Over the last few decades, various strategies have been suggested to increase the ZT values of thermoelectric materials. However, the industrial applications for thermoelectric devices have been relegated to niche applications. In general, the traditional method of fabricating thermoelectric generators (TEGs) involves synthesizing ingots followed various processes such as dicing, plating, cleaning, and soldering, which does not allow for complex geometry fabrication. Considering that most heat sources are not confined to flat surfaces, it is imperative to introduce an alternative fabrication method. 3D printing of thermoelectric materials using TE inks is the most prominent solution to tackle this issue. Thermoelectric materials that are sintered under pressure-less conditions would be conformable to various heat sources.
As a research group working on developing thermoelectric inks for many years, we were constantly on the lookout to fully utilize the advantage of ink processing. We successfully developed 3D inks for various working temperatures with superior thermoelectric performance compared with other literature values, (check our works on 3D printing of various TE materials: Nature Energy 3, 301–309, 2018; Journal of Rheology 63, 291, 2019; Nano Energy 81, 105638, 2021; Nature Communications 12, 3550, 2021; Advanced Energy Materials 11, 2100190, 2021) and we believed we were ready for another leap forward. Specifically, we were motivated to go above and beyond to produce TE ink with the highest resolution, which may have remarkable technological advancements in the thermoelectric community.
Despite recent advances in various methods of creating 3D TE materials, we found that the direct writing of TE material to produce μ-TE generator (μ-TEG) has not been demonstrated. The current method for creating μ-TEG is by MEMS technology which has the potential problem of costly multi-step complicated processes. Moreover, this technology is not suitable for fabricating structural three-dimensional (3D) TE legs with high aspect ratios in a μ-TEG; this 3D nature is especially critical in creating a large temperature gradient across a TE leg and obtaining high power in a μ-TEG. We saw this as an opportunity to produce a direct-writable ink for μ-TEG.
In this study, we realized the 3D direct ink writing of (Bi, Sb)2(Te, Se)3-based p- and n-type inks to build arbitrarily shaped 3D TE architectures with high aspect ratios toward the fabrication of high-power μ-TEGs. To obtain 3D-printed architectures with a high resolution and direct writability, precise control of the viscoelastic properties of the ink is essential. The rheological property of the colloidal system is dependent on various material parameters such as particle size, dispersity, shape, surface charge, volume fraction that affect the interparticle interaction. We systematically investigated the colloidal rheology of the ink by analyzing the correlation between the size and size distribution and the surface charge of the particles. We found that smaller average particle sizes and narrower particle size distribution produced a higher ink viscosity. Moreover, controlled surface oxidation of the TE particles reduced the “screen effect” caused by the ChaM additives and showed dramatic enhancement in the rheological properties.
With the in-depth comprehension and meticulous design of the colloidal inks, we were able to print 3D TE filaments with a high aspect ratio and fabricated high-performance μ-TEG. In our honest opinion, the TE performance would have been much higher if we found a way to further reduce the contact resistance between the Ag-containing adhesive and the TE materials. We hope that our 3D printing process paves a new way for cost-effective and rapid manufacturing of μ-TEG, accelerating the real-world application of TE technology. We believe this approach is an important advance toward a broad technological adaption of 3D printable materials in electronic device manufacturing.
For more information, please see our recent publication in Nature Electronics: "Direct ink writing of three-dimensional thermoelectric microarchitectures".