Physics doctoral student Michael Mroz uses a low-energy electron microscope to examine the physical properties of compounds that make up thermionic cathodes.
By Amanda Biederman
NQPI editorial intern
A new finding from the lab of Physics & Astronomy professor and Nanoscale and Quantum Phenomena Institute member Dr. Martin Kordesch has altered the scientific community’s understanding on the fundamental properties of materials in thermionic cathodes.
In a paper published in AIP Advances last June, Kordesch and doctoral student Michael Mroz collaborated with Mathematics professor and NQPI member Dr. Tatiana Savin to examine the properties of scandium on a tungsten surface.
Scandium is used in thermionic cathodes, a fundamental technology for satellites and space communication. For years, researchers assumed this material diffused freely across the cathode tube. However, Kordesch observed that scandium droplets actually form on its surface. This process is known as “dewetting.”
“Things that dewet can’t diffuse, but others have assumed that they are (diffusing),” Kordesch said. “This is so contrary to what people think is going on that we needed to get it on the record.” This particular case was unusual, Kordesch said, because the dewetting occurred at a solid-solid, rather than a liquid-solid, interface. The scandium clusters had formed in a crystalline state.
Puzzled by this finding, Kordesch reached out to Savin, who studies mathematical properties of materials. He spoke with Savin and her collaborator Dr. Alexander Nepomnyashchy, a professor at the Technion-Israel Institute of Technology & Northwestern University, who had visited Athens last January for the Condensed Matter and Surface Science Colloquium.
Based on Savin and Nepomnyashchy’s advice, Kordesch and Mroz tried altering the thickness of the scandium film. They found that dewetting was enhanced in thicker films.
“It was an eye-opener,” Kordesch said. “It was exactly the right thing to do. We stumble on these things because we have to deal with them (as experimentalists).”
Kordesch is currently working to examine how the addition of barium, a liquid-solid dewetter, alters the dewetting process. He said he hopes to continue to work with Savin’s group in the future, using mathematical computation to characterize physical phenomena.
The article titled “The structure of Scandium thin films on the W(100) surface observed using emission microscopy” was published at the 2018 31st International Vacuum Nanoelectronics Conference.
Abstract: Thermionic emission microscopy is used to observe scandium on W(100). Scandium thin films were deposited by thermal evaporation from a metal source onto W(100). The scandium films on W(100) were observed to dewet from the W(100) surface and to form droplets when the surface is heated: the dewetting temperature is thickness dependent, and spans a range from 500 to 800 C, far below the melting temperature (1541 C). Thinner films produce smaller droplets.
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