Microstructure evolution during reduction and sintering of 3D-extrusion-printed Bi2O3+TeO2 inks to form Bi2Te3

As an alternative to beam-based additive manufacturing, 3D ink-extrusion additive manufacturing is studied here for thermoelectric Bi2Te3, starting from Bi2O3+TeO2 oxide precursor powders. In situ synchrotron XRD in flowing H2 at elevated temperatures reveals the complex phase evolution upon co-reduction leading to the formation of Bi2Te3, Bi2TeO5 and Bi2TeO2. Sintering trials performed using optimal temperatures identified by in situ XRD show that low heating rates and extensive holding times are required to achieve full co-reduction to pure Bi2Te3. The formation of liquid Bi at the temperatures required for oxide reduction leads to local transient-liquid-phase sintering, creating a coarse-grained porous structure. To limit the amount of free Bi, compositional homogenization prior to reduction is achieved by oxide pre-sintering in air, allowing to access the high temperatures required for interdiffusion. After co-reduction of the complex Bi2Te4O11+Bi2Te2O7+TeO2 pre-sintered ceramic, fine-grained, Te-rich n-type Bi2Te3 is obtained, achieving a zT of ∼0.4 between 373 and 423 K. The present study demonstrates the feasibility of synthesizing thermoelectrics from oxide precursors, with the potential of simplifying the processing chain and reducing cost to obtain 3D-extruded thermoelectric parts with complex shapes and architectures.