Optimization and Analysis of Thermoelectric Properties of Unfilled Co1-x-yNixFeySb3 Synthesized via a Rapid Hydrothermal Procedure

A series of nanostructured co-doped Co1-x-yNixFeySb3 were fabricated using a rapid hydrothermal method at 170 +/- C for a duration of 12 h, followed by evacuated-and-encapsulated heating at 580 +/- C for a short period of 5 h. The resulting samples were characterized using powder X-ray diffraction, field emission scanning electron microscopy, bulk density, electronic and thermal transport measurements. The power factor of Co1-x-yNixFeySb3 is significantly enhanced in the high-temperature region due to significant enhancement of the electrical conductivity and absolute value of thermopower. The latter arises from the onset of bipolar effect being shifted to higher temperatures as compared with the non-doped CoSb3. The room temperature thermal conductivity falls in the range between 1.22 and 1.67 W m(-1) K-1 for Co1-x-yNixFeySb3. The thermal conductivity of both the (x,y) = (0.14,10) and (0.14,12) samples is measured up to 600 K and found to decrease with increasing temperature. The thermal conductivity of the (0.14,10) sample goes down to similar to 1.02 W m(-1) K-1. As a result, zT = 0.68 is attained at 600 K. The lattice thermal conductivity is analyzed to gain insight into the contribution of various scattering processes that suppress the heat transfer through the phonons in Co1-x-yNixFeySb3. The effect of the simultaneous presence of Co, Ni, and Fe elements on the electronic structure and transport properties of Co1-x-yNixFeySb3 is described using the quantum mechanical tunneling theory of electron transmission among the potential barriers.