The Critical Point of Average Grain Size in Phonon Thermal Conductivity of Fine-Grained Undoped Lead Telluride

Undoped PbTe was melted at 1123 K, ball milled (BM) at rotation speeds from 90 to 180 rpm and hot pressed (HP) at 147 MPa and 650 K. Milling at 120 rpm produced the minimum phonon thermal conductivity of 1.29 W m−1 K−1 and average grain size of 0.80 µm. Phonon thermal conductivity was constant from coarse grain size to fine grain size of 1 µm and decreased suddenly at 0.80 µm. This tendency of phonon thermal conductivity corresponded to theoretical calculations with grain boundary scattering. However, the observed critical point of 1 µm was much larger than the calculated value of 0.03 µm. There was a significant inverse relationship between phonon thermal conductivity and FWHM of X-ray diffraction peaks. The low phonon thermal conductivity was associated with not only grain boundary scattering but high internal strain

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 (Bi0.3Sb1.7Te3.1)1−xCx (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 Bi2Te3–Sb2Te3 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. (Bi0.3Sb1.7Te3.1)0.88C0.12 grains wrapped by carbon layers of thickness approximately 50 nm were observed. The thermal conductivity of (Bi0.3Sb1.7Te3.1)1−xCx increased with increasing x. It is considered that the presence of a large amount of carbon affected the thermal conductivity of the Bi0.3Sb1.7Te3.1 matrix because the thermal conductivity of carbon is much higher than that of Bi0.3Sb1.7Te3.1 and the carbon was dispersed inhomogeneously. Bi0.3Sb1.7Te3.1 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 (Bi0.3Sb1.7Te3.1)1−xCx. The results show that inhomogeneously dispersed carbon did not improve the thermoelectric properties of Bi0.3Sb1.7Te3.1

Synthesis and thermoelectric properties of bismuth antimony telluride thermoelectric materials fabricated at various ball-milling speeds with yttria-stabilized zirconia ceramic vessel and balls

p-type Bi0.3Sb1.7Te3.0 thermoelectric materials were fabricated at various ball-milling speeds with yttria-stabilized zirconia (YSZ) ceramic balls in an YSZ vessel, and then hot-pressed. The powders milled at speeds of or higher than 150 rpm were completely alloyed and single-phase Bi0.3Sb1.7Te3.0 was obtained. The grain size of a disk sintered at 350 ℃ was approximately 1 μm at a fracture surface. The Seebeck coefficients of sintered disks fabricated by YSZ milling were higher while their electrical conductivities were lower than those of disks fabricated by using a stainless-steel vessel and Si3N4 balls, as the YSZ milling suppressed the contamination by materials acting as carrier dopants in the Bi0.3Sb1.7Te3.0 bulk. The contamination from the YSZ vessel and milling balls did not affect the phonon thermal conductivities of the Bi0.3Sb1.7Te3.0 bulk materials. The dimensionless figure of merit ZT of the sample milled at 150 rpm with the YSZ vessel and balls and sintered at 350 ℃ was approximately 1.7 times that of the sample milled with the stainless-steel vessel and Si3N4 balls. ZT remained above 1.0 and reached the peak of 1.16 (α: 295 μV/K, σ: 4.16 ×104 S/m, κ: 0.94 W/(m K)) at room temperature for the sample milled at 130 rpm and hot-pressed at 350 ℃. Thus, the thermoelectric properties can be improved by selecting appropriate milling vessels and balls