ORALS

Recently, a huge magnitude of Seebeck coefficient was observed for metallic Cu₂Se under an unusual temperature gradient produced by an experimental set up consisting of the two-heaters. This material is characterized by a structure phase transition at about 400 K. Two heaters placed at the bottom and one side of sample produced a layered composite consisting of the low temperature phase (LTP) at the top surface and the high temperature phase (HTP) in the bottom. The composition of LTP varied with varying temperature during the phase transition, and eventually it became an insulator only in a very narrow temperature range. The insulating LTP staying on the metallic HTP allowed us to observe a large magnitude of Seebeck coefficient S exceeding a few mVK^-1 and rather small magnitude of electrical resistivity ρ as low as 1 mΩcm. These numbers led to a huge value of power factor PF = S²σ exceeding a few mWm^-1K^-2. Together with the small lattice thermal conductivity in association with the anharmonic lattice vibration, the dimensionless figure of merit ZT were increased to a surprisingly large value exceeding 470. [1] The variation of composition with varying temperature in the Cu₂Se LTP limits the large ZT-value only in a very narrow temperature range of a few K in width near 350 K, and this condition has prohibited us from utilizing it in practical applications. The problem about the narrow temperature range was eliminated by employing Ag₂S that also possess a structure phase transition between insulating low temperature phase and metallic high temperature phase. Despite that the magnitude of ZT was reduced to 20, the temperature width of the large ZT-value was extended to 40 K (390 K ≤ T ≤ 430 K). [2] The power generation from Ag₂S was also confirmed in the steady state condition, and the enhancement of power generation was found to appear above 390 K where ZT-value was strongly increased. The new phenomena introduced in this presentation provides us with a breakthrough in the technologies of thermoelectric power generation to contribute the construction of a low-energy-consuming, sustainable carbon neutral society.

References:
[1] D. Byeon et al., Nat. Comm.10, 1-7 (2019).
[2] G. Kim et al., J. Phys. D: Appl. Phys. 54, 115503 (2021).