ADVANCED MATERIALS & PROCESSES | MAY/JUNE 2023 5 GRAIN BOUNDARY ENGINEERING PROGRESS Researchers from the Max Planck Institute for Iron Research (MPIE), Germany, along with colleagues from Northwestern University and the Leibniz Institute for Solid State and Materials Research Dresden, recently tuned the microstructure of thermoelectric materials by doping the grain boundaries with titanium. Because thermoelectric materials are being considered for power generation—to transform waste heat into electricity—the team’s goal was to modify the grain boundaries so that only thermal conductivity is reduced, while electrical conductivity remains high. The scientists used a Ti-doped NbFeSb half-Heusler intermetallic compound, a new and promising thermoelectric alloy. It has excellent thermoelectric properties at mid to high temperatures, good thermal and mechanical robustness, and its elements are abundant. Because the grain size is small, the increased number of grain boundaries significantly reduces electrical conductivity. “By doping the alloy with titanium, we found that grain boundaries become titanium-rich and no longer resistive, so that we can fully utilize the beneficial low RESEARCH TRACKS / FEEDBACK The titanium-rich grain boundary phase provides a conductive path (left), while the iron-rich grain boundary phase is resistive to electrons (right). Courtesy of MPIE. thermal conductivity provided by the small grain size,” explains researcher Siyuan Zhang. After demonstrating the strategy of grain boundary engineering, the team is now exploring new ways to selectively dope these boundaries. www.mpie.de. MAKING MORE SENSE OF MOFs A new study from the University of Pittsburgh Swanson School of Engineering (Pitt) shows that metal-organic frame- works (MOFs) heat up substantially when they soak up gases—and if they get too hot, they quit working. “This study helps us determine which MOFs can soak up gases and dissipate that heat efficiently, ultimately moving MOFs closer to practical commercial implementation,” says researcher Chris Wilmer, associate professor of chemical and petroleum engineering. Researchers used computational screening of thermal conductivity in over 10,000 MOFs, a task that required more than a million hours of supercomputing power. They learned that MOFs with high densities, small pores, and 4-connected metal nodes are more capable of conducting heat. Conversely, those with extremely large pores are not. Wilmer and his colleagues, including researchers from Pitt, Colorado School of Mines, Carnegie Mellon University, and the University of California, Berkeley, are focusing on designing MOFs with excellent thermal properties. “There are millions of different types of MOFs one can design, so it can be hard to determine the best one for the job,” says Pitt researcher Meiirbek Islamov. “This study allows us to be more accurate as we create them in a lab.” www.engineering.pitt.edu. SIZING ERRATUM The news item “Measuring Thin Skin of Calcium Nuclei” in the March 2023 issue of AM&P stated that a femtometer is “just one billionth of a meter.” A femtometer is actually 10-15 m, or one quadrillionth of a meter. Kirk Cooper Worthington Industries Editor’s Note: We regret not catching the error that appeared on the original Department of Energy press release used as source material. We have notified them. FEEDBACK We welcome all comments and suggestions. Send letters to joanne.miller@asminternational.org. Building blocks used to build 10,194 hypothetical MOFs containing 1015 topologies. Courtesy of npj Computational Materials, 2023.
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