Enhancing Conversion Efficiency of Direct Ink Write Printed Copper (I) Sulfide Thermoelectrics via Sulfur Infusion Process

Copper (I) sulfide (Cu₂S) is a low-cost, earth-abundant, and non-toxic thermoelectric material for applications in the middle–high temperature range (>650 K). Although 3D printing these materials can simplify their manufacturing, elevated temperatures observed during sintering impair their crystal structure and energy conversion efficiency. In this study, we demonstrated a novel post-processing methodology to revert the thermoelectric properties of the 3D printed Cu_2-xS materials back to the unimpaired state via sulfur infusion. After printing and sintering, sulfur was infused into the specimens under vacuum to optimize their crystal structure and achieve high thermoelectric efficiency. Chemical analysis and X-ray Diffraction (XRD) tests showed that after the sulfur infusion process, the Cu/S ratio was reverted close to the stoichiometric level. The 3D printed Cu_2-xS showed p-type thermoelectric behavior with electrical conductivity peaking at 143 S-cm⁻¹ at 750 K and Seebeck coefficient of 175 µV-K⁻¹ at 627 K. The figure of merit (ZT) value of 1.0 at 780 K was achieved, which is the highest value ever reported for a 3D printed Cu_2-xS thermoelectrics at this temperature. The fabrication of environmentally friendly thermoelectric materials with extended dimensional freedom and conversion efficiency has the potential to impact the thermoelectric industry with new energy conversion applications and lowered manufacturing costs.