites, particularly cobalt triantimonide (CoSb3), have emerged as promising candidates due to their favorable electronic properties and tunable thermal conductivity. The unique crystal structure of skutterudites, combined with nanoscale engineering strategies, enables significant enhancements in thermoelectric performance, quantified by the dimensionless figure of merit (ZT). This article examines the structural characteristics, synthesis methods, and mechanisms underlying the improved thermoelectric properties of skutterudite nanomaterials, with a focus on high-temperature applications.

Skutterudites crystallize in a cubic structure with space group Im-3, characterized by large voids in the lattice. Unfilled CoSb3 consists of a framework of corner-sharing CoSb6 octahedra, forming cages that can host filler atoms. These fillers, typically rare-earth elements (e.g., Yb, La, Ce) or alkaline-earth metals (e.g., Ba, Sr), are incorporated to create filled skutterudites. The filler atoms "rattle" within the cages, scattering phonons and reducing lattice thermal conductivity while minimally disrupting electronic transport. This phonon-glass-electron-crystal (PGEC) behavior is central to the high ZT values observed in filled skutterudites. Unfilled CoSb3 exhibits higher thermal conductivity due to the absence of these phonon-scattering centers, making it less efficient for thermoelectric applications without further modification.

Synthesis of skutterudite nanomaterials involves a combination of techniques to achieve precise stoichiometry, reduced grain size, and controlled doping. Ball milling is widely used to produce nanopowders of CoSb3, with high-energy milling reducing particle sizes to the nanoscale (20–100 nm) and ensuring homogeneity. Subsequent consolidation via spark plasma sintering (SPS) preserves the nanostructure while achieving high-density bulk materials. SPS parameters, such as temperature (600–800°C), pressure (50–100 MPa), and duration (5–10 minutes), are optimized to minimize grain growth and enhance thermoelectric properties. Alternative methods like melt spinning and solvothermal synthesis have also been employed to produce skutterudite nanostructures with tailored morphologies.

The thermoelectric performance of skutterudites is quantified by ZT = (S²σT)/κ, where S is the Seebeck coefficient, σ is electrical conductivity, T is absolute temperature, and κ is thermal conductivity (κ = κe + κl, electronic and lattice contributions). Nanostructuring and doping strategies target each parameter to maximize ZT. For example, nanoscale inclusions (e.g., Yb2O3, CeO2) introduce additional phonon scattering at grain boundaries, reducing κl without significantly impairing σ. In filled skutterudites like Yb0.2Co4Sb12, κl can be reduced to below 1.5 W/m·K at 800°C, compared to 4–6 W/m·K for unfilled CoSb3. Simultaneously, optimizing carrier concentration through doping (e.g., Te at Sb sites, Ni at Co sites) enhances the power factor (S²σ). Double-filled skutterudites, such as Ba0.1Yb0.1Co4Sb12, achieve ZT values exceeding 1.5 at 800°C by balancing these effects.

High-temperature stability is crucial for skutterudite applications. Studies show that filled skutterudites retain their structural integrity and thermoelectric properties after prolonged exposure to 900°C in inert or reducing atmospheres. Oxidation resistance can be further improved by surface passivation or alloying with Fe. For instance, (Co,Fe)Sb3 compositions exhibit slower degradation rates in air while maintaining ZT > 1.2 up to 850°C.

Recent advances in skutterudite nanomaterials demonstrate the potential for ZT values approaching 2.0 at 900°C through hierarchical structuring. This involves combining micron-scale grains with embedded nanoscale precipitates and atomic-scale filler atoms to scatter phonons across multiple length scales. For example, a nanocomposite of Ce0.8Fe3CoSb12 with 5 vol% Sb nanoinclusions achieved a peak ZT of 1.8 at 850°C due to synergistic phonon scattering and optimized carrier mobility.

In summary, skutterudite thermoelectric nanomaterials leverage their unique PGEC behavior, nanostructuring, and doping to achieve high ZT values in the 500–900°C range. Filled skutterudites with rare-earth or alkaline-earth fillers, synthesized via ball milling and SPS, exhibit low lattice thermal conductivity and high power factors. Nanoscale inclusions and hierarchical structures further enhance performance, making these materials viable for industrial waste heat recovery and high-temperature power generation. Continued research focuses on optimizing filler compositions, nanostability, and scalable synthesis methods to enable commercial adoption.