Effects of magnetism and size of nano-oxide inclusions on the thermoelectric properties of Ge0.96Bi0.06Te

In this paper, we comparatively investigate the effects of compositing nanoparticles (NPs) with different sizes, contents, and magnetism on the thermoelectric properties of p-type Ge0.96Bi0.06Te (the NPs include diamagnetic TiO2 with the sizes of 30 and 100 nm, superparamagnetic 30 nm Fe3O4 and ferromagnetic 100 nm Fe3O4), with the purpose of distinguishing the influence of magnetism from that of nanostructures. Large-size inclusions (100 nm) reduce scattering factor r and increase lattice thermal conductivity due to their large size and low number density of NPs (N-NP = 10(18)-10(19) m(-3)), indicating that they have less chance to scatter phonons and carriers and can hardly optimize the thermoelectric performance regardless of whether the NP is magnetic or not. Without an external magnetic field, the 100 nm ferromagnetic Fe3O4 inclusions with multi-domain structures play a similar role to diamagnetic TiO2 inclusions of the same size. Small-size inclusions (30 nm) lead to a high N-NP (10(20)-10(21) m(-3)). Diamagnetic 30 nm TiO2 inclusions slightly enhance the r from -0.5 to about -0.48 due to the appropriate nanostructures at the interfaces, and single-domain superparamagnetic 30 nm Fe3O4 inclusions can greatly improve the r to -0.41 due to the additional multiple scattering effect by the random turning of magnetic moments. The results show that there is an optimal region of N-NP (2-9 x 10(20) m(-3)) for 30 nm nano-oxide inclusions, and thermoelectric properties will not be enhanced when N-NP is too large or too small. For 30 nm TiO2, 6.91 x 10(20) m(-3) inclusions (2 mol%) can lead to a large figure-of-merit (ZT) of 1.78 at 735 K in the composite, similar to 16% higher than that of the matrix. However, 2.53 x 10(20) m(-3) superparamagnetic 30 nm Fe3O4 inclusions (just 0.2 mol%) can result in a comparable ZT value of 1.77 at 735 K and the largest average ZT of 0.88 in the temperature range of 300-785 K. It is evidently clear from this work that only magnetic nano-oxide inclusions with appropriate size can optimize the thermoelectric properties of GeTe-based materials and incorporating a small content of magnetic inclusions can result in a similar effect of a large number of non-magnetic inclusions.