UVA Materials Scientists Study Xenotime’s Potential to Protect Turbine Engines in Extreme Environments
Pre-pandemic, air travel was the biggest contributor to greenhouse gas emissions. With passengers’ appetite for travel returning, the Environmental and Energy Study Institute estimates a tripling of commercial aircraft emissions by 2050.
But the news is not all bad. U.S. airlines have either met, or will meet, their 2030 target for reducing cruise fuel consumption, an indicator of carbon dioxide emissions through the tailpipe.
Airliners can go further and faster with less fuel thanks to new materials and coatings, like those being developed at the University of Virginia School of Engineering and Applied Science, that allow jet engines to run hotter and more efficiently. Modern gas turbine engines can reach 3,600° Fahrenheit — not hot enough to melt a diamond, but hotter than lava. This is what materials scientists call high-temperature, extreme and aggressive environments.
Padraig Stack, a UVA Ph.D. student of materials science and engineering, works on dual purpose thermal environmental barrier coatings that make turbine engines more robust and able to survive longer in these environments. Stack is working in the advanced high-temperature materials research group led by Elizabeth J. Opila, University of Virginia Rolls Royce Commonwealth Professor of Engineering and professor of materials science and engineering, with a courtesy appointment in mechanical and aerospace engineering. Opila’s research group combines strengths in corrosion and structural materials to increase engine efficiency and boost the protection of engine parts.
Stack developed an interest in high-temperature materials through an undergraduate research experience at the Oak Ridge National Laboratory while earning their bachelor’s degree in corrosion engineering at the University of Akron. Their desire to do more led them to Opila’s lab at UVA.
A ceramic material like silicon carbide can improve both efficiency and durability in combustion turbines, with some engineering enhancements.
“Silicon carbide is a strong, lightweight and heat-resistant material. This makes it highly desirable for making jet engines, but steam just eats away at it,” Stack said. “We are creating an environmental barrier coating to prevent that from happening.”
Stack is picking up research begun by Opila group member Mackenzie Ridley, who earned his Ph.D. in materials science and engineering at UVA Engineering in 2021. Ridley’s dissertation research demonstrated that a rare earth phosphate was an excellent candidate for an environmental barrier coating. This work pointed to a next-generation environmental barrier coating made with xenotime, a multi-component rare-earth phosphate mineral found in places as diverse as Brazil, Norway, Pakistan and the Democratic Republic of Congo. Unmined deposits of xenotime can be found throughout the world.
Xenotime has an added benefit: It comes out of the ground ready to use, relatively speaking. Other rare earth minerals must be heavily refined and processed to separate desirable chemical compounds from contaminants.
Xenotime is 80% yttrium phosphate with the rest being other rare earth phosphates, like ytterbium phosphate, a material that Ridley investigated and found promising.
“We want to cut out the middleman and reduce the number of steps,” Stack said. “With a tiny bit of processing, maybe we can get something we can coat with.”
Stack and third-year undergraduate materials science major Alex Uy decided to go digging. Luckily, Opila found xenotime samples at a Charlottesville mineralogist shop, which they have begun to characterize.
Using a technique called energy dispersive spectroscopy, Stack and Uy determined what elements are in the sample and their relative concentration. An additional technique, called x-ray diffraction, confirmed the structure of a single crystal.
Putting together the results of these two experiments, Stack and Uy could see what they had on their painter’s palette for a new coating.
“There are four other rare earth elements that have found their way into this mineral, so we want to find out how each of these play a part in making xenotime a good protective coating,” Stack said.
Using Ridley’s dissertation results as a baseline for comparison, Stack has begun a new suite of tests, going one element at time to see how they compare to one another; which has favorable properties; and which characteristics could apply to most any xenotime sample or mineral deposit. The process is painstaking but leaves Stack optimistic.
“It’s all novel, so even an unfavorable or null result tells us something useful,” Stack said.