Rheological Research of 3D Printable All-Inorganic Thermoelectric Inks for Direct Writing of Micro-Thermoelectric Generator

The thermoelectric (TE) generator is a solid-state device that directly converts the flow rate (temperature difference) into electrical energy and this energy transformation offers a unique solution for producing electricity from waste heat. For improving the performance and efficiency of the TE generator, 3d printing methods for TE materials are needed to create suitable geometries for heat sources. Usually, in the fabrication of TE inorganic 3D-printable inks due to their desired viscoelastic, non-toxicity, and low melting point, organic binders are widely used. This organic-inorganic hybrid system can be good for structure formability in the printing process. However, organic binders in the thermoelectric generator can act as impurities that lower the electrical properties of the printed TE structure.<br/>In this study, to attain high-quality 3D printing of TE inorganic materials, inorganic ionic binders have been used to achieve mild viscoelasticity in colloidal inks and perform layer-wise deposition of 3D TE structures without any degradation of TE performance. Our group has analyzed the rheological property of the BiTe-based TE inorganic inks to evaluate printability, thixotropy, and structure shape retention performance after printing regarding the properties of TE particles and binders to include the strong electrostatic interactions by various and comprehensive rheological analysis such as structural deformation parameters via three-interval thixotropy test (3ITT). Eventually, the optimized printing inks can be directly written into complex architectures having a high aspect ratio. And these printed micro-thermoelectric generators have exhibited large temperature gradients and a power density of 479.0 μW cm^–2.