Structure-Guided Enhancement of High-Temperature Thermoelectric Performance in Molten-Salt-Derived Weathered Biotite for Waste-Heat Recovery

Abstract Thermoelectric materials capable of operating at 600–900 °C are attractive for recovering industrial and nuclear waste heat toward a carbon-neutral society. Here, we report the structure-guided enhancement of high-temperature thermoelectric performance in natural weathered biotite (WB) through molten-salt treatment using a KCl–CaCl2 mixed salt (molar ratio 3:1) combined with systematic process optimization. The effects of WB particle size (<45 and 45–75 μm), salt addition ratio (1/100–1/1 by mass relative to WB), and compaction pressure (150–200 MPa) on crystal structure, densification behavior, and thermoelectric transport properties were systematically examined. Structural and compositional analyses revealed progressive incorporation of K and Ca with increasing salt addition while largely preserving the layered WB framework, together with additional diffraction features indicating reaction-induced structural modifications. Among the conditions investigated, samples prepared from WB with an intermediate particle size of 45–75 μm exhibited relatively high packing densities (>70%) at low salt addition levels and showed enhanced electrical conductivity, whereas excessive particle refinement (<45 μm) suppressed electrical transport, indicating that the transport response is sensitive to particle-size-dependent packing and processing-induced microstructural variation. High-temperature measurements further revealed exceptionally large Seebeck coefficients of up to ∼1.9 × 105 μV K–1, indicating that nonelectronic contributions may be involved in the observed thermovoltage. At present, the origin of this anomalously large response has not been conclusively established and requires further dedicated investigation. As a result of the cooperative optimization of particle size, salt addition, and compaction pressure, a maximum thermoelectric figure of merit of ZT = 0.29 at 667 °C was achieved, representing a marked improvement in this material system. These results demonstrate that microstructural control and densification strongly influence the measured thermoelectric response in layered silicate minerals and provide useful process–structure–property insights for natural mineral-derived high-temperature thermoelectric materials.