All-Mg3Sb2-based thermoelectric modules with high efficiency and excellent reliability

Thermoelectric (TE) power generation technology is a promising solution for improving the efficiency of fossil fuels and reducing environmental pollution. It offers advantages such as solid-state operation, minimal maintenance, and extended service life. While significant progress has been made in improving TE materials, the advancements in TE device technology have been slower.

To enable widespread applications of TE devices, the development of modules consisting of both n-type and p-type TE materials is necessary. However, this is a more challenging task compared to fabricating single legs. Several issues need to be addressed, including the development of matching n-type and p-type materials, optimization of TE leg geometry, welding and assembling of multiple legs, and evaluation of module efficiency and reliability. Additionally, the presence of rare or toxic elements in current TE components may hinder large-scale applications.

In recent years, Mg3Sb2-based compounds have gained attention in the TE community due to their non-toxic nature, abundance of constituent elements, and mechanical robustness. Research into these compounds has shown significant progress, leading to improvements in TE performance. Mg3Sb2-based compounds could serve as low-cost and environmentally friendly alternatives to current alloys containing rare or toxic elements.

Efforts have been made at the single-leg level, focusing on scalable synthesis of n-type Mg3Sb2, reliable junction interfaces, and barrier layers. Promising results have been achieved, with single-leg efficiencies of around 10% at a temperature difference of 400 K. This demonstrates potential for medium-temperature power generation applications.

At the module level, different p-type TE compounds have been paired with n-Mg3Sb2, resulting in outstanding power generation performance in the low and medium temperature range. However, the use of different-parent n- and p-type alloys complicates device design and the selection of suitable barrier layers. The differences in physical parameters between these materials, such as thermal expansion coefficient, can lead to high thermal stresses and device failure. Furthermore, variations in melting point and machinability pose challenges for welding and assembly processes.

There is a strong desire to develop efficient and robust TE modules using the same parent TE compounds to facilitate fabrication and ensure long-term stability. This approach has been successfully demonstrated with commercially available modules, such as Bi2Te3, PbTe, and SiGe modules used by NASA in deep space exploration, which are all made from the same parent n- and p-type TE materials.

In response to this challenge, a research team led by Professor Wan Jiang and Lianjun Wang from Donghua University and Doctor Qihao Zhang from the Leibniz Institute for Solid State and Materials Research Dresden has developed novel TE modules consisting of both n-type and p-type Mg3Sb2-based alloys. These alloys were fabricated using mechanical alloying and spark plasma sintering, resulting in well-matched TE and mechanical properties.

Finite element simulations and thermomechanical coupling calculations were conducted to optimize the module design and minimize thermal stresses. Iron was used as a diffusion barrier layer, and a one-step sintering process was adopted for TE joint fabrication. The researchers also developed a new joining process using Ag composite pastes for low-temperature assembly, capable of withstanding higher service temperatures.

These efforts led to the successful fabrication of fully Mg3Sb2-based modules with a high efficiency of 7.5% at a heat source temperature of 673 K. The modules exhibited exceptional reliability against thermal cycles, showcasing the potential of Mg3Sb2-based modules for efficient electricity generation from low-grade waste heat.

The findings have been published in the journal National Science Review.

Source: Science China Press

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