Determining the Structure-Property Relationships of Thermoelectric Materials Using Scanning Probe and In Situ TEM Imaging

Our scientific objective in this work is to determine the “structure” part of the structure-property relationship for key promising thermoelectric (TE) materials currently under development by the Jet Propulsion Laboratory’s Thermoelectric Materials Group, led by Dr. Sabah Bux. The broader impact of this project will be to train two pre-qualified graduate students in the Guiton Group in key characterization techniques, and facilitate them in the collection of preliminary data and development of expertise, with which they may apply for NASA graduate fellowships in Fall 2018 and Spring 2019. Dr. Bux has expressed her interest in these research projects, and willingness to support their fellowship applications. For the past 10 months, supported by our NASA KY RIDG grant, Guiton Group graduate student, Melonie Thomas, has been working to characterize a series of TE materials developed by the Bux Group, and in particular has focused on determining the interfacial relationship between nickel (Ni) nanoparticles and a lanthanum telluride (La3Te4) matrix within which they are embedded. The coherency of this interface, and the particular crystallographic orientation of each of the two components is key to the TE performance of the composite. This composite is particularly challenging to characterize due to its extreme air sensitivity; the material oxidizes within a few seconds to minutes exposure to air. Melonie has made recent breakthroughs in determining the interfacial structure using a focused ion beam instrument coupled with an air-protected transfer holder, to transfer “lift-out” cross-sections to the transmission electron microscope (TEM) without exposure to air. This work is performed at Oak Ridge National Laboratory (ORNL, at which Melonie is based full-time). The current award will enable two pre-qualified graduate students in UK Department of Chemistry’s doctoral program, Rose Pham and Andrew Schlereth, to build on and extend this collaboration, studying both the Ni-La3Te4 composite, as well as nanostructured silicon, also under development in the Bux Group. Rose proposes to use in situ heating in the TEM to determine the thermal stability of the active interfaces, and Andy proposes to use scanning probe microscopies – in particular scanning conductance microscopy – to elucidate percolation pathways and their effect on electrical transport. Melonie currently uses an Hitachi HF3300 TEM at ORNL (non-aberration-corrected), to be replaced in August by a state of the art, high-order aberration-corrected JEOL NEOARM, a specialized instrument intended to enable compositional analysis of light atoms with single-atom resolution. Rose has trained in in situ microscopy here at UK, using the Guiton group’s in situ holder with the (decades old) JEOL 2010F TEM, to be replaced in June/July with an FEI Talos, specialized for high resolution analysis. To complete this project Rose will work under the mentorship of Melonie, transitioning from the JEOL 2010F (UK) to the Hitachi HF3300 (ORNL) in June, to learn high-resolution analysis of the interface, and back to the FEI Talos (UK) in July, to implement in situ analysis of the same samples. Ultimately Melonie and Rose will work together using the JEOL NEOARM (ORNL) for single-atom resolution in situ imaging, to obtain single-atom resolution data from the interface in real-time. (The ORNL instruments are available via a current user proposal.) For the scanning probe microscopy experiments Andy will work in collaboration with the group of Dr. Douglas Strachan (UK Physics), making use of their Asylum Research atomic force microscope. Dr. Strachan serves on the thesis committees of both students.