Research on Preparation Process of N-type Nano Silicon-germanium Alloy for Isotope Batteries

Radioisotope thermoelectric generators (RTG) are critical and irreplaceable power systems for deep space exploration, where solar energy is scarce or unavailable. Among high-temperature thermoelectric materials, silicon-germanium (SiGe) alloys are recognized as the dominant candidate for the hot-side materials of RTG due to their excellent high-temperature stability, good mechanical strength, and proven operational reliability in extreme space environments. However, the intrinsic high thermal conductivity of SiGe alloys limits their thermoelectric conversion efficiency, constraining the overall performance and further development of next-generation RTG. To address this material-level challenge, this study focused on enhancing the thermoelectric properties of N-type SiGe through nanostructuring and process optimization. Specifically, N-type (Si_0.8Ge_0.2)_0.98P_0.02 alloys were synthesized using a two-step method combining high-energy ball milling (HEBM) and spark plasma sintering (SPS). HEBM was used to prepare nanoscale powders to introduce grain boundaries and lattice defects for phonon scattering and lattice thermal conductivity reduction. Subsequently, SPS enabled rapid consolidation of the powders at high temperatures under pressure, minimizing grain growth while ensuring high bulk density. Based on this two-step strategy, the effects of key fabrication parameters—including ball milling duration, SPS temperature, and holding time—on the microstructural evolution and thermoelectric transport properties of the resulting bulk materials were investigated. Measurements of the electrical conductivity, Seebeck coefficient, and thermal conductivity were performed over a temperature range from 300 K to 1 000 K. The results indicate that these processing parameters significantly influence the carrier concentration, grain size, and phonon scattering mechanisms, thereby modulating the power factor and thermal conductivity. An optimal combination of parameters is identified: a ball milling time of 8 h, an SPS temperature of 1 423 K (1 150 °C), and a holding time of 15 minutes. Under these conditions, the synthesized (Si_0.8Ge_0.2)_0.98P_0.02 sample exhibits a peak thermoelectric figure of merit (ZT) of approximately 1.218 at 1 000 K, representing a notable improvement compared to conventional unoptimized SiGe alloys. This enhancement is primarily attributed to a significant reduction in lattice thermal conductivity due to intensified phonon scattering at nanostructures and defects, while a reasonably high power factor is maintained. The findings confirm that precise control over nanostructuring via processing optimization is a viable and effective strategy for improving the ZT of SiGe-based thermoelectric materials. Consequently, this work provides an optimized material candidate and valuable processing insights for developing more efficient RTG, with potential implications for enhancing the energy sustainability and mission capability of future deep space exploration.