Microstructural Instability\nand Its Effects on Thermoelectric\nProperties of SnSe and Na-Doped SnSe

Effects of thermal cycling on the microstructure and\nthermoelectric\nproperties are studied for the undoped and Na-doped SnSe samples using\nX-ray computed tomography and property measurements. It is observed\nthat thermal cycling causes significant cracks to develop, which decrease\nboth the electrical and lattice thermal conductivities but do not\naffect the thermopower. The <i>zT</i> values are drastically\nreduced after the repeated heat treatment. It is important to account\nfor density changes during cycling to obtain accurate values of the\nthermal conductivity. Even before thermal cycling, the spark-plasma\nsintered (SPS) samples have a significant number of microcracks. The\norientation of cracks within the SPS pellets and their effect on the\nmicrostructure are influenced by the presence of a Na-rich impurity.\nThe SnSe and Sn<sub>0.995</sub>Na<sub>0.005</sub>Se samples without\nthe impurity develop cracks and exhibit grain growth parallel to the\npellet surface, which is also the plane of the 2D SnSe layers. The\nSn<sub>0.97</sub>Na<sub>0.03</sub>Se sample containing the impurity\ndevelops cracks that are orthogonal to the pellet surface. Such an\norientation of cracks in Sn<sub>0.97</sub>Na<sub>0.03</sub>Se inhibits\ngrain growth. All samples appear mechanically unstable after thermal\ncycling.