Direct Ink Printing of Thermoelectric Materials

3D ink extrusion printing (i.e., direct ink writing) enables powder deposition in a layer-by-layer fashion, in air at ambient temperature, to create 3D complex geometries. For thermoelectric materials (TEs), this versatile manufacturing technique can unlock the potential of fully printed TE modules consisting of p-type and n-type TE legs. Recently, pre-alloyed particles (e.g., Nb_1-xCoSb) were suspended in inks, printed and then densified via sintering; also, bismuth and tellurium oxide particles deposited from inks resulted in Bi₂Te₃ after hydrogen reduction. Here, we demonstrate inks, containing blended elemental powders, which are printed into TE legs and reacted during sintering to synthesize the high-temperature TE phases Yb₁₄MnSb₁₁ and La₃Te₄. To build mechanically robust TE module with high density and phase purity, several approaches are explored during post treatments of printed green bodies. Infiltration of lower-melting elements in pre-sintered structures followed by annealing can close open porosity and help achieve targeted compositions. By blending low-melting elements like Sb into precursor powders, liquid phases are formed which facilitate interdiffusion and densification during sintering. For blended elemental powders, sublimation of elements with high vapor pressure (such as Yb and Te) during sintering introduces deviations from phase stoichiometry and formation of secondary phases. We demonstrate strategies - <i>via</i> addition of excess elements and/or binary compounds in precursors - to mitigate elemental losses and increase phase purities in sintered TE legs. We discuss the effects of microstructure and phase purity on the thermoelectric figure of merit (zT), which is controlled by the Seebeck coefficients and the electrical and thermal conductivities.