Weak-ferromagnetism for room temperature thermoelectric performance enhancement in p-type (Bi,Sb)2Te3

ABSTRACT Unceasing efforts have been devoted to performance enhancement of thermoelectric (TE) materials, and recently reports suggested a new approach by utilizing magnetism to increase power factor. This work investigated the doping effect of Fe on the TE properties of Bi2Te3-based materials. A series of Fex(Bi,Sb)2-xTe3 bulk samples were synthesized via mechanical alloying combined with spark plasma sintering. XRD results and DFT calculations indicate that Fe2+ and Fe3+ would coexist and substitute Bi3+ site firstly and shrink lattice. The co-doped Fe2+/Fe3+ in (Bi,Sb)2Te3 is helpful to reduce carrier concentration (nH), band gap (Eg) and increase effective mass (m*). With further increase of x, ferromagnetic FeTe2 second phase was precipitated from (Bi,Sb)2Te3 matrix. A weak-ferromagnetism was observed in all Fe-doped samples via M-H curve, which could scatter low energy carriers along with little effect on high-energy carriers, whereby the mobility was enhanced effectively. Owing to the synergistic effect of weak-ferromagnetism and optimized band structure, the maximum power factor reaches up to 45 μWcm-1K-2 as x=0.05, which is 28.5% higher than that at x=0. The peak ZT value reaches up to 1.2 at 323 K along with a theoretical ηmax of 7.6% in the range of 303-473 K were achieved in x=0.05 sample. The results suggest that introducing weak-ferromagnetism is an effective way to enhance room temperature TE performance of Bi-Sb-Te based materials.