Ink casting and 3D-extrusion printing of the thermoelectric half-Heusler alloy Nb 1-x CoSb

Additive manufacturing of lattices with ∼600 μm diameter struts is achieved via extrusion printing of an ink containing prealloyed powder of the half-Heusler alloy Nb 1-x CoSb, followed by debinding and vacuum sintering to a relative density of ∼70–80%. The ink can also be poured and cast into square blocks, which, after debinding and sintering, achieve a similar relative density. Sintering in a Sb-rich atmosphere reduces Sb sublimation and decomposition of the half-Heusler phase. This is reflected in electrical conductivity values, for ink-cast specimens, similar to those of a sample created by vacuum hot-pressing of dry powders (1080 S/cm at 298 K). However, a lower Seebeck coefficient for the ink-cast specimens (92  vs. 150 µ V/K for a hot-pressed sample) leads to a reduced figure of merit ( zTmax = 0.10±0.015  vs. 0.26±0.04 at 873 K for a hot-pressed sample), where secondary phases, NbSb 2 and CoSb 3 were observed; these might be minimized, or even eliminated, via further optimization of the sintering conditions.