A study of thermoelectrics, materials that convert heat to electricity, demonstrates the importance of characterizing materials using several different methods. According to Vegard’s Law of Alloys, the size of a crystalline lattice (lattice parameter) changes linearly with composition. It is actually not a law, but an empirical observation that has been found to hold true for a majority of alloys. Researchers studied thermoelectrics made from lead, tellurium, and sulfur and varied the tellurium and sulfur content to answer the question “If a thermoelectric follows Vegard’s Law, does that make it an alloy?” The answer is: not necessarily. High-resolution synchrotron X-ray diffraction data taken at the Department of Energy’s Advanced Photon Source, probing long-range order, shows that this material adheres to Vegard’s Law. Solid-state tellurium (125Te) nuclear magnetic resonance (similar to magnetic resonance imaging) was used to determine the short-range structure and composition of the samples with high accuracy; sulfur shows strong effects on the resulting NMR spectra, with the Te peaks shifted depending on the number and arrangement of sulfur atoms near Te. Combined NMR and infrared spectroscopy results show that above 16% sulfur the samples are not alloys. Additionally, inhomogeneities improve the material’s ability to convert heat to electricity. These findings suggest that there may be many thermoelectric systems that are not alloys as reported, but instead are nanostructured. The work also demonstrates the importance of using probes of both long range and local chemical structure to differentiate between alloys and composites.
Wednesday, August 7, 2013
Advanced Functional Materials