Direct Ink Writing of Three-Dimensional Thermoelectric Microarchitectures

Micro thermoelectric modules can be used to develop unique components such as energy harvesters, active coolers, and thermal sensors in various integrated systems. However, the fabrication of these modules still relies on expensive traditional microfabrication processes that produce only two-dimensional (2D) thermoelectric films. This limitation severely limits the formation of a temperature gradient across the thermoelectric film, so that sufficient power is not generated to run the integrated system. Here, we present direct ink writing of a micro-scale three-dimensional (3D) thermoelectric architecture for fabricating high-performance micro-thermoelectric generators. We synthesized a Bi₂Te₃-based TE particle ink with extremely viscoelastic properties and applied it to a direct-write process to print microscale 3D TE filaments with high aspect ratios and controlled diameters ranging from 180 µm to 810 µm. Thereby, we could build highly performant 3D TE architectures such as arches and lattices standing on a substrate by computer-aided model design. Our group demonstrated that the addition of a Sb₂Te₄^2-based chalcogenidometallate (ChaM) ionic additive ensured the viscoelasticity of the TE particle colloidal ink, enabling 3D layer-by-layer deposition of the TE ink. However, the moderate viscoelasticity observed in these inks was not sufficient for the direct writing process. To overcome this problem, we introduced a new design principle for TE particles in terms of size, size distribution and surface state. First, we demonstrated that the smaller size and narrower size distribution of TE particles results in higher viscoelasticity due to an increase in the effective volume occupied by the particles. Second, the controlled oxidation of the TE particle surface minimizes the screen effect due to the ChaM additive, resulting in a higher viscoelasticity of the ink than that observed with the non-oxidized particle ink. Based on these results, TE inks were optimized to achieve the extremely high viscoelasticity required for 3D direct writing. In addition, the sample dispalyed ZT values of 1.1 for p-type and 0.5 for n-type, similar to the ZT values of typical Bi₂Te₃-based bulk ingots. The direct writability of TE inks allows for microscale TE leg design for optimized thermal management, maximizing temperature gradients and output power in μ-TEGs. We demonstrated the fabrication of μ-TEGs via direct 3D writing of a TE leg with an aspect ratio of 4.1 on a patterned electrode array. Upon heating, the high anisotropy of the TE leg created a large temperature difference of 82.9 °C, resulting in a power output in the order of μW for the unicouple of the TE leg.These results validate the practicability of our 3D direct writing process for fabricating high-performance μ-TE modules that can be integrated into the electronic systems