Preparation, thermoelectric properties, and crystal structure of boron-doped Mg2Si single crystals

摘要

Mg 2 Si is a potential thermoelectric (TE) material that can directly convert waste energy into electricity. In expectation of improving its TE performance by increasing electron carrier concentration, the element boron (B) is doped in Mg 2 Si single crystals (SCs). Their detailed crystal structures are definitely determined by using white neutron holography and single-crystal x-ray diffraction (SC-XRD) measurements. The white neutron holography measurement proves that the doped B atom successfully substitutes for the Mg site. The SC-XRD measurement confirms the B-doping site and also reveals the presence of the defect of Si vacancy (V Si ) in the B-doped Mg 2 Si SCs. The fraction of V Si increases with increasing B-doping concentration. In the case of B-doped Mg 2 Si polycrystals (PCs), V Si is absent; this difference between the SCs and PCs can be attributed to different preparation temperatures. Regarding TE properties, the electrical conductivity, σ , and the Seebeck coefficient, S , decreases and increases, respectively, due to the decrease in the electron carrier concentration, contrary to the expectation. The power factor of the B-doped Mg 2 Si SCs evaluated from σ and S does not increase but rather decreases by the B-doping. The tendencies of these TE properties can be explained by considering that the donor effect of the B atom is canceled by the acceptor effect of V Si for the B-doped Mg 2 Si SCs. This study demonstrates that the preparation condition of Mg 2 Si should be optimized to prevent the emergence of an unexpected point defect.