Effect of Sintering Temperature on the Thermoelectric Properties of Ag₂Se Fabricated by Spark Plasma Sintering with High Compression

Abstract

Silver selenide (Ag₂Se) is a high-performance thermoelectric (TE) material near room temperature. This research improves its TE figure-of-merit (ZT) by varying the sintering temperatures (423–723 K) in the spark plasma sintering (SPS) process with high compression pressure (300 MPa). The SPS compaction of Ag₂Se powders synthesized by wet chemical reaction leads to the fast fusion of particles so that the grain boundaries are hardly visible. Furthermore, the fast fusion causes nanopores at the grain surface and some cracks, particularly at higher sintering temperatures. These featured microstructures decrease carrier concentrations and affect the TE properties significantly. The TE measurements show that increasing sintering temperatures results in decreased electrical conductivity and increased magnitude of the Seebeck coefficient due to microstructural defects. Increasing SPS temperatures also suppresses the thermal conductivity from enhancing phonon scattering by defects. The bulk Ag₂Se sample sintered at 723 K shows the best TE performance with the maximum ZT of 0.90 with a slight variation from 300 to 400 K. Thus, the high-temperature SPS with high-compression pressure is likely to be the key for fabricating bulk Ag₂Se with high TE performance.