High-temperature thermoelectric materials are critical for aerospace and industrial applications where waste heat recovery and power generation in extreme conditions are required. These materials must exhibit exceptional thermal stability, oxidation resistance, and mechanical durability under harsh environments, including high temperatures, thermal cycling, and corrosive atmospheres. Oxides and silicides are among the most promising candidates due to their inherent stability at elevated temperatures, making them suitable for long-term operation in demanding scenarios.

Oxide-based thermoelectric materials, such as strontium titanate (SrTiO3), cobaltites (e.g., Ca3Co4O9), and zinc oxide (ZnO), have gained attention for their ability to withstand oxidizing environments. These materials exhibit high melting points, often exceeding 1500°C, and maintain structural integrity under thermal cycling. A key advantage of oxides is their natural oxidation resistance, eliminating the need for protective coatings that can degrade over time. Recent advancements in doping strategies, such as lanthanum substitution in SrTiO3, have improved their thermoelectric performance by optimizing carrier concentration and reducing thermal conductivity. For instance, La-doped SrTiO3 has achieved a ZT value of 0.4 at 1000°C, a significant improvement over undoped variants.

Silicides, including magnesium silicide (Mg2Si) and higher manganese silicides (MnSi1.7), are another class of high-temperature thermoelectric materials. These compounds are lightweight, cost-effective, and exhibit good thermoelectric properties at temperatures ranging from 500°C to 900°C. Silicides are particularly attractive for aerospace applications due to their low density and compatibility with silicon-based manufacturing processes. However, their oxidation resistance is inferior to oxides, necessitating protective layers or alloying with elements like aluminum to enhance durability. Recent research has demonstrated that nanostructuring MnSi1.7 with germanium doping can reduce thermal conductivity while maintaining electrical conductivity, resulting in ZT values approaching 0.8 at 800°C.

Thermal cycling stability is a critical parameter for high-temperature thermoelectric devices, as repeated heating and cooling can induce mechanical stress, phase separation, or interfacial degradation. Testing protocols involve subjecting materials to rapid temperature fluctuations between room temperature and operational extremes, often exceeding 1000 cycles. For example, oxide-based modules have shown less than 5% performance degradation after 1000 cycles between 200°C and 900°C, while silicide-based devices require careful interfacial engineering to prevent cracking or delamination. Advanced characterization techniques, such as in-situ X-ray diffraction and scanning electron microscopy, are employed to monitor microstructural evolution during cycling.

Oxidation resistance is another vital consideration, particularly for silicides and non-oxide materials. Accelerated aging tests in air or controlled atmospheres at elevated temperatures simulate long-term exposure. Oxide materials inherently excel in these tests, with negligible mass gain or phase changes after hundreds of hours at 1000°C. In contrast, silicides may form insulating oxide layers, necessitating protective barriers such as alumina coatings or encapsulation in inert atmospheres. Recent breakthroughs include the development of self-passivating silicide alloys that form a stable silica layer, slowing further oxidation.

Compatibility with harsh environments extends beyond thermal and oxidative stability. High-temperature thermoelectric devices must also resist chemical corrosion, mechanical vibration, and radiation exposure, especially in aerospace applications. For example, in jet engines or nuclear power systems, materials must endure aggressive chemical species like sulfur or chlorine compounds. Oxide thermoelectrics, particularly those based on perovskite structures, demonstrate superior resistance to such corrosive agents. Testing under simulated exhaust gas conditions has shown that doped SrTiO3 retains over 90% of its initial performance after exposure to sulfur dioxide at 800°C for 500 hours.

Device integration poses additional challenges, as electrode materials and interconnects must match the thermal expansion coefficients of the thermoelectric elements to minimize interfacial stress. Platinum and refractory metals like tungsten are commonly used, but their cost and weight drive research into alternatives such as conductive ceramics or layered composites. Recent work on graded electrodes, where the composition gradually transitions from the thermoelectric material to the metal interconnect, has improved adhesion and reduced interfacial resistance.

Recent breakthroughs in high-temperature thermoelectrics include the discovery of new oxide phases with intrinsically low thermal conductivity, such as layered cobaltites with phonon-blocking interfaces. Another advancement is the use of machine learning to optimize doping profiles and sintering conditions, leading to materials with tailored properties for specific temperature ranges. For silicides, progress in additive manufacturing has enabled the fabrication of complex geometries with reduced waste and improved thermal management.

Testing protocols for aerospace and industrial applications are rigorous, involving not only thermal cycling and oxidation tests but also vibration, shock, and vacuum exposure. Standards developed by organizations like NASA and ASTM provide guidelines for evaluating performance under simulated operational conditions. For example, a typical test sequence might include thermal cycling between -50°C and 900°C, followed by vibration testing at frequencies up to 2000 Hz, and finally, performance measurement in a vacuum chamber to simulate space conditions.

The future of high-temperature thermoelectric materials lies in further improving their efficiency and durability while reducing cost and weight. Multiphase composites, such as oxide-silicide hybrids, offer a promising path by combining the strengths of both material classes. Additionally, advances in nanotechnology, such as the incorporation of nanoscale precipitates or grain boundary engineering, continue to push the limits of ZT values and stability. As these materials mature, their adoption in aerospace power systems, industrial waste heat recovery, and deep-space missions will expand, enabling more efficient and sustainable energy utilization in extreme environments.