In recent decades, by continuously enhancing the figure of merit ZT of various thermoelectric (TE) materials, solid state TE technology has matured and is on the verge of making an impact in real industrial settings as a promising approach to harvest waste industrial heat and convert it to useful electricity. Nevertheless, actual TE module development has remained stagnant with rather poor efficiencies. This has raised an urgent need to design rational module structures that rely on complex parameter optimization and utilization of efficient integration technologies that minimize energy losses during bonding of various interfaces. Here, we demonstrate a three-dimensional numerical analysis model of a segmented TE power-generating device, which takes into account the temperature-dependent materials' properties and various parasitic losses. The model generates an optimized design with predictive performance to realize maximum conversion efficiency. Combined with the developed bonding schemes and assembly techniques, the segmented modules consisting of Bi2Te3-based alloys and CoSb3-based filled skutterudites were successfully fabricated with a record-high efficiency of up to 12% when operating under a temperature difference of 541 °C. The rational structure design based on the numerical analysis model and the extremely low thermal and electrical losses enable the heat-to-electricity conversion efficiency to reach up to 96.9% of the theoretical efficiency based on the TE materials themselves. These findings highlight the importance of the optimization strategy for TE power generation devices based on the TE materials' intrinsic properties and demonstrate that realistic high temperature TE modules with predictive high efficiency and high power density can be fabricated, which provides a useful guide to achieve a high conversion efficiency in large-scale TE applications.

Realizing a thermoelectric conversion efficiency of 12% in bismuth telluride/skutterudite segmented modules through full-parameter optimization and energy-loss minimized integration

Although the CoSb3-based skutterudite thermoelectric devices have been highly expected for wide uses such as waste heat recovery and space power supply, the limited long-term service stability majorly determined by the degradation of electrode interface obstructs its applications. Here, we built up an effective criterion for screening barrier layer based on the combination of negative interfacial reaction energy and high activation energy barrier of Sb migration through the formed interfacial reaction layer. Accordingly, we predicted niobium as a promising barrier layer. The experimental results show the skutterudite/Nb joint has the slowest interfacial reaction layer growth rate and smallest interfacial electrical resistivity. The fabricated 8-pair skutterudite module using Nb as barrier layer achieves a recorded conversion efficiency of 10.2% at hot-side temperature of 872 K and shows excellent stability during long-time aging. This simple criterion provides an effective guidance on screening barrier layer with bonding-blocking-conducting synergetic functions for thermoelectric device integration. Long-term service stability of thermoelectric devices is one of the major obstacles for their application. Here, the authors combine interfacial reaction energy and Sb migration activation energy barrier as a criterion to determine the interfacial reliability for skutterudite thermoelectric devices.

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Electrode interface optimization advances conversion efficiency and stability of thermoelectric devices

Recent progress in p-type thermoelectric magnesium silicide based solid solutions

Magnesium-based thermoelectric materials (Mg2X, X = Si, Ge, Sn) are considered one of the most attractive groups for large-scale application, due to their materials' high availability, low cost, low mass density, and reasonably high efficiency. In this work, we present an overview of the recent developments relating to magnesium-based thermoelectric materials and review the current approaches towards high thermoelectric efficiency.

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Recent progress in magnesium-based thermoelectric materials

Thermoelectric performance of Li doped, p-type Mg2(Ge,Sn) and comparison with Mg2(Si,Sn)

The durability of Ni electrodes, which are often used for Mg2Si thermoelectric chips, is poor at high working temperatures because of deposition of Mg at the Mg2Si/Ni interface and on the surface. Hence, a “Mg2Si/barrier material/Ni” structure was adopted instead of direct adhesion of Ni to Mg2Si. Ti, TiSi2, and TiN were selected as candidate materials for the barrier layer between Mg2Si and Ni, and the barrier effect, adhesion, and contact resistance of each of these materials were evaluated. After the samples had been annealed at 873 K for 1 h, Mg appeared on the Ti surface and TiSi2 deposited on Mg2Si; however, no Mg was detected on the surface of TiN or in the inner part of the Ni electrode. Continuous, low contact resistance was also observed for Mg2Si/TiN/Ni samples. TiN does not adhere strongly to Mg2Si but is a promising barrier material for Mg2Si/Ni interfaces.

Kiyohide Ichimi

Papers

Investigation of Barrier-Layer Materials for Mg 2 Si/Ni Interfaces