Carbon observation by electron energy-loss spectroscopy and thermoelectric properties of graphite added bismuth antimony telluride prepared by mechanical alloying-hot pressing

The effects of additional graphite in (Bi _0.3 Sb _1.7 Te _3.1 ) _1−x C _x (x = 0, 0.004, 0.012, 0.032, 0.06, and 0.12) prepared by mechanical alloying followed by hot pressing were investigated. Carbon was added to obtain a low thermal conductivity via phonon scattering. The samples were examined by X-ray diffraction, scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy, and electron energy-loss spectroscopy (EELS). EELS can be used to investigate the distributions of light elements such as carbon. The diffraction peaks indicated a single-phase Bi ₂ Te ₃ –Sb ₂ Te ₃ solid solution. All the specimens were p-type semiconductors and SEM and TEM images showed dense without coarse grains. Agglomeration along the grain boundaries and inhomogeneous dispersion of carbon was observed by EELS. (Bi _0.3 Sb _1.7 Te _3.1 ) _0.88 C _0.12 grains wrapped by carbon layers of thickness approximately 50 nm were observed. The thermal conductivity of (Bi _0.3 Sb _1.7 Te _3.1 ) _1−x C _x increased with increasing x. It is considered that the presence of a large amount of carbon affected the thermal conductivity of the Bi _0.3 Sb _1.7 Te _3.1 matrix because the thermal conductivity of carbon is much higher than that of Bi _0.3 Sb _1.7 Te _3.1 and the carbon was dispersed inhomogeneously. Bi _0.3 Sb _1.7 Te _3.1 without additional graphite had a maximum dimensionless figure of merit ZT = 1.1. The ZT value decreased, and varied from 0.8 to 1.0, for (Bi _0.3 Sb _1.7 Te _3.1 ) _1−x C _x . The results show that inhomogeneously dispersed carbon did not improve the thermoelectric properties of Bi _0.3 Sb _1.7 Te _3.1 .