Abstract:

Metal sulfide Ag₂S is an attractive semiconductor due to its excellent physical and chemical property that enable it with wide applications in fields of catalysis, sensing, optoelectronics in past years. In present work, ϕ18 mm× 50 mm Ag₂S ingot was successfully prepared using zone melting method and its thermoelectric (TE) behavior was investigated. Ag₂S has standard monoclinic P2₁/c space group (α-Ag₂S phase) below 450 K and transfer to cubic structure (β-Ag₂S phase) over this temperature. Ag₂S is a n-type semiconductor as the Seebeck coefficient S is always negative due to the Ag interstitial ions in the material that can provide additional electrons. S is about -1200 µV·K^-1near room temperature, declines to -680 µV·K ^-1 at 440 K and finally decreases to ~-100 µV·K ^-1at β-Ag₂S state. The electrical conductivity (σ) of α-Ag₂S is almost zero. However, the value sharply jumps to ~40000.5 S·m ^-1 as the material just changes to β-Ag₂S at 450 K and then gradually deceases to 33256.2 S·m ^-1 at 650 K. Hall measurement demonstrates that carrier concentration n_H of Ag₂S is suddenly increased from the level of ~10 ¹⁷ cm^-3 to ~10¹⁸ cm^-3during phase transition. Total thermal conductivity κ of α-Ag₂S is ~0.20 W·m ^-1·K^-1 but is ~0.45 W·m^-1·K^-1of β-Ag₂S. Ultimately, a maximum ZT=0.57 is achieved around 580 K that means Ag₂S might be a promising middle-temperature TE material.

Key words: Ag₂S, zone melting, thermoelectric material, phase transition