RESUMO
Effects of thermal cycling on the microstructure and thermoelectric properties are studied for the undoped and Na-doped SnSe samples using X-ray computed tomography and property measurements. It is observed that thermal cycling causes significant cracks to develop, which decrease both the electrical and lattice thermal conductivities but do not affect the thermopower. The zT values are drastically reduced after the repeated heat treatment. It is important to account for density changes during cycling to obtain accurate values of the thermal conductivity. Even before thermal cycling, the spark-plasma sintered (SPS) samples have a significant number of microcracks. The orientation of cracks within the SPS pellets and their effect on the microstructure are influenced by the presence of a Na-rich impurity. The SnSe and Sn0.995Na0.005Se samples without the impurity develop cracks and exhibit grain growth parallel to the pellet surface, which is also the plane of the 2D SnSe layers. The Sn0.97Na0.03Se sample containing the impurity develops cracks that are orthogonal to the pellet surface. Such an orientation of cracks in Sn0.97Na0.03Se inhibits grain growth. All samples appear mechanically unstable after thermal cycling.
RESUMO
The current work provides a comparative study of the thermoelectric properties of the Sn0.5Ge0.5Te phases doped with Sb and Bi and alloyed with Cu2Te. The Sn0.5Ge0.5Te composition was chosen based on the fact that it delivers the highest ZT value within the Sn1-xGexTe series (x≤ 0.5). Doping Sn0.5Ge0.5Te with electron-richer Sb and Bi improves both the charge transport properties and thermal conductivities. Alloying with Cu2Te optimizes the thermoelectric performance of the samples even further, yielding a ZT value of 0.99 for (Sn0.5Ge0.5)0.91Bi0.06Te(Cu2Te)0.05 at 500 °C. Hall measurements were performed to understand the effects of doping and alloying.
RESUMO
The Zn-Sb system contains two well-known thermoelectric materials, Zn1-δSb and Zn13-δSb10 ("Zn4Sb3"), and two other phases, Zn9-δSb7 and Zn3-δSb2, stable only at high temperatures. The current work presents the updated phases diagram constructed using the high-temperature diffraction studies and elemental analysis. All phases are slightly Zn deficient with respect to their stoichiometric compositions, which is consistent with their p-type charge transport properties. Either at room or elevated temperatures, Zn1-δSb and Zn13-δSb10 display deficiencies of the main Zn sites and partial Zn occupancy of the other interstitial sites. A phase pure Zn13-δSb10 sample can be obtained from the Zn13Sb10 loading composition, and there is no need to use a Zn-richer composition such as Zn4Sb3. While the Zn13-δSb10 phase is stable till its decomposition temperature of 515 °C, it may incorporate some additional Zn around 412 °C, if elemental Zn is present.