Controlling the Thermal Stability, Threshold Voltage, and Thermoelectric Properties of Cuprous Sulfide Thermoelectrics.

Abstract

Cu-ion liquid-like copper sulfide materials have excellent thermoelectric properties, while their applications are limited by their high-temperature decomposition and electric field-driven Cu precipitation issues. In particular, high thermoelectric properties and electric field-driven degradation are difficult to reconcile because liquid-like Cu ions are dominant in low κ and high ZT, while they cause electric field-driven degradation. Here, we control the sintering current and duration time to remove the Cu1.8S phase, thereby inhibiting the thermal decomposition of the copper sulfide samples, and introduce the Fe element into the sample matrix to improve its resistance to electric field-driven degradation. We reveal that the kinetic process of Cu1.8S phase decomposition can be suppressed by increasing the relative density of the sample or covering a layer of dense coating/film on the surface of the sample. However, as long as the Cu1.8S phase is present in the sample, it cannot maintain thermal stability above 450 °C. Furthermore, we find that the Fe element forms a nanogrid spinodal decomposition structure in the sample matrix, which acts as a barrier wall to prevent the long-range diffusion of liquid-like Cu ions and inhibit the electric field-driven degradation. The freely movable liquid-like Cu ions in the grid maintain a strong scattering of phonons in a short range, so the sample possesses low κ and high ZT. Then, a strategy to unify the high thermal decomposition temperature, high threshold voltage, and high thermoelectric performance of copper sulfide thermoelectric materials is proposed: transforming the Cu1.8S phase and introducing a liquid-like Cu ion migration barrier.