Rheological design of 3D printable all-inorganic inks using BiSbTe-based thermoelectric materials

In order to achieve high-quality 3D printing of inorganic materials, a thorough evaluation of appropriate rheological characteristics and methodologies for formulating all-inorganic inks is required. We recently reported all-inorganic inks using BiSbTe-based thermoelectric particles coupled with a chalcogenidometallate (ChaM) inorganic binder. In the current study, we analyzed the rheological behavior of the all-inorganic inks to assess printability and 3D structural retention with respect to the ChaM content. The stress sweep and three-interval thixotropy test (3ITT) were conducted to mimic a 3D printing and interpret the flow behavior under nonlinear viscoelastic region. The binder-free inks showed a sharp overshoot of the loss modulus (G″) followed by the fluctuation of both the storage modulus G′ and G″ in the stress range of 10–50 Pa. In addition, the inks developed stronger colloidal structure than the initial state after the 3ITT, resulting in the non-uniform jetting. The nonlinear flow of the inks became stable by incorporating ChaM. However, the excessive ChaM (37.5 wt. %) brought about stress-induced structure regeneration analogous to the binder-free inks. The 3D structure integrity was interpreted by yield stress and solid-like characteristics based on the frequency sweep results after undergoing deformation. Poor printability from the 12.5 wt. % ChaM-containing ink was correlated to low yield stress (2 Pa) and a high slope of the G′ curve. However, the 25 wt. % ChaM-containing ink gave a high yield stress of 48 Pa and a low G′ curve slope of 0.2 even after nonlinear deformation, resulting in high 3D shape retention and printability.