Scientists from the Shanghai Institute of Ceramics of Chinese Academy of Sciences (SICCAS) achieved simultaneously high thermoelectric (TE) performance and smooth volume variation during phase transitions by co-doping (Mg, Sb) into GeTe to push the pseudo-cubic angle of rhombohedral phase to 90°. They successfully fabricated a Ni/Ti/Ge_0.85Mg_0.05Sb_0.1Te TE uni-leg which shows high power output and good service stability.

This study, published in National Science Review, will promote the application of GeTe-based materials for power generation at middle-temperature range.

TE technology has attracted attentions for its capacity of converting waste heat directly into electricity, which is reliable and environmental friendly. Its conversion efficiency mainly depends on the material’s TE performance, which usually requires large Seebeck coefficient, high electrical conductivity as well as low thermal conductivity. However, these physical parameters are strongly interrelated, thus it is quite difficult to optimize them simultaneously.

GeTe is a p-type narrow band-gap semiconductor crystallizing with a rhombohedral structure (R3m) at room temperature. The TE performance of GeTe-based materials has developed rapidly in recent years. The highest ZT value has exceeded 2.0 and the theoretical energy conversion efficiency reaches 12%. Thus GeTe-based devices show the prospect in the application as thermoelectric generators.

By pushing the pseudo-cubic angle in the rhombohedral phase more toward 90°, scientists from SICCAS successfully improved the thermal expansion coefficient of rhombohedral phase from 11 ′ 10^-6 K^-1 for GeTe to 19 ′ 10^-6 K^-1 for Ge_0.85Mg_0.05Sb_0.1Te, which is close to that of cubic phase (23 ′ 10^-6 K^-1).

In addition, the negative thermal expansion coefficient of GeTe during the phase transition disappeared. Thus, the volume variation was very smooth around the phase transition temperature, which is beneficial for fabricating GeTe-based devices with high performance and high service stability.

Moreover, scientists synthesized a series of (Mg, Sb) co-doped GeTe-based compounds. (Mg, Sb) co-doping at Ge-sites introduced additional mass and strain field fluctuations to strongly scatter heat-carrying phonons, and thus lowered the lattice thermal conductivity. The simultaneous optimization for electron and thermal transport properties resulted in a maximum zT of 1.84 at 800 K and an average zT of 1.2 at 300-800 K for Ge_0.85Mg_0.05Sb_0.1Te.

By choosing Ni as the electrode and Ti as the diffusion layer, GeTe-based TE uni-leg was finally fabricated.

The results of the performance measurement of Ni/Ti/Ge_0.85Mg_0.05Sb_0.1Te TE uni-leg showed that the maximum power output reach 135 mW, and the internal resistance, output voltage, and power output were quite stable after even 450 thermal cycles (the hot side temperature is cycled between 473 K and 800 K). They indicated that Ge_0.85Mg_0.05Sb_0.1Te material facilitates the high service stability of TE uni-leg.